4886
J . Org. Chem. 1996, 61, 4886-4887
Ta ble 1. Red u ction P oten tia ls a n d Red u ction r a tes for a
Ser ies of Azoben zen es
Tita n iu m Com p lex-Ca ta lyzed Bor oh yd r id e
Red u ction of Azoben zen es
X
N
NaBH4
X
NH2 + H2N
N
Peter Dosa, Ian Kronish, J eremy McCallum,
J effrey Schwartz,* and Michael C. Barden
Cp2TiBH4
reduction potential
vs SCE (V)
reduction rate
X
EX - EH (V)
kobs (¥ 103)
kX/kH
Department of Chemistry, Princeton University,
Princeton, New J ersey 08544-1009
CF3
H
CH3
OMe
NEt2
-1.130
-1.332
-1.396
-1.456
-1.526
0.202
0
-0.064
-0.124
-0.194
5.51
1.39
1.21
0.66
0.73
3.95
1
0.87
0.47
0.52
Received May 13, 1996
We recently demonstrated that sodium borohydride
can be used for titanocene borohydride1 (Cp2TiBH4; 1)-
catalyzed reductive dechlorination of a broad class of
noxious haloaromatic substrates,2 including PCBs3 and
dioxins;4 this system was even found to be acitve for
reduction of PCBs in spiked soils.3 Nitroorganics are
another large category of aromatic organic pollutants that
can enter the environment in part as a result of muni-
tions fabrication. Reduction of these nitroorganics should
give amines which should, in turn, be readily biodegrad-
able.5 However, while classical laboratory procedures6
might be effective for reducing simple nitroorganics in
solution, they may not be appropriate for treatment of
contaminated environmental substrates, such as soils;
approaches to reductive destruction of bulk organics, for
example, those based on molten metal technologies,7
might also be inappropriate in the remediation context.
Sodium borohydride reduces nitrobenzene first to azoxy-
benzene and then to azobenzene,8 but further reduction
to aniline is not efficient with this simple reagent:
reduction of azobenzene to 1,2-diphenylhydrazine is slow.
We now report that NaBH4 can be used for efficient
reduction of azobenzenes to anilines under mild condi-
tions, when catalyzed by 1.
In a typical procedure, NaBH4 (1.236 g, 32.66 mmol,
6.0 equiv) and Cp2TiCl2 (250 mg, 1.00 mmol, 0.185 equiv)
were weighed under N2 and were added to 15 mL of
diglyme also under a nitrogen atmosphere (H2 is evolved).
The resulting deep purple solution of 1 was heated to 125
°C, and after 10 min, azobenzene (0.9865 g, 5.420 mmol,
1.0 equiv) and pentadecane (0.40 mL, GC internal
standard) in 10 mL of diglyme solution were added by
syringe. The reaction mixture turned brown. Aliquots
were withdrawn periodically, hydrolyzed, and analyzed
by GC. In this way it was found that reduction of
azobenzene to aniline occurred with a pseudo-first-order
rate constant kobs ) 1.39 × 10-3 s-1. A control procedure,
omitting 1, gave an orange solution, and reduction of
azobenzene occurred with a pseudo-first-order rate con-
stant kobs ) 2.03 × 10-6 s-1. Similarly, reduction of 1,2-
diphenylhydrazine was reduced by NaBH4 in the pres-
F igu r e 1. EX - E[H] vs ln (kX/kH).
Sch em e 1. Red u ction of Azoben zen e by
Coor d in a tion a n d In ser tion
Cp2TiBH4 (1)
N
N
– BH3
H
N
H
N
Cp2Ti
N
Cp2Ti
N
(1) (a) No¨th, H.; Hartwimmer, R. Chem. Ber. 1960, 93, 2338. (b)
Lucas, C. R. Inorg. Synth. 1977, 17, 91.
(2) Liu, Y.; Schwartz, J . J . Org. Chem. 1994, 59, 940.
(3) Liu, Y.; Schwartz, J .; Cavallaro, C. L. Environ. Sci. Technol.
1995, 29, 836.
2
3
(4) Liu, Y.; Schwartz, J . Tetrahedron 1995, 51, 4471.
(5) For an example of facile aerobic degradation of aniline by a
Pseudomonas strain, see: Konopka, A.; Knight, D.; Turco, R. F. Appl.
Environ. Microb. 1989, 55, 385. Aerobic metabolism of aniline, itself,
should be favorable relative to nitrobenzene by arene ring oxidative
cleavage. See: Kuznetsov, A. V. Mol. Biol. (USSR), Engl. 1991, 24,
1096. For measured relative rates, see: Tabak, H. H.; Desai, S.; Govind,
R. 44th Purdue Industiral Waste Conference Proceedings; Lewis
Publishers: Chelsea, MI, 1990; p 405.
(6) Fieser, L. F. Experiments in Organic Chemistry; D. C. Boston,
D. C. Heath: 1963; Chapter 26.
(7) For example, see: Schultz, C. G. Chem. Abstr. 1986, 105, 48445n.
(8) The reduction of nitrobenzene to azoxybenzene by sodium
borohydride in diglyme has been reported. See: Weill, C. E.; Panson,
G. S. J . Org. Chem. 1956, 21, 803.
ence of 1 with a pseudo-first-order rate constant kobs
)
2.47 × 10-3 s-1; omitting 1, kobs ) 2.62 × 10-5 s-1
.
Several p-substituted substrates (p-XC6H4N)NC6H4;
X ) CF3,9 CH3,9 OCH3, NEt2) were treated with NaBH4.
Rates for 1-catalyzed reduction of each were measured
and were correlated with electrochemical reduction po-
tentials measured in THF (Table 1). The correlation
observed between relative reduction potentials and re-
(9) Ogata, Y.; Takagi, Y. J . Am. Chem. Soc. 1958, 80, 3591.
S0022-3263(96)00871-7 CCC: $12.00 © 1996 American Chemical Society