Intramolecular dehydrofluorinative coupling of the asymmetric diphosphine Ph2PCH2CH2PPh(C5F4N-4) and pentamethylcyclopentadienyl ligands in a rhodium complex

Article abstract of DOI:10.1021/om020874p

The tetrafluoropyridyl-substituted diphosphine Ph₂PCH₂CH₂PPh(C₅F₄N-4) (1) has been prepared in three steps from dppe. Comparison of the spectroscopic properties between trans-[RhCl(CO){PPh₂(C₅F₄N-4)}₂] and analogous complexes of other fluorinated triaryl phosphines reveals the lower basicity of tetrafluoropyridylphosphines relative to pentafluorophenylphosphines. The reaction between [Cp*RhCl(μ-Cl)]₂ and 1 in the presence of [BF₄^-] yielded racemic diastereoisomers of [Cp*RhCl(1)][BF₄]. In the S_RhR_P and R_RhS_P pair of enantiomers the Cp* and tetrafluoropyridyl groups have a cis disposition about the Rh-P bond, and in the S_RhS_P and R_RhR_P pair the groups are trans. In situ NMR experiments reveal that the cis pair, in which the Cp* and tetrafluoropyridyl groups are close, underwent rapid dehydrofluorinative C-C coupling to give the respective enantiomers of [{η⁵,κP,κP-C₅Me₄CH₂- 2-C₅F₃N-4-PPhCH₂CH₂ PPh₂}RhCl][BF₄] (6·[BF₄]). The trans pair did not undergo coupling, but isomerized to the cis pair on heating in ethanol. The reaction between [Cp*RhCl(μ-Cl)]₂ and 1 in refluxing benzene afforded 6·[BF]₄ in 64% yield after anion metathesis. The structures of OPPh₂-(C₅F₄N-4), the cis isomer of [Cp*RhCl(1)][BF₄], and one enantiomer of 6·[BF₄], which crystallizes as a conglomerate, have been determined by single-crystal X-ray diffraction.