The preparation and use of polyHIPE-grafted reactants to reduce alkyl halides under free-radical conditions

PolyHIPE-supported tin hydride and thiol are synthesized to reduce alkyl bromides under free-radical conditions, taking advantage of the fully interconnected polyHIPE structure. Radical reduction of 1-bromoadamantane, 6-bromohex-1-ene and 1-allyloxy-2-bromobenzene are performed using two methodologies: one using supported tin hydride as reducing agent, and another using supported thiol as polarity-reversal catalyst in the presence of triethylsilane as reducing agent. The polyHIPE-supported tin hydride is used either in catalytic or stoichiometric amounts depending on the bromo compound to be reduced. In the case of unsaturated bromides, both methodologies lead to reductive cyclization, thus enabling radical rearrangement before hydrogen transfer. PolyHIPE-supported organotin hydride is a good alternative to tributyltin hydride to prevent tin contamination and facilitate product separation. PolyHIPE-supported thiol, in the presence of an excess of triethylsilane, shows good activity and selectivity toward reductive cyclization products. Results obtained by the two methodologies are compared and discussed.