Bi di,bis phosphines

Although they are often considered as poorer ligands than diphosphines, they lead also to very efficient and attractive enantioselective catalytic systems as exemplified here. As recent examples, diphosphinites 19 and 20 have been involved successfully in hydrogenation of olefins (mostly itaconate derivatives and enamides, up to 99.9 % ee) ([84-89] and functionalized ketones (21) (up to 86 % ee) [90], hydrocyanation (19) [91], standard Pd-mediated allylic alkylation (20) [92] (up to 86% ee) [93], and Diels-Alder reaction between a,/l-enals and dienes (eq. (4) 99 % ee) [94]. 

RajanBabu et al. has deeply explored the chemistry of carbohydrate phosphinite complexes [84, 95]. While the carbohydrate backbone provided the necessary stereochemical diversity, substitution patterns around phosphoms were used to vary the steric and electronic properties of the ligand. 

Many efforts are devoted to the design of new P(0)-containing biphosphine ligands. However, the most difficult task is still to reach very high enantioselectivities in the hydroformylation reaction, as well as high turnover numbers. 

Less electron-rich ligands could be applied efficiently in olefin hydrogenation (28) [118-122], Pd-catalyzed allylic alkylation (29) (up to 60% ee) [123], and hydroformylation (up to 77 % ee) [124]. 

Another major contribution of polydentate ligands is the creation of a chiral pocket around the catalytic center providing, an appropriate chiral environment. The chiral pocket concept has been introduced by Trost for catalytic enantioselective allylic alkylation with the tetradentate aminophosphine ligand 33 [133]. The nucleophile fits into the chiral environment created by the chiral ligand and the allyl Pd intermediate. As a result, the enantiocontrol of the newly formed chiral center is very effective. In addition, the chiral control is likely to be efficient even at positions remote from the chiral ligand. That auxiliary has been widely 

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