PennPhos

The PennPhos ligands, for example 108, complexed with rhodium, provide an excellent system for the hydrogenation of aryl alkyl ketones with ee values in the range of 94-96% (Eq. 362). Phenyl isopropyl ketone shows only a 72% ee under similar conditions. Dialkyl ketones exhibit ee values in the range of 73-94% with this system (Eq. 363).640... 

DuPhos. The exception for rhodium-catalyzed reductions are CnrPhos and BPE-4 [168, 264—268]. MalPhos has proven useful for the reductions of yS-acylamino-acrylates [260]. The ferrocene hybrid (FerroTANE) was referred to earlier (see Section 23.4.1) [167, 222]. The PennPhos ligand is useful for the reductions of cyclic enamides and enol acetates both classes of compounds are difficult for DuPhos itself to reduce with high selectivity [269, 270]. 

Catalyst performance was far superior to the corresponding BINAP or Me-Du-Phos systems, with both conversions and selectivities being higher. The hydrogenation of enol ethers using Rh-PennPhos catalysts has been reported in a patent by Zhang [67d]. Under mild conditions, high enantioselectivities were obtained (73-94% ee) for 1-aryl-l-methoxy-ethene derivatives 121, compared to Me-DuPhos (40-73% ee) and BINAP (46-48% ee). 

Jiang et al.4 have recently succeeded in hydrogenating both aryl alkyl and dialkyl ketones. High enantioselectivity was obtained using PennPhos (19)-coordinated Rh complex as the catalyst. This success is based on the finding that a weak base (such as 2,6-lutidine) can facilitate the Rh-catalyzed hydrogenation of simple ketones (Scheme 6 35). 

TABLE 6-8. Asymmetric Hydrogenation of Simple Ketones Catalyzed by Rh-PennPhos... 

The success of Burk s alkyl diphosphines spurred development of a number of other ligands with electron rich phosphines, such as Zhang s PennPHOS (7) [36-38], Marinetti s /Pr-CnrPHOS (8) [39], and Imamoto s BisP ligands (9) [40],... 

A Rh complex with (/f.S. /f.S -Me-PennPhos efficiently catalyzes asymmetric hydrogenation of simple ketones (Figure 1.22). Addition of catalytic amounts of 2,6-lutidine is crucial to achieve high enantioselectivity. This catalyst is also... 

Figure 1.22. As3mimetric hydrogenation of simple ketones catalyzed by a PennPhos-Rh complex.

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