1,2,4‐Triazole‐Based N‐Heterocyclic Carb­ene Nickel Complexes – Synthesis and Catalytic Application

Solvent- and functional-group-assisted tautomerism of 3-alkyl substituted 5-(2-pyridyl)-1,2,4-triazoles in DMSO-water

The tautomerism of a series of 5-alkyl substituted 3-(2-pyridyl)-1,2,4-triazoles in DMSO-d6-containing water has been investigated by 1H, 13C and 15N NMR spectroscopy. The populations of the three possible regioisomers in the tautomeric equilibrium (A [3-alkyl-5-(2-pyridyl)-1H], B [5-alkyl-3-(2-pyridyl)-1H] and C [5-alkyl-3-(2-pyridyl)-4H]) were determined. Isomers A (17-40%) and B (54-79%) are the major components and their ratio is insensitive to the substitution pattern, except for the unsubstituted and the methoxymethyl substituted derivatives. The isomer C (3-5%) has been fully characterised for the first time by NMR spectroscopy. Activation energies of tautomerisation (14.74-16.78 kcal mol-1) were determined by EXSY experiments, which also supported the involvement of water in the tautomerisation. Substituent effects on the 15N chemical shifts are relatively small. The DFT study of the tautomerism in DMSO-water showed that both A/B and B/C interconversions are assisted by the pyridine substituent and catalysed by solvent molecules. The NH-A/NH-B tautomerisation takes place via a relayed quadruple proton transfer mediated by three water molecules in the hydrogen-bonded cyclic substructure of a triazole·4H2O complex. The equilibrium B ⇄ C involves three steps: NH-B transfer to the pyridyl nitrogen mediated by a water molecule in a 1 : 1 cyclic complex, rotamerisation to bring the pyridinium NH close to N4 of the triazole catalysed by complexation to a DMSO molecule and transfer of the NH from the pyridinium donor to the N4 acceptor via a 1 : 1 complex with a bridging water molecule. This mechanism of 1,3-prototropic shift in triazoles is unprecedented in the literature.

First-Row d-Block Element-Catalyzed Carbon-Boron Bond Formation and Related Processes

Organoboron reagents represent a unique class of compounds because of their utility in modern synthetic organic chemistry, often affording unprecedented reactivity. The transformation of the carbon-boron bond into a carbon-X (X = C, N, and O) bond in a stereocontrolled fashion has become invaluable in medicinal chemistry, agrochemistry, and natural products chemistry as well as materials science. Over the past decade, first-row d-block transition metals have become increasingly widely used as catalysts for the formation of a carbon-boron bond, a transformation traditionally catalyzed by expensive precious metals. This recent focus on alternative transition metals has enabled growth in fundamental methods in organoboron chemistry. This review surveys the current state-of-the-art in the use of first-row d-block element-based catalysts for the formation of carbon-boron bonds.

Synthesis of the First Resorcin[4]arene-Functionalized Triazolium Salts and Their Use in Suzuki–Miyaura Cross-Coupling Reactions

Two bulky triazolium salts, namely 1-{4(24),6(10),12(16),18(22)-tetramethylenedioxy- 2,8,14,20-tetrapentylresorcin[4]arene-5-yl}-4-phenyl-3-methyl-1H-1,2,3-triazolium tetrafluoro borate (1) and 1,4-bis{4(24),6(10),12(16),18(22)-tetramethylenedioxy-2,8,14,20-tetrapentyl resorcin[4]arene-5-yl}-3-methyl-1H-1,2,3-triazolium iodide (2), have been synthesized and assessed in the palladium-catalyzed Suzuki–Miyaura cross-coupling of aryl chlorides, with aryl boronic acids. As a general trend, the reaction rates obtained with 1 were significantly higher (up to 5 times) than those observed for 2, this mainly reflected a sterically more accessible metal center in the catalytic intermediates formed with 1. The presence of flexible pentyl chains in these intermediates, which might sterically interact with the metal center, when the latter adopts an exo-orientation with respect to the cavity, were likely responsible for the observed good performance.

Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse

Although known for over 90 years, only in the past two decades has the chemistry of diboron(4) compounds been extensively explored. Many interesting structural features and reaction patterns have emerged, and more importantly, these compounds now feature prominently in both metal-catalyzed and metal-free methodologies for the formation of B-C bonds and other processes.