Elucidation of carbene ambiphilicity leading to the discovery of reversible ammonia activation.

An N,N'-diamidocarbene (DAC) was found to activate a broad range of primary as well as secondary aliphatic and aromatic amines. The relative rates measured for the insertion of the DAC into the primary amines were consistent with an electrophilic activation mechanism; in contrast, the DAC functioned as a nucleophile upon treatment with secondary aryl amines. Collectively, these results constituted the first ambiphilic process for an isolable carbene. By comparison, an analogous diaminocarbene was found to serve exclusively as a nucleophile under similar conditions and led to the discovery of the first organic reagent to reversibly activate ammonia.

Assessing the proton affinities of N,N'-diamidocarbenes.

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.

Characterization and dynamics of substituted ruthenacyclobutanes relevant to the olefin cross-metathesis reaction.

The reaction of the phosphonium alkylidene [(H(2)IMes)RuCl(2)=CHP(Cy)(3))](+) BF(4)(-) with propene, 1-butene, and 1-hexene at -45 °C affords various substituted, metathesis-active ruthenacycles. These metallacycles were found to equilibrate over extended reaction times in response to decreases in ethylene concentrations, which favored increased populations of α-monosubstituted and α,α'-disubstituted (both cis and trans) ruthenacycles. On an NMR time scale, rapid chemical exchange was found to preferentially occur between the ß-hydrogens of the cis and trans stereoisomers prior to olefin exchange. Exchange on an NMR time scale was also observed between the α- and ß-methylene groups of the monosubstituted ruthenacycle (H(2)IMes)Cl(2)Ru(CHRCH(2)CH(2)) (R = CH(3), CH(2)CH(3), (CH(2))(3)CH(3)). EXSY NMR experiments at -87 °C were used to determine the activation energies for both of these exchange processes. In addition, new methods have been developed for the direct preparation of metathesis-active ruthenacyclobutanes via the protonolysis of dichloro(1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene)(benzylidene) bis(pyridine)ruthenium(II) and its 3-bromopyridine analogue. Using either trifluoroacetic acid or silica-bound toluenesulfonic acid as the proton source, the ethylene-derived ruthenacyclobutane (H(2)IMes)Cl(2)Ru(CH(2)CH(2)CH(2)) was observed in up to 98% yield via NMR at -40 °C. On the basis of these studies, mechanisms accounting for the positional and stereochemical exchange within ruthenacyclobutanes are proposed, as well as the implications of these dynamics toward olefin metathesis catalyst and reaction design are described.