(Hive Addict)
05-23-03 11:06
No 434995
      Carbonyls from Nitroolefins with Al-NiCl2-THF-H2O
(Rated as: excellent)

A Facile Procedure for the Conversion of Nitroolefins into Carbonyl Compounds Using Al-NiCl2-6H2O-THF System

Maitreyee S. Bexbarua, Ghanashyam Bez, and Nabin C. Barua

Chemistry Letters 325 (1999)


It has been demonstrated that a variety of nitroolefins can be efficiently converted into the corresponding carbonyl compounds by the reaction with an Al-NiCl2-6H2O-THF system.

The versatility of the aliphatic nitro compounds as a precursor to a wide variety of building blocks and intermediates in organic synthesis is an established fact.1-7.  The reduction of conjugated nitroalkenes provides an easy access to a large spectrum of organic functionalities including nitroalkanes, 8 N-substituted hydroxylamines,9 amines, 10 ketones, 11,12 and oximes.13,14  Although the transformation of nitroolefins to ketones is an important protocol in organic synthesis, usage of expensive reagents, strong acid and bases, longer reaction time etc. always demands introduction of newer and cheaper reagents which can effect the conversion under very mild reaction conditions.

In recent years, more and more report of usage of different metal-metal salt combinations for bringing about useful organic transformations are appearing in the literature.8  In such combinations, elementary metal parts needs to be more electropositive the metal part of the salt.9  Such combinations, which gained importance as reducing systems applicable in organic synthesis, are Al-NiCl2-6H2O, 15 Fe-NiCl2-6H2, 16 etc.  The reducing property exhibited by these metal-metal salt combinations proceed through transfer of one electron from the metal surface (or metal in solution) to the substrate. Another important aspect of these combinations is the Nernst potential difference of M/M+ viz Cd/Cd+2, Mg/Mg+2, Sn/Sn+4 etc. on which activity as well as reactivity of these reagents depend.  The mild reducing property of Al-NiCl2-6H2O was reported by us for bringing about various organic transformations.15

In order to explore the versatility of this reducing system as well as our continued interest on the chemistry of nitroalphatics, 17 we attempted reduction of the nitroolefins to saturated nitroalkanes by using the Al-NiCl2-6H2O-THF system.  However, we observed that the products of the reactions were not the saturated nitroalkanes, instead we observed the corresponding carbonyl compounds were obtained in excellent yield.

This observation has been generalized through entries 1 to 9 in Table 1.

We believe that reaction with Al-NiCl2-6H2O proceeds through a SET process.  The nitroalkene being a strong Michael acceptor, traps electrons released during oxidation of Ni (0), produced in the reaction of NiCl2-6H2O and Al metal, to Ni+2.  The species B picks up protons from HCl generated from hydrolysis of AlCl3 to give C which gets converted to the corresponding carbonyl compound as shown in scheme 1.

To a freshly mixed solid mixture of Al powder (2.2mmol) and NiCl2-6H2 (4.2mmol) was added a solution of the substrate (0.23mmol) in freshly distilled THF (10ml) A vigorous reaction took place which subsided after 20-30minutes.  When TLC of the reaction mixture showed disappearance of the starting material, the reaction was diluted with THF (100ml) and filtered.  Filtrate was evaporated off and the residue purified by using preparative TLC.:

In order to establish the applicability of this transformation at a larger preparative scale, reactions of 1-nitro-2-phenyl-ethylene (entry 1) and 1-nitrocyclohexene (entry 5) were performed in 100mmol scale with this reagent system which gave the corresponding products 2-phenyl acetaldehyde and cyclohexanone in consistent yields.


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