The use of control experiments as the sole route to correct the mechanistic interpretation of mercury poisoning test results: The case of P,C-palladacycle-catalysed reactions
Graphical abstract
Introduction
In recent years, the need for comprehensive mechanistic studies for improving catalytic processes has become increasingly necessary. This problem is particularly relevant for the catalysis of cross-coupling and related reactions involving cyclopalladated precatalysts [1]. It has been nearly a quarter of a century since the introduction of palladacycles for use in catalysis [2], but the debate in this area has not subsided [3]. Recently, as an alternative to the widely accepted classical Pd(II)/Pd(0) pathway, the Pd(II)/Pd(IV) route was convincingly confirmed by detailed mechanistic studies [4].
As part of our programme of developing enantioselective versions of the Suzuki reaction catalysed by chiral C,N-palladacycles [5], we applied a mercury poisoning test to find conditions that guarantee the intact state of palladacycle during its participation in the catalytic cycle [5b]. We assumed that unlike many other diagnostic methods, which mainly determined the phase location of the catalysis (homo- or heterogeneous) [6], this test could provide information about the participants in the catalytic cycle in accordance with the conception of Jones [7].
The dramatic contradiction between the positive results of the mercury(0) test and other properties of our catalytic system [5b] forced us to conduct the first systematic investigation of metallic mercury reactions with palladacycles [8]. This research has shown that diverse types of cyclopalladated C,N-precatalysts react with mercury(0), even at room temperature, and form corresponding organomercuric chlorides via redox-transmetallation. We established the significant dependence of the effectiveness of this process on the palladacycle structure, reaction conditions, mercury/palladium ratio and base presence. The structures of all isolated organomercurials were convincingly confirmed by 1H, 13C{1H}, and 199Hg{1H} NMR spectroscopy in solutions and X-ray diffraction studies [5](b), [8]. A year later, a similar study was published on the interaction of mercury(0) with monodentate carbene complexes during both catalysis and control experiments with precatalysts [9]. These results and several previously published examples of the interaction of mercury(0) with metal complexes in elevated formal oxidation states [10] once again emphasize the categorical need for control experiments in the case of a positive mercury poisoning test.
Here, we report our studies of the reactivity of mercury(0) with P,C-palladacycles, which are known precatalysts of cross-coupling reactions, and we reveal our estimates of the dependence of their protective properties on the structures of P-donor ligands and the palladium formal oxidation state.
Section snippets
Results and discussion
At this stage, it is necessary (i) to determine whether phosphapalladacycles with a palladium(II) atom that is protected by soft phosphorus and anionic carbon donor atoms can resist the influence of mercury(0); (ii) to compare the reactivity with metallic mercury of bi- and zero-valent palladium compounds derived from precatalyst-formed reservoir or catalytic cycle intermediates, respectively; and (iii) to evaluate the potential of the mercury test for identifying the key intermediates in the
Conclusion
Our own and other known results of the application of mercury(0) diagnostic for catalytic systems with P,C- and P-donor precatalysts together with our own and other known control experiments enable us to draw the following important conclusions.
- (i)
First, this test is not applicable to the assessment of the mechanism of catalysis if the possibility of the interaction of metallic mercury with the precatalyst (instead of the putative intermediate) is not excluded by the control experiment.
- (ii)
Second,
General information
The 1H (400 MHz), 13C{1H} (100.6 MHz), 199Hg{1H} (71.5 MHz) and 31P{1H} (161.9 MHz) NMR spectra were recorded on the Agilent 400-MR spectrometer. The measurements were performed in a CDCl3 solution at ambient temperature (unless otherwise specified). The chemical shifts are reported in δ-scale in parts per million relative to the residual CHCl3 (δ 7.26 ppm for 1H or δ 77.16 ppm for 13C{1H} NMR) and relative to H3PO4 or (CH3)2Hg as an external references for the 31P{1H} or 199Hg NMR spectra,
Funding
The authors declare no competing financial interest.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Russian Foundation for Basic Research (project no. 18-03-01026a) and the Ministry of Science and Higher Education of the RF. The NMR part of this work was supported by M.V. Lomonosov Moscow State University “Programme of Development” (Y.K.G., V.A.R.). This work was also supported by the U.S. National Science Foundation (grant DMR-1523611 PREM) and RUDN University Programme “5–100” (VNKh).
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