Simplified access to low oxidation-state earth-abundant metals for catalytic application

A sustainable future lies in the use of first row, low cost, low toxicity, Earth-abundant metals. Despite this, the metals that are most abundant have yet to be widely adopted by the global community. The overarching aim at the outset of the project was to ask the question: Why is this? Why do expensive metals such as; platinum, palladium and rhodium dominate? Why does the synthetic chemist not instinctively use iron, manganese or cobalt? The simple answer: The non-expert chemist is simply not equipped to try. Many modern synthetic methods for the reductive functionalisation of alkenes and alkynes rely on the use of air- and moisture-sensitive pre-catalysts or reagents, which are challenging to handle, store and transport. In the ideal scenario, all reagents and pre-catalysts would be air- and moisture-stable solids that are easily handled, and applicable in large-scale processes with minimal associated hazards. This project entailed the development of a simple pre-catalyst activation protocol using a safe and easily handled reagent (NaOtBu) with wide commercial availability. This has allowed generic access to sustainable first-row transition metal (Fe, Co, Mn, Ni) low oxidation-state catalysis across a wide range of reductive alkene and alkyne functionlisation reactions (hydroboration, hydrosilylation, hydrogenation, hydrovinylation and [2π+2π] cyclisation reactions). Using this straightforward catalyst activation strategy a new regiodivergent cobalt-catalyst alkene hydrosilylation manifold was discovered and mechanistically explored. Taken together, all results are suggestive of a new and unique catalyst activation mechanism that is primed for future reaction, catalyst and mechanistic development

Cobalt‐Catalysed, Ligand‐Controlled Regiodivergent Alkene Hydrosilylation

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)

Nucleophile induced ligand rearrangement reactions of alkoxy- and arylsilanes

The ligand-redistribution reactions of aryl- and alkoxy-hydrosilanes can potentially cause the formation of gaseous hydrosilanes, which are flammable and pyrophoric. The ability of generic nucleophiles to initiate the ligand-redistribution reaction of commonly used hydrosilane reagents was investigated, alongside methods to hinder and halt the formation of hazardous hydrosilanes. Our results show that the ligand-redistribution reaction can be completely inhibited by common electrophiles and first-row transition metal pre-catalysts

Cobalt-catalysed Markovnikov selective hydroboration of vinylarenes

A bipyridiyl-oxazoline cobalt catalyst tBuBPOCoCl2 has been developed for the Markovnikov selective hydroboration of alkenes using pinacolborane and NaOtBu as the in situ activator with up to >98 : 2 branched : linear selectivity (24 examples, 45-92% isolated yield)

A Boron–Boron Double Transborylation Strategy for the Synthesis of gem-Diborylalkanes

Olefin hydroboration reactions provide efficient access to synthetically versatile and easily handled organoboronic esters. In this study, we demonstrate that the commercially available organoborane reagent 9-borabicyclo[3.3.1]nonane (H-B-9-BBN) can serve as a catalyst for the sequential double hydroboration of alkynes using pinacolborane (HBpin). This strategy, which is effective for a wide range of terminal alkynes, is predicated upon a key C(sp3)-B/B-H transborylation reaction. Transition-state thermodynamic parameters and 10-boron-isotopic labeling experiments are indicative of a σ-bond metathesis exchange pathway

Iron-catalysed C(sp²)-H borylation enabled by carboxylate activation

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

Iron‐catalysed C(sp²)‐H Borylation with Expanded Functional Group Tolerance

Arene C(sp2)-H bond borylation offers direct and efficient access to aryl boronic esters. Using in situ catalyst activation and photoirradiation, the iron-catalysed C(sp2)-H borylation reaction of carboarenes, pyrroles, and indoles has been developed using only bench-stable pre-catalysts and reagents. Good functional group tolerance was observed including those not reported using previous methods (ArNH2, ArOH, ArSiR3, ArP(O)(OR)2, ArC(O)NR2). Mechanistic studies revealed iron-catalysed reductive deoxygenation, C—F protodefluorination, and a demethylation of aryl methyl ethers by C—O sigma bond hydroboration

Borane-Catalysed Hydroboration of Alkynes and Alkenes

Simple, commercially available borane adducts, H 3 B·THF and H 3 B·SMe 2, 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

Synthesis of DBpin using Earth-abundant metal catalysis

The synthesis of DBpin was achieved using (EtBIP)CoCl2 or (tBuPNN)FeCl2 as pre-catalysts activated with NaOtBu. (EtBIP)CoCl2 was used as a pre-catalyst for the hydrogen isotope exchange of HBpin with D2, and (tBuPNN)FeCl2 for deuterogenolysis of B2pin2. The one-pot, tandem hydrogenolysis-hydroboration/deuterogenolysis-deuteroboration reaction of terminal alkenes could be catalysed by (tBuPNN)FeCl2 to give alkyl boronic esters

Iron-catalysed alkene and heteroarene H/D exchange by reversible protonation of iron-hydride intermediates

C–H functionalisation reactions offer a sustainable method for molecular construction and diversification. These reactions however remain dominated by precious metal catalysis. While significant interest in iron-catalysed C–H activation reactions has emerged, the isolation, characterisation and mechanistic understanding of these processes remain lacking. Herein the iron-catalysed C(sp(2))–H bond hydrogen/deuterium exchange reaction using CD(3)OD is reported for both heterocycles and, for the first time, alkenes (38 examples). Isolation and characterisation, including by single-crystal X-ray diffraction, of the key iron-aryl and iron-alkenyl C–H metallation intermediates provided evidence for a reversible protonation of the active iron hydride catalyst. Good chemoselectivity was observed for both substrate classes. The developed procedure is orthogonal to previous iron-catalysed H/D exchange methods which used C(6)D(6), D(2), or D(2)O as the deuterium source, and uses only bench-stable reagents, including the iron(ii) pre-catalyst. Further, a new mechanism of iron-hydride formation is reported in which β-hydride elimination from an alcohol generates the iron hydride. The ability to produce, isolate and characterise the organometallic products arising from C–H activation presents a basis for future discovery and development