Temperature Dependence of the Reactions of Singlet and Triplet Diphenylcarbene. Evidence for Reversible Ylide Formation in the Reaction with Alcohols

摘要

The insertion of diphenyscarbene (DPC) into the OH bonds of alcohols has received considerable attention. It is generally accepted that it is the singlet state of DPC (1DPC), in equilibrium with the ground triplet state (3DPC), that reacts with the alcohols. However, the reaction mechanism is not yet fully understood, and recent kinetic data have been viewed as inconsistent with the conventional mechanism shown in Scheme I. We now describe the effect of temperature on the products of the reaction of DPC with methanol, and with isoprene, in several solvents. These findings, together with measurements of product isotope effects, provide additional insight into the carbene-alcohol reaction and are consistent with an articulation of the conventional mechanism. DPC yields benzhydryl methyl ether upon reaction with methanol, and two isomeric cyclopropanes with iosprene. At sufficiently low concentrations of the carbene quenchers, equilibrium between 1DPC and 3DPC is more rapid than reaction. At sufficiently high concentrations, isoprene efficiently traps 3DPC but does not compete for 1DPC in the presence of methanol, which is a poor quencher of 3DPC, but a very efficient quencher of 1DPC. The methanol reaction monitors 1DPC and the isoprene reaction monitors 3DPC. In figure 1 it is shown the effect of temperature on the ration of the quantum yields for triplet (Q3) and singlet (Q1) product information in several solvents. In each solvent triplet products are more favored at low and high temperatures than at intermediate temperatures. In figure 2 are shown the results of similar experiments for CH3OD and t-butanol.