Benzylchlorocarbene:  Origins of Arrhenius Curvature in the Kinetics of the 1,2-H Shift Rearrangement

Abstract

Benzylchlorocarbene ( 1, BCC) was generated photochemically from benzylchlorodiazirine ( 2 ) in isooctane, methylcyclohexane (MCH), and tetrachloroethane (TCE) at temperatures from ∼30 to −75 °C. At −70 °C in isooctane, the identified products included Z / E -β-chlorostyrenes 4 (46.6%), α-chlorostyrene 5 (2.4%), 1,1-dichloro-2-phenylethane 6 (1.9%), a BCC-isooctane insertion product 8 (5.5%), carbene dimers 9 (3.8%), and azine 3 (30%). The significant incursion of intermolecular products 3, 8, and 9 implies that laser flash photolytic (LFP) kinetic data for the decay of BCC obtained at low temperature is biased and should not be employed in Arrhenius analyses. Accordingly, previously obtained curved Arrhenius correlations for BCC do not necessarily implicate quantum mechanical tunneling (QMT) in the 1,2-H shift rearrangement of BCC to 4 . Similarly in MCH, where BCC affords a solvent insertion product in ∼44−53% yield, the curved Arrhenius correlation (Figure 1) cannot be readily interpreted. In polar solvents such as TCE, clean H-shift reactions of BCC are obtained even at −71 °C; an Arrhenius correlation of LFP kinetic data is linear from 3 to −71 °C (Figure 2), affording E a = 3.2 kcal mol -1 and log A = 10.0 s -1 . Therefore, QMT does not appear to play a major role in the 1,2-H shift rearrangement of BCC at ambient or near ambient temperature in solution.