Kinetic data and activation parameters are reported for the reactions of a series of phosphites, arylphosphonites, diarylphosphinites, and triarylphosphines with octasulphur (S,) in toluene as solvent. For the phosphites the data are explained in terms of changes in p-character of the lone-pair orbital on phosphorus and inductive electron donation by the alkyl groups. The rates of reaction of series of arylphosphonites, diarylphosphinites, and triarylphosphines correlate with the Hammett 0 constants to give p values of -3.0, -3.0, and -2.5, respectively, and the results are discussed in terms of the
Reactivity-Selectivity Principle and the biphilic mechanism for insertion of trico-ordinate phosphorus into o-bonds.Commercial sulphur consists almost entirely of an octa-atomic cyclic structure, s8, and this crown-shaped molecule is the thermodynamically stable form of sulphur up to 95 "C.' The reaction of tertiary phosphines with s8 gives a quantitative yield of the corresponding tertiary phosphine sulphides ' with retention of configuration at p h o s p h ~r u s , ~ and a thorough investigation revealed a second-order reaction which was highly sensitive to solvent p ~l a r i t y . ~The rate was not directly proportional to the dielectric constant of the medium but was related to the hydrogen-bonding capacity of the solvent, suggesting that the rate increase was associated with the dispersal of negative charge. In addition, using a series of parasubstituted arylphosphines, Bartlett obtained a p value of -2.5 and proposed a mechanism involving rate-limiting attack of the phosphorus on sulphur, thus opening the sulphur ring to a linear octasulphide. The resultant dipolar ion then suffered a series of rapid nucleophilic displacements to yield eight moles of sulphide per mole of s8 (Scheme 1). The availability of a wide range of trico-ordinate phosphorus compounds, Ar,P(OR), -,(1) with n = 0-3, prompted us to investigate this reaction [equation (l)] more thoroughly, with the objective of assessing the effect of structural variations within the nucleophile on (i) the reaction rate for n = 0-3 and (ii) the Hammett p values for reactions involving n = 1-3.
Results and DiscussionReactions of Acyclic Phosphites with &-The reactions of trialkyl phosphites (1; n = 0, R = Me, Et, Pr", Pr', or Bun) with