Contributions of quantum mechanical tunneling to the rate of benzylic hydrogen atom abstraction reactions of triplet diarylcarbenes in fluid solution

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-biphenylphenylcarbene (BPC), 2-chlorodiphenylcarbene (ClDPC), 2-trifluoromethyldiphenylcarbene (FDPC), fluorenylidene (Fl), anthronylidene (AN), and diphenylcarbene (DPC). For 1-NPC, ClDPC, 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 Fl and AN the data were completely consistent with a purely classical atom-transfer process. © 1991 American Chemical Society.

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