SYNTHESIS OF BORON-SUBSTITUTED STANNACYCLOHEXADIENES BY A COMPETITIVEMETATHESIS

The reaction of phenyl borondibromide (PhBBr2) with 1,1-n-dibutylstann a-2,5-cyclohexadiene (8) produces, along with the expected phenylbora- 2,5-cyclohexadiene (1), a mixture of boron substituted stannacyclohexa diene compounds 2 and 3 in a ratio of 1:2 respectively. compounds 2 an d 3 are in equilibrium at room temperature. The initial formation of e ither 2 or 3 is presumably the result of a metathesis reaction of the PhBBr with the double allylic C-H bond of the starting 1, 1-n dibutyls tanna-2,5-cyclohexadiene (8), yielding compound 2, or with the allylic C-H bond of the 1,3-hydrogen isomerized starting stannacycle 8 (1,1-n -dibutylstanna-2,8-cyclohexadiene),: yielding compound 3. The pseudo-f irst order rate of conversion from 2 double right arrow 3 was measured by an NMR spin population transfer technique to be k(1) = 7.18 s(-1) (2, 0.034M; T-1 =1.6 s) while k(1) = was found to be 3.29 s(-1) (T-1 = 1.5 s), yielding an equilibrium constant of 2,18 at 298 K. The coales cence rate constant was calculated to be 235 s(-1) which was not reach ed by heating the sample up to 383 K.