Borane dimethylsulfide

Borane dimethylsulfide (BMS) is a complexed borane reagent that is used for hydroborations and reductions. The advantages of BMS over other borane reagents, such as borane-tetrahydrofuran, are its increased stability and higher solubility.[1] BMS is commercially available at much higher concentrations than its tetrahydrofuran counterpart (10 M neat) and does not require sodium borohydride as a stabilizer, which could result in undesired side reactions.[2] In contrast, borane·THF requires sodium borohydride to inhibit reduction of THF to tributyl borate.[2] BMS is soluble in most aprotic solvents.

Borane dimethylsulfide
Names
Other names
BMS, Borane-dimethyl sulfide
Identifiers
3D model (JSmol)
ECHA InfoCard 100.032.998
Properties
(CH3)2S•BH3
Molar mass 75.96 g/mol
Appearance colorless liquid
Density 0.801 g/mL
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Preparation and structure

Although usually purchased, BMS can be prepared by absorbing diborane into dimethyl sulfide:[3]

B2H6 + 2 SMe2 → 2 Me2SBH3

It can be purified by bulb to bulb vacuum transfer. Although a structure of BMS has not been determined crystallographically, (pentafluorophenyl)-borane dimethylsulfide (C6F5BH2SMe2), has been examined by X-ray crystallography.[4] The boron center adopts a tetrahedral geometry.

Reactions

Hydroborations

Due to the experimental ease of its use, BMS has become common in hydroboration reactions.[5] In hydroborations with BMS, the dimethylsulfide dissociates in situ, liberating diborane, which rapidly adds to the unsaturated bonds. The resulting organoborane compounds are useful intermediates in organic synthesis. Boranes add to alkenes in an anti-Markovnikov fashion and allow conversion of alkynes to the corresponding cis-alkenes.

Reductions

BMS has been employed for the reduction of many functional groups. Reductions of aldehydes, ketones, epoxides, esters, and carboxylic acids give the corresponding alcohols. Lactones are reduced to diols, and nitriles are reduced to amines. Acid chlorides and nitro groups are not reduced by BMS.

Borane dimethylsulfide is one of the most common bulk reducing agents used in the Corey–Itsuno reduction. The dimethylsulfide ligand attenuates the reactivity of the borane. Activation by the nitrogen of the chiral oxazoborolidine catalyst of the stoichiometric reducing agent allows for asymmetric control of the reagent. In general BMS does not lead to significantly greater enantiomeric selectivities than borane-THF, however its increased stability in the presence of moisture and oxygen makes it the reagent of choice for the reduction.[6]

Safety

Borane dimethylsulfide is flammable and reacts readily with water to produce a flammable gas. It also has an unpleasant smell.[7]

References
  1. Hutchins, Robert O.; Cistone, Frank (1981). "Utility and Applications of Borane Dimethylsulfide in Organic Synthesis. A Review". Organic Preparations and Procedures International. 13 (3–4): 225. doi:10.1080/00304948109356130.
  2. Kollonitisch, J. (1961). "Reductive Ring Cleavage of Tetrahydrofurans by Diborane". J. Am. Chem. Soc. 83 (6): 1515. doi:10.1021/ja01467a056.
  3. Braun, L. M.; Braun, R. A.; Crissman, R.; Opperman, M.; Adams, R. M. (1971). "Dimethyl Sulfide-Borane. A Convenient Hydroborating Reagent". J. Org. Chem. 36 (16): 2388–2389. doi:10.1021/jo00815a047.
  4. Fuller, Anna-Marie; Hughes, David L.; Lancaster, Simon J.; White, Callum M. (2010). "Synthesis and Structure of the Dimethyl Sulfide Adducts of Mono- and Bis(pentafluorophenyl)borane". Organometallics. 29 (9): 2194. doi:10.1021/om100152v.
  5. Atsushi Abiko (1925). "Dicyclohexylboron Trifluoromethylsulfonate". Organic Syntheses. 79: 103.; Collective Volume, 10, p. 273
  6. Corey, E.J.; Helal, C. J. (1998). "Reduction of Carbonyl Compounds with Chiral Oxazoborolidine Catalysts: A New Paradigm for Enantioselective Catalysis and a Powerful New Synthetic Method". Angew. Chem. Int. Ed. 37 (15): 1986–2012. doi:10.1002/(SICI)1521-3773(19980817)37:15<1986::AID-ANIE1986>3.0.CO;2-Z.
  7. "Sigma-Aldrich Material Safety Data Sheet". www.sigmaaldrich.com/. Retrieved 29 November 2014.
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