Crystallization-induced asymmetric transformation of a tertiary phosphine

An equilibrating diastereomer mixture of the tertiary phosphines 5 and 6 (2.5:1 equilibrium ratio) undergoes crystallization-induced asymmetric transformation upon slow evaporation of solvent from refluxing heptane to give a 20:1 ratio in favor of the more stable crystalline isomer 5. The process can also be carried out at room temperature by using iodine to catalyze the interconversion of 5 and 6 via a pentavalent intermediate 8. However, this variation is more sensitive to the purity of the starting phosphine. Crystalline 5 can be converted to the stable borane complex 3, and reductive cleavage of the fluorenyl group using lithium naphthalenide affords the corresponding lithio derivative 10. Alkylation with iodomethane or benzyl bromide affords 13 or 17, respectively, with retention of phosphorus configuration.