Reaction of 1,2,4,5-tetramethyl-1,4-cyclohexadiene with ozone - Competition between oxidative cleavage and oxydehydrogenation

Article abstract of DOI:10.1002/1099-0690(200105)2001:10<1899::AID-EJOC1899>3.0.CO;2-L

Treatment of 1,2,4,5-tetramethyl-1,4-cyclohexadiene (1) on polyethylene and in pentane with excess ozone afforded mixtures of the oxydehydrogenation product 2 and of the oxidative cleavage product 4 in ratios of 0.5:1 and 19:1, respectively. Epoxidation o

Full text of DOI:10.1002/1099-0690(200105)2001:10<1899::AID-EJOC1899>3.0.CO;2-L

FULL PAPER
Reaction of 1,2,4,5-Tetramethyl-1,4-cyclohexadiene with Ozone ؊ Competition
between Oxidative Cleavage and Oxydehydrogenation
In Chan Jung[a]
Keywords: Alkenes / Cleavage reactions / Dehydrogenation / Epoxidation / Ozonolysis
Treatment of 1,2,4,5-tetramethyl-1,4-cyclohexadiene (1) on
polyethylene and in pentane with excess ozone afforded
mixtures of the oxydehydrogenation product 2 and of the
oxidative cleavage product 4 in ratios of 0.5:1 and 19:1,
respectively. Epoxidation of 1 followed by ozonolysis of the
epoxide 6 provided the epoxyozonide 7.
Introduction
Treatment of 1 with excess ozone on polyethylene at Ϫ78
°C afforded a mixture containing 35% of 2 and 65% of the
diketoozonide 4; the latter was isolated in a yield of 23%.
Treatment of 1 with excess ozone at Ϫ78 °C in pentane
gave a mixture containing 95% of 2 and only 5% of 4, i.e.
oxydehydrogenation of 1 became the predominant reaction.
For the apparent solvent effect upon the ratio of oxydehyd-
rogenation and oxidative cleavage, viz. 65:35 in methanol
and 95:5 in pentane, there is a priori no plausible explana-
tion. The fact that ozonide 4 was obtained in considerably
higher yield on polyethylene than in pentane is in line with
the experience that ozonolysis of olefins on polyethylene is
conducive to ozonide formation probably due to immobil-
ization of the Criegee fragments at their place of origin.[8]
Ozonolysis reactions of substrates incorporating the 1,3-
cyclohexadiene moiety, such as 1,3-cyclohexadiene itself,[1,2]
1,4-disubstituted-1,3-cyclohexadienes,[3,4]
naphthalene[5]
and substituted naphthalenes,[5,6] have been examined un-
der a variety of aspects and conditions. The course and the
products of such reactions are in agreement with the Cri-
egee mechanism of olefin ozonolysis.[7a] The work described
below was carried out in order to examine whether ozone
treatment of a nonconjugated cyclohexadiene also follows
this pattern.
The structure of the liquid ozonide 4 was established by
Results and Discussion
1
its H and 13C NMR spectra, by reduction with triphenyl-
phosphane to give the expected diketone 5, and by inde-
pendent preparation through ozonolysis of the unsaturated
diketone 3 on polyethylene. The stereochemical identity of
4 was not proven. We assume, however, that ozonide forma-
tion occurs upon cleavage of the first double bond of 1 and
that, hence, ozonide 4 has a cis geometry.
1,2,4,5-Tetramethyl-1,4-cyclohexadiene (1) was treated
with 0.7 molar equivalents of ozone in methanol at ambient
temperatures and the crude product was admixed with tri-
phenylphosphane in order to reduce peroxidic components.
1H NMR analysis of the ensuing reaction mixture showed
the presence of 24% of residual 1, 49% of 1,2,4,5-tetra-
methylbenzene (2) and 27% of the unsaturated diketone 3.
Compounds 2 and 3 have been isolated in yields of 41%
1
and 11%, respectively, and characterized by their H NMR
and mass spectra. These results reveal that in the reaction
of ozone with 1 in methanol, oxydehydrogenation competes
favorably with oxidative double bond cleavage and that the
latter reaction occurs by sequential rather than by simultan-
eous attack of ozone at the two double bonds of 1.
Ozone cleavage of the second double bond of 1 to gener-
ate the two carbonyl groups of 4 is not in line with the
Criegee mechanism, since it cannot account for the third
oxygen atom of ozone. Such abnormal ozonolysis reactions
are, however, not unprecedented. Thus, ozonolysis of nor-
bornenes possessing dimethyl-substituted double bonds af-
forded the corresponding 1,3-diacetylcyclopentanes as the
major and the expected ozonides as the minor products.[9]
Furthermore, the thermal decomposition of such ozonides
yielded substantial proportions of the corresponding 1,3-
diacetylcyclopentanes.[9] It is, therefore, conceivable that 4
may have been formed by the spontaneous decomposition
of an unstable diozonide of 1.
The hitherto unknown oxydehydrogenation of 1 by ozone
is not restricted to reactions in the protic solvent methanol.
[a]
Department of Chemistry, Hanseo University
Seosan 356Ϫ820, Korea
Eur. J. Org. Chem. 2001, 1899Ϫ1901
WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001
1434Ϫ193X/01/0510Ϫ1899 $ 17.50ϩ.50/0
1899

I. C. Jung
FULL PAPER
extracted with ether and the ether was evaporated at reduced pres-
sure to leave 1.34 g of a liquid residue which contained 35% of 2
and 65% of 4 according to 1H NMR spectroscopy. Separation (col-
umn 2.5 ϫ 50 cm, 80 g of silica gel, pentane/ether, 7:3) afforded
134 mg (20%) of 2 and 248 mg (23%) of 4.
The reason for the abnormal behavior of substituted nor-
bornenes and of 1 in ozonolysis reactions may be steric
hindrance in the ozonides of such norbornenes or in the
diozonide of 1. This steric hindrance may even prevent
ozone cleavage of double bonds and instead lead to epox-
ides.[7b] A case in point is the ozonolysis of 1,2,3,4-tetra- 3,5-Dimethyl-3,5-bis(2-propanone)-1,2,4-trioxolane (4): Colorless li-
quid. Ϫ 1H NMR: δ ϭ 1.57 (s, 6 H), 2.23 (s, 6 H), AB-System with
methylcyclopenta-1,3-diene on polyethylene, which pro-
δA ϭ 2.81, δB ϭ 2.89 (JAB ϭ 14.4 Hz, 4 H). Ϫ 13C NMR: δ ϭ
vided the corresponding epoxyozonide to the exclusion of
23.28, 31.41, 51.03, 108.32, 203.93. Ϫ IR (neat): ν˜ ϭ 1724 cmϪ1
the expected diozonide.[10] In order to test whether the
(CϭO).
crude product from the ozonolysis of 1 contains the corres-
ponding epoxyozonide 7, we have independently prepared
Reduction of Ozonide 4: A solution of 4 (25 mg, 0.10 mmol) in 0.60
7. Thus, treatment of 1 with m-chloroperbenzoic acid pro- mL of CDCl3 was admixed with triphenylphosphane at ambient
temperature. 1H NMR analysis after 4 days showed that 5 was
vided epoxide 6 in a yield of 62%. Subsequent ozonolysis
formed as the sole detectable reduction product: δ ϭ 2.04 (s, 6 H),
of 6 on polyethylene at Ϫ78 °C gave a mixture containing
2.23 (s, 2 H).
84% of 7 and 16% of 8, from which 7 was isolated in a yield
1
of 38%. Its structure was established by H and 13C NMR
Ozonolysis of 1,2,4,5-Tetramethyl-1,4-cyclohexadiene (1) in Pentane:
spectroscopy and by reduction with triphenylphosphane to
A solution of 1 (0.20 g, 1.50 mmol) in 5 mL of pentane was treated
give the expected epoxy-diketone 8. With the help of the 1H with ozone at Ϫ78 °C until it turned blue. Excess ozone was flushed
out with nitrogen and the solvent was evaporated at reduced pres-
sure. 1H NMR analysis of the residue (0.26 g) showed the presence
of 95% of 2 [δ ϭ 2.19 (s), 6.91 (s)] and 5% of 4 [δ ϭ 1.57 (s), 2.24
(s), 2.87 (q)].
NMR spectroscopic data of 7 we could show that it was
not formed in detectable amounts in the ozonolysis of 1.
Ozonolysis of 4,5-Dimethyl-4-octene-2,7-dione (3) on Polyethylene
and Reduction of the Ozonolysis Product 4: Compound 3 (0.20 g,
1.19 mmol) on 20 g of polyethylene was treated with ozone at Ϫ78
°C for 3 h. The products were extracted with ether and the ether
was then evaporated at reduced pressure to leave 0.45 g of a liquid
residue which contained ozonide 4 as the sole product according
to 1H NMR analysis. A solution of the residue in 0.6 mL of CDCl3
1
was admixed with triphenylphosphane at ambient temperature. H
NMR analysis after 4 days showed that 5 was formed as the sole
detectable reduction product.
Experimental Section
General: NMR spectra were obtained in CDCl3 with TMS as in-
ternal reference on a Bruker AC 250 instrument. Mass spectra were
obtained on a HewlettϪPackard instrument 5985B and IR spectra
on a Beckman 4260 instrument. Products were isolated by flash
chromatography[11] on silica gel at 0.5 bar. Ozonolysis reactions on
polyethylene (Microthene FN 500, spherical particles, maximum
size 20 µm; supplier: Serva Feinbiochemica, Heidelberg, FRG) were
performed according to a known procedure.[12]
Epoxidation of 1,2,4,5-Tetramethyl-1,4-cyclohexadiene (1): A solu-
tion of 1 (1.00 g, 7.35 mmol) in 5 mL of ether was added dropwise
to
a stirred solution of m-chloroperbenzoic acid (1.00 g,
5.88 mmol) in 60 mL of ether at 0 °C and stirring was continued
for 1 h at 0 °C. The solution was then sequentially washed with a
5% solution of aqueous NaOH and water and dried with MgSO4.
Evaporation of the solvent at reduced pressure left 1.60 g of a solid
residue, from which 6 (0.74 g, 62%) was isolated (column 2.5 ϫ
50 cm, 80 g of silica gel, pentane/ether, 95:5).
Ozonolysis of 1,2,4,5-Tetramethyl-1,4-cyclohexadiene (1) in Meth-
anol: A solution of 1 (0.68 g, 5.00 mmol) in 30 mL of methanol
was treated with 0.70 molar equivalents of ozone at ambient tem-
peratures, admixed with triphenylphosphane (1.15 g, 4.40 mmol)
and left standing overnight. 1H NMR analysis showed the presence
of 1 (24%; δ ϭ 2.52), 2 (49%; δ ϭ 2.19) and 3 (27%; δ ϭ 1.76).
The solvent was evaporated at reduced pressure and the residue
(2.30 g) was separated (column: 4 ϫ 50 cm, 120 g of silica gel, pent-
ane/ether, 7:3) to give 190 mg (41%) of 2 and 65 mg (11%) of 3.
1,3,4,6-Tetramethyl-7-oxa-bicyclo[4,1,0]-3-heptene (6): Colorless
solid, m.p. 51Ϫ52 °C. Ϫ 1H NMR: δ ϭ 1.35 (s, 6 H), 1.57 (s, 6 H),
AB-System with δA ϭ 2.29, δB ϭ 2.38 (JAB ϭ 17.6 Hz, 4 H). Ϫ
13C (BB) NMR: δ ϭ 18.53, 19.43, 38.87, 61.59, 121.57. Ϫ EI/MS:
m/z (%) ϭ 152 (6) [Mϩ], 137 (37) [M Ϫ CH3]ϩ, 109 (100) [M Ϫ
CH3CO]ϩ, 43 (29) [CH3CO]ϩ. Ϫ C10H16O (152.24): calcd. C 78.90,
H 10.59; found C 78.91, H 10.43.
Ozonolysis of 1,3,4,6-Tetramethyl-7-oxabicyclo[4,1,0]-3-heptene (6)
on Polyethylene: Compound 6 (0.70 g, 4.60 mmol) on 100 g of poly-
ethylene was treated with ozone at Ϫ78 °C for 3 h. The products
were extracted with ether and the ether was then evaporated at
reduced pressure to leave 0.92 g of a residue which contained 84%
of 7 and 16% of 8, according to 1H NMR analysis. Separation
(column 2.5 ϫ 50 cm, 80 g of silica gel, pentane/ether, 90:10) af-
forded 64 mg (38%) of 7.
1,2,4,5-Tetramethylbenzene (2): Colorless solid. Ϫ 1H NMR: δ ϭ
2.19 (s, 12 H), 6.90 (s, 2 H). Ϫ EI-MS: m/z (%) ϭ 134 (37) [Mϩ],
119 (100) [M Ϫ CH3]ϩ.
1
4,5-Dimethyl-4-octene-2,7-dione (3): Colorless liquid. Ϫ H NMR:
δ ϭ 1.76 (s, 6 H), 2.13 (s, 6 H), 3.15 (s, 4 H). (ref.[13] 1H NMR:
δ ϭ 1.66, 2.08, 3.12). Ϫ 13C (BB) NMR: δ ϭ 19.84, 29.36, 49.94,
126.30, 206.52. Ϫ CI-MS: m/z (%) ϭ 169 (35) [M ϩ H]ϩ.
Ozonolysis of 1,2,4,5-Tetramethyl-1,4-cyclohexadiene (1) on Poly-
ethylene: Compound 1 (0.68 g, 5.00 mmol) on 68.0 g of polyethyl-
ene was treated with ozone at Ϫ78 °C for 8 h. The products were
1,3,5,7-Tetramethyl-4,8,9,10-tetraoxatricyclo-[5.2.1.03,5]-decane (7):
Colorless solid, m.p. 73Ϫ74 °C. Ϫ 1H NMR: δ ϭ 1.33 (s, 6 H),
1.51 (s, 6 H), AB-system with δA ϭ 2.28, δB ϭ 2.48 (JAB ϭ 16.1 Hz,
1900
Eur. J. Org. Chem. 2001, 1899Ϫ1901

Reaction of 1,2,4,5-Tetramethyl-1,4-cyclohexadiene with Ozone
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Received September 15, 2000
[O00479]
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1901