Mesoionic Carbenes: Reactions of 1,3-Diphenyltetrazol-5-ylidene with Electron-Deficient Alkenes, and Synthesis and Catalytic Activities of the (Tetrazol-5-ylidene)rhodium(I) Complexes

The reactions of 1,3-diphenyltetrazol-5-ylidene, a rare example of mesoionic carbenes, with electrondeficient unsaturated compounds were studied. The carbene reacted with dimethyl 1,2,4,5-tetrazine-3, 6-dicarboxylate to give a 5-tetrazoliomethylide, together with hydrazine derivatives. The reaction with tetracyanoethylene gave another methylide in low yield. On the contrary, the reactions with weaker electrophiles, such as 3,6-diphenyl-1,2,4,5-tetrazine, fumalonitrile, N-phenylmaleimide, and dimethyl acetylenedicarboxylate, did not give any coupling products, but gave phenylated products and/or Michael addition products via the degradation of the 1,3-diphenyltetrazole ring. Novel mesoionic mono and bis(carbene)-rhodium(I) complexes were synthesized and the structures were characterized by X-ray crystallography. Their catalytic activities for the decarbonylative addition reaction of benzoyl chloride to ethynylbenzene were investigated.