Transition Met. Chem., 16, 215-217 (1991) Nickel-catalysed reduction nitro compounds 215

Reduction of aromatic nitro compounds with NaBH, catalysed by nickel complexes of o-aminothiophenol Schiff base derivatives Anna Vizi-Orosz and L,'tszl6 Mark6

Research Group for Petrochemistry of the Hungarian Academy of Sciences, H-8200 Veszpr~m, Hungary

Summary

The nickel complexes of Schiff bases formed from o-aminothiophenol and c~-dicarbonyl compounds, Ni (o-SC6H4N=CRCR=-NC6H4S-o ) (R = H, Me, Ph) (Ia-c), catalyse the reduction of aromatic nitro compounds by NaBH 4. The reduced species [Ni(o- S C 6 H 4 N = C H C H = N C 6 H 4 S - o ) ] - (2) and [Ni(o- S C 6 H 4 N H C H 2 C H = N C 6 H 4 S - o ) ] - (3) were identified as intermediates in the catalytic cycle.

Introduction

Sodium borohydride is a widely used reducing agent in organic chemistry and its utility has been significantly expanded by the use of catalysts, mainly transition metal salts m. Nitro compounds are, for example, practically inert against NaBH 4 alone but in the presence of titanium c2), vanadium ~3), molybdenum (4), manganese (3), iron(3-5~, cobalt(3,*-11), rhodium(12), nickel(4,13-19), palladium (3,4), platinum (4), copper(4,19,zo), silver(4), gold (g), or tin ~21) compounds may be effectively reduced to amines, substituted hydroxytamines and other secondary reduction products.

Although such catalysed reductions with NaBH4 are experimentally facile and useful procedures, very little is known about the mechanism of these processes (3' 6.9,18, aa). The main reason for this is the fact that in most cases these systems are heterogeneous and contain a black metal "boride" as catalyst (4' 17, 2z-z v). We report now on a catalytic system for the reduction of aromatic nitro compounds by NaBH 4 using the N i - - S z N 2 type complexes glyoxalbis(2-mercaptoanil)nickel, Ni(gma), and analogues. This system remains homogeneous and therefore allows some insight into its mechanism.

Results and discussion

Aromatic nitro compounds were reduced with NaBH 4 in THF/ethanol (1/1) solution under mild conditions to the corresponding aromatic amines in the presence of the nickel(II)-N2S 2 complexes (la-c) (2s-31) as catalysts. The yields achieved with different substrates and catalysts are compiled in Tables I and 2. In most cases minor amounts of the corresponding azoxy and azo compounds were also formed as by-products. In the absence of catalyst only a negligible conversion of the nitro compounds (about 5%) was observed.

The reaction mixtures were dark greenish-brown solutions and remained homogeneous as long as the system was catalytically active. In those cases when the conversion of the nitro compound was significantly below 100% within the reaction time used (2 h) the decrease in catalytic activity was accompanied by the formation of a black, nickel- and sulphur-containing precipitate (11.5 and 19.8%, respectively) which was not investigated further.

Table 1. Reduction of nitrobenzene with NaBH4 in the presence of complexes (1) as catalysts a.

Complex Conversion Product yMds(%) (%) Aniline Azobenzene Azoxybenzene

(la) 64 16 10 38 (lb) 98 61 11 26 (Ic) 100 95 - -

aReaction conditions: 0.05 mmol complex, 0.5 mmol nitrobenzene, 5 ml ethanol, 5 ml THF, 50 ~ C, 3 h; bdetermined by glc.

Table 2. Reduction of aromatic nitro compounds with NaBH 4 in the presence of (Ib) as catalyst ".

Substrate Conversion Product yMds(%) (%) (%) Amine Azoxy Azo

compound compound

o-nitroaniline 77 77 - - m-nitroaniline 96 96 - - p-nitroaniline 100 100 - - p-chloronitrobenzene I00 86 14 - p-nitrotoluene 93 93 - - p-nitroanisole 80 67 13 - nitrobenzene 94 c 55 24 15

~Reaction conditions: 0.05mmol (lb), 0.5retool nitro compound, !.5mmol NaBH~, 5ml EtOH, 5ml THF, 50~ 2h; bdetermined by glc; ~under reflux 100% conversion to aniline.

As reported by Holm, Gray and coworkers (29-31), glyoxalbis(2-mercaptoanil)nickel, Ni(gma) (la), is reduced in T H F by sodium amalgam to the radical anion Ni(gma) (2) and by (Bu4N)(BH4) to the dihydro derivative of (2), the radical anion Ni(Hzgma)- (3). In contrast to the neutral complex (la) both anionic complexes are readily soluble in organic solvents, (2) forming dark brown and (3) forming dark green solutions. The two radical anions have characteristically different e.s.r. spectra(a~ (2) shows a ( g ) value of 2.0043 and the ( g ) value of (3) is 2.047.

R R in

-s s"'--P'A

(la), R = H , n = 0 (lb), R = M e , n = 0 (lc), R = P h , n = 0 (2), R = H, n = - 1

H~ C-OH2 u ]

(3)

The homogeneous character and the dark greenish- brown colour of our reaction mixtures strongly suggests that anionic complexes of types (2) and (3) may play

0340-4285/91 $03.00 + .12 �9 1991 Chapman and Hall Ltd

216 A. Vizi-Orosz and L. Mark6

an important role in our catalytic systems. To prove this assumption stoichiometric reactions were performed to model the individual steps of a possible catalytic cycle.

Reaction I

On reacting (3), in the form of (Bu4N)(NiH2gma), with nitrobenzene or nitrosobenzene in THF solution under argon (nickel:substrate molar ratio 1:2) the originally green solution turned brown within 20min. Aniline, azobenzene, and azoxybenzene were identified as reduction products, aniline being the main product (about 90%) in both cases. The reaction was also followed by e.s.r, spectroscopy with nitrobenzene as substrate: the ( g ) value 2.0465 of the signal shown by the original solution changed to (g)=2.0040 at the end of the experiment. These results furnish evidence for Reaction l:

(3) + PhNO2 (or PhNO)~ (2) + PhNH2,

PhN(O)NPh, PhNNPh (+H/O) (l)

Quantitative experiments performed with nitrosobenzene showed that if the substrate (the "oxidant") was applied in excess (PhNO:Ni = 2:1), the yield of aniline based on the amount of (3) exceeded 100% (three parallel experiments furnished 110, 111, and 126%). This may be due to some slow reduction of nitrosobenzene by (2) which is thereby probably oxidized to (la). Since (la) is reduced under the reaction conditions by NaBH 4 again to (3), this side reaction may also contribute to some extent to the overall reduction process.

Reaction 2

A THF solution of (2) prepared by reducing ( la) with sodium amalgam (3~) was treated with (BugN)(BH4). The brown solution gradually turned green and by following the reaction with visible spectroscopy (Figure 1) it could be shown that this colour change is due to the transformation of (2) into (3) (Reaction 2):

BH4

(2) > (3) (2)

Based on these observations the catalytic cycle presented in Scheme 1 is suggested for the reduction of aromatic nitro compounds with BH 4- catalysed by complexes of type (1). Since both reactions 1 and 2 are obviously not elementary reactions but represent a complex series of consecutive electron and hydrogen transfer processes the mechanism in Scheme 1 is highly simplified. It is nevertheless--to our knowledge--the first attempt to interpret the role of the transition metal complex in the catalytic reduction of nitro (and nitroso) groups by NaBH4.

A r NO reduct ion AFNO2~ .j,.1" products

Ni (gy~na) BH4 ~ black - [Ni(H2gma) ]- [Ni(gma)]- ~ precipitate

r e d u c t i o n ArNO products ArN02

Scheme 1. Proposed mechanism for the reduction of nitro (or nitroso) compounds.

Transition Met. Chem., 16, 215-217 (1991)

o% o

'o \

\. \ ' , ~ot \ YX, . Y \ / \ , ~ ' " - /~,-

\

i i

30000 25000

o f

?

?i " o

I J ~ ' ' i \ o o, ol I I t ~ - I

\ o ,, oo.O. i / " , / \ \ - - - S / - - -

F _ _ [

20000 15000 11000 crn -1

Figure 1. Reduction of Na[Ni(gma)] to Na[Ni(H2gma) ] by BH~- in THF solution. Visible spectra.

Na[Ni(gma)] (3. t0 - 8 tool din- 3) Na[Ni(gma)] + (Bu4N)(BH4) (1:0.5) after 5 min.

. . . . . . . . . . . Na[Ni(gma)] + (Bu4N)(BH4) (1:2) after 5 rain. -�9169169169 (BurN) [Ni(H2gma)]

Finally we should like to mention that stoichiometric reductions by (Bu4N)(NiHzgma) are not restricted to aromatic nitro or nitroso compounds; we have found that p-benzoquinone is also smoothly reduced by this complex to hydroquinone in THF solution.

Experimental All reactions were carried out under Ar using dried and deoxygenated solvents. Starting compounds were of commerical origin except for the Ni complexes <18-31) and Bu4NBH4 c32) which were prepared as described in the literature. Azoxybenzenes used as model compounds were prepared by literature methods (33). E.s.r. data were obtained using a Jeol-FE-3X spectrometer with 100 kHz field modulation. Optical spectra were measured on a Specord M40 u.v.-vis, spectrophotometer (VEB Carl Zeiss, Jena). Mass spectra were recorded on a Jeol MS-01-$6-2 mass spectrometer. The reaction products were analysed on a Pye Unicam t04 gas chromatograph at 100 ~ C using a 25 m capillary column of type OV-1/t01, and on an HP 1050 chromatograph with u.v. detector using an ODS Hypersil column.

Reduction of aromatic nitro compounds (general procedure)

The aromatic nitro compound (0.5 mmol), ( 1 b) (17.9 mg, 0.05 mmol) and NaBH4 (55 rag, 1.5 retool) were dissolved in 10 ml THF/EtOH (1/1) and stirred at 50~ under Ar. The reactions were monitored by glc; conversions and yields were calculated from the chromatographic data using model compounds as standards.

Reaction of (3) with nitrobenzene or nitrosobenzene

Into a flask containing (BugN)(NiHzgma) (100rag, 0.175 mmol) under Ar a solution of nitrobenzene (36 ~1,

Transition Met. Chem., 16, 215-217 (1991)

0.35mmol in 4ml THF) or nitrosobenzene (37.5mg, 0.35retool in 4ml THF) was injected. The reaction mixture was stirred for 20 min during which time the green cotour of the solution turned to brown. After evaporating the solvent the residue was extracted with E t20 and the extract analysed by glc-ms.

Reduction of p-benzoquinone to hydroquinone by (3)

(Bu4N)(NiH2gma) (41.5rag, 0.072mmol) was dissolved under Ar in T H F (1.5ml). To this green solution p-benzoquinone (7.8 rag, 0.072 mmol) dissolved in T H F (1 ml) was added and the mixture was stirred for 10rain. The colour of the reaction mixture rapidly changed to dark brown. The solvent was evaporated in vacuo, and the residue twice extracted with E t20 (2+ 2ml). The extract was qualitatively analysed by tlc on silica gel and by glc-ms. The amount of hydroquinone formed was determined by evaporating the etheral extract, dissolving the residue in water/methanol (1/1) and analysing the solution by hplc. Yield 0.42 mmol, 58%.

References

~11R. C. Wade, d. Mol. Catal., 18, 273 (1983). f2)S. Kano, Y. Tanaka, E. Sugino and S. Hibino, Synthesis,

695 (1980). (3)M. Eckert and Y. Kiesel, Angew. Chem., 93, 477 (1981). ~4)K. Yanada, H. Yamaguchi, R. Yanada, H. Meguri and S.

Uchida, Chem. Lett., 951 (1989). {StA. Ono, H. Sasaki and F. Yaginuma, Chem. Ind. (London),

480 (1983). (6~A. A. Vlcek and A. Rusina, Proc. Chem. Soc., 161 (1961). ~7)S. Satoh, S. Suzuki, Y. Suzuki, Y. Miyaji and Z. Imai,

Tetrahedron Lett., 4555 (1969). lS)T. Satoh, S. Suzuki, T. Kihushi and T. Okada, Chem. Ind.

(London), 1626 (1970). (9) A. E. Brearley, H. Gott, H. A. O. Hill, M. O'Riordan, J. M.

Pratt and R. J. P. Williams, Y. Chem. Soc (A), 612 (1971). ~xo)y. Arai, A. Mijin and A. Takahashi, Chem. Lett, 743 (1972). (~ 1t p. K. Das and A. K. De, Y. Indian Chem. Soc., 56, 562 (1979). ~12)F.J. McQuillin and I. Jardine, d. Chem. Soc., Chem.

Commun., 626 (1970).

Nickel-catalysed reduction nitro compounds 217

(13)K. Hanaya and T. Ono, Nippon Kagaku Zasshi, 92, 1225 (1971); Chem. Abstr., 76, 99253 (1972).

t14)K. Hanaya, N. Fujita and H. Kudo, Chem. Ind. (London), 794 (1973).

(as)B. Loubinou• J. J. Chanot and P. Caubere, d. Organomet. Chem., 88, C4 (1975).

(16)A. Nose and T. Kudo, Chem. Pharm. Bull., 29, 1159 (1981). /17) j. O. Osby and B. Ganem, Tetrahedron Lett., 26, 6413 (1985). (18)j. A. Cowan, Tetrahedron Lett., 27, 1205 (1986). (~9)K. Hanaya, T. Muramatsu, H. Kudo and Y. L. Chow, Y.

Chem. Soc., Perkin Trans., 1, 2409 (1979). (2~ Ono, M. Hiroi and K. Shimazaki, Chem. Ind. (London),

75 (1984). ~21)S. Satoh, M. Mitsuo, M. Mishiki, Y. Inoue and Y. Ooi,

Chem. Pharm. Bull., 29, 1443 (1981). ~22)S. W. Heinzman and B. Ganem, J. Am. Chem. Soe., 104,

6801 (1982). ~23)H. I. Schlesinger, H.G. Brown, A.E. Finholt, J.R.

Galbreath, H. R. Hoekstra and E. K. Hyde, J. Am. Chem. Soc., 75, 215 (1953).

(24)p. C. Maybury, R.W. Mitchell and M. F. Hawthorne, J. Chem. Soc., Chem. Commun, 534 (1974).

,,2s)j. A. Schreifels, P. C. Maybury and W. E. Schwartz, J. Or 9. Chem., 46, 1263 (198i).

~16)j. Ipaktschi, Chem. Ber., 117, 3320 (1984). ~27) M. Borbaruah, N. C. Barua and R. P. Sharma, Tetrahedron

Lett., 28, 5741 (1987). 12S)H. Jadamus, Q. Fernando and H. Freiser, J. Am. Chem.

Soe., 86, 3056 (1964). ~29)A. H. Maki, T. E. Berry, A. Davison, R. H. Holm and A. L.

Balch, J. Am. Chem. Soc., 88, 1080 (1966). 13o) F. Lalor, M. F. Hawthorne, A. H. Maki, K. Darlington, A.

Davison, H. B. Gray, Z. Dori and E. I. Stiefel, J. Am. Chem. Soc., 89, 2278 (1967).

~3~)R. M. Holm, A. L. Balch, A. Davison, A. H. Maki and T. E. Berry, J. Am. Chem. Soc., 89, 2866 (1967).

(32)L. V. Titov and L. A. Gavrilova, Zh. Neorgan. Khim., 15, 2899 (1970).

133)A. I. Vogel, A Textbook of Practical Organic Chemistry, 3rd Ed., Longmans and Green, 1961. p.631.

(Received 23 March I990) TMC 2282