Kinetic isotope effect in hydrogen isotope exchange between diphenylphosphine and methanol or 2-methylpropane-2-thiol in aprotic solvents

Abstract

The bimolecular rate constants for tritium exchange between (i) Ph₂PH and MeOH*, (ii) Ph₂PD and MeOD*, (iii) Ph₂PH and Me₃CSH*, and (iv) Ph₂PD and Me₃CSD* in MeCN solution were measured. In systems (i) and (ii) the HT/DT kinetic isotope effect depends on the direction of exchange and varies from 3.0 to 1.7 and from 4.8 to 2.4 at 323 and 250 K, respectively. In systems (iii) and (iv) the kinetic isotope effect is independent of the direction of exchange and changes from 2.2 at 323 K to 2.8 at 250 K. The rate constants for deuterium exchange between Ph₂PH and MeOH or Me₃CSH were measured in C₆H₆, C₂Cl₄, (CH₂)₄O, C₅H₅N and MeCN solutions. The reactions involve (i) desolvation of MeOH or Me₃CSH (ii) double hydrogen transfer in the four-centre cyclic complex, which is found to be the rate limiting-step. The contribution of tunnelling to hydrogen transfer is not significant.