THE STRUCTURE OF THE INTERMEDIATE RADICAL IN THE HYDROSTANNATION OF PHENYLACETYLENE

• Published in: Synthesis and reactivity in inorganic and metal-organic chemistry Vol. 31; no. 5; pp. 757 - 765
• Main Authors: Petraco, Nicholas D.K., Modarelli, David A., Cochran, John C.
• Format: Journal Article
• Language: English
• Published: Taylor & Francis Group 31.05.2001
• ISSN: 0094-5714; 1532-2440
• DOI: 10.1081/SIM-100104848

Hydrostannation of phenylacetylene, both thermal and free radical catalyzed, results in a Z/E mixture of β-(trialkylstannyl)styrenes with the Z isomer the kinetic product and the E isomer the thermodynamic product. The stereoselectivity of the kinetic product is a result of the structure of the intermediate vinyl radical. Semi-empirical calculations using the PM3 Hamiltonian indicate the radical species formed in the addition of trisubstituted tin hydride across the triple bond of phenylacetylene leads to an intermediate with an sp-geometry at the radical carbon. The reaction proceeds to form 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 quite sizable and does not readily fit in the reaction plane necessary to form the E product. Finally, the value of the Singly Occupied molecular Orbital (SOMO) is highest where radical attack occurs. The value of the SOMO is highest anti to the tin group in the sp-structures. Experimentally, the kinetic product isomerizes and the thermodynamically more stable E isomer is isolated.