Diastereoselectivity of borane addition to chiral triquinphosphoranes. NMR, X-ray, and molecular modeling studies

Series of new chiral tricyclic pentacoordinated phosphorus compounds, "triq uinphosphoranes" (13-18, 21, 22, and 24), were prepared from chiral enantio pure diamino diols that present a Ct symmetry axis. P-31 and C-13 NMR data are consistent either with a low-energy single-step Berry pseudorotation pr ocess between the two possible diastereomeric trigonal-bipyramidal structur es TBP(R-P) and TBP(S-P) or with the chiral square-pyramidal structure SP. Triquinphosphoranes reacted with borane to give two stable monoadducts with opposite configurations at the phosphorus center which do not undergo epim erization. The diastereoselectivity of this reaction depends strongly on th e nature and the position of the substituents, the highest diastereomeric e xcesses being obtained with 4,9-diisopropyl (28; 90% de) and 4,9-diisobutyl (29; 86% de) compounds. The X-ray structure of the major diastereomer 28M revealed that it is remarkably close to an ideal TBP, exhibiting an S-P abs olute configuration. Semiempirical AM1 MO calculations predict that TBP(R-P ) and TBP(S-P) ground-state species are in rapid equilibrium through a SP t ransition state, the activation barrier being about 5 kcal/mol. Calculation s predict a marked predominance of the R-P form in all cases, except for 6, 7-diPh triquinphosphorane 22. Steric interactions between the substituents and the tricyclic phosphorane skeleton explain the prevalence of one diaste reomer. The diastereoselectivity observed in borane addition on 15 is ratio nalized in terms of a kinetically controlled process in which the minor tri quinphosphorane diastereomer reacts faster than the major one to afford (S- P)-28M.