Generation of Hydrofluoronickelacycles from Trifluoroethylene and Ni(0): Ligand Effects on Regio-/Stereoselectivity and Reactivity.

Treatment of Ni(0) complexes 1a-e with sub-atmospheric pressures of trifluoroethylene (TrFE) affords hydrofluoronickelacyclopentanes L2Ni(C4F6H2) 2a-e (L = PPh3, P(O-o-tol)3, PPh2Me, PPhMe2, PMe3). Fluorine NMR analysis of 2a-e demonstrates predominant formation of three (of the possible six) isomers upon oxidative cycloaddition of TrFE: the cis and trans head-tail isomers and the trans head-head isomer, where the CHF group is defined as the TrFE "head". The respective ratios of L2Ni(C4F6H2) isomers are influenced by the nature of L, with smaller phosphines favoring the thermodynamically preferred (from DFT calculations) trans head-head isomer (cf. 50% with PMe3) and the largest affording small amounts of the tail-tail isomers. Lewis and Brønsted acids induce a surprising double C-F bond activation in 2c-d, affording small functionalized hydrofluoroalkenes. Interestingly, varying the acid employed dictates the organic product obtained from the head-tail isomers: BF3·OEt2 is selective for 1,1,2,3-tetrafluorocyclobutene, whereas Me3SiOTf and N,N-dimethylanilinium bromide yield (Z,E)-1,1,3,4-tetrafluorobutadiene as the major fluorinated product. Reaction intermediates were isolated, and possible pathways are discussed.