Polydentate Metal Complexes and Asymmetric Syntheses

Over the past 25 years there has been a marked trend towards marketing pharmaceuticals (and pesticides) as enantiometrically pure compounds. Desired bioeffects are almost always produced by only one isomer, and the other is usually unwanted ballast. This can sometimes produce undesirable side effects. 

Enantioselective catalysts are currently much sought after, even though they are usually expensive because of their complexity, and the expensive metals they usually contain (e.g. Rh, Re, Ru, Ir, 

Ni and W). In addition, desirable catalyst/snbstrate ratios are needed and much work has gone into their recovery and re-cycling. Enantiometric phosphorus compounds are today prominent in this field of activity and some typical ones are (12.372). Some degree of catalytic activity has been reported in many diverse reactions but information is now widely scattered in the literature [38,54]. 

Polydentate polyphosphine metal complexes freqnently show greater catalytic activity than the monophosphine-type complexes so far discussed. In addition, asymmetric polydentate complexes (whether the asymmetry arises on the P atom or another atom) may catalyse asymmetric syntheses, that is, the production of a single optical isomer, sometimes of nearly 100% optical purity (Chapter 13.2). In the presently known field of asymmetric catalytic ligands, P componnds figure prominently and are of ever-growing importance. BINAP (12.372d) represents a family of such ligands, many of which have C2 symmetry (12.373). 

Over the past decade, many efficient catalysts have been derived from cyclometallated pincer-type ligands (8.144) [55]. 

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