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.