Cobalt Boride1,2

Co2B

[12045-01-1]  · BCo2  · Cobalt Boride  · (MW 127.8)

(a selective hydrogenation catalyst;3 reduces nitriles to amines,4,5 nitroarenes to azobenzenes,6 sulfoxides to sulfides,7,8 and, in the presence of CO, carbonylates aryl9 and benzyl10 halides)

Physical Data: mp 1265 °C.11

Solubility: insol aqueous base and most organic solvents; reacts with aqueous acid.

Form Supplied in: black granules (a mixture of Co2B and Co3B).

Preparative Method: a solution of CoCl2 in methanol is treated with five mol equiv of NaBH4 at 0 °C; the black solid which is produced can be isolated after 15 min and is air stable. Drying in vacuo (6 h) produces a pyrophoric solid.

Handling, Storage, and Precautions: while the solid can be easily handled when wet with solvent, most preparations use Co2B generated in situ. Care must be exercised when handling CoII salts as they are known to be toxic in man (e.g. large amounts of CoCl2 depress erythrocyte production).

Catalyst Composition and Structure.

The composition of the catalyst produced by the reaction of Cobalt(II) Chloride and Sodium Borohydride is dependent on the reaction conditions (temperature, solvent, presence of a ligand, etc.). The amount of salts which precipitates onto the surface of the boride during preparation greatly affect their selectivity. Therefore the solubility of the byproduct salt (e.g. NaCl), in the medium in which the boride is prepared is important.1 Early studies of the action of NaBH4 on transition metal salts12 (FeII, CuII, PdII, NiII, CoII, etc.) showed that either the metal is produced (as in the case of PdCl2) or a granular black solid results (as in the case of CoCl2); in either case, hydrogen gas is evolved. Analysis of the black solid formed from CoCl2 suggested, … {the catalyst} to be a cobalt boride….12,13

When CoII salts react with NaBH4 in a CO saturated solution, no boride forms (the solution remains green); instead, a cobalt carbonyl complex forms which carbonylates aryl9 and benzylic10 halides. Similarly, CoCl2/NaBH4/Ph3P acts as a hydroboration or bond isomerization catalyst.14

Hydrogenation of Alkenes and Alkynes.

From early reports12,15 it was clear that the CoII salt/borohydride catalyst was both a selective reagent for the reduction of alkenes, as well as a convenient source of hydrogen gas. Rhodium, ruthenium, and platinum salts were the most effective in the decomposition of NaBH4 to generate H2;16 however, NiII provided the most active and selective hydrogenation catalyst for simple hydrocarbons. CoII salts/NaBH4 selectively reduces mono- and disubstituted alkenes (trisubstituted alkenes are unaffected) as well as alkynes, as shown in Tables 1 and 2.3,17

While there has been a tremendous amount of speculation as to the species responsible for these reactions, it has been shown that use of hydrogen gas over preformed Co2B works as well as the CoII/NaBH4 reagent and that under conditions which disfavor hydrogen gas evolution (THF-MeOH, 12:1) limonene is not reduced with the CoII/NaBH4 reagent.5

Other CoII/boron hydride mixtures which are effective selective reducing agents include t-butylamine borane (TAB)/Co2B (in refluxing methanol)18 and dimethoxyborane/Co2B (or Ni2B)19 (4-6 equiv hydride, 0.1 equiv boride, 1 equiv alkene).

Several CoII/boron hydride mixtures are effective catalysts for the reduction of a,b-unsaturated systems as shown in Table 3.18,20-25

The b-thioenone example is noteworthy for several reasons:

  • 1)The ketone survives in the presence of an excess of NaBH4; in the absence of CoCl2, an allylic alcohol is formed.
  • 2)In general, reactions which use catalytic amounts of CoCl2 are more effective than those which use stochiometric amounts of the chloride.
  • 3)NiCl2 is somewhat less effective (10% lower yield). CeCl3.7H2O gives the inverted enone as shown in the last entry of Table 3. FeCl3, CuI and CuCl2 had no activity in this system.

    Chiral C2-symmetric semicorrin/CoII complexes (such as 1), in combination with NaBH4, reduce b-substituted acrylates enantioselectively (eq 1).26

    These results are comparable to those obtained with Noyori's RuII-BINAP27 but does not require a free alcohol, carboxylate, or amide in the substrate.

    Reduction of Nitrogenous Functional Groups.

    Nitriles.

    One of the most widely used reactions of the CoCl2/NaBH4 system is the reduction of nitriles to primary amines.4 This method has been used in the synthesis of 11C28 and 3H23 labelled radiodiagnostics. Dimethoxyborane has been substituted for NaBH4 in the reduction of nitriles and aldehydes19,29 in the presence of transition metals (NiII, PdII, and PtII have also been used).4

    Studies of the nitrile reduction5,18 with the CoII/NaBH4 reagent has provided some insight into the mechanism of the reaction:

  • 1)Co2B acts as a heterogenous catalyst (Co2B/H2 works as well).
  • 2)Co2B activates the nitrile towards reaction with uncomplexed NaBH4.
  • 3)The reaction is pH sensitive (the optimal pH range is 8-9).

    This last observation led to the development of the Co2B/t-butylamine/borane system for the reduction of nitriles.18 Example of the reduction of nitriles to amines are shown in Table 4.4,28,30-34

    Nitro Groups.

    While nitroarenes are reduced to anilines in modest yields (30-50%) using the CoII/NaBH4 system,4 nitroalkanes are reduced slowly in good yields to amines (NiII 1 and CuII 35 are preferred reagents for this reduction). Some examples of this reduction are given in Table 5.32,34,36,37

    Other Nitrogenous Functional Groups.

    Satoh4 has reported the use of CoCl2/NaBH4 in methanol for the reduction of primary amides to amines: however, others1,18,38 have reported problems with reproducibility in this reaction. Azobenzenes are reduced to hydrazobenzenes and hydrazobenzenes to anilines by the action of CoCl2/NaBH4.39 Similarly, imines (and enamines) can be reduced to secondary amines40 by CoCl2/NaBH4. By contrast, oximes are converted into secondary alcohols by CoCl2/NaBH4.41

    Miscellaneous Reactions.

    As noted above, the course of reaction involving CoII and various borohydrides is sensitive to reaction conditions. When NaBH4 is added to a solution of CoCl2 saturated with CO, no precipitate of Co2B is observed; in the presence of a base, this mixture carbonylates benzylic halides (eq 2).10

    When an aryl bromide or iodide9 is used, a mixture of a-keto acid, glycolic acid, and benzoic acid results (the a-keto acid predominates). These reactions are thought to proceed via a Co(CO)4 anion generated in situ (Sodium Sulfide and NaSMe have also been used in the place of the NaBH4).10 Under phase transfer conditions this reaction has been used to prepare malonates from chloroacetates in high yield.42

    The CoII/NaBH4 reduction system deoxygenates aromatic and aliphatic sulfoxides in good to excellent yields.7 This mixture also desulfurizes sulfides, sulfoxides, and disulfides8 in good to excellent yields (however, Nickel Boride is a generally superior desulfurizing agent). NaBH4, Lithium Triethylborohydride, and NaBHEt3 have been used in combination with CoCl2 or Iron(II) Chloride to desulfurize thiols and thioketones43 (FeCl2 is the preferred reagent for this reaction).

    Adams44 reported the reduction of an aldehyde in the presence of a ketone using the CoII/NaBH4 reagent in DMSO solution (Cerium(III) Chloride also works in this system, a reversal of the Luche reduction protocol).45 Satyanarayana found CoCl2/NaBH4 to be an effective hydroboration17 and alkene isomerization catalyst14 (in the presence of Triphenylphosphine as a ligand). The same group also found that the CoCl2/Ph3P/NaBH4 system is an effective catalyst for the synthesis of trans dienes via reductive dimerization of alkynes.46


    1. (a) Ganem, B.; Osby, J. O. CRV 1986, 86, 763. (b) Wade, R. C. J. Mol. Catal. 1983, 48, 763.
    2. It should be noted that Co2B represents a nominal stoichiometry of the reagent prepared by the action of NaBH4 on a cobalt(II) salt as described above. Several CoxBy species have been described in the literature. Chemical Abstracts uses the registry number 12619-68-0 to designate cobalt boride of unspecified stochiometry. Furthermore, the reagent prepared from a cobalt(II) salt in the presence of a ligand (such as CO, Ph3P, and glyoximate) does not always involve a boride species.
    3. Chung, S.-K. JOC 1979, 44, 1014.
    4. Satoh, T.; Suzuki, S.; Suzuki, Y.; Miyaji, Y.; Imai, Z. TL 1969, 4555.
    5. Osby, J. O.; Heinzman, S. W.; Ganem, B. JACS 1986, 108, 67.
    6. Satoh, T.; Suzuki, S.; Kikuchi, T.; Okada, T. CI(L) 1970, 1626.
    7. Chung, S.-K.; Han, G. SC 1982, 12, 903.
    8. Back, T. G.; Baron, D. L.; Yang, K. JOC 1993, 58, 2407.
    9. Itoh, K.; Miura, M.; Nomura, M. BCJ 1988, 61, 4151.
    10. Satyanarayana, N.; Periasamy, M. TL 1987, 28, 2633.
    11. This is the melting point of Co2B prepared by the fusion of the elements (AC 1961, 32, 53). Material prepared by the reduction of CoCl2 with NaBH4 begins to decompose at 100 °C, when heated in vacuo, with the liberation of hydrogen (Maybury, P. C.; Mitchell, R. W.; Hawthorne, M. F. CC 1974, 534).
    12. Schlesinger, H. I.; Brown, H. C.; Finholt, A. E.; Gilbreath, J. R.; Hoekstra, H. R.; Hyde, E. K. JACS 1953, 75, 215.
    13. A boride of the same composition had been previously described (Stock, A.; Kuss, E. CB 1914, 47, 810; CA 1914, 8, 2129).
    14. Satyanarayana, M.; Periasamy, M. JOM 1987, 319, 113.
    15. Paul, R.; Buisson, P.; Joseph, N. CR(C) 1951, 232, 627 (CA 1951, 45, 10 436h).
    16. Brown, H. C.; Brown, C. JACS 1962, 84, 1493.
    17. Satyanarayana, M.; Periasamy, M. TL 1984, 25, 2501.
    18. Heinzman, S.; Ganem, B. JACS 1982, 104, 6801.
    19. Nose, A.; Kudo, T. CPB 1990, 38, 1720.
    20. Iwata, C.; Takemoto, Y.; Kubota, T.; Imanishi, T. TL 1985, 26, 3231.
    21. Satoh, T.; Nanba, K.; Suzuki, S. CPB 1971, 19, 817.
    22. Satoh, D.; Hashimoto, T. CPB 1976, 24, 1950.
    23. Bracchini, B.; Adams, E. W.; Eagan, L. A.; Murtiashaw, M. J. Labelled Comp. Radiopharm. 1992, 31, 384. Note: when NaBT4 was used the chemical yield was 49% with a radiochemical purity of 89%.
    24. Ihara, M.; Tokunaga, Y.; Fukumoto, K. JOC 1990, 55, 4497.
    25. Nishio, T.; Omote, Y. JCS(P1) 1981, 934.
    26. Leutenegger, U.; Madin, A.; Pfaltz, A. AG(E) 1989, 28, 60. Note: it is not known whether this reaction involves a boride intermediate or not.
    27. (a) Takaya, H.; Ohta, T.; Sayo, N.; Kumobayashi, S.; Akutagawa, S.; Inoue, S.; Kasahara, I.; Noyori, R. JACS 1987, 109, 1596. (b) Ohta, T.; Takaya, H.; Kitamura, M.; Nagai, K.; Noyori, R. JOC 1987, 52, 3174.
    28. (a) Ding, Y. S.; Antoni, G.; Fowler, J. S.; Wolf, A. P.; Langstrom, B.; J. Labelled Comp. Radiopharm. 1989, 27, 1079. (b) Antoni, G.; Langstrom, B.; J. Labelled Comp. Radiopharm. 1989, 27, 571.
    29. Nose, A.; Kudo, T. CPB 1989, 37, 808.
    30. Gonzales, F. S.; Bereguel, A. U.; Mateo, F. H.; Mendoza, P. G. Carbohydr. Res. 1991, 209, 131.
    31. Beaulieu, P. L.; Schiller, P. TL 1988, 29, 2019.
    32. Acevedo, O. L.; Krawczyk, S. H.; Townsend, L. B. TL 1983, 24, 4789.
    33. Brimble, M. A.; Rowan, D. D. CC 1988, 978.
    34. Midland, M. M.; Lee, P. E. JOC 1985, 50, 3237.
    35. Cowan, J. A. TL 1986, 27, 1205.
    36. Delaszlo, S. E.; Ley, S. V.; Porter, R. A. CC 1986, 344.
    37. Nose, A.; Kudo, T. CPB 1988, 36, 1529.
    38. Baird, D. B.; Baxter, I.; Cameron, D. W.; Phillips, R. A. JCS(P1) 1973, 832.
    39. Avar, G; Kisch, H. M 1978, 109, 89.
    40. Periasamy, M.; Devasagayaraj, A.; Satayanarayana, N.; Narayana, C. SC 1989, 19, 565.
    41. Shoji, N.; Kondo, Y.; Takemoto, Y. H 1975, 3, 147.
    42. Kantam, M. L.; Choudary, B. M.; Reddy, N. P. SC 1990, 20, 2631.
    43. (a) Alper, H.; Prince, T. L. AG(E) 1980, 19, 315. (b) Alper, H.; Ripley, S.; Prince, T. L. JOC 1983, 48, 250.
    44. Adams, C. SC 1984, 14, 1349.
    45. Gemal, A. L.; Luche, J.-L. JOC 1979, 44, 4187.
    46. Satyanarayana, N.; Periasamy, M. TL 1986, 27, 6253.

    Thomas J. Caggiano

    Wyeth-Ayerst Research, Princeton, NJ, USA



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