INTRAMOLECULAR REACTIVITY OF ARYLCARBENES - DERIVATIVES OF O-TOLYLCARBENE

Various CH(2)X groups have been attached to the ortho position of phenylcarbene. If 2'- or 3'-C-H bonds are present, as in 23 (X = Me), 71 (X = CMe(3)), and 58 (X = SiMe(3)), C-H insertion leading to five- or six-membered rings predominates in the gas phase and competes with intermolecular reactions in solution. The formation of benzocyclobutenes via insertion into 1'-C-H bonds is a very minor reaction path of 23 and 71. In contrast, 58 produces substantial amounts of the benzocyclobutene 59 by way of C-Si insertion, particularly in the gas phase. Insertion into the Si-Me bonds of 58 also occurs while the C-F bonds of 37 (X = CF3) and 50 (X = Fl are inert. In the gas phase, 37 and 50 give mainly benzocyclobutenes whereas intermolecular reactions prevail in solution. The effects of sensitization and of solvent polarity suggest that benzocyclobutenes arise from singlet arylcarbenes. The amount of carbene-carbene rearrangement decreases in the order 7 (X = H) > 37 (X = CF3) > 23 (X = Me); no rearrangement was observed with 50 (X = Fl, 58 (X = SiMe(3)), and 71 (X = CMe(3)).