Reductive amination amino borane complexes

A variety of optically active amines and amino alcohols have been used as chiral auxiliaries for borane reductions. With few exceptions9, early results gave poor to modest asymmetric induction. For example, a variety of amino alcohols derived from a-amino acids gave reduction products of up to 60% ee1U. These reagents presumably used one equivalent of borane per mole of amino alcohol. In 1983 it was shown that the ratio of borane to amino alcohol was important and that two equivalents of borane were required for maximum asymmetric induction. It was postulated that an amino-borane complex of an oxazaborolidinc was involved in the reduction11. 

Scheme 26 shows the supposed mechanism for the reduction of amides by BH3 [30]. The reaction proceeds weU up to the formation of aminobo-rane 257. Its complexation with three additional equivalents of borane occurs faster than the reduction of fmther amides. 258 is stable and can only be transformed to 259b at high temperatmes (110 °C in toluene). The amino-borane is converted to the secondary amine 260 by acid treatment. These conditions are not suitable for solid phase chemistry and make a different workup procedure necessary. If reduction is the terminal step of a synthesis, complex 258 can easily be destroyed during the cleavage from the resin with strong acids. 

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... 

Chiral modification is not limited to boronate and aluminate complexes. Boranes or alanes are partially decomposed with protic substances such as chiral amines, alcohols or amino alcohols to form useful reagents for enantioselective reduction of carbonyl compounds. For example, reduction of acetophenone with borane modified with the amines (65) to (67) gives (5)-l-phenylethyl alcohol with 3.5-20%... 

The complexes of chiral amino-alcohols of type (13) with borane are reported to reduce aryl alkyl ketones to alcohols in optical yields up to 60% [for propiophenone reduction with (13 R = CHMe2)]/ whereas the asymmetric reduction of phenyl alkyl ketones with amine-boranes formed from chiral amines (such as 1-phenylethylamine or various L-a-amino-esters) has been found to give low optical yields of alcohols, i.e. up to 23%. ... 

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