Amine-boranes, steric effects

On the theoretical side, study of the dissociation of addition compounds of amines with trimethylborane, boron trifluoride, and borane provide a new quantitative approach to steric strains. These studies quickly removed doubts as to the importance of steric effects in chemical behavior. 

Reduction of aldehydes and ketones. Earlier work on amine borane reagents was conducted mainly with tertiary amines and led to the conclusion that these borane complexes reduced carbonyl compounds very slowly, at least under neutral conditions, and that the yield of alcohols is low. Actually complexes of borane with primary amines, NHj or (CH3)3CNH2, reduce carbonyl compounds rapidly and with utilization of the three hydride equivalents. BH3 NH3 is less subject to steric effects than traditional complex hydrides. A particular advantage is that NH3 BH3 and (CH3)3CNH2 BH3 reduce aldehyde groups much more rapidly than keto groups, but cyclohexanone can be reduced selectively in the presence of aliphatic and aromatic acyclic ketones. 

The reactivity of BX3 fragments toward bifunctional Tewis bases (e.g. R2ENR2, RE(NR2)2, and E(NR2)3 E=P, As) are also of fundamental interest. It is clear that a combination of substituent inductive effects, E- N p r-d r electronic effects, as well as steric influences are responsible for the BX3 coordination site selectivity and selected bondbreaking chemistry found in these complexes. The mechanism for hydrolytic decomposition of amine-borane complexes also continues to be of interest. A comparison of the hydrolysis of benzylamine-borane with the hydrolysis of various arylamine-boranes has been reported, and the results are important for the selection of borane adducts as practical hydridic reagents in aqueous phase reductions. 

Brown s investigation of the addition compounds of trimethyl borane, diborane, and boron trifluoride with amines has provided a quantitative estimation for steric strain effects in chemical reactions. He also investigated the role of steric effects in solvolytic, displacement, and in elimination reactions. His results demonstrate that steric effects can assist, as well as hinder, the rate of a chemical reaction. 

Steric Effects In reactions between Lewis acids and bases such as amines and boranes or boron... 

In reactions between Lewis acids and bases such as amines and boranes or boron halides, bulky substituents on one or both species can affect the stability of the acid-base adduct. Perhaps the most straightforward type of effect is simple steric hindrance between substituents on the nitrogen atom and similar large substituents on the boron atom. Figure 9.3 is a diagrammatic sketch of the adduct between molecules of tripropylamine and triethylborane. This phenomenon is known as front or F-... 

Preparation of B-fluoroborazines cannot be effected from trifluoro-borane and amines by a Brown-Laubengayer type of reaction. However, direct synthesis of B-fluoroborazine rings has been reported recently and involves the dehydrofluorination of primary amine-trifluoroborane adducts with the aid of the adducts of sterically hindered tertiary amines with trifluoroborane. 2,4,6-Trifluoroborazines and ammoniumtetra-fluoroborates sl) are obtained in excellent yield. 

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