Borane-Dimethyl Sulfide1


[13292-87-0]  · C2H9BS  · Borane-Dimethyl Sulfide  · (MW 75.96)

(hydroborating and reducing agent)

Alternate Name: BMS.

Physical Data: d204 = 0.801 g cm-3.

Solubility: sol dichloromethane, benzene, toluene, xylene, hexane, diethyl ether, diglyme, DME, and ethyl acetate; insol but reacts slowly with water; reacts with alcohols, acetone.

Form Supplied in: neat complex, colorless liquid, ca. 10 M in BH3; contains slight excess of dimethyl sulfide.

Analysis of Reagent Purity: active hydride is determined by hydrolysis of an aliquot in glycerol-water-methanol mixture and measuring the hydrogen evolved according to a standard procedure.3a 11B NMR (CH2Cl2) d -20.1 ppm (q, JB-H = 104 Hz).4

Preparative Method: Diborane generated by the reaction of Sodium Borohydride with Boron Trifluoride Etherate in diglyme3 is absorbed in Dimethyl Sulfide.

Purification: commercial reagent can be purified and freed of excess dimethyl sulfide by vacuum transfer.2

Handling, Storage, and Precautions: use in a fume hood; flammable liquid with stench; reacts with atmospheric moisture forming a crust of boric acid. Store and handle under nitrogen or argon. Stable indefinitely when kept at 0 °C. Stable for prolonged periods at rt.


Borane-dimethyl sulfide (BMS) parallels Borane-Tetrahydrofuran in hydroboration and reduction reactions. Its advantages are stability, solubility in various solvents, and higher concentration.


Directive effects and stereoselectivity in the hydroboration of representative alkenes with BMS are shown in Figure 1.5,6

Oxidation of organoborane intermediates with a standard alkaline hydrogen peroxide in the presence of dimethyl sulfide requires a higher concentration of alkali to suppress slow concurrent oxidation of dimethyl sulfide. When its presence is not desirable, it can be removed by selective oxidation with sodium hypochlorite.7 The oxidation of primary trialkylboranes with Pyridinium Chlorochromate in methylene chloride and with Chromium(VI) Oxide in acetic acid provides the corresponding aldehydes and acids, respectively. The method works best for a-alkyl-substituted alkenes undergoing hydroboration with high regioselectivity (eq 1).8 BMS finds application for the synthesis of various hydroborating and reducing agents derived from hindered alkenes and other precursors.9

The reaction of internal alkynes with BMS (also with Borane-Tetrahydrofuran) can be controlled to give the corresponding (Z)-alkenylborane. Photochemical isomerization of such alkenylboranes has been reported (eq 2).10

The hydroboration of cyclic dienes with BMS gives mixtures of products. Dihydroboration predominates for seven- and eight-membered ring dienes, whereas cyclopentadiene and 1,3-cyclohexadiene give mainly the homoallylic and allylic organoborane, respectively.11 Acyclic a,o-dienes are transformed into bora heterocycles by cyclic hydroboration with 9-Borabicyclo[3.3.1]nonane and BMS (eq 3).9c,12

The hydroboration reaction tolerates many functional groups. A high level of acyclic diastereoselection is achieved in certain highly functionalized alkenes with BMS (eq 4)13a and also with borane-THF.13b,c Vinylic and allylic derivatives containing oxygen, sulfur, or nitrogen substituents react with high regioselectivity, placing boron at the b-position (Figure 2).6,14 Although such organoboranes are prone to elimination reactions, pure products can often be obtained by a careful control of the reaction conditions.

Generally, the hydroboration of allylic amines and sulfides proceeds only to the monoalkylborane stage due to intramolecular complexation.15,16 The amino group can be protected by Boc, benzyloxycarbonyl, trimethylsilyl, or phosphoramido groups14,17,18 (eq 5).17 Allylic N-phosphoroamidates are readily transformed into g-haloamines via hydroboration-halogenolysis.18

The hydroboration of b-monosubstituted enamines with BMS in THF affords [(2-dialkylamino)alkyl]boranes as the major product, which can be transformed into the corresponding boronates, boronic acids, amino alcohols, or alkenes, depending on the reaction conditions (eq 6).16,19 Aldehydes and ketones can be stereoselectively transformed into (E)- or (Z)-alkenes by this method (eq 6).19

Directive effects in the addition of BMS to vinylic and allylic silicon derivatives are opposite to those observed for the oxygen and nitrogen derivatives (Figure 3).20

Several stereodefined organoboranes containing two different metals, boron and silicon, in a 1:3 relationship have been synthesized and transformed into 1,3-diols.21 The hydroboration of alkynylsilanes provides a convenient access to acylsilanes (eq 7).20b,22

Following the analogy to cyclic hydroboration of dienes with 9-BBN/BMS, pharmacologically important 1-silacyclohexan-4-one has been synthesized via cyclic hydroboration of the divinylsilane derivative.23,24

Reduction of Functional Groups:1,25,26 Carboxylic Acids and Derivatives.

Carboxylic acids,16,27 thioacids,28 and amino acids29 are transformed into the corresponding alcohols, thiols, and amino alcohols. Aromatic acids are reduced in the presence of trimethoxyborane.30 They may also undergo reduction to hydrocarbons.31 Oxidation of the intermediate alkoxyboranes yields aldehydes.27a Since alkoxyboranes are readily prepared from alcohols,32 the PCC oxidation of alkoxyboranes may be advantageous if water-sensitive groups are present (eq 8).27a,32b

Carboxylic groups can be protected from BMS reduction as trialkylsilyl esters.33 Aliphatic esters and lactones are rapidly reduced in refluxing THF. Aromatic esters react at a slower rate.26 Site selective reductions of a-hydroxy esters in the presence of other ester groups have been achieved in both aliphatic and aromatic systems34 (eq 9).34a,b

Amides and lactams are reduced to amines35 (eq 10).35b a,o-Amido esters undergo reductive cyclization.36 Formylation-reduction of primary amines provides convenient access to monomethylated amines.37 For the isolation of amines, a decomplexing agent such as BF3 is added, or the amine is isolated as a hydrochloride salt.26 Alternatively, if a theoretical amount of BMS is used in refluxing toluene, neither distillation of dimethyl sulfide nor a decomplexing agent is necessary.38 Both aliphatic and aromatic nitriles undergo fast reduction.26

Ketones and Derivatives.

BMS is used as a source of borane in asymmetric reductions of prochiral ketones and derivatives catalyzed by oxazaborolidines (see Borane-Tetrahydrofuran). It is also used for the synthesis of the catalysts.39

In stoichiometric ratios, BMS is a highly selective reducing agent for a-halo ketones and a-halo imines.40 In the reduction of aldoximes and ketoximes, the reagent offers the advantage of a simpler isolation procedure as compared to reductions with borane-THF.1b Selective reduction of an aldehyde in the presence of a ketone and a,b-unsaturated aldehydes and ketones to the corresponding allylic alcohols has been demonstrated.1a Acetals are tolerated by the reagent and serve as protective groups for ketones in selective hydroborations and reductions. Efficient acetal opening is achieved upon activation with Trimethylsilyl Trifluoromethanesulfonate (eq 11).41

Cleavage of Single Bonds.

Simple ethers appear to be stable to BMS, for example THF is not cleaved when refluxed with BMS for long periods. However, methoxy groups attached to aromatic rings may undergo cleavage to phenols.35c

BMS is an efficient reagent for the direct reduction of ozonides to alcohols in methylene chloride solution.42

Other Applications.

BMS is used for the synthesis of various borane complexes, e.g. with amines,43 phosphines,44 azo compounds,45 and oligonucleotides.46 A novel staining method for transmission electron microscopy is based on the reduction (BMS)-oxidation of ester groups in polymeric materials.47

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Marek Zaidlewicz

Nicolaus Copernicus University, Torun, Poland

Herbert C. Brown

Purdue University, West Lafayette, IN, USA

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