We report LiBH₄ as a preferred, simple and effective reagent for reductive boronation of achiral and racemic chlorophosphonium salts (CPS) and for diastereomeric alkoxyphosphonium salts (DAPS), both of which are, in turn, easily generated from either the corresponding phosphane or, more conveniently, the phosphane oxide. Further, we have shown that the DAPS reduction/boronation could be achieved with complete stereocontrol to give scalemic phosphane–borane directly in excellent yield and enantiomeric excess (ee). This new methodology was employed to investigate the effects of aryl substitution on the outcome of dynamic kinetic resolution of arylmethylphenylphosphanes and phosphane oxides via DAPS. It was found that substitution at the ortho position strongly affects the degree of stereoselection. However, surprisingly, we confirmed that there was no variation of stereoselectivity seen with the electronic effect of substituents on the para position.

Within the currently accepted mechanism of the Li-salt-free Wittig reaction, the phenomenon of stereochemical drift remains the one remaining “loose end” in an otherwise internally consistent explanation of a large body of diverse observations. The term describes the nonstereospecific decomposition of the oxaphosphetane (OPA) intermediate in reactions of certain alkylides with certain aldehydes. In this paper, it is shown that the previous examples in which drift occurs are not merely isolated aberrations from the observed norm, but rather that there is a general phenomenon in reactions of ethylides with benzaldehydes. Variable-temperature NMR (VTNMR) spectroscopy was used to establish that the amount and diastereomeric ratio of the OPA intermediates do not change below a certain temperature. At and above the temperature at which OPA decomposition to alkene and phosphine oxide begins to occur, the alkene shows a different diastereomeric ratio to the OPA, which indicates the occurrence of stereochemical drift. In one example, owing to an apparent remarkable coincidence of rates, the diastereomeric ratio of the OPA does not change above the decomposition temperature, even though stereochemical drift occurs in the formation of the alkene product. An alternative mechanism for drift involving its catalysis by aldehyde was not confirmed. Drift was also shown not to occur in similar Wittig reactions of structurally related longer-chain alkylides by stereospecific decomposition of OPA intermediates generated from β-hydroxyphosphonium salts (β-HPSs). The extremely useful (and generally applicable) NMR techniques, ¹H–³¹P HMBC and selective ¹H{³¹P}, which we have utilised to establish kinetic diastereomeric ratios, are described in full for the first time. Details of the determination of the relative stereochemistry of two β-HPSs (derived from acid quenching of OPAs) by X-ray crystallography are also given.

In contrast to tertiary phosphine oxides, the deoxygenation of aminophosphine oxides is effectively impossible due to the need to break the immensely strong and inert PO bond in the presence of a relatively weak and more reactive PN bond. This long-standing problem in organophosphorus synthesis is solved by use of oxalyl chloride, which chemoselectively cleaves the PO bond forming a chlorophosphonium salt, leaving the PN bond(s) intact. Subsequent reduction of the chlorophosphonium salt with sodium borohydride forms the P^III aminophosphine borane adduct. This simple one-pot procedure was applied with good yields for a wide range of PN-containing phosphoryl compounds. The borane product can be easily deprotected to produce the free P^III aminophosphine. Along with no observed PN bond cleavage, the use of sodium borohydride also permits the presence of ester functional groups in the substrate. The availability of this methodology opens up previously unavailable synthetic options in organophosphorus chemistry, two of which are exemplified.

An efficient one-pot synthesis has been developed of enantioenriched P-stereogenic phosphanes and phosphane boranes from the corresponding racemic phosphanes in excellent yield under asymmetric Appel conditions. The chiral auxiliary (menthol) can also be recovered unchanged. The simple and efficient protocol significantly expands the scope of our asymmetric Appel process. The crucial step in the preparation involves stereospecific reduction of intermediate diastereomeric alkoxyphosphonium salts, which are obtained in the reaction of phosphane, hexachloroacetone, and menthol. Thereby, reaction with LiAlH₄ or NaBH₄ gives the corresponding phosphanes or phosphane boranes, respectively.

The effects of aryl ring substitution on the dynamic resolution of aryl(methyl)phenylphosphanes under asymmetric Appel reaction conditions have been studied. As expected, substitution at the ortho position strongly affects the degree ofstereoselection that can be achieved. Unexpectedly, however, there was no variation of stereoselectivity with the electronic nature of the para substituents, which suggests that there are two selection processes in operation. An unusual halogen-exchange process was observed in the arylphenylphosphinous chlorides on route to the required tertiary phosphane substrates.