Nickel(II)‐Carbene Intermediates in Reactions of Geminal Dihaloalkanes with Nickel(0) Reagents and the Corresponding Carbene Capture as the Phosphonium Ylide

In a previous study of geminal bond cleavages of substrates of the type R 2 CE 2 [E 2 = X 2 , O, S, Li(SO 2 Ph)] by nickel(0) reagents [L n Ni, L n = (Cod) 2 , (Et 4 P) 4 ], leading to R 2 C=CR 2 as products, the tentative hypothesis had been proposed that such reactions likely proceed via nickel(0)‐carbene intermediates (J. J. Eisch, Y. Qian, M. Singh, J. Organomet. Chem. 1996 , 512 , 207). Because such proposed nickel(0)‐carbenes do not satisfactorily account for reactions encountered with such α‐eliminations, a detailed reexamination of the reaction of these nickel(0) reagents with geminal dihalides has been undertaken. For example, two reactions of such presumed nickel(0)‐carbenes remained anomalous: (1) the failure of 5,5‐dibromotetraphenylcyclopentadiene to form its expected dimer, octaphenylphenylfulvalene and instead the formation of triethylphosphonium tetraphenylcyclopentadienide in its reaction with (Et 3 P) 4 Ni; and (2) the presumed capture of intermediate R 2 C=Ni 0 in presence of the trapping agent (benzaldehyde or benzophenone). As to the first anomaly, a detailed study has shown that no trace of octaphenylfulvalene was formed. As to the second anomaly, the R 2 C fragment could be trapped by the carbonyl reagents only in reactions involving (Et 3 P) 4 Ni, but not in reactions with (Cod) 2 Ni. This finding compels one to conclude that the carbonyl reagent is capturing the Wittig reagent, R 2 C=PEt 3 , and not R 2 C=Ni 0 . Based upon all present data, the mechanism of such C=C bond dimerizations is best explicable in terms of nickel(II)‐carbenes. The triethylphosphonium tetraphenylcyclopentadienide formed here has by single‐crystal X‐ray structure determination, complemented by 13 C NMR spectroscopic data, been found to have the zwitterionic structure Ph 4 Cp – – + PEt 3 as its paramount resonance contributor.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)