LASER FLASH PHOTOLYSIS STUDY OF PHENYLCARBENE,o-TOLYLCARBENE AND MESITYLCARBENE

Laser flash photolysis (LFP, XeCl, 308 nm, 20 ns) of phenyldiazomethane (PDM), o-tolydiazomethane (TDM) and mesitydiazomethane (MDM) produces phenylcarbene (PC), o-tolycarbene (TC) and mesitylcarbene (MSC), respectively. Transient spectra of PC and TC could not be detected in pentane at ambient temperature; however, these carbenes could be trapped with pyridine to form UV–Vis-active ylides. The rate of formation of these ylides was resolved in CF2ClCFCl2 (Freon-113) and yields, after analysis, values of kpyr and τ, where kpyr is the absolute rate constant of the reaction of the spin equilibrated carbene with pyridine and τ is the carbene lifetime in Freon-113 in the absence of pyridine. LFP of MDM produces the transient spectra of both triplet MSC and 3,5-dimethyl-1,2-benzoquinodimethane (3,5-BQM). The lifetime of 3MSC in pentane is ca 500 ns. 3MSC reacts with pyridine with an apparent rate constant of kpyr=1·35×107 M−1 s−1. The decay of 3MSC in a variety of solvents does not lead to increased absorption of 3,5-BQM in solution, nor does the presence of a carbene scavenger (methanol) lead to a reduction in yield of 3,5-dimethyl-1,2-benzoquinodimethane upon LFP of precursor. Thus, the decay of 3MSC in solution does not lead to the formation of quinodimethane. This species (3,5-BQM) is formed, instead, by a hydrogen atom transfer in the excited state of the precursor. Product analysis reveals that the excited state rearrangement which forms xylylene is a minor process. Photolysis of MDM in a nitrogen matrix at 10 K leads to the UV–Vis spectra of matrix-isolated triplet mesitylcarbene and 3,5-BQM. The carbene rearranges slowly to 3,5-BQM in the cryogenic matrix, in contrast to its behavior in solution.