Quantitative Description of Structural Effects on the Stability of Gold(I) Carbenes

Abstract

The gas-phase bond-dissociation energies of a SO2 -imidazolylidene leaving group of three gold(I) benzyl imidazolium sulfone complexes are reported (E0 =46.6±1.7, 49.6±1.7, and 48.9±2.1 kcal mol(-1) ). Although these energies are similar to each other, they are reproducibly distinguishable. The energy-resolved collision-induced dissociation experiments of the three [L]-gold(I) (L=ligand) carbene precursor complexes were performed by using a modified tandem mass spectrometer. The measurements quantitatively describe the structural and electronic effects a p-methoxy substituent on the benzyl fragment, and trans [NHC] and [P] gold ligands, have towards gold carbene formation. Evidence for the formation of the electrophilic gold carbene in solution was obtained through the stoichiometric and catalytic cyclopropanation of olefins under thermal conditions. The observed cyclopropane yields are dependent on the rate of gold carbene formation, which in turn is influenced by the ligand and substituent. The donation of electron density to the carbene carbon by the p-methoxy benzyl substituent and [NHC] ligand stabilizes the gold carbene intermediate and lowers the dissociation barrier. Through the careful comparison of gas-phase and solution chemistry, the results suggest that even gas-phase leaving-group bond-dissociation energy differences of 2-3 kcal mol(-1) enormously affect the rate of gold carbene formation in solution, especially when there are competing reactions. The thermal decay of the gold carbene precursor complex was observed to follow first-order kinetics, whereas cyclopropanation was found to follow pseudo-first-order kinetics. Density-functional-theory calculations at the M06-L and BP86-D3 levels of theory were used to confirm the observed gas-phase reactivity and model the measured bond-dissociation energies.

Carbon‐Atom Extrusion from Halobenzenes and Its Coupling with a Methylene Ligand to Form Acetylene

“…In addition to the vast number of condensed‐phase studies on metal carbenes, there exists also a considerable number of reports dealing with various aspects of gaseous metal–carbene complexes;3 in the latter, not only their generation in the gas phase has been addressed, but also their reactivity towards a large variety of substrates has been explored. The reactions observed in these studies can be classified in three different types: 1) metathesis processes have been observed in the reactions of metal carbenes [MCR 2 ] + with olefins;3a, c, e, o 2) mechanistic details have been uncovered for the carbene transfer from the metal center to olefins, in a stepwise or concerted manner to result in the formation of cyclopropanes;3r, u 3) metal–carbene complexes have been found to participate in CN and CO coupling processes 3l–n. Here, we report a novel type of reaction: when the metal carbene [LaCH 2 ] + is exposed to halobenzenes C 6 H 5 X (X=F, Cl, Br, I), in this thermal reaction an unprecedented carbon‐atom transfer from the phenyl ring to the methylene ligand is observed, resulting in the formation of ethyne.…”

Reaction Intermediates Kinetics in Solution Investigated by Electrospray Ionization Mass Spectrometry: Diaurated Complexes

“…Reactive intermediates that are often present at very low concentrations can be detected, isolated in the gas phase and can have their reactivity probed. [8][9][10] The structure of the intermediates can be interrogated in collisional experiments or with ion spectroscopy approaches. 11,12 The limitations of ESI-MS always have to be considered.…”

The chemistry of the carbon-transition metal double and triple bond: Annual survey covering the year 2014

“…1444) through ketene formation followed by CO loss. Related studies on gasphase generation and trapping of gold carbenes from precursor complex 1445 were also reported [1268]. Several papers report on mechanistic aspects of Group 11-carbene complex intermediates generated from organic diazo compounds (see Scheme 118).…”

“…conversion of enyne azide 1253 to bicyclic compound 1255) was employed in the total synthesis of stemonamine alkaloids [1188]. 1267 led to a spectroscopically observable gold metallacyclopropane complex (1268), which transforms to the observed product (1269) through Bronsted acid catalysis [1193]. Mechanistic hypotheses were supported through deuterium labeling studies.…”