INTRAMOLECULAR ADDITION-REACTIONS OF FUNCTIONALIZED ARYLCARBENES TO DOUBLE-BONDS

Arylcarbenes with unsaturated ortho substituents (vinyloxy, 1-propenyloxy, allyl, 2-butenyl, 2,4-pentadienyl) were generated in methanol by photolysis of diazo precursors. Intermolecular OH insertion was found to compete with intramolecular addition to the double bonds. The product distributions are strongly affected by triplet sensitization, in contrast to those of analogous intermolecular reactions. The singlet and triplet components of the intramolecular processes were dissected and evaluated in terms of k(S)/k(ST) and k(T)/k(TS) rate ratios. Intramolecular cycloaddition reactions of singlet arylcarbenes are rather inefficient, probably due to the need for rotation about the exocyclic C-CH: bond. By contrast, the nonconcerted intramolecular addition of triplet arylcarbenes is strongly accelerated relative to intermolecular analogues. Most likely, the in-plane sigma-orbital is involved in the first step of the triplet addition process.