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Under aprotic conditions, the stoichiometric reaction of N-heterocyclic carbenes (NHCs) such as imidazolidin-2-ylidenes with aldehydes affords Breslow Intermediates (BIs), involving a formal 1,2-C-to-O proton shift. We herein report kinetic studies (NMR), complemented by DFT calculations, on the mechanism of this kinetically disfavored H-translocation. Variable time normalization analysis (VTNA) revealed that the kinetic orders of the reactants vary for different NHC-to-aldehyde ratios, indicating different and ratio-dependent mechanistic regimes. We propose that for high NHC-to-aldehyde ratios, the H-shift takes place in the primary, zwitterionic NHC-aldehyde adduct. With excess aldehyde, the zwitterion is in equilibrium with a hemiacetal, in which the H-shift occurs. In both regimes, the critical H-shift is auto-catalyzed by the BI. Kinetic isotope effects observed for R-CDO are in line with our proposal. Furthermore, we detected an H-bonded complex of the BI with excess NHC (NMR).

A Bronsted acid catalyzed method was developed for the synthesis of.-cyanoketones from sulfonyl cyanides, olefins and ketones. The reaction is believed to proceed via intermediate formation of alkenyl peroxides by condensation of ketones with tert-butylhydroperoxide. These unstable compounds decompose by homolytic O-O bond cleavage, generating ketone-derived radicals which add to the olefins and generate the final products after reaction with the sulfonyl cyanide, thereby forming two new C-C bonds. A range of different ketones and olefins can be used, including steroidal ketones and simple alkyl olefins. The products can be further transformed to substituted lactones and piperidines, including a tetracyclic one. This reaction can thus be utilized to gain access to complex molecules from simple starting materials in only a few synthetic steps.

In this work, the reactivities of acetonyl and benzoyl radicals in aromatic substitution and addition reactions have been compared in an experimental and computational study. The results show that acetonyl is more electrophilic than benzoyl, which is rather nucleophilic. A Hammett plot analysis of the addition reactions of the two radicals to substituted styrenes clearly support the nucleophilicity of benzoyl, but in the case of acetonyl, no satisfactory linear correlation with a single substituent-related parameter was found. Computational calculations helped to rationalize this effect, and a good linear correlation was found with a combination of polar parameters (sigma(+)) and the radical stabilization energies of the formed intermediates. Based on the calculated philicity indices for benzoyl and acetonyl, a quantitative comparison of these two radicals with many other reported radicals is possible, which may help to predict the reactivities of other aromatic radical substitution reactions.

Molecular iodine, an easy to handle solid, has been successfully employed as a catalyst in different organic transformation's for more than 100 years. Despite being active even in very small amounts, the origin of this remarkable catalytic effect is still unknown. Both a halogen bond mechanism as well as hidden Bronsted acid catalysis are frequently discussed as possible explanations. Our kinetic analyses reveal a reaction order of 1 in iodine, indicating that higher iodine species are not involved the rate-limiting transition state. Our experimental investigations rifle out hidden Bronsted acid catalysis by partial deconiposition of I-2 to HI and suggest a halogeri bond activation instead. Finally, molecular iodine turned out to be a similar if not superior,catalyst for Michael additions compared with typical Lewis acids.

The acid-catalyzed reactions of photochemically generated tetrahydrocarbazole peroxides with anilines have been studied experimentally and computationally to identify the underlying reaction mechanism. The kinetic data indicate a reaction order of one in the hydroperoxide and zero in the aniline. Computational investigations using density functional theory support the experimental findings and predict an initial tautomerization between an imine and enamine substructure of the primarily generated tetrahydrocarbazole peroxide to be the rate controlling step. The enamine tautomer then loses hydrogen peroxide upon protonation, generating a stabilized allylic carbocation that is reversibly trapped by solvent or aniline to form the isolated products.