Borane reagent, chiral

The field of transition metal-catalyzed hydroboration has developed enormously over the last 20 years and is now one of the most powerful techniques for the transformation of C=C and C=C bonds.1-3 While hydroboration is possible in the absence of a metal catalyst, some of the more common borane reagents attached to heteroatom groups (e.g., catecholborane or HBcat, (1)) react only very slowly at room temperature (Scheme 1) addition of a metal catalyst M] accelerates the reaction. In addition, the ability to manipulate [M] through the judicious choice of ligands (both achiral and chiral) allows the regio-, chemo-, and enantioselectivity to be directed. 

Scheme 13) (35). This high selectivity can be obtained with the 1 2 amino alcohol-borane reagent, however, the 1 1 reagent is less reactive and affords only low stereoselectivity (36). The continuous-flow reaction using a polymer-bound amino alcohol provides evidence for the catalytic nature of the reduction with respect to the chiral ancillary. The reduction is accelerated by the presence of the amino alcohol-borane adduct, and the product is not bound to the complex. 

Hydrogenation of 2,2,2-trifluoroacetophenone and its derivatives with a mixture of trans-RuCl2[(S)-xylbinap][(S)-daipen] and (CH3)3COK in 2-propanol gives the S alcohols quantitatively with a high optical purity (Scheme 1,64) [258]. Unlike with many chiral borane reagents [264], the sense of enantioface discrimination is the same as in hydrogenation of acetophenone. The electronic effects of 4 -substituents on the enantioselectivity are small. These chiral fluorinated alcohols are useful as components of new functionalized materials [265]. 

Thought Experiments II and III on the Hydroboration of Chiral Alkenes with Chiral Boranes Reagent Control of Diastereoselec-tivity, Matched/Mismatched Pairs, Double Stereodifferentiation... 

The pioneering studies by Itsuno [1] and Corey [2] on the development of the asymmetric hydroboration of ketones using oxazaborolidines have made it possible to easily obtain chiral secondary alcohols with excellent optical purity [3]. Scheme 1 shows examples of Corey s (Corey-Bakshi-Shibata) CBS reduction. When oxazaborolidines 1 were used as catalysts (usually 0.01-0.1 equiv), a wide variety of ketones were reduced by borane reagents with consistently high enan-tioselectivity [2]. The sense of enantioselection was predictable. Many important biologically active compounds and functional materials have been synthesized using this versatile reaction [2-4]. 

Chiral borane reagents similar to those described above were independently reported by Yamamoto and co-workers in the same year (Eq. 2) [2]. These reagents promote the reaction of juglone and a variety of dienes in the synthesis of anthracycli-nones. The borane reagent used is trimethyl borate. Different tartrate derivatives have been tried and (i ,f )-(+)-tartaric acid diamide has proved to be the most efficient. The best results are obtained in the reaction of juglone and (triethylsiloxy)buta-... 

Chiral allylic boranes and chiral aldehydes. The possibilities for double diastereo-selection with allylic borane reagents was first demonstrated by Hoffmann in the combination of chiral 2-butenylboronates with chiral aldehydes [123]. Reaction of... 

Since the first asymmetric reduction of ketones with chiral borohydrides by Itsuno et al. [ 1 ], a number of studies on the asymmetric reduction of ketones with chiral borane reagents have been demonstrated [2]. Corey s oxazaborolidines are some of the most successful reagents [3 ]. The effect of fluorine substituents was examined in the asymmetric reduction of acetophenone with LiBH4 by the use of chiral boronates (73) obtained from substituted phenyl boronic acid and tartaric acid [4]. Likewise, 3-nitro, fluorine, and trifluoromethyl groups on the 3- or 4-position provided enhanced stereoselection (Scheme 5.20). 

Another Cz symmetric chiral borane reagent has teen utilized by Corey et alP for the construction of syn aldol products in a highly enantioselective manner. When an enolate prepared from 5-phenyl pro-... 

Highly enantioselective aldol reactions of diethyl ketone have been recorded with the use of the chiral borane reagent (1). As shown in Scheme A, syn aldol products of 95-98% ee have been obtained upon reaction with various aldehydes with 94-98% diastereoselectivities. 

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