ON THE TRANSITION-STATE OF THE THERMAL PROPENE ELIMINATION FROM ALLYLPHOSPHINES

Diallyltetramethylpiperidinophosphine 1, diallyldiisopropylaminophosph ine 2, and diallylmesitylphosphine 3 were pyrolyzed in a stirred flow reactor over the temperature range 400-450 degrees C, using toluene as carrier gas, producing, in all cases, mixtures of phosphorus-containi ng products. The pyrolysis of 1 produced 1-tetramethylpiperidino-1-pho sphabutadiene in addition to a mixture of phosphines and polyphosphine s. Compound 2 produced the azadiphosphole 4, the phosphinine 5, allylp hosphine, and diisopropylaminophosphine as major phosphorated products . The pyrolysis of 3 produced a mixture of phosphines and polyphosphin es. The complex mixture generated by the three diallylphosphines indic ates the formation of free radicals during their pyrolyses. AM1 calcul ations on the transition state of an expected retroenetype propene eli mination reaction showed that, due to the phosphaalkene character of t he transition state, the structure of the latter is very rigid and sen sitive to steric effects. Steric hindrance of the substituents on the phosphorus atom compels the molecule to distort the half-chair transit ion structure, causing a rise on the activation energy to values in th e range of a homolytic P-C bond dissociation energy. (C) 1997 John Wil ey & Sons, Inc.