Simple unprecedented conversion of phosphine oxides and sulfides to phosphine boranes using sodium borohydride

Abstract

A variety of phosphine oxides and sulfides can be efficiently converted directly to the corresponding phosphine boranes using oxalyl chloride followed by sodium borohydride. Optically active P-stereogenic phosphine oxides can be converted stereospecifically to phosphine boranes with inversion of configuration by treatment with Meerwein's salt followed by sodium borohydride.

A convenient and mild chromatography-free method for the purification of the products of Wittig and Appel reactions

“…We were encouraged in this because we had recently discovered that both chlorophosphonium salts and alkoxyphosphonium salts 40 could be subjected to reduction and in-situ boranation using NaBH 4 , thus providing facile conversion from phosphine oxide to phosphine-borane. 32 Drawing on this work and the work of Imamoto and co-workers, 38 we investigated LiAlH 4 as reductant, and found it to be successful for the overall reduction of the 45 phosphine chalcogenides shown in Chart 1. In cases where a small amount of phosphine oxide contaminant was shown to be present in the sample of phosphine, elution through a short plug of silica using cyclohexane, ethyl acetate or dichloromethane removed the phosphine oxide and furnished 75 clean phosphine.…”

“…30 We were already interested in such salts because of their involvement in other chemistry in our laboratory . 31,32 They can also be generated by the reactions of phosphine with chlorine (in various solvents), 33,34,35,36 and phosphine oxide with sulfuryl 45 chloride, methanesulfonyl chloride, thionyl chloride or trichloromethyl chloroformate. 30,32 The structure of the entity produced seems to depend heavily on the solvent.…”

“…31,32 They can also be generated by the reactions of phosphine with chlorine (in various solvents), 33,34,35,36 and phosphine oxide with sulfuryl 45 chloride, methanesulfonyl chloride, thionyl chloride or trichloromethyl chloroformate. 30,32 The structure of the entity produced seems to depend heavily on the solvent. It adopts a chlorophosphonium chloride structure in acetonitrile and chlorinated solvents, 34 and a dichlorotriphenylphosphorane 50 structure in benzene and diethyl ether, 34 although it solubility in the latter solvent is poor, especially at low temperature.…”

Identification of a key intermediate in the asymmetric Appel process: one pot stereoselective synthesis of P-stereogenic phosphines and phosphine boranes from racemic phosphine oxides

“…We were intrigued to discover that it had been known for a long time that they can be easily obtained from the phosphine oxide by reaction with oxalyl 70 chloride. 14 Recently we utilised this reaction in a novel direct (achiral) conversion of phosphine oxides to the corresponding boranes, 15,16 and Denton and co-workers have also used it in their catalytic Appel reaction. 17 When treated with chiral alcohol (at -78 ˚C), these independently generated CPS indeed yielded the 75 same diastereomeric alkoxyphosphonium salts as in the asymmetric Appel reaction (DAPS-B, Scheme 2).…”

“…The use of enantiomerically pure phosphine ligands in 15 asymmetric catalysis is a popular strategy for asymmetric synthesis 1 and much effort has been directed towards the design, synthesis and testing of new enantiomerically pure phosphines. 2 Several methodologies have been developed for the synthesis of enantiomerically pure P-stereogenic phosphines 3 and a large 20 number of such ligands have been reported in the literature.…”