Metallation of sensitive fluoroarenes using a potassium TMP-zincate supported by a silyl(bis)amido ligand

Transition Metal-Free Catalytic C-H Zincation and Alumination

C-H metalation is the most efficient method to prepare aryl-zinc and -aluminium complexes that are ubiquitous nucleophiles. Virtually all C-H metalation routes to form Al/Zn organometallics require stoichiometric, strong Brønsted bases with no base-catalyzed reactions reported. Herein we present a catalytic in amine/ammonium salt (Et3N/[(Et3N)H]+) C-H metalation process to form aryl-zinc and aryl-aluminium complexes. Key to this approach is coupling an endergonic C-H metalation step with a sufficiently exergonic dehydrocoupling step between the ammonium salt by-product of C-H metalation ([(Et3N)H]+) and a Zn-H or Al-Me containing complex. This step, forming H2/MeH, makes the overall cycle exergonic while generating more of the reactive metal electrophile. Mechanistic studies supported by DFT calculations revealed metal-specific dehydrocoupling pathways, with the divergent reactivity due to the different metal valency (which impacts the accessibility of amine-free cationic metal complexes) and steric environment. Notably, dehydrocoupling in the zinc system proceeds through a ligand-mediated pathway involving protonation of the β-diketiminate Cγ position. Given this process is applicable to two disparate metals (Zn and Al), other main group metals and ligand sets are expected to be amenable to this transition metal-free, catalytic C-H metalation.

Unlocking the Metalation Applications of TMP-powered Fe and Co(II) bis(amides): Synthesis, Structure and Mechanistic Insights

Typified by LiTMP and TMPMgCl.LiCl, (TMP=2,2,6,6-tetramethylpiperidide), s-block metal amides have found widespread applications in arene deprotonative metalation. On the contrary, transition metal amides lack sufficient basicity to activate these substrates. Breaking new ground in this field, here we present the synthesis and full characterisation of earth-abundant transition metals M(TMP)2 (M=Fe, Co). Uncovering a new reactivity profile towards fluoroarenes, these amide complexes can promote direct M-H exchange processes regioselectively using one or two of their basic amide arms. Remarkably, even when using a perfluorinated substrate, selective C-H metalation occurs leaving C-F bonds intact. Their kinetic basicity can be boosted by LiCl or NBu4Cl additives which enables formation of kinetically activated ate species. Combining spectroscopic and structural studies with DFT calculations, mechanistic insights have been gained on how these low polarity metalation processes take place. M(TMP)2 can also be used to access ferrocene and cobaltocene by direct deprotonation of cyclopentadiene and undergo efficient CO2 insertion of both amide groups under mild reaction conditions.

Synthesis and Modular Reactivity of Low Valent Al/Zn Heterobimetallics Supported by Common Monodentate Amides

Recent advances on low valent main group metal chemistry have shown the excellent potential of heterobimetallic complexes derived from Al(I) to promote cooperative small molecule activation processes. A signature feature of these complexes is the use of bulky chelating ligands which act as spectators providing kinetic stabilization to their highly reactive Al-M bonds. Here we report the synthesis of novel Al/Zn bimetallics prepared by the selective formal insertion of AlCp* into the Zn-N bond of the utility zinc amides ZnR2 (R=HMDS, hexamethyldisilazide; or TMP, 2,2,6,6-tetramethylpiperidide). By systematically assessing the reactivity of the new [(R)(Cp*)AlZn(R)] bimetallics towards carbodiimides, structural and mechanistic insights have been gained on their ability to undergo insertion in their Zn-Al bond. Disclosing a ligand effect, when R=TMP, an isomerization process can be induced giving [(TMP)2AlZn(Cp*)] which displays a special reactivity towards carbodiimides and carbon dioxide involving both its Al-N bonds, leaving its Al-Zn bond untouched.

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.

Applying Na/Co(II) bimetallic partnerships to promote multiple Co-H exchanges in polyfluoroarenes

Heterobimetallic base NaCo(HMDS)₃ [HMDS = N(SiMe₃)₂] enables regioselective di-cobaltation of activated polyfluoroarenes under mild reaction conditions. For 1,3,5-C₆H₂X₃ (X= Cl, F), NaCo(HMDS)₃ in excess at 80 °C impressively induces the collective cleavage of five bonds (two C-H and three C-X) of the substrates via a cascade activation process that cannot be replicated by LiCo(HMDS)₃ or KCo(HMDS)₃.

Synthesis and Catalysis of Anionic Amido Iron(II) Complexes

Low-coordinate, open-shell 3d metal complexes have attracted great attention due to their critical role in several catalytic transformations but have been notoriously difficult to prepare and study due to their high lability. Here, we report the synthesis of a heteroleptic tri-coordinate amidoferrate that displays high catalytic activity in the regioselective hydrosilylation of alkenes.