Theoretical insight on the treatment of ß-hexachlorocyclohexane waste through alkaline dehydrochlorination.

The occurrence of 4.8-7.2 million tons of hexachlorocyclohexane (HCH) isomers stocked in dumpsites around the world constitutes a huge environmental and economical challenge because of their toxicity and persistence. Alkaline treatment of an HCH mixture in a dehydrochlorination reaction is hampered by the low reactivity of the ß-HCH isomer (HCl elimination unavoidably occurring through syn H-C-C-Cl arrangements). More intriguingly, the preferential formation of 1,2,4-trichlorobenzene in the ß-HCH dehydrochlorination reaction (despite the larger thermodynamical stability of the 1,3,5-isomer) has remained unexplained up to now, though several kinetic studies had been reported. In this paper, we firstly show a detailed Density Functional study on all paths for the hydroxide anion-induced elimination of ß-HCH through a three-stage reaction mechanism (involving two types of reaction intermediates). We have now demonstrated that the first reaction intermediate can follow several alternative paths, the preferred route involving abstraction of the most acidic allylic hydrogen which leads to a second reaction intermediate yielding only 1,2,4-trichlorobenzene as the final reaction product. Our theoretical results allow explaining the available experimental data on the ß-HCH dehydrochlorination reaction (rate-determining step, regioselectivity, instability of some reaction intermediates).

Comparison of Chemical and Enzymatic Methods for the Transesterification of Waste Fish Oil Fatty Ethyl Esters with Different Alcohols.

Fatty acid esters of 2-ethyl-1-hexanol (EH), 2-hexyl-1-decanol (HD), and isopropanol have been obtained from a mixture of ethyl esters obtained as a fish oil byproduct. Homogeneous base catalysis with alkaline hydroxides and alkoxides has been compared with the use of two commercially available immobilized lipases. The enzymatic methodology is more efficient in the case of the largest alcohol (HD) mainly because of the high stability of the immobilized enzymes upon recovery and reuse. In contrast, the use of a base as a catalyst is highly favorable in the case of isopropanol because of the rather poor activity of the lipases and the low price of the bases. With EH, the activity of lipases is good but the recoverability is not as efficient; hence, basic catalysts are again the most attractive alternative. The mixtures of esters obtained may be useful as hydraulic liquids given their viscosity values.

Copper-catalyzed cyclopropanation reaction of but-2-ene.

The mechanism of the copper(I)-catalyzed cyclopropanation reaction for methyl diazoacetate with both (Z)- and (E)-but-2-ene stereoisomers has been studied using the 6-311++G(d,p) basis set by means of M06-2X and O3LYP functionals. According to both methods, the rate-limiting step is the formation of a copper-carbene intermediate, formed by association between methyl diazoacetate and bis(acetonitrile)-copper(I) ion with the concomitant extrusion of dinitrogen. Cis/trans diastereoselectivity for the cyclopropanation reaction of a 1,2-disubstituted alkene ((Z)-but-2-ene) has been theoretically studied for the first time through the proper location of transition states on the potential-energy surface with the O3LYP method, since no transition structures could be found with the M06-2X functional due to the extreme flatness of the potential-energy surface. The calculated stereoselectivities involving two acetonitrile ligands or one dichloromethane molecule show qualitative agreement with experimental data. This study allows attributing the origin of the selectivity to steric interactions between the ligands of the catalyst system and the olefin substituents. The comparison between the corresponding activation barriers for the direct insertion step shows a higher reactivity for the Z stereoisomer of but-2-ene, consistently with the larger reactant destabilization through steric interactions.

Polytopic bis(oxazoline)-based ligands for recoverable catalytic systems applied to the enantioselective Henry reaction.

Several kinds of polytopic chiral ligands (including ditopic, tritopic and tetratopic), based on the bis(oxazoline) and azabis(oxazoline) motifs, have been tested in the preparation of recoverable catalytic systems for the Henry reaction. The results obtained with the different ligands are, in general, good, but they point to the existence of a delicate balance between the coordinating ability of the ligand, the catalytic activity and the recovery of the catalyst by formation of the coordination polymer, related to the easiness to form oligomeric species in solution.

Bis(oxazoline)-based coordination polymers: a recoverable system for enantioselective Henry reactions.

An efficient release-capture strategy for the recovery and reuse of enantioselective catalysts in the Henry reaction is described. This strategy is based on the precipitation of an insoluble coordination polymer at the end of each reaction, allowing easy separation from products. The coordination polymer is formed through aggregation of Cu(II) ions with ditopic bisoxazoline-based chiral ligands. Up to 11 catalytic cycles have been conducted keeping good yields and enantioselectivities.

Enantioselective catalysis with chiral complexes immobilized on nanostructured supports.

In this tutorial review, we discuss how recycling and stability advantages alone are frequently not sufficient to justify the effort necessary to immobilize chiral complexes unless additional advantages are found. Nanostructured solids with well-controlled surfaces and pores may act as nanoreactors, hindering or even blocking some of the reaction channels, and hence modifying the stereochemical result of the reaction. The use of support effects to improve or change the enantioselectivity is emerging as an interesting field, whose understanding might allow, in the near future, the design of chiral ligands better adapted to this strategy.

Metal-mediated cyclization of aryl and benzyl glycidyl ethers: a complete scenario.

Enantiomerically pure aryl and benzyl glycidyl ethers undergo stereospecific cyclizations leading to 3-chromanols or to tetrahydrobenzo[c]oxepin-4-ols under mild conditions in the presence of a catalytic amount of FeBr3/3AgOTf. The same processes are also mediated, albeit in a much less efficient manner, by AuCl3/3AgOTf. The set of active mediators in this group of processes (BF3 x Et2O, FeBr3, FeBr3/3AgOTf, AuCl3/3AgOTf) shows its nature as Friedel-Crafts reactions and disfavors the intermediacy of arylgold species.

Surface-enhanced stereoselectivity in Mukaiyama aldol reactions catalyzed by clay-supported bis(oxazoline)-copper complexes.

Surface effects on a laponite-exchanged bis(oxazoline)-copper complex modify the stereochemical course of the Mukaiyama aldol reaction between 2-(trimethylsilyloxy)furan and alpha-ketoesters, leading up to 90% ee (dr 86 : 14) becoming the best overall result obtained for this reaction and significantly improving the homogeneous process.

The first synthesis of organic-inorganic hybrid materials with chiral bis(oxazoline) ligands.

Condensation of tetraethoxysilane with silane-functionalized bis(oxazolines) in the presence of dodecylamine leads to hybrid materials whose textural and catalytic properties depend on both the ligand and the spacer structure.

The role of binding constants in the efficiency of chiral catalysts immobilized by electrostatic interactions: the case of azabis(oxazoline)-copper complexes.

As shown by theoretical calculations, azabis(oxazoline)-copper complexes are considerably more stable than the analogous bis(oxazoline)-copper complexes. This enhanced stability allows them to be efficiently immobilized by means of electrostatic interactions to different anionic supports, such as clays and nafion-silica nanocomposites, without the loss of a ligand, as is observed for bis(oxazolines). As a result, enantioselectivities of around 90 % ee are obtained in the cyclopropanation reaction between styrene and ethyl diazoacetate. Moreover, the solid catalyst is easily recoverable.