Epoxidation of limonene

Bregeault and co-workers have reported supporting [HP04 W0(02)2 2]2 species on resins and silica (Table 4.6).64 Amberlyst A26 was the macro-reticular resin used. The PW2 species was supported onto dehydrated porous silica. The catalysts were found to be highly selective for the epoxidation of limonene by hydrogen peroxide. 

Table 4.6 Epoxidation of limonene with supported heteropolyacid species...

Limonene cfr-epoxide (549) is favored by employing molybdenum-catalyzed epoxidation of limonene, and it undergoes franj-diaxial opening with sodium phenylselenide. Both the ring-opened selenides were readily converted to trans-... 

Previous studies indicated that the structure of the alkyl hydroperoxide in molybdenum catalyzed epoxidations has only a minor effect on the rate and selectivity [10]. Hence, we were initially surprised to observe that PHP failed to give the expected epoxidation of cyclohexene (1) and limonene (2) in the presence of a molybdenum catalyst (Tablel). Epoxidation of limonene with TBHP as oxidant, in contrast, gave the epoxide of the more highly substituted double bond in 84% selectivity, consistent with nucleophilic attack of the olefin on the alkylperoxomolybdenum(Vl) [3,5]. We tentatively concluded that this low reactivity of PHP is a result of steric hindrance in the putative alkylperoxomolybdenum(VI) intermediate. This prompted us to carry out a systematic investigation [8] of steric effects of the alkyl substituents in the alkyl hydroperoxide on the rate of molybdenum catalyzed epoxidations. 

FIGURE 19.45 Epoxidation of limonene (68), a pinene (4), and 3-carene (336) with p38 from the cultured cells of Nicotiana tabacum. (Modi ed from Yawata, T. et al., Epoxidation of monoterpenes hy the peroxidase from the cultured cells oiNicotiana tabacum. Proceedings of 42nd TEAC, 1998, pp. 142-144.)... 

New nitrogen-containing ligands and additives have been developed in recent years. Michel et al. employed t-butylpyridine for the selective epoxidation of (+)-limonene with H2O2 as an oxidant in a two-phase system. 1-Methylimidazole resulted in a suitable additive to avoid hydrolysis and rearrangement of acid-sensitive epoxides produced by MTO-catalyzed epoxidation (eq 54). Epoxides from styrenes, chromene, and bis(homoallylic) alcohols were obtained in good yields within reasonable reaction times. ... 

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