Cis-trans Photoisomerization of Nerol in the Presence of Some Additives

Photoisomerization of nerol to more useful geraniol has been carried out in the presence of acetophenone derivatives, halides, and thiols. p-Bromoacetophenone was shown to be an excellent catalyst in comparison with other acetophenone derivatives bearing hydrogen, fluorine or chlorine atom at the para-position. Other bromides such as bromotrichloromethane and 1-bromonaphthalene were also effective for the isomerization, although the reaction selectivity was low. The bromine atom produced by fission of the C-Br bond was presumed to be involved in the bromine-catalyzed reaction. Isomerization in the presence of thiols proceeded without an accompanying side reaction to give an equilibrium mixture of geraniol/nerol of about 60/40. The addition of radical-producing compounds accelerated the isomerization, suggesting that the reaction proceeded via an intermediate formed between the thiyl radical and nerol. Because of the high selectivity and the appreciable rate of the reaction even in bulk, the thiol-catalyzed reaction was concluded to be one of the most promising methods for industrial conversion of nerol to geraniol.