Mechanistic aspects of the lycopodine Michael-Claisen domino cyclization.

The Michael-Claisen domino (MCD) cyclization used in the lycopodine synthesis by Stork, was evaluated mechanistically using DFT calculations. Calculations suggest that a dianion is not formed, which conforms to classical dianion formation normally requiring strong kinetic bases. Instead ethoxide in ethanol produces a monoanionic species driving the MCD cyclization.

Carbon dioxide insertion into diamines: a computational study of solvent effects.

We studied computationally, on the model compound ethylenediamine, the insertion of carbon dioxide into diamines, yielding cyclic urea compounds. Two mechanisms were elaborated, depending on the value of the dielectric constant (DC) of the solvent. Accordingly, reaction mixtures with a high DC lead to carbamates, whereas lower DC values result in the preferred product cyclic urea.

Enantioselective total synthesis of the mexicanolides: khayasin, proceranolide, and mexicanolide.

The enantioselective total synthesis of the limonoids khayasin, proceranolide and mexicanolide was achieved via a convergent strategy utilizing a tactic aimed at incorporating natural products as advanced intermediates. This extended biomimetically inspired approach additionally achieved the enantioselective total synthesis of the intermediates azedaralide and cipadonoid B.

A combined DFT and NMR investigation of the zinc organometallic intermediate proposed in the syn-selective tandem chain extension-aldol reaction of β-keto esters.

The tandem chain extension-aldol (TCA) reaction of β-keto esters provides an α-substituted γ-keto ester with an average syn:anti selectivity of 10:1. It is proposed that the reaction proceeds via a carbon-zinc bound organometallic intermediate potentially bearing mechanistic similarity to the Reformatsky reaction. Evidence, derived from control Reformatsky reactions and a study of the structure of the TCA intermediate utilizing DFT methods and NMR spectroscopy, suggests the γ-keto group of the TCA intermediate plays a significant role in diastereoselectivity observed in this reaction.

Thermal decomposition of branched silanes: a computational study on mechanisms.

The initial steps of the thermal decomposition of silanes in the gas phase were examined by DFT-B3LYP calculations, with particular attention being paid to the way in which the reactivity pattern changes with the degree of branching of the silane. Besides the established pathways-1,2-hydrogen shift, H(2) elimination, and homolytic dissociation-1,3-hydrogen shift was also explored as an initial reaction step which leads to disilene structures. Subsequent silylene insertion and initial steps of radical chain reactions were also studied.

Metal-mediated reaction modeled on nature: the activation of isothiocyanates initiated by zinc thiolate complexes.

On the basis of detailed theoretical studies of the mode of action of carbonic anhydrase (CA) and models resembling only its reactive core, a complete computational pathway analysis of the reaction between several isothiocyanates and methyl mercaptan activated by a thiolate-bearing model complex [Zn(NH(3))(3)SMe](+) was performed at a high level of density functional theory (DFT). Furthermore, model reactions have been studied in the experiment using relatively stable zinc complexes and have been investigated by gas chromatography/mass spectrometry and Raman spectroscopy. The model complexes used in the experiment are based upon the well-known azamacrocyclic ligand family ([12]aneN(4), [14]aneN(4), i-[14]aneN(4), and [15]aneN(4)) and are commonly formulated as ([Zn([X]aneN(4))(SBn)]ClO(4).

A mechanistic investigation into the zinc carbenoid-mediated homologation reaction by DFT methods: is a classical donor-acceptor cyclopropane intermediate involved?

An extensive density functional theory (DFT, M05-2X) investigation has been performed on the zinc carbenoid-mediated homologation reaction of β-keto esters. The mechanistic existence of a classical donor-acceptor cyclopropane intermediate was probed to test the traditional school of thought regarding these systems. Calculations of the carbenoid insertion step, following enolate formation, unmasked two possible pathways.

The zinc complex catalyzed hydration of alkyl isothiocyanates.

Based upon our preceding studies of the hydration of CO(2), COS and CS(2), accelerated by the carbonic anhydrase (CA) using simplified [ZnL(3)OH](+) complexes as model catalysts, we calculated the hydration mechanisms of both the uncatalyzed and the [ZnL(3)OH](+)-catalyzed reactions (L = NH(3)) of isothiocyanates RNCS on the B3LYP/6-311+G(d,p) level of theory. Interestingly, the transition state for the favored metal mediated reaction with the lowest Gibbs free energy is only slightly higher than in the case of CO(2) (depending on the attacking atom (N or S). Calculations under inclusion of solvent corrections show a reduction of the selectivity and a slight decrease of the Gibbs free energy in the rate-determining steps.

Allene as the parent substrate in zinc-mediated biomimetic hydration reactions of cumulenes.

The aim of our present investigation is to unravel the general mode of biomimetic activation of a wide variety of cumulenes by carbonic anhydrase (CA) models. Carbonic anhydrases allow the specific recognition, activation and transfer not only of CO2 but also of heteroallenes X=C=Y such as the polar or polarizable examples COS, CS2, H2CCO, and RNCS. Therefore, this enzyme class fulfils the requirements of excellent catalysts with a wide variety of important applications.