Nucleophilic and Electrophilic Properties of Carbenes, II. 4‐Biphenylyl‐4‐pyridylcarbene

Flash pyrolysis of 4‐biphenylyl‐4‐pyridyldiazomethane ( 4 gave 7‐phenyl‐2‐azafluorene) 5 , which was also synthesized from 3‐mesitoylpyridine in four steps. 4‐Biphenylyl‐4‐pyridyl‐[ 13 C]‐diazomethane ( 9 ) was prepared from isonicotinic [ 13 C]‐acid chloride in three steps. Flash pyrolysis of 9 established that 4a‐ and 4b‐[ 13 C]‐7‐phenyl‐2‐azafluorenes are formed in a carbene‐carbene rearrangement in which ring expansion of the biphenyl part dominates over that of the pyridine ring. These results support the postulate that carbene‐carbene rearrangements are favoured by a nucleophilic interaction between the filled singlet carbene sp 2 (σ) orbital and the lowest unoccupied molecular orbital (LUMO) of the aromatic ring.