Late-stage peptide C–H alkylation for bioorthogonal C–H activation featuring solid phase peptide synthesis

Methods for the late-stage diversification of structurally complex peptides hold enormous potential for advances in drug discovery, agrochemistry and pharmaceutical industries. While C-H arylations emerged for peptide modifications, they are largely limited to highly reactive, expensive and/or toxic reagents, such as silver(I) salts, in superstoichiometric quantities. In sharp contrast, we herein establish the ruthenium(II)-catalyzed C-H alkylation on structurally complex peptides. The additive-free ruthenium(II)carboxylate C-H activation manifold is characterized by ample substrate scope, racemization-free conditions and the chemo-selective tolerance of otherwise reactive functional groups, such as electrophilic ketone, bromo, ester, amide and nitro substituents. Mechanistic studies by experiment and computation feature an acid-enabled C-H ruthenation, along with a notable protodemetalation step. The transformative peptide C-H activation regime sets the stage for peptide ligation in solution and proves viable in a bioorthogonal fashion for C-H alkylations on user-friendly supports by means of solid phase peptide syntheses.peerReviewe

Mapping out the key carbon–carbon bond-forming steps in Mn-catalysed C–H functionalization

Detailed understanding of the mechanistic processes that underpin transition metal-catalysed reactions allows for the rational and de novo development of complexes with enhanced activity, efficacy and wider substrate scope. Directly observing bond cleaving and forming events underpinning a catalytic reaction is non-trivial as the species that facilitate these steps are frequently short-lived and present at low concentrations. Here we describe how the photochemical activation of a manganese precatalyst, [Mn(ppy)(CO)4], results in selective loss of a carbonyl ligand simulating entry into the catalytic cycle for Mn-promoted C–H bond functionalisation. Time-resolved infra-red spectroscopy (ps-ms timescale) allows direct observation of the species responsible for the essential carbon-carbon bond formation step and an evaluation of the factors affecting its rate. The mechanistic information prompted discovery of a new photochemically initiated manganese-promoted coupling of phenylacetylene with 2-phenylpyrindine. This study provides unique insight into the mechanistic pathways which underpin catalysis by an Earth-abundant metal, manganese