Arylmalonic esters and arylmalonodinitriles can be coupled oxidatively to the corresponding bibenzyls. Good yields of dimers are obtained when a para substituent (CH3, Cl) is introduced, which inhibits the formation of higher oligomers through benzylic C-para C coupling. Substitution at both ortho positions and the para position (CH3) in phenylcyanoacetic esters completely inhibits C-C coupling by steric crowding. Ketene imines are formed instead by C-N coupling. Substitution at one ortho position (CHa) partially gives the usual C-C coupling together with benzylic C-para C coupling (oligomer formation) in case of a free para position and C-N coupling (ketene imine formation) in case of a CHs-substituted para position. The thermal dissociation of the dimers into radicals is confirmed by esr analysis. From nmr line width measurements kinetic parameters for the dissociation reaction are obtained.
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