Synthesis of Boron Substituted Stannacyclohexadienes by a Competitive Metathesis

Abstract

The reaction of phenyl borondibromide (PhBBr 2 ) with 1,1-n-dibutylstanna-2,5-cyclohexadiene (8) produces, along with the expected phenylbora-2,5-cyclohexadiene (1), a mixture of boron substituted stannacyclohexadiene compounds 2 and 3 in a ratio of 1:2 respectively. Compounds 2 and 3 are in equilibrium at room temperature. The initial formation of either 2 or 3 is presumably the result of a metathesis reaction of the PhBBr 2 with the double allylic C-H bond of the starting 1,1-ndibutylstanna-2,5-cyclohexadiene (8), yielding compound 2, or with the allylic C-H bond of the 1,3hydrogen isomerized starting stannacycle 8 (1,1-n-dibutylstanna-2,4-cyclohexadiene), yielding compound 3. The pseudo-first order rate of conversion from 2 => 3 was measured by an NMR spin population transfer technique to be ki = 7.18 s" 1 (2, 0.034M; Ti =1.6 s) while k-1 = was found to be 3.29 s' 1 0Ί = 1.5 s), yielding an equilibrium constant of 2.18 at 298 K. The coalescence rate constant was calculated to be 235 s" 1 which was not reached by heating the sample up to 383 K.Brought to you by | University of Arizona Authenticated Download Date | 5/30/15 7:29 AM