Environmentally friendly reagents

The dihydro-2//-1-benzopyran skeleton is found in many biologically active compounds. Such moieties have also been used in the synthesis of other biologically active molecules [16-18]. Hence their synthesis has received attention. Some catalysts used for formation of dihydro-2//-1-benzopyran include (Ph3P)2PdCl2, [19] I2 [20], and Sc(OTf)3 [21]. Few of these methods are highly catalytic in nature or have been reported to use environmentally friendly reagents. For example,... 

Is there a more environmentally friendly reagent available to accomplish the oxidation of alcohols Recently, it has been shown that sodium hypochlorite (NaOCI) in acidic solution is an excellent reagent for the oxidation of secondary alcohols to ketones. Examples are shown in the following equations ... 

None of the known examples met the requirements for high yields at short reaction times with environmentally friendly reagents. Therfore, various supported noble metal catalysts and solvents were tested under the same reaction conditions in a catalyst screening program. 

The development of aqueous-phase reactions using environmentally friendly reagents is one of the active fields in organic synthesis [3-5]. Several protocols for reactions of hypervalent iodine reagents in water and environmentally-friendly aqueous solvent systems have been developed. 

The term phase-transfer catalysis was introduced in 1971 by Starks, explaining the critical role of tetraalkylammonium or phosphonium salts to promote reactions between two substances located in different immiscible phases (325). Over the years, the use of achiral quaternary ammonium salts as phase-transfer catalysts (PTCs) has attracted widespread interest not only in academia but also for industrial applications (326, 327). Some of the most important benefits of phase-transfer catalysis are simple experimental conditions, which are usually easily scalable, in addition to mild reaction conditions, and the use of inexpensive and environmentally friendly reagents and solvents. 

Economical, in terms of yields (100%), use of environmentally-friendly reagents, and requiring few steps. 

It years, there has been a considerable growth of interest in the catalysis of organic IS by inorganic reagents supported on high surface areas, Envirocat, a new family of supported reagents, which exhibits both Bronstead and Lev/is acid character, are ideal for environmentally friendly chemistry. These reagents are non-toxic powders that can be easily... 

In order to overcome these problems, attention was focused on the use of heterogeneous catalysis. We have found that functionalized solid materials, e.g., ionic liquids or tin triflates immobilized into mesoporous materials, can be used in N-acylation reactions as environmentally friendly replacements for traditional homogeneous acids which are useful but environmentally unacceptable catalysts [17, 18]. They had comparable activity to homogeneous reagents but can offer greater stability, safer and easier handling and can be... 

Low reagent consumption (environmental friendly) and equipment maintenance (long lasting capillary columns)... 

The third contribution presents the combination of electrochemistry and enzymatic synthesis for the selective formation of complex molecules. This quite young field of research is developing rapidly because the application of the reagent-free electrochemical procedure combined with the regio- and stereoselectivity of enzymes offers the possibility of establishing new environmentally friendly process even on a technical scale. 

The results presented here demonstrate the first successful applications of the concept of electroenzymatic synthesis. However, it is also obvious that many more systems are potentially very interesting for synthetic applications. This area is wide open for further creative research and only the tip of the iceberg has surfaced. What is especially promising is the fact that electroenzymatic synthesis is an environmentally friendly technique using the electrode as a clean reagent and the enzymes for high selectivity. Closed systems for technical applications are easier to realize than in many other areas. The author is convinced that industrial syntheses of valued small scale products will be seen in the near future. 

Electrolysis is an environmentally friendly technology. Properly designed electrochemical processes which use a mass-free reagent (the electron without the need of additional chemicals at ambient temperature) do not produce unwanted effluents, they often do not need toxic compounds (such as CN ) and do not introduce hazards of their own. 

Copper-catalyzed Suzuki cross-coupling reactions using mixed nanocluster catalysts have been studied recently. Copper-based catalysts were shown to be effective as reagents that can present an inexpensive and environmentally friendly alternative to noble metal catalysts. In the hydrogenation of cinnamic acid to corresponding alcohol, the selectivity can be varied by doping Sn with Rh colloid catalysts. A selectivity of 86% was achieved using a colloidal Rh/Sn (Rh/Sn = 1.5 1) catalyst on... 

Oxidation of alcohols to carbonyl compounds is an important reaction. Stoichiometric oxidants such as chromates, permanganates and MO4 (M = Ru, Os) are the commonly used reagents [19a,59,60]. However, they are going out of favour increasingly because they create heavy metal wastes . In view of this, development of environmentally friendly heterogeneous catalysts for alcohol oxidation is very important. In the use of catalytic amounts of transition metal salts or complexes as homogeneous catalysts for the oxidation of alcohols [61-64], separation of the catalyst from the reaction mixture and its subsequent recovery in active form is cumbersome. Heterogeneous catalysts for this kind of reaction are therefore necessary [65]. Clearly, encapsulation and/or immobilization of known... 

In this context, Czekelius and Carreira have recently documented a convenient heavy-metal-free transformation of optically active nitroalkanes 12 to chiral aldoximes 13 at room temperature by employing inexpensive reagents benzyl bromide, KOH and 5 mol% n-BU4NI (Scheme 14). This provides an environmentally friendly reaction that excludes the potential contamination of the products by metal impurities. 

Typically, stoichiometric amounts of a Lewis acid such as AICI3 are required and produce stoichiometric amounts of salts and mineral acids (HX) as side products. Furthermore, undesired side reactions such as multiple alkylations and a low functional group tolerance are observed. With the need for more environmentally and economically benign processes, the development of Friedel-Crafts-type reactions using catalytic amounts of a Lewis acid catalyst is desirable. In addition, the substitution of benzyl halides for other environmentally friendly alkylating reagents constitutes an attractive goal. In particular, benzyl alcohols are suitable... 

This chapter is an update (2003 to present) of the main applications of Bi(III) Lewis acids in organic synthesis developed and, in some cases, co-developed, by French and Portuguese research groups. Thus, in this chapter, the preparation of Bi(III) catalysts and their application to chemical transformations ranging from electrophilic addition to cyclization reactions, will be reviewed. The development of new environmentally friendly chemical processes, using Bi(III) reagents and catalysts, with direct application to steroid chemistry and related compounds will also be considered. 

A phosgene-free route to aromatic isocyanates, such as M DI and TDI, was reported by Fernandez et al. [42] (Scheme 5.7) According to the patent, the one-pot synthesis involves the use of an immobilized Schiff base type of ligand catalyst that facilitates the oxidative carbonylation of aromatic amines to the corresponding isocyanates. However, 2,2,2-trifluoroethanol (TFE), 1,2-dichlorobenzene, and carbon monoxide were used in this process, so this would not be a totally environmentally friendly process even if these reagents could be recycled and reused. 

---