Imidazolyl Alanes - Synthesis, Structures, and Reactivity Studies – Imidazolyl Alanes - Synthesis, Structures, and Reactivity Studies

Targeting the synthesis of Al/C based ambiphilic molecules, we investigated the dehydrohalogenation of a series of (benz)imidazole alane adducts. Depending on the steric bulk of the heterocycle, different dimeric products with various ring sizes were obtained. Dehydrohalogenation of the adduct of 1‐mesityl imidazole ($^{Mes}$Im) and 0.5 [tBu$_{2}$AlBr]$_{2}$ furnished the dimer 2, featuring a “classical” N‐heterocyclic carbene (NHC) and a mesoionic or “abnormal” NHC (aNHC) subunit within a single molecule. The dimer is bound loosely enough to allow thermally induced isomerization of 2 into the isomers 2$^{NHC}$ (all NHC) and 2$^{aNHC}$ (all aNHC). Dehydrohalogenation of the adduct of 1‐mesityl‐2‐methyl imidazole ($^{Mes}$Im$^{Me}$) and 0.5 [tBu$_{2}$AlBr]$_{2}$ (4) yielded the dimeric compound 5 consisting of two N‐heterocyclic olefin (NHO) subunits. Although these six‐ and eight‐membered heterocycles show no FLP‐type reactivity towards small molecules like H$_{2}$, CO or CO$_{2}$, we observed an ambiphilic behavior of the imidazolyl alanes during our studies. Salt metathesis reactions using $^{Mes}$Im resulted in the formation of 3, which can be viewed as tBu$_{2}$AlBr adduct of an Al/N ambiphile. Utilizing heterocycles such as benzimidazole or spiroindole provided the entry point to C–H (7, 9) and N–H (10) activation products, most likely resulting from a reactivity of intermediate species as Al/C ambiphiles

Use of ring-expanded diamino- and diamidocarbene ligands in copper catalyzed azide-alkyne "click" reactions

The two-coordinate ring-expanded N-heterocyclic carbene copper­(I) complexes [Cu­(RE-NHC)₂]⁺ (RE-NHC = 6-Mes, 7-<i>o</i>-Tol, 7-Mes) have been prepared and shown to be effective catalysts under neat conditions for the 1,3-dipolar cycloaddition of alkynes and azides. In contrast, the cationic diamidocarbene analogue [Cu­(6-MesDAC)₂]⁺ and the neutral species [(6-MesDAC)­CuCl]₂ and [(6-MesDAC)₂(CuCl)₃] show good activity when the catalysis is performed on water

Borylated N-heterocyclic carbenes: rearrangement and chemical trapping

This study details attempts to access N-heterocyclic carbenes (NHCs) featuring the diazaborolyl group, {(HCNDipp)2 B}, as one or both of the N-bound substituents, to generate strongly electron-donating and sterically imposing new carbene ligands. Attempts to isolate N-heterocyclic carbenes based around imidazolylidene or related heterocycles, are characterized by facile N-to-C migration of the boryl substituent. In the cases of imidazolium precursors bearing one N-bound diazaborolyl group and one methyl substituent, deprotonation leads to the generation of the target carbenes, which can be characterized in situ by NMR measurements, and trapped by reactions with metal fragments and elemental selenium. The half-lives of the free carbenes at room temperature range from 4-50 h (depending on the pattern of ancillary substituents) with N-to-C2 migration of the boryl function being shown to be the predominant rearrangement pathway. Kinetic studies show this to be a first-order process that occurs with an entropy of activation close to zero. DFT calculations imply that an intramolecular 1,2-shift is mechanistically feasible, with calculated activation energies of the order of 90-100 kJ mol-1 , reflecting the retention of significant aromatic character in the imidazole ring in the transition state. Trapping of the carbene allows for evaluation of steric and electronic properties through systems of the type LAuCl, LRh(CO)2 Cl, and LSe. A highly unsymmetrical (but nonetheless bulky) steric profile and moderately enhanced σ-donor capabilities (compared with IMes) are revealed

Synthetic, structural and reaction chemistry of N-heterocyclic germylene and stannylene compounds featuring N-boryl substituents

This study details the syntheses of N-heterocyclic germylenes and stannylenes featuring diazaborolyl groups, {(HCDippN)2B} (Dipp = 2,6-iPr2C6H3), as both of the N-bound substituents, with a view to generating electron rich and sterically protected metal centres. The energies of their key frontier orbitals - the group 14-centred lone pair and orthogonal pπ-orbital (typically the HOMO-2 and LUMO) have been probed by DFT calculations and compared with a related acyclic analogue, revealing (in the case of the stannylenes) a correlation with the measured 119Sn chemical shifts. The reactivity of the germylene systems towards oxygen atom transfer agents has been examined, with 2 : 1 reaction stoichiometries being observed for both Me3NO and pyridine N-oxide, leading to the formation of products thought to be derived from the activation of C-H bonds by a transient first-formed germanone

Remarkable stability of copper(II)-N-heterocyclic carbene complexes void of an anionic tether

A library of pyridyl- and picolyl-substituted imidazolium salts have been synthesized and coordinated to copper, via transmetalation from silver(I)-N-heterocyclic carbenes (NHCs), to prepare several copper(I)- and copper(II)-NHC complexes. The copper(I)-NHCs are complexes of the type Cu(NHC)Br, with the solid-state structures revealing a variety of coordination environments around the copper centers. The stability of the copper(II) complexes is particularly unusual, given the absence of a "hard" anionic tethering group appended to the ligands. The stability has been attributed to the pyridyl substituent, with the complexes being extremely stable, while those with an appended anionic group tend to be more sensitive to air/moisture. The ligands and complexes have been examined in an Ullmann-type etherification reaction and exhibit improved activity in comparison to copper in the absence of a ligand or the common Cu(I)-NHC complexes Cu(IMes)Cl and [Cu(IMes)2]PF6, indicating stabilization of higher oxidation state species by the ligands during the catalytic cycle