Current Green Chemistry - Volume 1, Issue 2, 2014

The first three-component synthesis of phosphane sulfides from diphenylphosphane, elemental sulfur, and α,β-unsaturated carbonyl compounds is presented. Reactions proceed under solvent-free conditions in the absence of catalyst, providing the expected β-diphenylphosphorothioyl-functionalised products in moderate-to-high yields. Different experiments revealed that the process takes place through the in situ formation of diphenylphosphane sulfide and subsequent 1,4-addition to the substrate.

The present review summarizes the synthetic application of tribromoisocyanuric acid [1,3,5-tribromo-1,3,5- triazine-2,4,6-(1H,3H,5H)-trione] in organic synthesis and focuses on its green aspects. It is a safe, stable and easily prepared solid. Electrophilic bromination reactions involving alkenes, arenes, dicarbonyl compounds and the bromodecarboxylation of cinnamic acids (Hunsdiecker reaction), as well as benzylic bromination and some oxidations involving tribromoisocyanuric acid are presented and discussed. In accordance with green chemistry principles, the use of tribromoisocyanuric acid enables bromination without bromine or other harmful or dangerous reagents.

According to the current literature, a dramatic increase in new published data or some-like ,,renaissance“ in the synthesis and fabrication of zeolitic and other industrial adsorbents, in a broad sence porous adsorption materials, has been reported over the last quarter of the century. Some of them are expected to be very useful and perspective adsorbents also in environmental control and remediation processes. Zeolites synthetic as well as natural, based on their beneficial properties and economic value, represent for a long time recognized industrial commodities. Advances in nanoscale science and engineering and development of novel functional materials, nanostructured membranes, bioactive nanoparticles and other surface engineered materials, microstructure of which has been recognized from evolutionary unique anatomy in nature are providing another unprecedent opportunity to develop more cost effective and environmentally acceptable adsorption materials of today.

The title compounds were synthesized starting from tosyl ether derivative of glycerol using KF/Al2O3 and PEG-400 as solvent. The reactions were performed under mild conditions with a range of aliphatic and aromatic thiols and the catalytic system was reused without previous treatment and with comparable activity for one additional reaction.

Bis-allyl ruthenium(IV) complexes containing N-heterocyclic ligands [Ru(η3:η3-C10H16)Cl2(Κ1-N-L)] (L = pyrazole (1a), benzimidazole (1b)) are active catalysts in the redox isomerization of allylic alcohols into saturated carbonyl compounds using the ionic liquid [BMIM][BF4] (BMIM = 1-butyl-3-methylimidazolium) as a non-conventional reaction media in the absence of additional base. Complex 1b has been found to be the most efficient catalyst for the isomerization of a series of primary and secondary allylic alcohols (9 substrates). In addition, we have studied the effect of the counter anion in the ionic liquid finding that non coordinating anions (BF4-, PF6- and SbF6-) led to the best reactions times and conversion, when compared with the coordinating chloride anion (Cl-). It is important to note that the catalytic system could be recovered through seven catalytic cycles with only partial loss of the initial activity in the first five cycles. Since this catalytic system: i) shows high efficiency and selectivity, ii) involves no presence of phosphines or any base as co-catalyst, iii) proceeds without the formation of by-products (atom economy), and iii) is readily recovered, it can be considered as a genuine example of a Green chemical process.

An efficient, inexpensive and selective catalytic system for the industrially relevant isomerization of estragole to trans-anethole has been developed using the arene-ruthenium(II) dimer [{RuCl(μ-Cl)(η6-p-cymene)}2] in combination with P(OMe)3. As an example, using 100 mg of this complex and 38 μL of P(OMe)3 (1 equiv. per Ru), 10 mL of estragole could be quantitatively converted into anethole (98% trans-selectivity) after heating the mixture at 80ºC for 24 h. In addition, after separation of the anethole product by distillation, the catalytic system could be reused without loss of efficiency and selectivity.

We describe here a simple method for the synthesis of several substituted benzimidazoles using niobic acid Nb2O5.5H2O as heterogeneous catalyst and under solvent-free conditions. This new protocol is general and a range of aromatic and aliphatic aldehydes were condensed with o-phenylenediamine furnishing the corresponding 1,2-disubstituted benzimidazoles selectively in good yields. The catalyst was reused up four times giving the respective products in good yields but with a little decreasing in the selectivity.

The biocatalytic synthesis of anisyl acetate fragrance was carried out by direct esterification of acetic acid with anisyl alcohol in sponge-like ionic liquids (SLILs), e.g. N,N,N,N-hexadecyltrimethylammonium bis(trifluoromethylsulfonyl) imide ([C16tma][NTf2], N,N,N,N-octadecyltrimethylammonium bis(trifluoromethylsulfonyl)imide ([C18tma] [NTf2]), etc. as reaction/separation media under conventional and microwave (MW) heating. These SLILs are temperatureswitchable ionic liquid/solid phases that behave as sponges. As liquid phases, they are excellent monophasic reaction media for the lipase-catalyzed synthesis of anisyl acetate, the product yield being improved up to 100% for 2 hour reaction under the appropriate reaction conditions (i.e. SLIL concentration, alcohol: acid molar ratio, enzyme amount, dehydrating molecular sieves, temperature and MW heating). As a function of the phase behaviour of different SLIL/anisyl acetate mixtures, a new clean separation protocol based on the centrifugation of the solid IL/flavour ester through nylon membranes was proposed, which provided a nearly full separation of the solid SLIL and the easy recovery of the reaction mixture. The enzymatic synthesis of anisyl acetate in [C16tma] [NTf2] under MW assistance, followed by the separation step of the solid SLIL, provided a nearly solvent-free fragrance product with up to 0.89 g/mL concentration. The catalytic activity of the enzyme / SLIL system remained unchanged for ten consecutive operational cycles. This work reports a straightforward and sustainable approach for producing anisyl acetate as a natural flavour and demonstrates to be suitable for scaling-up, providing a high potential for practical application.

The use of biocatalyzed processes is increasingly becoming recognized as a very important part inside Green Chemistry, because those synthetic routes mediated by enzymes or cells are generally conducted under mild reaction conditions, at ambient temperature and can use water as reaction medium in many cases. Their high selectivity avoids the need of functional group activation and protection/deprotection steps usually required in traditional organic synthesis. Thus, biocatalysis provides processes which are shorter, produce less waste and reduce manufacturing costs and environmental impact. These features are even more significant in drug synthesis, because it is well known that Pharma Industry is the most waste-producers. In this review some biocatalyzed protocols for the preparation of bioactive molecule and/or chiral building blocks for drug synthesis will be presented, focusing only in those used for treatment of noncommunicable diseases (NCDs).

The possibilities for the deoxygenation of 3-methyl-1-phenyl-3-phospholene 1-oxide were investigated using different silanes. Cl3SiH and PhSiH3 were efficient, but they have disadvantages regarding user-friendliness and price, respectively. Tetramethyldisiloxane and polymethylhydrosiloxane that are cheap and user-friendly, but are of lower reactivity could be applied at 110 °C in toluene solution, without the use of any catalyst successfully. Then, the deoxygenation was extended to other 1-phenylphospholene oxides and phospholane oxides. The cyclic phosphines useful as ligands in transition metal complexes were identified as their sulfides and boranes. The catalyst-free method elaborated seems to be of more general value.