Borane-tetrahydrofuran complex

Description

Borane tetrahydrofuran complex (BH3-THF) is widely used as a reducing agent in organic synthesis. It is also used as a reagent in hydroboration reactions.It is a charge-transfer complex that is a useful surrogate for diborane1 in organic synthesis. It can be used to reduce carboxylic acids to alcohols or nitriles to primary amines. It reacts with olefins to add the BH2 functional group. Alkyl- or arylboranes formed in this way can further react with unsaturated compounds such as olefins, imines, ketones, and alkynes (the hydroboration reaction) to make useful boron-containing intermediates.

Description

Borane–tetrahydrofuran (BH3–THF) is a charge-transfer complex that is a useful surrogate for diborane1 in organic synthesis. It can be used to reduce carboxylic acids to alcohols or nitriles to primary amines. It reacts with olefins to add the BH2 functional group. Alkyl- or arylboranes formed in this way can further react with unsaturated compounds such as olefins, imines, ketones, and alkynes (the hydroboration reaction) to make useful boron-containing intermediates.
Charge-transfer complexes such as BH3–THF form when a Lewis base (THF in this case) “donates” its unshared electron pair to an electron-deficient Lewis acid such as borane. The electrostatic attraction between the components is not as strong as a covalent bond; but in many cases it is sufficient to make the complex behave as a covalent compound. As an example, BH3–THF can be distilled without decomposing.
BH3–THF’s relative stability makes it a convenient way to use extremely toxic and flammable diborane. But it is not without its own risks. In particular, it reacts violently with water to produce copious amounts of hydrogen. As a result, BH3–THF is normally sold as a 1 M (≈10 wt%) solution in THF. It is available from numerous chemical supply houses worldwide.
1. Under standard conditions, borane dimerizes to produce the more electronically stable diborane.

Description

Borane-tetrahydrofuran (BH3-THF) is a complex of borane with tetrahydrofuran, and is generally purchased as a solution in THF. BH3-THF can decompose violently, therefore BH3-THF is typically only available in 1 M concentration. It is recommended that BH3-THF solutions be stored at 0-5 C, and that reactions are performed at under 35 C. Borane-dimethylsulfide (BH3-SMe2) is more stable than BH3-THF and is available in much higher concentrations (10 M).

Chemical properties

colourless liquid

The Uses of Borane-tetrahydrofuran complex

Borane-tetrahydrofuran complex is used to reduce Nylon surface amide groups to secondary amines.It is an important reagent used in the reduction of certain functional groups viz. aldehyde, ketone, carboxylic acid, amide, oxime, imine and nitrile. It is also used as hydro borating agent. It acts as a borane source for oxazaborolidine catalyzed asymmetric reductions.

The Uses of Borane-tetrahydrofuran complex

To a 0.24M THF suspension of the SM (71.7 mmol) at 0 C was added BH3-THF (1M, 220 mmol). The reaction mixture was stirred under Ar at RT for 66 h then quenched by addition of EtOH (15 mL) at 0 C and stirred an additional 15 min. The mixture was poured into H2O and extracted with DCM. The combined organics were washed with brine, dried (Na2SO4), and concentrated to provide the crude product as a white solid. [10.16 g, 62%]

The Uses of Borane-tetrahydrofuran complex

Borane-tetrahydrofuran complex is an important reagent used in the reduction of certain functional groups viz. aldehyde, ketone, carboxylic acid, amide, oxime, imine and nitrile. It is also used as hydro borating agent. It acts as a borane source for oxazaborolidine catalyzed asymmetric reductions. It is utilized to reduce nylon surface amide groups to secondary amines.

What are the applications of Application

New, Safer, NIMBA-Stabilized BH3 THF Solutions
BH3-THF can be used as a reducing agent for the reduction of various functional groups such as carboxylic acids, aldehydes, ketones, esters, acid chlorides, nitriles, epoxides, amides, lactones, oximes, and imines into corresponding alcohols and amines. Grignard reagents, arylmercury, arylthalium, and allyl and propargyllithium compounds react with BH3?THF to give organoboranes, which can be oxidized to the corresponding alcohols, phenols, and 1,3-diols.
It can also be used:
To synthesize the chiral borane catalyst, which is used in the enantioselective halo-aldol reaction.
To prepare 9-unsubstituted acridines by reduction of corresponding acridones.
To reduce nylon surface amide groups to secondary amines.

Reactions

Borane-tetrahydrofuran complex (BTHF) is a valuable reagent for the reduction of functional groups and for hydroboration reactions with carbon-carbon double and triple bonds. Functional groups that are readily reduced by BTHF include aldehyde, ketone, carboxylic acid, amide, oxime, imine, and nitrile. The carboxylic acid group is reduced at a faster rate than most groups including non-conjugated alkene. Conjugated α,β-unsaturated carboxylic acids give saturated alcohols as the major products.
Ketones and the carbonyl of enones are effectively reduced with borane-tetrahydrofuran. The addition of borohydride to the reaction solution is advantageous for accelerated reduction as well as higher selectivity towards carbonyl reduction in conjugated and non-conjugated enones.

Asymmetric ketone reduction using chiral oxazaborolidine catalysts was recently reviewed. Work at Callery with BTHF improved on reaction conditions to provide consistent results in the reduction.

General Description

Borane tetrahydrofuran complex (BH₃-THF) is widely used as a reducing agent in organic synthesis. It is also used as a reagent in hydroboration reactions.

Precautions

Air and moisture sensitive. Forms explosive peroxides in contact with air. Incompatible with acids, acid chlorides, acid anhydrides, oxidizing agents and alcohols. On hydrolysis, it forms hydrogen and boric acid.