Elusive Arylalkylcarbenes in Solution and in Crystals: Facile 1,2-R (R = H, Ph) Migrations of 1,2,2-Triphenylethylidene.

Highly reactive arylalkylcarbenes generated in solution by photolysis of their aryldiazoalkane precursors tend to undergo competing inter- and intramolecular reactions to yield a complex mixture of products. Having previously shown the use of crystals to effectively control the reactivity of arylalkylcarbenes to afford high yields of a single product, it was of interest to investigate whether the crystalline environment could also enable spectroscopic detection of these intermediates en route to the photoproduct. Using 1,2,2-triphenyldiazoethane ( 3 ) as a model substrate to probe the effect of alternative reaction trajectories that yield triphenylethylene ( 5 ) by competing 1,2-H shift or 1,2-Ph migration, we report selectivities consistent with reaction from a spin-equilibrated carbene 4 in solution, while reactions in crystals primarily afford alkene 5 via a lattice-controlled 1,2-H shift. Attempts to detect 1,2,2-triphenylethylidene 4 in crystals by nanosecond laser-flash photolysis or by triplet-triplet fluorescence at 77 K were unsuccessful, indicating that arylalkylcarbenes possessing α-H substituents undergo facile 1,2-H shifts both in solution and in the solid state. However, related tert -butylphenylmethylene with no α-H substituents could be observed by triplet-triplet fluorescence at 77 K in glassy matrices.