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BenzvaleneЧProperties and Synthetic Potential.

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F. G. Riddell. J. M. Lehn. J. Wagner, Chem. Commun. 1968, 1403.
F. G. Riddell, J. E. Anderson, J . Chem. SOC.Perkin Trans. 2 1977, 588.
F. G. Riddell, E. S. Turner, A. Boyd, Tetrahedron 35, 259 (1979).
J . E. Anderson, A. C. Oehlschlager, Chem. Commun. 1968, 284.
[I21 J . M. Lehn, Fortschr. Chem. Forsch. IS, 311 (1970).
[I31 M. Dauis, H. M. Hiigel, R. Lakhan. B. Ternai. Aust. J . Chem. 29, 1445
(1976).
1141 S. F. Nelsen. G. R. Weisman, E. L. Cleman, V. E. Peacock, J. Am.
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1151 S. F. Nelsen. G. R. Weisman, J . Am. Chem. SOC.98, 1842 (1976).
1161 F. G. Riddell, A. J. Kidd. J. Chem. SOC.Perkin Trans. 2 1977, 1816.
[I71 D. Hofner, I. Tamir. G. Binsch, Org. Magn. Reson. 11, 172 (1978).
[I81 D. Hofner, S. A. Lesko, G. Binsch. Org. Magn. Reson. 11, 179 (1978).
1191 F G. Riddell. M . H . Berry, E. S. Turner, Tetrahedron 34, 1415 (1978).
I201 A. R. Katritzky, R. C. Patel, J . Chem. SOC.Perkin Trans. 2 1980, 279.
[21] S. F Nelsen, Acc. Chem. Res. 11. 14 (1978).
[22] R. A. Y. Jones, A. R. Katritzky, R. Scattergood, Chem. Commun. 1971,
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[23] G . R. Weisman, S. F. Nelsen, J. Am. Chem. SOC.98, 7007 (1976).
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(251 A. R. Katritzky, R. C. Patel, J . Chem. SOC. Perkin Trans. 2 1979, 984.
1261 H.-J. Schneider, L. Stunn, Angew. Chem. 88,574 (1976); Angew. Chem.
Int. Ed. Engl. 15, 545 (1976).
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Perkin Trans. 2 1979, 143.
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Trans. 2 1980, 1734.
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[9]
[lo]
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1291 A. R. Katritzky. V. J. Baker, F. M.S. Brito-Palma, I. J . Ferguson, L. Angiolini, J. Chem. SOC.Perkin Trans. 2 1980, 1746.
1301 V. J . Baker, I. J . Ferguson. A. R. Katritzky. R. Patel, S. Rahimi-Rastgo.
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[31] I. J. Ferguson. A. R. Katritzky, R. Patel, J . Chem. SOC.Perkin Trans. 2
1976, 1564.
I321 J. E. Anderson. J . Am. Chem. SOC.91,6374 (1969).
[33] A. R. Katritzky, R. C. Patel, D. M. Read, Tetrahedron Lett. 1977,
3803.
[34] I. J. Ferguson. A . R. Katritzky. D. M. Read, J. Chem. SOC.Perkin Trans.
2 1976, 1861: A . R. Katritzky, R. C. Patel, F. M . S. Brito-Palma. F. G.
Riddell, E. S. Turner, Isr. J . Chem. 29, 150 (1980).
[35] R. A. Y. Jones, A. R. Katritzky, A. R . Martin, S. Saba, J . Chem. SOC.Perkin Trans. 2 1974, 1561.
[36] A. R. Katritzky, R. C. Patel, F. G. RiddelI. E. S. Turner, unpublished results.
[37] A. R. Katritzky, R. C. Patel, Heterocycles 9,263 (1978); F. G. Riddell, E.
S. Turner, ibid. 9,267 (1978); A. R. Katritzky, R. C. Patel, 3. Chem. SOC.
.Perkin Trans. 2 1979,993; F. G. Riddell. E. S. Turner, J . Chem. Res. (S)
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[38] F. G. RiddeIl, E. S. Turner, Heterocycles 9, 267 (1978): F. G. Riddell, E.
S. Turner, A . R. Katritzky, R. C. Patel, F. M. S. Brito-Palma, Tetrahedron 35, 1391 (1979).
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J . Am. Chem. SOC.98, 5748 (1976).
1401 A. R. Katritzky, I. J. Ferguson, R. C. Patel, J . Chem. SOC.Perkin Trans. 2
1979,981.
Benzvalene- Properties and Synthetic Potential
By Manfred Christl[*'
Dedicated to Professor Siegfried Hiinig on the occasion of his 60th birthday
Today, thanks to the versatile synthesis developed by Katz et al., benzvalene is not only the
most extensively studied valence isomer of benzene but also one of the most easily synthesized bicycle[ 1.1.O]butane derivatives. The double bond in this highly strained hydrocarbon
is particularly reactive owing to interactions between the 0 system and the double bond.
Benzvalene is one of the most reactive olefins toward electron deficient substrates. Furthermore, the compound is bifunctional, since after addition to the TI system the ring strain of
the c system provides the driving force for rearrangement or further addition reactions.
This paper summarizes the spectroscopic properties and the reactivity of benzvalene. In order to demonstrate the importance of benzvalene and its derivatives as building blocks in
organic synthesis the chemistry of compounds arising from benzvalene is also discussed.
The article concludes with a summary of substituted benzvalenes.
1. Introduction
For the research chemist to be interested in a particular
compound, three important requirements must be met:
first, it must be easily accessible, it should be highly reactive, and the reaction products must show interesting properties. Cyclooctatetraene, norbornene and without doubt
benzvalene (I) are examples of compounds satisfying these
conditions.
[*I Prof. Dr. M. Christ1
Institut fur Organische Chemie der Universitgt
Am Hubland, D-8700 Wikrzburg (Germany)
Angew. Chem. Int. Ed. Engl. 20. 529-546 (1981)
(1)
The substituted derivatives (243)"l and (245)'*I (Section
4) were already known when in 1967 Wilzback et uI.'~] identified benzvalene ( I ) as a photoproduct of benzene. Four
years later, Katz et a1.[4.51
reported a versatile synthesis
starting from lithium cyclopentadiene, dichloromethane,
and methyllithium, which made it possible to obtain 20 g
quantities of (I). This opened up the way for intensive
studies of its physical and chemical properties and also
0 Verlag Chemie GmbH. 6940 Weinheim, 1981
0570-0833/81/0707-529% 02.50/0
529
permitted an experimental evaluation of theoretical prediction~[~,'].
2. Spectroscopy of Benzvalene
The presence in benzvalene ( I ) of two functionalities in
close proximity, namely the olefinic double bond and the
bicycle[ I . 1.OJbutane system, leads to strong interactions.
This results in a UV absorption band with a maximum at
h= 217 nm (E= 2500)[81. In contrast, 1,2-dialkylethylenes
and bicyclo[ l.l.O]b~tane[~l
show no significant absorption
above 200 nm. This relatively longwave band is due, at
least in part, to the increased energy of the double bond TI
orbital (4b2) as a result of interaction with an occupied orbital of the proper symmetry in the bicyclo[l.l.O]butane
system. This follows from the photoelectron spectrum
where the first ionization potential appears at 8.55 eV[10-121
(for comparison: cyclopentene 9.01, 9.18 eV['3a1;norbornene 8.97 eVr131).
Scheme 1 shows the three highest occupied orbitals of (I). The ordering is based on calculations[6,'01,according to which the second absorption band
at 9.75 eV is due to the orbital (lOa,) localized almost exclusively in the central bond of the bicyclo[l. l.O]butane
system. The la, orbital (corresponding to the third ionization band at 10.83 eV) is an important contributor to the
four peripheral bicyclo[l. 1.O]butane bonds. The second
and third ionization potentials of ( I ) are increased and decreased, respectively, in comparison with those of the unsubstituted bicyclo[l.l.0]butane (9.39 and 11.30 eV["l).
This is a consequence of the decreased angle between the
two three-membered rings due to the etheno bridgef1']. It is
interesting to note that in homobenzvalene (118) the order
of the two highest occupied orbitals is interchanged: here,
the HOMO is the G orbital of the central bicyclo[l.l.O]butane bond"', I4I.
gative end of the dipole is on the endo side of the bicyclo[l.l.O]butane system, i.e. in (1) at the site of the C=C
double bond, which itself, however, contributes only 20%
to the dipole moment.
The IRC2']and Raman spectra'"] of ( 1 ) have been thoroughly analyzed. The exceptionally low frequency (1556
cm-') of the C=C stretching vibration as well as the
NMR spectra (Scheme 2) undoubtedly reflect the conjugation between the double bond and the strained CT system.
The signal for the allylic protons (2,5-H) appears at
6 = 1.84, i.e. at considerably higher field than the protons
in the 1,6-position (6=3.53)[']. The unusually low field absorption of 1,6-H compared to the saturated system (2yz1
is also reflected in the corresponding I3C chemical shifts:
(2) shows signals at 6=2.4 and 34.0 which are typical values for the central and lateral hydrogen atoms in bicy~lo[l.l.O]butanes~~~*~~~.
In (I) the sequence is inverted. Although the position of the C2,5 signal is more or less unchanged, the C 1,6 signal appearing at 6=48.3 is shifted by
45.9 ppm to lower field[24.2s1.This low field shift arises
from the interaction between the x* orbital of the double
bond and the a, orbital of the bicycle[ 1.1.O]butane system.
This is also found in other rigid envelope-shaped cyclopentenes, the magnitude depending on the degree of ring
Also the I3C--'H coupling constants in ( I ) and
(2) show significant differences.
3-53
1.69
I
(168.8)
1.28
133.7
(112.8)
(1)
Scheme 2. 'H- (at positions 6,2,3) and I3C-NMR data (at positions 1,5,4, in
brackets JucxH in Hz) of benzvalene ( I ) and dihydrobenzvalene (2). 6 values,
measured in C6H6and CDCI,, respectively.
3. Reactions of Benzvalenes
3.1. Metalation
4b2 (8.55eV)
l@a,(975 eV)
la (10.83 eV )
Scheme 1. The three highest occupied orbitals of benzvalene (1).
The structural parameters of ( 1 ) were established using
microwave spectroscopy[l5]and electron diffraction"61. The
results are in agreement with the exception of the CH bond
lengths (1.08 A['51and 1.14 A[161,resp.). The dihedral angle
is reduced from 121.7' in bicyclo[l.l.0]butane~'71
to 106" in
(I). Therefore, the central bond arising from atomic orbitals possessing more than 90% p-character is shortened,
from 1.497 in bicyclo[l.l.O]butane (C1-C3)['71 to ca. 1.45
A in (I) (Cl-C6). This trend is even more evident in the
case of the bicycloI1 .l.O]butanes where the dihedral angles
are even smaller[1s1.The dipole moment of (1). 0.88 D"91,
obtained from the Stark effect in the microwave spectrum,
is somewhat larger than that of bicyclo[l.l.O]butane (0.68
D)[I7].Calculationsr6]show that for both molecules the ne-
As in other bicyclo[l. l.O]b~tanes[*~~
the bridgehead hydrogen atoms 1,6-H in ( I ) are comparatively acidic and
therefore react easily with butyllithium to give the metalated product 1-benzvalenyllithium (3)[271.This in turn
reacts with methyl iodide to give 1-methylbenzvalene (4JZs1
or with heavy water to give I-deuteriobenzvalene (la). The
A
530
(4)
D
Angew. Chem. Int. Ed. Engl. 20, S29-546 (1981)
latter can undergo a further metalation, followed by treatment with D20, giving 1,6-dideuteriobenzvalene (16y7].
(la), which can also be obtained from CD,Cl, by the Katz
synthesis1z91,
as well as (lb)are highly useful in mechanistic
studies.
3.2. Isomerization and Formation
Hii~ke"~'~ mentioned
the formula for tricyclo[3.1.0.02,6]hex-3-enealready in 1937. The trivial name,
benzvalene, is, however, due to Viehe[',3'' who wished to
convey not only the relationship with benzene, but also the
possibility of degenerate valence isomerism (automerization)-e.g. through a 1,3-shift of the C6-C2 bond. According to the principle of the conservation of orbital symm e t r ~ [ ~at
" , the time unknown, the process is thermally forbidden and also unobserved due to the migration being
forced to occur suprafacially and with retention of configuration at the migrating C atom.
cal course of the reaction has been established. The AgOcatalyzed rearrangement of tricyclo[4.1.0.02.']heptane to
1,3-cycloheptadiene derivatives (the so-called a-rearrangement) is analogous to the benzvalene is~merization"~~.
Whereas metallic silver and iron transform ( I ) into benzene, platinum, palladium, gold, nickel, and copper[31catalyze the isomerization to fulvene (8f341.When (la) reacts
with copper, after ca. 100 sec at 25 " C , the benzvalene derivatives (la), (lc), and (Id) are isolated in the ratio:
81 :12 :7. It was postulated in this case that the reaction
proceeds by reversible 1,2- or 1,4-retrocarbene addition via
the copper cyclopentadienylcarbene complexes (7a)-(7c).
A second scrambling process must be involved because
the ratio of the fulvene products (Sa):(86) :(8c) (56 :24 :20),
deviates strongly from that of ( l a ):(lc) :(ld). This second
3.2.1. Silver Ions and Metals
Burger and Mazenod have shown that silver(1) ions[331
and ~ o p p e r ( o ) [catalyze
~~I
an automerization of fly5].
Age
ions catalyze the isomerization of ( 1 ) to benzene. When the
isomerization of ( l a ) is interrupted after one half-life, the
deuterium label is found distributed over positions 1, 2
and 3 [(la). (lcj, and (ldn. The fact that the starting benzvalene is labeled in only one position means that a reversible reaction, involving two cations of the type (5). must be
involved[331.This mechanism also explains the formation
of 0-,rn-. and pdideuteriobenzene when (lb) is treated
with silver salts[3sJ.The end product of the ( l a ) isomerization reaction, deuteriobenzene, is probably formed from
the cation (5) via argentobenzenium ions of the type (6).
The endo attack by Age on (la), (lc), and (Id) is analogous
to protonation (see Section 3.3. I), where the stereochemi-
,R!
\
+
Angew. Chem. Int. Ed. Engl. 20, 529-546 (1981)
process must be a [1,5]-H migration leading to an equilibrium between (7b), (7c). and (7d). The irreversible [1,2]-H
shift to the exocyclic carbon atom completes the formation
of the fulvene. The gas chromatography of ( l a ) on a copper column reportedly gives a quantitative yield of
( 8 ~ ) ~ ~ ~ ~ ~ .
3.2.2. The Mechanism of the Katz Benzvalene Synthesis
The above processes do not play a role in the Katz synthesis of ( I ) because when labeled dichloromethane is
used, the resulting benzvalene is labeled in the I-position
onlylzgl,and benzene is the only side product. A likely intermediate is the cyclopentadienylcarbene (9) which undergoes ring expansion to benzene, however, in the main reaction cyclizes to ( I ) , the latter process being an extremely
rare 1.4-addition of a carbene.
531
The fact that, apart from toluene and spiro[2,4]hepta-4,6diene only 1-methylbenzvalene (4) is formed in the reaction of (10)(formed from 5-chloromethyl-5-methylcyclopentadiene), excludes participation of the equally possible
1,2-addition as shown by Burger and GandiIlod2’]. Early
reports of benzvalenes labeled in all possible positions”5.37b1 in
. the synthesis with D- or 13C-labeled dichloromethane do not, therefore, concern the synthesis itself, but rather the subsequent metal-catalyzed automerization.
the triplet state of benzene (ca. 82 kcab’mol). It follows,
therefore, that Dewar benzene cannot be an intermediate
in this reaction. The lack of chemiluminescence in pericyclic reactions is taken as evidence for a concerted reaction,
since theory predicts the crossing of the energy surfaces for
ground and triplet states and hence chemiluminescence in
forbidden pericyciic reactions. As expected for a one-step
reaction, the thermolysis of (Ib) gives odideuterioben~ene[~~].
3.2.4. Photochemistry and Photochemical Formation
3.2.3. Thermolysis and Thermochemical Data
The half-life of the thermal isomerization of (I) to benzene in ether at 30°C is 48 hcZ8l.In n-heptane an enthalpy
of activation of 25.9 kcal/mol was measured. The reaction
enthalpy for the Age-catalyzed process is -67.5 kcal/
m01‘~’~.Since the transformation to benzene is so exothermic, and the activation energy so low, it is not surprising
that pure (1) is susceptible to detonation[41.Yet, it can be
safely handled in dilute solution.
An approximate value of the heat of formation of ( I j
(87.3 kcal/mol) can be calculated from the standard heat
of formation of benzene (19.8 k~al/mol)[~~].
Group increment~‘~’]
yield a AH! value of 10.4 kcal/mol for strain free
(1). In other words, the strain energy of (1) is ca. 77 kcaI/
mol, or a little higher than the sum of the strain energies of
bicyclo[ 1.1.0Jbutane (67 kcal/mol) and cyclopentene (5.9
k~al/mol)[~~].
According to MIND0/3 calculations the value of AH:
was at first estimated to be 114[401and then 102 kcal/
m01[10,411.
When this is compared with the MIND0/3
value of A H : for benzene, then a reaction enthalpy which
is slightly too high, i. e. 73 k ~ a l / m o l for
~ ~ the
~ ] transformation of ( I ) into benzene, is found. Although a one-step
thermal transformation of an endo.endo-bridged bicyclo[ 1. I.O]butane into a &,cis-butadiene is forbidden[321,not
only MIND0/3[4z1,but other calculation^^^^.^^ as well, favor a concerted thermal transformation of (1) into benzene.
This is because the double bond is involved in a six-elecactivation energy of 21.5
tron process. The
kcal/mol is somewhat too low.
Experimental findings are also in agreement with the
idea of a one-step process. Two plausible intermediates in
the stepwise processes are the diradical (11) and Dewar
benzene formed oia cis,cis,trans-~yclohexatriene~~~~.
When
the activation energy is compared with those of the thermal rearrangements of dihydrobenzvalene (2) and homobenzvalene (118)r4“1,
or with the AH: calculated for (11)
using group
then the activation enthalpy of
25.9 kcal/mol does not seem to be large enough to generate (11) from (I). The benzene formed in the thermolysis of
Dewar benzene (by a forbidden process)[321
is chemiluminesin spite of the fact
That formed from ( I ) is
that the energy liberated from the transition state (ca. 93
kcal/mol) would be more than sufficient for creation of
532
In contrast to the above, direct irradiation as well as
photolysis in the presence of singlet (pyrene) or triplet sensitizers (triplet energy, &, between 53 and 63 kcal/mol)
leads to automerization, i.e. formation first of (le) and
then benzvalenes with two statistically scrambled deuterium at0ms[~’1.
D
n
This is the [1,3] shift already mentioned, which Viehe[1,311
had in mind when naming the compound. A second benzvalene triplet state is populated using higher energy triplet
sensitizers. The second triplet does not significantly decay
to the first, but rather to the triplet state of benzene. The
end product is odideuteriobenzene, formed with a quantum yield of ca. 4[481,presumably because triplet benzene
sensitizes the isomerization of ( I ) to benzene[”], thereby inducing a chain process.
7
As mentioned in the introduction, (1) was first obtained
by photolysis of benzene13], a process in which fulvene (8)
had already been discoveredr491.Neither of these isomers
can be obtained in a yield higher than 300-500 mg per
liter of pure benzene on direct irradiation at 254 nm14’I.
The yields of ( I ) and (8)can be increased by dilution of the
benzene with alkaned3I. As common intermediate, the diradical (11) has been proposed[453So1.
However, (8)is possibly
derived from (I) in a quartz-catalyzed r e a ~ t i o n ~ ~( I~) ,is~ ’ ~ .
obtained in a measurable concentration in the direct gasphase photolysis of benzene only in the presence of additives, which facilitate the vibrational relaxation of (I) and
inhibit its benzene-sensitized reverse reaction[”]. The irradiation of liquid benzene at 160-200 nm gives a mixture
of ( l j and (8)and Dewar benzenecsz1.
Angew. Chem. Int. Ed. Engl. 20, 529-546 (1981)
3.3. Stepwise Additions to Bennalene
3.3.1. Oxymercuration and Action of Acids
A possible intermediate in the o x y r n e r c u r a t i ~ nof~ ~(I)
~~
is the cation (12)which is analogous to the intermediate (5)
in the Age-catalyzed isomerization. The cation (12). however, is trapped by nucleophiles such as acetate or methanolate ions (R= CO-CH3 or CH,). Work-up with sodium
borohydride gives low yields of the bicycloI3. I.O]hexenyl
acetates or the methyl ethers (13).
Thiophenol reacts with ( I ) in the presence of boron trifluoride etherate like an acid, giving a high yield of predominately ex0-(16a)~’~~.
In agreement with the above, the
use of (16) as starting material leads to the specific appearance of one deuterium atom in the e x 0 4 position; the
other deuterium atom is evenly distributed over positions I
and 5, thus giving a 1 : 1 mixture of (166) and ( 1 6 ~ )The
~~~~.
presence of a deuterium atom at one bridgehead carbon
means that in contrast to (14a), (146) is unsymmetrical.
Hence, there are two different but equivalent centers for
attack by the sulfur nucleophile.
3.3.2. Halogens
Acids also react with (1) to give bicycloI3.1.O]hexene derivatives. Even before the intermediate formation of ( I ) was
clarified, compounds of the type (13) were isolated when
benzene was photolyzed in acidified hydroxylated solvents
(ROH =CH3C02H, CF3CH20H, CH,OH, H20)1s41.The 6substituted bicyclo[3.1.O]hexene derivatives (15) also
formed originate from the benzene-sensitized vinylcyclopropane-cyclopentene rearrangement of (13).
The behavior of (I) toward the halogens is dependent
both on the nature of the halogen and the reaction conditions employed. With ether as solvent, iodine in CHCI3,
iodine and tetrapropylammonium bromide or tetraethylammonium chloride in acetonitrile, all react with (I) to
give the trans adducts (18)-(20)[581.Under these conditions, the initially formed iodonium ion is opened by an
iodide, bromide, or chloride ion. Only a small quantity of
the trans adduct ( I 7)[’*’ is formed with either pyridinium
(17),X = Y
=
BR
(18), X = Y = I
(19), X = I, Y = BR
(20), x = I, Y = CI
(21), x
(22), X
(23), X
Y
=
=
=
c1
Br
I
2
X
Using deuterium-labeled substrate^'^.^^] it has been established that, in analogy to other bicycle[ 1.l.O]b~tanes~”~,
a proton attacks at the bridgehead carbon, with retention,
from the endo side. Whether or not the nucleophile ROH
enters in the ex0 or endo position of the bicyclo[3.3.0]hexenyl cation (14) depends on the acidity of the medium. In a
strongly acidic medium “free” (14)is present; thus, the nucleophiles can attack from the more sterically favorable
ex0 side. On the other hand, in weakly acidic media the
proton transferring, and hence endo oriented nucleophile
has a better chance of attacking; the result is that up to
50% endo-(l3) is
hydrobromide perbromide or the dimethyl sulfide-bromine
complex. The main product is the endo,anti-5,6-dibromobicyclo[2.l.l]hexene (22) which in the reaction of (I) with
bromine in CC14 is the only product, isolated in quantitative yield[27,’9,601.
Similarly, with chlorine in CCI4, or iodine
in acetonitrile, only the compounds (21)59,601
and (23)[’*],
respectively, are isolated. Formally, the compounds (21)(23)are adducts of the halogens to the C1-C6 bond of ( I ) .
Mechanistically, however, this is only partially correct as
seen in the bromination or chlorination of (lb)[27,601.
87%
r
L
(14a). R = H
(14h), R = D
f/6a), R = H
(16h), R = D
Angew. Chem. In!. Ed. Engl. 20. 529-546 (1981)
fi6cj, R = D
of the bromine attacks the double bond. The resulting bromonium ion (24) undergoes a Wagner-Meenvein rearrangement to give (25)which is none other than the 6-bicyclo[2.l.l]hexenyl cation (26a) written in the classical form.
The backside attack of a bromide ion at the C6 atom leads
to (22a). (22b)is probably formed from (lb) by bromine ca-
533
tion attack at C1. The central bond of bicyclo[l.l.O]butane
is then thought to rupture through inversion to give the
non-classically stabilized cation (26b). The final step giving
(22b) is analogous to that for (26a)- (22a).
The fact that the halogen adducts are reactive is synthetically useful in the cases of (21) and (22). Already at 50 C,
(21) is transformed into the bicyclo[3.l.O~hexene (27). (22)
reacts four times faster[601.When (21) reacts with silver trifluoroacetate in benzene, the bistrifluoroacetate (28) is isolated[581.On the other hand, with potassium acetate in acetonitrile or silver acetate in benzene, the bicyc10[2.2.0.0~~~]hexane
derivative (30) is formed, which can
transform either into (29) on heating, or into (31) on treatment with HCl[59,601.The bicyclo[2.1. llhexene derivative
(31) also rearranges thermally to (29). In acetonitrile, with
an excess of potassium cyanide, (21) gives (32). When only
one equivalent of potassium cyanide is used, the intermediate (33) is isolable; this in turn isornerizes thermally to
(34). The detailed study of the rearrangements of the bicyclo[2.l.l]hexene derivatives (21), (22). (31), and (33) led to
the discovery of a manifold of unexpected complex reaction mechanisms[601.
,
OC OC H3
CF3COOh
A&
c1
C HnC 00
KCN
OC OC H3
OC OC H3
The reaction of (22) with an excess of LiAlH, afforded
an economical pathway[z71to the known, but difficultly accessible C6Hs hydrocarbon (37); (36) is the side product.
(37a) is obtained when LiAlD, is used. Thus, both deuterium atoms are endo, which suggests the bicyclo[2.l.l]hexenyl cation as a plausible intermediate[37b1.(22) gives (39)
when less than a fivefold excess of LiAIH, is
Both
silver salts and [Rh(CO),CI], catalyze the isomerization of
(37) to bicyclo[3.1 .O]hexene (36) and 4-methylenecyclopentene (38); with Age formation of (36)is favored, with Rh(1)
(38) is the main product.
3.3.3. 4-Phenyl-l,2,4-triazoline-3,5-dione
(40)
and Chlorosulfonyl Isocyanate
The mechanism of the addition of the triazolinedione
(40) to benzvalene (1) is similar to that of the addition of
bromine to (lj6'].This assumption is supported by the observation made when (40) reacts with (Ib). It is possible
that the non-classical zwitterion (41) is formed in an electrophilic addition followed by a Wagner-Meerwein rearrangement. The final step leads to (42). This compound allows the preparation of the azo compound (43) which is
also obtainable in an alternative synthesid6". Compound
(43) affords only benzene on t h e r m o l y s i ~ [ ~ ~but
" , ~its
~ ]pho,
tolysis represents the only route to the unsnbstituted prismane (44f6'], which detonates when pure but in solution
isomerizes slowly to benzene at 90°C.
The direct irradiation of (43) at 25 " C gives mainly Dewar benzene and benzene, as well as small amounts of ( I ) ,
(44) and 1,s-diazacyclooctatetraene (45)[63a1.
The low temperature photolysis, on the other hand, gives apart from
benzene and (451, only small amounts of (1) and Dewar
benzene; no (44) is formed(641.In the presence of the sensitizer acetophenone, (43) decomposes to (45) in 67% yield,
together with benzene and nitrogen in 33% yield.
\
-
+ J
O
N- N
+?=Lo
0LNAO
I
N
I
C6H5
The double bond in the bicyclo[2.1. llhexene derivative
(22) was catalytically hydrogenated; subsequent replacement of the bromine atoms by hydrogen using triphenyltin
hydride afforded bicyclo[2.1. Ilhexane (35j27.601.
A proof of the bicyclo[3.l.O]hexene structure was obtained when the hydrocarbon (36) was isolated after stepwise debromination of the bromo analog of (27). first with
LiAIH, and then with sodium in tert-butyl alcohol/tetrahydrofuran.
(35), R
(39), R
534
=
H
=
Br
(36)
(37), R = H
(37a), R = D
(38)
f 401
hu
145)
+
benzene
+
D e w a r benzene
+ (1)
1431
A secondary decomposition of (45) gives rise to the latter
two p r o d ~ c t s [ ~ ~
The
, ~ ~synthesis
].
and properties of (45)
have been described in
(45) is probably derived
from the first triplet state of (43). whereas the C6H6 isomers arise from the singlet state through cleavage of a C-N
Angew. Chem. Int. Ed. Engl. 20. 529-546 (1981)
and (56) to the corresponding sulfones. From these, 3benzvalenylphenyl sulfone (SS), as well as the explosive
bis-3-benzvalenyl sulfone (57), can be obtained by HCI
elimination using sodium bis(trimethylsi1yl)amide.
bond. The dependence of the product pattern on temperature and solvent observed in the direct photolysis of (43) is
evidence for a competition between singlet-triplet transition and C-N bond cleavage; this bond breaking process
requires thermal activationi63a1.
Chlorosulfonyl isocyanate (CSI) attacks the strained (3
bond system in bicyclo[l. l.O]butane derivatives[661,whereas
it adds to the double bond in ( I ) . The reaction of ( l b ) with
CSI supports the postulated mechanism[671.The first intermediate is probably the zwitterion (46) containing the tricyclo[3.1.0.02,6]hexylcation structure. In this intermediate
Benzvalenothiirane (60)"" can be prepared using a new
general method for the synthesis of thiiraneP9]. N-(chlorothio)succimide reacts with (1) giving a mixture of (58)and
(59).The succinimide moiety probably improves the leaving group ability of the sulfur in the intermediate episulfonium ion, thereby causing partial rearrangement to (58).
The main product (59) reacts smoothly with LiAlH, to give
(60). a stable isomer of the unknown parent thiepin.
(so),
(49), R = SOzCl
(Sl), R = H
R
/
o
R
(-52). R
=
sozci
= H
both the cis and trans-with respect to the substituentoriented bicyclo[l.l.O]butane bonds (C5-C6 and Cl-CS)
will migrate to the neighboring cationic center. This should
give a mixture of the zwitterions (47) and (48) which cyclize
to the N-chlorosulfonyllactams (49) and (50). These are hydrolyzed by sodium hydroxide to the N-unsubstituted Iactams (51)and (.52), whereas dimethylformamide transforms
them into (33) and an isomer thereof.
The addition of thiophenol to ( I ) gives four products:
exo- and endo-(lba) in a ratio of 6:1, together with (61)
and (62)[563.The formation of the bis-adduct (62) is the result of a slow secondary reaction between (61) and thiophenol following rapid addition of C6H5SHto ( I ) ; there is
precedence for such
(16a) is also formed in
the acid catalyzed reaction between ( 1 ) and C,H,SH (see
Section 3.3.1). In the uncatalyzed reaction, however, it is
formed by a different mechanistic route as has been shown
using (1b)[56,571.
3.3.4. Sulfenyl Chlorides and Mercaptans
The mechanism of the addition of sulfenyl chlorides to
olefinic double bonds is controversial. The participation of
a free episulfonium ion such as (53) is not uncontestedi6*].
At any rate, the quantitative formation of the trans adduct
(54)[671from ( I ) and benzenesulfenyl chloride shows that
rearrangement does not occur and therefore proves that an
open carbocation analogous to (46) cannot be involved.
t
(53)
1) 3-C1C6H4C03H;
(54)
r
D
1
D
I
>c1
2 ) NaN[Si(CH,),]z
Sulfur dichloride reacts with two molecules of ( I ) to give
(56). rnChloroperbenzoic acid oxidizes the thioethers (54)
Angew. Chem. Int. Ed. Engl. 20. 529-546 (1981)
The reaction proceeds via a radical chain m e ~ h a n i s r n ~ ~ ~ l ,
in which the phenylthio radical, formed by air-oxidation of
thiophenol, adds to the double bond in (Jb). The resulting
(63) can react in two ways: hydrogen abstraction from thiophenol gives (61a); a cyclopropylmethyl-homoallyl radical
rearrangement gives (64), which abstracts a hydrogen atom
from thiophenol, non-stereospecifically, giving (16c) and
535
(16d) in the ratio 1 :3[571.The dependence of the ratio
(Ida) :(61) on the thiophenol concentration supports the
above mechanistic scheme. This means that it is possible,
starting from (1) with suitable choice of reaction conditions, to obtain (16). (61) or (62) in essentially pure form
and high yields'561.Methylthiol reacts with (1) in the same
way as thiophenol, giving the methyl analogs of (16) and
(61)["].
acid to (74). presumably via the ketene hydrate complex
(73). This complex is regenerated from (74) at -30°C
through the action of fluorosulfonic acid, and at - 10°C is
further dehydrated to (72). The thermolysis of (66) in methanol at 40°C gives benzene and the methyl ester of
(74).
0
(C 0)zFe-Fe(
CO),
Fe-Fe
3.3.5. Carbonyliron Complexes
Low temperature photolysis of pentacarbonyliron in the
presence of (1) gives the x complex (65), which rapidly
isomerizes to the acyl complex (66) above 10"C'741.Excess
trifluoroacetic acid protonates (65) at -78°C into the
complexed bicyclo[3.1.0]hexeny1 cation (67). Above
50"C, (67) looses a molecule of CO and undergoes ring
expansion to the complexed cyclohexadienyl cation (68).
R
0
( 75)
(76), R
=
H; (77), R = CH3
In contrast to the photochemically induced reaction of
(1) with Fe(CO), no primary product can be obtained in
the thermal reaction with Fe2(C0)9 at 20°C[74,751.The
hexacarbonylfulvenediiron (75) is obtained together with
the two-center cyclopentadienyl and methylcyclopentadienyl complexes (76) and (77), presumably via the common
precursor (66).
3.3.6. Orientation of Electrophilic Attack
(67)
(65) reacts with only one equivalent of trifluoroacetic
acid to give the neutral complex (69) with an endo-oriented
trifluoroacetoxy group. At - 10°C an equilibrium is established between exo- and endo-(69) in which the ex0 isomer
predominates. At the same time decay of (69) to (68) is also
observed.
So far, in all the reactions described for (I), the reaction
partners have been electrophiles; even the thio radicals
can be so described. Most electrophiles selectively attack
the double bond in (1): 4-phenyl- 1,2,4-triazolinedione,
chlorosulfonyl isocyanate, sulfenyl chlorides, thio radicals,
and carbonyliron complexes. Halogens also transfer a cation to the double bond in most cases, although with bromine and chlorine an endo attack on C-1 with inversion appears to compete to a small extent.
Protons, mercuric acetate and probably also silver ions
attack C-1, as is usual for bicyclo[l.l.O]butanes, from the
endo side. Retention of configuration is, however, observed.
The reactions at the double bond of (1) are probably
HOMO controlled, since the HOMO is represented by the
71 orbital (see Section 2). The endo addition to the (T system
of ( I ) might be favored by the partial negative charge on
the endo side. These ideas are opposed by the fact that
mercury and silver ions are soft electrophiles.
3.4. Concerted Additions to the Benzvalene Double Bond
Electrophiles adding to (1)in multistep reactions can do
so by attack at either the (T or n system. In contrast, the (T
system is not capable of undergoing concerted additions.
Consequently, all products arising from a concerted reaction retain the tricyclo[3. 1.0.02,6]hexaneframework (2).
3.4.1. Hydrogenation with Diimine and cis-Hydroxylation
Protonation of (66) with fluorosulfonic acid in S02CIF
at -78 "C gives the cation (70) which reacts with methanol
to the tricarbonyliron complex (71) of cyclopentadienylacetic acid methyl ester. (70) looses a molecule of CO at
- 10°C giving the cationic ketene complex (72). Thus, (1)
has been transformed into the cyclopentadienyl ligand
while coordinated to the iron atom. (72) is hydrolyzed by
536
The double bond in (1) cannot be selectively saturated
by catalytically activated hydrogen, since under these conditions dihydrobenzvalene (2) is transformed into methylcyclopentane (78J76I. Furthermore, (1) should rearrange to
fulvene in the presence of the usual hydrogenation catalysts (see Section 3.2.1) more rapidly than hydrogen is added. In contrast, diimine smoothly transfers two hydrogen
Angew. Chem. Inl. Ed. Engl. 20, 529-546 (1951)
atoms to the double bond of (I)[221.Thus, (2), which is also
formed in low yield together with other C6H8isomers on
irradiation of 3-cyclopentenyldiazomethane[761,
can be obtained in 10 g quantities for use in a variety of reactions
(cf. Scheme 3).
o>
OCH3
/
1. n G H 9 L i
2. TosCl
RLI
(81)
n-C4Hg
sH5
The cis-glycol (90) can be prepared from ( I ) and KMn04
using standard reaction conditions, or even better with
tert-butyl hydroperoxide and OsO, as catalyst[78! The ditosylate is formed when tosyl chloride in pyridine is employed. This in turn is solvolyzed in buffered aqueous acetone to the tricyclic diol (91) isomeric with (90)[701.
The cyclic carbonate (92) is formed from (90) and phosgene in
pyridine. A mixture of two ortho esters (93)is formed from
(90) and trimethyl orthoacetate when benzoic acid is used
as a catalyst. The chlorohydrin acetate (94) can be prepared by reaction of (93) with trityl chloride and transformed into benzvalene oxide (95)1781.
+
3.4.2. [2 I]-Cycloaddition Reactions:
Epoxidation and Halocarbene Additions
I lS0T
(87)
(90)
(88)
Scheme 3. Products derived From tricyclo[3.1.0.0z~6]hexane(2)
At 20°C silver ions catalyze the isomerization of (2) to
1,3-cyclohexadiene (79)'*']. This reaction requires at least
230 "C in the absence of a catalyst. The kinetics of the latter reaction has been studied'461.Aluminum chloride transforms (2) into bicyclo[3.1.O]hexene (36). Thiophenol adds
to (2) giving the bicyclo[2.l.l]hexane derivative (80)171,721.
In analogy to ( l ) ,(2) is also metalated with n-butyllithium.
Treatment with D 2 0 causes deuteration and a repetition of
this procedure gives the 1,6-dideuterio derivativeiz2'. Using
lithiated (2) and tosyl chloride Szeimies et
synthesized the I-chloro derivative (81); this in turn reacts with
organolithium compounds, to the reactive intermediate
(82) which is an isomer of (1). As a highly strained bridgehead olefin, (82) readily adds n-butylltihium, phenyllithium, and tricycloj4.1.0.02~7]hept-l-yllithium(84), giving
the hydrocarbons (83). (86) and (88), respectively. The latter compound isomerizes at 160°C to give a quantitative
yield of the substituted acetylene (89). When (82) is generated in the presence of anthracene, a Diels-Alder addition
occurs giving the propellane (85) which rearranges to the
diene (87) at 150°C.
Angew. Chem. Int. Ed. Engl. 20, 529-546 (1981)
Attempts at the direct epoxidation of ( I ) using the usual
peracids have so far met without success. Benzoylperoxycarbamic acid is the only reagent which affords (95) in
good yield1781
together with benzamide und carbon dioxide. Preliminary thermolyses in solution at 150"C indicate
a rearrangement to the oxepin-benzene oxide system (97)
via the 2-oxabicyclo[3.2.0]hexa-3,6-diene intermediate
(96)'"'. On irradiation, thiophenol adds to (95) to give the
rearranged tricyclic compound (98); (95) is reduced by
complex hydrides to give a 1 : 1 ratio of the isomeric alcohols (99) and (100).
The synthesis of a large variety of new small-ring polycycles is made possible by the addition of halocarbenes to
(I). Difluoro-, chlorofluoro-, dichloro-, dibromo-, chloro-,
chlorophenyl-, and bromophenylcarbene all react with (I)
to give the tetracyclo[4.1.0.0z~4.03~5]heptane
derivatives
(101)-(107)~79Jas well as (108) and (109)[801.
537
4
m
R'
1W
R
2
I
I(101)
(102) (103)
(104)
(105)
(106) (107) f10Si
(1091
F
C1
C1
Br
C1
H
C1
F
C1
Br
H
C1
CsH5
Br
CsH5
C1
The known reactions of (105) and products derived from
it are summarized in Scheme 4. As in the case of (104), reduction with sodium in liquid ammonia furnishes the parent hydrocarbon (129)r79r.
The I3C-NMR spectra of this
compound, as well as those of the heterocyclic analogs
(60). (95) and (207). are remarkable because of the large
difference in the chemical shifts of C-3 and C-4[241.The
thermal isomerization of (129) via tricyclo[3.2.O]hepta-2,6diene (114)to cycloheptatriene only occurs at temperatures
above 180°C. The mechanism has been the subject of kinetic and deuterium labeling
Cycloheptatriene
is formed by the rapid isomerization of (129) caused by a n
Ago-catalyst already at 0°C. A simple synthesis of 3,4-dideuteriocycloheptatriene is available by using [3,4-D2](129) as the starting material[791. Thiophenol can add to
(129) in two different ways: in the presence of boron trifluoride etherate it adds to one of the lateral bonds of the
bicyclo[ 1. I.O]butane molecule to give (130). Irradiation
causes addition to occur at the central bond with formation of (131), which can be desulfurized to the unsubstituted tricyclo[3.1.1 .02,4]heptane(126)1721.
Partial dehalogenation of (104) and (105) using triphenyltin hydride with irradiation leads to the formation of
c H3
(122)
r
QB
OC H3
(127)
(135)
CaHsCNO
0 -N
(137)
(142)
grz
(1431
(144)
Scheme 4. Products derived from 7,7-Dibromotetracyclo[4.I.0.02.4.0'.s]heptane(105).
538
Angew. Chem. Int. Ed. Engl. 20, 529-546 (1981)
the monochloro compounds (106) and (107) and the monobromo derivatives (120) and (125). respectively. Similarly,
(102) and (103)are reduced with sodium in liquid ammonia, giving the monofluorine analogs. The endo-bromo derivative (120) is, like the corresponding chloride (106), thermally labile. Already at 20"C, it is transformed into the 7bromonorbornadiene (115). The probable intermediates in
cation (147)
this reaction are the tricycl0[4.1.O.O~~~]heptenyl
and the 7-norbornadienyl cation (148). NMR spectroscopy
shows evidence of a dead end equilibrium with (121)[''].
(148), R = H
(!SO), R = B r
(1471, R = H
(149), R = B r
R
The transformation of (105) into (128)[8'1and (124)'s21,
as
well as that of (128)into (124), proceed via the intermediate
cation (149), which is trapped by either bromide ions or
water before rearranging to give (150). Starting from (128)
this rearrangement can be initiated either by heating to
80 "C in acetonitrile[''], giving norbornadiene (133), or
Ago-catalyzed[821,whereby on work-up with methanol
(132) is the product. With sodium in liquid ammonia, (124)
gives the debrominated alcohol (119)az1
which can also be
obtained by an alternative route['". The reaction of the alcohol (119) with fluorosulfonic acid at - 120°C[841,
as well
as the solvolysis of its 3,5-dinitrobenzoic acid ester["] provide evidence for the rearrangement (147)- (148).
There are no skeletal changes involved when (128) reacts
with methyllithium, sodium methylate, or LiAIH,, giving
(122)[571,
(127)[821,
and (123)[''],respectively. (103) and (104)
rearrange in a series of reactions analogous to that of
(105)+(128)+(123). The unsubstituted tricycl0[4.1.0.0~~~]
hept-3-ene (118) is obtained in good yield by
dehalogenation of (123) and the corresponding chloro
compound with sodium in liquid ammonia1811,or else by
reduction of the dichloro compound analogous to (128)
with sodium and tert-butyl a l ~ o h o l This
~ ~ ~synthesis
~.
from
benzvalene i s useful for preparing (118) in quantity; there
are, however, two other albeit less productive route^[^^,'^].
When (118) is heated at 135°C (half-life = 1 h), it is
transformed ca. 500 times quicker than the isomer (129jJ6];
the only product formed is (114). Use of 1,7-dideuteriated
(118)leads selectively to (114) labeled in positions 6 and 7.
On the other hand, the Age-catalyzed reaction already occurs at room temperature, giving 2,3-dideuteriocycloheptatriene[811.In contrast to benzvalene (see Section 3.3.4),
(118) adds thiophenol exclusively at the bicyclo[l. I.O]butane central bond to give the norpinene derivative (113)561.
The reaction is accelerated by irradiation. The different
nature of the HOMO'S of ( 1 ) and (118) is regarded as responsible for the rather different reactions of these two
substrates. The epoxidation of (118) with benzoylperoxycarbamic acid gives the epoxide (l12)871;dibromocarbene
addition gives the dibromocyclopropane ( I 1 7)18s1.
The latter is much more stable than (105) and when heated gives
This rearthe dibromo-trans-bishomobenzene ( I 11)157,891.
Angew. Chem. Int. Ed. Engl. 20, 529-546 (1981)
rangement is said to be concerted[891although there is
strong evidence that it is acid-catalyzed, e.g. by traces of
HBr[571.Indeed, ( I 1 7) is transformed into ( I l l ) in the presence of boron trifluoride etheratet5']. The isomerization of
(110) to the trans-bishomobenzene is also acid-catal y ~ e d [ ~Compound
~l.
(110) can be prepared by reaction of
(117) with sodium in liquid
or via a shorter,
more efficient route1901.
On the other hand, (110) rearranges
thermally at 21 0 C to give trans-tricyclo[4.2.0.0z~4]oct-7ene as the main product[881.
The homologous dibromocyclopropanes (105) and ( I 1 7)
behave very differently with methyllithium. It is probable
that (117) first gives rise to a cyclopropylidene which stabilizes itself through skeletal rearrangement to the acetylenic compound (116)[881.Both isotope labeling[881and methyl
enabled clarification of the mechanism
of this reaction. The cyclopropylidene arising from (105)
undergoes ring expansion to give the new reactive intermediate (134). a strained allene, which can be trapped by activated alkenes in what are probably two-step [2+4]- and
[2 + 2]-cycloaddition reactions. (134) reacts with 1,2-bismethylenecyclohexane to give the pentacyclic compound
(137)[871,
and with styrene to give two isomers of the tetracyclic molecule (138)921.The double bond in (138) is highly
reactive; it can be epoxidized'''], smoothly hydrogenated
by diimineLg2],and adds dibromocarbene to give the spiro
compound (142). The cycloheptatriene (143) is obtained
from (138) using Age catalysis[871
and also by an alternative
route[931.Furthermore, (143) is formed by purely thermal
means at 80°C from (138); in comparison to the rearrangement (118)-(114) this is very unusual. Presumably, this is
due to the lability of the cyclobutane moiety[a71.The 1,l-diphenylethylene a d d ~ c t ' analogous
~~]
to (138) exhibits the
same behaviorlS7].In contrast, the two isomers of the cyclopentadiene adduct (139) formed from (134)[921
rearrange
thermally to (144)["l in a manner analogous to (118). Age
catalyzes the formation of the two cycloheptatrienes (145).
The products which are formed from (134) and furan, or
1,3-cyclohexadiene are analogous both in structure[921,and
thermal decay1871
to (139). The butadiene adduct (135) behaves abnormally but similarly to (138j[921.Heating at
80°C leads not only to the formation of the cycloheptatriene system but also to a vinylcyclobutane-cyclohexene
rearrangementfa7' which gives rise to (146). When Age is
used, the bicycle[ 1.1.O]butane skeleton isomerizes at
- lO"C, and the resulting compound (140) rearranges also
to (146)on heating at 80 C. The reactivity of the methylenecyclobutane double bond manifests itself in the benzonitrile oxide addition of (135) to give (136) and (141). In spite
of its greater substitution it is more reactive than the vinyl
group.
+
3.4.3. 12 21-Cycloadditions:
Dichloroketene and Singlet Oxygen
As yet, only the addition of one ketene to benzvalene ( I )
is known: dichloroketene gives 8,s-dichlorotetracyc10[4.2.0.0~~~.0~~~]octan-7-one
(152) in high yieldrg4].Monochloroketene does not react with (I). Scheme 5 summarizes the reactions of (152) which have so far been carried
out.
539
NaBy
1
+
HIN-NH-Tos
(167)
(168)
I
T 0s
(1 70)
(169)
fl7J)
Scheme 5. Products derived from 8,8-Dichlorotetracyclo~4.2.O.O2~~.O3~s]octan-7-one
(152). (153). (156)and (159).n= 2; (154). (157) and (160).n= 3; (155). (158) and
(161). n=4.
Ammonia opens the dichlorocyclobutanone ring to give
the amide (151); dimethylamine and hydrazine react analogously. The partial or complete dehalogenation of (152)
can be carried out with triphenyltin hydride; this gives
(162) or (163). The ketone (163) is reduced with NaBH, to
the endo alcohol (167);the tosylate of the latter compound
does not undergo a 0-elimination to the olefin (1 70). However, the ketone (163) permits access to the desired new
C8H8isomer ( I 70) via the tosylhydrazone (168). Here, the
use of lithium 2,2,6,6-tetramethylpiperidide as a base afforded the best yield. The reaction of (168) with methyllithium gives a mixture of (169) and (I 70). the ratio of which
is solvent dependent. (I 70) isomerizes to cyclooctatetraene
in Ag@-catalyzed, photochemical, or purely thermal
(> 140") reactions. 4-Phenyl-1,2,4-triazoline-3,5-dione
(40)
adds to ( I 70) in a stepwise manner. In an initially formed
zwitterion, the cationic part rearranges and then reacts
with the anionic nitrogen atom of the heterocycle giving
the known (166) as the main product. One of the side products was identified as (171). A Wolff-Kishner reduction of
(163) leads smoothly to the saturated hydrocarbon (153).
which in the gas phase at 430°C decomposes to a mixture
of known CsHlo isomers and products derived therefrom.
(163) undergoes two successive ring expansions with diazomethane, giving cyclopentanone (164) and cyclohexanone (165).The positions of the carbonyl groups are uncert a i r ~ ' ~Wolff-Kishner
~].
reduction of these ketones allows
the preparation of the corresponding hydrocarbons (154)
and (155). Thiophenol adds photochemically to the central
bond of the bicyclo[l.l.O]butane system in (153)-(155);
the resulting tricycloalkylphenyl thioethers (156)-(158)
undergo reductive desulfurization with lithium in ethylamine to give the tricyclic hydrocarbons (159)-(161) possessing annelated four-, five- and six-membered rings. These
compounds, together with (126), and the corresponding tricycl0[3.1.0.0~~~]hexane
derivatives (129) and (153)-(155).
540
were the subject of a study of the effect of annelation on
13C-NMR chemical shifts of strained c y c l ~ p e n t a n e s [ ~ ~ ~ .
The mechanism of the addition of singlet oxygen to olefins with dioxetane formation is still controversial[961.
Whereas norbornene reacts sluggishly with 102971,(1)
reacts already at - 30 "C in a [2 21-cycloaddition giving
the dioxetane (172)178J;
this is probably due to the higher
energy HOMO (see Section 2). As is characteristic for compounds of this type, ( I 72) decomposes to the dialdehyde
( I 73) in a [2 21-cycloreversion accompanied by chemilumines~ence"~~.
Proof of the structure of the dialdehyde
( I 73) is provided by its reduction with LiAlH, to give, depending on reaction conditions, the products (174)-(176)
(see Section 3.4.4).
+
+
+
3.4.4. [2 31-Cycloadditions: 1,3-Dipolar Cycloadditions
A polymeric ozonide is formed from ( I ) and O3 at
0C[z21.Successive 1,3-dipolar cycloaddition to the
- 78
polymeric
ozonide
L~MHI
- 30°C
OH
il74j
I
I
- 30=c
Angew. Chern. Int. Ed. Engl. 20, 529-S46 (1981)
double bond, cycloreversion, and finally another cycloaddition reaction should leave the bicycle[ 1.1 .O]butane skeleton intact in accord with the findings after reductive workup with LiAlH,['*].
When three equivalents of LiAIH, are employed in tetrahydrofuran at -3O"C, a 1 :3 mixture of the hemiacetal
(174) and the diol (175) is obtained. In the reaction using
six equivalents of LiAIH,, (I 75) is the only product. Surprisingly, when the reaction temperature is raised to 35 "C,
the bicyclo[l.l.O]butane central bond in (175) is broken to
give the known cyclobutane derivative ( I 76).
tanes is the result of the photochemical nitrogen extrusion
from the I-pyrazolines (181)-(187). The parent substrate
(129) is best obtained via the dichloro compound (104)
where the yield is higher, but for the 7-alkyl [(188) and
(191n[991
and the 7-aryl compounds [(189), (190). (192n[991,
as well as the 7-carboxylic acid methyl esters (193fIoo1,the
photolysis of the I-pyrazolines is by far the method of
choice.
bN
R
The nitrone 3,4-dihydroisoquinoline N-oxide reacts with
(I 77); (1) and diphenylnitrile imine combine to give the 2-pyrazoline derivative
( I 78)[981.
Benzonitrile oxide and its 2,4,5-trimethyl analog
react with (1) giving the isoxazolines (179) and (180), respectively.
(1) to give the hexacyclic adduct
A
f J 8 J ) (182) (183) (184)
R' H
R2 H
CH3
CH3
(J85j (186)
C&5 bipheCH3
C6H5 nylylene H
C6H5
H
fJ87)
CH3
COzCH3
The addition of diazoalkanes to (1)has been more thoroughly investigated. Symmetrical compounds such as diazomethane, 2-diazopropane, diphenyldiazomethane and
diazofluorene all react with (1) to give the homogeneous 1pyrazolines (181)[981-(184)[99J.
Unsymmetrical substrates
such as diazoethane and phenyldiazomethane each give a
mixture of the two possible isomers (18s) and (186). Ethyl
diazoacetate is no longer reactive enough to compete with
the isomerization of (1) to benzene[991.On the other hand,
methyl 2-diazopropionate does react with ( I ) to give the
mixture (187JIoo1.
Tetrachlorodiazocyclopentadiene reacts only very
slowly with (1)and the unobserved primary addition product eliminates nitrogen already at room temperature to
give the fulvene ( 1 9 3 ~ ) ' ~ ~ ' .
( I 93a)
Besides the additions of halocarbenes to ( I ) (see Section
3.4.2) a second synthesis of tetracyclo[4. 1.0.02,4.03,5]hepAngew. Chem. In!. Ed. Engl. 20, 529-546 (1981)
In view of the high reactivity of azides toward norbornene, their ready addition to (1) was not entirely unexpected. Phenyl, p-nitrophenyl, mesityl, and benzyl azides,
as well as ethyl and tert-butyl azidoformate react with ( I )
to give the triazolines (194)-(199)[70~1011
in acceptable
yields. Tosyl azide reacts with (I) to give as the immediate
product 7-p-toluenesulfonyl-7-azatetracyclo[4.l.O.O2~4.O3~5]heptane (206), which is probably derived from the initially
formed triazoline (200) by nitrogen elimination. (201)(203) can be synthesized photochemically from (194),(196).
and (197), respectively; in the thermal reaction, the triazolines (198) and (199) give rise to (204) and (ZOS), respectively. The unsubstituted aziridine (207) is obtained by LiAIH,
reduction of (204). or from (206) with naphthalene- or,
even better, biphenyl-sodium. It reacts via the N-metallated derivative to give the benzylated and sulfonated compounds (203) and (206). respectively.
When (199) is treated with aluminum oxide of activity
grade I , the aziridine (205) is obtained. When aluminum
oxide of activity grade I11 is employed, nitrogen elimination also occurs. However, after rearrangement, probably
via cationic intermediates, and addition of water, the
endo,endodisubstituted tricycl0[2.2.0.0~~~]hexane
(208) can
be isolated"021.(206) reacts with acetic acid and thiophenol
to give also compounds of the type (208)"*]. Presumably, the nitrogen is protonated; this is followed by C-N
bond cleavage and Wagner-Meerwein rearrangement to
give a cation of the type (41), which then adds a nucleophile to give (209) or (210).
541
accompanied by nitrogen elimination; the ketene then further rearranges to the tetracyclic compound (221)L’051.
This
latter step involves a novel addition of a ketene function to
the neighboring bicyclo[l.l.O]butane bond.
Whereas the N-phenyl compound (201) reacts with acetic acid to give the rearranged compound (211). with thiophenol it gives the unrearranged molecule (213). Apparently the aziridinium ion formed from (202) does not rearrange as easily as protonated (206) because NHC6HSis a
poorer leaving group than NHTos. Therefore, thiophenol
directly opens the three-membered ring giving (213). In
contrast, in the presence of the less nucleophilic acetic
acid, the rearrangement of protonated (201) occurs faster
than the nucleophilic attack. With still weaker nucleophiles, the anilino group competes with the nucleophile,
and dimerization to (214) results; this is the case, for example, on treatment of (201) with a catalytic amount of silver
perchlorate or trifluoroacetic acid.
The thermolysis of (201) at 150°C leads to cleavage of
three o bonds and the formation of three 7c bonds giving
N-phenylazepine (212)r‘0’1
which can also be prepared via
an alternative route. As is general for N-acylaziridines,
(204) undergoes an expansion only of the aziridine ring on
heating to 120°C; this gives the oxazoline derivative (215)
with retention of the tricycl0[3.1.0.0~~~]hexanemoiety.
3.4.5. [2 +4]-Cycloadditions: Diels-Alder Additions
Due to the high energy of the 71 orbital, benzvalene is
predestined to be a good dienophile in Diels-Alder additions with inverse electron demand. 3,6-Bismethoxycarbonyl- and 3,6-diphenyl-1,2,4,5-tetrazine
add to ( I ) in a manner typical of these dienes. A subsequent nitrogen elimination gives the dihydropyridazines (216) and (217I1O3].With
excess ( I ) , (216) undergoes also a Diels-Alder addition
leading to the octacyclic azo compound (218)11031,
the configuration of which has been established by X-ray
analysis”’].
( 2 1 6 / , R = COzCH3
(2171, R CsH5
(2181, R = COzCH3
The newly reported 2,5-diphenyl-l,3,4-oxadiazin-6-one
can be used as a diene[lo41.Here, the reaction with ( I ) presumably gives (219) as the primary product, from which
the ketene (220)is formed in a 102+ 02 + 02] cycloreversion,
542
Although cyclopentadiene is unreactive toward ( I ) ,
hexachlorocyclopentadiene and tetrachlorocyclopentadienone dimethylketal react in accord with Alder’s endo rule
to give the pentacycles (222)[’03]or (223)”06a1in good
yields. On reaction with sodium in liquid ammonia or sodium and tert-butanol the chlorine atoms in these products
are replaced by hydrogen with formation of (224) and
(225),respectively. The double bond in the ketal (225) is reduced with diimine to form (226).
c1
R&
c1
k
(2221, R = C 1
( 2 2 3 ) , R = OCH3
(227), R
(228), R
= C1
= H
MRU
OC H3
1
R
(2241, R
(2251, R
= H
\
OC H3
(226)
= OCH3
(229), R
(230), R
= H
= OCH3
It is worth noting the thiophenol additions to (223)(225) which probably involve radical chain process[‘06a1.
Triggered by the phenylthiolyl radical the reaction of (223)
begins at the bicyclo[l .l.O]butane ring, continues in a
transannular ring closure with participation of the double
bond, and finally gives the pentacyclic thioether (227).
When the starting compound is (225), only about half the
molecules follow this pathway giving (228). The other half
of the starting molecules add thiophenol to the double
bond to give (230). The analogous product (229) is formed
as the sole product on reaction of (224) with thiophenol.
Freed from steric hindrance due to the syn-methoxy group
in (223) and (2251, and from the inductive effect of the
chlorine atoms in (223). the double bond in (224) is far
Angew. Chem. i n t . Ed. Engl. 20, 529-546 (1981)
more reactive toward thiophenol than is the case for the
central bond in the bicyclo[ 1 . 1 .O]butane system. This can
be understood when one considers the energies of the
(2)""
highest occupied orbitals in tricyclo[3.1.0.02~6]hexane
and n o r b ~ r n e n e ' ' ~which
~',
serve as models for the different functionalities of (224).
4. Substituted Benzvalenes
R
(23S), R
R
= C1
R
(233j, R = C1
(234). R = H
(237). R = C 1
(238). R = H
Despite the fact that substituted benzvalenes are not as
easily accessible as ( I ) , they are of great importance because of their sometimes extremely varied properties. The
first benzvalene derivative, tri-tert-butyltrifluorobenzvalene (243), was found, by Viehe et aZ!'],along with other
products in the spontaneous trimerization of tert-butylfluoroacetylene. 1,2,4-Tri-tert-butylbenzvalene (244) is
formed when 1,3,5-tri-tert-butylbenzeneis photoly~edl'~~';
on further irradiation it partially rearranges to the 1,3,6isomer (245), which is also formed in the photolysis of
1,2,4-tri-~ert-butylbenzene~~'.
n
(243)
Tetrachloro-o-benzoquinonereacts instantaneously with
(1). whereas the addition of o-benzoquinone itself needs
15 h at 20°Cf'031.The configuration of the pentacyclic a-diketones (231) and (232) was established from the NMR
spectra of the quinoxalines (233) and (234) which are easily
obtained on treatment with o-phenylenediamine. Photochemical elimination of two CO molecules from the a-diketones (232) and (231) gives the new CloHlohydrocarbon
(236) and its tetrachloro derivative (235), respectively. Further irradiation now at shorter wavelengths leads to an
electrocyclization forming the isomeric cyclobutene derivatives (238) and (240). or (237) and (239), respectively. At
120°C they isomerize back to (236) and (235). respectivel ~ [ ' ~ The
' . double bonds in (236) each accept two hydrogen
atoms from diimine, thus providing a second synthetic
pathway to (155)721(see Section 3.4.3).
cr-Pyrone does not react with ( I ) ; tetrachloro-a-pyrone,
on the other hand, gives a high yield of a lactone, for which
configuration (241) is proposed"06a'. At 150 C in solution,
it looses C02, and (235) is formed. With sodium methylate
in methanol (241) undergoes opening of the lactone
ring and elimination of HCl, to give the first substituted
naphthvalene (242).
Dimethyl dithionooxalate-a 1,4-dithia-1,3-butadienereacts with (1) to give the Diels-Alder adduct (242a)['O6".
Angew. Chem. Int. Ed. Engl. 20, 529-546 (1981)
When 1,2,4,5-tetrakis(trimethylsilyl)benzene is irradiated, the substituted benzvalenes (246) and (247) as well
as other isomers, are obtained"'''.
Irradiation of hexakis(trifluoromethy1)benzene causes isomerization to the
benzvalene (248Jio9.' lo],in quantitative yield. This product
has been the subject of extensive studies.
Thermolysis of (248) causes reverse rearrangement to
hexakis(trifluoromethyl)benzene~'091. This process has a
considerably higher activation enthalpy (38.0 kcall
mol)lll'l, than the isomerization of ( I ) (see Section
3.2.3). (248) reacts analogously to ( I ) with phenyl azide to
give the corresponding triazoline["21. Because of the
change in electronic properties due to the trifluoromethyl
groups, (248) reacts with other dienes than does ( I ) : cyclopentadienef''21, pyrrole["*], butadiene" 131 and its methyl
substituted deri~atives[''~J,
furan and methylfuran1''41 as
well as cyclobutadiene["5,' I6l. 1,3-Cyclohexadiene does not
add to (248); instead the double bond is hydrogenated" 14].
543
Also in contrast to ( I ) , (248) forms the monomeric ozonide[1'7Jwith which a variety of interesting reactions are
made possible[118J.Acetone-sensitized irradiation of (248)
in the presence of dialkylacetylenes leads to [2 + 21-cydoaddition" 19].
The CF,-substituted 2,5-diphosphabenzvalene (249) is
formed when tetrakis(trifluoromethyl)-1,4-diphosphabenzene is irradiated"201. The 2- and 3-methylbenzvalenes
(250) and (251), as well as some toluene, are the products
formed when methylcyclopentadienyllithium reacts with
dichloromethane and methyllithium[28J. Although l-methylbenzvalene (4) is not formed in this reaction, it can be
synthesized by other methods (see Sections 3.1 and 3.2.2).
Hexamethylbenzvalene (252) is the proposed intermediate in the thermal isomerization of hexamethylprismane
to hexamethylbenzene"211. Its derivatives (253)[Iz2],
(254)['231,(255)[1231,and (256)"241,were synthesized by Hogeveen et al. by reaction of 1,2,5,6-tetramethyl-3,4-bismethylenetricyclo[3.1 .0.02,6]hexane with tetracyanoethylene,
sulfur dioxide, and the triazolinedione (40), respectively.
Analogously to ( I ) , benzobenzvalene (naphthvalene)
(257) is synthesized from the indenyl anion and chlorocarbeneL41.The mechanism of its formation'291,as well as the
Age- or Cu-catalyzed rearrangernentI3,] to naphthalene
and benzofulvene have been investigated. (2.57) is thermally more stable than ( I ) ; in CCl, at 80°C it rearranges to
benzofulvene with a half-life of ca. 30 hIa1. Acids add to
one of the lateral bonds of the bicyclo[l.l.O]butane systernt4].Under radical reaction conditions, thiophenol does
not react with (2S7/157.7'J.
544
The anthracene isomers (2S8) and (259) are formed from
the s-indacene dianion and chlorocarbene in a manner
analogous to (257)["']. The heterocyclic compound (260)
can also be prepared by this method using 4-azapentalenyl
anion['261. On thermolysis, (258) rearranges to fulveno[ blnaphthalene. (259) reacts via benzvalenobenzofulvene
to give a mixture of 1,5- and 1,7-bismethyIenedihydro-s-ind a ~ e n e [ ' Other
~ ~ ~ . substituted benzvalenes can be synthesized from (I): the benzvalene sulfones (55) and (57) (see
Section 3.3.4) and the naphthvalene (242) (see Section
3.4.5).
1260)
5. Conclusion and Outlook
The findings presented in this paper should give ample
evidence that although benzvalene was first discovered
only about a decade ago, it is more than a curiosity among
the hydrocarbons. Its ready availability and high reactivity
allows the synthesis of a multitude of new small-ring polycyclic compounds. Furthermore, the rearrangements and
intermediates encountered are of mechanistic interest.
Spectroscopic measurements on benzvalene and products
derived therefrom provide information on 0-z and 0-0interactions. A deeper understanding of these effects may
contribute to the overall knowledge of the chemical
bond.
This work is not a final review, however. The synthetic
utilization of the new compounds has certainly not been
exhausted, and so far benzvalene has mainly been treated
with classical double bond reagents only. It is possible that
new substrates will be found which are inert to normal olefins but reactive toward the benzvalene double bond.
Only a small number of reagents attack the bicyclo[l.l .O]butane moiety in benzvalene and related molecules. This is probably due to the fact that the chemistry of
the bicycle[ 1.1.O]butane system as such is little explored,
the required starting materials, apart from a few exceptions, being difficultly accessible. Via benzvalene a variety
of bicyclo[1.1.O]butane derivatives is available, therefore
this area has been opened for more active study.
The chemistry of benzvalene demonstrates that the study
of small-ring hydrocarbons is more than a playground for
old-fashioned esoterics. Advances in the understanding of
chemical properties have often been made by means of
small and simple molecules.
I wish to acknowledge the enthusiastic collaboration of H .
Leininger, R . Lang, E. Brunn, S . Brengel, R . Herbert, G .
Briintrup, M . Lechner, G . Freitag, and P. Kemmer. Generous financial support by the Deutsche Forschungsgemeinschaft and the Fonds der Chernischen Industrie is gratefully
acknowledged, also on behalf of my co-workers. lihanks are
also due to Union Rheinische Braunkohlen Kraftstoff AG for
gifts of dimethyl ether used in the preparation of ( I ) .
Angew. Chem. Inr. Ed. Engl. 20, 529-546 (1981)
Received: October 20, 1980 [A 367 IE]
German version: Angew. Chem. 93, 515 (1981)
Translated by Edeline Wentrup-Byme, Marburg
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Intermolecular ForcesAn Example of Fruitful Cooperation of Theory and Experiment
By Peter Schuster“]
New experimental techniques and extensive a6 initio calculations have rendered it possible
to gain a detailed knowledge of aggregates of atoms, small ions and/or small molecules in
the vapor phase which are held together by intermolecular forces. Theory and experiment
are often complementary regarding reliable predictions. The results obtained for complexes
of three or more constituents are still fragmentary; nevertheless, they can be used to explain
some properties of the condensed phase.
1. Introduction
For more than two decades most theoretical chemists
have been engaged in developing and testing various numerical methods in quantum chemistry. An enormous
amount of effort has been expended, involving many millions of hours of electronic computing time. The primary
goal in this field has been and still is the calculation of precise approximations of the Schrodinger equation for the
stationary states of atoms, molecules and molecular complexes. The limitations of the numerical methods available
are already apparent: the computational problems concerning ground states of molecules have more or less been
solved“’. This is true for most molecular properties. The
major obstacle opposing general applicability of ab initio
calculations is the size of the molecules and molecular associations. The computer time required increases enormously with the size of the molecular structures to be studied.
[*] Prof. Dr. P. Schuster
Institut fur Theoretische Chemie und Strahlenchemie der Universitat
Wahringerstrasse 17, A-I090 Wien (Austria)
546
0 Verlag Chemie GmbH. 6940 Weinheim, 1981
The central role of intermolecular forces in any theory
of the three states-gaseous, liquid and solid-is readily
understood: in a world free of attractive forces between
molecules or atoms there exists only ideal gases. Consequently, the investigation of intermolecular forces represents an extremely important but difficult chapter of chemical physics. The theory of intermolecular forces fits very
well into the requirements and possibilities of quantum
chemistry. Experimental data which are easy to interpret
can be expected for very small and structurally simple molecular associates only. These systems, however, are notoriously difficult to investigate by conventional experimental techniques. Enormous experimental efforts and developments of new spectroscopic methods, e. g . “molecular
beam electric resonance” spectroscopy[’] or high-pressure
mass spe~trometry‘~]
were necessary in order to obtain reliable experimental data on simple aggregates of small molecules. These small systems, however, are amenable also
to precise numerical computation. The progress recently
achieved in this field was a direct result of theoretical
“predictions” and “precalculations” eventually being accurate enough for use by the experimentalist.
The interactions most commonly studied at present are
those between rare gas atoms or between unpolar mole-
0570-0833/81/0707-0546 $ 02.50/0
Angew. Chem. Int. Ed. Engl. 20, 546-568 (1981)
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