James Sanderson

It's electrifying! Electrochemical cross-coupling of aryl compounds provides a one-step approach to unsymmetrical biaryl compounds. A boron-doped diamond electrode is used as the anode to oxidise the phenolic hydroxy group to a phenoxy radical, and 1,1,1,3,3,3-hexafluoroisopropylalcohol (HFIP) and tetraalkylammonium methylsulfate are used as the solvent and electrolyte. Since no reagents are required, no waste is formed from this source.

The homocoupling of aryl halides and the heterocoupling of aryl halides with either aryl bromides or arenes bearing an ortho-lithiation directing group are presented. The use of a Pd catalyst, in combination with t-BuLi, allows for the rapid and efficient formation of a wide range of polyaromatic compounds in a one pot procedure bypassing the need for the separate preformation of an organometallic coupling partner. These polyaromatic structures are obtained in high yields, in 10 min at room temperature, with minimal waste generation (E-factors as low as 1.5) and without the need for strict inert conditions, making this process highly efficient and practical in comparison to classical methods. As illustration, several key intermediates of widely used BINOL-derived structures are readily prepared including the highly desired precursor to the chiral TRIP phosphoric acid.

Quick-and-clean: The cross-coupling of distinct (hetero)arenes is achieved in a rapid and efficient manner under ambient conditions with very little waste. By using a Pd catalyst and t-BuLi, many polyaromatic compounds are obtained including highly sterically hindered ones. Of these, several are advanced intermediates for widely used chiral Brønsted acid catalysts normally obtained via a rather cumbersome process.

The activation of readily prepared, air-stable cobalt(II) bis(carboxylate) pre-catalysts for the functionalization of C(sp²)−H bonds has been systematically studied. With the pyridine bis(phosphine) chelate, ^iPrPNP, treatment of 1-(O₂C^tBu)₂ with either B₂Pin₂ or HBPin generated cobalt boryl products. With the former, reduction to (^iPrPNP)Co^IBPin was observed while with the latter, oxidation to the cobalt(III) dihydride boryl, trans-(^iPrPNP)Co(H)₂BPin occurred. The catalytically inactive cobalt complex, Co[PinB(O₂C^tBu)₂]₂, accompanied formation of the cobalt-boryl products in both cases. These results demonstrate that the pre-catalyst activation from cobalt(II) bis(carboxylates), although effective and utilizes an air-stable precursor, is less efficient than activation of cobalt(I) alkyl or cobalt(III) dihydride boryl complexes, which are quantitatively converted to the catalytically relevant cobalt(I) boryl. Related cobalt(III) dihydride silyl and cobalt(I) silyl complexes were also synthesized from treatment of trans-(^iPrPNP)Co(H)₂BPin and (^iPrPNP)CoPh with HSi(OEt)₃, respectively. No catalytic silylation of arenes was observed with either complex likely due to the kinetic preference for reversible C−H reductive elimination rather than product- forming C−Si bond formation from cobalt(III). Syntheses of the cobalt(II) bis(carboxylate) and cobalt(I) alkyl of ^iPrPONOP, a pincer where the methylene spacers have been replaced by oxygen atoms, were unsuccessful due to deleterious P−O bond cleavage of the pincer. Despite their structural similarity, the rich catalytic chemistry of ^iPrPNP was not translated to ^iPrPONOP due to the inability to access stable cobalt precursors as a result of ligand decomposition via P−O bond cleavage.