25 years of N-heterocyclic carbenes: activation of both main-group element-element bonds and NHCs themselves

From Imidazole toward Imidazolium Salts and N-Heterocyclic Carbene Ligands: Electronic and Geometrical Redistribution

Investigations dealing with N-heterocyclic carbenes and their derivatives are usually centered on the influence that they exert by acting as catalysts, ionic liquids, or metallodrugs and consequently on their capabilities to tune the properties and reactivity of these systems. In this context, we aimed to focus on the internal molecular changes undergone by imidazole derivatives, from electronic and geometrical points of view. This work represents an empirical evidence of the molecular modifications that an imidazole skeleton undergoes upon protonation, alkylation, and metalation.

Reversible Oxidative Addition at Carbon

The reactivity of N-heterocyclic carbenes (NHCs) and cyclic alkyl amino carbenes (cAACs) with arylboronate esters is reported. The reaction with NHCs leads to the reversible formation of thermally stable Lewis acid/base adducts Ar-B(OR)₂ ⋅NHC (Add1-Add6). Addition of cAAC^Me to the catecholboronate esters 4-R-C₆ H₄ -Bcat (R=Me, OMe) also afforded the adducts 4-R-C₆ H₄ Bcat⋅cAAC^Me (Add7, R=Me and Add8, R=OMe), which react further at room temperature to give the cAAC^Me ring-expanded products RER1 and RER2. The boronate esters Ar-B(OR)₂ of pinacol, neopentylglycol, and ethyleneglycol react with cAAC at RT via reversible B-C oxidative addition to the carbene carbon atom to afford cAAC^Me (B{OR}₂ )(Ar) (BCA1-BCA6). NMR studies of cAAC^Me (Bneop)(4-Me-C₆ H₄ ) (BCA4) demonstrate the reversible nature of this oxidative addition process.

Electronic Structure and Excited-State Dynamics of an Arduengo-Type Carbene and its Imidazolone Oxidation Product

We describe an investigation of the excited-state dynamics of isolated 1,3-di-tert-butyl-imidazoline-2-ylidene (tBu₂ Im, C₁₁ H₂₀ N₂ , m/z=180), an Arduengo-type carbene, by time- and frequency-resolved photoionization using a picosecond laser system. The energies of several singlet and triplet excited states were calculated by time-dependent density functional theory (TD-DFT). The S₁ state of the carbene deactivates on a 100 ps time scale possibly by intersystem crossing. In the experiments we observed an additional signal at m/z=196, that was assigned to the oxidation product 1,3-di-tert-butyl-imidazolone, tBu₂ ImO. It shows a well-resolved resonance-enhanced multiphoton ionization (REMPI) spectrum with an origin located at 36951 cm^-1 . Several low-lying vibrational bands could be assigned, with a lifetime that depends strongly on the excitation energy. At the origin the lifetime is longer than 3 ns, but drops to 49 ps at higher excess energies. To confirm formation of the imidazolone we also performed experiments on benzimidazolone (BzImO) for comparison. Apart from a redshift for BzImO the spectra of the two compounds are very similar. The TD-DFT values display a very good agreement with the experimental data.

Ring expansion of a ring expanded carbene

Reaction of phenylsilane with the ring expanded carbene 6-Mes results in facile Si–H oxidative addition to the carbenic carbon at room temperature and ring opening on heating.

A catalytic intramolecular nitrene insertion into a copper(i)–N-heterocyclic carbene bond yielding fused nitrogen heterocycles

We report a catalytic NHC–imido/nitrene cyclization allowing an easy access to diverse nitrogen-rich heterocyclic scaffolds.

Azido- and amido-substituted gallium hydrides supported by N-heterocyclic carbenes

The synthesis of azido- and amido-gallanes supported by hindered N-heterocyclic carbene donors is reported.

Adduct Formation, B-H Activation and Ring Expansion at Room Temperature from Reactions of HBcat with NHCs

We report the reactions of catecholborane (HBcat; 1) with unsaturated and saturated NHCs as well as CAAC(Me) . Mono-NHC adducts of the type HBcat⋅NHC (NHC=nPr2 Im, iPr2 Im, iPr2 Im(Me) , and Dipp2 Im) were obtained by stoichiometric reactions of HBcat with the unsaturated NHCs. The reaction of CAAC(Me) with HBcat yielded the B-H activated product CAAC(Me) (H)Bcat via insertion of the carbine-carbon atom into the B-H bond. The saturated NHC Dipp2 SIm reacted in a 2:2 ratio yielding an NHC ring-expanded product at room temperature forming a six-membered -B-C=N-C=C-N- ring via C-N bond cleavage and further migration of the hydrides from two HBcat molecules to the former carbene-carbon atom.

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.