Reductions Other than Dehalogenation

Aromatic nitro compounds include both important explosives and a number of agrochemicals. Concern with their fate has motivated extensive examination of their reduction to amines under a range of conditions. 

Reduction of monocyclic aromatic nitro compounds has been demonstrated (a) with reduced sulfur compounds mediated by a naphthoquinone or an iron porphyrin (Schwarzenbach et al. 1990), and (b) by Fe(II) and magnetite produced by the action of the anaerobic bacterium Geobacter metallireducens (Heijman et al. 1993). Quinone-mediated reduction of monocyclic aromatic nitro compounds by the supernatant monocyclic aromatic nitro compounds has been noted (Glaus et al. 1992), and these reactions may be signihcant in determining the fate of aromatic nitro compounds in reducing environments (Dunnivant et al. 1992). 

The reduction of 2,4,6-trinitrotoluene with Fe° has been extensively studied (references in Bandstra et al. 2005), and it has hnally produced 2,4,6-triaminotoluene that could undergo polymerization. 

A number of other abiotic reductions have been described  

Cell-free supernatants may mediate reductions. The reduction of aromatic nitro compounds by SH was mediated by the hltrate from a strain of Streptomyces sp. that is known to synthesize 2-amino-3-carboxy-5-hydroxybenzo-l,4-quinone (cinnaquinone) 

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The reaction proceeds until each chlorine ion is removed. For example, carbon tetrachloride would be reduced to chloroform, then to methylene chloride, and finally to methane (the reduction of methylene chloride takes several months, however). No degradation products other than the parent compounds were found therefore, degradation is simple, reductive dechlorination, with the zero-valent iron serving as an electron donor. The reaction was pseudo first-order and the reaction constant, k, decreased with each additional dehalogenation step (Gillham and O Hannesin, 1994). 

Reduction of NHC-borane adducts has proven useful for purposes other than the synthesis of boron-boron multiple bonds. Bissinger et al. [124] successfully employed this technique to prepare a trapped form of borylene, BH. NHC-stabi-lized dichloroborane (IMe-BHCl2) was prepared from BHCl2 SMe2 and then subsequentiy dehalogenated with 2 equivalents of sodium naphthalenide (NaN), yielding the naphthalene-trapped IMe BH adduct (Scheme 15.10). The authors propose that this reaction occurs via a [2+1] cycloaddition pathway and provide quantum chemical calculations to support this notion. 

Try to explain qualitatively the observed differences in reactivity. Are there compounds in this table for which other reactions than reductive dehalogenation may be important under these conditions If yes, which ones, and what kind of reaction do they undergo ... 

Lithium triethylborohydride (Super-Hydride) is a much more powerful reducing agent than lithium aluminium hydride. It is useful for the reductive dehalogenation of alkyl halides, but unlike lithium aluminium hydride does not affect aryl halides. It is available as solution in tetrahydrofuran in sealed containers under nitrogen. The solutions are flammable and moisture sensitive and should be handled with the same precautions as are taken with other organometallic reagents (see Section 4.2.47, p. 442). 

The first polysilanes were synthesized more than 75 years ago by Kipping through Wurtz-type reductive dehalogenation of dihalodiorganosilanes. This procedure, which is alkali metal intensive, has in the last decade been better understood and has consequently acquired a degree of control that was not previously considered possible. It remains the easiest and most widely used route to a polysilane and it will be discussed later in this section after consideration has been given to three other methods of synthesis that have been researched in recent years. 

Even though the Jourdan-Ullmann condensation has been known for more than a century, the mechanism is still not quite clear. On the one hand, the reaction may proceed via a free-radical mechanism, pertaining to the reductive dehalogenation of aryl halides and the acceleration of the reaction rate by ultraviolet irradiation, as outlined in Scheme 1. On the other hand, the reaction may involve halonium and proceed as a simple aromatic nucleophilic substitution, as displayed in Scheme 2. However, for the reaction of 6>-halobenzoic acid, it is believed that the copper ion coordinates with both carboxyl and... 

Contrarily, the nickel-catalysed SM reactions are always carried out in dry solvents. Since the arylnickel complexes have more ionic character than the corresponding arylpalladiums, the former are generally more reactive and sensitive to water (and other protic compounds) and furnish the protonated (reductive dehalogenation) arene from the parent aryl halide. However, the most common base in the nickel-catalysed SM reactions is K3P04 nH20. The fact, that a small amount of dehalogenation products was isolated, supports the arylnickel protonation side-reaction, either with water (from solvent(s) or K3P04 H20) or any other protic source. 

Understanding the factors that lead to different reductive dehalogenation products is very relevant, since certain products of reductive dehalogenation are regulated pollutants that are more toxic than their parent compounds, while other products are relatively benign. For the transformation PCE and TCE by FeS, for example, parallel transformation by both vicinal dichloroelimination... 

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