Reduction chirally modified hydride reagents

Reviews on chiral hydride reagents M. Nishizawa and R. Noyori, Reduction of C=X to CHXH by Chirally Modified Hydride Reagents, in B. M. Trost and I. Fleming, eds., Comprehensive Organic Synthesis, Vol. 8, Chap. 1, p. 159, Per-gamon Press, Oxford, 1991 V. K. Singh, Synthesis, 605 (1992). 

Reduction of C=X to CHXH by Chirally Modified Hydride Reagents... 

Nishizawa M, Noyori R (1991) Reduction of C=X to CHXH by chirally modified hydride reagents. In Trost BM, Fleming I (eds) Comprehensive organic synthesis Pergamon, Oxford New York Seoul Tokyo, p 159... 

In 1979, Noyori and co-workers invented a new type of chiral aluminum hydride reagent (1), which is prepared in situ from LiAlEE, (S)-l, E-bi-2-naphthol (BINOL), and ethanol. The reagent, called binaphthol-modified lithium aluminum hydride (BINAL-H), affects asymmetric reduction of a variety of phenyl alkyl ketones to produce the alcohols 2 with very high to perfect levels of enantioselectivity when the alkyl groups are methyl or primary1 (Scheme 4.3a). 

In 1951 Bothner-By first attempted asymmetric reductions based on the conversion of lithium aluminum hydride (LAH) into a chiral alkoxy derivative by reaction with (+)-camphor. Since this pioneering work, the use of chirally modified LAH reagents has been the focus of much attention. In 1979, the first virtually complete enantiofacial recognition of prochiral carbonyl compounds was accomplished by using LAH modified with optically pure 2,2 -dihydroxy-1,1 -binaphthyl and a simple alcohol (BINAL-H). Asymmetric reduction with chiral 2,5-dimethylborolane also gave alcohols in high optical yields." Recently, excellent results have been obtained using a chirally modified sodium borohydride... 

Asymmetric Carbonyl Reductions. Examples of hydride reagents modified with a chiral auxiliary published during the past year Include... 

One approach to enantioselective reduction of prochiral carbonyl compounds is to utilize chiral ligand-modified metal hydride reagents. In these reagents, the number of reactive hydride species is minimized in order to get high chemo-selectivity. Enantiofacial differentiation is due to the introduced chiral ligand. 

Asymmetric reduction of ketones or aldehydes to chiral alcohols has received considerable attention. Methods to accomplish this include catalytic asymmetric hydrogenation, hydrosilylation, enzymatic reduction, reductions with biomimetic model systems, and chirally modified metal hydride and alkyl metal reagents. This chapter will be concerned with chiral aluminum-containing reducing re-... 

As a consequence of the wide choice of hydride reagents the classical methods such as reduction with sodium in ethanol almost fell into oblivion [579, 520]. Nevertheless some old reductions were resuscitated. Sodium di-thionite was found to be an effective reducing agent [262], and the reduction by alcohols [309] was modified to cut down on the temperature [755] or the time required [527], or to furnish chiral alcohols ( in good yields and excellent optical purity ) by using optically active pentyl alcohol and its aluminum salt [522]. Formation of chiral alcohols by reduction of pro-chiral ketones is... 

In particular, reduction of unsymmetric ketones to alcohols has become one of the more useful reactions. To achieve the selective preparation of one enantiomer of the alcohol, chemists first modified the classical reagents with optically active ligands this led to modified hydrides. The second method consisted of reaction of the ketone with a classical reducing agent in the presence of a chiral catalyst. The aim of this chapter is to highlight one of the best practical methods that could be used on an industrial scale the oxazaborolidine catalyzed reduction.1 1 This chapter gives an introductory overview of oxazaborolidine reductions and covers those of proline derivatives in-depth. For the oxazaborolidine derivatives of l-amino-2-indanol for ketone reductions see Chapter 17. 

The enantioselective reduction of unsymmetrical ketones to produce optically active secondary alcohols has been one of the most vibrant topics in organic synthesis.8 Perhaps Tatchell et al. were first (in 1964) to employ lithium aluminum hydride to achieve the asymmetric reduction of ketones9 (Scheme 4.IV). When pinacolone and acetophenone were treated with the chiral lithium alkoxyaluminum hydride reagent 3, generated from 1.2 equivalents of 1,2-0-cyclohexylidene-D-glucofuranose and 1 equivalent of LiAlHzt, the alcohol 4 was obtained in 5 and 14% ee, respectively. Tatchell improved the enantios-electivity in the reduction of acetophenone to 70% ee with an ethanol-modified lithium aluminum hydride-sugar complex.10... 

Although -3-pinanyl-9-borabicyclo[3.3.1]nonane and related substances have also been developed as efficient asymmetric reducing agents for carbonyl compounds (Volume 8, Chapter 1.3), we discuss here only asymmetric reductions using chirally modified metal hydride reagents. The asymmetric hydrosilyl-ation of a carbonyl group catalyzed by a chirally modified transition metal is mentioned briefly. 

Rational designing of efficient chiral reducing agents. Highly enantioselective reduction of aromatic ketones by binaphthol-modified lithium aluminium hydride reagents, R.Noyorl, I.Tomino, Y.Tanimoto and M.Nishizawa, J. Am. Chem. Soc., 1984, 106, 6709. 

Pioneers in the asymmetric reduction area developed suitable reagents by modifying lithium aluminum hydride (LAH) (7) or by developing chiral versions of Meerwdn-Ponndorf-Verley reductions with chiral aluminum alkoxides (8) or chiral Giignard reductions with chiral alkyl magnesium halides (9). 

Borane and aluminum hydrides modified by chiral diols or amino alcohols are well-known, effective reagents for the stoichiometric enan-tioselective reduction of prochiral ketones and related compounds (34). Reduction of prochiral aromatic ketones with the Itsuno reagent, which is prepared from a chiral, sterically congested /3-amino alcohol and borane, yields the corresponding secondary alcohols in 94-100% ee... 

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