Dibromoborane-Dimethyl Sulfide1

BHBr2.SMe2

[55671-55-1]  · C2H7BBr2S  · Dibromoborane-Dimethyl Sulfide  · (MW 233.77)

(hydroborating agent providing access to alkyl-2 and alkenyldibromoboranes3 and boronic acids2,4)

Alternate Name: DBBS.

Physical Data: mp 30-35 °C; bp 75 °C/0.1 mmHg.

Solubility: sol dichloromethane, carbon disulfide, carbon tetrachloride.

Form Supplied in: white solid or liquid, 7.8 M in BHBr2.

Preparative Methods: by redistribution of BH3.SMe2 and BBr3.SMe2 or BH3.SMe2 and BBr3;5 by the reaction of bromine with BH3.SMe2 in CS2.6

Analysis of Reagent Purity: 1H NMR (CCl4) d 2.48, (CS2) 2.69 ppm; 11B NMR (CCl4) d -7.3 (d, JB-H = 160 Hz),5 (CS2) -8.2 ppm.6 Hydrolysis of an aliquot and measuring the hydrogen evolved according to the standard procedure.7

Handling, Storage, and Precautions: corrosive liquid; air and moisture sensitive; flammable; stench. Handle and store under nitrogen or argon. Stable indefinitely when stored under nitrogen at 25 °C. Reacts violently with water. This reagent should be handled in a fume hood.

Hydroboration of Alkenes.

Dibromoborane-dimethyl sulfide hydroborates alkenes directly without need for a decomplexing agent (see Dichloroborane Diethyl Etherate) (eq 1).2

Regioselectivity of DBBS in the hydroboration of alkenes and derivatives is high, approaching 9-Borabicyclo[3.3.1]nonane, e.g. 1-hexene, styrene, 2-methyl-1-pentene, 2-methyl-2-butene and 4-(dimethylphenylsilyl)-2-pentene, react by placing the boron atom at the less hindered position with &egt;99% selectivity.2,8a Lower regioselectivity of the hydroboration-oxidation is observed if an excess of alkene is used, due to hydrobromination, which is a side reaction in the hydrolysis-oxidation step.8b The reactivity of DBBS toward structurally different alkenes and alkynes is different from that of other hydroborating agents (Table 1).3,9

Reactions of Alkyldibromoboranes.

Alkyldibromoboranes are versatile synthetic intermediates. They are resistant to thermal isomerization, a feature of considerable importance for the regio- and stereoselective synthesis of organoborane intermediates from highly labile alkenic structures.10

Standard oxidation of alkyldibromoboranes with alkaline Hydrogen Peroxide affords alcohols.2,8b Conversion of terminal alkenes to carboxylic acids using alkyldibromoboranes works well, although hydrolysis prior to oxidation is needed.11 Chiral alkyldibromoboranes have been used as catalysts for the asymmetric Diels-Alder reaction.12,13

The hydridation-stepwise hydroboration procedure provides a convenient general approach to monoalkylbromoboranes, mixed dialkylbromoboranes, dialkylboranes, totally mixed trialkylboranes, ketones, alcohols (eq 2),14 and stereodefined alkenes, dienes, and haloalkenes (see below). Mixed alkylalkenylalkynylboranes are also available by this methodology.15

Hydrolysis and alcoholysis of alkyldibromoboranes provide simple access to alkylboronic acids and esters respectively,4 which are important synthetic intermediates and reagents for protection of hydroxy groups of diols16a-f and derivatizing agents for GC and GC-MS analysis.16g,h

Selective Hydroboration of Dienes and Enynes.

The opposite reactivity trends of DBBS and other hydroborating agents makes possible the selective hydroboration of dienes (eq 3)9 and enynes.9 In conjugated systems, however, bromoboration of the triple bond is observed.17

Reactions of Alkenyldibromoboranes and Alkenylalkylbromoboranes.

Synthesis of Alkenylboronic Acids, Alkenes, Aldehydes, and Ketones.

Alkenyldibromoboranes undergo many of the characteristic reactions of alkenylboranes. The presence of dimethyl sulfide does not interfere in their transformations. Protonolysis with acetic acid in refluxing dichloromethane gives the corresponding alkene. Oxidation leads to aldehydes or ketones (eq 4).3 Hydrolysis and alcoholysis yields alkenylboronic acids and esters, respectively.

The (E)-alkenylboronic acids are directly available from 1-alkynes by hydroboration-hydrolysis (eq 4).4 The (Z)-isomers are prepared from 1-bromo-1-alkynes by hydroboration-hydride reduction (eq 5).18

Synthesis of (E)- and (Z)-Alkenes, Trisubstituted Alkenes, 1,2-Disubstituted Alkenyl Bromides, Ketones, and Enolborates.

The synthesis of (Z)-alkenes, according to eq 4, is a simple, convenient method, provided the alkynic precursor is readily available. A general Zweifel (E)- and (Z)-alkene synthesis starts with 1-alkynes and 1-bromo-1-alkynes, respectively. The precursors are hydroborated with monoalkylbromoborane to give the corresponding alkylalkenylbromoboranes. Migration of the alkyl group completes the formation of the carbon skeleton (eq 6).19-21 The procedure allows full utilization of the alkyl group.

(Z)-9-Tricosene (muscalure), the sex pheromone of the housefly (Musca domestica), has been prepared by this method in 69% yield and >99% purity.20,22 Extension of the methodology to trisubstituted alkenes is based on the iodine-induced migration of the second alkyl group R3 (eq 7).23

The key alkenylboronate intermediate used for the introduction of the R3 group (eq 7) is of the same structure as the one shown in eq 6. The procedure works well both for alkyl and aryl R3 groups. However, a methyl group shows poor migratory aptitude in these reactions.20,23 If the two trans-alkyl groups in the product alkene are the same or differ significantly in steric bulk, an internal alkyne may serve as a starting material (eq 8).24

Other approaches to trisubstituted alkenes via organoboranes involve alkynyltrialkyl borates,25 alkenyltrialkyl borates26 or the cross-coupling reaction of alkenylboronic acids with alkyl halides.27

Both (E)- and (Z)-1,2-disubstituted alkenyl bromides can also be prepared by the methodology shown in eq 7.28 The Boron Trifluoride Etherate-mediated 1,4-addition of 1,2-disubstituted alkenylboronates affords g,d-unsaturated ketones (eq 9).29 The boronates can also be converted into chiral enolborates for the enantioselective addition to aldehydes.30

Synthesis of (E)- and (Z)-1-Halo-1-alkenes and a-Bromoacetals.

Stereodefined alkenyl halides are important starting materials for the synthesis of alkenyl Grignard31 and lithium32 derivatives, pheromones,33 and (E,E)-, (E,Z)-, (Z,E)- and (Z,Z)-dienes by the cross-coupling reaction.34 An efficient, general (E)- and (Z)-1-halo-1-alkene synthesis starts with 1-alkynes and 1-halo-1-alkynes respectively, which are converted into the corresponding (E)- and (Z)-alkenylboronic acids or esters via alkenyldibromoboranes, according to eqs 4 and 5. The alkenylboronic acids and esters react with halogens directly35-38 or via alkenylmercurials39 to give the haloalkenes in high stereochemical purity in 70-100% yield (eq 10). In some of these halogenation reactions, alkenyldibromoboranes can be used directly.35,36 Alternatively, (Z)-1-halo-1-alkenes are simply obtained by hydroboration-protonolysis of 1-halo-1-alkynes with 9-BBN or Disiamylborane.40

Synthesis of Conjugated Dienes.

The cross-coupling reaction of alkenylboronates with alkenyl halides is a general method for the synthesis of stereodefined 1,3-dienes (eq 11).41

Other Applications.

DBBS is an excellent precursor for the formation of bulk powders and ceramic fiber coatings of boron nitride.42 It has been used for the synthesis of the small carborane closo-2,3-Et2C2B5H5,43 3-O-carboranylcarbene,44 silver and sodium isocyanoborohydrides,45 and dications based on the hydrotris(phosphonio)borate skeleton.46

Related Reagents.

Dichloroborane-Dimethyl Sulfide.


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Marek Zaidlewicz

Nicolaus Copernicus University, Torun, Poland

Herbert C. Brown

Purdue University, West Lafayette, IN, USA



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