Current Green Chemistry - Volume 1, Issue 3, 2014

The aim of this review was to collect the results published in 2009-2013 on asymmetric aldol reactions taking place in aqueous media (water, brine) and in the absence of solvents (neat), to highlight the catalysts identified as best, and, in the course of doing so, to present the trends of research in this field. The five groups of organocatalysts discussed are: 1. Amino acids and esterified hydroxyamino acids; 2. Carboxamides of amino acids; 3. Peptides and derivatives; 4. Peptide-based alcohols; 5. 1, 2-Diamines and derivatives. The efficiency of the catalysts is characterized by their concentration in the reaction, reaction time, their selectivity (anti/syn) and their enantioselectivity.

In this review with broad coverage of recent literature references we present some general aspects of green chemistry as well as a brief overview of transformations of 2H-pyran-2-ones and fused pyran-2-ones under green condition. The emphasis is given on microwave-assisted transformations under various conditions, such as aqueous condition, neat reactions, reactions in ionic liquids and, to provide a comparison, also the reactions under high pressure are presented.

One-pot reactions where several reaction sequences are conducted in the same reaction flask are one of the methods that can be used in order to conduct synthesis in a greener fashion. The chemistry is greener due to the reduction of work-up procedures and purification steps required compared to a more stepwise approach. In reactions that require a catalyst it is possible to combine several catalytic processes in the same reaction vessel. By highlighting a few of the recent success stories where multiple synthetic conversions are conducted in one reaction flask this minireview is aiming to show the power and benefits of one-pot multicomponent reactions.

Following an important principle of Green Chemistry, namely the use of a benign solvent, the reaction of azines, namely pyridine, isoquinoline and phenanthridine with phenacyl bromides or with t-butyl bromoacetate has been accomplished in water to generate an aqueous solution of the (substituted methyl)azinium bromides, which on further reacting with dimethyl acetylenedicarboxylate (DMAD) and potassium carbonate furnish annelated pyrroles through tandem formation of the azinium methylides followed by 1,3-dipolar cycloaddition. In one case, maleic anhydride could also be used as 1,3-dipolarophile. The products are obtained in good yields and have been well characterized.

In the present study, two analytical methodologies, one based on direct thermal degradation of samples followed by atomic absorption analysis (TDA-AAS), and another one, which consists on acid microwave-assisted digestion (MAD) followed by cold vapour atomic fluorescence determination (CV-AFS), have been applied to determine mercury in mushrooms. Both methodologies found the same concentration values, from 258 to 570 ng g-1 (dry weight), for nine samples. Analysis based on thermal degradation provided a limit of detection of 3 ng g-1 similar to that found by CV-AFS, and a relative standard deviation of 3 %, three times higher than that of CV-AFS. However, the direct analysis of untreated samples agrees well with green analytical principles because it involves less reagents consume and reduces waste generation. In fact, at analytical Eco-Scale, TDA-AAS procedure involves a green score of 92 points in front of the 59 points obtained for CV-AFS method.

This work demonstrates that low antimicrobial toxicity C2-substituted imidazolium-based ILs, with either ester or amide groups in the side chain, can successfully replace harmful volatile organic compounds (VOCs) in the asymmetric carbonyl-ene reaction of phenylglyoxal with four alkenes catalysed by {[(R)-BINAP]Pd}(SbF6)2. Furthermore, the Pd(II) catalyst immobilised in these ILs was recycled and reused up to 5 times without loss of enantioselectivity, although a decrease in yield was observed.

The microporous, clinoptylolite-based zeolites (Ersorb products) are suitable catalysts in a number of condensation reactions yielding heterocyclic compounds. The catalysts can be simply removed from the reaction mixtures and reused several times without any significant loss of activity.

The synthesis of cyclic carbonates from polyols is an important reaction from a sustainability point of view as polyols which are by-products of industrial transformations are used as reagents to produce useful chemical products with many applications. This review is focused on the transformation of polyols with carbon dioxide, urea or carbon monoxide to cyclic carbonates.