Diborane sulfide complexes

Any of these BH3 compounds adds readily to most alkenes at room temperature or lower temperatures. The reactions usually are carried out in ether solvents, although hydrocarbon solvents can be used with the borane-dimethyl sulfide complex. When diborane is the reagent, it can be generated either in situ or externally through the reaction of boron trifluoride with sodium borohydride ... 

Various polymer-supported hydrides have been applied successfully to reductions of both carbonyl and olefin groups. Rajasree and Devaky13 describe a cross-linked polystyrene-supported ethylenediamine borane reagent for the selective reduction of aldehydes in the presence of ketones (entry 9). This borane reagent is easily prepared and can be recycled after completion of the reaction. This is a practical alternative to standard borane reagents such as diborane, borane-amine, or borane-sulfide complexes. 

Odorless borane-sulfide complexes 36a-e65-68 and borane-amine complexes 37a-g69-75 have been recently prepared because of the growing importance of diborane for the synthesis of pharmaceuticals or other compounds and certain inconveniences of well-established reagents, for example, the low concentration and instability of BH3-THF and high volatility, flammability, and unpleasant odor of dimethyl sulfide from BH3-SMe2. Trialkylsilanes, for example, Me3SiH, were found to be an efficient reducing reagent for the in situ preparation of... 

For the synthesis of trialkylboranes. hydroboration is carried out with a borane solution in THF or with the borane-methyl sulfide complex (BMS) in THF, diethyl ether, or dichloromethane. Diborane reacts rapidly and quantitatively with alkenes to produce a solution of trialkylborane [16] (eq (13)). The addition of dialkylboranes. such as 9-borabicyclo[3.3.1]nonane (9-BBN. 3). disiamylborane (1), or dicyclohexylb-orane (2), to alkenes or alkynes gives mixed alkylboron compounds. The high regio-. stereo-, or chemoselectivity in the additions of these borane reagents unsaturated C—C bonds have been extensively used in organic syntheses. 

Malate esters are easily reduced in a highly selective fashion using either diborane [40,41] or borane—methyl sulfide complex [42— 45] in the presence of a catalytic amount of sodium borohydride (5 mol%) to give diol esters 45. Yields of 45a and 45b generally range from 80-97%, while 45c is formed in 60% yield [46]. Dimethyl (5)-malate is reduced with 99 1 selectivity as regards C-1 to C-4 ester reduction, while diethyl ( S)-malate shows even greater selectivity (200 1). 

Diborane can be generated in situ from sodium borohydride and boron trifluoride. Solutions in tetrahydrofuran are commercially available. An alternative commercially available reagent is the borane-dimethyl sulfide complex ... 

Diborane [19287-45-7] the first hydroborating agent studied, reacts sluggishly with olefins in the gas phase (14,15). In the presence of weak Lewis bases, eg, ethers and sulfides, it undergoes rapid reaction at room temperature or even below 0°C (16—18). The catalytic effect of these compounds on the hydroboration reaction is attributed to the formation of monomeric borane complexes from the borane dimer, eg, borane-tetrahydrofuran [14044-65-6] (1) or borane—dimethyl sulfide [13292-87-0] (2) (19—21). Stronger complexes formed by amines react with olefins at elevated temperatures (22—24). 

A boron analog - sodium borohydride - was prepared by reaction of sodium hydride with trimethyl borate [84 or with sodium fluoroborate and hydrogen [55], and gives, on treatment with boron trifluoride or aluminum chloride, borane (diborane) [86. Borane is a strong Lewis acid and forms complexes with many Lewis bases. Some of them, such as complexes with dimethyl sulfide, trimethyl amine and others, are sufficiently stable to have been made commercially available. Some others should be handled with precautions. A spontaneous explosion of a molar solution of borane in tetrahydrofuran stored at less than 15° out of direct sunlight has been reported [87]. 

Opening of a bottle where some particles of lithium aluminum hydride were squeezed between the neck and the stopper caused a fire [68]. Lithium aluminum hydride must not be crushed in a porcelain mortar with a pestle. Fire and even explosion may result from contact of lithium aluminum hydride with small amounts of water or moisture. Sodium bis(2-methoxy-ethoxy)aluminum hydride (Vitride, Red-Al ) delivered in benzene or toluene solutions also may ignite in contact with water. Borane (diborane) ignites in contact with air and is therefore kept in solutions in tetrahydrofuran or in complexes with amines and sulfides. Powdered lithium borohydride may ignite in moist air. Sodium borohydride and sodium cyanoborohydride, on the other hand, are considered safe. ... 

In the presence of ethers (and especially dimethyl sulfide) diborane can dissociate into a complexed borane ... 

Reduction of esters, nitriles, and amides. These groups are rapidly reduced by horanc-dimcthyl sulfide in refluxing THF (b.p. 67°) if the dimethyl sulfide (b.p. 38°) is removed as liberated. Under these conditions, the reagent is comparable to uncomplexed diborane. Reduction of secondary and tertiary amides is best effected in the presence of boron trifluoride etherate otherwise, excess reagent is utilized for formation of complexes with the products. 

Two stable diborane complexes, borane-tetrahydrofurane and borane-dimcthyl sulfide, are available from Aldrich. BHj THF. should be stored at 0 BHj (CHj)2S can be stored at room temperature. 

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