First-row, earth-abundant metals offer an inexpensive and sustainable alternative to precious-metal catalysts. As such, iron and cobalt catalysts have garnered interest as replacements for alkene and alkyne hydrofunctionalization reactions. However, these have required the use of air- and moisture-sensitive catalysts and reagents, limiting both adoption by the non-expert as well as applicability, particularly in industrial settings. Here, we report a simple method for the use of earth-abundant metal catalysts by general activation with sodium tert-butoxide. Using only robust air- and moisture-stable reagents and pre-catalysts, both known and, significantly, novel catalytic activities have been successfully achieved, covering hydrosilylation, hydroboration, hydrovinylation, hydrogenation and [2π+2π] alkene cycloaddition. This activation method allows for the easy use of earth-abundant metals, including iron, cobalt, nickel and manganese, and represents a generic platform for the discovery and application of non-precious metal catalysis.

A bipyridiyl-oxazoline cobalt catalyst BPOCoCl has been developed for the Markovnikov selective hydroboration of alkenes using pinacolborane and NaOBu as the in situ activator with up to >98 : 2 branched : linear selectivity (24 examples, 45-92% isolated yield).

Site-predictable and chemoselective C–H bond functionalization reactions offer synthetically powerful strategies for the step-economic diversification of both feedstock and fine chemicals. Many transition-metal-catalyzed methods have emerged for the selective activation and functionalization of C–H bonds. However, challenges of regio- and chemoselectivity have emerged with application to highly complex molecules bearing significant functional group density and diversity. As molecular complexity increases within molecular structures the risks of catalyst intolerance and limited applicability grow with the number of functional groups and potentially Lewis basic heteroatoms. Given the abundance of C–H bonds within highly complex and already diversified molecules such as pharmaceuticals, natural products, and materials, design and selection of reaction conditions and tolerant catalysts has proved critical for successful direct functionalization. As such, innovations within transition-metal-catalyzed C–H bond functionalization for the direct formation of carbon–carbon bonds have been discovered and developed to overcome these challenges and limitations. This review highlights progress made for the direct metal-catalyzed C–C bond forming reactions including alkylation, methylation, arylation, and olefination of C–H bonds within complex targets.

BBN) has been used as a catalyst for the sequential double hydroboration of alkynes with pinacolborane (HBpin) to give alkyl gem-di-pinacol boronic esters. This strategy, which is effective for a wide range of terminal alkynes, is predicated upon a key C(sp 3)-B / B-H transborylation reaction. Transition-state thermodynamic parameters and 10-boron-isotopic labelling experiments are indicative of an σ-bond metathesis exchange pathway. File list (2) download file view on ChemRxiv Manuscript ChemRxiv PDF.pdf (1.39 MiB) download file view on ChemRxiv Supporting information ChemRxiv PDF.pdf (8.28 MiB)

Aryl boronic esters are bench-stable, platform building-blocks that can be accessed through metal-catalyzed aryl C­(sp2)–H borylation reactions. C­(sp2)–H bond functionalization reactions using rare- and precious-metal catalysts are well established, and while examples utilizing Earth-abundant alternatives have emerged, manganese catalysis remains lacking. The manganese-catalyzed C–H borylation of furan and thiophene derivatives is reported alongside an in situ activation method providing facile access to the active manganese hydride species. Mechanistic investigations showed that blue light irradiation directly affected catalysis by action at the metal center, that C­(sp2)–H bond borylation occurs through a C–H metallation pathway, and that the reversible coordination of pinacolborane to the catalyst gave a manganese borohydride complex, which was as an off-cycle resting state.

Arene C(sp2)-H bond borylation reactions provide rapid and efficient routes to synthetically versatile boronic esters. While iridium catalysts are well established for this reaction, the discovery and development of methods using Earth-abundant alternatives is limited to just a few examples. Applying an in situ catalyst activation method using air-stable and easily handed reagents, the iron-catalysed C(sp2)-H borylation reactions of furans and thiophenes under blue light irradiation have been developed. Key reaction intermediates have been prepared and characterised, and suggest two mechanistic pathways are in action involving both C-H metallation and the formation of an iron boryl species.

Alkene hydrosilylation is amongst the largest industrial homogenous catalysis processes. Cobalt catalysis offers a sustainable alternative to commonly used platinum catalysts to achieve this transformation. Using two bisiminopyridine cobalt(II) catalysts the regiodivergent hydrosilylation of alkenes has been developed. Variation of pre‐catalyst activator and ligand substituents were investigated to enable the controlled, regiodivergent hydrosilylation of both aryl‐ and alkyl‐substituted alkenes with phenylsilane. In contrast to other regiodivergence strategies, excellent regioselectivity for either isomer was achieved using the same ligand class but differing by a single methyl group (ethyl vs isopropyl).

Simple, commercially available borane adducts, H3B·THF and H3B·SMe2, have been used to catalyse the hydroboration of alkynes and alkenes with pinacolborane to give the alkenyl and alkyl boronic esters, respectively. Alkynes and terminal alkenes underwent highly regioselective hydroboration to give the linear boronic ester products. Good functional group tolerance was observed for substrates bearing ester, amine, ether and halide substituents. This catalytic process shows comparable reactivity to transition-metal-catalysed hydroboration protocols.