INTRAMOLECULAR REACTIVITY OF ARYLCARBENES - BIPHENYL-2-YLCARBENES

Biphenyl-2-ylcarbenes, 2-ArC(6)H(4)CR, were generated photolytically a nd thermally from diazo precursors. Cyclization, leading to fluorenes, competes with capture of the carbenes by methanol but proceeds faster than intramolecular hydrogen shifts (with R = Me) and intermolecular C-H insertion reactions (with R = H in cyclohexane). By comparison of product ratios with kinetic data for related carbenes from the literat ure, the cyclization rate is estimated as ca 10(11) s(-1). The intramo lecular reactivity of biphenyl-2-ylcarbenes is nor significantly atten uated by variation of R (R = H, Me, Ph). Very minor effects of triplet sensitization and methanol quenching indicate that fluorenes arise fr om spin-equilibrated biphenyl-2-ylcarbenes, presumably from the single t state. When Ar = mesityl, the carbene predominantly inserts into C-H bonds of the 2'-methyl groups, giving rise to a dihydrophenanthrene. Formation of a fluorene derivative, by formal insertion into C-C bonds , occurs as a minor process. This unprecedented reaction points to int ervention of an o-xylylene in which the methyl group migrates. Laser f lash photolysis (LFP) of 2-PhC(6)H(4)CN(2)Ph generates a transient abs orption which is due to the T-0-->T-n transition of 9-phenylfluorene r ather than to the presumed o-xylylene. On LFP of 2-ArC(6)H(4)CN(2)Ph i n trifluoroethanol-acetonitrile, protonation of the carbenes gives ris e to carbocations, 2-ArC(6)H(4)CH(+)Ph. The transient absorption spect ra of these cations are strongly influenced by twisting about the Ar-A r bond (Ar = Ph < o-tolyl < mesityl) whereas the rates of nucleophilic capture vary only slightly. Biphenyl-2-ylcarbenium ions (Ar = R = Ph) cyclize more slowly than the analogous carbenes, by a factor of great er than or equal to 10(4).