Vicinal diphosphoniums: electrostatic repulsion under covalent constraint

Abstract

A series of five vicinal bis(alkyl-triarylphosphoniums) derived from o-bis(diphenyl-phosphino)benzene (o-dppb) is described. Each of them have been prepared by specific methods, and their formal electrostatic and possible van der Waals strain is compared through the P⁺⋯P⁺ distances in the crystal state. According to X-ray diffraction analyses, while the conformations of the dimethyl dication is C₂ symmetric, the conformation of alka-1,n-diyl-diphosphoniums (n = 1, 2, 3) is pseudo-C_s symmetric. The solution structure of the dimethyldiphosphonium 2 was studied by NMR techniques after metathesis of the triflate counterions with enantiomerically pure X-PHAT chiral anions (X = TRIS, BIN). [(Δ)-BINPHAT] was indirectly shown to discriminate between the enantiomers of the diphosphonium, but no effective enantio-differentiation could be measured down to −70 °C in CD₂Cl₂. When associated to [(Δ)-TRISPHAT] counterions, progressive cleavage of the diphosphonium afforded the corresponding methyl-triphenyl-monophosphonium salt. In the bridged series, the diphosphoniacyclopentene dication was found to be stable in spite of the very short transannular P⁺⋯P⁺ distance (2.83 Å). The naturally more relaxed homologous diphosphoniacyclohexene dication (P⁺⋯P⁺ = 3.28 Å) was found to be flexible in solution, as indicated by the equivalence of the four protons of the ⁺P–CH₂CH₂–P⁺ bridge from 20 °C to −80 °C in acetone-d₆. The two diphosphoniacycloheptene dications exhibit no P⁺⋯P⁺ steric compression (ca 3.64 Å), just as the dimethyl dication (3.70 Å). In solution, the seven-membered ring of the ⁺P–CH₂CR₂CH₂–P⁺ dications (R = H, Me) remains rigidly C_s symmetric: no interconversion of the pseudo-axial and pseudo-equatorial R groups is evidenced at the NMR time scale at −95 °C (coalescence at −20 °C for R = H, at −65 °C for R = Me). According to DFT calculations at the B3PW91/6-31G** level in 1,2-dichloroethane (DCE: PCM, ε = 10.36), the lowest energy conformation is indeed C_s-symmetric. However a quasi-isoenergetic C₂-symmetric conformation which is the most stable in the gas phase is also found, and is thus a likely intermediate in the observed C_s⇄C_s interconversion. This ⁺P–CH₂CMe₂CH₂–P⁺diphosphonium was obtained by dialkylation of o-dppb with 2,2-dimethylpropan-1,3-diyl bistriflate in DCE at 80 °C. The use of this solvent was the key for the synthesis of this dication, which was however competitively produced along with some ⁺P–CH₂CH₂–P⁺ dication. The latter was also obtained by a totally different route, by heating a sulfinylethyl monophosphonium salt of o-dppb in the presence of the cationic complex [Rh(cod)₂][PF₆]. A mechanism for this peculiar process is proposed.