Poly-N-haloacrylamides as polymeric solid phase synthetic reagents for α-halogenation of ketones

Abstract

Poly-N-bromoacrylamide (PNBA), poly-N-chloroacrylamide (PNCA) and poly-N-iodoacrylamide (PNIA) were prepared starting from commercially available polyacrylamide (PA) and were developed as new solid phase polymeric reagents for halogenation of ketones. PNBA was prepared by the bromination of polyacrylamide with bromine and potassium hydroxide in a suspension in carbontetrachloride. PNCA and PNIA were prepared by the reaction of t-butylhypochlorite and t-butylhypoiodite, respectively, on polyacrylamide. The active halogen contents of these reactive polymers, determined by iodometric titration, ranged from 3.8 to 7 meq g^-1. These insoluble, but swellable, reagents were found to effect preparative level conversion of ketones to α-haloketones. The spent reagents were regenerated and reused for five cycles of the halogenation reaction. The advantages of these polymeric halogenating agents include operational simplicity, longer shelf life, and the elimination of the hazardous nature characteristic of the analogous low molecular halogenating reagents like t-butylhypohalites. The reactivity of the PNBA was found to be much greater than the N-bromoamide function incorporated in a polystyrene matrix or the t-butylhypobromite function attached onto a polystyrene matrix. This points to the role of the polar polymeric matrix in facilitating the halogenation reaction by the N-haloamide function which is favoured in a polar environment.