Elusive Arylalkylcarbenes\nin Solution and in Crystals:\nFacile 1,2‑R (R = H, Ph) Migrations of 1,2,2-Triphenylethylidene

Highly reactive arylalkylcarbenes\ngenerated in solution\nby photolysis\nof their aryldiazoalkane precursors tend to undergo competing inter-\nand intramolecular reactions to yield a complex mixture of products.\nHaving previously shown the use of crystals to effectively control\nthe reactivity of arylalkylcarbenes to afford high yields of a single\nproduct, it was of interest to investigate whether the crystalline\nenvironment could also enable spectroscopic detection of these intermediates\nen route to the photoproduct. Using 1,2,2-triphenyldiazoethane (<b>3</b>) as a model substrate to probe the effect of alternative\nreaction trajectories that yield triphenylethylene (<b>5</b>) by competing 1,2-H shift or 1,2-Ph migration, we report selectivities\nconsistent with reaction from a spin-equilibrated carbene <b>4</b> in solution, while reactions in crystals primarily afford alkene <b>5</b> via a lattice-controlled 1,2-H shift. Attempts to detect\n1,2,2-triphenylethylidene <b>4</b> in crystals by nanosecond\nlaser-flash photolysis or by triplet–triplet fluorescence at\n77 K were unsuccessful, indicating that arylalkylcarbenes possessing\nα-H substituents undergo facile 1,2-H shifts both in solution\nand in the solid state. However, related <i>tert</i>-butylphenylmethylene\nwith no α-H substituents could be observed by triplet–triplet\nfluorescence at 77 K in glassy matrices.