The structure of the intermediate radical in the hydrostannation of phenylacetylene

Hydrostannation of phenylacetylene, both thermal and free radical catalyzed , results in a Z/E mixture of beta-(trialkyl-stannyl)styrenes with the Z is omer the kinetic product and the E isomer the thermodynamic product. The st ereoselectivity of the kinetic product is a result of the structure of the intermediate vinyl radical. Semi-empirical calculations using the PM3 Hamil tonian indicate the radical species formed in the addition of trisubstitute d tin hydride across the triple bond of phenylacetylene leads to an interme diate with an sp-geometry at the radical carbon. The reaction proceeds to f orm the Z product for a number of reasons. First, there is a small barrier to tin group rotations hindering hydrogen abstraction syn to tin. There is a nearly unhindered reaction plane anti to tin. A trialkyltin hydride is qu ite sizable and does not readily fit in the reaction plane necessary to for m the E product. Finally, the value of the Singly Occupied Molecular Orbita l (SOMO) is highest where radical attack occurs. The value of the SOMO is h ighest anti to the tin group in the sp-structures. Experimentally, the kine tic product isomerizes and the thermodynamically more stable E isomer is is olated.