Asymmetric hydrogenation phospholane ligands

Very recently, chiral 6-DPPon derivatives, the phospholanes 4b and 5b and the phosphepine 6b, have been prepared and studied as ligands in asymmetric hydrogenation (Figure 2.4) [14]. For comparison, the corresponding 2-alkoxypyridine systems 4a-6a were studied, too. The latter should behave as truly monodentate ligands while the pyridine systems 4b-6b should allow for complementary hydrogenbonding. 

In addition to hydrogenation reactions, modular phospholane ligands are being applied in a growing rank of other useful asymmetric catalytic transformations. For instance, Jiang and Sen reported the discovery of a dicationic Me-DuPhos-Pd catalyst for the alternating copolymerization of aliphatic a-olefins and carbon monoxide (Scheme 13.21).67... 

A chiral ligand system based on C2-symmetric chiral bis(phospholanes) (5 and 6) has shown to be a powerful ligand in the asymmetric hydrogenation of various substrates that include enamides, enol acetates, C=N bond reduction, and P-keto esters. Detailed discussions of this class of ligands are included Chapter 18. 

Chiral Phospholane Ligands for Rh-Catalyzed Asymmetric Hydrogenation... 

Table 9.1 Rh catalyzed asymmetric hydrogenation of model substrates with phospholane ligands., R...

Since the diphosphine is appreciably more electron-rich than is BINAP, the major ruthenium complex is a more active hydrogenation catalyst than the parent. Increased electron-rich ligation may be the reason for the success of heterocyclic analogues of BINAP in which the binaphthalene is replaced by a bi(ben-zothiophene) or biindolyl the resulting Ru complexes are effective both in terms of enantioselectivity and reactivity [139]. Readers of the related Chapter 6.1 on the asymmetric hydrogenation of carbonyl compounds will encounter the Ru complexes of ligands in the DUPHOS family, where the ease of modification of the alkyl substituents of the phospholane enhances the power of the system, since it permits the easy optimization of ee for any substrate [140]. 

The cationic chiral catalyst [RhCcodlC )]" showed high activity and selectivity in the presence of C02-philic counterions such as the BARF or triflate anion (Scheme 4.7-3) [21]. The DuPHOS-type ligand 7 is a fairly nonpolar alkyl phospholane and is especially useful in asymmetric hydrogenation of dehydroamino acids in conventional solvents [38]. The enantioselectivities in SCCO2 compared well to those in methanol for most substrates, and the asymmetric induction was considerably higher for p,p-disubstituted substrates (Table 4.7-2). 

We therefore focused on the development of a modular synthesis of bi- and tridentate chiral phospholane ligands and selected the 2,5-dimethyl (DMP) and 2,5-diphenyl (DPP) substituted phospholanes as stereodirecting groups, as these two privileged chiral units had shown remarkable reactivity and enantioselectivi-ties with special impact in catalytic asymmetric hydrogenations [66]. 

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