Chiral bisphosphites

Asymmetric versions of the cyclopropanation reaction of electron-deficient olefins using chirally modified Fischer carbene complexes, prepared by exchange of CO ligands with chiral bisphosphites [21a] or phosphines [21b], have been tested. However, the asymmetric inductions are rather modest [21a] or not quantified (only the observation that the cyclopropane is optically active is reported) [21b]. Much better facial selectivities are reached in the cyclopropanation of enantiopure alkenyl oxazolines with aryl- or alkyl-substituted alkoxy-carbene complexes of chromium [22] (Scheme 5). 

Rhodium (I) complexes of chiral phosphines have been the archetypical catalysts for the hydrocarbonylation of 1-alkenes, with platinum complexes such as (61) making an impact also in the early 1990s[1461. More recently, rhodium(I)-chiral bisphosphites and phosphine phosphinites have been investigated. Quite remarkable results have been obtained with Rh(I)-BINAPHOS (62), with excellent ee s being obtained for aldehydes derived for a wide variety of substrates1 471. For example, hydroformylation of styrene gave a high yield of (R)-2-phenylpropanal (94% ee). The same catalyst system promoted the conversion of Z-but-2-ene into (5)-2-methylbutanal (82% ee). 

Ir(cod)Cl]2> reactions afforded enantiomerically enriched cyclopentenones when (S)-tolBINAP was employed as ligand. The corresponding cyclopentenones were obtained in yields up to 85% and ee values ranging from 82 to 98%. Interestingly, Co2(CO)8 in combination with a chiral bisphosphite also gives access to chiral Pauson-Khand products [66]. Here, yields were observed up to 97% however, in most cases the ee was rather low (< 20%). Equation 11 summarizes some representative examples of enantioselective PKR. 

At this stage, Rh(I) catalysts became the most promising candidates for the asymmetric hydroformylation in comparison with Pt(II) catalysts. In 1992, the authors reported the use of a chiral bisphosphite derived from binaphthol for the... 

The historic discovery of Rh complexes of chiral bisphosphites and phosphine-phosphites dramatically raised the enantioselectivities of asymmetric hydrocarbonylation from -50% ee to almost quantitative values in the first half of the 1990s. The successes with Rh catalysts seemed to replace the earfier used Pt catalysts which often suffered from extensive side reactions such as hydrogenation and isomerization, and low selectivity to fso-aldehydes. At this stage, asymmetric hydroformylation has reached the level of enantioselectivity of asymmetric hydrogenation, the most studied asymmetric reaction. 

Bisphosphite ligands were originally discovered at Union Carbide and have been extensively studied for olefin hydroformylation. Structural modification of bisphosphite ligands allows their use in asymmetric catalysis. These chiral bisphosphites lead to the highest combination of branched regioselectivity (b/1) and enantioselectivity (%ee) of any asymmetric hydroformylation catalyst reported to date. These catalysts have been applied to a highly efficient synthesis of... 

Naproxen. In addition, the use of chiral bisphosphites in asymmetric olefin hy-drocyanation is described. Details of the structural features responsible for hi b/1 and %ee in asymmetric hydroformylation and hydrocyanation are presented. 

Fig. 2.10 The ligand universe A stunning breadth of structural variation with members representing various motifs, e.g. atropisomeric biaryls, ferrocene-based bisphosphanes, P-chiral bisphosphanes, bisphosphites, monodentate phosphoramidites, AJ,P-ligands.

The effects of bisphosphite ligand structure on regioselectivity and enantio-selectivity in asymmetric styrene hydroformylation are shown in Table 1. Catalytic reactions were preformed at ambient temperature and 130 psi CO/H2. Hydroformylation regioselectivity was determined by GC of the product aldehydes. Enantioselectivity was determined by chiral GC after conversion to the carboxylic acid (eqn 1). The, i -enantiomer of the bisphosphites in Figure 1 all produced the... 

Chiral bulky bisphosphite ligands with C2 symmetry such as 44 were claimed to lead to a high, 50 1 = iso normal ratio and up to 90% enantiomeric excess of the 2-phenyl-propionaldehyde product in rhodium-catalyzed styrene hydroformylation (272). 

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