A new method to investigate the reaction kinetics of intermediates in solution by electrospray ionization mass spectrometry is presented. The method, referred to as delayed reactant labeling, allows investigation of a reaction mixture containing isotopically labeled and unlabeled reactants with different reaction times. It is shown that we can extract rate constants for the degradation of reaction intermediates and investigate the effects of various reaction conditions on their half-life. This method directly addresses the problem of the relevance of detected gaseous ions toward the investigated reaction solution. It is demonstrated for geminally diaurated intermediates formed in the gold mediated addition of methanol to alkynes. Delayed reactant labeling allows us to directly link the kinetics of the diaurated intermediates with the overall reaction kinetics determined by NMR spectroscopy. It is shown that the kinetics of protodeauration of these intermediates mirrors the kinetics of the addition of methanol demonstrating they are directly involved in the catalytic cycle. Formation as well as decomposition of diaurated intermediates can be drastically slowed down by employing bulky ancillary ligands at the gold catalyst; the catalytic cycle then proceeds via monoaurated intermediates. The reaction is investigated for 1-phenylpropyne (Ph-CC-CH3) using [AuCl(PPh3)]/AgSbF6 and [AuCl(IPr)]/AgSbF6 as model catalysts. Delayed reactant labeling is achieved by using a combination of CH3OH and CD3OH or Ph-CC-CH3 and Ph-CC-CD3.

The product of a revealed transformation—NHC‐ethynyl coupling—was observed as a catalyst transformation pathway in the Sonogashira cross‐coupling, catalyzed by Pd/NHC complexes. The 2‐ethynylated azolium salt was isolated in individual form and fully characterized, including X‐ray analysis. A number of possible intermediates of this transformation with common formulae (NHC)nPd(C2Ph) (n=1,2) were observed and subjected to collision‐induced dissociation (CID) and infrared multiphoton dissociation (IRMPD) experiments to elucidate their structure. Measured bond dissociation energies (BDEs) and IRMPD spectra were in an excellent agreement with quantum calculations for coupling product π‐complexes with Pd0. Molecular dynamics simulations confirmed the observed multiple CID fragmentation pathways. An unconventional methodology to study catalyst evolution suggests the reported transformation to be considered in the development of new catalytic systems for alkyne functionalization reactions.

Diaurated intermediates of gold-catalysed reactions have been a long-standing subject of debate. Although diaurated complexes were regarded as a drain of active monoaurated intermediates in catalytic cycles, they were also identified as the products of gold-gold cooperation in dual-activation reactions. This study shows investigation of intermediates in water addition to alkynes catalysed by [(IPr)Au(CH 3 CN)(BF 4 )].Electrospray ionisation mass spectrometry (ESI-MS) allowed us to detect both monoaurated and diaurated complexes in this reaction. Infrared photodissociation spectra of the trapped complexes show that the structure of the intermediates corresponds to a-gold ketone intermediates protonated or aurated at the oxygen atom. Delayed reactant labelling experiments provided the half life of the intermediates in reaction of 1-phenylpropyne ($7 min) and the kinetic isotope effects for hydrogen introduction to the carbon atom (KIE $ 4-6) and for the protodeauration (KIE $ 2). The results suggest that the ESI-MS detected monoaurated and diaurated complexes report on species with a very similar or the same kinetics in solution. Kinetic analysis of the overall reaction showed that the reaction rate is firstorder dependent on the concentration of the gold catalyst. Finally, all results are consistent with the reaction mechanism proceeding via monoaurated neutral a-gold ketone intermediates only.Scheme 1 Intermediates in gold(I)-mediated nucleophile addition to an alkyne. Red and blue code independent pathways to monoaurated and diaurated intermediates, respectively.Scheme 2 Reaction pathway for [(IPr)Au] + mediated hydration of alkynes. The ESI-MS experiments revealed that the detected monoaurated and diaurated complexes report on the identical intermediate in solution, a-gold ketone. Small differences in the measured kinetics are most probably associated R and R 0 . The GC kinetic experiments showed that the rate of the formation of the product depends on the first order of the gold catalyst concentration.This journal is

This study shows that gold salts [(L)AuX] (L = PMe, PPh, JohnPhos, IPr; X = SbF, PF, BF, TfO, TfN) act as bases in aqueous solutions and can transform acetone to digold acetonyl complexes [(L)Au(CHCOCH)] without any additional base present in solution. The key step is the formation of digold hydroxide complexes [(L)Au(OH)]. The kinetics of the formation of the digold complexes and their mutual transformation is studied by electrospray ionization mass spectrometry and the delayed reactant labelling method. We show that the formation of digold hydroxide is the essential first step towards the formation of the digold acetonyl complex, the reaction is favoured by more polar solvents, and the effect of counter ions is negligible. DFT calculations suggest that digold hydroxide and digold acetonyl complexes can exist in solution only due to the stabilization by the interaction with two gold atoms. The reaction between the digold hydroxide and acetone proceeds towards the dimer {[(L)Au(OH)]·[(L)Au(CHCOCH)]}. The monomeric units interact at the gold atoms in the perpendicular arrangement typical of the gold clusters bound by the aurophilic interaction. The hydrogen is transferred within the dimer and the reaction continues towards the digold acetonyl complex and water.