Hydride, chiral complexes

Another possibility for asymmetric reduction is the use of chiral complex hydrides derived from LiAlH. and chiral alcohols, e.g. N-methylephedrine (I. Jacquet, 1974), or 1,4-bis(dimethylamino)butanediol (D. Seebach, 1974). But stereoselectivities are mostly below 50%. At the present time attempts to form chiral alcohols from ketones are less successful than the asymmetric reduction of C = C double bonds via hydroboration or hydrogenation with Wilkinson type catalysts (G. Zweifel, 1963 H.B. Kagan, 1978 see p. 102f.). 

Probably the first non-covalent immobilization of a chiral complex with diazaligands was the adsorption of a rhodium-diphenylethylenediamine complex on different supports [71]. These solids were used for the hydride-transfer reduction of prochiral ketones (Scheme 2) in a continuous flow reactor. The inorganic support plays a crucial role. The chiral complex was easily... 

Iridium(III) hydride forms complexes with DIOP, BDPP (2,4-bis(diphenyl-phosphino)pentane), NORPHOS, and BINAP ligands to produce amines in 11 -80% ee.679 Similar modest results are obtained in the reduction of N-arylketimines with an iridium(HI) complex with (2S,3 S) -C HIRA PHOS as the chiral ligand.680 The indium complexes with chiral phosphinodihydrooxazoles catalyze the enantioselective hydrogenation of imines in supercritical carbon dioxide with up to 80% ee, but generally lower ee values are observed in... 

Haubenstock, H., Asymmetric Reductions with Chiral Complex Aluminum Hydrides and Tricoordinate Aluminum Reagents, 14, 231. 

ASYMMETRIC REDUCTIONS WITH CHIRAL COMPLEX ALUMINUM HYDRIDES AND TRICOORDINATE ALUMINUM REAGENTS 231... 

Transformation of ketones to alcohols has been accomplished by many hydrides and complex hydrides by lithium aluminum hydride [55], by magnesium aluminum hydride [89], by lithium tris tert-butoxy)aluminum hydride [575], by dichloroalane prepared from lithium aluminum hydride and aluminum chloride [816], by lithium borohydride [750], by lithium triethylboro-hydride [100], by sodium borohydride [751,817], by sodium trimethoxyborohy-dride [99], by tetrabutylammonium borohydride [771] and cyanoborohydride [757], by chiral diisopinocampheylborane (yields 72-78%, optical purity 13-37%) [575], by dibutyl- and diphenylstannane [114], tributylstanrume [756] and others Procedure 21, p. 209). 

Asymmetric Reductions with Chiral Complex Aluminum Hydrides and... 

Hydridotris(3,5-dimethyl-l-pyrazolyl)borate]molybdenum-(i72-acyl) complexes, such as 1, are deprotonated by butyllithium or potassium hydride to generate enolate species, such as 488.8> jjie overa]] structure of these chiral complexes is similar to that of the iron and rhenium complexes discussed earlier the hydridotris(3,5-dimethyl-l-pyrazolyl)borate ligand is iso valent to the cyclopentadienyl ligand, occupying three metal coordination sites. However, several important differences must be taken into account when a detailed examination of the stereochemical outcome of deprotonation-alkylation processes is undertaken. 

Asymmetric reduction of a,/ -unsaturated carbonyl compounds using chiral complexes (Section 5.4.1, p. 521) could feasibly lead to optically active allylic alcohols. Other reducing agents which have some merit of regioselectivity, but not stereoselectivity, are sodium cyanoborohydride,244 and sodium boro-hydride in the presence of lanthanide salts.245... 

H. Haubenstock, Asymmetric Reductions with Chiral Complex Aluminium Hydrides and Tricoordinate Aluminium Reagents, Top. Stereochem. 1983, 14, 213. 

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... 

Hoshino et al. [33] reported the first example of an enantioselective radical reaction employing a chiral Lewis add complex. The enantioselective reduction of a-methoxy-methyl-a-iodolactone 41 with tributyltin hydride (BusSnH) in the presence of stoichiometric amounts of the chiral complex of a chiral diamine 42 and Mgl2, gave the reduced product 43 in 88 % yield with 62 % ee (Sch. 17). Reaction using Mg(C104)2, TiCU, Znl2,... 

The oxidative addition of silicon hydride to cyclopentadienyl manganese carbonyl also provide a convenient route to chiral complexes (213,250) (eq. [78]). 

Asymmetric reductions with chiral complex metal hydrides and tricoordinate hydride reagents are rare. Iminium salts25 26 and imines27 have been reduced by chiral complex aluminum hydrides. Optically active 2-substituted Ar-methylpiperidine was obtained by reduction of the corresponding 3,4,5,6-tetrahydropyridinium perchlorate with (—)-menthol lithium aluminum hydride. The optical purity for the -propyl derivative was 12% in favor of the S configuration. Similar reductions of imines prepared from acetophenone and propiophenonc with (-)-mcn-thol-lithium aluminum hydride and ( + )-borneol-lithium aluminum hydride reagents resulted in low (<10%) optical yields in those examples where optical yields could be calculated. 

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