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

Abstract

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.

Direct Evidence for the Origin of Bis‐Gold Intermediates: Probing Gold Catalysis with Mass Spectrometry

“…In our experiment, when formic acid was added to the reaction mixture containing 4 mm Ph 3 PAuNTf 2 and 200 mm propargyl acetate alkyne 1 to adjust the solution pH to 4.5, the reaction occurred and ions of m/z =193 and 1109 were all observed (Figure 7S in the Supporting Information), thereby excluding the possibility of involvement of L-Au-OH species in the reaction. Such an acid effect was also noted in the recent report by Roithova et al [17] …”

“…Maier and Zhdanko’s study suggested gem-diaurated species cannot directly undergo protodeauration, which is not directly involved in the catalytic cycle, whereas Roithova and co-worker’s results showed the alkyne could assist the dissociation of diaurated species, which was involved in the reaction cycle. [16,17] Houk, Vilhelmsen, and Hashmi computed that in some reactions the gem-diaurated species is in the catalytic cycle. [6g,h] Inspired by the previous work, we hypothesize that the bis-gold intermediate would be more difficult to form if the gold catalyst concentration is reduced.…”

The preference for dual-gold(i) catalysis in the hydro(alkoxylation vs. phenoxylation) of alkynes

“…In conclusion, this study provides useful information that will enable future in silico studies of mechanism and structure/activity relationships. By DFT calculations it is shown that the dual gold mechanism is favoured, but this does not exclude the monogold catalyst, as stated recently, 47 especially if the catalyst loading is low. It will depend on the facility to join two metal moieties.…”

“…46,47 However, there is much ongoing debate about the relative competence of mono-and digold mechanisms for many gold-catalysed reactions. Even in situations where dual metal-catalysed mechanisms were not thought to be operative, there are some recent examples where calculations revealed cooperative effects; for example, there are examples of heterogeneous catalysis where changing the surface allowed adjacent sites to achieve cooperative catalysis.…”

The Intermediates in Lewis Acid Catalysis with Lanthanide Triflates

“…[12] They are also very useful as catalysts for multicomponent reactions [13][14][15] and in the synthesis of heterocycles, [16] and are widely embedded in the concept of asymmetric catalysis. [30] Moreover, ESI-MS can be coupled to ion spectroscopy, [31][32][33][34][35][36] that can provide conclusive information about the structure of trapped ions in the gas phase. As far as we know, there are no reports concerning the identity of the lanthanide intermediates formed in a reaction medium.…”