An efficient method for the selective reduction
of 2-aryl-1-nitroalkenes to 2-aryl-1-nitroalkanes
by 2-phenylbenzimidazoline

Synth. Commun. 15(6), 527-533 (1985)

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Recently we have reported that 2-phenylbenzimidazoline (PBI) prepared in situ from o-phenylenediamine and benzaldehyde was a good reducing agent for the reduction of α,β-unsaturated dinitriles [see abstract below]. Our interest in the selective reducing properties of this reagent prompts us to study the capability of PBI for the reduction of 2-aryl-1-nitroalkenes. In this paper we wish to report the facile reduction of 2-aryl-1-nitroalkenes to the corresponding nitroalkanes without side reactions.

Table 1
Substrate
Rxn
Time
Yield
β-Nitrostyrene
1 h
88%
p-NO2-β-Nitrostyrene
1 h
91%
p-Cl-β-Nitrostyrene
1 h
82%
p-Me-β-Nitrostyrene
3 h
85%
p-MeO-β-Nitrostyrene
3 h
93%
p-OH-β-Nitrostyrene
6 h
78%
β-Methyl-β-Nitrostyrene
6 h
91%
3,4-MeO-β-Nitrostyrene
6 h
84%

In order to optimize the reaction conditions, the reduction of β-nitrostyrene with PBI was examined in a variety of solvents. The reduction with PBI was accelerated by the use of alcoholic solvent. However the refluxing in butanol gave best results in term of the rapidity, due to a rise of the reaction temperature. Thus, the reduction of various nitroalkenes were carried out with o-phenylenediamine and benzaldehyde in butanol at reflux temperature. The results are summarized in Table 1. Although the kind of aromatic substituent in nitroalkene influenced the rate of reduction, a variety of nitroalkenes employed here were reduced to the corresponding nitroalkanes in excellent yields.

In all cases carbon-carbon double bonds of nitroalkenes were reduced in complete selectivity and no formation of dimer or other by-products could be observed. In particular, the successful reduction of 3-nitrovinylindole to the corresponding nitroethylindole is of interest in conection with the synthesis of functionalized polycyclic indoles and natural products of ergoline class. The use of 1.2 equiv. of o-phenylenediamine and benzaldehyde was sufficient to conduct the reduction, and 2-phenylbenzimidazole formed by the reaction could be easily removed by filtration.

An attempt to reduce 1-nitro-2-methylpropene with PBI under the the present conditions did not produce 1-nitro-2-methylpropane. This result suggests the present method may be limited to 2-aryl-1-nitroalkenes.

Experimental

General procedure

To a stirred solution of β-nitrostyrene (5 mmol) and benzaldehyde (0.64g, 6 mmol) in 25 ml butanol under nitrogen at room temperature, o-phenylenediamine (0.65g, 6 mmol) was added. After refluxing for the appropriate time (1 hour), the solvent was evaporated under reduced pressure. Methylene chloride (DCM) was added to the residue and insoluble 2-phenylbenzimidazole was filtered off. The methylene chloride solution was washed thoroughly with 0.1 N HCl and concentrated to give a crude product. The crude product was purified by short column chromatography on sillica gel to give pure reduced product (2-aryl-1-nitroethane). In a large scale synthesis, the purification by distillation was also effective.

In situ generation and synthetic application of 2-phenylbenzimidazoline to the
selective reduction of carbon-carbon double bonds of electron-deficient olefins
Chikashita, Hidenori; Nishida, Shuichi; Miyazaki, Makoto; Morita, Yasuhiro; Itoh, Kazuyoshi
Bull. Chem. Soc. Jpn. 60(2), 737-46 (1987) (In English)

Abstract

The title compd. (I), a mild, selective, and convenient reducing agent, was prepd. in situ from o-(H2N)2C6H4 and BzH in alcs. A general method for the selective redn. of C=C double bonds of a variety of electron-deficient olefins with an alc. soln. of I is described. The redn. of α,β-unsatd. ketones to the corresponding satd. ketones was accomplished less effectively with I and a Lewis-acid catalyst. Condensation of 2-O2NC6H4CHO with 2-MeNHC6H4NH2 gave benzimidazoline II in 92% yield. Redn. of PhCH=C(CN)2 with II gave PhCH2CH(CN)2 and benzimidazole III in 85 and 97% yields, resp. This shows the validity of I to be the actual reducing species in the redn. system.