Pergamon

Hecrrochimica Acfa, Vol. 42, Nos. 13-14, 2177-2180, pp. 1997 0 1997 Elsevier Science Ltd. All rights reserved

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Cathodic reduction of 2-bromo-2-nitropropane in the presence of dipolarophiles

Guillermo Montero, Gloria Quintanilla and Fructuoso Barba*

Department of Organic Chemistry, University of Alcala de Henares, Madrid, Spain

(Received 24 September 1996)

Abstract-Cathodic reduction of 2-bromo-nitropropane in aprotic medium in the presence of dimethyl acetylene dicarboxylate led to 5,5-dimethyl-2,3,4-trimethoxycarbonyl-S-hydropyrro~e and dimethyl 2-( l- nitroisopropyl)-2-butenedioate. In the presence of methyl propiolate a mixture of 2-nitropropane, 2,3-dimethyl-2,3-dinitrobutane, methyl (E)-4-methyl-4-nitro-2-pentenoate and an organomercurial compound was obtained. 0 1997 Elsevier Science Ltd.

Key words: Cathodic reduction, 2-bromo-2-nitropropane, dimethylacetylene dicarboxilate, methyl propiolate.

INTRODUCTION

In the electrochemical reduction of a-halogenated nitroalkanes the carbon-halogen bond is usually reduced at a less negative potential than the nitro group.

Thus, the cathodic reduction of a-halonitroalkanes in protic media leads to the corresponding nitronic acids, which can react further depending on their structure and on the electrochemical conditions [ 1, 21 while in aprotic media dimeric products are obtained

[31. Nitronic acids and their salts have been profusely

studied by organic chemists [46]. Chemically generated nitronate anions can react as 1,3-dipoles towards different dipolarophiles leading to cycloaddi- tion products such as S-substituted isoxazolidines [7], 4-isoxazolidines and aziridines [8].

In the present work we carried out the cathodic reduction of 2-bromo-2-nitropropane in aprotic medium (CH~C~~/BQNBF~) in the presence of two potential dipolarophiles such as dimethylacetylene dicarboxilate (DMAD) or methyl propiolate, expect- ing that the possibly electrogenerated anion (II) could act as l-3 dipole towards the present dipolarophiles. However, the result of the reaction was completely unexpected. Structures of the products and possible ways for the reaction to occur are discussed.

*Author to whom correspondence should be addressed.

EXPERIMENTAL

General electrolysis proc&re

Electrochemical reductions were carried out using the following conditions: Anode, platinum; anolyte, BQNBF, (2.63 g, 8 mmol) in dry dichloromethane (20 ml) (0.85 g, 5 mmol); cathode, mercury pool; catholyte, BQNBF~ (2.63 g, 8 mmol) in dry dichloromethane (30 ml) and 2-bromo-2-nitro- propane (0.85 g, 5 mmol). Alternatively either DMAD (1.42 g, 10 mmol) (Process A) or methyl propiolate (0.84 g, 10 mmol) (Process B) were also added to the catholyte. Electrolysis cell, divided cell equipped with a magnetic stirrer containing a piece of glass tubing with a glass frit of medium porosity at one end (anode compartment). Solid sodium thiosulfate was added to the anode compart- ment for in situ neutralization of the generated chlorine.

A constant cathodic potential of -0.45 V (vs see) was applied.

After the electrolysis the crude product was worked up by removing the solvent (dichloromethane) under vacuum and further extraction with ethyl ether and water. The ethereal phase was dried over anhydrous sodium sulfate and stripped to dryness.

Process A. (Scheme 1) When DMAD was employed as dipolarophile the charge consumption was 1.5 F mol-I. The residue of the ethereal extract was chromatographed over silica-gel with dichloromethane-ethyl acetate (14: 1). Leftover

2177

2178 G. Montero et al.

Process A

(CHa)&-NO, - O*” “’ Hg

Br

.

CH&l@,NBF,

- (Cl-&&*@o- -1 I

MeCWy =CHCOOMe +

C(CH&N02

1 2 Scheme 1.

DMAD together with compounds (1) (0.17 g, 35% yield) and (2) (0.41 g, 64% yield) were isolated.

Compound (1) was characterized by its analytical and spectroscopic data as dimethyl 2-( 1 nitroiso- propyl)-2-buthenedioate, previously described in the literature [9]. Physical and spectroscopic data for compound (2) are given as follows:

5,5-dimethyl-2,3,4-trimethoxycarbonyi-S-hydropir- role (2). B.p. = 16&163”C (yellow liquid), IR (vmaxcm-’ ): 2956, 1727, 1631, 1438, 1319, 1264, 1208, 1169, 1041. ‘H-NMR (300 MHz, CDCl3) 6 (ppm): 1.56 (s 6H, CHJ), 3.84 (s 3H, OCHJ), 3.93 (s, 3H, OCHJ), 3.95 (s, 3H, GCH3). 13C-NMR (300 MHz, CDClj), S (ppm): 21.9 (q, J = 132 Hz, CH3), 52.1 (q, J = 148 Hz, OCHg), 52.5 (q, J = 148 Hz, OCHs), 52.9 (q, J = 148 Hz, 0CH3), 81.4 (s, CMez), 135.6 (s, c---c), 158.3(s, C==C), 159.3 (s, C==C), 160.5 (s, CO), 160.8 (s, CO), 163.4 (s, CO). MS (70eV), m/e(rel.int.): 269(M+, 18) 254(13), 238(54), 224(75) 210(68), 194(28), 180(35), 166(47), 154(37), 148(32), 126(30), 93(53), 59(100); Analysis for CIIHENO~. Calc.: C(53.53), H(5.62), N(5.20). Found: C(53.3), H(5.5), N(5.6).

Catalytic hydrogenation of 2 on PdJC. 0.08 g of (2) was placed into a flask and dissolved in 25 ml of absolute methanol. 1 g Pd/C catalyst was added, a balloon filled with hydrogen fitted to the flask and the

system purged several times. The mixture was allowed to stand with stirring under hydrogen atmosphere for 24 h. The solution was filtered and the solvent eliminated under reduced pressure. 0.07 g of a yellow liquid was obtained. The main product (0.057 g) was isolated and purified by chromatog raphy in dichloromethane:ethyl acetate (14:l). Physical and spectroscopic data for the hydrogenated compound are given as follows:

5,5 - dimethyl - 2,3,4 - trimethoxycarbonylpirrolidine. IR (vmax cm -‘): 3324, 2957, 1740, 1435, 1344, 1260, 1204, 1107, 1030; ‘H-NMR (300 MHz, CDCI,): S (ppm,): 1.26 (s, 3H, CH3), 1.27 (s 3H, CH,), 3.0 (d, lH, J = 7.3 Hz, CH) 3.55 (t, lH, J = 7.3 Hz, CH), 3.65 (s, 3H, OCH,), 3.66 (s, 3H, OCHs), 3.73 (s, 3H, OCH,), 4.0 (d, lH, J = 7.3 Hz, CH); ‘)C-NMR (300 MHz, CDCls): 6 (ppm): 24.7 (q, J = 126 Hz, CH,), 30.8 (q, J = 126 Hz, CH3), 51.1 (d, J = 138 Hz, CH), 51.7 (q, J = 149 Hz, OCH,), 52.0 (q, J = 149 Hz, OCH3), 52.3 (q, J = 149 Hz, OCHs), 57.5 (d, J = 138 Hz, CH), 61.8 (d, J = 138 Hz, CH), 62.2 (s, CMez), 170.8 (s, CO), 171.4 (s, CO), 171.7 (s, CO); MS-Chemical ionization (70 eV), m/e (rel.int.): 314 (M+’ + 41,6), 302 (M+ +29, lo), 274 (M+ + 1, loo), 242 (3), 214 (5).

Process B. (Scheme 2) As methylpropiolate was used as dipolarophile, the consumed charge was 1 F

“b=c’ CooMe (CH3)2C--C(CH3)2 +

IGO bo

+ &ooc--CCIC-Hg-c-cc-COOW

2 2 02N(CH&C’ ‘H

3 4 6

Scheme 2.

Cathodic reduction of 2-bromo-2nitropropane 2179

mol-I. The crude residue obtained after removing the ether was treated with cold methanol and a white solid (3) precipitated. Compound (3) (0.15 g) was characterized by its properties as 2,3-dimethyl-2,3- dinitrobutane [3]. The methanol solution was extracted and chromatographed over silica-gel with dichloromethane. Leftover methyl propiolate to- gether with methyl (E)-4-methyl-4-nitro-2-pentenoate (4) [9] (0.17 g), nitropropane and the organomercurial (5) (0.16 g) were isolated.

Physical and spectroscopic data for compound (5) are given as follows:

Bis(melhoxycarbonylacetylide) mercury(U) (5). M.p.: 163%165°C. (white solid). IR (v,,, cm-‘) 3013, 2960, 2169, 1714, 1431, 1238, 1183, 873, 672. ‘H-NMR (300 MHz, acetone-&), 6 (ppm): 3.84 (s, OCHj). i3C-NMR (300 MHz, acetone-&), 6 (ppm): 51.4 (q, J = 148 Hz, OCH3), 93.0 (s, C=C), 119.4 (s, C=C), 152.2 (s, CO). MS (70 eV), m/e (rel.int.): 368 (M+, 2), 367(l), 366 (2) 365 (l), 340 (l), 339 (24), 338 (9) 337 (100) 336 (52),335 (83), 334 (61), 333 (32) 254 (31) 253 (15) 252 (24) 202 (22) 200 (20) 76 (32). Analysis for CsHhOsHg: Calc. C (26.2) H (1.6). Found, C (26.2) H (1.8).

RESULTS AND DISCUSSION

When 2-bromo-2-nitropropane (I) was electro- chemically reduced at -0.45 V (vs see) in aprotic medium on a mercury cathode in the presence of dimethyl acetilenedicarboxylate (DMAD) as dipolar- ophile (Process A) none of the expected cycloaddition products were obtained. Both the charge consump- tion (1.5 F mol-‘) and the obtained products were completely surprising. Compound (1) was character- ized as dimethyl 2-( I-nitroisopropyl)-2-butenedioate, previously described by Anderson [9].

The structure of compound (2) was determined as follows. IR spectroscopy shows a carbonyl band at

1727 cm-’ from the DMAD, and a band at 1600 cm-’ from a carbon-carbon double bond, but there is a total absence of nitro group bands. The very simple ‘H-NMR spectrum presents just three singlets (3.84, 3.93 and 3.95 ppm, 3H each) corresponding to three methoxy groups from the DMAD and a singlet (1.56 ppm, 6H) from two methyl groups. Although the nitro group has apparently disappeared during the process, the presence of this last singlet discards the possibility that compound (2) could be just a reaction product of DMAD with itself. Its analytical and mass spectrometry data led to the formula C,ZH~SNO,,. In 13C-NMR the signals corresponding to three olefinic and three carbonyl carbons, as well as those corresponding to the methyl and methoxy carbons and a signal at 81 ppm from a quaternary carbon also support the structure of the 5,5-dimethyl- 2,3,4-trimethoxycarbonyl-5-hydropyrrole (2).

In order to confirm the proposed structure both a 15N-NMR spectrum and a catalytic hydrogenation on PdjC of (2) were carried out. A unique signal at 7.9 ppm in i5N-NMR which can correspond to a carbon-nitrogen double bond in a cyclic system and all data from the hydrogenated compound (see experimental) are consistent with the structure of (2).

Scheme 2 (Process B) shows the result of the reduction of 2-bromo-2-nitropropane under the same conditions as mentioned above, but using methyl propiolate as the potential dipolarophile. The consumed charge was 1 F mol-’ and no cycloaddi- tion products were obtained. Compound (3) identified as 2,3-dimethyl-2,3-dinitrobutane, is the result of the electrodimerization as described by Simonet [3]. Compound (4) was characterized by its analytical and spectroscopic data as methyl (E)-4- methyl-4-nitro-2-pentenoate previously described by Anderson [9]. An undetermined amount of nitro- propane was also detected.

R-C=CH Hk=CHR

4

/ 1e- Dimer. V43)2C - WW2

Wd2~-W - - Br’

Wd2G-N02 - o,k ILO,

Br 3

I

(CH&CHN02 + R-C=C .

Hg

R=COOCH3

Scheme 3.

(R-C=C)2Hg

5

2180 G. Montero et al.

(C&)2 f- NO2 -+- H.

(CHJ~C--NO~ - RC=CR R:=CHR

Br WhCN02

I II

Scheme 4.

1

RS2OOMe

Compound (5) was a white solid whose IR spectrum shows a carbonyl band of a conjugated ester at 1714 cm-‘. The acetylenic C-H band at 3247 cm-’ present in the methyl propiolate, has now disappeared but there remains a sharp band at 2619 cm-’ (carbon-carbon triple bond). In ‘H-NMR there appears a unique singlet at 3.84 ppm (-0CH3). i3C-NMR spectroscopy shows a methoxy carbon, two acetylene signals at 93 and 119 ppm and a carbonyl carbon. The clue to solve the structure was given by the mass spectrum, in which, both the M+ peak at 368 and specially the characteristic isotopic distribution of the mercury atom led us to identify the product as bis(methoxycarbonylacetide) mercury(H)

(5). Scheme 3 shows a possible way for the reaction in

Process B to occur, bearing in mind the consumed charge.

The starting material I, instead of being reduced until the expected nitronate anion, could lead to radical II which would evolve as indicated in the Scheme to give (3), (4) and (5).

A similar process could be designed in Process A (Scheme 4) in order to explain the synthesis of product (1). However an explanation for the formation of (2) is at the moment unclear.

ACKNOWLEDGEMENTS

We thank DGICYT, PB 94-0341 for the financial support.

5.

6.

REFERENCES

J. Armand, Bull. Sot. Chim. Fr. 543 (1966). N. Limosin and E. Lavison, Bull. Sot. Chem. Fr. 4189 (1969). J. Armand, J. Pinson and J. Simonet, Anal. Lett. 4,219 (1971). A. T. Nielsen, in The Chemistry of the Nitro and Nifrosos Groups (Edited by H. Fever) p. 349. J. Wiley & Sons, New York (1969). E. Brever, in The Chemistry of Amino, Nitroso and Nitro Compoundr and Their Derivatives (Edited by S. Patai) p. 459. J. Wiley & Sons, New York (1982). K. B. G. Torssel, in Nitrile Oxides, Nitrones and Nitronates in Organic Synthesis (Edited by M. Fever) p. 95. VCH, New York (1988). E. Brever, in The Chemistry of Amino, Nitroso and Nitro Compoun& and Their Derivatives (Edited by S. Patai) p. 544-46. J. Wiley & Sons, New York (1982). R. Gr& and R. Car%, J. Am. Chem. Sot. 99, 6667 (1977). D. A. Anderson and J. R. Hwu, J. Org. Chem. 55, 511 (1990).