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The Burgess reagent ([methoxycarbonylsulfamoyl]triethylammonium hydroxide) has historically found utility as a dehydrating agent. Herein we show that, in the presence of dimethyl sulfoxide, the Burgess reagent efficiently and rapidly facilitates the oxidation of a broad range of primary and secondary alcohols to their corresponding aldehydes and ketones in excellent yields and under mild conditions, and can be combined with other transformations (e.g., Wittig olefinations). A mechanism similar to those described for the Pfitzner-Moffatt and Swern oxidations is proposed.

The gas-phase proton affinities (PAs) of a series of novel diamidocarbenes (DACs) were assessed and compared to various imidazolylidene-based N-heterocyclic carbenes (NHCs) through experimental and computational methods. Apart from a perfluorinated-phenyl derivative (PA = 233 kcal/mol), the calculated and measured PAs for a range of DACs (256-261 kcal/mol) were comparable to those of the NHCs (260-266 kcal/mol). Proton transfer from the protonated carbene to various reference bases, as observed by mass spectrometry, was inhibited by steric bulk and precluded the direct measurement of the PA for the known DACs, N,N'-dimesityl-4,6-diketo-5,5-dimethylpyrimidin-2-ylidene and N,N'-diisopropylphenyl-4,6-diketo-5,5-dimethylpyrimidin-2-ylidene. However, DACs featuring less hindered N-aryl substituents facilitated proton transfer, and the measured PA values were found to be consistent with density functional theory calculations (B3LYP/6-31+G(d)). Notably, the PAs of the DACs studied were similar to those of the NHCs, indicating that the former retain many of the nucleophilic characteristics intrinsic to their parent diaminocarbenes and that the observed differences in chemical reactivity may be primarily attributed to an enhanced electrophilicity.

Coupling of various 4-substituted phenyl azides with two distinct quinone-containing N-heterocyclic carbenes (NHCs) afforded the respective mono- and ditopic 1,3-disubstituted acyclic triazenes in moderate to excellent yields (38-92%). Depending on their pendant substituents (derived from the azides), the acyclic triazenes exhibited intense absorptions in the visible spectrum (359-428 nm), which were bathochromically shifted by up to Δλ=68 nm upon reduction of the quinone moiety on the component derived from the NHC. Cyclic voltammetry confirmed that the aforementioned redox processes were reversible, and a related set of UV-vis spectroelectrochemical experiments revealed that bulk electrolysis may also be used to switch reversibly the colors exhibited by these triazenes., (© 2011 American Chemical Society)

Using stopped flow and other spectroscopic techniques, the thermodynamic parameters of the coupling reaction between 1,3-dimesitylimidazolylidene and phenyl isothiocyanate were determined (H(o) = -96.1 kJ mol(-1) and ΔS(o) = -39.6 J mol(-1) K(-1)). On the basis of these data which indicated that the reaction was reversible (K(eq) = 5.94 × 10(14) M(-1) at 25 °C; k(f) = 252 M(-1) s(-1); k(r) = 4.24 × 10(-13) s(-1)), the adduct formed from the two aforementioned coupling partners was used as a latent catalyst to facilitate the [2 + 2 + 2] cyclotrimerization of phenyl isocyanate and the polymerization of DL-lactide.

Treatment of an N-heterocyclic carbene that features two amide groups N-bound to the carbene nucleus with various organic isocyanides afforded a new class of ketenimines in yields of up to 96% (isolated). DFT analyses revealed that the carbene exhibits a unique, low-lying LUMO, which may explain the atypical reactivity observed.

A new class of 1,3-disubstituted-triazenes were synthesized by coupling functionalized benzimidazol-2-ylidenes, as their free N-heterocyclic carbenes or generated in situ from their respective benzimidazolium precursors, to various aryl azides in modest to excellent isolated yields (36-99%). Electron delocalization between the two coupled components was studied using UV-vis spectroscopy, NMR spectroscopy, and X-ray crystallography. Depending on the complementarity of the functional groups on the N-heterocyclic carbenes and the organic azides, the respective triazenes were found to exhibit lambda(max) values ranging between 364 and 450 nm. X-ray crystallography revealed bond alteration patterns in a series of triazenes characteristic of donor-acceptor compounds. Triazene thermal stabilities were studied using thermogravimetric analysis and found to be strongly dependent on the sterics of the benzimidazol-2-ylidene component and the electronics of the azide component. Triazenes possessing bulky N-substituents (e.g., neo-pentyl, tert-butyl, etc.) were stable in the solid-state to temperatures exceeding 150 degrees C, whereas analogues with small N-substituents (e.g., methyl) were found to slowly decompose at room temperature. Triazenes featuring electron-rich phenyl azide components decomposed at higher temperatures than their electron-deficient analogues. Products of the thermally induced triazene decomposition reaction were identified as molecular nitrogen and the respective guanidine. Using an isotopically labeled triazene, the mechanism of the decomposition reaction was found to be analogous to the Staudinger reaction.