Reactive intermediates and mass spectrometry in homogeneous oxidation catalysis [electronic resource]
Keywords: Chemistry.

There has been a burst in the number and variety of available ionization techniques to use mass spectrometry to monitor chemical reactions in and on liquids. Chemists have gained the capability to access chemistry at unprecedented timescales, and monitor reactions and detect intermediates under most any set of conditions. Herein, recently developed ionization techniques that facilitate mechanistic studies of chemical processes are reviewed. This is followed by a discussion of our perspective on the judicious application of these and similar techniques in order to study reaction mechanisms. Sodium periodate (NaIO4) is added to Cp*IrIII (Cp* = C5Me5-) or (cod)IrI (cod = cyclooctadiene) complexes, which are water and C--H oxidation catalyst precursors, and the resulting aqueous reaction is investigated from milliseconds to seconds using desorption electrospray ionization, electrosonic spray ionization, and cryogenic ion vibrational predissociation spectroscopy. Extensive oxidation of the Cp* ligand is observed, likely beginning with electrophilic C--H hydroxylation of a Cp* methyl group followed by nonselective pathways of further oxidative degradation. Evidence is presented that the supporting chelate ligand in Cp*Ir(chelate) precursors influences the course of oxidation and is neither eliminated from the coordination sphere nor oxidatively transformed. Isomeric products of initial Cp* oxidation are identified and structurally characterized by vibrational spectroscopy in conjunction with DFT modeling. Less extensive but more rapid oxidation of the cod ligand is also observed in the (cod)IrI complexes. The observations are consistent with the proposed role of Cp* and cod as sacrificial placeholder ligands that are oxidatively removed from the precursor complexes under catalytic conditions. The regio- and chemoselective oxidation of unprotected vicinal polyols with [LPd(OAc)]2(OTf)2 (L = neocuproine = 1,9-dimethylphenanthroline) occurs readily under mild reaction conditions to generate -hydroxy ketones. As part of a larger mechanistic study, in situ mass spectrometry utilizing desorption electrospray ionization mass spectrometry (DESI-MS) revealed several key intermediates in the proposed mechanism for diol oxidation. The bridging trinuclear palladium species [(LPd(II))3(mu3-O)2]2+ was identified during the [LPd(OAc)]2(OTf)2 catalyzed aerobic oxidation of 1,2-propanediol. Independent synthesis, structural characterization, and catalytic studies of the trinuclear compound show that it is a product of oxygen activation by reduced palladium species and a competent intermediate in the catalytic aerobic oxidation of alcohols. The formation and catalytic activity of the trinuclear Pd3O2 species illuminates a multinuclear pathway for aerobic oxidation reactions catalyzed by Pd complexes. Hydrogen peroxide disproportionation with the cationic Pd-acetate catalyst, [LPd(OAc)]2(OTf)2, was studied by a series of kinetics, electrospray mass spectrometry, and isotope labelling experiments. The data suggest multiple catalytic paths for H2O2 disproportionation occur that involve formation of multinuclear Pd species. The trinuclear compound [(LPd)3(mu3-O)2]2+ is a product of dioxygen activation that is formed during aerobic oxidations of alcohols catalyzed by [LPd(OAc)]2(OTf)2. It is also a product of hydrogen peroxide activation during disproportionation reactions catalyzed by [LPd(OAc)]2(OTf)2. The role of this compound in catalysis and its mechanism of formation have been investigated by isotope labelling and reactivity studies. The results suggest the trinuclear compound is involved in one of multiple simultaneous mechanisms for the reduction of oxygen and/or the disproportionation of hydrogen peroxide during oxidation catalysis. Electrospray mass spectrometry of hydrogen peroxide disproportionation reactions suggest the presence of other multinuclear Pd-O2 species in solution, and theoretical studies of these compounds yield some insight into their structure.