On the divergent reactivity of allenylstannanes generated from the O-directed free radical hydrostannation reaction of (±)-trans-3-(2-phenylcyclopropyl)prop-2-yn-1-ol. EPR evidence for the reversible addition of Ph3Sn radicals to vinyl triphenyltins†‡

Abstract

(±)-trans-3-(2-Phenylcyclopropyl)prop-2-yn-1-ol () undergoes O-directed rt free radical hydrostannation with 2 equiv. of Bu₃SnH or Ph₃SnH in PhMe to produce the α-cyclopropyl-β-stannylvinyl radicals and , which rapidly ring-open to give the benzylic stannylhomoallenyl radicals and . These, in turn, react with the excess stannane that is present to provide and as primary reaction products. The triphenylstannylallene also undergoes a competitive Ph₃Sn˙ addition to its central allene carbon. This affords the allylically-stabilised radical , which itself reacts with the stannane to produce (Z)-6-phenyl-2,3-bis(triphenylstannyl)hex-3-en-1-ol (). EPR studies of the reaction of with Ph₃SnH (1 equiv.) and cat. Et₃B/O₂ in PhMe at 250 K have failed to identify the radicals and in the reaction mixtures. Rather, a sharp dd is always observed whose multiplicity is consistent with it being the tris-Ph₃Sn-stabilised free radical . The latter is suggested to arise from a reversible O-directed Ph₃Sn˙ addition to . The radical has ¹H^β values of 1.32 mT (13.2 G) and 0.57 mT (5.7 G) and a g of 2.0020.