Intramolecular reactivity of arylcarbenes: Biphenyl-2-ylcarbenes

Abstract

Biphenyl-2-ylcarbenes, 2-ArC6H,CR, were generated photolytically and 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 literature, the cyclization rate is estimated as cu 10" s-'. The intramolecular reactivity of biphenyl-2-ylcarbenes is not significantly attenuated by variation of R (R = H, Me, Ph). Very minor effects of triplet sensitization and methanol quenching indicate that fluorenes arise from spin-equilibrated biphenyl-2-ylcarbenes, presumably from the singlet state. When A r = 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 intervention of an o-xylylene in which the methyl group migrates. Laser flash photolysis (LFP) of 2-PhC6H,CN,Ph generates a transient absorption which is due to the To -+ T, transition of 9-phenylfluorene rather than to the presumed o-xylylene. On LFP of 2-ArC,H,CN,Ph in trifluoroethanol-acetonitrile, protonation of the carbenes gives rise to carbocations, 2-ArC,H,CH 'Ph. The transient absorption spectra of these cations are strongly influenced by twisting about the Ar-Ar bond (Ar = P h i 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 310,.

Preparation of a Copper Ion Complex of Sterically Congested Diphenyldiazomethanes Having a Pyridine Ligand and Characterization of Their Photoproducts

“…Analysis of the spent solution showed the presence of 2-azafluorene compound ( 3 ) as a main product obviously formed as a result of formal insertion of diphenylcarbene into the C−H bond of the o -pyridyl group. The observation is very similar with the photochemistry of o -arylated diphenyldiazomethanes leading to fluorenes as a result of formal insertion of diphenylcarbene into the C−H bond of the o -aryl group. , The transient bands detected in LFP of o -arylated diphenyldiazomethanes are shown to be triplet fluorenes. Spectroscopic features (broad bands centered around 400−420 nm) and decay kinetics (the first order with 7−9 × 10 5 s -1 ) observed for triplet fluorenes are in good agreement with those observed in the present system.…”

Ultrafast Study of p-Biphenylyldiazomethane and p-Biphenylylcarbene

“…To our knowledge, the transient spectrum of cation C has not been reported previously. Kirmse observed the absorption of the p -biphenylylphenylcarbenium ion at 515 nm, which shows a bathochromic shift relative to the absorption of diphenylcarbenium ion at 430 nm. Benzyl cation absorbs at 363 nm; thus, absorption of quinoidal cation C at 450 nm does not seem unreasonable.…”

Rearrangement of Biaryl Monoaminocarbenes via Concerted Asynchronous Insertion into Aromatic C−H Bonds

“…In order to assess the influence of the amino group on the ease and asynchronicity of the insertion reaction, similar calculations were then performed on the biphenyl hydrogenocarbene 1 * 15,19. As expected from the removal of the strongly donating amino group, carbene 1 * adopts a triplet ground state, with a singlet—triplet separation of 8.7 kcal/mol 20.…”