Polyphosphazenes for the Stille reaction: A new type of recyclable stannyl reagents

Abstract

The random phosphazene copolymer {[N=P((CH2)7-Br)Ph]0,5[N=PMePh]0,5}n (2) and the block copolyphosphazene {[N=P((CH2)7-Br)Ph]25[N=PMePh]20}-b-[N=P(O2C12H8)]55 (5), having a branch with two randomly distributed units, have been synthesized and used as precursors for the stannyl derivatives {[N=P((CH2)7-SnBu2An)Ph]0,5[N=PMePh]0,5}n (3) and {[N=P((CH2)7-SnBu2An)Ph]25[N=PMePh]20}-b-[N=P(O2C12H8)]55 (6, An = p-MeOC6H4). Polymers 3 and 6 were tested as recyclable tin reagents in the Stille cross-coupling reaction with C6H5I, using various Pd catalysts and different experimental conditions. Polymer 6 can be recycled without significant release of tin, but its efficiency decreased after three consecutive cycles. This effect was explained by studying the self-assembly of the polymer under the same conditions used for the catalytic experiments, which evidenced the progressive coalescence of the polymeric vesicles (polymersomes) leading to stable and bigger core-shell aggregates by the attraction of the [N=P(O2C12H8)] rich membranes, thus decreasing the accessibility of the tin active centers.