The olefin aziridination reactions catalyzed by copper and silver complexes bearing hydrotris(pyrazolyl)borate (Tp(x)) ligands have been investigated from a mechanistic point of view. Several mechanistic probe reactions were carried out, specifically competition experiments with p-substituted styrenes, stereospecificity of olefins, effects of the radical inhibitors, and use of a radical clock. Data from these experiments seem to be contradictory, as they do not fully support the previously reported concerted or stepwise mechanisms. But theoretical calculations have provided the reaction profiles for both the silver and copper systems with different olefins to satisfy all experimental data. A mechanistic proposal has been made on the basis of the information that we collected from experimental and theoretical studies. In all cases, the reaction starts with the formation of a metal-nitrene species that holds some radical character, and therefore the aziridination reaction proceeds through the radical mechanism. The silver-based systems however hold a minimum energy crossing point (MECP) between the triplet and closed-shell singlet surfaces, which induce the direct formation of the aziridines, and stereochemistry of the olefin is retained. In the case of copper, a radical intermediate is formed, and this intermediate constitutes the starting point for competition steps involving ring-closure (through a MECP between the open-shell singlet and triplet surfaces) or carbon-carbon bond rotation, and explains the loss of stereochemistry with a given substrate. Overall, all the initially contradictory experimental data fit in a mechanistic proposal that involves both the singlet and the triplet pathways.

Carbene transfer reactions can be performed in batch and flow with a highly active, chemically stable heterogenized tris(triazolyl)methyl copper(i) catalyst.

Silver(I) promotes the highly chemoselective N-amidation of tertiary amines under catalytic conditions to form aminimides by nitrene transfer from PhI═NTs. Remarkably, this transformation proceeds in a selective manner in the presence of olefins and other functional groups without formation of the commonly observed aziridines or C-H insertion products. The methodology can be applied not only to rather simple tertiary amines but also to complex natural molecules such as brucine or quinine, where the products derived from N-N bond formation were exclusively formed. Theoretical mechanistic studies have shown that this selective N-amidation reaction proceeds through triplet silver nitrenes.

The sequential reaction of two different furans and PhI=NTs produces 1,2-dihydropyridines in a quantitative manner in the presence of Tp Br3 Cu(NCMe) (Tp Br3 = hydrotris(3,4,5-tribromopyrazolyl)borate) as the catalyst under very mild conditions. The use of a furan and ethyl vinyl ether with PhI=NTs has led to the corresponding 1,2,3,4-tetrahydropyridine. ■ INTRODUCTIONThe metal-mediated catalytic transfer of nitrene (NR) groups to saturated or unsaturated substrates constitutes a useful tool in organic synthesis, in both inter-and intramolecular fashions, the main transformations being the C−H amination or the C=C aziridination reactions (Scheme 1). 1 During the course of our investigations in this area, we found a novel reaction 2 in which alkyl-substituted furans could be selectively converted into 1,2-dihydropyridines, in a process involving two molecules of the heterocycle and one molecule of PhI=NTs (Ts = ptoluenesulfonyl) and with Tp x M complexes (Tp x = hydrotris(xsubstituted-pyrazolyl)borate ligand; M = Cu, Ag) as the catalyst. Mechanistic studies showed that this transformation takes place along four consecutive catalytic cycles, in which at least three of them were metal-mediated. The first step consisted of the aziridination of the furan (2-methylfuran in Scheme 2) and subsequent spontaneous opening of the aziridine AZ to give the aldehyde O1. This aldehyde was converted into the imine O2 throughout a transimination catalytic step (either metal-or water-induced). The third cycle involved in this transformation was a metal-induced aza-Diels− Alder reaction (ADAR) that afforded the bicyclic compound BC, which finally converted into the 1,2-dihydropyridine upon a metal-catalyzed H-elimination−migration process.Dihydropyridines (DHPs) are essential intermediates in the synthetic routes of organic compounds with important biological activity. 3 1,4-Dihydropyridines are starting materials for a variety of drugs such as Nifedipine 4 and AK-2-38 5 as Special Issue: Copper Organometallic Chemistry