Sodium borohydride chirally modified

Nonmetallic systems (Chapter 11) are efficient for catalytic reduction and are complementary to the metallic catalytic methods. For example lithium aluminium hydride, sodium borohydride and borane-tetrahydrofuran have been modified with enantiomerically pure ligands161. Among those catalysts, the chirally modified boron complexes have received increased interest. Several ligands, such as amino alcohols[7], phosphino alcohols18 91 and hydroxysulfoximines[10], com-plexed with the borane, have been found to be selective reducing agents. 

Isosorbide (3) and isomannide (4) act as chiral auxiliaries for the sodium borohydride reduction of some prochiral ketones optical yields of up to 20% were achieved. It seems that the isohexides form chiral complexes with sodium borohydride, whereby the chiral information is transferred to the substrate.219 Optical active alcohols were obtained by reduction of appropriate ketones with sodium or lithium borohydride in the presence of isosor-bide.219 Asymmetric reduction of propiophenone using sodium borohydride, modified with (+)-camphoric acid and isosorbide, resulted in C -phenylethylcarbinol in 35% enantiomeric excess.2,9b... 

A combination of chiral cobalt-catalyst and sodium borohydride was successfully applied to the asymmetric reduction of aromatic ketones. A chiral cobalt complex 164 (5 mol%), prepared from the corresponding salen-type chiral bisketoaldimine and cobalt(II) chloride, catalyzed the reduction of dimethylchromanone 165 in the presence of sodium borohydride (1.5 equiv to ketone) in chloroform, including a small amount of ethanol at -20°C for 120 h to give alcohol 166 92% ee (S ) in 94% yield (Scheme 2.18) [94], Addition of tetrahydrofurfuryl alcohol (THFFA) to the reaction system or the use of pre-modified borohydride, NaBH2(THFFA)2, improved the catalyst activity, that is, using this protocol, the reactions of ketone 165 and... 

The oxazolidin-2-ones 53 (R = H=CCH=CH2 or COEt) are obtained in a one-pot reaction of amino alcohol carbamates 52 with sodium hydroxide, followed by allyl bromide or propi-onyl chloride (94TL9533). A modified procedure for the preparation of chiral oxazolidin-2-ones 56 from a-amino acids 54, which avoids the hazardous reduction of the acids with borane and the intermediacy of water-soluble amino alcohols, is treatment of the methyl ester of the amino acid with ethyl chloro-formate to give 55, followed by reduction with sodium borohydride and thermal ring-closure of the resulting carbamate f95SC561). The 2-prop-ynylcarbamates 57 (R = Ts, Ac, Bz, Ph or allyl) cyclize to the methyleneoxazolidinones 58 under the influence of silver cyanate or copper(I) chloride/triethylamine (94BCJ2838). 

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... 

A suspension of sodium borohydride is essentially inert to chiral amino alcohols and is unable to reduce ketoxime O-alkyl ethers. On the other hand, when combined with a Lewis acid (e.g., ZrCb), sodium borohydride reacts with chiral amino alcohols with evolution of hydrogen to form a chiral borohydride reagent. This chirally modified borohydride has the ability to reduce the C —N double bond of ketone O-alkyl ethers to give chiral primary amines in 78-95% yield with 43-90% ee34. 

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