Nickel Boride1

Ni2B
(Ni2B)2

[12007-01-1]  · BNi2  · Nickel Boride  · (MW 128.19)

(selective hydrogenation catalyst,1a,c,3 desulfurization catalyst;4 reduces nitro5 and other functional groups;1a dehalogenation catalyst;1b,6 hydrogenolysis7 catalyst)

Physical Data: mp 1230 °C.8

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

Form Supplied in: black granules, stoichiometry varies with supplier.

Preparative Methods: to a stirred suspension of 1.24 g (5 mmol) of powdered Nickel(II) Acetate in 50 mL of 95% ethanol is added 5 mL of a 1 M solution of Sodium Borohydride in 95% ethanol at room temperature (control frothing). Stirring is continued until the gas evolution ceases (usually 30 min). The flask is used directly in the hydrogenation.1c,9a This catalyst is nonpyrophoric.

Handling, Storage, and Precautions: caution must be taken in handling nickel salts. Ingestion of soluble nickel salts causes nausea, vomiting, and diarrhea. Nickel chloride has an LD50 (iv) = 40-80 mg kg-1 in dogs. Many nickel salts will sublime in vacuo. Nickel metal is carcinogenic and certain nickel compounds may reasonably be expected to be carcinogenic.

Catalyst Composition and Structure.

The composition of the catalyst produced by the reaction of NiII salts and Sodium Borohydride is dependent on reaction conditions (solvent, stoichiometry, temperature, etc.).9 X-ray photoelectron spectroscopy10 showed that the main difference between the P1 form of nickel boride (P1 Ni) and the P2 form of nickel boride (P2 Ni) is the amount of NaBO2 adsorbed on to the surface of the catalyst. P1 Ni (which is prepared in water) has an oxide:boride ratio of 1:4, while P2 Ni (which is prepared in ethanol) has a ratio of 10:1. Early studies of the reaction of borohydrides with transition metal salts11 (FeII, CuII, PdII, NiII, CoII, etc.) showed that the reaction product is either the metal (as in the case of PdII) or a black granular solid (as in the case of NiII); in both cases, H2 is evolved.11c,12 Analysis of the black solid formed from the NiII suggested the catalyst to be a boride.11c,13 Paul et al.11b examined several NiII salts and found nickel acetate to be most acceptable.

Hydrogenation of Alkenes and Alkynes.

Brown has described two forms of nickel boride (P1 Ni and P2 Ni)9 which are hydrogenation catalysts. In a comparison of P1 Ni to W2 Raney Nickel (Ra Ni) as a hydrogenation catalyst, P1 Ni was found to be somewhat more active (as measured by the t1/2 for hydrogenation of several alkenes).9a What is more important in the comparison of Ra Ni and P1 Ni is the lower incidence of double-bond isomerization observed with P1 Ni vs. Ra Ni (3% vs. 20%). P1 Ni reduces mono-, di-, tri-, and tetrasubstituted alkenes under mild conditions (1 atm H2, rt) while leaving many groups unaffected (e.g. a phenyl ring). There is a significant difference in the rate of reduction among the various substituted alkenes allowing for selectivity. However, P2 Ni is very sensitive to steric hindrance and to the alkene substitution pattern. Little or no hydrogenolysis of allylic, benzylic, or propargylic substituents is observed with this catalyst; partial reduction of alkynes and dienes are also possible. Some examples of the use of P2 Ni as a hydrogenation catalyst are shown in Table 1.

Under more forcing conditions (30 psi in a Parr apparatus), Russell19 was able to reduce unsaturated ethers, alcohols, aldehydes, esters, amines, and amides to their saturated counterparts without hydrogenolysis. Unsaturated nitriles19b were reduced to primary amines while epoxides were unaffected by the reagent. Both dimethoxyborane (eq 1)20 and Lithium Aluminum Hydride (eq 2)21 can replace NaBH4 in these reactions.

Heteroarenes.

Nose and Kudo22 examined the reduction of quinaldine (1) with a variety of transition metal salts (CoCl2, NiCl2, CuCl2, CrCl3) in the presence of NaBH4; only Nickel(II) Chloride was effective (eq 3).

Partial reduction of a series of heteroaromatics was examined using NiCl2/NaBH4 in methanol at room temperature (Table 2). The authors suggest that the reduction proceeds through a NiCl2 complex of the arene; however, other workers1a dispute this mechanism.

Desulfurization.

While Raney nickel23 is the traditional reagent for desulfurization reactions, it has several drawbacks (i.e. strongly basic, pyrophoric, sensitivity to air and moisture). In 1963, Truce and Roberts24 reported the use of NiCl2/NaBH4 in the partial cleavage of a dithioacetal (eq 4).

Since then, there have been numerous examples of the use of NiII salt/NaBH4 in desulfurization reactions;4 in many cases the yields are greater than those seen with Raney nickel25 (eq 5) (note: caution must be exercised when using NaBH4 in DMF).

Boar et al.26 used nickel boride in a protection-deprotection scheme for triterpenoid ketones (eq 6).

NiII/NaBH4 is an effective reagent for desulfurization of thioamides,27 thioethers,28 and sulfides.4,29 Back and co-workers4,30 has reported extensive studies of the scope, stereochemistry, and mechanism of nickel (and cobalt) boride desulfurizations. In general, nickel boride is a more effective desulfurization catalyst than Cobalt Boride (other metals such as Mo, Ti, Cu, and Fe were completely ineffective). Lithium Borohydride can be used in place of NaBH4 while Sodium Cyanoborohydride cannot. Sulfides, thioesters, thiols, disulfides, and sulfoxides are reduced to hydrocarbons by Ni2B, while sulfoxides are stable. Esters, chloro groups, and phenyl groups are stable to Ni2B. Iodides, nitro groups, nitriles, and alkenes are reduced completely by Ni2B, while bromides, aldehydes, ketones, and cyclopropanes show variable reactivity (eqs 7-10).

Using deuterium labelling, Back showed that desulfurization occurs with retention of configuration, unlike Raney nickel, which involved a radical mechanism. The suggested mechanism of desulfurization involves an oxidative addition-reductive elimination sequence via a nickel hydride intermediate.

Reduction of Other Nitrogenous Functional Groups.

Primary, secondary, and tertiary aliphatic nitro groups are reduced to amines with NiCl2/NaBH4.5c Hydrazine hydrate has also been used with Ni2B to reduce both aryl and aliphatic nitro groups in a synthesis of tryptamine (eqs 11 and 12).31

Reductive cleavage of thioethers and reduction of nitro groups has been combined in a synthesis of pyrrolidones (eq 13).32

Like Co2B, Ni2B5b reduces nitroarenes to anilines and azoxybenzenes to azobenzenes (Table 3); unlike Co2B, Ni2B reduces oximes33 to amines (Table 4).

Reduction of Other Nitrogenous Functional Groups.

Borane-Tetrahydrofuran/NiCl2 has been used to reduce chiral cyanohydrins to ethanol amines in high yield.35 Azides are cleanly reduced to amines in good yield with nickel boride.36 Azides are reduced in preference to hindered aliphatic nitro groups (eq 14).37

Isoxazoles are reduced to b-amino enones in high yield using the NiCl2/NaBH4 system.38 Dihydroisoxazolones are reduced with a high degree of diastereoselectivity with the NiCl2/NaBH4 system.39

Dehalogenation.

Many a-bromo ketones6a are cleanly reduced to the parent ketone with nickel boride in DMF (caution). Vicinal dibromides are reduced to alkenes (eq 15).

Aryl and certain alkyl chlorides can be dehalogenated1a,1b,6b with a variety of NiII/hydride agents (e.g. NaBH2(OCH2CH2OMe)2, Triethylsilane, NaBH4). Lin and Roth have effected the clean debromination of aryl bromides40 using Dichlorobis(triphenylphosphine)nickel(II)/NaBH4 in DMF (caution); Tris(triphenylphosphine)nickel(0) is assumed to be the active catalyst. Russel and Liu41 demonstrated that reductive cleavage of an iodide goes with retention when NiCl2/NaBH4 is used (cf. inversion seen with LiAlH4; eq 16).

Hydrogenolysis.

Ni2B has been used to hydrogenolyze benzylic (eqs 17-19),7a allylic (eqs 20-22),7b,42 and propargylic (eq 23)7b esters in good yields.

Enol tosylates and aryl tosylates are deoxygenated in good to excellent yields43 (eqs 24 and 25)

A variety of allylic functional groups44 (alcohols, esters, silyl ethers, ketones, and hydroperoxides) have been reduced with Ni2B. The combination of Chlorotrimethylsilane/Ni2B will selectively reduce an aldehyde in the presence of a ketone.45

Selenides46 and tellurides47 are reductively cleaved by Ni2B with retention of stereochemistry. The phenyl selenyl group is cleaved in preference to the thio phenyl group.


1. (a) Ganem, B.; Osby, J. O. CRV 1986, 86, 763. (b) Wade, R. J. Mol. Catal. 1983, 48, 273. (c) Hudlicky, M.; Reductions in Organic Chemistry; Wiley: New York, 1984.
2. It should be noted that Ni2B represents a nominal stoichiometry for the reagent prepared by the action of NaBH4 on a NiII salt. Several NixBy species have been described in the literature. Chemical Abstracts uses the registry number [12619-90-8] to designate nickel boride of unspecified stoichiometry. [12007-02-2] and [12007-00-0] are the registry numbers for Ni3B and NiB, respectively. These are the most widely cited synthetically useful reagents.
3. (a) Brown, C. A. JOC 1970, 35, 1900. (b) Brown, C. A.; Ahuja, V. K. JOC 1973, 38, 2226.
4. (a) Back, T. G.; Baron, D. L.; Yang, K. JOC 1993, 58, 2407. (b) Back, T. G.; Yang, K.; Krouse, R. H. JOC 1992, 57, 1986.
5. (a) Nose, A.; Kudo, T. CPB 1989, 37, 816. (b) Nose, A.; Kudo, T. CPB 1988, 36, 1529. (c) Osby, J. O.; Ganem, B. TL 1985, 26, 6413. (d) Nose, A.; Kudo, T. CPB 1981, 29, 1159.
6. (a) Sarma, J. C.; Borbaruah, M.; Sharma, R. P. TL 1985, 26, 4657. (b) Tabaei, S-M. H.; Pittman, C. V. TL 1993, 34, 3264.
7. (a) He, Y.; Pan, X.; Wang, S.; Zhao, H. SC 1989, 19, 3051. (b) Ipaktschi, J. CB 1983, 117, 3320 (CA 1985, 102, 94 904x).
8. This is the melting point of Ni2B formed by fusion of the elements Adv. Chem. Ser. 1961, 32, 53). Material prepared by the reduction of NiCl2 with NaBH4 begins to decompose at 100 °C when heated in vacuo with liberation of H2 (Maybury, P. C.; Mitchell, R. W.; Hawthorne, M. F. JCS(C) 1974, 534).
9. (a) This procedure provides the P2 form of nickel boride, which is a selective hydrogenation catalyst. Brown, H. C.; Brown, C. A. JACS 1963, 85, 1005. (b) Brown, H. C.; Brown, C. A. JACS 1963, 85, 1003. This paper reports the preparation and properties of P1 nickel boride. P1 nickel boride is more active, in some applications, than Raney nickel. (c) Destefanis, H.; Acosta, D.; Gonzo, E. Catal. Today 1992, 15, 555. This group describes the use of BH3.THF complex to prepare Ni3B and Ni4B3 using Ni(OAc)2 and NiCl2, respectively, and their use as hydrogenation catalysts.
10. Schreifels, J. A.; Maybury, C. P.; Swartz, W. E. JOC 1981, 46, 1263.
11. (a) Paul, R.; Buisson, P.; Joseph, N. Ind. Eng. Chem. 1952, 44, 1006 (CA 1952, 46, 9960e). (b) Paul, R.; Buisson, P.; Joseph, N. CR(C) 1951, 232, 627 (CA 1951, 45, 10 436h). (c) Schlesinger, H. R.; Brown, H. C.; Finholt, A. E.; Gilbreath, J. R.; Hoekstra; Hyde, E. K. JACS 1953, 75, 215.
12. Brown, H. C.; Brown, C. A. JACS 1962, 84, 1493.
13. A boride of the same composition had been previously described (Stock, A.; Kuss, E. CB 1914, 47, 810 (CA 1914, 8, 2129).
14. Jefford, C. W.; Jaggi, D.; Bernardinelli, G.; Boukouvalas, J. TL 1987, 28, 4041.
15. Novak, J.; Salemink, C. A. JCS(P1) 1982, 2403.
16. Miller, J. G.; Ochlschlager, A. C. JOC 1984, 49, 2332. This reaction uses TMEDA as an additive.
17. Kido, F.; Abe, T.; Yoshikoshi, A. JCS(C) 1986, 590.
18. Lee, K-H.; Ibuka, T.; Sims, D.; Muraoka, O.; Kiyokawa, H.; Hall, I. H.; Kim, H. L. JMC 1981, 24, 924. When Pt2O was used, only 20% of the desired product was isolated; the major product was the tetrahydro compound.
19. (a) Russell, T. W.; Hoy, R. C. JOC 1971, 36, 2018. (b) Russell, T. W.; Hoy, R. C.; Cornelius, J. E. JOC 1972, 37, 3552.
20. Nose, A.; Kudo, T. CPB 1990, 38, 1720.
21. Jung, M.; Elsohly, H. N.; Croon, E. M.; McPhail, D. R.; McPhail, A. T. JOC 1986, 51, 5417.
22. Nose, A.; Kudo, T. CPB 1984, 32, 2421.
23. Pettit, G. R.; van Tamelen, E. E. OR 1962, 62, 347.
24. Truce, W. E.; Roberts, F. E. JOC 1963, 28, 961.
25. Zaman, S. S.; Sarmah, P.; Barus, N. C.; Sharma, R. P. CI(L) 1989, 806.
26. Boar, R. B.; Hawkins, D. W.; McGhie, J. F.; Barton, D. H. R. JCS(P1) 1973, 654.
27. Guziec, F. S.; Wasmund, L. M. TL 1990, 31, 23.
28. (a) Euerby, M. R.; Waigh, R. D. SC 1986, 16, 779. (b) Euerby, M. R.; Waigh, R. D. JCS(C) 1981, 127.
29. Truce, W. E.; Perry, F. M. JOC 1965, 30, 1316.
30. Back, T. G.; Yang, K. JCS(C) 1990, 819.
31. Lloyd, D. H.; Nichols, D. E. JOC 1986, 51, 4294.
32. Posner, G. H.; Crouch, R. D. T 1990, 46, 7509.
33. Ipaktschi, J. CB 1984, 117, 856 (CA 1984, 101, 22 611f).
34. Seltzman, H. H.; Berrang, B. B. TL 1993, 34, 3083.
35. Lu, Y.; Meit, C.; Kunesch, N.; Poisson, J. TA 1990, 1, 707.
36. Sarma, J. C.; Sharma, R. P. CI(L) 1987, 764.
37. Guilano, R. M.; Deisenroth, T. W. J. Carbohydr. Res 1987, 6, 295.
38. (a) Koroleva, E. V.; Lakhvich, F. A.; Yankova, T. V. KGS 1987, 11, 1576 (CA 1988, 109, 928 546). (b) Oliver, J. E.; Lusby, W. R. T 1988, 44, 1591.
39. (a) Lakhvich, F. A.; Koroleva, E. V.; Antonevich, I. Q.; Yankova, T. V. ZOR 1990, 26, 1683 (CA 1991, 114, 81 311). (b) Annunziata, R.; Cinquini, M.; Cozzi, F.; Gilardo, A.; Restelli, A. JCS(P1) 1985, 2289.
40. Lin, S. T.; Roth, J. A. JOC 1979, 44, 309.
41. Russel, R. N.; Liu, H. W. TL 1989, 30, 5729.
42. Jiang, B.; Zhao, H.; Pan, X.-F. SC 1987, 17, 997.
43. Wang, F.; Chiba, K.; Tada, M. JCS(P1) 1992, 1897.
44. (a) Sarma, D. N.; Sharma, R. P. TL 1985, 26, 2581. (b) Zaman, S. S.; Sarma, J. C.; Sharma, R. P. CI(L) 1991, 509. (c) Sarma, D. N.; Sharma, R. P. TL 1985, 26, 371.
45. Borbaruah, M.; Barua, N. C.; Sharma, R. P. TL 1987, 28, 5741.
46. (a) Back, T. G.; Birss, V. I.; Edwards, M.; Krishna, M. V. JOC 1988, 53, 3815. (b) Back, T. G. JCS(C) 1984, 1417.
47. (a) Barton, D. H. R.; Fekih, A.; Lusinchi, X. TL 1985, 26, 6197. (b) Barton, D. H. R.; Bohe, L.; Lusinchi, X. TL 1990, 31, 93.

Thomas J. Caggiano

Wyeth-Ayerst Research, Princeton, NJ, USA



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