Phosphine redox catalysis

Phosphines play a vital role as promoters in many transformations classically used in organic chemistry, in which the phosphine is usually oxidized (Wittig, Mitsunobu, Staudinger reactions, etc.). While these reactions are highly efficient and used daily in academic research laboratories and industry for the formation of carbon-carbon and carbon-heteroatom bonds, they also generate large quantities of chemical waste, notably phosphine oxide. This by-product often complicates the purification stage. To reduce the amount of phosphine used, several researchers around the world have developed P(III)/P(V) redox catalytic processes involving in situ regeneration of trivalent phosphine. Silanes, which can also be waste products from the silicone industry, are used as reducing agents. So it is now possible to move towards a more atom-efficient chemistry, and thereby reduce the environmental impact and energy demand of certain chemical reactions. In this article, after detailing the first catalytic Wittig reaction to phosphine, all the operating conditions required for a successful transformation are outlined. Subsequently, several redox catalytic processes using phosphines are described and presented by major class of reactions: Wittig olefination and its derivatives, Mitsunobu (nucleophilic substitution on an alcohol), Appel halogenation, Staudinger (transformation of an azide into an amine or formation of amides). Examples of organic synthesis, including the total synthesis of natural products such as (S)-vasicinone and dichrocephones A and B, are presented. Finally, in cases where the products formed have stereogenic centers, it is possible to use a small amount of chiral phosphines, and thus obtain optically active products by asymmetric catalysis, including chiral cyclobutenes and complex tricyclic nitrogen compounds.

At the end of the article, readers will find a glossary and a table of the notations used.