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Issue 40, 2009
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Vicinal diphosphoniums: electrostatic repulsion under covalent constraint

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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-Cs 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[upper bond 1 start]–CH2CH2–P[upper bond 1 end]+ 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[upper bond 1 start]–CH2CR2CH2–P[upper bond 1 end]+ dications (R = H, Me) remains rigidly Cs 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 Cs-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 CsCs interconversion. This +P[upper bond 1 start]–CH2CMe2CH2–P[upper bond 1 end]+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[upper bond 1 start]–CH2CH2–P[upper bond 1 end]+ 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.

Graphical abstract: Vicinal diphosphoniums: electrostatic repulsion under covalent constraint

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The article was received on 25 Feb 2009, accepted on 06 Jul 2009 and first published on 06 Aug 2009

Article type: Paper
DOI: 10.1039/B903933C
Citation: Dalton Trans., 2009,0, 8493-8508
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    Vicinal diphosphoniums: electrostatic repulsion under covalent constraint

    M. Abdalilah, R. Zurawinski, Y. Canac, B. Laleu, J. Lacour, C. Lepetit, G. Magro, G. Bernardinelli, B. Donnadieu, C. Duhayon, M. Mikolajczyk and R. Chauvin, Dalton Trans., 2009, 0, 8493
    DOI: 10.1039/B903933C

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