Oximes, borane reduction

Organometallic compounds asymmetric catalysis, 11, 255 chiral auxiliaries, 266 enantioselectivity, 255 see also specific compounds Organozinc chemistry, 260 amino alcohols, 261, 355 chirality amplification, 273 efficiency origins, 273 ligand acceleration, 260 molecular structures, 276 reaction mechanism, 269 transition state models, 264 turnover-limiting step, 271 Orthohydroxylation, naphthol, 230 Osmium, olefin dihydroxylation, 150 Oxametallacycle intermediates, 150, 152 Oxazaborolidines, 134 Oxazoline, 356 Oxidation amines, 155 olefins, 137, 150 reduction, 5 sulfides, 155 Oxidative addition, 5 amine isomerization, 111 hydrogen molecule, 16 Oxidative dimerization, chiral phenols, 287 Oximes, borane reduction, 135 Oxindole alkylation, 338 Oxiranes, enantioselective synthesis, 137, 289, 326, 333, 349, 361 Oxonium polymerization, 332 Oxo process, 162 Oxovanadium complexes, 220 Oxygenation, C—H bonds, 149... 

D. The use of chiral oxazaborolidines as enantioselective catalysts for the reduction of prochiral ketones, imines, and oximes, the reduction of 2-pyranones to afford chiral biaryls, the addition of diethylzinc to aldehydes, the asymmetric hydroboration, the Diels-Alder reaction, and the aldol reaction has recently been reviewed.15b d The yield and enantioselectivity of reductions using stoichiometric or catalytic amounts of the oxazaborolidine-borane complex are equal to or greater than those obtained using the free oxazaborolidine.13 The above procedure demonstrates the catalytic use of the oxazaborolidine-borane complex for the enantioselective reduction of 1-indanone. The enantiomeric purity of the crude product is 97.8%. A... 

The in situ generated catalyst from ATBH and trimethyl borate has also been used in the stereoselective reduction of a-oxoketoxime ethers to prepare the corresponding chiral 1,2-amino alcohols. Thus the asymmetric borane reduction of buta-2,3-dione monoxime ether followed by acidic work-up and subsequent reaction with benzyloxycarbonyl chloride affords a 90% yield of 7V-(Z)-3-aminobutan-2-ol with excellent enantioselectivities (eq 5). A trityl group in the oxime ether is required for high enantioselectivity. This method has been successively applied to both cyclic and acyclic a-oxoketoxime ethers. 

Based on a comparative study for asymmetric reduction of oxime ethers of acetophenone and 2-hepitanone using different classes of OABs, such as la, 2a, and 6a, Cho and Ryu reported the 6a-induced borane reduction of ketoxime O-trimethylsilyl ethers in moderate to good enantioselectivities (Scheme 11.19) [83a, bj. Similarly, ben-zylic amine derivatives were obtained with high enantioselectivities by stoichiometric reduction of oxime ethers using ent-6a-BHj or la-BH (Scheme 11.20) [84-86],... 

DFT has been used to investigate the mechanism of enantioselective borane reduction of -acetophenone 0-methyl oxime, using a stable chiral spiroborate ester. ... 

DFT investigation of two reaction pathways in the borane reduction of the ( )-acetophenone O-Me oxime catalysed by spiroborate ester at the B3LYP/6-31G(d,p) level of theory indicates that the reaction is accomplished via four steps. The reactants, products, transition states and intermediates are optimized at the B3LYP/6-31G(d,p) level. The solvent effect considered at the B3LYP/6-31G(d,p) level using the PCM indicates that THF has no great influence on the enantioselectivity of this reduction. ... 

Oximes are generally reduced to amines (19-44), ° but simple reduction to give hydroxylamines can be accomplished with borane or sodium cyanoborohy-dride. ... 

In the asymmetric reduction of ketones, stereodifferentiation has been explained in terms of the steric recognition of two substituents on the prochiral carbon by chirally modified reducing agents40. Enantiomeric excesses for the reduction of dialkyl ketones, therefore, are low because of the little differences in the bulkiness of the two alkyl groups40. In the reduction of ketoxime ethers, however, the prochiral carbon atom does not play a central role for the stereoselectivity, and dialkyl ketoxime ethers are reduced in the same enantiomeric excess as are aryl alkyl ketoxime ethers. Reduction of the oxime benzyl ethers of (E)- and (Z)-2-octanone with borane in THF and the chiral auxiliary (1 R,2S) 26 gave (S)- and (R)-2-aminooctane in 80 and 79% ee, respectively39. 

O-linked polymer-bound Af-substituted hydroxylamines are prepared by reduction of resin-bound oximes with borane-pyridine complex in the presence of dichloroacetic acid (Scheme 94). Other reducing systems commonly used for imine or oxime reduction are ineffective, including borane-pyridine in the presence of acetic acid. Subsequently, the A-substituted products are acylated and cleaved from the resin to afford Af-substituted hydroxamic acids 220. ... 

Enantioselective reduction of oxime ethers promoted by chiral spiroborate esters (10) with an O3BN framework is reported. In the presence of (R,S)-10, aralkyloxime ethers are reduced by borane-THF at give (S)-l-aralkylamine in high yield and excellent enatiomeric excess (up to 98% ee). A possible mechanism (Scheme 13) of the catalytic reduction is suggested.310... 

Enantioselective reduction of ketones by boranes and an enantiomeric catalyst oxazaboro-lidine (the CBS catalyst) is known as the Corey, Bakshi and Shibata method . Both enantiomers of 2-methyl-CBS-oxazaborolidine (6.52 and 6.53) are used for the reduction of prochiral ketones, imines and oximes to produce chiral alcohols, amines and amino alcohols in excellent yields and enantiomeric excesses. 

Application as a Component of an Asymmetric Catalyst. Amino alcohol (1) has proven to be a highly versatile ligand for use in asymmetric catalysts for a series of reactions. One of the most comprehensively studied uses is as an ox-azaborolidine derivative such as 8 for the asymmetric control of the reduction of ketones by borane. Although its use was first described with stoichiometric levels of 1 being employed for the reduction of both ketones and oximes, development of the system has delivered a catalytic method requiring only 5-10 mol % catalyst. Enantiomeric excesses of over 85% and as high as 96% have been achieved for a range... 

Enantioselective Reduction of Oxime 0-Ethers. In addition to the reduction of prochiral ketones, oxazaborolidine (3) has been used (both stoichiometrically and catalytically with borane-THF) to catalyze the enantioselective reduction of prochiral ketoxime O-ethers to the corresponding amine (eq Unlike the ketone reduction described above, the... 

Several other 1,3,2-oxazaborolidines have been successfully used as chiral catalysts or reagents in borane-promoted reduction of ketones, imines and oxime ethers, and lactones as well as in aldol condensations, Diels-Aldercycloadditions, and ally Imetal additions to aldehydes. ... 

In this model the sense of asymmetric induction is controlled by two principle factors (i) coordination of borane on the least-hindered face of the bicyclic ring system and (ii) coordination of the Lewis acid syn to the small group (R ). The latter point is in good agreement with the structural data that has been presented in this chapter, and is further supported by results from the asymmetric reduction of oxime ethers (Figure 51). As predicted by the model (75 and 78), (E) and (Z)-oxime ethers afford enantiomeric amines upon reduction by the reagent derived from (-)-norephedrine and borane (2 equiv.). Here, Lewis acid coordination is dictated by the ( )/(Z) stereochemistry of the oxime ether rather than by the rule of coordination syn to the small group. 

The iridium(I) salt containing 185 has also been employed as catalyst in hydrogenation of imines. In the hydrogenation of 2-substitufed quinolines to provide chiral tetrahydro derivatives iridium complexes of SYNPHOS and DIFLUORPHOS prove to he effective catalysts. Oximes undergo enantioselective reduction (and N—O bond cleavage) on treatment with borane and spirocychc boronate 186. ... 

---