Multicomponent and Multicatalytic Reactions are those processes that try to imitate the way the enzymatic machinery transforms simple building blocks into complex products. The development of asymmetric versions of these...

The potential of foundry sands, industrial waste from the iron foundry industry, was evaluated for the removal of Cr (VI) using discontinuous assays. Chemical foundry sands are composed of fine silica sand, furanic resins as binder, chemical catalyst and residual iron particles. The influence ofpH, agitation rate and metal concentration on the removal process was investigated. Kinetic and equilibrium tests were conducted to determine Cr (VI) removal from aqueous solutions at a temperature range of 25-55 degrees C. Cr (VI) removal of 40-100% for a range of pH 6-1.6 was obtained. This removal was attributed to the presence of a large number of protonated silanol and aluminol groups. Cr (VI) adsorption in foundry sands follows a pseudo-second-order kinetic reaction (Ho model, r2 > 0.999) reaching kinetic constants of 0.341, 0.551, 0.775 and 0.920 g/mg h at 25, 35, 45 and 55 degrees C, respectively. The adsorption data were fitted to the Langmuir adsorption isotherm model (r > 0.99) obtaining adsorption capacities (q(max)) of 1.99, 2.40, 2.50, and 3.14 mg Cr (VI)/g sand at 25, 35, 45 and 55 degrees C, respectively. Calculated Gibbs free energy change (deltaG0), adsorption energy (E) and activation energy (E(a)) values indicate that a physisorption mechanism governs Cr (VI) adsorption process in foundry sands.

The stereoselective synthesis of cis-disubstituted cyclopropanes by the Au(I)/PPh 3 -catalyzed cycloaddition of propargylic esters and styrene has been studied using density functional theory calculations. The computed mechanistic scheme involves the rate-limiting 1,2-rearrangement of the propargylic ester with the π-coordinated gold complex, followed by the (2 + 1)-cheletropic reaction of styrene with the alkenyl−Au(I) carbene intermediate to afford the cis-disubstituted cyclopropane derivative in a high cis/ trans diastereomeric ratio. With a (R)-di-chloro,di-gold-DTBM-SEGPHOS complex as the catalyst, computations are consistent with a rate-determining (2 + 1)-cheletropic reaction, in which facial discrimination is proposed to result from a combination of subtle steric and electronic effects in the SiRe facial approach transition structure, which favor the formation of the cis-cyclopropane diastereomer of 1R,2S absolute configuration, as experimentally observed.

Structural variants of the (iso)‐Nazarov and aza‐(iso)Nazarov electrocyclic reactions, including monofunctional and difunctional pentadienyl cations with permutations of the heteroatom‐containing functional groups have been computationally studied at the ωB97XD/Def2TZVPP(SMD, THF)//ωB97XD/Def2TZVP level of theory. The relative location of two heteroatoms at either odd‐ or even‐numbered positions of the conjugated pentadienyl cation, including the Piancatelli and iso‐Piancatelli reactions and their variants, determines the feasibility of their rearrangement. In particular 4πe–‐electrocyclic reactions of formal enamines/protonated ketones are more favorable than those of the alternative enols/iminium ions. Both the classical aza‐Piancatelli and the aza‐iso‐Piancatelli electrocyclizations are highly favored. The key electrocyclic aza‐iso‐Piancatelli and aza‐Piancatelli steps of the few experimental systems successfully reported show activation energies that are in agreement with those of the simple model systems. On line with the computational predictions, the stabilization of the intermediates are key to ensure the success of these concerted electrocyclizations due to the positional location of the two heteroatoms and other factors (such as the release of strain of reacting allenes).

The aryl transmetalation processes between cis-[PdRf2(AsPh3)2] (Rf = C6Cl2F3) and [AuPf(AsPh3) (Pf = C6F5) has been studied experimentally and by DFT calculations. Aryl exchange with or without isomerization of the Pd geometry occur by ligand displacement of one AsPh3 ligand by an AuAr(AsPh3) molecule, which coordinates using the Au-Ar bond electron density, followed or not by a second switch to the next Ar group. The transition states are bridged Ar-Au(AsPh3)-Ar' structures with fairly planar geometry. Alternatively, a direct switch of the Au(AsPh3) fragment to either cis or trans Ar groups on Pd can be achieved from a square-pyramidal [(AsPh3)Au-PdAr3(AsPh3)] intermediate or transition state. The later pathway is less favorable for the case studied (M = Pd), but it is preferred for the same chemical system with M = Pt. The study provides some clues on exchanges that can be relevant in organic syntheses catalyzed by bimetallic systems.