Which cations form upon CSI or ESI of solutions of Grignard reagents?

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The combination of CoCl and 1,3-dienes is known to catalyze challenging alkyl-alkyl cross-coupling reactions between Grignard reagents and alkyl halides, but the mechanism of these valuable transformations remains speculative. Herein, electrospray-ionization mass spectrometry is used to identify and characterize the elusive intermediates of these and related reactions. The vast majority of detected species contain low-valent cobalt(I) centers and diene molecules. Charge tagging, deuterium labeling, and gas-phase fragmentation experiments elucidate the likely origin of these species and show that the diene not only binds to Co as a π ligand, but also undergoes migratory insertion reactions into Co-H and Co-R bonds. The resulting species have a strong tendency to form anionic cobalt(I) ate complexes, the superior nucleophilicity of which should render them highly reactive toward electrophilic substrates and, thus, presumably is the key to the high catalytic efficiency of the system under investigation. Upon the reaction of the in situ formed cobalt(I) ate complexes with organyl halides, only the final cross-coupling product could be detected, but no cobalt(III) species. This finding implies that this reaction step proceeds in a direct manner without any intermediate or, alternatively, that it involves an intermediate with a very short lifetime.

Section: Resultssupporting

confidence: 86%

Low‐Valent Ate Complexes Formed in Cobalt‐Catalyzed Cross‐Coupling Reactions with 1,3‐Dienes as Additives

Kreyenschmidt

Koszinowski

2017

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“…The analysis of solutions of Grignard reagents RMgCl by positive ion‐mode ESI mass spectrometry only led to the observation of [Mg 3 Cl 3 (OMe) 2 (THF) n ] + ions ( n =3–5). As reported previously, these ions originated from reactions with surprisingly long‐lasting MeOH contaminations present in the ESI source …”

Section: Resultssupporting

confidence: 58%

“…()]. As reported previously, the inferred organomagnesium cations have so far eluded detection by ESI mass spectrometry because they apparently undergo extremely fast protolysis reactions with traces of protic solvents remaining in the ionization source . We have, however, previously observed analogous or similar intact organometallic cations in solutions of organozinc and ‐indium halides …”

Section: Discussionsupporting

confidence: 51%

“…Solutions of i PrMgCl and t BuMgCl also afforded prominent trinuclear ate complexes of the type [R n Mg 3 Cl 7− n ] − , namely [ i Pr 5 Mg 3 Cl 2 ] − and [ t Bu 3 Mg 3 Cl 4 ] − , respectively (see the Supporting Information, Figures S20–S24). Additional ions contained alkoxy groups, which presumably resulted from reactions with traces of dioxygen and the consecutive cleavage of the O‐OR bonds by the nucleophilic attack of metal‐bound R groups . Control experiments with sample solutions deliberately exposed to O 2 indeed found a dramatic increase of alkoxide complexes (see the Supporting Information, Figures S25 and S26).…”

Section: Resultsmentioning

confidence: 95%

“…As the present findings clearly show, this perturbation can result in drastic shifts of the aggregation equilibria. Furthermore, very weakly bound complexes, such as THF adducts of the magnesium ate anions, do not survive the ESI process (in marked contrast to their more stable cationic analogues) …”

Section: Discussionmentioning

confidence: 99%

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Association and Dissociation of Grignard Reagents RMgCl and Their Turbo Variant RMgCl⋅LiCl

Schnegelsberg

Bachmann

Kolter

et al. 2016

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Grignard reagents RMgCl and their so-called turbo variant, the highly reactive RMgCl⋅LiCl, are of exceptional synthetic utility. Nevertheless, it is still not fully understood which species these compounds form in solution and, in particular, in which way LiCl exerts its reactivity-enhancing effect. A combination of electrospray-ionization mass spectrometry, electrical conductivity measurements, NMR spectroscopy (including diffusion-ordered spectroscopy), and quantum chemical calculations is used to analyze solutions of RMgCl (R=Me, Et, Bu, Hex, Oct, Dec, iPr, tBu, Ph) in tetrahydrofuran and other ethereal solvents in the absence and presence of stoichiometric amounts of LiCl. In tetrahydrofuran, RMgCl forms mononuclear species, which are converted into trinuclear anions as a result of the concentration increase experienced during the electrospray process. These trinuclear anions are theoretically predicted to adopt open cubic geometries, which remarkably resemble structural motifs previously found in the solid state. The molecular constituents of RMgCl and RMgCl⋅LiCl are interrelated via Schlenk equilibria and fast intermolecular exchange processes. A small portion of the Grignard reagent also forms anionic ate complexes in solution. The abundance of these more electron-rich and hence supposedly more nucleophilic ate complexes strongly increases upon the addition of LiCl, thus rationalizing its beneficial effect on the reactivity of Grignard reagents.

Mechanisms of Cobalt/Phosphine‐Catalyzed Cross‐Coupling Reactions

Kreyenschmidt¹,

Meurer²,

Koszinowski³

2019

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>The combination of CoCl2 with bidentate phosphines is known to catalyze challenging cross‐coupling and Heck‐type reactions, but the mechanisms of these valuable transformations have not been established. Here, we use electrospray‐ionization mass spectrometry to intercept the species formed in these reactions. Our results indicate that a sequence of transmetalation, reductive elimination, and redox disproportionation convert the cobalt(II) precatalyst into low‐valent cobalt complexes. These species readily transfer single electrons to alkyl bromides, which thereupon dissociate into alkyl radicals and Br−. In cross‐coupling reactions, the alkyl radicals add to the cobalt catalyst to form observable heteroleptic complexes, which release the coupling products through reductive eliminations. In the Heck‐type reactions, the low abundance of newly formed ionic species renders the analysis more difficult. Nonetheless, our results also point to the occurrence of single‐electron transfer processes and the involvement of radicals in these transformations.