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

Abstract

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.

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

“…Additionally, we observed polynuclear complexes. The present experiments do not establish the extent to which these complexes are already present in the sample solutions or result from a shift of the aggregation equilibria due to an increase in concentration during the ESI process . Relatively few of the higher aggregates were homonuclear.…”

Alkene Metalates as Hydrogenation Catalysts

“…In addition, the potassium‐bound dimer [K{Co(anthracene) 2 } 2 ] − was also observed. Presumably, this species was not present in the diluted sample solution, but formed due to the concentration increase during the ESI process; similar behavior has been found in other cases as well . ESI of a solution of heteroleptic complex 3 produced not only [Co(η 4 ‐cod)(η 4 ‐naphthalene)] − , as well as small quantities of [K{Co(η 4 ‐cod)(η 4 ‐naphthalene)} 2 ] − , but also its homoleptic counterpart [Co(η 4 ‐cod) 2 ] − (Figure S7 in the Supporting Information).…”

Molecular Weight Estimation of Molecules Incorporating Heavier Elements from van‐der‐Waals Corrected ECC‐DOSY

“…The calculated MW of the n BuMgBr + LiBr system ( MW calc =392 gmol −1 ) indicates the association of two solvent molecules ( MW det,cor ([ n BuMgBr (THF) 2 LiBr])=404 gmol −1 ). However, the slightly raised MW det might indicate a dynamic equilibrium for the coordination of THF . Note that n BuMgBr (THF) 3 LiBr would invoke lithium's and magnesium's preferred coordination number four.…”