INFLUENCE OF PHOSPHORUS–SULFUR BONDING IN THE FORMATION OF OCTAHEDRALLY COORDINATED CYCLIC PENTAOXYPHOSPHORANES

Abstract

Formation of new pentaoxyphosphorane X[R₁R₂C₆H₂O]₂P(OPh)₃ (X = S, R₁ = R₂ = t-Bu for 1; X = S, R₁ = t-Bu, R₂ = Me for 2; X = S, R₁ = R₂ = Me for 3; X = CH₂, R₁ = R₂ = t-Bu for 4) containing sulfur-bridged eight-membered rings was achieved by oxidative addition reactions of triphenyl phosphite with diols in the presence of N-chlorodiisopropylamine in yields ranging from 70 to 80%. 1 contains 0.5 mol of Et₂O of crystallization. X-ray analysis of 1–3 revealed molecular structures having varying degrees of P–S bond formation, which results in distorted octahedral configurations. The P–S distances, 2.880(1) (3), 2.744(2) (2), and 2.640(2) Å (1), when included in a series with those of related cyclic oxyphosphoranes studied earlier, showed a uniform decrease which parallels a structural coordinate measuring the formation of an octahedron from a square pyramid, i.e., a trajectory proceeding from five- to six-coordinated phosphorus. The decrease in P–S distance along the series is attributable to an increase of electronic factors supplied by the alkyl substituents on the eight-membered ring, t-Bu > Me, and the electron-withdrawing ability of the appendent ligands, (OC₆H₄)₂ > OCH₂CF₃ > OPh. ¹H and ³¹P solution-state NMR data are also reported. The structures of 1–3 contrast with that for ananalogous derivative S(Me₂C₆H₂O)₂P(OCH₂CF₃)₃ (B), having the same sulfur-containing eight-membered ring which resides in a trigonal bipyramid with the ring located diequatorially and possessing no P–S bond (P–S = 3.504(3) Å). The results have important mechanistic implications for enzyme hydrolysis of phosphates at active sites having interacting sulfur residues. Oxyphosphorane 1 crystallizes in the triclinic space group P 1 with a = 9.696(3) Å, b = 10.814(2) Å, c = 22.113(3) Å, α = 82.62(1)°, β = 85.38(2)°, γ = 88.08(2)°, and Z = 2. Oxyphosphorane 2 crystallizes in the monoclinic = 4. Oxyphosphorane 3 crystallizes in the triclinic space group P 1 with a = 8.757(2) Å, b = 10.047(1) Å, c = 17.614(3) Å, α = 103.88°, β = 100.69(2)°, γ = 92.65(2)°, and Z = 2. The final conventional unweighted residuals are 0.056 (1), 0.031 (2), and 0.032 (3).