Diborane

B2H6

[19287-45-7]  · B2H6  · Diborane  · (MW 27.67)

(strong reducing agent for many functional groups; extremely efficient hydroborating reagent)

Physical Data: bp -92.5 °C; mp -165.5 °C; d 0.437 g cm-3 (liquid at -92.5 °C); heat of vaporization 3.41 kcal mol-1 (at -92.5 °C).

Solubility: slightly sol pentane and hexane; forms BH3 adduct with DMS, THF, and other ethers; reacts with H2O and protic solvents, releasing flammable hydrogen gas.

Form Supplied in: commercially available as a compressed gas.

Analysis of Reagent Purity: supplied as either 99.99% or >99.0% B2H6.

Handling, Storage, and Precautions: diborane is a toxic, pyrophoric gas and must be stored and handled accordingly. Feed lines and reactors should be flushed with N2 and kept free of moisture. Cylinders should be hard-piped directly to reactor. When not in use, cylinder valve should be securely closed and capped. Cold (-20 to 0 °C) storage is recommended to ensure product purity. Decomposition products include higher boron hydrides and hydrogen gas.

Diborane and Borane Reagents.

Borane complexes have been used in industrial-scale applications for years. In addition to their unique ability to add across carbon-carbon multiple bonds, boron hydrides reduce ketones, carboxylic acids, amides, and nitriles (eqs 1-3).1 By the appropriate choice of borane reagent and reaction conditions, a significant degree of chemical selectivity can be achieved. This high level of selectivity and reactivity has found utility not only in classical organic synthesis,2,3 but also in the area of asymmetric synthesis with the Corey-Itsuno catalyst serving as a prime example of a highly reactive and enantioselective reagent.4

Chemically, diborane is the parent of all the borane complexes. Diborane will react with a Lewis base to form an adduct in which the electron pair from the Lewis base is donated to the borane. This complexation alters the reactivity of the borane, subsequently changing its physical form. Diborane is a gas with a boiling point of -92.5 °C, whereas the borane complexes are either liquids or solids. As illustrated in the molecular model (1), diborane has some unusual bonding characteristics. The boron is so electron deficient that it resorts to sharing the electrons in the B-H bond, giving rise to bridging hydrogens. This electron deficiency allows formation of borane complexes with Lewis bases such as tetrahydrofuran (THF), dimethyl sulfide (DMS), and amines (Scheme 1).

In the search for better borane reagents, a variety of complexes have been synthesized; almost all fit under one of the three following classifications: ether-borane complexes, sulfide-borane complexes, or amine-borane complexes.

A general trend of borane complexes is that the stronger the Lewis base, the weaker the reducing power of the resulting borane.5 Borane-Tetrahydrofuran (THFB) is more reactive than Borane-Dimethyl Sulfide (DMSB), which in turn is more reactive than most amine-boranes. When reducing a carboxylic acid or hydroborating a carbon-carbon double bond, THFB and DMSB are both effective reagents, while amine-boranes generally are not suitable choices.

Comparison of Borane Reagents.

THFB and DMSB, however, do have drawbacks which can be of some concern in an industrial setting. The maximum workable concentration for THFB is 1 molar. Therefore a large reactor volume is required to achieve the desired reduction. The low loading in a given size reactor, and ultimately solvent waste, increase the cost-per-pound of product.

DMSB is commercially available as a neat solution (~10M); thus solvent volume is determined by reactor loading of other reactants (substrate). However, anyone who has had the opportunity to use dimethyl sulfide can attest to the fact that the human nose can detect DMS in the ppm range. Various schemes have been used in an attempt to scrub the DMS from reaction streams, but none are truly effective.

When diborane is added to an ether solvent, ether-borane complexes are formed. If the solvent is THF, then THFB is formed in situ, and reacts in the same manner without the high dilution effect of using the 1M THFB solution. By using diborane rather than THFB, reactor loading can be increased, thereby reducing costs. Use of diborane will also reduce the amount of waste solvent remaining at the end of the process. The advantage of diborane over DMSB is the avoidance of odor control problems inherent in dimethyl sulfide, again reducing costs.

Although much of the literature describing borane reductions involves coordinating solvents such as ethers, diborane can be employed in nonpolar hydrocarbon solvents.6-9 Table 1 shows an appreciable solubility of diborane in hexanes. It should be noted that the reactivity of diborane in hydrocarbons may be distinctly different from the reactivity of borane complexes. For example, because it is uncomplexed, diborane is generally much more reactive than sulfide boranes such as DMSB.

However, diborane should not be used in every reaction where THFB or DMSB are currently in use.

  • 1)DMSB is a good choice if the reduction requires heat to push the reaction to completion, because DMSB is stable at higher temperatures than ether-borane complexes.
  • 2)THFB is a good choice when addition of substrate to an excess of borane is required, and reactor loading is not a major concern. THFB solutions should not be heated above 50 °C, since diborane will be lost from the complex and reductive ring opening of the THF will occur.
  • 3)Diborane is a good choice when it is desirable to add borane to the substrate in the reactor. With diborane, higher reactor loadings can be achieved than with either of the other two reagents.

    Generally, if the reaction is fast at or below ambient temperature, diborane is an effective replacement for either THFB or DMSB.

    The classic synthesis of 9-Borabicyclo[3.3.1]nonane (9-BBN) from cis,cis-1,5-cyclooctadiene (cod) serves as a good model for hydroboration reactions (eq 4). The route described in the literature10 advises the use of DMSB in dimethoxyethane (DME). Following the hydroboration and removal of DMS, crystalline 9-BBN precipitates out of the DME solvent. Unfortunately, the resulting product is contaminated with the odor of dimethyl sulfide which is not readily removed from the 9-BBN. The use of THFB as the hydroborating reagent does alleviate the DMS odor, but the product remains solubilized as a dilute solution in THF and DME. When diborane is added as a gas to a DME solution of cod, the resulting 9-BBN is obtained as a clean crystalline product with a typical purity of >96%. No further processing is required from the diborane-produced 9-BBN.11

    Reduction of a substituted xanthone to a xanthene, shown in eq 5, was accomplished using three borane reagents.12 THFB was an effective reagent for this transformation, and was the most desirable for small-scale preparations because of its ease of use. The reduction utilizing DMSB could not be driven to completion. Diborane proved to be the most effective reagent and was highly desirable for commercial-scale synthesis of the xanthene. With the higher loading achievable using diborane, the THFB route was considered to be economically unfeasible.

    Practical Considerations.

    Introduction of diborane into a reactor requires some careful consideration of the reactor system. Diborane, like almost all hydrides, is very reactive. It is critical that the reactor be under an inert gas, such as nitrogen, since diborane is a pyrophoric material. The addition of diborane to a solution is best accomplished by feeding gaseous diborane under the surface of the solvent. If the product formed by reaction of diborane is a solid, diborane can be added to the reactor head space provided that a back pressure on the reactor keeps it contained. Given a reactor which is free of oxygen and moisture, diborane is used as any other compressed gas.

    Diborane cylinders are packaged in dry ice for shipment. The vapor pressure of diborane at -78 °C is 15 psig (776 Torr). This allows for a convenient method to control the pressure of diborane gas being fed to a reactor. The rate at which diborane is fed into the reactor is dictated by the thermodynamics and kinetics of the reaction, and the temperature constraints of the system.

    Conclusion.

    In many cases, diborane can be used as a cost effective reagent for reduction or hydroboration. Use of diborane in place of THFB can significantly increase the loading in a given reaction. Used in place of DMSB, it can eliminate the handling of dimethyl sulfide as a waste. All three reagents (diborane, THFB, and DMSB) have application in organic synthesis, and each should be evaluated for potential use.

    Related Reagents.

    Borane-Ammonia; Borane-Dimethyl Sulfide; Borane-Pyridine; Borane-Tetrahydrofuran; Ephedrine-borane; Norephedrine-Borane.


    1. Brown, H. C. Boranes in Organic Chemistry; Cornell University Press: Ithaca, NY, 1972.
    2. Brown, H. C.; Krishnamurthy, S. Aldrichim. Acta 1979, 12(1), 3.
    3. Brown, H. C.; Choi, Y. M.; Narasimhan, S. S 1981, 8, 605.
    4. Quallich, G. J.; Woodall, T. M. TL 1993, 34, 785.
    5. Zaidlewicz, M. In Comprehensive Organometallic Chemistry; Wilkinson, G.; Stone, F. G. A.; Abel, E. W., Eds.; Pergamon: Oxford, 1982; Vol. 7, pp 162-163.
    6. Schechter, W. H., Adams, R. M. and Jackson, C. B. Boron Hydrides and Related Compounds, Callery Chemical Company, 1951.
    7. Schlesinger, H. I., University of Chicago, Final Report Navy Contract N173s-9058 and 9820, 1944-5.
    8. Boldebuck, E. M.; Elliot, J. R.; Roedel, G. F.; Roth, W. L. Solubility of Diborane in Ethyl Ether and in Tetrahydrofuran; General Electric Company, Project Hermes Report No. 55288; Nov 19, 1948.
    9. Reed, J. W.; Masi, J. F. Solubility of Diborane in n-Pentane and in n-Butane; Callery Chemical Company, Report No. CCC-454-TR-300; Sept 30, 1958.
    10. Soderquist, J.; Negron, A. OS 1991, 70, 169.
    11. Corella, J. A., Callery Chemical Company; unpublished results.
    12. Burkhardt, E. R., Callery Chemical Company; unpublished results.

    Joseph M. Barendt & Beth W. Dryden

    Callery Chemical Company, Pittsburgh, PA, USA



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