| dc.description.abstracteng | Organocuprates are important reagents with numerous applications in preparative organic chemistry, which has led to extensive investigations regarding their molecular composition. However, while lithium organocuprates have been thoroughly studied, the solution speciation of magnesium organocuprates is only poorly understood. Organocuprates have also been proposed as important intermediates in certain copper-mediated trifluoromethylation reactions, but their origin as well as the overall reaction mechanism remained unclear. The higher homologues – organoargentates and -aurates – have been characterized to a much lesser extent, and only little is known about their speciation and aggregation. Despite the potential of silver to promote cross-coupling reactions, the mechanisms of these reactions and the role of organoargentates in these synthetically valuable but rarely used transformations are unknown.
In the present work, electrospray-ionization (ESI) mass spectrometry and collision-induced dissociation (CID) experiments were used to identify and characterize organocuprates, -argentates, and -aurates. Furthermore, the copper-mediated trifluoromethylation of terminal alkynes as well as silver-mediated cross-coupling reactions were investigated by ESI mass spectrometry and CID experiments.
The results show great similarities in the speciation of solutions of organocuprates, -argentates, and -aurates that were prepared from the respective coinage-metal cyanide and organolithium reagents in tetrahydrofuran (THF). In particular, the formation of polynuclear, lithium-containing heterobimetallic organometallates was observed for all three coinage metals. These complexes closely resemble species that were detected in previous ESI mass spectrometric studies of cyanocuprates and Gilman cuprates in ethereal solvents. In contrast, organocuprates that were prepared by transmetallation of a copper precursor with Grignard reagents in THF showed a different behavior. ESI mass spectra of these reagents did not exhibit heterobimetallic cuprates in significant amounts, but were dominated by monometallic species. This difference between organocuprates prepared from organolithium or organomagnesium reagents is attributed to the formation of different lithium- and magnesium counter-ions. ESI mass spectra of organoargentate and -aurate solutions prepared from silver cyanide or gold cyanide, respectively, and Grignard reagents also showed monometallic organoargentates and -aurates, which is in agreement with the outcome of mass spectrometric analysis of the magnesium organocuprates. However, these reactions were, in part severely, affected by hydrolysis reactions.
The unimolecular gas-phase reactivity of the observed organometallates was studied by CID experiments. The polynuclear, lithium-containing heterobimetallic complexes of all three coinage metals prefer similar deaggregation reactions, while the mononuclear coinage metallates show different reaction pathways that apparently depend on the actual coinage metal present in the complex. Presumably, the overall structure of the larger heterobimetallic species governs the outcome of their fragmentation reactions, while the intrinsic reactivity of the individual coinage metals becomes the determining factor in the fragmentation behavior of the mononuclear metallates. Polynuclear but monometallic cuprates that were prepared from Grignard reagents also showed deaggregation reactions. In addition to that, beta-hydride elimination and beta-methyl elimination reactions were obseverd in two cases.
In order to investigate the copper-mediated trifluoromethylation of alkynes, solutions of copper(I) iodide, potassium fluoride, and trimethyl(trifluoromethyl)silane (the Ruppert-Prakash reagent) were examined with and without the addition of a terminal alkyne. In the absence of an alkyne component, homoleptic trifluoromethyl cuprates could be identified in ESI mass spectra of these solutions. The addition of terminal alkynes additionally afforded heteroleptic trifluoromethyl cuprates bearing three trifluoromethyl groups and an alkynyl ligand. In gas-phase fragmentation experiments, these key copper(III) intermediates underwent reductive eliminations that led to the exclusive release of the trifluoromethylated alkyne as cross-coupling product. Furthermore, the results suggest that oxygen is involved as oxidizing agent, and, apparently, the heteroleptic cuprates did not originate from the homoleptic ones, but presumably from neutral precursor species that cannot be observed by ESI mass spectrometry.
ESI mass spectrometric experiments also demonstrated that the addition of organic iodides RI to solutions containing methyl argentate(I) complexes resulted in the formation of argentate(III) complexes. Upon collisional activation in the gas-phase, the latter species underwent reductive eliminations in which they preferentially released the cross-coupling product RMe. These findings point to the crucial role of argentates(I) and (III) in silver-mediated cross-coupling reactions. Moreover, the observed gas-phase reactivities significantly differ from those of previously studied organocpurates(III). Thus, the use of argentates may provide a future alternative to the popular organocuprates in synthesis. | de |