Reduction of secondary and tertiary phosphine oxides to phosphines

Abstract

Achiral or chiral phosphines are widely used in two main domains: ligands in organometallic catalysis and organocatalysis. For this reason, the obtention of optically pure phosphine has always been challenging in the development of asymmetric catalysis. The simplest method to obtain phosphines is the reduction of phosphine oxides. The essential difficulty is the strength of the P=O bond which involves new procedures to maintain a high chemio- and stereoselectivity. The reduction can occur with retention or inversion of the stereogenic phosphorus atom depending on the nature of the reducing agent and the presence of additives. In fact, the reactivity of the phosphine oxides and the mechanism of the reduction are not always well understood. Since the first work in the 1950's, numerous studies have been realised in order to develop methodologies with different reagents or to understand the mechanism of the reaction. In the last decade, efficient stereospecific methodologies have been developed to obtain optically pure tertiary phosphines from P-stereogenic phosphine oxides. In this review, we intend to provide a comprehensive and critical overview of these methodologies.

Chemoselective Reduction of Phosphine Oxides by 1,3‐Diphenyl‐Disiloxane

“…[1] Reduction of stable phosphine(V) oxides remains the most straightforward preparation of phosphines, and is often essential for the construction of easily-oxidized electron-rich phosphines. [3] Unfortunately, current methods are extremely harsh, which limits the substrate scope and precludes reaction telescoping. [4] …”

Phosphine functionalized polyphosphazenes: soluble and re-usable polymeric reagents for highly efficient halogenations under Appel conditions

“…Secondly, atom efficiency has also been a concern, and improvements in this regard have recently gained increasing attention. The two basic approaches here are the recovery of the initial phosphine by reductive methods [22], which often requires hazardous reagents and is not always efficient or the direct re-use of the phosphine oxides as reagents or catalysts [23][24][25][26][27].…”

Metal-Free Reduction of Phosphine Oxides Using Polymethylhydrosiloxane

“…Using more reactive silanes proved to be beneficial and excellent conversions and yields were obtained by the group of Werner using hexyl silane as the reducing agent and triflic acid as the catalyst [17]. The most commonly used reductants for P=O bonds include silanes, boranes and metal hydrides based on Al and Ca [7]. Among silanes, as early as 1964, Fritzsche, et al [8][9][10] reported that phenylsilane, diphenylsilane, triphenylsilane and PMHS (polymethylhydrosiloxane, Scheme 1) could be used in excess stoichiometry to reduce alkyl, aryl phosphine oxides and dihydrophosphole oxides (Scheme 2).…”

“…The most commonly used reductants for P=O bonds include silanes, boranes and metal hydrides based on Al and Ca [7]. Among silanes, as early as 1964, Fritzsche, et al [8][9][10] reported that phenylsilane, diphenylsilane, triphenylsilane and PMHS (polymethylhydrosiloxane, Scheme 1) could be used in excess stoichiometry to reduce alkyl, aryl phosphine oxides and dihydrophosphole oxides (Scheme 2).…”

“…Phosphine oxides are also generated in large quantities as by-products of industrial scale applications such as the Wittig reaction [5] or in water-phase catalysis [6]. Even though many chemical methods that reduce R 3 P=O to R 3 P are known in the literature [7], they usually need harsh reaction conditions, stoichiometric amounts of toxic or dangerous reagents and are often scarcely functional-group tolerant. Limited solubility of phosphine oxides and cumbersome separation of products from reagents are often discouraging for widespread applications.…”