Reduction Reaction with Borane

There are also a variety of asymmetric catalytic reactions which may produce optically active alcohols. Metalo-organic compounds have been reported to play an essential role in some of them. One of these reactions is enantioselective reduction of pro-chiral ketones with borane in which BHj-SMe adduct are used as reductant. It also needs a chiral ligand along with aluminum isopropoxide to prepare enantioenrich compounds (reaction 7.7). Fu and Uang used (R)-binaphthol as chiral ligand and without use of AKOTrlj, reached 50% yield with no enantioselectivity in production of phenyl ethanol 63, from acetophenone 62, by BH -SMe even after 24 hours [54]. But in the presence of both, at 40°C, 96% yield with 74% 

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Stereoselective reduction of some triazolodiazines (derivatives of ring systems 33 and 37) bearing chiral terpene residues has been elaborated by Groselj el al. <2006TA79>. With catalytic hydrogenation, partial saturation of the six-membered ring was experienced, while reaction with borane-methyl sulfide resulted in formation of triazole-boron complexes. 

The reduction of epoxides withborane is noteworthy as it gives rise to the less substituted alcohol as the major product (7.96), in contrast to reduction with complex hydrides (compare with Scheme 7.71). The reaction is catalysed by small amounts of sodium or lithium borohydride and high yields of the alcohol are obtained. With 1-alkylcycloalkene epoxides, the 2-alkylcycloalkanols produced are entirely cis, and this reaction thus complements the hydroboration-oxidation of cycloalkenes described in Section 5.1, which leads to trans products. Reaction with borane in the presence of boron trifluoride has also been used for the reduction of epoxides and for the conversion of lactones and some esters into ethers. 

Nitrophenol 141 during alkylation with 2-bromo-2-methylpropionic acid ethyl ester in the presence of cesium carbonate in acetonitrile underwent Smiles rearrangement to afford the corresponding propionamide 142. Further upon reduction of 144, which was obtained from 141 in a three step reaction, with borane dimethyl sulfide complex in THF, a Smiles rearrangement was observed furnishing the propanol 146. ... 

The desired pyridylamine was obtained in 69 % overall yield by monomethylation of 2-(aminomethyl)pyridine following a literature procedure (Scheme 4.14). First amine 4.48 was converted into formamide 4.49, through reaction with the in situ prepared mixed anhydride of acetic acid and formic acid. Reduction of 4.49 with borane dimethyl sulfide complex produced diamine 4.50. This compound could be used successfully in the Mannich reaction with 4.39, affording crude 4.51 in 92 % yield (Scheme 4.15). Analogous to 4.44, 4.51 also coordinates to copper(II) in water, as indicated by a shift of the UV-absorption maximum from 296 nm to 308 nm. 

Primary dialkylboranes react readily with most alkenes at ambient temperatures and dihydroborate terminal acetylenes. However, these unhindered dialkylboranes exist in equiUbtium with mono- and ttialkylboranes and cannot be prepared in a state of high purity by the reaction of two equivalents of an alkene with borane (35—38). Nevertheless, such mixtures can be used for hydroboration if the products are acceptable for further transformations or can be separated (90). When pure primary dialkylboranes are required they are best prepared by the reduction of dialkylhalogenoboranes with metal hydrides (91—93). To avoid redistribution they must be used immediately or be stabilized as amine complexes or converted into dialkylborohydtides. 

Catalytic hydrogenation of triple bonds and the reaction with DIBAL-H usually give the eis alkene (15-11). Most of the other methods of triple-bond reduction lead to the more thermodynamically stable trans alkene. However, this is not the case with the method involving hydrolysis of boranes or with the reductions with activated zinc, hydrazine, or NH2OSO3H, which also give the cis products. 

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