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Molecules Undergoing Fast Reversible Valence-Bond Isomerization.

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Molecules Undergoing Fast, Reversible Valence-Bond Isomerization [11
(Molecules with Fluctuating Bonds)
BY PR1V.-DOZ. DR. G. SCHRODER
INSTITUT FUR ORGANISCHECHEMIE DER TECHNISCHEN HOCHSCHULE KARLSRUHE(GERMANY)
DR. J. F. M. OTH AND D1PL.-ING. R. MERENYI
UNION CARBIDE EUROPEAN RESEARCH ASSOCIATES, BRUSSELS (BELGIUM)
Compounds which undergo fast and reversible valence-bond isomerization (i.e. which have
fluctuating cyclopropyl and/or double bonds) are taken to be those in which the average lifetime of the valence-bond isomers at 0 “Cis of the order of 100 seconds at most, and f o r which
the maximum activation energy Gf the bond migration is 20 kcallmole. The fast, reversible
bond shift can scarcely be detected by chemical means, but can be recognizedby NMR
spectroscopy, since the time for which a proton occupies a position with a given magnetic
environment has a pronounced influenceon the N M R spectrum. Some examples of molecules
with fluctuating bonds are cyclooctatetraene and its derivatives, unsaturated seven-membered
ring systems, homotropdidene, bridged hc motropilidene systems, bullvalene, and substituted
bullvalenes. The last class is particularly interesting, since here the carbon atoms are continua& changing their relative positions and neighbors.
I. Introduction
In order to reconcile the equivalence of all six carboncarbon bonds in benzene with his cyclohexatriene formula, Kekuli [2] assumed that “one carbon atom is
doubly bonded to one of the adjacent carbon atoms for
one unit of time, and to the other adjacent carbon atom
for the next . . .” Thus according to the Kekulk picture,
the bonds in benzene jump backwards and forwards,
i. e. they oscillate or fluctuate. This attempted explanation is now known to be untenable. The actual structure
of benzene is somewhere between two fictitious limiting
cyclohexatriene structures ; the .r;-electrons in benzene
are delocalized. However, Kekulg’s basic postulate of the
equivalence of all six carbon-carbon bonds in benzene is
correct.
Kekule’s oscillation theory is currently attractin,p interest
again, since it is capable of describing the behavior of
molecules with fluctuating bonds. The bonding electrons
in these molecules, however, unlike those in benzene,
are localized, and depend on valence isomerization for
their sudden shift. Valence isomerization is a reorganization of TC,or of TC and Q, electrons accompanied by
changes in interatomic distances and bond angles but
without migration of atoms or groups [3]. It is an iriiramolecular rearrangement which obeys first-order kinetics, is practically uncatalysable, is independent of solvent, and can be influenced only by temperature. It
belongs to the class of “no-mechanism” reactions [4].
[I] Part VII. Part V1: J. F. M. Oth, R. Merinyi, J. Nielsen, and
G . Schroder, Chem. Ber., in press.
[2] A. KekuU, Bull. SOC.chim. France 3 , 98 (1865); Liebigs Ann.
Chem. 137, 129 (1866); H . A. Staab, Angew. Chem. 70,37 (1958).
[3] E. Vogel, Angew. Chem. 74, 829 (1962).
[4] S. J. Rhoads in P . de Mayo, Molecular Rearrangements.
Wiley, New York/London 1963, p. 655.
752
Of the many molecules that exhibit valence isomerization, we shall be concerned only with those in which the
phenomenon is fast and reversible, i.e. in which the
mean lifetime of the valence-bond isomers at 0 “C is of
the order of 100 seconds or less and the activation
energy AE* of the isomerization is not greater than
20 kcal/mole. This arbitrary convention is based on the
use of NMR spectroscopy as the means of detection.
Molecules of this type may be said to have fluctuating
bonds. This concept may be illustrated with cyclooctatetraene ( I ) as an example.
This compound can undergo three different isomerizations,
the kinetic data for which are given in Table 1 :
1. The ring inversion ( l a ) Z ( l a ) (see Section 111, I ) is due
to rotation about the C-C single bonds. There is no reorganization of the bonding electrons. Consequently this is
not a valence - bond isomerization.
2. The reaction ( l a ) & ( l b ) (see Section 111, I), on the other
hand, is characterized by a reversible bond migration involving all four double bonds [ 5 ] . The molecular structure
remains unchanged, but if we number the carbon atoms, the
structures ( l a ) and (Ib) are different. The two valence
isomers have the same energy and the same lifetime At(,,,
of about 1 . 4 ~10-2 sec at 0 ‘C. Thus according to our arbitrary
condition, this is an example of a fast, reversible valencebond isomerization.
3. As far as the type of reaction is concerned, there is fundamentally no difference between ( I n ) z (Zb) and the dynamic
[5] F. A. L. Anet, J. Amer. chem. SOC.84, 671 (1962).
Angew. Chem. internat. Edit. 1 V d . 4 (1965) I No. 9
equilibrium of cyclooctatetraene with bicyclo[4.2.0]octa2,4,7-triene (2) [6,7]. This is again a reversible valence isomerization (equilibrium constant K = [ ( 1 ) ] / [ ( 2 ) ];t: lO4), b u t
it is not fast, since the lifetimes of (2) and ( I ) (carbon atoms
not distinguished) at 0 "C are more than 100 sec (see Table 1).
The temperature-dependence of the NMR spectra of
tricyclo[3,3,2,04~6]deca-2,7-diene(dihydrobullvalene) [9]
is shown in Fig. 1. The valence isomers in this case are
structurally identical. However, if the carbon (or hydrogen) atoms are distinguished from one another, the two
coexisting structures become different.
Table 1 . Kinetic data for the isomerizations of cyclooctatetraene 15-71.
k - rate constant, AE* = activation energy, At = lifetime.
~
katOOC
Structure
AE'
[kcal/
moll
__
~
( 1 ) [a1
1 . 7 ~109
( l a ) o r (16)
1.4%10-
L I X 103
Ring inversion
(la) Z (la)
Valence
isomerization
(la)
(Ib)
Valence
isomerization
(1)
(2)
ki
%
10 Ib]
k z = 70
-
13.7
a c p s
200
300
I
I
I
I
100
0
I
I
27.2
18.7
*
[a] C atoms not distinguished
[b] Estimated from measurements on substituted cyclooctatetraene ( 1 5 )
(see Section II1,I).
11. Detection of Valence-Bond Isomerization
Molecules with fluctuating bonds can scarcely be recognized as such by chemical means. The simplest and
best method of detection is NMR spectroscopy. The
lifetime At of a structure, or in general, the time during
which a position with a given magnetic environment is
occupied by a proton, has a strong influence on the
NMR spectrum.
In infrared and ultraviolet spectroscopy, the energy
difference, AE = hAv, between the individual absorption bands is so large that, even for short-lived structures, the spectrum obtained reflects all the structures
present. The chemical shifts in the NMK spectra, on the
other hand, correspond to very small energy differences,
the measurement of which requires an appreciable time.
It follows that the lifetime At of a compound plays an
important part in N M R spectroscopy. The product
At.Av determines the NMR spectra of dynamic systems,
where Av (in c.P.s.) = \ J ~ - v,; v A and vB are the chemical shifts of the same proton in two valence isomers
in the absence of exchange [8].
Molecules with fluctuating bonds have temperaturedependent spectra provided that 1 , the rate constant for
the valence isomerization is such that 10-3 < At.Av <
10, i.e. 10-3 < k-1.Av < 10; and 2. the valence isomers
in equilibrium with one another have identical structures (equilibrium constant K = I), or if the structures
are different, they are present in comparable concentrations (equilibrium constant K < 20).
161 R . Huisgen, Lecture at the International Symposium on
Reaction Mechanisms in Organic Chemistry, July 20th-25th,
1964 in Cork (Ireland). Cf. Angew. Chem. 76, 928 (1964);
R . Huisgen and F. Mietzsch, Angew. Chem. 76, 36 (1964); Angew. Chem. internat. Edit. 3, 83 (1964).
[71 E. Vogel, H. Kiefer, and W. R . Roth, Angew. Chem. 76, 432
(1964); Angew. Chem. internat. Edit. 3, 442 (1964).
[8] H. S . Gutowsky and C. H. Holm, J. chem. Physics 25, 1228
(1956).
Angew. Chem. internat. Edit. / Vol. 4 (1965)
/ No. 9
A,B'
ILI
2
C:O' B:'C:'D"
I
I
I
5
6
7
[51
131
8
I
9
10
TFig. I. N M R spectruni of dihydrobullvalene (tricy~loI3.3.2.0.~.~]
deca2,7-diene) [9, LO]. Internal standard: tetramethylsilane.
A hydrogen atom B, for example, is attached t o a cyclopropane group in one valence isomer, while in the other it is
attached to a n olefinic group. The average lifetime of the
isomers at -lOO°C is so long (At.Av > 10) that the N M R
spectrum contains relatively sharp bands corresponding to
the states of the protons. The spectrum can now be said t o
correspond t o the structural formula.
As the average lifetime decreases with rising temperature
(10 > At.Av >10-3), the bands for the protons B first become
indistinct and then spread until they finally coalesce t o form
one wide band. Only one maximum is observed at -50 "C.
If the vaIence isomerization is accelerated by further heating
(At.Av < IO-S), the N M R spectrum again exhibits sharp
bands. As a result of the fast and reversible valence isomerization, pairs of protons become identical and the chemical shifts and coupling constants now have average values.
For example, a t 25 "C the spectrum contains a band centered
at about T = 6.32, corresponding t o the four B protons.
At higher temperatures, therefore, the N M R spectrum
reflects the dynamic situation (valence isornerization), while
~~~
191 R . MerPnyi, J . F. M . Oth, and G.Schroder, Chem. Ber. 97,3150
(1964).
[lo] The numbers in square brackets indicate the relative intensities of the signals. The sensitivity of the instrument was occasionally varied during the recording of a spectrum.
753
a t lower rernperatures it reflects t he static situation, i.e. the
structure of the c ompound.
The valence isomerization very probably passes through
a planar cyclooctatetraene in the transition state.
T h e kinetic d a t a for t he valence isomerization can be deduced
f r o m th e f o r m o f t he N M R signal at various temperatures [S]
(for examples, see T abl e 3).
111. Known Molecules with Fluctuating Bonds
Several molecules with fluctuating bonds have recently
become known. These include cyclooctatetraene (1) and
some of its derivatives, such as ( 3 ) , (14), (I5), and (17),
a number of unsaturated seven-membered ring systems
such as ( 4 ) t(5), (23) t(24), bicyclo[5.1.O]octa-2,5diene (6) (homotropilidene), bridged homotropilidene
systems of the type ( 7 ) , (8) & (9), ( l o ) , (11) 2 (12),
and bullvaleiie (46) and some of its derivatives (see
Scheme 1).
R
(3), R
(14), R
(IS), R
OR
= H
= F
The bonds in the eight-membered ring also fluctuate in
substituted clclooctatetraenes. Thus the fluorine resonance signal of fluorocyclooctatetraene (3) undergoes a
striking change between room temperature and -65 "C
[Ill. The free energy of activation for the valencebond isomerization at -33 "Cis about 12 kcal/mole [ I 11.
Anet et al. [12] prepared the derivatives (14) and (15).
[51
(111
A
= COOCzH5 [ l Z ]
= C(CH3)zOH [ 1 2 ]
(17), R = 0 - A l k y l [13]
COOCzH5
D
CH,MgI
D
H
D
The kinetic data k2 for the bond migration in (14) and
(15) and kl for the ring inversion in (15) were deduced
from the temperature-dependence of the IH-NMR
spectra with additional irradiation at the frequency OF
deuterium [*I. One of the two (non-equivalent) methyl
groups in (15) is distinguished by an asterisk.
(461,
R
=
(13), R =
(35), R =
(591, R =
(61) - (64), R =
H 19, 19, 21, 231
CH3 [l, 241
C6H5 [ 1, 241
B r 119, 251
0 - A l k y l [ 191
Schcme 1.
1. Cyclooctatetraene and Derivatives
The fluctuation of the bonds in cyclooctatetraenc was
recently recognized by Anet [S] and Roberts [l I] from
the NMR spectra. The 1 3 C satellites in the proton resonance spectrum of this compound (resultiiig from a
spin-spin coupling of the H atoms with the natural 13C
in the cyclooctatetraene) change with temperature. The
rate constant k for the valence isomerization ( l a ) z
( l b ) was deduced from the teniperature-dependence of
the satellites. The free energy of activation is about
13.7 kcal/niole at --IO"C [ S ] .
754
It can be seen from Table 2 that the bond migration in
the ethyl cyclooctatetraene carboxylate (14) is faster
thaii i.; the cyclooctatetraenyldiniethylcarbinol(15), and
that the free energy of activation for the ring inversion
in (15) is less than that for the bond migration.
[ I 11 J . D. Roberts, Angew. Chem. 75, 20 (1963); Angew. Chem.
internat. Edit. 2, 53 (1963); D. E. Gwynn, G. M . Whitesides,
and J. D. Robei-fs, J. Amer. chem. SOC.87, 2862 (1965).
[I21 F. A . L . A n e f , A . J . R. Bourn, and Y . S . Lin, J. Amer. chem.
SOC.86, 3576 (1964).
[ * ] Simpler spectra a re obtained by this additional irradiation
because the H/D coupling is eliminated.
Angew. Chem. internat. Edit.
VoI. 4 (1965) 1 No. 9
The valence isomers (4) and ( 5 ) have different structures.
Since the equilibrium constant is about 4, the valence-bond
isomerization can be recognized by the temperature-dependence of the N M R spectrum. At room temperature the
IH-NMR spectrum contains a complex multiplet between
T = 3.2 and 3.8 (protons on C-2, C-3, C-4, and C-5 and a
doublet centered at about z= 5.30 (protons on C-I) and C-6).
This doublet widens as the temperature is lowered, and
ultimately gives two new signals: a doublet at 7 = 4.60
The cyclooctatetraenyl alkyl ethers (17), which are prepared
by the action of potassium alkoxides on the dibromide (16),
also have temperature-dependent N M R spectra [ I 31. The
resonance signal of the proton Ha in ( I 70) is clearly separated
in thc low-temperature spectrum from the signals of the other
Table 2. Kinetic data for the bond migration kZ and the ring inversion
k l in compounds ( 14) and ( 1 5 ) . AF’,
AH*, l S * = free energy,
enthalpy, and entropy of activation, respectively.
Con’pound
I 1
Temp.
[ “C]
I
I
React’on
1 1
rsec-11
I
AF*
[kcal/mole]
I
cal.rnole-1.deg-1
I -8.0
I
protons attached to the eight-membered ring. At higher
temperatures, the N M R method cannot distinguish
between Ha and H b . This result shows that the cyclooctatetraenyl alkyl ethers undergo a rapid and reversible valencebond isomerization (17a) z? (17b).
116)
2. Unsaturated Seven-Membered Ring Systems
a) A C y c 1o h e p t a t r i e n e / N o r c a r a d i e n e
Derivative
-6.3
[protons on C-1 and C-6 in (4)] and a fairly broad singlet at
7.01 [protons on C-1 and C-6 in ( 5 ) ] , the areas being i n
the ratio 4 : 1.
7 =
NMR measurements on cycloheptatriene (18) at various temperatures give no indication of the presence of
norcaradiene (19) [26]. On the basis of Cigunek’s
results, it may be assumed that a reversible valence isomerization occurs, but that cycloheptatriene is present
to the extent of practically 100%.In the case of a 7,7dicyano derivative, o n the other hand, the equilibrium
mixture consists almost entirely of the norcaradiene
structure (21) [27].
L
Cycloheptatriene and norcaradiene were shown by
Ciganek [14] to be related by a rapid and reversible
valence isomerization in the case of a 7-cyano-7-trifluoromethyl derivative (4 ) z ( 5 ) .
J
L
120)
(21)
b) O x e p i n / B e n z e n e O x i d e
( 4 ) 80%
15) 20%
[I31 G. Schroder, Th. Martini, J. F. M . Oth, and R. Me r i nj i ,
unpublished work.
[I41 E. Ciganek, J. Amer. chem. SOC.87, 1149 (1965).
[I51 E. Vogel, W . A . Ed//, and H. Giinther, Tetrahedron Letters
IY65. 609.
[16a] W. v. E. Doering, Zh. Vses. Khim. Obshchestva irn D.I.
Mendeleeva, 7, 308 (1962).
[16b] W. Y . E. Doering and W. R. Roth, Angew. Chern. 7 5 , 27
(1963); Angew. Chem. internat. Edit. 2, 115 (1963).
[ I71 W. v. E. Doering and W. R. Roth, Tetrahedron 19, 71 5 ( 1963).
[I81 W . v. E. Doering and E. Ferrier, footnote in [16b, 171.
[I91 W . v. E. Doering and G. Klumpp, unpublished work
[20] G. Sclirocier, Chem. Ber. 97, 3131 (1964).
[2I] G. SclirBdcr, Chern. Ber. 97, 3140 (1964).
[221 H. Rottele, Diploma Thesis, Technische Hochschule Karlsruhe, 1965.
[23] G. Sciiroder, Angew. Chern. 75, 722 (1963); Angew. Chern.
internat. Edit. 2, 481 (1963).
[24] G. Schroder, R. M e r i n y i , and J . F. M . Oth, Tetrahedron
Letters 1964, 773.
[251 G. SchrBder, Angew. Chem. 77, 682 (1965); Angew. Chem.
internat. Edit. 4, 695 (1965).
Awgrw.
Cllern. internat. Edit. / V d . 4 (1965) j N o . 9
Vogel, BdI, and Giinther [IS] found that the dehydrohalogenation of 1,2-epoxy-4,S-dibromocyclohexane(22)
leads to a non-phenolic, orange product in good yield.
The temperature-dependence of the NMR spectrum
indicates a fast and reversible valence isomerizztion
between oxepin (24) and benzene oxide (23). The e q u librium constant appears to be affected by the temperature and (as ultraviolet absorption studies show) by the
solvent [*I.
Br
I
123)
i22)
(24)
[26] F. A . L . Anet, J . Amer. chem. SOC. 86, 458 (1964); F. R . Jensen and L. A. Smith, ibid. 86, 956 (1964).
[27] E. Ciganek, J. Amer. chem. SOC.87, 652 (1965).
[ * ] Note added in proof: The following kinetic data (at 0 “C)
were determined from the half-widths of the a-proton signals
( H . Giinther, personal com.nunication):
ka
(23) s ( 2 4 )
ka
kb
kb
i
14.1
106
sec
2.29
106 sec 1 ;
1;
AE,+
AEc
=
9.1 kcaljmole
7.2 kcal/mole.
755
c) H o m o t r o p i 1id e n e
(Bicyclo[S. 1 .O]octa-2,S-diene)
d) Bridged H o m o t r o p i l i d e n e Systems
of t h e T y p e (7)
The reaction of cycloheptatriene (18) with diazomethane
leads inter alia to bicycl0[5.1.0]octa-2,5-diene (homotropilidene (6) [16,17].
The reaction of the acid chloride of cycloheptatrien-7carboxylic acid (29) with diazomethane yields a diazoketone (30), which by elimination of nitrogen forms
tricyclo[3.3.1.04~6]nona-2,7-dien-9-one
(7) [18] [*I.
16)
CIS)
Homotropilidene contains the cis-l,2-divinylcyclopropane
grouping. Attempts at the preparation of (27) [16,17,28]
resulted in the formation of 1,4-cycloheptadiene (28), the
valence-bond isomer of (27).
The formation of (28) was explained by postulating the
occurrence of cis-1,2-divinylcyclopropane (27) as an intermediate; this presumably undergoes a very fast Cope rearrangement. The valence-bond isomerization (27) + (28)
is made possible by the reduction of the activation energy
owing to the cis arrangement of the two vinyl groups on the
three-membered ring. The geometry of the transition state is
already largely present in the ground state [17,28]. The
geometry of the trans-isomer, on the other hand, does not
favor the rearrangement 131; this isomer can therefore be
isolated, and rearranges into (28) only at 190' C .
The lifetime of the compound (27) at -40'C is evidently
still so short that the compound cannot be isolated 1171. If
the two vinyl groups in (27) are linked by one methylene
group, the resulting structure is homotropilidene (6).
The structure of (6) is not changed by valence isomerization. If the carbon (or hydrogen) atoms are distinguished
from one another, however, the two structures (6a) and
(6b) existing in equilibrium are different.
The homotropilidene part of the tricyclic ketone (7) is fixed
in the cisoid configuration by the carbonyl bridge. This
compound undergoes a reversible and extremely fast valencebond isomerization. The N M R spectrum of (7) shows little
change as a result of cooling to -30 "C. The chemical shifts
and coupling constants found for the pairs of identical
protons resulting from the valence-bond isomerization
(7u) z? (7b) are average values. The spectrum comprises
three groups of bands corresponding to three groups of
equivalent protons. Only at about -90 "C does the average
lifetime of the isomers becomes sufficiently long to give a
spectrum corresponding to the structural formula. The
kinetic data for (7) were determined by Lambert [30] from
the N M R spectra. The activation energy AE* of the reaction
( 7 4 s (7b) is 8.2 kcal/mole (cf. Table 3).
e) B r i d g e d H o m o t r o p i l i d e n e Systems
of t h e T y p e (8)
Doering and Klumpp [19] synthesized the tricyclic ketone
(8) by the AlzO,-catalysed ring expansion of (7) with
diazomethane.
r
8
l6a) cisoid
f 6 b ) cisoid
2
2
The temperature-dependence of the N M R spectrum of
homotropilidene shows it to undergo fast and reversible
valence-bond isomerization [29]. The dynamic equilibrium between the two valence-isomeric structures is
superposed on an equilibrium between two conformational isomers (cisoid and rransoid). This makes it difficult to determine the kinetic data for the valence isomerization from the NMR spectra.
[281 E. Vogel, K.-H. O t f , and K . Gajek, Liebigs Ann. Chem. 644,
172 (1961).
[291 Doering and Rofh [16,17] coined the term "fluctuating
structure" in a study of hornotropilidene.
756
Valence-bond isornerization should lead to the formation of the structurally different (9) from (8). The NMR
spectrum of (8) is independent of temperature [19],
suggesting that the equilibrium is doniinated by a
single isomer. Conjugatioii between the carbonyl group
and the three-membered ring evident11 makes (8) by
far the more stable of the two isomers.
f) B r i d ge d H o m o t r o p i 1i d e n e Sy s t e m s
of t h e T y p e (10)
Homotropilidene systems of the type (8) contain an
unsymmetrical bridge consisting of two carbon atoms.
The two valence-bond isomers therefore have different
[30] J. B. Lambert, Tetrahedron Letters 1963, 1901.
[ * ] Note added in proof: Reduction of (7) with hydrazine
hydrate in diethylene glycol/KOH gives inter a/ia tricyclo[3.3.1.04,6]nona-2,7-diene, which undergoes reversible valenceisomerization with AE* = 8.76 kcal/mole, k (-80°C) = 8016
sec-1. ( W . v . E. Doering and J . H. Hartenstein, personal communication).
Angew. Chem. internat. Edit. / Vol. 4 (1965) / No. 9
structures. Bridged homotropilidene systems of the type
(lo), on the other hand, contain a symmetrical bridge of
t w o carbon atoms, and therefore the va!ence isomers are
structurally identical, i. e. the equilibrium constant K is
unity. The valence-bond isonicrizatiorl can be studied
with the aid of the temperature-dependent NMR spectra, as in the case of tricyclo[3.3.2.04*6]deca-2,7-diene
(Section 11). The kinetic data for bridged homotropilidenes of the types (7) and ( l o ) ,as well as for bullvalene,
are also given in Table 3.
the chemical shifts of the proton resonance signal change
appreciably with temperature.
The previous interpretation [20] of the mode of formation of
(31) was based on the incorrect structure (32) proposed by
Jones [33] for a dimeric cyclooctatetraene of m.p. 53 ' C . This
structure should be replaced [37a] by (33). Surprisingly,
cyclooctatetraenedimerizes a t 100°C by 1,2:1,2 cycloaddition
t o give an all-cis-cyclobutane derivative.
Table 3. Kinetic data for bridged homotropilidene systems.
Compound
1
171
I 4360
k [sec-l] at
ooc
-400c
[a]
I
58.320 [ a ]
6150
3920
3210
2140
187 800
140 900
1
I
I
AE* Ikcal/mole]
8.1
i 0.4
(32)
1
Ref.
I 1301
l 2 . 9 * 0.2
13.2L 0.6
(33)
When (31) is treated with strong bases (e.g. potassium
t-butoxide in dimethyl sulfoxide), it isomerizes readily
to form a hydrocarbon (34), m.p. 69 "C.The reaction of
(34) with N-bromosuccinimide yields a dibromide,
which loses both bromine atoms on treatment with zinc
to form phenylbullvalene (35) [25].
29 tal
13.1 -C 1.0
11.7 f 0 . 2
(46)
[a] Extrapolated values.
The best synthetic route to bridged homotropilidene
systems of the type (10) is by way of cyclooctatetraene,
which dimerizes on heating or on prolonged standing at
room temperature [311. Four dimers of cyclooctatetraene, C16H16, are known with certainty, their melting
points being 38.5, 41.5, 53, and 76 "C [321. The C16H16
melting at 76 "C [33] is obtained by heating cyclooctatetraene at 100 "C for 68 h ; the yield is 37 % [37] based
on the cyclooctatetraene which has reacted. This dimer
(31) [9,20,23] consists of a bicyclo[4.2.0]octa-2,4-diene
unit and a homotropilidene unit (shown by thick lines).
The homotropilidene system is responsible for the
valence-bond isomerization.
The structures (31a) and (316) are identical. Two x
bonds and one of the CT bonds of the cyclopropane ring
fluctuate in the homotropilidene system [as in (6), (7),
and probably (S)], as can be seen from the teniperaturedependence of the NMR spectrum [9]. The areas and
[31] W. Reppe, 0 . Schlichting, K . Klager, and T.Toepel, Liebigs
Ann. Chem. 560, 1 (1948).
[32] G. Schroder: Cyclooctatetraen. Verlag Chemie, Weinheim/
Bergstr., 1965.
[33] W. 0.Jones, Chem. and Ind. 1955, 16.
[34] J. F. M . Oth, J . M . Gilles, and R. MerPnyi, unpublished
work.
I351 M. Saunders, Tetrahedron Letters 1963, 1699.
[36] J. M . Gilles and J . F. M . Oth, unpublished work.
[ 3 7 ] B. Fliigel, Diploma Thesis, Technische Hochschule Karlsruhe, 1964.
I37a1 G. Schroder and W. Martin, unpublished results.
Angew. Chem. internat. Edit.
I
Vol. 4 (1965) NO.9
+
COOR
COOR
COOR
(36)
(37)
Compound (31) reacts extremely readily with dienophiles. The adduct (36) formed form (31) and dimethyl
acetylenedicarboxylate decomposes on heating into dimethyl phthalate and a hydrocarbon (37) 19,201, in
which the homotropilidene system is bridged by a cyclobutene ring. The NMR spectrum of (37) is temperaturedzpendent [9], showing that this is also a molecule with
fluctuating bonds.
The compound ( 3 1 ) reacts with iron pentacarbonyl at
160-170 "C to form infer atia the complex C & I ~ - F ~ ( C O ) ~ ,
m.p. 118 @C.The temperature-dependent N M R spectrum of
this complex agrees with the structure (38) [38].X-ray analysis
led t o the structure (39) for a complex having the same
overall formula [39]. It was later found, however, that the
complex used for the X-ray analysis, obtained by irradiating a
mixture of cyclooctatetraene and cyclooctatetraene-Fe(CO),,
has m. p. 172 "C [40].
(381
(391
Other bridged homotropilidene systems of the type (10) are
derived from bullvalene [cf. compounds (53), (56), and
(57)l. These are discussed in Section 111.3.
[38] C . N . Schrartrer, P. Chckner, and R . MerPnyi, Angew. Chem.
76, 498 (1964); Angew. Chem. internat. Edit. 3, 509 (1964).
[39] A . Robson and M . R . Truter, Tetrahedron Letters 1964, 3079.
[40]M. R . Truter, personal communication.
757
g) B r i d g e d H o m o t r o p i l i d e n e S y s t e m s
of t h e T y p e ( I I )
Doering and Klumpp [191 later synthesized bullvalene
(and monodeuterated bullvalene) from the tricyclic
ketone (8).
The hydrocarbon (37) reacts with one mole of bromine
to form the dibromide (40), which loses HBr under the
influence of bases (potassium alkoxide) with formation
of the monobromide C12H1IBr (4/)[22]. This is an unsymmetrically substituted homolropilidene system with
a cyclobutene bridge.
b) T h e ” B u l l v a l e n e Principle“
137)
140)
Valence isomerization of (41) should lead to the structural isomer (42) with the bromine attached to the cyclopropane ring. Compounds (41) and (42) therefore have
different energies. The NMR spectrum of C l ~ H l l B ris
independent of temperature, showing that the equilibrium mixture consists almost entirely of (41). The latter
reacts with potassium t-butoxide to form C12HllOC4Hg.
The NMR spectrum of the ether (45) corresponding to
the bromide (41) should be independent of temperature.
Since, however, the spectrum of ClzH110C4H9 is temperature-dependent, this must be a mixture of (44) and
(45) [41], probably formed via the intermediate (43).
Compounds (44) and (45) are evidently formed by an
elimination-addition mechanism.
o-[a]=
Bullvalene is an unusual member of the class of molecules with fast and reversible valence isomerization. In
this molecule, no two carbon atoms remain bonded to
each other for any length of time. The ten C atoms are
continually changing places and neighbors. No precedent for a molecule with properties of this kind is
known in organic chemistry [16,17]. Any carbon atom
in bullvalene can be bonded to any other as a result of
valence isomerization, as shown in Scheme 2 [42].
t
t
J.
Scheme 2.
Br
141)
(44)
(43)
145)
3. Bullvalene and Some of its Derivatives
In homotropilidene (6)and the bridged homotropilidene
systems of the types (7), (81, (lo), and ( I I ) , only one
cyclopropyl bond can be broken, and only two valence
isomers are involved. Bullvalene, on the other hand,
possesses a threefold axis of symmetry. The three cyclopropyl bonds are equivalent and have equal statistical
weights in a valence isomerization, i.e. all three bonds
are equally likely to be broken. Thus instead of giving
only two structurally identical valence isomers, bullvalene gives 10!/3, i.e. about 1.2 million 116,171.
The photolysis of the dimeric cyclooctatetraene (31)
gives a very good yield of benzene and tricyclo[3.3.2.04,6]deca-2,7,9-triene (46), m.p. 96 “C121,231. Doering and
Roth [ 16,171,on the basis of theoretical considerations,
postulated the structure (46),predicted its NMR-spectroscopic behavior, and suggested the name “bullvalene”.
In the course of a few valence isomerizations, each proton in the bullvalene molecule passes through cyclopropyl, olefinic, and bridgehead positions (e.g. the C-5
proton in Scheme 2). If the valence isomerization is
sufficiently fast. the ten protons of bullvalene become
equivalent from the point of view of NMR spectroscopy.
At 100 “C the NMR spectrum of bullvalene consists of a
single sharp proton resonance signal at T = 5.78 with a
line width of 1.5 C.P.S. (Fig. 2) [9]. This confirms the
predictions of Doering and Roth [16,17].
[41] The valence isomers of ( 4 4 ) are structurally identical.
1421 Regarding the bullvalene principle see also [16,17].
a) P r e p a r a t i o n of B u l l v a l e n e
758
Angew. Chem. internat. Edit. / V d . 4 (1965) No. 9
-cps
300
200
100
0
I
I
I
I
1
Bullvalene itself contains three types of hydrogen atom
(olefinic, cyclopropyl, and bridgehead protons). There
is thus the question as to which type of C-H bond is so
strongly activated that a base catalysed H / D exchange
147)
-85°C
-
I
1
I
I
I
I
I
,
I
4
5
6
7
8
9
10
I---
Fig. 2. N M R spectra of CloHto, bullvalene (461 (0.4 mole/l in
CS2) 19,101. Internal standard letramethylsilane.
The low-temperature spectrum of bullvalene comprises two
groups of bands, the areas of which are in the ratio 3:2. A
multiplet is found a t T = 4.35, corresponding t o the six olefinic
protons of bullvalene. The relatively broad band at T = 7.42
must be assigned to the three cyclopropyl protons and the
bridgehead protons. The two groups of bands coalesce at
13 "C, giving the single signal at T = 5.78 above this temperature (cf. Fig. 2).
The rate constant k and the activation energy AE* of the
valence isomerization of bullvalene shown in Table 3 [35,36]
were determined from the N M R spectra at various temperatures with the aid of the line-width theory developed by
Anderson [431 and Sack [44].
The single proton resonance signal in the high-temperature spectrum of bullvalene is physical proof of the constant statistical change of position of the ten C atoms in
the molecule. Chemical evidence can be obtained from
the base-catalysed hydrogen-deuterium exchange in the
tricyclic ketone (8) [19] and in bullvalene [21].
is possible. Bicyclo[3,2,2]nona-2,5-diene(SO) takes up
deuterium in thesystemROK/ROD at about 160 "C [45].
In this case only the olefinic protons are replaced. By
analogy it is assumed that the most strongly acidic protons in bullvalene are also the olefinic hydrogen atoms.
Deuterium entering bullvalene at the double bond
should immediately become randomly distributed over
the entire molecule. Bullvalene undergoes an H/D exchange in the system ROK/ROD at 160 "C. The infrared
spectrum of the product contains distinct C-D stretching vibrations in the olefinic and aliphatic regions [21].
c) T h e C h e m i s t r y of B u l l v a l e n e [21]
Bullvalene is exceptionally stable. It decomposes only at
about 400 "C;the reaction takes about 10 min and yields
naphthalene and hydrogen.
Catalytic hydrogenation over palladium results in the
uptake of four moles of hydrogen to form (SI), m.p.
180 "C. The action of sodium in liquid ammonia followed by addition of ethanol reduces bullvalene to (521,
m.p. 45 "C. Thus the two sodium atoms add in the 1,4position to the vinylcyclopropyl system in bullvalene.
The three double bonds of (46) are hydrogenated in
succession by diiniine. Dihydrobullvalene (53), m.p.
62"C,can be prepared in this way. This is also a molecule
Both hydrogen atoms of the methylene group of (8) are
relatively acidic, and a base-catalysed H / D exchange
proceeds via the anion (47). Similarly, in the case of
bullvalene, deuterated hydroxybullvalene (48) is formed
as an intermediate. The transposition of the C-D and
C-H units should therefore lead to (49), with random
distribution of the deuterium in (8). In deuterium oxide
at room temperature, all the hydrogen atoms in (8) are
replaced by deuterium in the presence of 0.1 N base [191.
[43] P. W. Anderson, J. physic. SOC.Japan 9, 16 (1954).
1441 R. A. Suck, Molecular Physics I, 163 (1958).
Angew. Chem. internat. Edit. 1 Vol. 4(196.5)
1 No. 9
[451 G. Schroder, unpublished work.
759
with a fluctuating bond, and is the simplest homotropilidene bridged by two carbon atoms. The NMR spectrum of (53) is temperature-dependent (Section 11). A
large excess of diimine leads to (54), m.p. 126 "C.
R = CsH5 (35)
R = CH3 (13)
The three-membered ring in (46) can be detected chemically both by the formation of the compound (54) and by
degradation. The ozonolysis of bullvalene, followed by
reduction with sodium borohydride and esterification of
the trio1 (which is not isolated), leads to cis-1,2,3-tris(acetoxymethy1)cyclopropane (55).
Another synthesis of phenylbullvalene (35) has already
been mentioned in Section 111,2f.
A useful route to monosubstituted bullvalenes is the
bromination of bullvalene [l, 241. The dibromide (58)
loses hydrogen bromide in the presence of potassium
alkoxide, giving good yields of bromobullvalene (59),
which is used for the preparation of other monosubstituted bullvalenes. Thus (59) reacts with alcohol-free
potassium alkoxides in diniethyl sulfoxide, presumably
via dehydrobullvalene (60) [*] to form the alkoxybullvalenes (61) to (641, with R = C(CH3)3, CH(CH&,
CzH5, and CH3 respectively.
Bullvalene reacts smoothly with dichlorocarbene and
with osmium teiroxide/mannitol to form the products
(56) and (57) respectively. As expected, the NMR
spectra of (56) and (57) are temperature-dependent
[9,34]. Kinetic data for (56) are given in Table 3.
d) S u b s t i t u t e d B u l l v a l e n e s
A substituted bullvalene has about 1.2 x 106 valencebond isomers which unlike those of bullvalene are not
all structurally identical [l].
Four positional isomers are possible for monosubstituted
bullvalene. Whereas the valence isomerization of bullvalene
is characterized by one rate, seven elementary rates of
isomerization must be distinguished in monosubstituted
bullvalenes. Scheme 3 shows the four positional isomers
( C : substituent o n the cyclopropane ring; Oc and o b : on the
olefinic double bond near the cyclopropane ring and near the
C
f) N M R S t u d i e s o n
M o n o s u b s t i t u t e d B u l l v a l e n e s 111
In bullvalene and substituted bullvalenes there are three
courses open to each valence-bond isomerization, i. e.
oc
QQ
R
B
o b
o b
oc
oc
R
C
Scheme 3.
e) P r e p a r a t i o n of S o m e
Monosubstituted Bullvalenes
any one of the three cyclopropyl -ends can break. Bullvalene itself has a threefold axis of symmetry and each
of the three cyclopropyl bonds has an equal probability
of being broken. The isomers C, O,, and 0, (Scheme 3)
possess only a plane of symmetry, so that two isomerization processes are possible in each case. The isomer B
also has a threefold axis of symmetry and therefore only
a single rate of isomerization. The high-temperature
spectra of all the monosubstituted bullvalenes studied so
far contain only one singlet for the nine bullvalyl proions. On the one hand this proves the bullvalene skele-
Phenylbullvalene and niethylbullvalene were prepared
by the reaction of a Grignard compound with (8),followed by the elimination of water from the adduct 1191.
which exhibits a temperature-dependent N M R
spectrum, can be isolated ( H . Rdttele, Doctorate dissertation,
Technische Hochschule Karlsruhe).
bridgehead respectively; B: on the bridgehead) and the
seven rates of isomerization (kl-k7) of a monosubstituted
bullvalene.
The number of positional isomers and the number of possible
valence isomerizations increase with the number of substituents. In a bullvalene with nine different substituents,
there are 10!/6 = 604800 different chemical environments for
the single remaining bullvalyl proton.
760
[*I Note added in proof: In the presence of furan an adduct
C14H120,
Angew. Chem. internat. Edit. Vol. 4 (1965) 1 No. 9
ton, while on the other it shows that the molecule passes
through all the possible valence-isomeric structures.
Table 4. N M R signals of the bullvalyl protons and average rates of
isomerization in monosubstituted bullvalene derivatives 1461.
7 = chemical shift, 7 = 10 for tetrarnethylsilane as internal standard.
1 Preferred
-60°C
Substituent
__
__
~-
-~
-
Line
width
r
olef.
T
4.35
4.17
7.92
7.27
7.80
7.35
7.79
7.90
5.78
5.72
2.8
6.0
5.73
6.8
5.97
19.0
7.92
6.00
41.0
7.92
6.00
55.0
7.90
6.02
72.0
aliph.
- __
4.22
4.33
4.99
4.33
5.50
4.35
5.52
4.37
5.48
from the temperature-dependent NMR spectra of
monosubstituted bullvalenes (see Table 4); for a discussion cf. [l].
The isomers of monosubstituted bullvalenes in which
the substituent occupies an olefinic position (0, and 0,)
are preferentially formed. This is mainly due to the fact
that substituents are more firmly bonded to an olefinic
than to an aliphatic carbon atom (bond effect).
In the compounds (35), (62), (63), and (64), 0, is
strongly preferred owing to the possibility of conjugation between the substituent and the vinylcyclopropyl
system of the bullvalene skeleton. In (61), 0, and 0,
are present in approximately equal quantities. Owing to
steric hindrance, the bulky t-butyl group in this case is
twisted into a configuration which does not favor optimum conjugation, so that the conjugation effect is subordinate to the bond effect. It cannot be decided whether
0, and O,, or 0, alone predominate in the equilibrium
mixture in (59).
__ 2.p.s.
___
[a] Cf. Scheme 3
IV. Conclusion
The line width of the singlet and the average rate of isomerization I; at a given temperature depend on the
substituent (see Table 4).
If all seven rate constants were equal, 30 % of the molecules on the average would be in each of the C, O,, and
0, structures, while 10% would have the B structure.
Certain valence isomerizations, however, are at a disadvantage with respect to others, so that the distribution
of the positional isomers depends on the rate constants.
Information 011 rate constants kl-k7 can be obtained
1461 E is that rate constant which, in the case of bullvalene,
would give the same line width a s the one observed for the
derivative studied.
We speak of molecules with fluctuating bonds when
the valence isomers have a mean lifetime of less than
about 100 sec at 0°C. Such molecules can so far be
unambiguously recognized and studied only by NMR
spectroscopy. The study of the bond shifts in substituted cyclooctatetraeiies and bullvalenes is particularly
interesting since the rate of shift depends on the substituent. This gives information on the relative importance af bonding, conjugation, steric, and inductive
effects.
[A 4691245 IE]
Received: June 9th. 1965
German version: Angew. Chem. 77, 774 (1965)
Translated by Express Translation Service, London
Conformational Analysis in Mobile Cyclohexane Systems
BY PROF. DR. E. L. ELIEL
DEPARTMENT OF CHEMISTRY, UNIVERSITY OF NOTRE DAME, NOTRE DAME, INDIANA (U.S.A.)
By “Conformational Analysis” is meant the analysis of the physical and chemical properties
of a compound in terins of its preferred“conformations”, i. e. rotational arrangements about
single bonds. This particular review deals with cyclohexanoid compounds capable of existing
in two or more stable conformations.
1. Historical
That cyclohexane is a puckered, chair- (or boat-)shaped
molecule was first suggested by Sachse [l] 75 years ago.
Because of a lack of appreciation of the restrictions to
rotation about single bonds and other misunderstandings, Sachse’s theories were rejected for nearly 30 years,
[I] H. Sachse, Ber. dtsch. chem. Ges. 23, 1363 (1890); Z. physik.
Chem. 10, 203 (1892).
Angew. Chem. internat. Edit.
VoI. 4(1965)
1 No. 9
to be revived only in 1918 by Mohr 121 and soon to be
confirmed by the classical work of W. Huckel on decalin
[3] and of Boeseken and coworkers on the acetone and
boric acid derivatives of cyclic diols [4].Still the ideas
[2] E. Mohr, J. prakt. Chem. [2] 98, 315 (1918); Ber. dtsch. chem.
Ges. 55, 230 (1922).
[3] W. Huckel, Liebigs Ann. Chem. 441, 1 (1925).
[4] J. Boeseken and J. van Giffen, Recueil Trav. chirn. Pays-Bas
39, 183 (1920); J. Boeseken, ibid. 40, 553 (1921); Ber. dtsch.
chem. Ges. 56, 2409 (1923).
76 1
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