Ene-reactions, singlet oxygen

That was the situation encounter in 1986 when Pettit and Fox56 reported nearly identical selectivity in the singlet oxygen ene reaction of 1-methylcylclohexene, 5, using either a soluble sensitizer (Ru(bpy)3+/MeOH) or a zeolite-embedded sensitizer (Ru(bpy)f+ NaY) in a methanol slurry (Fig. 8). 

Fig. 11 Steric effect in the intrazeolite singlet oxygen ene reaction.

Fig. 13 Substituent effects on the regiochemistry of the intrazeolite singlet oxygen ene reactions of trimethylstyrenes.

A striking feature of the intrazeolite singlet oxygen ene reaction is the rate enhancement often observed in these reactions.57,67 This rate enhancement is nicely accounted for by stabilization of the incipient perepoxide as depicted by Model C in Fig. 12. This rate enhancement can be used to promote the ene reaction at the expense of other reaction modes.68 An interesting example of this was reported by Stratakis and Rabalakos.69 Photooxygenations of alkenylarenes, 11 and 12 (Fig. 15) are dominated by [2 + 2] and [4 + 2] reactions in solution but react predominately by the ene mode in intrazeolite reactions. 

Fig. 16 Comparison of intrazeolite and solution singlet oxygen ene reactions of electron poor alkenes.

Fig. 18 Influence of a phenyl ring as an alternative binding site in intrazeolite singlet oxygen ene reactions.

Fig. 20 Cation effect on an intrazeolite singlet oxygen ene reaction in Y-zeolite.

Clennan, E.L. and Sram, J.P. (2000). Photochemical reactions in the interior of a zeolite. Part 5. The origin of the zeolite induced regioselectivity in the singlet oxygen ene reaction. Tetrahedron 56, 6945-6950... 

Clennan, E.L., Zhang, D. and Singleton, J. (2006). A comparison of intrazeolite and solution singlet oxygen ene reactions of allylic alcohols. Photochem. Photobiol. 82, 1226-1232... 

Houk, K.N., Williams Jr., J.C., Mitchell, P.A. and Yamaguchi, K. (1981). Conformational control of reactivity and regioselectivity in singlet oxygen ene reactions Relationship to the rotational barriers of acyclic alkylethylenes. J. Am. Chem. Soc. 103, 949-951... 

Stephenson, L.M. (1980). The mechanism of the singlet oxygen ene reaction. Tetra hedron Lett. 21, 1005-1008... 

Charge transfer (CT) intrazeolite photooxygenation, 253-254, 255, 257 Cw-effect, in singlet oxygen ene reaction, 240, 243... 

These intrazeolite singlet oxygen ene reactions have synthetic potential because the cis effect observed in solution is suppressed in the zeolite [13]. Consequently, allylic hydroperoxides which are inaccessible by other routes may be available via this new technology. For example, photo-oxidations of aryl-substituted alkenes, 7, in sensitizer-doped NaY react to generate the allylic hydroperoxides as the major or exclusive product [17]. In contrast, in solution, the hydroperoxides are formed in only 5-20% yields, with 2-1-2 and 4-1-2 adducts dominating the reaction mixtures. In the case of 2-methyl-5-phenyl-2-hexene, 8, the regio-selectivity for 8b and 8c improved from 47% to 94% and the diastereoselectivity from 10% to 44% as the reaction is moved from solution into the zeolite [18] ... 

Selective formation of allylic hydroperoxides via singlet oxygen ene reaction... 

Selective fomation of allylic hydroperoxides via singlet oxygen ene reaction 837 TABLE 2. Syn/anti regioselectivity in the photooxygenation of aUyUc alcohols 10- and 10-Z... 

Previous studies have shown that the rate of the O2 ene reaction with alkenes shows neghgible dependence on solvent polarity . A small variation in the distribution of the ene products by changing solvent was reported earlier . However, no mechanistic explanation was offered to account for the observed solvent effects. It is rather difficult to rationahze these results based on any of the currently proposed mechanisms of singlet oxygen ene reactions. Nevertheless, product distribution depends substantially on solvent polarity and reaction temperature only in substrates where both ene and dioxetane products are produced ° . 

By using the same method, Katsumura and coworkers isolated the enantiomerically pure allene segment in 97a and 97b (Scheme 26)". The one-step singlet oxygen ene reaction pathway, to produce the allene moiety from the twisted l,3,3-trimethylcyclohexenyl-2-vinyl derivatives, was considered as a possible biomimetic route towards the synthesis of carotenoids. 

SCHEME 29. Singlet oxygen ene reaction of selected bicycUc monoterpenes... 

SCHEME 31. Diastereoselection in the singlet oxygen ene reaction of 7-isopropylidenenorbornenes... 

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