SegPhos

Iridium-catalyzed transfer hydrogenation of aldehyde 73 in the presence of 1,1-dimethylallene promotes tert-prenylation [64] to form the secondary neopentyl alcohol 74. In this process, isopropanol serves as the hydrogen donor, and the isolated iridium complex prepared from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and (S)-SEGPHOS is used as catalyst. Complete levels of catalyst-directed diastereoselectivity are observed. Exposure of neopentyl alcohol 74 to acetic anhydride followed by ozonolysis provides p-acetoxy aldehyde 75. Reductive coupling of aldehyde 75 with allyl acetate under transfer hydrogenation conditions results in the formation of homoallylic alcohol 76. As the stereochemistry of this addition is irrelevant, an achiral iridium complex derived from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and BIPHEP was employed as catalyst (Scheme 5.9). 

The in situ generation of CuH from organosilanes in the presence of either a BIPHEP (99) or a SEGPHOS (100) type ligand represents a general method for the asymmetric hydrosilylation of aryl alkyl ketones at low temperatures. 

The chiral hydrosilylation of -substituted a,/3-unsaturated esters to their saturated counterparts is the subject of reports by two groups. The combination of triphenylphosphinecopper hydride and () )-DTBM-SEGPHOS is reported to give excellent yields of the -substituted esters (Eq. 353).598 Comparable yields, but with lower ee values, are reported for this transformation.599 600... 

The copper-catalyzed chiral reduction of -substituted ,/Tunsaturated lactones with PMHS and (S)-/ -Tol-BINAP in the presence of a hindered alcohol can be carried out in moderate to good yields with moderate ee values.599 The reaction is useful for both butenolides and pentenolides. Inferior results are realized with diphenylsilane as the reducing agent. Excellent results employing PMHS and the DTBM-SEGPHOS ligand are possible (Eq. 354).598... 

Vinylsilane to copper transmetallation has entered the literature,93 93a,93b and a system suitable for catalytic asymmetric addition of vinylsilanes to aldehydes was developed (Scheme 24).94 A copper(l) fluoride or alkoxide is necessary to initiate transmetallation, and the work employs a copper(ll) fluoride salt as a pre-catalyst, presumably reduced in situ by excess phosphine ligand. The use of a bis-phosphine was found crucial for reactivity of the vinylcopper species, which ordinarily would not be regarded as good nucleophiles for addition to aldehydes. The highly tailored 5,5 -bis(di(3,5-di-tert-butyl-4-methoxyphenyl)phosphino-4,4 -bis(benzodioxolyl) (DTBM-SEGPHOS) (see Scheme 24) was found to provide the best results, and the use of alkoxysilanes is required. Functional group tolerance has not been adequately addressed, but the method does appear encouraging as a way to activate vinylsilanes for use as nucleophiles. 

An interesting approach to investigating the relationship between the position of enantiodescriminating sites in a number of chiral ligands and enantioselec-tivity in enantioselective hydrogenation has been proposed by Saito et al. [50]. In this report, (aS,S,S)-MPL-SEGPHOS (21) was used for the reduction of... 

---