Journal of Applied Sciences and Environmental Management World Bank assisted National Agricultural Research Project (NARP) - University of Port Harcourt ISSN: 1119-8362 Vol. 6, Num. 2, 2002, pp. 59-60

Journal of Applied Sciences & Environmental Management, Vol. 6, No. 2, Dec, 2002, pp. 59-60

Photo-oxidation of 2-Methyl-1-phenylcyclohexene

*FEKARURHOBO,  G K   OBOMANU, F G

Department of Chemistry, Rivers State University of Science and Technology, P.M.B. 5080, Port Harcourt.
*Corresponding author

Code Number: ja02029

ABSTRACT: Singlet oxygen (¹O₂) reacted with 2-methyl-1-phenylcyclohexene (1) in the 'ene' mode only.  The products of the reaction were isolated and identified as 2-methylene-1-phenylcyclohexyl hydroperoxide (2)(20.7%) and 1-methyl-2-phenylcyclohex-2-enyl hydroperoxide (3)(22.6%).  A mixture of the two hydroperoxides (39.8%) was also isolated. @ JASEM      

It has been established that no significant amount of degradation of petroleum fractions can occur without the preliminary photo-oxidative reactions to pre-dispose the hydrocarbon constituents to microbial attacks (Simiullah, 1985). This necessitates the need to understand the modes of photo-oxidation reactions of hydrocarbons structurally related to constituents of crude oils and their processed fractions.(Anderson, 1992).The photo-oxidation reaction of 6-methyl-1-phenylcyclohexene showed that the tri-substituted cycloalkene reacted in the 'ene' and [2 + 4] cycloaddition modes to give hydroperoxide and bis(endoperoxide) products respectively (Fekarurhobo, 1997).. In the present work, the photo-oxidation reaction of the tetra-substituted 2-methyl-1-phenylcyclohexene (1) is reported.

MATERIALS AND METHODS

¹H n.m.r. spectra were measured in CDCl₃  on a Jeol MH-100 spectrometer using TMS as an internal standard.  Infra-red spectra were recorded on a Perkin-Elmer Infra-red spectrophotometer model 597.  The oils were smeared on sodium chloride discs.

Preparation of 2-Methyl-1-phenylcyclohexene (1): The cycloalkene (1) was prepared by the dehydration of 2-methyl-1-phenylcyclohexanol following a method already published (Fekarurhobo, 1997); d_H: 7.1 (5H, m, aromatic), 2.3 - 1.8 (4H, m, H-3 and H- 6), 1.7 (4H, m, H-4 and H-5) and 1.50 (3H, s, CH₃) ppm.

Photo-Oxidation of 2-Methyl-1-phenylcyclohexene (1): The cycloalkene (1) (0.5g) was photo-oxidized following the usual procedure (Fekarurhobo, 1997).  The tlc. of the crude reaction mixture after 2.5h showed that it had been completely converted to two closely separated products.  Concentration (rotary) and column chromatography (silica, gradient elution with ethyl acetate/petroleum spirit) gave the products, which were identified as 2-methylene-1-phenylcyclohexyl hydroperoxide (2) (0.123g, 20.7%) and 1-methyl-2-phenylcyclohex-2-enyl hydroperoxide (3) (0.134g, 22.6%), both as colourless oils.  A 1:1 mixture (by ¹H n.m.r.) of the two hydroperoxides (0.236g, 39.8%) was also isolated; (2) d_H: 7.7 - 7.1 (6H, m, OOH and aromatic), 5.0 (1H, s, ½ x =CH₂), 4.6 (1H, s, ½ x =CH₂) and 2.7 - 1.0 (8H, m, 4 x CH₂) ppm;  n_max: 3350 (OOH), 1435 (aromatic) and 890 (0-0)cm^-1; (3) d_H: 8.1 (1H, s, OOH), 7.3 (5H, m, aromatic), 6.0 (1H, t, J = 4Hz, H -3), 2.7 - 1.3 (6H, m, 3 x CH₂  and 1.2 (3H, s, CH₃)ppm.

RESULTS AND DISCUSSION

Photo-oxidation of (1) gave two products (by t.l.c.), which were isolated unrearranged by column chromatography and identified (by ¹H n.m.r.) as hydroperoxides.  The higher R_f  product showed no methyl signal in its ¹H n.m.r. spectrum but the signals at 5.0 (1H, s) and 4.6 (1H, s) ppm were assignable to vinyl methylene protons.  The product was consequently assigned the structure of 2-methylene-1-phenylcyclohexyl hydroperoxide (2) (Scheme 1). The other product displayed signals due to a vinylic proton at 6.0 (1H, t, J = 4Hz) and a methyl singlet at 1.2ppm [cf. 1.5 for vinylic methyl in (1)], which enabled its identification as 1-methyl-2-phenylcyclohex-2-enyl hydroperoxide (3) rather than (4) (Scheme 1).

The formation of the hydroperoxides (2) and (3) in the 'ene' reaction of (1) is explainable on the basis of abstractions of the axial H_a and H_c respectively in the half-chair conformation (5).  Thus the attack of ¹O₂  at C-2 leads to the abstraction of H_c  to form (3).  Although the attack of  ¹O₂  at C-1 could, a priori, cause the abstraction of H_a  and H_b  to form the hydroperoxides (2) and (4) respectively, the results obtained indicate that only the former was formed.  The relative ease of abstraction of H_a  ,    compared to  H_b ,  probably reflects the ease with which the methyl protons can adopt the axial-orientation pre-requisite of the 'ene' reaction (Fekarurhobo, 2000).  Thus while H_a  can become axial by rotation, H_b  can only be axial by the more energy-demanding flip of conformation.  Another factor that could contribute to the exclusive abstraction of H_a is its statistical advantage over H_b  , the protons being in the ratio 3:1 respectively.  However, this would also have resulted in a product excess of (2) over (3), instead of the 1:1 ratio observed for the products.  Thus the non-regeoselectivity observed in the 'ene' reaction is indicative of restricted rotation of the methyl group, possibly by the adjacent phenyl ring.  In substituted acyclic alkenes, such restricted rotations caused by cis substituents have been used to explain the 'cis' effect-the phenomenon where abstractions of protons occur predominantly on the more sterically crowded side of an alkene (Houk, 1981).

The photo-oxidation reaction of (1) differs remarkably from that of 6-methyl-1-phenylcyclohexene (Fekarurhobo, 1997) in that no bis(endoperoxide) was formed in the present reaction.  Bis(endoperoxides) are formed from cisoid 1,3-dienes in a concerted mechanism, which requires the coplanarity of the reacting  olefinic bonds (Wasserman, 1979). Thus the non-formation of an endoperoxide in the present reaction suggests that the substrate (1) cannot adopt a conformation in which the phenyl ring is coplanar with the cyclohexenyl double bond.  Molecular models reveal that such a conformation represents an energy maximum for the alkene, corresponding to maximum steric interaction between the phenyl ring and the methyl protons.

Acknowledgement:  The authors wish to thank Dr. H.A.J. Carless (Brikbeck College, University of London) for his useful suggestions and assistance with ¹H n.m.r. Spectra.

REFERENCES

• Anderson, J. T; Bobinger, S; (1992), Polycyclic Aromatic Sulphur Heterocycles II: Photochemical Oxidation of Benzo[B]thiophene in AqueousSolution, Chemosphere,   24, 383.
• Fekarurhobo, G. K; Obomanu, F. G; Carless, H.A.J; (1997), Photo-oxidation of 6-Methyl-1- phenylcyclohexeene, J. Chem. Soc. Nigeria, 22, 119.
• Fekarurhobo, G. K; Obomanu, F. G; Carless, H.A.J; (2000), Photo-oxidation of 4-t-butylcyclohexene, J. Chem. Soc. Nigeria, 25,46.
• Houk, K. N. Williams Jr, J. C; Mitchell, P. A; Yamaguchi, K; (1981), Conformational Control of Reactivity and Regioselectivity in Singlet Oxygen Ene Reactions: Relationship to the Rotational Barrier of Acyclic Alkylethylenes,  J. Am. Chem. Soc. 103, 949.
• Simiullah, Y; Jones, K.C; (1985); Deer Antlerrs and Pollution Monitors in Deer, Monitoring and Assessment Research Center, 6, 253.
• Wasserman, H.H; Murray, R.W; (1979), Singlet Oxygen, Academic Press, New York.

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