Since the discovery of copper catalyzed cycloaddition of organic azides and alkynes into Click triazoles (1,2,3-triazoles), the corresponding triazolylidenes (1,2,3-triazol-5-ylidenes) have emerged as a powerful subclass of N-heterocyclic carbene ligands possessing unique complexation ability to transition metals. Especially attractive are the ligands of advanced architecture such as pyridyl-triazolylidenes, which can through variety of bi- and multidentate coordination fine-tune the catalytic, spectroscopic and electrochemical properties of the metal centre. Despite the tremendous potential of pyridyl-triazolylidenes their preparation, even starting from easily derived Click triazoles, remains a significant challenge due to some serious selectivity issues that are connected to the existing methods. In this communication we reported on highly selective and efficient protocol to easily access a variety of isomeric and homologous pyridyl-triazolium salts. It is based on a simple, yet carefully selected, reliable and selective protection of pyridine functionalized Click triazoles through the pyridine N-oxidation with subsequent triazole ring alkylation and deprotection. Our methodology offers a platform for the preparation of structurally diverse pyridyl-triazolylidenes almost at will. The concept and the applicability was demonstrated on in situ generated palladium N-heterocyclic carbene complex of this type and its use in Suzuki–Miyaura catalysis. Turnover of 9000 was achieved in selected C–C coupling reactions at room temperature in the environmentally benign water as a solvent, with as low as 0.01 mol% loading of the in situ generated catalyst. This article was highlighted in Chemistry & Industry 2013, 77 (12), p. 57), DOI: 10.1002/cind.7712_19.x (http://onlinelibrary.wiley.com/doi/10.1002/cind.7712_18.x/abstract)
COBISS.SI-ID: 1623855
One of the most remarkable aspects in the chemistry of organoazides is the loss of dinitrogen, which can occur thermally, photochemically, or by acid or transition-metal catalysis. Only few transition metals are capable of this transformation and this is the first report on platinum-mediated dinitrogen liberation from an organoazide. In a combined experimental and theoretical analysis we scrutinized this phenomenon on simple model substrate, 2-picolyl azide. Its interaction with Pt(II) precursor results in the azide Nα coordination, Nα−Nβ bond cleavage with dinitrogen liberation followed by β-C−H activation. The transformation involves a highly reactive intermediate, which is calculated to exhibit a uniquely short Pt−Nα distance of 1.837 Å. Although this short distance could be interpreted as Pt=N double bond the structure of the intermediate can also be imagined as the association of a Pt(III) and a nitrene radical ligand, strongly coupled antiferromagnetically. Scrutinizing the electronic structure of the “nitrene-like” intermediates for platinum and compared to those for copper, we found that they are very similar and can be best described as a metal(III) interacting with a nitrene radical ligand. This is also in line with the assigned cobalt(III)-nitrene radical intermediate detected recently in a similar reaction of cobalt(II) porphyrin. On the basis of these results, one can confidently assume intermediates with similar electronic structure in analogous transformations with other metals, where the mechanism is not completely determined or the intermediate is too short-lived to be detected. Also, in general, metals, which form stable complexes in oxidation state +3, may also form detectable intermediates with controllable and tunable reactivity toward, for example, C−H activation.
COBISS.SI-ID: 36619781
This account deals with recent advances in the chemistry of hydrogen trioxide, HOOOH, the simplest member of the family of polyoxides of the general formula ROnR, where R stands for hydrogen or other atoms or groups and n ≥ 3. These species, which may be regarded as higher homologues of hydrogen peroxide, are believed to be key intermediates in the low-temperature oxidations, atmospheric and environmental chemistry, chemistry of combustion and in biochemical oxidations. Various chemical methods were used for the preparation of relatively highly concentrated solutions of HOOOH, thus enabling unambiguous identification (1H and 17O NMR, IR (matrix and solution) and microwave spectroscopy, and state-of-the-art ab initio calculations). Theoretical and NMR spectroscopic evidence indicates that (HOOOH)n (n = 2, 3, 4, ...) assemblies are the characteristic structural feature of the polyoxide in the gas phase and in inert (nonpolar) solvents. Organic oxygen bases (B) as solvents are capable of disrupting these assemblies by forming intermolecularly hydrogen-bonded complexes, HOOOH−B. Water plays a crucial role in the decomposition of this polyoxide by acting as a bifunctional catalyst and accelerates the decomposition of HOOOH () 1000 times) to produce water and singlet oxygen. Hydrogen trioxide is more lipophilic than water and hydrogen peroxide, and stronger acid than HOOH as well. Protonation of terminal oxygen atoms (the most basic sites in HOOOH) gives HOOO(H)H+, a short-lived intermediate, which rapidly decomposes to produce H3O+ and singlet oxygen. HOOOH (together with the HOOO• radical and the HOOO— anion) may be regarded as an effective reactive oxygen species involved in the “peroxone process” and in the atmosphere. This polyoxide can also seriously damage different important biomolecules including DNA, lipids, and proteins (atherosclerosis, cancer, neurodegenerative disorders).
COBISS.SI-ID: 1615407