Ring-opening of Epoxides Mediated by Frustrated Lewis Pairs

Abstract

Treatment of the preorganized frustrated Lewis pairs (FLPs) tBu(2)PCH(2)BPh(2) (1) and o-Ph2P(C6H4)BCat (Cat=catechol) (4) with 2-methyloxirane, 2-phenyloxirane and 2-(trifluoromethyl)oxirane resulted in epoxide ring-opening to yield the six- and seven-membered heterocycles 2a-c and 5a-c, respectively. These zwitterionic products were characterized spectroscopically, and compounds 2a, 2b, 5a and 5c were structurally characterized by single-crystal X-ray structure analyses. Based on computational and kinetic studies, the mechanism of these reactions was found to proceed via activation of the epoxide by the Lewis acidic borane moiety followed by nucleophilic attack of the phosphine of a second FLP molecule. The resulting chain-like intermediates afford the final cyclic products by ring-closure and concurrent release of the second equivalent of FLP that behaves as catalyst in this reaction.