Ultrafast Carbene Dynamics

Carbenes have fascinated physical organic chemists for over a century. These highly unstable, but important reaction intermediates frequently undergo rapid intersystem crossing (ISC) due to their small singlet-triplet splittings. We have been studying the ultrafast dynamics of carbenes generated upon photolysis of diazo precursors in solution in collaboration with Prof. Matthew Platz at Ohio State. Intersystem crossing occurs in a few ps for the carbene produced from an aryl diazo ester [1]. This rate is considerably faster than for any previously studied carbene.

We have conducted a detailed study of singlet diphenylcarbene in different solvents [2]. In acetonitrile and cyclohexane ISC dynamics were followed by transient absorption. Singlet carbenes are powerful, photogenerated bases that are protonated by alcohols. By detecting the rise and decay of the diphenyl methyl cation, we have recently determined that proton transfer occurs in neat methanol with a time constant of 9.0 ps. This is one of the fastest known intermolecular proton transfer reactions to carbon. In O-deuterated methanol proton transfer occurs in 15.0 ps. Slower rates were observed in the longer alcohols. The protonation times correlate reasonably well with solvation times in these alcohols, suggesting that solvent fluctuations are the rate-limiting step. In all alcohols studied, the carbocations decay on a somewhat slower timescale to yield diphenylalkyl ethers.

References

1. Peon, J.; Polshakov, D.; Kohler, B., "Solvent Reorganization Controls the Rate of Proton Transfer from Neat Alcohol Solvents to Singlet Diphenylcarbene," J. Am. Chem. Soc. 2002, 124, 6428-6438. 2. Hess, G. C.; Kohler, B.; Likhotvorik, I.; Peon, J.; Platz, M. S., "Ultrafast carbonylcarbene formation and spin-equilibration," J. Am. Chem. Soc. 2000, 122, 8087-8088.