General Papers ARKIVOC 2002 (i) 71-75
predominant products, i.e. 3j (83%), 3d (53%), 3i (37%), and 3e (38%), respectively. On the other hand, nitrobenzene 1a, 2-nitrobiphenyl 1h, 3,4-dimethylnitrobenzene 1k and 2,4dimethylnitrobenzene 1n gave anilines as predominant products, i.e. 2a (51%), 2h (50%), 2k (43%), and 2l ( 31%), respectively. Separation of the azoxyarenes 3 from anilines 2 was easily accomplished by simple acid-base extraction methodology. Interestingly, yields of 2 and 3 did not vary when the reduction was carried out at -70 oC, although the reactions were much slower, requiring stirring overnight for completion. Attempts to reduce 2- and 4-nitropyridine using LDA failed.
In most cases, products 2 and 3 were known compounds and thus identified by comparison to their physical properties and/or spectral data to those of known compounds. In the case of new compounds, structures were identified on the basis of spectral properties and elemental analysis.
In conclusion, the major finding of this study is that LDA reduces nitrobenzenes to azoxybenzenes and anilines at low temperatures. Prior to our work, LDA had never been implicated as a reducing agent for nitro compounds. This finding is significant in its own right. However, since the reduction occurs at low temperatures to azobenzenes as major products, it might also serve as a valuable alternative to previously reported methods all of which requires room or elevated temperatures and some require several steps. For example, one method requires a two step process at 45 0C which involves the conversion of nitrobenzenes to hydroxylamines followed by reduction with I2 or FeCl3.7 Catalytic reduction using NaBH48 or NaBH4/LiCl in diglyme at 125-162 oC 9,, as well as reductions in basic alcoholic refluxing media, are also commonly used.10-12 Recently a BiCl3-Zn promoted reduction has been performed at room temperature.13
Experimental Section
General Procedures. Melting points were taken on a Mel-Temp capillary apparatus and are uncorrected with respect to stem correction. IR spectra were recorded on a Nicolet Magna-IRTM 550 FTIR instrument. 1H and 13C NMR spectra were recorded on a 400 MHz Bruker AVANCE DRX-400 Multi-nuclear NMR spectrometer; chemical shifts were referenced to TMS as internal standard and coupling constants J are given in Hz.
Materials. These were obtained from commercial sources. The LDA used was obtained as a 2.0M solution in hexanes.
General procedure for LDA reduction of nitroarenes 1a-m
A 12.5 ml portion of 2.0 M solution of LDA (25 mmol) was added to a flame-dried flask flushed with nitrogen and containing 10 ml of THF and the resulting mixture was cooled to -40 oC. The appropriate nitroarene (5 mmol) was then added, and the resulting solution was first stirred for 5 min then allowed to warm to rt where it was quenched with sat. aq. NH4Cl (30 mL). The
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