摘要:
Kinetic isotope effects for the benzylic hydrogen atom abstraction reactions of the triplet states of several diarylcarbenes with toluene-toluene-d8 in fluid solution were determined by measuring the ratio of 1,2-diphenylethane to 1,2-diphenylethane-d7 produced in the reaction. Eight carbenes were studied: dibenzocycloheptadienylidene (DBC); 1-naphthylphenylcarbene (1-NPC), 4-biphenylphenlcarbene (BPC), 2-chlorodiphenylcarbene (C1DPC), 2-trifluoromethyldiphenylcarbene (FDPC), fluorenylidene (F1), anthronylidene (AN), and diphenylcarbene (DPC). For 1-NPC, C1DPC, and FDPC the differential kinetic isotope effects were much larger than predicted by complete loss of all zero point energy in the transition state, indicating that there is a contribution of quantum mechanical tunneling to the H(D) atom transfer process. For DPC, DBC, and PBC the differential kinetic isotope effects were barely consistent with a completely classical atom-transfer reaction. For F1 and AN the data were completely consistent with a purely classical atom-transfer process.