Phosphorus prochiral

A stereochemical property of compounds arising from the ability of an enzyme s active site to distinguish between two chemically identical substituents covalently bound to a tetrahedral center (usually carbon and, in some cases, phosphorus). Prochirality is also termed prostereoisomerism. The classical example is citrate with its two carboxymethyl group substituents. Likewise, the Cl carbon atom of ethanol has two prochiral hydrogens. See Chirality Chirality Probes... 

In general, of the mixed phosphorus-thioether ligands that have been used in the asymmetric hydrogenation of prochiral olefins, the thioether-phosphinite ligands have provided some of the best results. As an example, a new class of thioether-phosphinite ligands developed by Evans et al. has recently proved to be very efficient for the rhodium-catalysed asymmetric hydrogenation of a... 

The development of chiral phosphorus ligands has made undoubtedly significant impact on the asymmetric hydrogenation. Transition metal catalysts with efficient chiral phosphorus ligands have enabled the synthesis of a variety of chiral products from prochiral olefins, ketones, and imines in a very efficient manner, and many practical hydrogenation processes have been exploited in industry for the synthesis of chiral drugs and fine chemicals. 

Although the latter product is a solvated mononuclear [Rh(MeOH)2(diphos)]+ cation, in the solid state it is isolated as a binuclear complex of formula [Rh2 (diphos)2](BF4)2, in which each rhodium center is bonded to two phosphorus atoms of a chelating bis(diphenylphosphino)ethane ligand, and to a phenyl ring of the bis(diphenylphosphino)ethane ligand of the other rhodium atom. This dimer reverts to a mononuclear species on redissolving. The mechanism of hydrogenation of the prochiral alkene methyl(Z)-a-acetamidocinnamate, studied in detail by Halpern [31], is depicted in Scheme 1.7. 

Scheme 4.15 Desymmetrization products of some phosphorus containing prochiral diols.

It must be pointed out than, in the case of phosphonium salts in which the phosphorus atom is a prochiral centre, attempted alkaline hydrolyses with asymmetric induction... 

This process is stereospecific and has the advantage of giving a quantitative conversion. Although AMP, the usual acceptor substrate for adenylate kinase, is achiral at phosphorus and so not subject to stereospecific phosphorylation, the phosphorus in AMPS is a prochiral center. Adenylate kinase catalyzes the phosphorylation of the pro-R oxygen exclusively to produce the (Sp) configuration. 

These compounds are important in asynunetric catalysis, in which a prochiral substrate is converted into a chiral product. There are basically three types of chiral phosphines. Firstly, there are phosphines of the type PRR R", where the chiral center is the phosphorus atom. Secondly, the substituent(s) or the molecule as a whole may be chiral. Finally, a coordinated phosphine complex may be chiral at the metal center. 

The synthesis of various new chiral (o-hydroxyaryl)oxazaphospholidine oxides (139), derived from (S)-proline derivatives, from precursors (140) have been elaborated. This two-step reaction involves an unstable metallated intermediate that undergoes a fast 1,3-rearrangement with the formation of phosphorus-carbon bond. These catalysts have been successfully applied to the catalytic asymmetric borane reduction of numerous prochiral ketones with enantiomeric excess up to 84% ee (Scheme 35). ... 

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