On the mechanism of the digold(I) hydroxide-catalyzed hydrophenoxylation of alkynes

Herein we present a detailed investigation of the mechanistic aspects of the dual gold-catalysed hydrophenoxylation of alkynes, using both experimental and computational methods. The dissociation of [{Au(NHC)}2(µ-OH)][BF4] is essential to enter the catalytic cycle; this step is favored in the presence of bulky, non-coordinating counterions. Moreover, in silico studies confirmed that phenol does not only act as a reactant, but as a co-catalyst, lowering the energy barriers for several transition states. A gem-diaurated species might form during the reaction, but this lies deep within a potential energy well, and is likely to be an ‘off-cycle’ rather than an ‘in-cycle’ intermediate

Mechanism of the transmetalation of organosilanes to gold

Density functional theory (DFT) calculations were carried out to study the reaction mechanism of the first transmetalation of organosilanes to gold as a cheap fluoride-free process. The versatile gold(I) complex [Au(OH)(IPr)] permits very straightforward access to a series of aryl-, vinyl-, and alkylgold silanolates by reaction with the appropriate silane reagent. These silanolate compounds are key intermediates in a fluoride-free process that results in the net transmetalation of organosilanes to gold, rather than the classic activation of silanes as silicates using external fluoride sources. However, here we propose that the gold silanolate is not the active species (as proposed during experimental studies) but is, in fact, a resting state during the transmetalation process, as a concerted step is preferred

Orbital decomposition of the carbon chemical shielding tensor in gold(I) N-heterocyclic carbene complexes

The good performance of N-heterocyclic carbenes (NHCs), in terms of versatility and selectivity, has called the attention of experimentalists and theoreticians attempting to understand their electronic properties. Analyses of the Au(I)-C bond in [(NHC)AuL](+/0) (L stands for a neutral or negatively charged ligand), through the Dewar-Chatt-Duncanson model and the charge displacement function, have revealed that NHC is not purely a sigma-donor but may have a significant pi-acceptor character. It turns out, however, that only the sigma-donation bonding component strongly correlates with one specific component of the chemical shielding tensor. Here, in extension to earlier works, a current density analysis, based on the continuous transformation of the current density diamagnetic zero approach, along a series of [(NHC)AuL](+/0) complexes is presented. The shielding tensor is decomposed into orbital contributions using symmetry considerations together with a spectral analysis in terms of occupied to virtual orbital transitions. Analysis of the orbital transitions shows that the induced current density is largely influenced by rotational transitions. The orbital decomposition of the shielding tensor leads to a deeper understanding of the ligand effect on the magnetic response properties and the electronic structure of (NHC)-Au fragments. Such an orbital decomposition scheme may be extended to other magnetic properties and/or substrate-metal complexes

Catalytic Role of Nickel in the Decarbonylative Addition of Phthalimides to Alkynes

Density functional theory calculations have been used to investigate the catalytic role of nickel(0) in the decarbonylative addition of phthalimides to alkynes. According to Kurahashi et al. the plausible reaction mechanism involves a nucleophilic attack of nickel at an imide group, giving a six-membered metallacycle, followed by a decarbonylation and insertion of an alkyne leading to a seven-membered metallacycle. Finally a reductive elimination process produces the desired product and regenerates the nickel(0) catalyst. In this paper, we present a full description of the complete reaction pathway along with possible alternative pathways, which are predicted to display higher upper barriers. Our computational results substantially confirm the proposed mechanism, offering a detailed geometrical and energetical understanding of all the elementary steps

The "innocent" role of Sc3+ on a non-heme Fe catalyst in an O-2 environment

Density functional theory calculations have been used to investigate the reaction mechanism proposed for the formation of an oxoiron(iv) complex [Fe-IV(TMC)0](2+) (P) (TIvIC = 1,4,8,11-tetramethylcyclam) starting from a non-heme reactant complex-EFell(TMC)1(2+) (R) and 02 in the presence of acid H+ and reductant BPV. We also addressed the possible role of redox-inactive Sc3+ as a replacement for H+ acid in this reaction to trigger the formation of P. Our computational results substantially confirm the proposed mechanism and, more importantly, support that Sc3+ could trigger the 02 activation, mainly dictated by the availability of two electrons from BPV, by forming a thermodynamically stable Sc3+-peroxo-Fe3+ core that facilitates 0-0 bond cleavage to generate P by reducing the energy barrier. These insights may pave the way to improve the catalytic reactivity of metal-oxo complexes in 02 activation at non-heme centers

The driving force role of ruthenacyclobutanes

DFT calculations have been used to determine the thermodynamic and kinetic preference for ruthena-cyclobutanes resulting from the experimentally proposed interconversion pathways (olefin and alkylidene rotations) through the investigation of cross-metathesis reaction mechanism for neutral Grubbs catalyst, RuCl2 (=CHEt) NHC (A), with ethylene and 1-butene as the substrates. Our results show that although the proposed interconversions are feasible due to the predicted low energy barriers (2-6 kcal/mol), the formation of ruthenacyclobutane is kinetically favored over the competitive reactions involving alkylidene rotations. In comparison with catalyst A, the reaction energy profile for cationic Piers catalyst [RuCl2 (=CHPCy3)NHC+] (B) is more endothermic in nature with both ethylene and 1-butene substrates

Deconstructing Selectivity in the Gold-Promoted Cyclization of Alkynyl Benzothioamides to Six-Membered Mesoionic Carbene or Acyclic Carbene Complexes

We demonstrate that the experimentally observed switch in selectivity from 5-exo-dig to 6-endo-dig cyclization of an alkynyl substrate, promoted by Au-I and Au-III complexes, is connected to a switch from thermodynamic to kinetic reaction control. The Au-III center pushes alkyne coordination toward a single Au-C(alkyne) (sigma-bond, conferring carbocationic character (and reactivity) to the distal alkyne C atom

Mechanistic Insights of a Selective C-H Alkylation of Alkenes by a Ru-based Catalyst and Alcohols

Density functional theory calculations have been used to investigate the reaction mechanism for [(C6H6)(PCy3)(CO) RuH](+) (1; Cy, cyclohexyl) mediated alkylation of indene substrate using ethanol as solvent. According to Yi et al. [ Science 2011, 333, 1613] the plausible reaction mechanism involves a cationic Rualkenyl species, which is initially formed from 1 with two equivalents of the olefin substrate via the vinylic C-H activation and an alkane elimination step. Once the active catalytic species is achieved the oxidative addition step is faced. The latter step together with the next C-C bond formation might display the upper barrier of the catalytic cycle. Having these experimental insights at hand, we investigated in detail the whole reaction pathway using several computational DFT approaches including alternative pathways, higher in energy