The chemoselective reduction of nitro compounds

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A. Cyr, P. Huot, G. Belot, J. Lessard; Electrochim. Acta; 15(1) 147 (1990). 4. A. Velin Prikidanovics, J. Lessard; J. Appl. Electrochem.; 20 527 (1990). 5. A. Cyr, P.
JChimPhys (1996) 93, 601-610 ©Elsevier, Paris

The chemoselective reduction of nitro compounds: scope of the electrochemical method JMChapuzet, RLabrecque, MLavoie, EMartel, J Lessard Centre de recherche en électrochimie et électrocatalyse, Département de chimie, université de Sherbrooke, Sherbrooke (Québec) J1K2R1 Canada

RESUME L'électrohydrogénation sélective de groupements nitrés aliphatiques et aromatiques de molécules possédant d'autres groupes fonctionnels faciles àhydrogéner (double liaison activée, liaison carbone-iode, nitrile) à été réalisée avec succés dans des solutions de methanol-eau légèrement acides (pH= 3) ou neutres (pH = 5-6) sur des électrodes de cuivre de Devarda et de cobalt de Raney électrocodéposés. Les synthèses électrochimiques d'une quinolone 15 et d'une quinoxaline 18 sont également présentées. Les résultats préliminaires de l'électroréduction du 5-nitroindole 21 sur Hg, dans une solution hydrométhanolique contenant HBr comme électrolyte support sont présentés pour lapremière fois. Mots clés :hydrogénation électrocatalytique, composés nitrés, chimiosélectivité, quinolone, triazole, nitroindole. ABSTRACT The selective electrohydrogenation of nitro aliphatic and nitro aromatic functional groups in molecules containing other groups that are easy to hydrogenate (activated double bond, carbon-iodine bond, nitrile,) has been sucessfully carried out in slightly acidic (pH = 3) or neutral (pH = 5-6) methanol-water solutions at Devarda copper and Raney cobalt electrodes. The electrochemical synthesis of a quinolone 15 and a quinoxaline 18 is also reported. Preliminary results on the preparative electroreduction of 5-nitroindole 21 on Hg in aqueous methanol with HBr as supporting electrolyte are presented and dicussed for the first time. KEY-WORDS:electrocatalytic hydrogenation, nitro compounds, chemoselectivity, quinolone, triazole, nitroindole.

—602 — INTRODUCTION We have shown that aliphatic (1, 2) and aromatic (1, 3-6) nitro compounds can be electrochemically reduced to the corresponding amines inhigh chemical and electrochemical yieldsat Raney metal electrodes (Raney Ni, Devarda Cu and Raney Co) both in neutral and basic protic solutions. The electrochemical reduction of nitrobenzene and its derivatives (phenylhydroxylamine, azo and azoxybenzene) was also performed in neutral and basic aqueous methanolic solutions at polycristalline copper and nickel electrodes (6 ). In this paper, we report examples of selective reduction of nitroalkyl and nitroaryl groups inthe presence of other functional groups that areeasily hydrogenated (double bond, nitrile and carbon iodide bond). The electrochemical formation of3carbomethoxy-2-quinolone (15) and of ethyl 4,5-dihydro-4-oxo-5-hydroxy-l,2,4 triazolo[1,5-α] quinoxaline-2-carboxylate (18) are presented for the first time. The possibility of synthesizing substituted aminoindoles by trapping the diiminoquinone intermediates with a nucleophile (Scheme 2) led us to study the electrochemical reduction of nitroindoles. Results on the preparative electrolyses of 5-nitroindole 21 on Hg in acidic methanol-water solution (MeOH-H2O 93:7 w/w; HBr 0.15 M; pH= 0.3) are presented. To our knowledge, there has been only one brief report on the electrochemistry of nitroindoles (7).

RESULTS AND DISCUSSION Selective electrohydrogenation of nitroalkanes The preparative electrolytic reduction of primary, secondary and tertiary nitroalkanes inadd and neutral solutions containing alcohol at nickel (8 ), lead amalgam(9) and mercury cathodes (10, 11) was reported to give the corresponding N-alkylhydroxylamines with high yields and good current efficiencies in a four-electron process. As pointed out in the Introduction, at Raney metal electrodes however, the corresponding amines were obtained in high chemical and electrochemical yields both in neutral and basic solutions (1, 2). Since aliphatic hydroxylamines are not easily reduced electrolytically in neutral or slightly acidic medium and not reduced in basic medium, the most probable mechanism for the electrohydrogenolysis of the intermediate hydroxylamine is an electrocatalytic hydrogenation (ECH) mechanism as already proposed (3, 4). This mechanismis shown below where M represents the metallic surface, M(H) chemisorbed hydrogen and M(Y=Z) the adsorbed organic substrate. The electroreduction of water (or hydronium ions) with the formation of chemisorbed hydrogen on the electrode surface (Volmer reaction [1]) is followed by reaction of the adsorbed substrate M(Y=Z) with M(H) (reaction [3]). One may note that thereisa

—603 — competitionbetween hydrogenation (reaction [3]) and desorption of hydrogen by an electrochemical (Heyrovskyreaction [5]) and/or chemical (Tafel reaction [6 ]) pathway. [1]

2H2O(H3O+) + 2e-+M→2 (H)M+2OH(H2O)

[2]

Y=Z+M→(Y=Z)M

[3]

(Y=Z)M+ 2(H)M→(YH-ZH)M

[4]

(YH-ZH)M→YH-ZH+M

[5]

(H)M+H2O+ e-→H2 +M+OH-

[6 ]

(H)M+ (H)M→H2 +M

The results of entry 1 in Table 1 show that a tertiary nitroalkane can be converted into the corresponding amine (conversion of 1 into 2 ), in a high chemical yield (91%) in the presence of an aryl cyano group by electrohydrogenation at Devarda Cu in neutral aqueous methanol, thus show that the electrohydrogenolysis of the hydroxylamine was faster than the electrohydrogenation of the arylcyanogroup. Work is in progress to elucidate the source and mechanismof formation of alcohol 3(9%). Selective electrohydrogenation of nitroaryl groups The electrochemical reduction of nitroaryl groups (especially nitrobenzene) has been extensively studied and reviewed (see ref. 3 for the most pertinent references from 1900). There are only a few examples of the successful electroreduction of nitrobenzene to aniline which generally requires well defined conditions: acid concentration, cathode material, potential and stirring. Phenylhydroxylamine has been obtained inneutral solutions. Inanalkaline medium, chemicai follow upreactions involving phenylhydroxylamine and/or nitrosobenzene leads to condensation products. We previously reported that aniline was obtained in high chemical and current yields by electrohydrogenation of nitrobenzene at Devarda copper and Raney nickel electrodes, in neutral and basicaqueous methanolic solutions (3). Devarda Cu electrodes are very interesting fromthe point of view of chemoselectivity since only functional groups with two or more heteroatoms linked together are readily reduced on these electrodes (1, 12) whereas many types of functional groups are reducedat similar rates on Raney nickel electrodes (1, 12).

—604 — Table 1: Selective controlled potential electrohydrogenation of nitroalkyl and nitroaryl groups in methanol-water (93:7 w/w) solutions at Raney metal (D-Cu or R-Co) electrodes a Entry

1

Substrate

Cathode

D-Cu

Products

pH

Eb (Vvs SCE)

Yield (%)

5c

-0.90

91d

Curr. Eff. (%) 33

9

2

3g

D-Cu or R-Co D-Cu or R-Co

6e

-0.90

100f

>98

3h

-0.70

90-97f

36-48

0-6f

a: Devarda copper and Raney cobalt particles embedded in a Ni matrix (1, 3). b: Potential corresponding to zero current for H2evolution (see ref. 3). c: NaCl 0.1 Min the catholyte, AcOH0.54 M/ AcONa 0.37 Min the anolyte. d: Yield of isolated products. e: AcOH0.54 M/ AcONa 0.37 Minboth compartments, f: Determined by VPC. g: Taken fromref. 6. h: Pyridine 0.3 M/ HCl 0.15 Minboth compartments.

Results of selective controlled potential electrohydrogenation of nitroaryl groups are gathered in Table 1. They showthat a nitroaryl group can be converted into the corresponding amine in the presence of a cyano group (conversion of 4 into 5; entry 2) or a carbon-iodide bond (conversion of 6 into 7; entry 3) with a high chemical yield (90-100 %). In the case of oiodonitrobenzene 6, small quantities of aniline were detected. It is noteworthy that oiodonitrobenzene which possesses the weakest carbon-halogen of all the halonitrobenzenes (fromthe rate constants of halide ion dissociation fromthe radical anion: kortho> kpara> kmetaand kI>> kB>kCl

—605 — kF(13-15)) was reduced to o-iodoaniline (2) with avery high selectivity (94-97 %) both on a D-Cu andaR-Co electrode (entry3; Table 1). Such high selectivities and yields has never been obtained before to our knowledge (16). Since unprotonated arylhydroxylamines are not reducible (17-19), the reduction of the intermediate hydroxylamines to the amine, at Raney metal electrodes, must involve an ECHmechanism(3, 4). This is not surprising since Raney metal electrodes are charged withchemisorbed hydrogen (reaction [1]). Electrochemical synthesis of quinolone 15 and quinoxaline 18 Quinolones can be formed by reduction of ortho substituted nitrobenzylidenes via the intramolecular addition of the corresponding intermediate hydroxylamine or amine to functional groupsuch as ketones, aldehydes esters or nitriles. Thus, Lund et al. (20) have performed electrochemical cyclisations with ketones, esters, carboxylic acids and nitriles. More recently, Tallec (21) reportedthat the electrochemical, reductionof 9 onHg, inanacetic buffered medium(Ep = 0.90Vvs SCE) gives l-hydroxy-3-carboethoxy-2-quinolone (10). Quinolones have also been

9

10

obtained by chemical reductions (22-24). The aim of this study was to demonstrate that such intramolecular additions can be performed via the electrohydrogenation of the corresponding nitro compoundat Raney metal electrodes even inthe presence of aneasily hydrogenated functional group suchas a conjugated double bond. The electrohydrogenation of the o-nitrobenzylidene 11 (see the Experimental Part for its preparation), at a D-Cu electrode and in a neutral buffered medium(pH= 6), led to quinolone 15 in good chemical yield and current efficiency ( 53 %and 46 %respectively: entry 1; Table 2). Since the hydroxy quinolone 10 (provided by Pr. André Tallec) was not electrohydrogenated under the same conditions, it is reasonable to assume that 1-hydroxy -3carbomethoxy-2-quinolone (13). which was not detected during the ECHof 11, was not formed. As aconsequence, quinolone 15 was formed by intramolecular addition of the intermediate amine 14 to theester bond (Scheme 1).

—606 —

Scheme 1

It is noteworthy that the chemical reduction of 11 by iron in the presence of ammonium chloride gave quinolone 15 with a maximumyield of 20 %(25). The electrohydrogenation of 16(for its preparation see refs 26 and 27 ) at aDevarda copper cathode ina neutral buffered medium(pH= 6 ) gave ethyl 4,5-dihydro-4-oxo-5-hydroxy-1,2,4-triazolo [1,5-a] quinoxaline-2 -carboxylate (18) (entry 2; Table 2) with trace amount of quinoxaline 20. Since N-hydroxy quinolones are not hydrogenolized under these conditions, quinoxaline 2 0 was most probably formed by cyclisationof the intermediate amine 19. Thus, the cyclisation of hydroxylamine 17 to 18 is faster than its hydrogenolysis to 19. It is noteworthy that the yield of the electrohydrogenation of 16 (85 %:entry 2; Table 2) is higher thanthat obtained by catalytic hydrogenation on Pt (65 %in ref. 27). However, it is important to mentione that the chemical reduction of 16 with iron inglacial acetic acid gave the quinoxaline 20 ina 70 %yield (27).

—607 — Table2: Selective controlled potential electrohydrogenation of nitro compounds 11 and 16 at Raney metal (D-Cu or R-Co) electrodesainaneutral buffered mediumb Entry

substrate

Cathode pH

Ec (Vvs SCE)

Product

Yieldd Curr. Eff. (%) (%)

1

2

D-Cu

6

-0.69

53

46

R-Co

6

-0.71

85

64

a: Devarda copper or Raney cobalt particles embedded in a Ni matrix (1, 3). b: MeOH/ H2O(93:7 w/w); AcOH0.54 M/ AcONa 0.37 Min both compartments, c: Potential corresponding to zero current for H2evolution (see ref. 3). d: Yield of isolated products.

Preparative electroreduction of 5-nitroindole (21) To our knowledge, there has been only one brief report on the electrochemistry of nitroindoles by Person et al. (7). They reported that the polarographic behavior of nitroindoles shows a 6F/mol wave in acidic media (pH< 5), two distinct waves (4F/mol and 2F/mol) in neutral media (5 < pH < 8) and a 4F/mol wave in basic media (pH > 8). However, microcoulometric measurements indicated (7) that only four electrons are transfered in acidic solutions and five electrons in alkaline solutions. No preparative electrolysis was reported. In a preliminary study we havecarried out the preparative electroreduction (Ep= -0.620 Vvs SCE) of 5-nitroindole (21) inan acidic (pH = 0.3) methanol-water (93:7 w/w) solution containing HBr (0.15 M) as supporting electrolyte. After a charge corresponding to 4 moles of electrons / mole of 21, 4-bromo-5aminoindole (24) was formed ina nearly quantitative yield (>99 %) with trace amount (< 1%) of 5aminoindole (25). 4-Bromo-5-aminoindo!e (24) is formed via the nucleophilic attack of Br on the

— 608 —

Scheme 2 intermediate diiminoquinone 23. Since, at pH ≈ 0.3, arylhydroxylamines are protonated and reducible, the dehydration of 5-hydroxylaminoindole (22) into diiminoquinone 23 is thus muchfaster than its protonation and reduction into 5-aminoindole 25. The trace amount of 25 could also result fromthe electroreduction of 23. The formation of a substituted aminoindole, however in a much lower yield, has been reported by Somei et al. (28) inthe case of the chemical reduction of 4-nitro-3indoleacetonitrile (26) by TiCl3: 4- amino-7-chloro-3-indoleacetonitrile (27) was formed (27.4 % ) together with 4-amino-3-indoleacetonitrile (28) (43.7 %). Work is now in progress to evaluate the influence of the presence of other nucleophiles (Cl-, MeO-, RS- etc...) and the influence of the position of the nitro group (4-, 5-, 6 - and 7-nitroindole) on the selectivity and efficiency of diiminoquinone trapping.

CONCLUSION In this paper we have reported chemoselective electrohydrogenations of (i) a tertiary nitroalkyl group in the presence of an arylcyano group, (ii) a nitroaryl group in the presence of a carbone-iodide bond, (iii) a nitroaryl group in the presence of other unsaturated groups (conjugated double bond , nitrile, ester). A quinolone and a quinoxaline have been obtained by electroreduction of a nitroaryl group with subsequent cyclisation of the corresponding amine or the intermediate

—609 — hydroxylamine respectively. The quantitative electrochemical formation of 4-bromo-5-aminoindole (24) by electroreduction of 5-nitroindole (21) in a mediumcontaining HBr is also presented for the first time.

EXPERIMENTAL The preparative electrolyses were carried out as previously described (see refs 1, 6 and references therein): two-compartment cell (120 mL in each compartment); electrolytes composition andreference electrode, see Tables; auxiliary electrode, Pt or glassy carbon; substrate concentration, 10-2M; oxygen-free nitrogen atmospher. The preparation of the Raney cobalt and Devarda copper workingelectrodes (geometrical area:3*4 cm2) has been described as well as the apparatus used (see refs 1,6). -2(4 -cyanophenyI)-2 -nitropropane (1): this compound was prepared and characterized according to ref. 29. - 2 (4 -cyanophenyl)-2 -aminopropane (2): IR (CHCl3) cm-1: 3683 (NH2), 2230 (CN); 1H NMR (CDCl3) δppm: 7.47-7.75 (m, 4H, arom.), 1.55-1.65 (m, 2H, NH2), 1.44 (s, 6H, CH3); HRMS: m1/z =145.0782 (M+-CH3), calculated: 154.0766; m2/z = 144.0826 (M+-ΝΗ2), calculated: 144.0813. -2(4-cyanophenyl)-2 -hydroxypropane (3): IR(CHCl3) cm-1: 3608-3500 (OH), 2230 (CN); 1HNMR (CDCI3) δ ppm: 7.60-7.65 (m, 4H, arom.), 1.77-1.90 (broad peak, 1H, OH), 1.58 (s, 6 H,CH3); ms (CI)= 162. -Products 4, 5, 6 , 7 and 8 have been purchased formAldrich andwere used as authentic samples. -Nitrobenzylidene 11 was prepared according to the procedure described inref. 25: m. p.= 68-70 °C (litt. (25): 68 °C); ER(CHCl3) cm-1: 1732 (C=Oester), 1528 (N=Onitro), 1346 (N=Onitro), 1265 (C-Oester); 1HNMR(CDCI3) δ ppm: 8.22 (m, 1H, CH), 7.51 (m, 3H, arom.), 7.40 (m, 1H, arom.), 3.89(s, 3H, CH3), 3.61 (s,3H, CH3); HRMS: m/z = 234.0406, calculated 234.0402. - 3-Carbomethoxy-2-quinolone (15): m. p. = 186-187 °C (litt. (25): 186-188 °C); ER(CHCl3) cm-1: 3360 (NH), 1740 (C=Oester), 1654 (C=Oamide), 1296 (C-N amide), 1222 (C-O ester); 1HNMR (CDCl3) δ ppm: 8.61 (s, 1H, NH), 7.64 (m, 2H, arom.), 7.45 (m, 1H, arom.), 7.27 (t, 2H, arom.), 3.97(s, 3H, CH3); 13CNMR(CDCl3) δ ppm: 165.0 (C1), 161.5 (C2), 146.2 (C3), 140.0 (C10), 133.1 (C9), 128.5 (C8), 123.1 (C6), 121.5 (C5), 117.9 (C4), 116.1 (C7), 52.5 (C12); ms: M+= 203, M+-CH3 =188, M+-OCH3 = 172, M+-CO2CH3 = 143. - Diethyl 1-(2-nitrophenyl)-l,2,4-triazolo-3,5-dicarboxylate (16) was prepared according to a proceduredescribed inrefs 26 and27: m. p. = 111-112 °C(litt. (27): 115-116 °C); ER(CHCl3) cm-1: 1737 (C=Oester); 1H NMR (CDCl3) δ ppm: 8.32-8.41 (m, 1H, arom.), 7.70-7.88 (m, 2H, arom.), 751-7.59 (m, 1H, arom.), 4.54 (q, 2H, CH2), 4.37 (q, 2H, CH2), 1.47 (t, 3H, CH3), 1.34 (t, 3H, CH3); HRMS: m/z = 305.0519 (M+-C2H5), calculated 305.0522. - Ethyl 4,5-dihydro-4-oxo-5-hydroxy-1,2,4 triazolo [1,5-a] quinoxaline -2-carboxylate (18) was extractedandisolated according to the procedure described inref. 26: m. p. = 251-253 °C(litt. (27): 250-252 °C); IR(CHCl3) cm-1: 1738 and 1684 (C=0); 1HNMR(DMSO-d6) δ ppm: 12.1 (s, 1H, NOH), 8.2 (dd, 1H, arom), 7.65-7.85 (m, 2H, arom.), 7.5 (t, 1H, arom.), 4.48 (q, 2H, CH2), 1.41 (t, 3H,CH3); HMRS: m/z = 274.0705, calculated: 274.0502. - 5-nitroindole (21) and 5-aminoindole (25) were purchased fromAldrich. -4-bromo-5-aminoindole (24) was extracted using diethyl ether and purified by chromatography on silicagel (ether saturated with NH3 / Hexane: 50/50): decomposes at 140-142 °C; IR (CHCl3) cm-1: 3479 and 3692 (NH2), 1218 (C-N) ; 1HNMR (DMSO-d6) δ ppm: 10.98 (s, 1H, NH), 7.22 (t, 1H, arom), 6 6 6 (d, 1H, arom.), 6.13 (t, 1H, arom.), 4.68 (s, 2H, NH2); 13CNMR (DMSO-d6) δ ppm: 138.16 (C5), 129.33 (C7a), 128.71 (C3a), 125.47 (C2), 1 1 2 .1 0 (C6), 111.41 (C7), 99.90 (C3), 97.41 (C4).

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