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Hexamethyl(Dewar Benzene) (Hexamethylbicyclo[2.2

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Investment Costs for the
Separation Nozzle Process
Investment Costs for the
Gaseous Diffusion Process.
Thus, in spite of the fact that, according to the estimate
above, the specific suction volume is greater by a
factor of 8.8 one would not expect an appreciably
greater investment cost for the separation nozzle
process than for the gaseous diffusion process.
It is expected that the separation nozzle process will
have a real advantage compared to the gaseous diffusion process with respect to the equilibrium time of a
separation cascade. By the term, equilibrium time,
one refers to the time required between the start-up of
a cascade and the attainment of the product concentration, i.e. the time during which no product can
be withdrawn. The equilibrium time for cascades
which enrich 235U to medium or high concentrations
can be so long that they may not be neglected in the
economic considerations. Foragiven separation job, the
equilibrium time is directly proportional to the average
residence time of a UF6 molecule in a stage and inversely proportional to the square of the elementary
separation effect [121. Since the average residence time
will be at least no greater, and probably appreciably
less, in the separation nozzle process than in the
diffusion process, one can expect that the equilibrium
time will be shorter by at least an order of magnitude
in the former owing to the larger elementary separation
effect.
W e wish to thank G. Frey for his assistance in the
development of the Roots compressor and the test loop
as well as Miss B. Gebauer and J. Drsge, W. Mannherz,
and G. Schuler for their help in carrying out the e.xperiments. We also wish to thank Dr. van HalIe for
valuable discussions and for the translation of this report,
Received: December 8th and 9th, 1966
[B 2228 and 2230 IE]
German version: Chemie-1ng.-Techn. 39, I , 80 (1967)
Hexamethyl(Dewar Benzene) (Hexamethylbicyclo[2.2.O]hexa-2,5-diene)
BY W. SCHAFER AND H. HELLMANN [*I
Dedicated to Professor Friedrich Asinger on the occasion of his 60th birthday
Hexamethyl(Dewar benzene) was first obtained by bicyclotrimerization of 2-butyne in
the presence of aluminum chloride. This preparation provided a basis for a comprehensive
study of the chemical reactivity of Dewar benzene derivatives. Interesting information
concerning the reactivity of the permethylated Dewar benzene system can be deduced
from its behavior on hydrogenation and oxidation, and, in particular, from the addition
of H-acidic compounds and from the substitution and elimination reactions of the resulting
adducts.
1. Introduction
:a:
Dewar benzene (Dervar, 1867)[**I
Bicyclo[2.2.0]hexa-2,5-diene131
4
Dewar benzenes, benzvalenes, and prismanes, derivatives of the three non-planar “valence isomers” of
benzene, are now, 100 years after Kekult’s conception
of his benzene formula, emerging from the ranks of
the esoteric organic compounds (see also [I ,21). Fears
that the unstable valence-isomeric forms would readily isomerize into benzene derivatives, because of
their steric strain, are now recognized as being almost
groundless. Derivatives of all three “valence isomers”
have been prepared in recent years; these compounds
can be handled without difficulty in the absence of
[*I
Dr. W. Schafer and Prof. Dr. H. Hellmann
Forschungslaboratorien der Chemische Werke Hiils AG.
4370 Marl (Germany)
111 E. E. van Tamelen, Angew. Chem. 77, 759 (1965); Angew.
Chem. internat. Edit. 4, 738 (1965).
[2] H . G. Viehe, Angew. Chem. 77, 768 (1965); Angew. Chem.
internat. Edit. 4,746 (1965).
518
:@
6
-
Benzvalene (Hiickd, 1937)
TricycIof3.1.0.0~~6]hex-3-ene
[3)
Prismane (Ladenburg, 1869)
Tetracyclo[2.2.0.02603.s]hexane [3]
4
rearrangement catalysts. It still remains to be seen
whether this is also true of the three parent compounds.
Of these, the only one synthesized so far is the un-
[**I The name “Dewar benzene” rather suggests a resonance
structure of benzene. A trivial name, similar to benzvalene and
prismane, has not been assigned to this compound although the
structure and unsaturated character are well described by the
name “tectadiene” (latin, tectum = roof).
131 Concerning nomenclature, cf. J . Meinwald and J . K . Crandall, 3. Amer. chern. SOC.88, 1300 (1966).
Angew. Chem. internat. Edit.
Vol. 6 (1967)
No. 6
substituted Dewar benzene 14,4aI, which is less stable
than its derivatives (half-life 2 days at 20 “C).
From the point of view of preparation, the Dewar
benzene derivatives occupy a special place among the
derivatives of the valence isomers of benzene, since all
the preparations of prismanes yet accomplished are
based on the photochemical excitation of Dewar
benzene derivatives.
The irradiation of a 20 vol: ”/, solution of cyclobutene
in benzene with UV light (A = 2537 A) at room temperature gave rise to a novel cycloaddition. The reaction afforded the tetracyclic hydrocarbon CloHl2
(50 % yield), the formation of which can be explained
by the addition of cyclobutene to the Dewar benzene
that is formed as an intermediate“21 (eq. (b)).
Dewar benzenes can be synthesized by the following
five methods.
1.1. Photoisomerization of Benzene Derivatives
A benzene derivative is irradiated with high frequency
UV light.
The first Dewar benzene derivative was synthesized by
van Tamelen and Pappas 151, who obtained 1,2,5-tri-tbutyl(Dewar benzene) by photoisomerization of 1,2,4tri-t-butylbenzene in ether. 1,2,3,5-Tetra-t-butyl(Dewar benzene) was obtained, together with the corresponding benzene derivative, by irradiation of 1,2,4,5tetra-t-butylbenzene [GI. The photochemical rearrangement of hexafluorobenzene to hexafluoro(Dewar
benzene) proceeds particularly smoothly (60 ”/,
yield) [7,83 ;octafluorotoluene, on the other hand, gives
a mixture of 1- and 2-trifluoromethylpentafluoro(Dewar benzenes) 181.
On the other hand, no Dewar benzene derivative can
be isolated on irradiation of the “aromatically destabilized” o-di-t-butylbenzene; the product is in fact
a mixture of m- and p-di-t-butylbenzenes in a ratio
that depends on the irradiation time[l3,141, a result
that can again be explained by the formation of Dewar
benzene and prismane intermediates.
1.2. Degradation of Bicyclo[2.2.0]hex-5-ene2,3-dicarboxylic Anhydride with Lead Tetraacetate
U V irradiation of cis-l,2-dihydrophthalic anhydride in
ether yields bicyclo[2.2.0]hex-5-ene-2,3-dicarboxylic
anhydride, which has been converted into the unstable
Dewar benzene in 2 0 % yield by degradation with
lead tetraacetate under mild conditions 141.
Van Tamelen a n d Pappas chose 1,2,4-tri-t-butylbenzenea s
starting material because they deduced that th e steric interaction o f the bulky t-butyl groups situated ortho to each
other (“aromatic destabilization”, cf. e.g. 191) would favor
the Dewar form. in which these groups a r e farther apart.
However, the success of th e photoisomerization is not tied
to steric conditions of this nature[lol, as is shown by t h e
photoisomerizations of hexafluorobenzene a n d octafluorotoluene, a s well a s by a n u m b er o f reactions in which t h e
intermediate formation of Dewar benzenes h as been postulated in order t o explain th e structure of th e products. A n
example of such a reaction was described by Kaplan et al. [ I l l ,
who obtained [1,2,4- I4Cjltrimethylbenzene via Dewar
benzenes o n photolysis of [1,3,5-14C3]trimethylbenzene;t h e
course of this reaction also shows th at t h e migration of t h e
methyl groups does n o t involve free radicals.
[4] E. E. van Tamelen and S . P. Pappas, J. Amer. chem. SOC.88,
3297 (1963).
[4a] Note added in proof: I(. E. Wilzbach, J . S . Rifscher, and
L . Kaplan, J. Amer. chem. Soc. 89, 1031 (1967), were able to
separate henzvalene from the photoisomerization products
(A = 2537 A) of benzene in the liquid phase by gas chromatography. Benzvalene rearomatizes slowly at room temperature.
[5] E. E. van Tamelen and S . P. Pappas, J . Amer. chem. SOC. 84,
3789 (1962).
[6] E. M. A r n e f t and J . M. Bollinger, Tetrahedron Letters 1964,
3803.
[7] I. Huller, J. Amer. chem. SOC. 88, 2070 (1966).
[8] G. Carnaggi, F. Gozzo, and G. Cevidulli, Chem. Commun.
1966, 313.
[9] A . W . Burgstahler, P.-L. Chien, and M . 0 . Ahdel-Rahman, J.
Amer. chem. SOC. 86, 5281 (1964).
[lo]Cf., however, D . Seebacli, Angew. Chem. 77, 119 (1965).
especially p. 122; Angew. Chem. internat. Edit. 4 , 121 (1965).
[ll]L. Kaplan, K . E. Wilzbach, W . G. Brown, and S . S . Yang,
J. Amer. chem. SOC.87, 675 (1965).
Angew. Chem. internat. Edit.
1 Vol. 6 (1967) 1 No.
6
1.3. Addition of Cyclobutadiene in situ to
Dihalogenomaleic Anhydride, Esterification, and
Dehalogenation
Criegee and Zanker [I51 have obtained dimethyl tetramethyl(Dewar phthalate) ( I ) in accordance with
equation (d). The last step is reminiscent of a method
used for the preparation of a Dewar anthracene derivative [I61 I *I.
( I ) can be converted into another Dewar benzene
derivative, namely, octsrnethylbicyclo[2.2.0]hexa-2,5diene-2,3-dimethanol, by reaction with an excess of
methylmagnesium iodide 1171.
[12] R. Srinivasan and K . A . Hill, J. Amer. chem. SOC.87, 4653
(1965);cf., however, K. E. Wilzbach and L. Kaplan, ihid. 88,2066
(1966).
[131 K . Kirk, Dissertation, University of Wisconsin (1963).
1141 A . W . Burgstahlet and P.-L. Chien, J. Amer. chem. SOC.86,
2940 (1964).
[15] R. Criegee and F. Zanker, Chem. Ber. 98, 3838 (1965);Angew. Chem. 76, 716 (1964); Angew. Chem. internat. Edit. 3, 695
(1964).
[16] D . E. Applequist and R. Searle, J . Amer. chem. SOC.86,
1389 (1964).
[*I I n all the following formulae, lines attached to the ring
denote methyl groups, and crosses denote t-butyl groups.
[17] R. Criegee and R. Askani, Angew. Chern. 78, 494 (1966);
Angew. Chem. internat. Edit. 5, 519 (1966).
519
(d)
CO O C H,
(1)
COOCti?
(Dewar benzene) and 1,3,6-tri-t-butyl-2,4,5-trifluorobenzvalene were formed in a yield of 33 % each. The
trimerization of t-butylfluoroacetylene proceeds via
dimeric intermediates, which can be intercepted with
an excess of acrylonitrile or styrene. The origin of
small quantities of tetramers observed in the oligomerization of t-butylfluoroacetylene can also be
traced back to the dimerization of the dimeric intermediates. Viehe explained the reaction as proceeding
formally via biradicals (eq. (9) and (h)) [211.
OH
1.4. Addition of Cyclobutadiene in situ to Acetylenes
All attempts to isolate cyclobutadiene or its derivatives as such have so far been unsuccessful. However,
there are a number of stable cyclobutadienemetal
complexes
of which tricarbonylcyclo butadieneiron,
which can be prepared[l9Jin40Xyield by the reaction of
cis-3,4-dichlorocyclobutenewith Fe2(C0)9, can be
used for the synthesis of Dewar benzenes[201. When
the complex is decomposed by oxidation with salts of
Ce(1v) in the presence of acetylene derivatives, the
resulting cyclobutadiene is intercepted to form the
Dewar benzene system (eq. (e)).
[a]
R-C-C-R',
i.e(CO),
R
mR
(e)
'Rl
I R'
Ref
1.5. Bicyclotrimerization of Acetylenes
The formally most simple method for the preparation
of Dewar benzenes is the bicyclotrimerization of
acetylenes in accordance with equation (f).
In 1964, Viehe et al. [2,231 reported a spontaneous trimerization of the extremely reactive compound tbutylfluoroacetylene. 1,2,6-Tri-t-butyl-3,4,5-trifluoro1181 P . M . Maitlis, Advances organometallic Chem. 4, 95 (1966).
[19] G. F. Emerson, L. Watts, and R . Pettit, J. Amer. chem. SOC.
87, 1 3 1 (1965).
1201 Chem. Engng. News 43, No. 34, p. 38 (1965).
[21] L. Watts, J . D . Fitzpatrick, and R . Pettit, J. Amer. chem.
SOC.87, 3253 (1965).
1221 G. D. Burt u. R . Pettit, Chem. Commun. 1965, 517.
[23] H . G. Viehe, R. MerPnyi, J. F. M . Oth, J. R . Senders, and
P. Valange, Angew. Chem. 76, 922 (1964); Angew. Chem. internat. Edit. 3, 755 (1964).
520
2. Synthesis of Hexamethyl(Dewar Benzene) (3)
It can be seen from the above survey that the preparation of Dewar benzenes requires the use of processes
involving several steps. In the photochemically induced
valence isomerizations, the yields and the uniformity
of the reaction course generally leave much to be
desired.
Early in 1966, we discovered a new synthesis that is
free from these disadvantages, i.e. the bicyclotrimerization of 2-butyne in the presence of aluminum
chloride to form hexamethyl(Dewar benzene) (3) f251;
the reaction proceeds formally in accordance with
equation (f).
Optimization experiments showed that the synthesis
is best carried out at 35-40 "C in benzene or methylene
chloride. The catalyst is added in the proportion of
about 5 wt-% (based on the 2-butyne), and the reaction time at 35 "C is 5-7 hours. The brown reaction
mixture is poured onto ice and washed with dilute
alkali, and (3) is isolated in a very pure state by
fractional distiIlation of the organic phase. The conversion of 2-butyne is about SO%, and the yield is
60 to 70 %, based on the reacted 2-butyne. Hexamethylbenzene and an approximately equal quantity of a
dark oil are formed as by-products. Longer reaction
times give higher conversions, but the yield of (3)
decreases in favor of hexamethylbenzene ( 5 ) . This
suggests that (3) is an intermediate and that ( 5 ) is the
true stable end product of the AlCI3-catalysed trimerization of 2-butyne. ( 5 ) can be obtained in yields
of up to 85 % by trimerization of 2-butyne with AlCl3
in boiling benzene. Even at low temperatures, the
reaction leads exclusively to ( 5 ) when the reaction
time is sufficiently long.
The course of the reaction is formally as shown in
equation (i).
1241 Cf. [Z], p. 771 und 772.
[25] W. Schufer, Angew. Chem. 78, 716 (1966); Angew. Chem.
internat. Edit. 5, 669 (1966).
Angew. Chem. internat. Edit.
Vol. 6 (1967) NO. 6
isomeric hydrocarbons C16H24, which have been identified
as syn- (9) and anti-octamethyltricyclo[4.2.0.0~~~]octa-3,7diene (8) and octamethylcyclooctatetraene (ZO).
When ( 3 ) is brought into contact with AlCl3, it reacts
extremely vigorously to form ( 5 ) . Quantitative exothermic rearrangement into ( 5 ) takes place when (3)
is added dropwise to a slurry of AICl3 in benzene. It
must therefore be concluded that in the bicyclotrimerization of 2-butyne the reaction mixture does not
contain free AlC13, at least so long as the concentration of 2-butyne is sufficiently high.
2-Butyne reacts immediately with AlCl3 to form a
brown complex, which is readily soluble in methylene
chloride but only slightly soluble in benzene. The UV,
IR, and NMR spectra of (2) are very similar to
the spectra of the 4-chloro-l,2,3,4-tetramethylcyclobutenyl cation (6), which was prepared by Gold and
Kutz [261 by the action of AICl3 on 3,4-dichloro-l,2,3,4tetramethylcyclobutene in methylene chloride.
By analogy, therefore, the cationic part of the solvated
polar complex intermediate (2) can be formulated as
(7). Thus ( 2 ) is not comparable to the stable cyclobutadiene complexes, which have a central dS ion of
Feo, Co, NiII, or PdII [181.
Since the reaction mixture contains an excess of
2-butyne, AlC13 is slowly displaced from (2), with
immediate formation of fresh (2) and liberation of
the unstable tetramethylcyclobutadiene, which is also
instantaneously intercepted in a diene synthesis with
2- butyne to form hexamethyl(Dewar benzene).
In addition to hexamethyl(Dewar benzene) (3) (60-70 %)
and hexamethylbenzene (5) (12-18 %) (yields based on
These hydrocarbons had previously been obtained by
Criegee (271 as dimeric products formed by secondary reaction of the unstable tetramethylcyclobutadiene. We regard
their formation in the AlCI3-catalysed bicyclotrimerization
of 2-butyne as important evidence of the occurrence of
tetramethylcyclobutadiene as a n intermediate.
Finally, reaction scheme (i) raises the question of how
the course of the reaction is affected by the presence
of hexamethylbenzene, which also forms an addition
complex ( 4 ) with AlCl3. To solve this problem an
excess of hexamethylbenzene was added to the slurry
of aluminum chloride in benzene, and 2-butyne was
then introduced. The reaction proceeded normally,
showing that 2-butyne decomposes the complex ( 4 )
with formation of ( 5 ) and (2).
The bicyclotrimerization of 2-butyne is catalysed not
only by AlC13, but also by other aluminum compounds
particularly alkylaluminum dichlorides (alkyl = ethyl,
n-propyl, n-hexyl, s-butyl). The reaction time required
is several times as long, and the amount of catalyst
used must be increased to correspond to the increased
molecular weight. AlBr3 and alkylaluminum dibromides also gave positive results. On the other hand,
dialkylaluminum chlorides are catalytically inactive,
as are other Lewis acids (e.g. FeC13, TiC14, SnC14,
ZnClz, or BF3).
3. Reactions with Hexamethyl(Dewar Benzene)
reacted 2-butyne in both cases), the trimerization of 2butyne also yields about 2 % of a crystalline mixture, which
separates out (contaminated with hexamethylbenzene) from
the very non-homogeneous residual oil (10-15 %) on cooling.
Repeated fractional crystallization of this mixture gives three
The bicyclotrimerization of 2-butyne made it possible
for the first time to prepare a Dewar benzene derivative on the kg scale by a simple one-step reaction.
Apart from its ready availability, hexamethyl(Dewar
benzene) has two properties that favor an investigation
of its chemical reactivity: its appreciable thermal
stability (when heated for 48 hours at 100°C, the
hexamethylbenzene content is less than 1 %) and the
fact that it carries only methyl substituents. Consequently, in chemical changes in the Dewar benzene
system, there is no need to take into account the
effects of different substituents. On the other hand, the
chemical reactivity of hexamethyl(Dewar benzene)
cannot be regarded u priori as typical of Dewar benzenes carrying fewer substituents.
M.p. 127 "C
The physical constants of hexamethyl(Dewar benzene)
are listed in Table 1.
M.p. 196 "C
M.p. 113 "C
[26] E. H . Gold and Th. J. Kafz,J. org. Chemistry 31, 372 (1966);
see also D . E. Applequist and J . D . Roberts, J. Amer. chem. SOC.
78, 4012 (1956).
Angew. Chem. internat. Edit.
Vol. 6 (1967)
1 No. 6
[271 R . Criegee, Angew. Chem. 74, 703 (1962); Angew. Chem.
internat. Edit. 1 , 519 (1962).
52 1
Table 1. Physical constants of hexamethyKDewar benzene) ( 3 )
B.p.
ng
M.p.
d:o
Flash point (Pensky-Marrerrs)
Ignition point
Viscosity 20 "C
Heat of combustion
U V spectrum (end absorption only)
I R spectrum
N M R spectrum
-
Thermal stability:
(5)
half-life ( 3 )
6 0 "C/2Omm ( m 152 "C/760mrn)
1.4479
7.5 "C
0.8125
42 " C
238 " C
1.47 cP; 1.81 cSt
1765 I 1.1 kcal mole-1
250 nm, E = 0.76 1 g-1 cni-1
240 nm, E = 2.9 1 g-1 cm-1
230 nm, E == 7.3 1 g-1 cm-1
220 niii, E = 13.7 I g-1 cm-1
1680, 1370, 1280, 1220, 1060
735, 660 cni-J
T
8.45 and 8.95 (2: 1)
3.2. endo-1,2,3,4,5,6-Hexamethylbicyclo[2.2.0]hex2-ene 1321
Hydrogenation of hexamethyl(Dewar benzene) (3) to
form endo-l,2,3,4,5,6-hexamethylbicyclo[2.2.0]hex-2ene ( 1 5 ) proceeds smoothly under normal pressure
on Pt/active charcoal at room temperature, or in an
autoclave at 50 "C and 20 atm with Raney nickel as the
catalyst when ethyl acetate is used as the solvent. When
the reaction is carried out without solvent, the temperature and pressure must be increased (60-100 "C/
100 atm).
(inCCI4; internal standard:TMS)
105 h a t 120 O C
5.5 h a t 140'C
2.1 h a t 1 5 0 O C
3.1. Valence Isomerization
The physical constants of the compounds (15) to (45)
are listed in Table 2.
Hexamethyl(Dewar benzene) (3) can be quantitatively
isomerized to hexamethylbenzene ( 5 ) by AlC13 (cf.
Section 2). This isomerization can also be brought
about by other Lewis acids, such as FeCl3, SbClS,
TiC14, or, on gentle heating, by SbC13, SnC14, and
ZnC12.
Small quantities of (15) had already been obtained by
another route by Criegee et a / . [331. T h e thermal rearrangement of (15) into hexamethylcyclohexa-1,3-diene(16) had
also been studied in the course of kinetic investigations on t he
valence isomerization of cyclobutenes [341, though t h e compound was not isolated. T h e rearrangement of (15) into (16)
in a stainless steel (V4A)autoclave a t 200 "C takes 3 hours;
a t 250 O C , hexamethylbenzene (5) is also formed.
When t h e pyrolysis of (15) is carried o u t a t 250°C in a
sealed glass tube, pentamethylbenzene an d methane a r e
formed a s well as (16). Thus the course of t h e pyrolysis is
influenced by metal ions.
Irradiation of hexamethyl(Dewar benzene) with a low
pressure UV lamp again leads mainly to hexamethylbenzene; however, in this case, the main product is
accompanied by a 20-25 % yield [based on reacted (3)]
of a second valence isomer, hexamethylprismane (13),
m.p. 91 "C [28,29J.
Thus three prismane derivatives are now known [30,31J,
though (11) has so far been obtained only in a purity
of 82 %. They were all obtained by irradiation of the
corresponding Dewar benzenes with UV light of
wavelength
254 nm.
-
Table 2. Physical constants of the products formed by secondary
reactions of hexarnethyl(Dewar benzene) ( 3 ) .
Compound
B.p. ("Cimrn)
64/20
91-92/11
54-5510.13
132- 134/12
73/12
94/10
63-64/14
M.p. ( " C )
1.4498
40-42
1.447 1
1.447s
88-92/11
When heated, (13) is converted into hexamethyl(Dewar benzene) (3) and hexamethylbenzene (5) 128,291.
It reacts with the strong dienophile 4-phenyl-l,2,4triazoline-3,5-dione in strongly polar solvents t o give
(14) (e.g. (k)) 1281.
110-1 12/19
76-77/9
63-6411
118119
12010.6
128/0.2
87-90/13
143-145/8
72-73/9
(14)
[28] D . M . Lemal and J. P . Lokensgard, J . Amer. chem. SOC.88,
5934 (1966).
[29] W . Schafer, R . Criegee, R . Askani, and H . Griiner, Angew.
Chem. 79, 54 (1967); Angew. Chem. internat. Edit. 6, 78 (1967).
[30] K . E. Wilzbach and L . Kaplan, J. Amer. chem. SOC.87, 4004
(1965).
[31] R . Criegee and R . Askani, Angew. Chem. 78, 494 (1966);
Angew. Chem. internat. Edit. 4,519 (1966).
522
90-921 I8
6411 1
71/11
70-7 1 / I 2
3.5
1.4979
1.5183
1.4809
87
155-157
50-52
53-54
184-186
57
72-73
-3
62-64
3-5
43-44
36-40
1.4986
1.4761
1.4962
1.4570
10
70
1.5054
1.4670
1.4665
I .4898
194
268-269
[32] H.-N. Junker, W . Schafer, and H . Niedenbruck, unpublished.
[33] R . Criegee, H . Kristinsson, D . Seebach, and F. Zanker,
Chem. Ber. 98, 2331 (1965).
[34] R . Criegee, D . Seebach, R . E. Winter, B. Borretzen, and
H.-A. Brune, Chem. Ber. 98, 2339 (1965).
Angew. Chem. internat. Edit./ Vol. 6 (1967)
/ No. 6
Ozonization of ( 1 5 ) in methylene chloride at 50°C
leads quantitatively to the distillable ozonide (17),
reductive cleavage of which gives 1,2,3,4-tetramethyl1,2-diacetylcyclobutane (18). (18) is also formed
directly from ( 1 5 ) in low yields by oxidation with
KMn04.
merization leads to the formation of pentamethyl-5-acetylcyclopentadiene (24). Similar acid-catalysed rearrangements
to form three-membered rings have been observed with
epoxycyclobutanes 136-371. (21) gives good yields of hexamethyl-2,3;5,6-bisepoxybicyclo[2.2.O]hexane(22) on reaction with perbenzoic or peracetic acid. (22) is stable under
the conditions used for the hydrolysis of the monoepoxide
( 2 1).
When the stoichiometric quantity of ozone is passed
into a solution of (3) in CHzClz at -50 "C, the monoozonide (25) is formed, while a further mole of 0 3
gives the diozonide (26).
(15) can be epoxidized with perbenzoic acid in benzene (76 % yield) or with peracetic acid in methylene
chloride in the presence of anhydrous sodium carbonate (65 % yield).
Unlike (25), (26) is insoluble in methanol (separation
method), and is so stable thermally that it melts
without decomposing. The hydrogenative cleavage of
(26) proceeds non-uniformly, and leads mainly to
tetramethylfuran and acetic acid.
Treatment of the epoxide (19) with 1 % sulfuric acid
in dimethyl sulfoxide leads, not to hydrolysis, but to
On oxidation of (3) with KMn04 in aqueous alkaline
isomerization to 1,2,3,4,5-pentamethyl-3-acetylcyclobutanol at O"C, the main product was the steampentene (20).
volatile 1,4,5,6,7-pentamethylbicyclo[3.2.0]hepta-3,6dien-2-one (28) (yield 50 %), which is formed by
intramolecular aldol condensation of the primary
3.3. Oxidation Reactions with
product, tetramethyl-3,4-diacetylcyclobutene(27), in
Hexamethyl(Dewar Benzene) ( 3 ) 1351
the alkaline solution.
Hexamethyl(Dewar benzene) (3) is autoxidizable, and
is therefore best stored under an inert gas. Old,
partially oxidized samples, which give an acidic reaction, are washed with aqueous alkali before distillation, since rearrangement into hexamethylbenzene ( 5 )
may otherwise take place, sometimes very violently.
When (3) is allowed to stand in air, colorless crystals
of hexamethyl bicyclo[2.2.0]hex-2-ene-5,6-diol(23)separate after only 1-2 days (yield M 20 %). (23) is also
(231
n
If the oxidation of (3) is carried out with KMn04 in
aqueous acetone and with the addition of MgS04 to
intercept the OHQ ions, (27) can be isolated in good
yield in the form of colorless crystals (m.p. 57 "C).
]To"
OH
formed from hexamethyl-5,6-epoxy-bicyclo[2.2.0]hex2-ene (21) by hydrolysis e.g. with an ether-water
mixture in the presence of H @ions. The formation of
(23) on autoxidation of (3) presumably also proceeds
via the monoepoxide.
The epoxide (21) is obtained by the action of perbenzoic acid on an equimolar quantity of (3). Peracetic acid (in the presence of anhydrous sodium
carbonate) may also be used.
The epoxidation IS accompanied by a side reaction, in which
proton-catalysed ring contraction followed by valence iso_ _
[35] H . - N . Junker, W. Schafer, and H . Niedenbriick, Chem.Ber.,
in press.
Angew. Chem. internat. E d i t .
1 Val. 6 (1967) / N o . 6
3.4. The Action of H-Acidic Compounds on
Hexamethyl(Dewar Benzene) ( 3 ) I381
The literature contains several references to the sensitivity of Dewar benzenes to acids. In fact this lability
does exist, but the Dewar benzenes should not be
expected to isomerize exclusively to the benzene
derivatives.
The action of HCI on hexamethyl(Dewar benzene) ( 3 )
in methylene chloride at -30, 0, or +40 "C leads to a
[36] R . Criegee and K . N o / / , Liebigs Ann. Chem. 627, 1 (1959).
[37] J.-L.RipoNand J.M.Conia,Bull. Soc. chim. France 1965,2755.
1381 H. Meister, W. Schiifer, and U. Sage, unpublished.
523
new vacuum-distillable compound (yield 80-85 %;
b.p. 110-112 "Cj19 mm), elementary analysis of which
shows that (3) has taken up 1 mole of HCI. The
products also include a lower-boiling isomer having
m.p. 62-64 OC, which is formed in a yield of 0-15 %,
depending on the reaction conditions. Only a small
amount of hexamethylbenzene is formed. The addition
of HCl to (3) can also be carried out with concentrated
HCI, though the yields obtained are lower.
The lower-boiling HCl adduct is hexamethyl-5-chloro-6Hbicyclo[2.2.0]hex-2-ene (30), while the higher-boiling adduct
is hexamethyl-5-chloro-6 H-cyclohexa-l,3-diene (29). The
course of the reaction is as indicated in equation (r).
(3)
/
B
A
\
C
reaction conditions and on the nature of the attacking
reagent.
3.5.1. H e x a m e t h y 1 b e n z e n e
The structure of (29) can be deduced not only from
spectral data, but also from the dehydrochlorination
by Lewis acids such as ZnClz or AlC13. Hexamethylbenzene ( 5 ) is obtained in quantitative yield even
under very mild conditions. Gentle heating with zinc
dust or iron powder leads to the same result.
7
(29) can add a further mole of HCI to form a colorless,
salt-like compound, which is moderately stable at low temperatures only, and which readily decomposes into HCl and
(29) '
Addition in accordance with equation (r) is not confined to
hydrogen chloride, though it proceeds particularly uniformly
and smoothly in this case. We were even able to add carboxylic acids to (3).
(29) can take part in diene syntheses, e.g. with maleic
anhydride or tetracyanoethylene; the reactions proceed
at temperatures up to 40 "C.
3.5.2. B i cyc 1o [ 2.2.01 h e x - 2 - e n e D e r i v a t i v e s
Br
OOCH
OOCCHj
OOCCHzCl
00CCHC12
OCHi
65
50
26
64
65
45
The addition of acids is always accompanied by a deep
violet coloration of the reaction mixture, indicating an ionic
mechanism. It is interesting to note that even methanol
reacts with hexamethyl(Dewar benzene) (3) when the
reactants are refluxed for several days in a n atmosphere of
nitrogen. After 48 hours, 45 of (36) and 15 %, of (5) could
be isolated. (3) also reacts with methanol at room temperature, but the reaction is much slower. The reaction is
completely suppressed by a few drops of sodium hydroxide
solution.
3.5. Elimination and Substitution Reactions with
HexarnethyI-5-chloro-6H-cyclohexa1,3-diene (29) 1391
In our attempts to find chemical proof of the structure
of the readily obtainable compound (29), we obtained
a series of unexpected reaction products. The formation of these products can be explained by the equilibrium of the carbonium ions A, B, C, and D as shown
in equation (s). These ions can be formed either by
protonation o f (3) or by elimination of C1Qfrom (29).
The questions of whether stabilization takes place by
substitution or by deprotonation and whether the
product is a derivative of A, B, C, or D depend on the
[39] H . Meister and W. Schufer, unpublished.
524
When (30) is refluxed for several hours with morpholine, the substitution product hexamethyl-5-morpholino-6 H-bicyclo[2.2.O]hex-2-ene(37) and a mixture of elimination products, including hexamethylbenzene, are obtained in a yield of about 50 % each.
n
-50%
(30) behaves in a similar manner on treatment with
methanolic sodium methoxide solution; hexamethyl5-methoxy-6 H- bicyclo[2.2.0]hex-2-ene (38) is formed
in a yield of about 50%, while the remainder of the
products includes hexamethylbenzene.
Treatment of (30) with LiAlH4 results in replacement
of the chlorine atom by H to give exo-l,2,3,4,5,6hexamethylbicyclo[2.2.O]hex-2-ene(39).
(39)
k
3.5.3. Bicyclo[3.1 .O]hex-2-ene D e r i v a t i v e s
The action of amines on (29) leads to dehydrochlorination. Two isomeric hydrocarbons ClzHls may
be formed, depending on the p& value of the amine.
Angew. Chem. internat. Edit. 1 Vol. 6 (1967)
1 No. 6
Primary, secondary, and tertiary amines having pKb
values lower than 8 give only pentamethyl-4-methylene6 H-bicyclo[3.1 .O]hex-2-ene (40). Examples of suitable
strongly basic amines are bields of (40) in brackets]:
morpholine (86 %), cyclohexylamine (76 %), piperidine
(80
diethylamine (77 %), N-methylmorpholine
(72
triethylenediamine (1,4-diazabicyclo[2.2.2]octane) (75 X), and ethylenediamine (79 Z). To obtain
(40), (29) is heated with the amine for several hours
at 6O-20O0C, if necessary with the addition of an
inert solvent (dioxane).
dehydrochlorination product pentamethyl-5-vinyl-cyclopentadiene (43) is formed in 85-90 % yield.
(29)
DM F
9
0
%
(40)
DMF-HCl
~
6 %
(43)
x),
x),
G*
(-1
- HCl(R,N)[*]
a
H
(w)2
+HCI
(291
(40)
[‘I R3N = strongly basic amine.
It should be noted that this elimination can be reversed
by the introduction of HC1 gas into a solution of (40)
in CHzCl2 at 0 “C. (29) is formed in 80 % yield.
Substituted bicyclo[3.1 .O]hex-2-ene derivatives are
obtained when (29) is refluxed for several hours with
LiAlH4 or CH3MgI in ether.
LiAlHa
DMF.HCI
75%
DMF
74%
(3)
(30)
The dehydrochlorination can also be effected, though
in lower yields, by e.g. formamide, quinoline, pyridine,
or urea. The purely thermal dehydrochlorination of
(29) (3 hours at 180-205 “ C )in an atmosphere of N.2
proceeds in the same direction, but less uniformly.
About 50 % of (43) and about 40 % of hexamethylbenzene ( 5 ) are obtained.
(43) can be also obtained in almost quantitative yield
from the methyl ester (36) by refluxing with D M F
hydrochloride (cf. eq. (y)).
Further possibilities can be predicted from the
equilibrium (s) and confirmed by experiment. Thus
(43) is obtained in very good yields when the bicyclic
compounds (30) and (40) are heated with D M F or
DMF.HC1. Even (3) can be isomerized to (43) in a
single-vessel reaction by heating with DMF-HC1 in
D M F , the product then being extracted with ligroin.
The structure of (43) is established e.g. by the very
smooth diene synthesis with rnaleic anhydride, in
which the adduct (44) is formed in 90 % yield.
(44)
&
0
R
(CHhC
;{
R+:,
H
H
R
R”
H
H
I R’
R
OCHJ
OCHzCF3
H
OOCCH,
OH
H
H
OCHICF,
H
H
Ref.
I401
1
I
“W
~411
1411
3.5.4. C y c 1o p e n t a d i e n e D e r i v a t i v e s
Stable derivatives of carbonium ion D (eq. (s)) are to
be expected only under reaction conditions that offer
the ions a “chance of survival”. Such conditions
prevail e.g. during the action of weak amines on (29).
A suitabIe amine is dimethylformarnide; when the
components are refluxed for about 8-10 hours, possibly in the presence of an inert solvent (toluene), the
1401 L . Kaplan, J . S. Ritschei, and K . E. Wilzbach, J. Amer.
chem. SOC.88, 2881 11966).
[411 E. Farenhorst and A. F. Bickel, Tetrahedron Letters 1966,
5911.
Angew. Chem. internat. Edit.
No substitution product derived from D (eq. (s)) has
so far been isolated.
1 Vol. 6 (1967) 1 No. 6
3.6. Action of Dienophiles on HexamethylDewar
Benzene) (3)
The reactions of (3) discussed in the foregoing sections
are only selected examples. It should finally be mentioned that (3), by analogy with norbornadiene, may
be able to add to strong dienophiles. Lemal and Lokensgard[28] reported that (14) could be obtained,
not only from the prismane derivative (13) (cf. Section 3.1), but also directly from (3).
(45)
We obtained the compound (45) (m.p. 268-269°C)
by reaction of ( 3 ) with tetracyanoethylene.
Received: March 14th. 1967
[A 584 IE]
German version: Angew. Chem. 79, 566 (1967)
Translated by Express Translation Service, London
525
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