NICKEL BORIDE

1

Nickel Boride

1

Ni

2

B

(Ni

2

B)

2

[12007-01-1]

BNi

2

(MW 128.19)

InChI = 1/B.2Ni/rBNi2/c1-2-3-1
InChIKey = WRLJWIVBUPYRTE-QAXWXQGDAA

(selective hydrogenation catalyst,

1a,c,3

desulfurization catalyst;

4

reduces nitro

5

and other functional groups;

1a

dehalogenation

catalyst;

1b,6

hydrogenolysis

7

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 sup-

plier.

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 con-
tinued until the gas evolution ceases (usually 30 min). The flask
is used directly in the hydrogenation.

1c,9a

This catalyst is non-

pyrophoric.

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
LD

50

(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 carcino-
genic.

Catalyst Composition and Structure. The composition of

the catalyst produced by the reaction of Ni

II

salts and Sodium

Borohydride is dependent on reaction conditions (solvent, sto-
ichiometry, temperature, etc.).

9

X-ray photoelectron spectro-

scopy

10

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 NaBO

2

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
salts

11

(Fe

II

, Cu

II

, Pd

II

, Ni

II

, Co

II

, etc.) showed that the reaction

product is either the metal (as in the case of Pd

II

) or a black granu-

lar solid (as in the case of Ni

II

); in both cases, H

2

is evolved.

11c,12

Analysis of the black solid formed from the Ni

II

suggested the

catalyst to be a boride.

11c,13

Paul et al.

11b

examined several Ni

II

salts and found nickel acetate to be most acceptable.

Hydrogenation of Alkenes and Alkynes.

Brown has de-

scribed 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 t

1/2

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 iso-
merization observed with P1 Ni vs. Ra Ni (3% vs. 20%). P1 Ni

reduces mono-, di-, tri-, and tetrasubstituted alkenes under mild
conditions (1 atm H

2

, 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 se-
lectivity. 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.

Table 1

Reduction of alkenes, dienes, and alkynes with P2 Ni

Substrate

Product

Yield (%) Ref.

1-Hexyne

Hexane

16

9a

1-Hexene

68

Starting material

16

3-Hexyne

Hexane

1

9a

cis

-3-Hexene

96

trans

-3-Hexene

3

2-Methyl-1,5-hexadiene

2-Methylhexane

2

9a

2-Methyl-1-hexene

96

Other methylhexenes

2

1,3-Cyclohexadiene

Cyclohexane

2

9a

Cyclohexene

89

Benzene

9

1-Penten-3-ol

3-Pentanol

100

9a

OH

O

O

OH

O

O

94

14

F

F

CO

2

H

F

F

CO

2

H

75

15

CO

2

H

TBSO

H

( )

4

( )

3

CO

2

H

TBSO

H

( )

4

( )

3

100

16

O

O

O

O

O

O

70

17

O

O

O

HO

H

H

H

O

O

O

HO

H

H

H

50

18

Under more forcing conditions (30 psi in a Parr apparatus),

Russell

19

was able to reduce unsaturated ethers, alcohols, alde-

hydes, esters, amines, and amides to their saturated counterparts
without hydrogenolysis. Unsaturated nitriles

19b

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 NaBH

4

in these reactions.

Avoid Skin Contact with All Reagents

2

NICKEL BORIDE

(MeO)

2

BH (8 equiv)

NiB

2

(0.5 equiv)

(1)

O

O

MeOH

93%

CO

2

H

CO

2

Me

H

H

H

H

(2)

1. LiBH

4

(0.5 equiv)

NiCl

2

(0.5 equiv)

2. CH

2

N

2

95%

Heteroarenes. Nose and Kudo

22

examined the reduction of

quinaldine (1) with a variety of transition metal salts (CoCl

2

,

NiCl

2

, CuCl

2

, CrCl

3

) in the presence of NaBH

4

; only Nickel(II)

Chloride was effective (eq 3).

(3)

NiCl

2

(1.4 mmol)

NaBH

4

(32 mmol)

N

N
H

(1)

MeOH, rt

93%

Partial reduction of a series of heteroaromatics was examined

using NiCl

2

/NaBH

4

in methanol at room temperature (Table 2).

The authors suggest that the reduction proceeds through a NiCl

2

complex of the arene; however, other workers

1a

dispute this mech-

anism.

Table 2

Reduction of heteroaromatics with NiCl

2

/NaBH

4

Substrate

Product

Yield (%)

N

N
H

83

N

N

N
H

H
N

99

N

NH

96

N

N
H

83

N

N

N
H

H
N

54

N

N

N
H

H
N

52

Desulfurization. While Raney nickel

23

is the traditional re-

agent for desulfurization reactions, it has several drawbacks (i.e.
strongly basic, pyrophoric, sensitivity to air and moisture). In

1963, Truce and Roberts

24

reported the use of NiCl

2

/NaBH

4

in

the partial cleavage of a dithioacetal (eq 4).

(4)

NiCl

2

, NaBH

4

O

PhS SPh

O

PhS

reflux

71%

Since then, there have been numerous examples of the use of

Ni

II

salt/NaBH

4

in desulfurization reactions;.

4

in many cases the

yields are greater than those seen with Raney nickel

25

(eq 5) (note:

caution must be exercised when using NaBH

4

in DMF)

.

OAc

S

S

OAc

H

H

H

H

(5)

NiCl

2

, NaBH

4

DMF

90%

Raney Ni gave 85%

Boar et al.

26

used nickel boride in a protection–deprotection

scheme for triterpenoid ketones (eq 6).

S

O

HO

(6)

O

O

NiCl

2

, NaBH

4

EtOH, H

2

O

+

mixture is not separated

87% from acetal

CrO

3

H

3

BO

3

Ni

II

/NaBH

4

is an effective reagent for desulfurization of thioa-

mides,

27

thioethers,

28

and sulfides.

4,29

Back and co-workers

4,30

has reported extensive studies of the scope, stereochemistry, and
mechanism of nickel (and cobalt) boride desulfurizations. In gen-
eral, 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 NaBH

4

while Sodium Cyanoborohydride cannot.

Sulfides, thioesters, thiols, disulfides, and sulfoxides are reduced
to hydrocarbons by Ni

2

B, while sulfoxides are stable. Esters,

chloro groups, and phenyl groups are stable to Ni

2

B. Iodides, ni-

tro groups, nitriles, and alkenes are reduced completely by Ni

2

B,

while bromides, aldehydes, ketones, and cyclopropanes show vari-
able reactivity (eqs 7–10).

(7)

Ph

SPh

Ph

Ni

2

B

C

7

H

15

Ph

+

52%

24%

(3.5 equiv)

Ni

2

B

Ph

O

SPh

(8)

PhCH

2

OH

(3.5 equiv)

91%

A list of General Abbreviations appears on the front Endpapers

NICKEL BORIDE

3

(9)

Ni

2

B

S

H
N

H
N

(3.5 equiv)

68%

(10)

Ni

2

B

S

Br

S

+

biphenyl

11%

77%

(3.5 equiv)

Using deuterium labelling, Back showed that desulfurization

occurs with retention of configuration, unlike Raney nickel, which
involved a radical mechanism. The suggested mechanism of desul-
furization involves an oxidative addition–reductive elimination se-
quence via a nickel hydride intermediate.

Reduction of Other Nitrogenous Functional Groups. Pri-

mary, secondary, and tertiary aliphatic nitro groups are reduced
to amines with NiCl

2

/NaBH

4

.

5c

Hydrazine hydrate has also been

used with Ni

2

B to reduce both aryl and aliphatic nitro groups in a

synthesis of tryptamine (eqs 11 and 12).

31

Ni

2

B

N

2

H

2

•H

2

O

(11)

N
H

BnO

NO

2

BnO

N

91%

Ni

2

B

N

2

H

2

•H

2

O

(12)

N
H

BnO

N
H

BnO

NO

2

NH

2

71%

Reductive cleavage of thioethers and reduction of nitro groups

has been combined in a synthesis of pyrrolidones (eq 13).

32

(13)

+

1:1

MeOC(O)

CO

2

Me

PhS

NO

2

SPh

N
H

CO

2

Me

O

N
H

CO

2

Me

O

NiCl

2

MeOH

57%

Like Co

2

B, Ni

2

B

5b

reduces nitroarenes to anilines and azoxy-

benzenes to azobenzenes (Table 3); unlike Co

2

B, Ni

2

B reduces

oximes

33

to amines (Table 4).

Reduction of Other Nitrogenous Functional Groups.

Borane–Tetrahydrofuran/NiCl

2

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

NiCl

2

(1 equiv)

NaBH

4

(4 equiv)

(14)

O

OMe

NO

2

N

3

O

OMe

NO

2

H

2

N

EtOH

Table 3

Reduction of nitroarenes with Ni

II

/borohydride reagents

Substrate

Reagent

Product

Yield (%)

PhNO

2

2 equiv Ni

2

B, MeOH

PhNH

2

3

5a

PhNO

2

1 equiv Ni

2

B, 15N NH

4

OH PhNH

2

96

5a

PhNO

2

0.1 equiv Ni

2

B, 5 equiv

Azoxybenzene

89

5b

NaBH

4

4-ClC

6

H

4

NO

2

1 equiv Ni

2

B, 3N HCl

4-ClC

6

H

4

NH

2

96

5a

4-CNC

6

H

4

NO

2

2 equiv Ni

2

B, 3N HCl

4-CNC

6

H

4

NH

2

60

5a

6-Nitroquinoline

1 equiv Ni

2

B, 15N NH

4

OH 6-Aminoquinoline 86

5a

1-Nitronaphthalene NiCl

2

–NaBH

4

(2:1)

1-Aminonaphtha- 85

5d

lene

4-IC

6

H

4

NO

2

4 equiv Ni

2

B, IN HCl

4-IC

6

H

4

NH

2

76

34

NO

2

CO

2

Me

I

4 equiv Ni

2

B, IN HCl

NH

2

CO

2

Me

I

85

34

Table 4

Reduction of oximes to amines with NiCl

2

/NaBH

4

Substrate

Product

Yield (%)

NOH

NH

2

NH

2

92

NHOH

NH

2

1:2

95

NOH

NH

2

NH

2

90

NOH

NH

2

1:1

70

Isoxazoles are reduced to β-amino enones in high yield us-

ing the NiCl

2

/NaBH

4

system.

38

Dihydroisoxazolones are reduced

with a high degree of diastereoselectivity with the NiCl

2

/NaBH

4

system.

39

Dehalogenation.

Many α-bromo ketones

6a

are cleanly re-

duced to the parent ketone with nickel boride in DMF (caution).
Vicinal dibromides are reduced to alkenes (eq 15).

(15)

NiCl

2

, NaBH

4

O

O

HO

O

Br Br

O

O

HO

O

DMF

80%

Aryl and certain alkyl chlorides can be dehalogenated

1a,1b,6b

with a variety of Ni

II

/hydride agents (e.g. NaBH

2

(OCH

2

CH

2

OMe)

2

, Triethylsilane, NaBH

4

). Lin and Roth have effected the

Avoid Skin Contact with All Reagents

4

NICKEL BORIDE

clean debromination of aryl bromides

40

using Dichlorobis(tri-

phenylphosphine)nickel(II)/NaBH

4

in DMF (caution); Tris(tri-

phenylphosphine)nickel(0) is assumed to be the active catalyst.
Russel and Liu

41

demonstrated that reductive cleavage of an

iodide goes with retention when NiCl

2

/NaBH

4

is used (cf. in-

version seen with LiAlH

4

; eq 16).

O

O

O

OMe

OBz

I

Ph

(16)

O

O

O

OMe

OBz

Ph

NaBD

4

, NiCl

2

D

EtOH

94%

Hydrogenolysis. Ni

2

B has been used to hydrogenolyze ben-

zylic (eqs 17–19),

7a

allylic (eqs 20–22),

7b,42

and propargylic

(eq 23)

7b

esters in good yields.

(17)

OAc

Me

2

N

Me

2

N

NiCl

2

, NaBH

4

(2:1)

95%

NiCl

2

, NaBH

4

(18)

OAc

CO

2

Me

CO

2

Me

(2:1)

76%

NiCl

2

, NaBH

4

(19)

OAc OAc

OAc

(2:1)

83%

NiCl

2

, NaBH

4

OAc

(20)

(2:1)

93%

NiCl

2

, NaBH

4

(21)

OAc

+

1:1

2:1

95%

OH

O

HO

O

O

MeO

OH

HO

O

MeO

O

H

H

H

H

H

(22)

NiCl

2

, NaBH

4

(2:1)

100%

(23)

OAc

AcO

OAc

NiCl

2

, NaBH

4

(2:1)

35%

Enol tosylates and aryl tosylates are deoxygenated in good to
excellent yields

43

(eqs 24 and 25)

(24)

NHTs

OTs

NiCl

2

(1 equiv)

NaBH

4

(20 equiv)

OTs

NHTs

MeOH

95%

NiCl

2

(1 equiv)

NaBH

4

(20 equiv)

(25)

O

OTs

OH

MeOH

77%

A variety of allylic functional groups

44

(alcohols, esters, silyl

ethers, ketones, and hydroperoxides) have been reduced with
Ni

2

B. The combination of Chlorotrimethylsilane/Ni

2

B will se-

lectively reduce an aldehyde in the presence of a ketone.

45

Selenides

46

and tellurides

47

are reductively cleaved by Ni

2

B

with retention of stereochemistry. The phenyl selenyl group is
cleaved in preference to the thio phenyl group.

1.

(a) Ganem, B.; Osby, J. O., Chem. Rev. 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 Ni

2

B represents a nominal stoichiometry for the

reagent prepared by the action of NaBH

4

on a Ni

II

salt. Several Ni

x

B

y

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 Ni

3

B and NiB, respectively. These are the most widely cited

synthetically useful reagents.

3.

(a) Brown, C. A., J. Org. Chem. 1970, 35, 1900. (b) Brown, C. A.; Ahuja,
V. K., J. Org. Chem. 1973, 38, 2226.

4.

(a) Back, T. G.; Baron, D. L.; Yang, K., J. Org. Chem. 1993, 58, 2407.
(b) Back, T. G.; Yang, K.; Krouse, R. H., J. Org. Chem. 1992, 57, 1986.

5.

(a) Nose, A.; Kudo, T., Chem. Pharm. Bull. 1989, 37, 816. (b) Nose, A.;
Kudo, T., Chem. Pharm. Bull. 1988, 36, 1529. (c) Osby, J. O.; Ganem, B.,
Tetrahedron Lett. 1985

, 26, 6413. (d) Nose, A.; Kudo, T., Chem. Pharm.

Bull. 1981

, 29, 1159.

6.

(a) Sarma, J. C.; Borbaruah, M.; Sharma, R. P., Tetrahedron Lett. 1985,
26

, 4657. (b) Tabaei, S-M. H.; Pittman, C. V., Tetrahedron Lett. 1993,

34

, 3264.

7.

(a) He, Y.; Pan, X.; Wang, S.; Zhao, H., Synth. Commun. 1989, 19, 3051.
(b) Ipaktschi, J., Chem. Ber. 1983, 117, 3320 (Chem. Abstr. 1985, 102,
94 904x).

8.

This is the melting point of Ni

2

B formed by fusion of the elements Adv.

Chem. Ser. 1961

, 32, 53). Material prepared by the reduction of NiCl

2

with NaBH

4

begins to decompose at 100

◦

C when heated in vacuo with

liberation of H

2

(Maybury, P. C.; Mitchell, R. W.; Hawthorne, M. F., J.

Chem. Soc. (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., J. Am.
Chem. Soc. 1963

, 85, 1005. (b) Brown, H. C.; Brown, C. A., J. Am. Chem.

Soc. 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 BH

3

˙

THF complex

to prepare Ni

3

B and Ni

4

B

3

using Ni(OAc)

2

and NiCl

2

, respectively,

and their use as hydrogenation catalysts.

10.

Schreifels, J. A.; Maybury, C. P.; Swartz, W. E., J. Org. Chem. 1981, 46,
1263.

11.

(a) Paul, R.; Buisson, P.; Joseph, N., Ind. Eng. Chem. 1952, 44, 1006
(Chem. Abstr. 1952, 46, 9960e). (b) Paul, R.; Buisson, P.; Joseph, N.,
C. R. Hebd. Seances Acad. Sci., Ser. C 1951

, 232, 627 (Chem. Abstr.

1951, 45, 10 436h). (c) Schlesinger, H. R.; Brown, H. C.; Finholt, A. E.;
Gilbreath, J. R.; Hoekstra Hyde, E. K., J. Am. Chem. Soc. 1953, 75, 215.

A list of General Abbreviations appears on the front Endpapers

NICKEL BORIDE

5

12.

Brown, H. C.; Brown, C. A., J. Am. Chem. Soc. 1962, 84, 1493.

13.

A boride of the same composition had been previously described (Stock,
A.; Kuss, E., Chem. Ber. 1914, 47, 810 (Chem. Abstr. 1914, 8, 2129).

14.

Jefford, C. W.; Jaggi, D.; Bernardinelli, G.; Boukouvalas, J., Tetrahedron
Lett. 1987

, 28, 4041.

15.

Novak, J.; Salemink, C. A., J. Chem. Soc., Perkin Trans. 1 1982, 2403.

16.

Miller, J. G.; Ochlschlager, A. C., J. Org. Chem. 1984, 49, 2332. This
reaction uses TMEDA as an additive.

17.

Kido, F.; Abe, T.; Yoshikoshi, A., J. Chem. Soc. (C) 1986, 590.

18.

Lee, K-H.; Ibuka, T.; Sims, D.; Muraoka, O.; Kiyokawa, H.; Hall, I.
H.; Kim, H. L., J. Med. Chem. 1981, 24, 924. When Pt

2

O 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., J. Org. Chem. 1971, 36, 2018. (b) Russell,
T. W.; Hoy, R. C.; Cornelius, J. E., J. Org. Chem. 1972, 37, 3552.

20.

Nose, A.; Kudo, T., Chem. Pharm. Bull. 1990, 38, 1720.

21.

Jung, M.; Elsohly, H. N.; Croon, E. M.; McPhail, D. R.; McPhail, A. T.,
J. Org. Chem. 1986

, 51, 5417.

22.

Nose, A.; Kudo, T., Chem. Pharm. Bull. 1984, 32, 2421.

23.

Pettit, G. R.; van Tamelen, E. E., Org. React. 1962, 62, 347.

24.

Truce, W. E.; Roberts, F. E., J. Org. Chem. 1963, 28, 961.

25.

Zaman, S. S.; Sarmah, P.; Barus, N. C.; Sharma, R. P., Chem. Ind.
(London) 1989

, 806.

26.

Boar, R. B.; Hawkins, D. W.; McGhie, J. F.; Barton, D. H. R., J. Chem.
Soc., Perkin Trans. 1 1973

, 654.

27.

Guziec, F. S.; Wasmund, L. M., Tetrahedron Lett. 1990, 31, 23.

28.

(a) Euerby, M. R.; Waigh, R. D., Synth. Commun. 1986, 16, 779.
(b) Euerby, M. R.; Waigh, R. D., J. Chem. Soc. (C) 1981, 127.

29.

Truce, W. E.; Perry, F. M., J. Org. Chem. 1965, 30, 1316.

30.

Back, T. G.; Yang, K., J. Chem. Soc. (C) 1990, 819.

31.

Lloyd, D. H.; Nichols, D. E., J. Org. Chem. 1986, 51, 4294.

32.

Posner, G. H.; Crouch, R. D., Tetrahedron 1990, 46, 7509.

33.

Ipaktschi, J., Chem. Ber. 1984, 117, 856 (Chem. Abstr. 1984, 101, 22
611f).

34.

Seltzman, H. H.; Berrang, B. B., Tetrahedron Lett. 1993, 34, 3083.

35.

Lu, Y.; Meit, C.; Kunesch, N.; Poisson, J., Tetrahedron: Asymmetry 1990,
1

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Thomas J. Caggiano

Wyeth-Ayerst Research, Princeton, NJ, USA

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