Polyphosphazenes for the Stille reaction: a new type of recyclable stannyl reagent

Abstract

A random phosphazene copolymer {[NP((CH₂)₇–Br)Ph]_0.5[NPMePh]_0.5}_n (2) and a block copolyphosphazene {[NP((CH₂)₇–Br)Ph]_0.5[NPMePh]_0.5}₄₅-b-[NP(O₂C₁₂H₈)]₅₅ (5), having a branch with two randomly distributed units, have been synthesized and used as precursors for the stannyl derivatives {[NP((CH₂)₇–SnBu₂An)Ph]_0.5[NPMePh]_0.5}_n (3) and {[NP((CH₂)₇–SnBu₂An)Ph]_0.5[NPMePh]_0.5}₄₅-b-[NP(O₂C₁₂H₈)]₅₅ (6, An = p-MeOC₆H₄). Polymers 3 and 6 were tested as recyclable tin reagents in the Stille cross-coupling reaction with ArI, using various Pd catalysts and different experimental conditions. Polymer 6 can be recycled without a 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 [NP(O₂C₁₂H₈)] rich membranes, thus decreasing the accessibility of the tin active centers.