Characterization of the zwitterionic intermediate in 1,1‐carboboration of alkynes

The reaction of a Lewis acidic borane with an alkyne is a key step in a diverse range of main group transformations. Alkyne 1,1‐carboboration, the Wrackmeyer reaction, is an archetypal transformation of this kind. 1,1‐Carboboration has been proposed to proceed through a zwitterionic intermediate. We report the isolation and spectroscopic, structural and computational characterization of the zwitterionic intermediates generated by reaction of B(C6F5)3 with alkynes. The stepwise reactivity of the zwitterion provides new mechanistic insight for 1,1‐carboboration and wider B(C6F5)3 catalysis. Making use of intramolecular stabilization by a ferrocene substituent, we have characterized the zwitterionic intermediate in the solid state and diverted reactivity towards alkyne cyclotrimerization

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

Markovnikov-Selective, Activator-Free Iron-Catalyzed Vinylarene Hydroboration

Two series of structurally related alkoxy-tethered NHC iron­(II) complexes have been developed as catalysts for the regioselective hydroboration of alkenes. Significantly, Markonikov-selective alkene hydroboration with HBpin has been controllably achieved using an iron catalyst (11 examples, 35–90% isolated yield) with up to 37:1 branched:linear selectivity. <i>anti</i>-Markovnikov-selective alkene hydroboration was also achieved using HBcat and modification of the ligand backbone (6 examples, 44–71% yields). In both cases, ligand design has enabled activator-free low-oxidation-state iron catalysis

A graceful break-up: serendipitous self-assembly of a ferromagnetically coupled [NiII14] wheel

Agapaki E, Singh MK, Canaj AB, Nichol GS, Schnack J, Brechin EK. A graceful break-up: serendipitous self-assembly of a ferromagnetically coupled [NiII14] wheel. Chemical Communications. 2022.The complex [NiII14(HL2)12(HCOO)14Cl14(MeOH)(H2O)] describes an aesthetically pleasing wheel displaying ferromagnetic nearest neighbour exchange

[Fe15]: A Frustrated, Centred Tetrakis Hexahedron

The combination of two different FeIII salts in a solvothermal reaction with triethanolamine results in the formation of a high symmetry [FeIII15] cluster whose structure conforms to a centred, tetrakis hexahedron

Enhanced N-directed electrophilic C-H borylation generates BN-[5]- and [6]helicenes with improved photophysical properties.

Helicenes are chiral polycyclic aromatic hydrocarbons (PAHs) of significant interest, e.g. in supramolecular chemistry, materials science and asymmetric catalysis. Herein an enhanced N-directed electrophilic C–H borylation methodology has been developed that provides access to azaborine containing helicenes (BN–helicenes). This borylation process proceeds via protonation of an aminoborane with bistriflimidic acid. DFT calculations reveal the borenium cation formed by protonation to be more electrophilic than the product derived from aminoborane activation with BBr(3). The synthesised helicenes include BN-analogues of archetypal all carbon [5]- and [6]helicenes. The replacement of a CC with a BN unit (that has a longer bond) on the outer helix increases the strain in the BN congeners and the racemization half-life for a BN–[5]helicene relative to the all carbon [5]helicene. BN incorporation also increases the fluorescence efficiency of the helicenes, a direct effect of BN incorporation altering the distribution of the key frontier orbitals across the helical backbone relative to carbo-helicenes

C–H Borylation Catalysis of Heteroaromatics by a Rhenium Boryl Polyhydride

Transition metal complexes bearing metal–boron bonds are of particular relevance to catalytic C–H borylation reactions, with iridium polyboryl and polyhydrido-boryl complexes the current benchmark catalysts for these transformations. Herein, we demonstrate that polyhydride boryl phosphine rhenium complexes are accessible and catalyze the C–H borylation of heteroaromatic substrates. Reaction of [K(DME)(18-c-6)][ReH4(Bpin)(η2-HBpin)(κ2-H2Bpin)] 1 with 1,3-bis(diphenylphosphino)propane (dppp) produced [K(18-c-6)][ReH4(η2-HBpin)(dppp)] 2 through substitution of two equivalents of HBpin, and protonation of 2 formed the neutral complex [ReH6(Bpin)(dppp)] 3. Combined X-ray crystallographic and DFT studies show that 2 is best described as a σ-borane complex, whereas 3 is a boryl complex. Significantly, the boryl complex 3 acted as a catalyst for the C(sp2)–H borylation of a variety of heteroarenes (14 examples including furan, thiophene, pyrrole and indole derivatives) and displayed similar reactivity to the iridium analogues

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