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 (5) undergoes O-directed rt free radical hydrostannation with 2 equiv. of Bu₃SnH or Ph₃SnH in PhMe to produce the α-cyclopropyl-β-stannylvinyl radicals 26 and 27, which rapidly ring-open to give the benzylic stannylhomoallenyl radicals 28 and 29. These, in turn, react with the excess stannane that is present to provide 21 and 23 as primary reaction products. The triphenylstannylallene 23 also undergoes a competitive Ph₃Sn˙ addition to its central allene carbon. This affords the allylically-stabilised radical 31c, which itself reacts with the stannane to produce (Z)-6-phenyl-2,3-bis(triphenylstannyl)hex-3-en-1-ol (24). EPR studies of the reaction of 5 with Ph₃SnH (1 equiv.) and cat. Et₃B/O₂ in PhMe at 250 K have failed to identify the radicals 27 and 29 in the reaction mixtures. Rather, a sharp dd is always observed whose multiplicity is consistent with it being the tris-Ph₃Sn-stabilised free radical 33. The latter is suggested to arise from a reversible O-directed Ph₃Sn˙ addition to 24. The radical 33 has ¹H^β values of 1.32 mT (13.2 G) and 0.57 mT (5.7 G) and a g of 2.0020.