Bull. Chem. Soc. Jpn. への投稿のすすめ

Azobenzenes are ubiquitous motifs very important in many areas of science. Azo compounds display crucial properties for important applications, mainly for the chemical industry. Because of their discovery, the main application of aromatic azo compounds has been their use as dyes. These compounds are excellent candidates to function as molecular switches because of their efficient cis–transisomerization in the presence of appropriate radiation. The classical methods for the synthesis of azo compounds are the azo coupling reaction (coupling of diazonium salts with activated aromatic compounds), the Mills reaction (reaction between aromatic nitroso derivatives and anilines) and the Wallach reaction (transformation of azoxybenzenes into 4-hydroxy substituted azoderivatives in acid media). More recently, other preparative methods have been reported. This critical review covers the various synthetic methods reported on azo compounds with special emphasis on the more recent ones and their mechanistic aspects (170 references).

http://www.chtf.stuba.sk/~szolcsanyi/education/files/Organicka%20chemia%20II/Prednaska%202_Aminy_Reakcie/Doplnkove%20studijne%20materialy/Azobenzenes/Synthesis%20of%20azobenzenes%20-%20the%20coloured%20pieces%20of%20molecular%20materials.pdf

Synthesis of azobenzenes: the coloured pieces of molecular materials

A novel zinc-catalyzed reduction of tertiary amides was developed. This system shows remarkable chemoselectivity and substrate scope tolerating ester, ether, nitro, cyano, azo, and keto substituents.

Zinc-Catalyzed Reduction of Amides: Unprecedented Selectivity and Functional Group Tolerance

In the last decade, an increasing number of useful catalytic reductions of carboxylic acid derivatives with hydrosilanes have been developed. Notably, the combination of an appropriate silane and catalyst enables unprecedented chemoselectivity that is not possible with traditional organometallic hydrides or hydrogenation catalysts. For example, amides and esters can be reduced preferentially in the presence of ketones or even aldehydes. We believe that catalytic hydrosilylations will be used more often in the future in challenging organic syntheses, as the reaction procedures are straightforward, and the reactivity of the silane can be fine-tuned. So far, the synthetic potential of these processes has clearly been underestimated. They even hold promise for industrial applications, as inexpensive and readily available silanes, such as polymethylhydrosiloxane, offer useful possibilities on a larger scale.

Selective reduction of carboxylic acid derivatives by catalytic hydrosilylation.

Amines constitute an important class of compounds in chemistry and biology. They are widely used in the pharmaceutical industry for crop protection and natural product synthesis, as well as for the preparation of advanced materials. Among the different procedures for their synthesis, the reduction of amides is one of the most fundamental methods. In general, reactive alkali metal hydrides or boron hydrides are used for such reduction processes. However, their air and moisture sensitivty, their low functional group tolerance, and tedious purification procedures are drawbacks. Interestingly, Cole-Hamilton and co-workers showed that catalytic hydrogenation of amides using molecular hydrogen constitutes a highly attractive access to amines, but the vigorous reaction conditions, that is, high pressures and elevated temperature, limit this methodology. During the last decade, metal-catalyzed hydrosilylations of amides have received considerable interest. Although various catalyst systems including Rh, Ru, Pt, Pd, Ir, Os, Re, Mn, Mo, In, and Ti have proven to be effective for this reduction, the development of cost-efficient and environmentally benign catalysts for this transformation is still desirable. In addition, most of the known catalyst systems either require expensive silanes or have limited functional group tolerance. During our recent studies on iron-catalyzed dehydrations of primary amides in the presence of hydrosilanes, we isolated the corresponding secondary amine as a by-product. Thus, our attention focused on this side-reaction of reducing carboxamides to amines. The abundant availability of iron makes it a highly attractive candidate for catalysis and a “cheap metal for noble tasks”. 7] Herein we report the first general iron-catalyzed hydrosilylation of amides to generate amines. Initially, the reaction of N,N-dimethylbenzamide (1 a) with PhSiH3 in toluene was investigated as a model system to identify and optimize critical reaction parameters (Table 1). As expected the reaction did not occur in the absence of any catalyst (Table 1, entry 1). In contrast, when using 2 mol% of cheap [Fe3(CO)12], an excellent yield (97%) of N,N-dimethylbenzylamine (2a) was obtained (Table 1, entry 3). Other iron sources, such as [Fe2(CO)9], Fe(OAc)2, [Fe(acac)2], and [Fe(acac)3] are also reactive but resulted in lower product yields (Table 1, entries 2 and 4–9). Next, we investigated the influence of different silanes on the reaction. To our delight the reduction also took place in the presence of inexpensive polymethylhydrosiloxane (PMHS) to give the desired product in 93 % yield (Table 1, entry 16). Advantageously, this silane is also easily separated from the reaction mixture. Additional studies revealed that the reduction proceeded best in the presence of an excess of four equivalents of Si H (Table 1, entry 17). Among the different solvents tested, toluene and di-n-butyl ether gave the best results (Table 1, entries 17 and 19). With respect to the mechanism, we propose that the ironcatalyzed reduction of tertiary amides proceeds somewhat similarly to the ruthenium-catalyzed procedure reported by Nagashima and co-workers. Reaction of the hydrosilane with the iron precursor should yield an activated species, Table 1: Iron-catalyzed reduction of N,N-dimethylbenzamide.

A convenient and general iron-catalyzed reduction of amides to amines.

Highly selective conversion of nitrobenzenes using a simple reducing system combined with a trivalent indium salt and a hydrosilane

One-step conversion to tertiary amines : InBr3/Et3SiH-mediated reductive deoxygenation of tertiary amides

An indium(III) bromide-triethylsilane reagent system promotes direct reduction of esters to produce the corresponding unsymmetrical ethers. This simple catalytic system accommodated other carbonyl compounds, such as a tertiary amide and a carboxylic acid.

Direct Reduction of Esters toEthers with an Indium(III) Bromide/TriethylsilaneCatalytic System

The reducing system consisting of a trivalent indium salt and a hydrosilane allows the selective conversion of aromatic nitro compounds (I) into azoxybenzenes (II), azobenzenes (III), diarylhydrazines (IV) and anilines (V).

Highly Selective Conversion of Nitrobenzenes Using a Simple Reducing System Combined with a Trivalent Indium Salt and a Hydrosilane.

SYNTHESIS 2008, No. 21, pp 3533–3536xx.xx.2008 Advanced online publication: 16.10.2008 DOI: 10.1055/s-0028-1083191; Art ID: F14308SS © Georg Thieme Verlag Stuttgart · New York Abstract: An indium(III) bromide–triethylsilane reagent system promotes direct reduction of esters to produce the corresponding unsymmetrical ethers. This simple catalytic system accommodated other carbonyl compounds, such as a tertiary amide and a carboxylic acid.

Kohji Fujii

Papers

Highly selective conversion of nitrobenzenes using a simple reducing system combined with a trivalent indium salt and a hydrosilane

One-step conversion to tertiary amines : InBr3/Et3SiH-mediated reductive deoxygenation of tertiary amides

Direct Reduction of Esters toEthers with an Indium(III) Bromide/TriethylsilaneCatalytic System

Highly Selective Conversion of Nitrobenzenes Using a Simple Reducing System Combined with a Trivalent Indium Salt and a Hydrosilane.

Direct Reduction of Esters to Ethers with an Indium(III) Bromide/Triethylsilane Catalytic System.