Carbon‐Halogen Bond Activation with Powerful Heavy Alkaline Earth Metal Hydrides

Opportunities with calcium Grignard reagents and other heavy alkaline-earth organometallics

More than a century old, magnesium Grignard reagents remain essential to the toolbox of organic chemists. Although similar reagents with the neighbouring group 2 metal Ca have been explored, the considerably higher polarity and reactivity of the Ca-C bond result in undesired decomposition pathways. Ca Grignard reagents have found academic interest but have never fully developed into an established synthetic tool. Recent research activities, however, provide facile access to these highly reactive organocalcium species, including in situ preparation and ball milling approaches to tackle the challenge of controlling their extreme sensitivity. Heavier Grignard reagents are not just more reactive but profit from unique chemical transformations. Insight into the transition metal-like properties of Ca, Sr and Ba is only just emerging. Considering the rapidly developing field of alkaline-earth metal-mediated catalysis, heavy Grignard reagents will probably have a bright future.

Main group metal-mediated strategies for C-H and C-F bond activation and functionalisation of fluoroarenes

With fluoroaromatic compounds increasingly employed as scaffolds in agrochemicals and active pharmaceutical ingredients, the development of methods which facilitate regioselective functionalisation of their C-H and C-F bonds is a frontier of modern synthesis. Along with classical lithiation and nucleophilic aromatic substitution protocols, the vast majority of research efforts have focused on transition metal-mediated transformations enabled by the redox versatilities of these systems. Breaking new ground in this area, recent advances in main group metal chemistry have delineated unique ways in which s-block, Al, Ga and Zn metal complexes can activate this important type of fluorinated molecule. Underpinned by chemical cooperativity, these advances include either the use of heterobimetallic complexes where the combined effect of two metals within a single ligand set enables regioselective low polarity C-H metalation; or the use of novel low valent main group metal complexes supported by special stabilising ligands to induce C-F bond activations. Merging these two different approaches, this Perspective provides an overview of the emerging concept of main-group metal mediated C-H/C-F functionalisation of fluoroarenes. Showcasing the untapped potential that these systems can offer in these processes; focus is placed on how special chemical cooperation is established and how the trapping of key reaction intermediates can inform mechanistic understanding.

Beryllium compounds for the carbon-halogen bond activation of phenyl halides: the role of non-innocent ligands

Beryllium is a metallomimetic main-group element, i.e., it behaves similarly to transition metals (TMs) in some bond activation processes. To investigate the ability of Be compounds to activate C-X bonds (X = F-I), we have computationally investigated, using DFT methods, the reaction of (CAAC)2Be (CAAC = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) and a series of five-membered heterocyclic beryllium bidentate ligands with phenyl halides. We have analysed all plausible reaction mechanisms and our results show that, after the initial C-X oxidative addition, migration of the phenyl group occurs towards the less electronegative heteroatom. Our theoretical study highlights the important role of bidentate non-innocent ligands in providing the required electrons for the initial Ph-X oxidative addition. In contrast, the monodentate ligand, CAAC, does not favour this oxidative addition.

Formation and Reactivity of a Hexahydridosilicate [SiH6]2- Coordinated by a Macrocycle-Supported Strontium Cation

The cationic benzyl complex [(Me₄TACD)Sr(CH₂Ph)][A] (Me₄TACD=1,4,7,10‐tetramethyltetraazacyclododecane; A=B(C₆H₃‐3,5‐Me₂)₄) reacted with two equivalents of phenylsilane to give the bridging hexahydridosilicate complex [(Me₄TACD)₂Sr₂(thf)₄(μ‐κ³ : κ³‐SiH₆)][A]₂ (3 a). Rapid phenyl exchange between phenylsilane molecules is assumed to generate monosilane SiH₄ that is trapped by two strontium hydride cations [(Me₄TACD)SrH(thf)_x]⁺. Complex 3 a decomposed in THF at room temperature to give the terminal silanide complex [(Me₄TACD)Sr(SiH₃)(thf)₂][A], with release of H₂. Upon reaction with a weak Brønsted acid, CO₂, and 1,3,5,7‐cyclooctatetraene SiH₄ was released. The reaction of a 1 : 2 mixture of cationic benzyl and neutral dibenzyl complex with phenylsilane gave the trinuclear silanide complex [(Me₄TACD)₃Sr₃(μ₂‐H)₃(μ₃‐SiH₃)₂][A], while ⁿOctSiH₃ led to the trinuclear (n‐octyl)pentahydridosilicate complex [(Me₄TACD)₃Sr₃(μ₂‐H)₃(μ₃‐SiH₅ⁿOct)][A].