Diborane, dimethyl sulfide complex

Two stable diborane complexes, borane-tetrahydrofurane and borane-dimethyl sulfide, are available from Aldrich. BH3 THF should be stored at 0° BH3 - (CH 3)2S can be stored at room temperature. 

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]. 

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

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 ... 

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. 

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... 

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 ... 

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