Environmentally benign solvent substitutes

The Design of Technologically Effective and Environmentally Benign Solvent Substitutes... 

Alternative solvents are relatively environmentally benign solvents that can be substituted for the more hazardous choices, which, while they may be traditionally used, may have established precedence without environmental safety or toxicity concerns in mind. 

Ethyl lactate (ethyl 2-hydroxypropionate) is a nontoxic biodegradable compoxmd naturally found in a variety of food and industrially produced via esterification of lactic acid with ethanol [149]. Both of the components for the production of ethyl lactate are derived from renewable raw material (including low-cost agricultural waste) mainly by the fermentation process. This environmentally benign solvent with high boiling point (154 °C) and low volatility has properties superior to many conventional petroleum-based solvents and can substitute them as an eco-friendly alternative in many reactions [150,151]. The applications of ethyl lactate as a medium for the multicomponent heterocyclic synthesis are exemplified below. 

Direct bromination of toluene and ethylbenzene form the corresponding benzyl bromides in high yield. The observed selectivity in SC-CO2 is similar to that observed in conventional organic solvents. Also, SC-CO2 is an effective alternative to carbon tetrachloride for use in the classical Ziegler bromination with N-bromosuccinimide. Reaction yields are high, side products are minimized, and bromine-atom selectivities are observed. Thus, SC-CO2 must be useful as a viable, environmentally benign substitute for many of the solvents typically used for free-radical reactions (Tanko and Blackert, 1994). 

The use of supercritical fluids, particularly carbon dioxide, as a substitute solvent for chemical synthesis is an area of rapidly growing importance. (1-5) Carbon dioxide, when compressed to a liquid or, al ve its critical point (Jq = 31.rC, Pc = 1071 psi. Pc = 0.468 g/ml), to a supercritical fluid (SCF), represents an environmentally benign alternative to organic solvents. Since C02is nontoxic, nonflammable, inexpensive, and unregulated it can replace hazardous organic solvents and thereby provide a valuable pollution prevention tool. Moreover, as discussed below, there is a significant potential for improved synthetic chemistry via faster rates and/or enhanced selectivity for a number of reactions. 

The Friedel-Crafts reaction involves an electrophilic aromatic substitution that facilitates the alkylation or acylation of arenes (135) and heterocyclic compounds catalyzed by acidic catalysts. Zinc oxide has been found to be an effective catalyst for the Friedel-Crafts acylation of activated and nonactivated aromatic compounds (135) (Hosseini-Sarvari and Sharghi 2004) under solvent-free and room temperature conditions (Scheme 9.44). The catalyst provides a large surface area for the reaction. This Friedel-Crafts reaction is a safe and environmentally benign method which requires simple workup, mild reaction conditions and a short reaction time. 

The environmentally benign synthesis of arylated thiophenes 147 in pure water is possible via desulfinylative cross-coupling between nucleophilic 2-thienylsulfinates 146 and differently substituted bromoarenes (Scheme 58, Table 36) [356]. The reaction proceeds efficiently within short reaction times in the presence of tetrakis (triphenylphosphine)palladium(O) (Pd(PPh3)4) without the need for additives, base, cocatalysts, or water-soluble ligands. Additionally, the apphcation of a highly aqueous solvent mixture consisting of water and iV,A -dimethylformamide at a ratio of 3 1... 

---