Mechanism of the diphenyldisulfone-catalyzed isomerization of alkenes. Origin of the chemoselectivity: experimental and quantum chemistry studies

Polysulfone- and diphenyldisulfone-catalyzed alkene isomerizations are much faster for 2-alkyl-1-alkenes than for linear, terminal alkenes. The mechanism of these reactions has been investigated experimentally for the isomerization of methylidenecyclopentane into 1-methylcyclopentene, and theoretically CCSD(T)/6-311++G(d,p)//B3LYP/6-311++G(d,p) calculations] for the reactions of propene and 2-methylpropene with a methanesulfonyl radical, MeSO2*. On heating, polysulfones and (PhSO2)2 equilibrate with sulfonyl radicals, RSO2*. The latter abstract allylic hydrogen atoms in one-step processes giving allylic radical/RSO2H pairs that recombine within the solvent cage producing the corresponding isomerized alkene and RSO2*. The sulfinic acid, RSO2H, can diffuse out from the solvent cage (H/D exchange with MeOD,D2O) and reduce an allyl radical. Calculations did not support other possible mechanisms such as hydrogen exchange between alkenes, electron transfer, or addition/elimination process. Kinetic deuterium isotopic effects measured for the (PhSO2)2-catalyzed isomerization of methylidenecyclopentane and deuterated analogues and calculated for the H abstraction from 2-methylpropene and deuterated analogues by CH3SO2* are consistent also with the one-step hydrogen transfer mechanism. The high chemoselectivity for this reaction is not governed by an exothermicity difference but by a difference in ionization energies of the alkenes. Calculations for CH3SO2* + propene and CH3SO2* + 2-methylpropene show a charge transfer of 0.34 and 0.38 electron, respectively, from the alkenes to the sulfonyl radical in the transition states of these hydrogen abstractions.