Borane-dimethyl sulphide

A convenient route to both saturated and unsaturated acylsilanes lies in the hydroboration-oxidation of alkynylsilanes (Chapter 7). Recent improvements (6) to this method involve the use of the borane-dimethyl sulphide complex for hydroboration, and of anhydrous trimethylamine A-oxide for the oxidation of the intermediate vinyl boranes. 

Bis(trimethylsilyl)propyne, 141 Bis(trimethylsilyl)terminai alkynols, 97 Borane-dimethyl sulphide. 84 Bromination-dcsilicobromination, 17-18... 

A most convenient procedure for the conversion of carboxylic acids to aldehydes results from their initial treatment with borane-dimethyl sulphide to give first, the triacyloxyborane (9), which is then reduced further to the intermediate (10) the trimer of (10) is the trialkyloxyboroxine (11). Oxidation of (11) with PCC then yields the aldehyde.110 The method is illustrated for the preparation of octanal (Expt 5.81), but it has been applied to cyclohexanecarboxylic acid and to various aromatic carboxylic acids (X-C6H4C02H, X = p-Cl, NOz, OMe, ni-CN). 

Diborane may be generated in situ from NaBH4 and AICI3 or BF3 but many sources of borane are commercially available, e.g. borane-tetrahydrofuran (H3B.THF), borane-dimethyl sulphide (H3B.SMe2). 

Phosphoramidates and their analogues of type (111) are hydroborated with borane-dimethyl sulphide to give boranes which are sources of the halogenopropylphosphoramidates (112). 

Borane-dimethyl sulphide solution (73 mmol, 2.74 m in THF) was added dropwise, with stirring, to the neat alkynylsilane (200mmol), keeping the reaction temperature below 20°C by use of an ice bath. The mixture was stirred for 2 h at 0-5 °C. At this time, the solution of the trialkenyl borane was diluted with THF (200 ml), and then anhydrous trimethylamine A/-oxide (240 mmol, dried by azeotropic distillation with toluene) was added over 5 min at ambient temperature. The resultant slurry was heated (bath temperature 75 °C) for 4h, and then cooled to ambient temperature. The enol borinate thus formed was hydrolysed by the addition of water (100 ml), followed by stirring for 5 min. The layers were separated, and the aqueous phase was extracted with ether (2x 100ml). The combined organic extracts were washed with brine and dried. Concentration and distillation gave the acylsilane (75-91%). 

Conjugate addition of nitroalkanes to a,6-unsaturated carbonyl compounds has been achieved using basic alumina, in the absence of a solvent.36 3-Nitropropanal has been prepared by conjugate addition of nitrite anion to acrolein reduction to 3-nitropropanol was effected with borane-dimethyl sulphide.36 -Nitro-acids and u-nitro-esters have been prepared by ring cleavage of 2-nitrocycloalkanones (Scheme 163).362... 

The reduction of ketals to protected secondary alcohols is readily accomplished in high yield with borane-dimethyl sulphide upon activation with TMSOTf at —78 °C (eq 85). Other Lewis acids require higher temperatures, and 1 equiv of TMSOTf is essential for complete conversion of the ketal. The solvent has... 

Boron.—Preparation of Organoboranes. The year 1977 was one of consolidation in hydroboration. Detailed investigations of the reactions of 9-borabicy-clo[3,3,l]nonane (9-BBN) with cycloalkenes and non-conjugated dienes were reported in full. The reagent shows a remarkable degree of positional and stereochemical selectivity during hydroboration. Details of the applications of borane-dimethyl sulphide have appeared, and hydroborations using dimethyl... 

Dodecane-l,12- C has been synthesized by the procedure shown in equation 32. 1,10-Dibromodecane treated with cyanide- in aqueous ethanoF gave the dinitrile which was hydrolyzed in situ to the acid, 20, and in turn reduced with a borane-dimethyl sulphide complex to l,12-dodecanediol-l,12- C in 90% yield. The bis-tosylate of 20 has then been reduced to the required dodecane (1,12- C) in 30% yield, enriched with up to 88.9%. 

Reductive Methods.—Aldehydes from Acid Equivalents. In a two-step process which formally constitutes a reduction, carboxylic acids are first reduced to the corresponding alkyl borate ester by the borane-dimethyl sulphide complex, which in turn is oxidized in high overall yield to the aldehyde [equation (1)]. ... 

Improved procedures for the reduction of amides, and nitriles to the corresponding amines have been reported. The requirement of no more borane reagent than the calculated quantity for the borane-dimethyl sulphide reduction of primary amides is a consequence of distilling dimethyl sulphide out of the reaction mixture (Scheme 2), and represents a significant improvement in... 

Y. Nagai, Hydrosilanes as Reducing Agents , Org, Prep. Proc. Int., 1980,12,13. R. O. Hutchins and F. Cistone, Utility and Applications Borane Dimethyl-sulphide in Organic Synthesis , Org. Prep. Proc. Int., 1981,13, 225. 

The hydroboration of cw-but-2-ene and c -hex-3-ene with di-isopinocampheyl-borane and monoisopinocampheylborane (both prepared from (+)-a-pinene) has been studied as has the hydroboration of a/ -unsaturated carbonyl corn-compounds by borane-dimethyl sulphide, ... 

Reduction of Carbonyl Groups. Rapid reduction of salts of carboxylic acids to alcohols with 2 molar equivalents of borane in tetrahydrofuran (THF) has been reported. Full details have appeared of the improved procedure for reduction of esters to primary alcohols, using borane dimethyl sulphide (6,157) in THF at reflux, which allows the dimethyl sulphide that is liberated to distil off during the reaction. Further suggested procedures for the reduction of esters to primary alcohols use lithium borohydride in diethyl ether or THF, calcium borohydride in THF, and sodium borohydride in a t-butyl alcohol-methanol mixed solvent. The reactions using lithium borohydride-diethyl ether can be catalysed by lithium 9-boratobicyclo[3.3.1]nonane (1), lithium triethylboro-hydride, or the Lewis acid 9-methoxy-9-borabicyclo[3.3.1]nonane (2). ... 

Hydroboration.—Borane-l,4-oxathiane (1) is a convenient hydroborating agent with reaction characteristics similar to those of borane-tetrahydrofuran (BHj.THF) or borane-dimethyl sulphide (BMS). Like BMS, but unlike BH3.THF, it can be handled as a neat, stable material and can be utilized in a variety of solvents. Its odour is less unpleasant than that of BMS, and the oxathiane can be removed from many organic solvents by washing with water, particularly if it is first oxidized by means of hypochlorite solution. Hence,... 

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