Facile a-Halogenations of cyclic enones and linear enals.

Abstract

A facile methodology for the formations of α -chloro, α -bromo and α -iodo cyclic enones and linear enals has been developed. In our method, a combination of pyridine or pyridine-Noxide, and various N-halosuccinimides are employed to convert enones or enals in various solvents at room temperature to α -haloenones or α -haloenals. Previously, α -iodination of enones, both cyclic and linear, could be affected by small amounts of pyridine and stoichiometric iodine.(1) α -Iodination of several cyclic and linear enones were also reported to be effective with combination of iodine and excess quantities of pyridine using carbon tetrachloride as co-solvent.(2)Other reagents for α -iodination of enones include morpholine,(3) and pyridinium dichromate (PDC) and iodine.(4) With linear enones, the combination of pyridine and I2 required refluxing acetonitrile to obtain a maximum 62% yield for (Z)-4-iodo-4-hexen-3-one.1 With PDC and iodine, (E)-4-hexen-3-one reacted to yield less than 5% conversion to its α -iodo analogue.4 Extending the reactions to linear enones and to α -bromination and α -chlorination are limited due to low yields and uses of toxic bromine and chlorine. Previous methods for α -chlorination include hydrochloric acid and m-chlorobenzoic acid.(5) Reagents for α -bromination are iodobenzene diacetate with tetrabutylammonium bromide,(6) and Rhodium (III) complex combined with acid halides or benzyl halides.(7) In our general methodology for α -halogenation of both cyclic enones and linear enals, α - halogenations proceeded facilely with good to excellent yields and with retention of geometry across the double bond for the linear enals and also with retention of the sensitive aldehyde group.