Boranes complex hydrides

Then we established that borane and its derivatives are acidic reducing agents (31-34), in contrast to the basic reducing agents, the complex hydrides. 

The reduction of dialkylketones and alkylaryl ketones is also conveniently accomplished using chiral oxazaborolidines, a methodology which emerged from relative obscurity in the late 1980s. The type of borane complex (based on (,V)-diphenyl prolinol)[39] responsible for the reductions is depicted below (10). Reduction of acetophenone with this complex gives (/ )-1 -phenylethanol in 90-95% yield (95-99% ee) [40]. Whilst previously used modified hydrides such as BiNAL-H (11), which were used in stoichiometric quantities, are generally unsatisfactory for the reduction of dialkylketones, oxazaborolidines... 

ALKYLALUMINIUM DERIVATIVES, ALKYLBORANES, ALKYLHALOBORANES ALKYLHALOPHOSPHINES, ALKYLHALOSILANES, ALKYLMETALS ALKYLNON-METAL HYDRIDES, ALKYLPHOSPHINES, ALKYLSILANES ARYLMETALS, BORANES, CARBONYLMETALS, COMPLEX ACETYLIDES COMPLEX HYDRIDES, HALOACETYLENE DERIVATIVES HEXAMETHYLNITRATODIALUMINATE SALTS, METAL HYDRIDES NON-METAL HYDRIDES, ORGANOMETALLICS, PYROPHORIC ALLOYS PYROPHORIC CATALYSTS, PYROPHORIC IRON-SULFUR COMPOUNDS PYROPHORIC METALS... 

The mechanism of reduction by boranes and alanes differs somewhat from that of complex hydrides. The main difference is in the entirely different chemical nature of the two types. Whereas complex hydride anions are strong nucleophiles which attack the places of lowest electron density, boranes and alanes are electrophiles and combine with that part of the organic molecule which has a free electron pair [119]. By a hydride transfer alkoxyboranes or... 

The domain of hydrides and complex hydrides is reduction of carbonyl functions (in aldehydes, ketones, acids and acid derivatives). With the exception of boranes, which add across carbon-carbon multiple bonds and afford, after hydrolysis, hydrogenated products, isolated carbon-carbon double bonds resist reduction with hydrides and complex hydrides. However, a conjugated double bond may be reduced by some hydrides, as well as a triple bond to the double bond (p. 44). Reductions of other functions vary with the hydride reagents. Examples of applications of hydrides are shown in Procedures 14-24 (pp. 207-210). 

Similar reductions are achieved by trialkylboranes [S26]. These reactions, although different in nature from the reductions by hydrides and complex hydrides, were amongst the first applications of boranes and alanes for the reduction of organic compounds. 

Benzylic halides are reduced very easily using complex hydrides. In a-chloroethylbenzene lithium aluminium deuteride replaced the benzylic chlorine by deuterium with inversion of configuration (optical purity 79%) [537]. Borane replaced chlorine and bromine in chloro- and bromodiphenylme-thane, chlorine in chlorotriphenylmethane and bromine in benzyl bromide by hydrogen in 90-96% yields. Benzyl chloride, however, was not reduced [5iSj. Benzylic chlorine and bromine in a jy/n-triazine derivative were hydrogeno-lyzed by sodium iodide in acetic acid in 55% and 89% yields, respectively [5i9]. 

Reagents of choice for reduction of epoxides to alcohols are hydrides and complex hydrides. A general rule of regioselectivity is that the nucleophilic complex hydrides such as lithium aluminum hydride approach the oxide from the less hindered side [511, 653], thus giving more substituted alcohols. In contrast, hydrides of electrophilic nature such as alanes (prepared in situ from lithium aluminum hydride and aluminum halides) [653, 654, 655] or boranes, especially in the presence of boron trifluoride, open the ring in the opposite direction and give predominantly less substituted alcohols [656, 657,658]. As far as stereoselectivity is concerned, lithium aluminum hydride yields trans products [511] whereas electrophilic hydrides predominantly cis products... 

Reduction of aromatic carboxylic acids to alcohols can be achieved by hydrides and complex hydrides, e.g. lithium aluminum hydride 968], sodium aluminum hydride [55] and sodium bis 2-methoxyethoxy)aluminum hydride [544, 969, 970], and with borane (diborane) [976] prepared from sodium borohydride and boron trifluoride etherate [971, 977] or aluminum chloride [755, 975] in diglyme. Sodium borohydride alone does not reduce free carboxylic acids. Anthranilic acid was reduced to the corresponding alcohol by electroreduction in sulfuric acid at 20-30° in 69-78% yield [979],... 

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. 

Several complex hydrides have been prepared, such as AgBH4 these were all thermally unstable.383 More stable complexes were obtained with substituted boranes when isolated as phosphine derivatives.386-388... 

Weber and coworkers reported the synthesis of novel substituted borazoles. The reaction of boron trichloride with 1,2-diimines 367 provide a mixture of products. The formation of dimeric borazole 369 was explained based on a nucleophilic attack of the chloroborane 370 on the ate complex 368 followed by dehydrohalogenation. Alternatively, a controlled addition of BCR to the diimine 367 results in a clean formation of the imine-borane complex 373 that upon reduction with sodium amalgam and calcium hydride provided the chloroborazole 375 (Scheme 59) <2006EJI5048>. 

D Ulivo, A. 2004. Chemical vapor generation by tetrahydroborate (III) and other borane complexes in aqueous media—A critical discussion of fundamental processes and mechanisms involved in reagent decomposition and hydride formation. Spectrochim. Acta B 59 793-825. 

Various reducing agents have been used for the generation of selenolates from diselenides or selenocyanates. Alkali metals M (M = Li, Na, K)149,150 or alkali hydrides MH (M = Li, Na, K)151,152 can generate the corresponding selenolate anions such as 80 these are more reactive than the borane complexes of type 77 (Scheme 15). Diaryl diselenides are easier reduced than dialkyl diselenides, but the mechanism for the reduction of selenocyanates is complex and can lead to either diselenides or selenolates.153,154... 

The hydride nucleophile prefers to attack the carbonyl group of the borane complex IBBU2 from the top face following the Biirgi-Dunitz trajectory.3 The... 

Thus, using L-amino add oxidase from P. myxcfaciens and various amine-borane complexes or D-amino acid oxidase from porcine kidney and sodium cyanoboro-hydride, the preparation of several natural and non-natural enantiopure D- and L-amino adds was achieved, respectively [51]. In a more recent report, several P- and y-substituted a-amino adds were deracemized using D-amino add oxidase from Trigonopsis variahilis and sodium cyanoborohydride or sodium borohydride [52] (Scheme 13.20). 

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