Intermediates of N-Heterocyclic Carbene (NHC) Dimerization Probed in the Gas Phase by Ion Mobility Mass Spectrometry: C−H⋅⋅⋅:C Hydrogen Bonding Versus Covalent Dimer Formation

N-Heterocyclic carbenes (NHCs, :C ) can interact with azolium salts ( C−H⁺ ) by either forming a hydrogen-bonded aggregate ( CHC⁺ ) or a covalent C−C bond ( CCH⁺ ). In this study, the intramolecular NHC–azolium salt interactions of aromatic imidazolin-2-ylidenes and saturated imidazolidin-2-ylidenes have been investigated in the gas phase by traveling wave ion mobility mass spectrometry (TW IMS) and DFT calculations. The TW IMS experiments provided evidence for the formation of these important intermediates in the gas phase, and they identified the predominant aggregation mode (hydrogen bond vs. covalent C−C) as a function of the nature of the interacting carbene–azolium pairs.     

Differences and Commonalities in the Gas‐Phase Reactions of Closed‐Shell Metal Dioxide Clusters [MO2]+ (M=V,Nb, and Ta) with Methane

High‐level electronic structure calculations, in combination with Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometric studies, permit the mechanism by which closed‐shell, “naked” [TaO₂]⁺ brings about C?H bond activation of methane to be revealed. These studies also help to understand why the lighter congeners of [MO₂]⁺ (M=V, Nb) are unreactive under ambient conditions.     

ON THE REARRANGEMENT OF N-ARYL SULFIMIDES

Abstract

N-Aryl-S,S-dialkylsulfimides, 1, with R¹ = alkyl other than CH₃, have been rearranged by heating in ethanol yielding o-alkylthiomethyl-anilines, 2, as main products. Isomeric o-methylthioalkyl-anilines, 14, are formed in minor amounts only. Reactions of sulfimides, 1, with R¹ = CH₃, with certain alkylating or acylating agents yielded o-methylthiomethylated, N-alkylated or -acylated products 9. Mechanistic considerations are discussed. The rearrangement of sulfimides 1 has been assumed to occur via [2,3]-sigmatropic reactions of intermediate azasulfonium ylids 3. Attempts to resolve (+)-camphor-10-sulfonates of N-aryl sulfimides failed, but optically active N-aryl sulfimides could be obtained by reaction of anilines with optically active sulfoxides and P₄O₁₀. Optically active 2,6-disubstituted sulfimides, 1, could be rearranged in ethanolic KOH to yield optically active cyclohexadienimines 12, indicating a transfer of asymmetry from sulfur to carbon and supporting the assumption of a sigmatropic rearrangement.     

Aliphatic Poly(ether amide)s by Polycondensation of Activated Sebacic Acid Derivatives

Poly(ether amide)s were prepared by polycondensation of 1,13‐diamino‐4,7,11‐trioxatridecane (DTT) with the bisimidazolide or with the bis N‐hydroxysuccinimide ester of sebacic acid. Four different solvents and three different temperatures were compared. The highest molecular weights were obtained with the bisimidazolide in dimethylsulfoxide (DMSO) at 60°C. MALDI‐TOF mass spectra revealed the existence of cyclic oligoamides and polyamides in all samples. The molar fraction of cycles considerably increased with higher molecular weights of the entire sample. The polycondensations were repeated under optimum conditions in the presence of α‐cyclodextrin to prepare polydisperse catenanes consisting of α‐cyclodextrin threaded on cyclic polyamides. Yet, despite broad variation of the reaction conditions, only cylic polyamides free of cyclodextrin were isolated. Furthermore, a pseudorotaxane was prepared from DTT and α‐cyclodetrin and polycondensed with bis‐(4‐chlorophenyl)sebacate. Again, only cyclic polyamides free of cyclodextrin were detectable.