The synthesis and reactivity of a sterically unhindered phosphanylidene-phosphorane & the reduction of 1,3,2,4-dithiadiphosphetane-2,4-disulfides to primary and tertiary phosphines

Abstract

The reactivity of peri-substituted phosphine-phosphine donor acceptor complexes was explored and resulted in the formation of the first crystallographically characterised phosphanylidene bis(borane) adduct B3. The borane “protecting groups” in B3 were removed to form the intramolecular phosphanylidene-0⁻⁴-phosphorane B4: the only peri-stabilised phosphanylidene-0⁻⁴-phosphorane reported to date.

A study into the reactivity of B4 followed, highlighting its ability to coordinate to transition metals, chalcogens and electrophiles utilising either one or two of the lone pairs located on the phosphanylidene P atom. In addition to coordination complexes, ‘free’ phosphinidene type reactivity was demonstrated with B4 via the formation of the side-coordinated diphosphene palladium complex B11.

The reactivity of the 1,3,2,4-dithiadiphosphetane-2,4-disulfides (R₂P₂S₄; R = aryl) towards reducing agents (R’Li) was explored and resulted in the formation of the corresponding tertiary phosphines (RPR’₂) in yields ranging from 20 to 79 % (where R’ = Me, nBu, sBu, Ph). It was discovered that changing the nature of the organolithium employed from alkyls to phenyl results in different by-products, with the former affording the corresponding dialkyl sulfide and the latter diphenylsulfide and biphenyl. Reaction mechanisms were proposed that account for the product distribution when alkyllithiums were employed and for when phenylithium was, respectively.

A separate study probed the reduction of Fc₂P₂S₄ with the hydridic reducing agent LiAlH4, with the aim of producing the fundamental but elusive synthon ferrocenyl phosphine (FcPH₂) This was successful and FcPH₂ was formed in high yield (80%). Chlorination followed and afforded dichloroferrocenyl phosphine (FcPCl₂) in a quantitative yield. The scale of these syntheses was successfully increased to a >100 g scale and both FcPH₂ and FcPCl₂ were supplied to the major fine chemical companies. Due to the work reported herein ferrocenyl- and dichloroferrocenyl phosphine are now for the first time available commercially.

Date of Award	24 Jul 2013
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Petr Kilian (Supervisor) & J Derek Woollins (Supervisor)