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.