Activation of an alkyl C-H bond geminal to an agostic interaction:An unusual mode of base-induced C-H activation

Deuterium labeling studies indicate that base-induced intramolecular C-H activation in the agostic complex 2-D proceeds with exclusive removal of a proton from the methyl arm of an iPr substituent on the N-heterocyclic carbene (NHC) ligand. Computational studies show that this alkyl C-H bond activation reaction involves deprotonation of one of the C-H bonds that is geminal to the agostic interaction, rather than the agostic C-H bond itself. The reaction is readily accessible at room temperature, and a computed activation barrier of ΔE† calcd = +11.8 kcal/mol is found when the NHC 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene is employed as the external base. Charge analysis reveals that the geminal hydrogens are in fact more acidic than the agostic proton, consistent with their more facile deprotonation.</p

Smooth C(alkyl)-H bond activation in rhodium complexes comprising abnormal carbene ligands

Rhodation of trimethylene-bridged diimidazolium salts induces the intramolecular activation of an alkane-type C-H bond and yields mono- and dimetallic complexes containing a formally monoanionic C,C,C-tridentate dicarbene ligand bound to each rhodium centre. Mechanistic investigation of the Calkyl-H bond activation revealed a significant rate enhancement when the carbene ligands are bound to the rhodium centre via C4 (instantaneous activation) as compared to C2-bound carbene homologues (activation incomplete after 2 days). The slow C-H activation in normal C2-bound carbene complexes allowed intermediates to be isolated and suggests a critical role of acetate in mediating the bond activation process. Computational modelling supported by spectroscopic analyses indicate that halide dissociation as well as formation of the agostic intermediate is substantially favoured with C4-bound carbenes. It is these processes that discriminate the C4- and C2-bound systems rather than the subsequent C-H bond activation, where the computed barriers are very similar in each case. The tridentate dicarbene ligand undergoes selective H/D exchange at the C5 position of the C4-bound carbene exclusively. A mechanism has been proposed for this process, which is based on the electronic separation of the abnormal carbene ligand into a cationic N-C-N amidinium unit and a metalla-allyl type M-C-C fragment.</p

Computational Studies on the Mechanism of the Gold(I)-Catalysed Rearrangement of Cyclopropenes

Density functional theory calculations have been employed to investigate the mechanism of gold(i)-catalysed rearrangements of cyclopropenes. Product formation is controlled by the initial ring-opening step which results in the formation of a gold-stabilised carbocation/gold carbene intermediate. With 3-phenylcyclopropene-3-methylcarboxylate, the preferred intermediate allows cyclisation via nucleophilic attack of the carbonyl group and hence butenolide formation. Further calculations on simple model systems show that substituent effects can be rationalised by the charge distribution in the ring-opening transition state and, in particular, a loss of negative charge at what becomes the β-position of the intermediate. With 1-C3H3R cyclopropenes (R = Me, vinyl, Ph), ring-opening therefore places the substituent at the β-position.</p

Experimental and computational investigation of C-N bond activation in ruthenium N-heterocyclic carbene complexes

A combination of experimental studies and density functional theory calculations is used to study C-N bond activation in a series of ruthenium N-alkyl-substituted heterocyclic carbene (NHC) complexes. These show that prior C-H activation of the NHC ligand renders the system susceptible to irreversible C-N activation. In the presence of a source of HCl, C-H activated Ru(I iPr2Me2)′(PPh3) 2(CO)H (1, IiPr2Me2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) reacts to give Ru(I iPrHMe2)(PPh3)2(CO)HCl (2, I iPrHMe2 = 1-isopropyl-4,5-dimethylimidazol-2-ylidene) and propene. The mechanism involves (i) isomerization to a trans-phosphine isomer, 1c, in which hydride is trans to the metalated alkyl arm, (ii) C-N cleavage to give an intermediate propene complex with a C2-metalated imidazole ligand, and (iii) N-protonation and propene/Cl- substitution to give 2. The overall computed activation barrier (ΔEcalcd) corresponds to the isomerization/C-N cleavage process and has a value of +24.4 kcal/mol. C-N activation in 1c is promoted by the relief of electronic strain arising from the trans disposition of the high-trans-influence hydride and alkyl ligands. Experimental studies on analogues of 1 with different C4/C5 carbene backbone substituents (Ru(IiPr2Ph2)′(PPh 3)2(CO)H, Ru(IiPr2)′(PPh 3)2(CO)H) or different N-substituents (Ru(IEt 2Me2)′(PPh3)2(CO)H) reveal that Ph substituents promote C-N activation. Calculations confirm that Ru(I iPr2Ph2)′(PPh3) 2(CO)H undergoes isomerization/C-N bond cleavage with a low barrier of only +21.4 kcal/mol. Larger N-alkyl groups also facilitate C-N bond activation (Ru(ItBu2Me2)′(PPh 3)2(CO)H, ΔEcalcd = +21.3 kcal/mol), and in this case the reaction is promoted by the formation of the more highly substituted 2-methylpropene.</p

The Influence of N-Heterocyclic Carbenes (NHC) on the Reactivity of [Ru(NHC)(4)H](+) With H-2, N-2, CO and O-2

The five-coordinate ruthenium N-heterocyclic carbene (NHC) hydrido complexes [Ru(IiPr2Me2)4H][BArF 4] (1; IiPr2Me2 = l,3-diisopropyl4,5- dimethylimidazol-2-ylidene ; ArF = 3,5-(CF3) 2C6H3), [Ru(IEt2Me2) 4H]-[BArF4] (2; IEt2Me2 = l,3-diethyl-4,5-dimethylimidazol-2-ylidene) and [Ru(IMe4) 4H][BArF4] (3; IMe4=l,3,4,5- tetramethylimidazol-2-ylidene) have been synthesised following reaction of [Ru(PPh3)3HCl] with 4-8 equivalents of the free carbenes at ambient temperature. Complexes 1-3 have been structurally characterised and show square pyramidal geometries with apical hydride ligands. In both dichloromethane or pyridine solution, 1 and 2 display very low frequency hydride signals at about (3 -41. The tetramethyl carbene complex 3 exhibits a similar chemical shift in toluene, but shows a higher frequency signal in acetonitrile arising from the solvent adduct [Ru(IMe4)4(MeCN)H] [BArF4], 4. The reactivity of 1-3 towards H2 and N2 depends on the size of the N-substituent of the NHC ligand. Thus, 1 is unreactive towards both gases, 2 reacts with both H2 and N2 only at low temperature and incom-pletely, while 3 affords [Ru(IMe4)4(η2H2)H] [BAr F4] (7) and [Ru(IMe4)4(N 2)H][BArF4] (8) in quantitative yield at room temperature. CO shows no selectivity, reacting with 1-3 to give [Ru(NHC) 4(CO)H][BArF4] (911). Addition of O2 to solutions of 2 and 3 leads to rapid oxidation, from which the Ru III species [Ru(NHC)4(OH)2][BAr F4] and the RuIV oxo chlorido complex [Ru(IEt2Me2)4(O)Cl][BArF 4] were isolated. DFT calculations reproduce the greater ability of 3 to bind small molecules and show relative binding strengths that follow the trend CO ≫ O2 &gt; N2 &gt; H2.</p

Formation of [Ru(NHC)₄(η²-O₂)H] ⁺:An unusual, high frequency hydride chemical shift and facile, reversible coordination of O₂

(Chemical Equation Presented) Reaction of the purple tetrakiscarbene ruthenium cation [Ru(IiPr2Me2) 4H]+ (1, IiPr2Me2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) with oxygen affords the pink η2-O2 hydride species [Ru(IiPr 2Me2)4(η2-O2)H] + (2). 2 displays (i) a very facile, reversible O2 coordination and (ii) an unexpectedly positive hydride chemical shift, and both these features can be predicted and explained by density functional theory (DFT) calculations.</p