Mild, Aqueous α-Arylation of Ketones: Towards New Diversification Tools for Halogenated Metabolites and Drug Molecules

Electronically flexible PYA ligands for efficient palladium-catalyzed α-arylation of ketones

Palladium-catalyzed cross-coupling chemistry and in particular ketone α-arylation has been relying on a rather narrow range of supporting ligands with almost no alternatives to phosphines and N-heterocyclic carbenes. Here we introduce a class of well-defined palladium(II) complexes supported by N,N'-chelating and electronically flexible pyridylidene amide (PYA)-pyridyl ligands as catalysts for efficient α-arylation of ketones. Steric and electronic variations of the N,N'-bidentate ligand indicate that the introduction of an ortho-methyl group on the pyridinum heterocycle of the PYA ligand enhances the arylation rate and prevents catalyst deactivation, reaching turnover numbers up to 7300 and turnover frequencies of almost 10 000 h-1, which is similar to that of the best phosphine complexes known to date. Introducing a shielding xylyl substituent accelerates catalysis further, however at the expense of lower selectivity towards arylated ketones. Substrate scope investigations revealed that both electron-rich and -poor aryl bromides as well as a broad range of electronically and sterically modified ketones are efficiently converted, including aliphatic ketones. Mechanistic investigations using Hammett and Eyring analyses indicated that both, oxidative addition and reductive elimination are relatively fast, presumably as a consequence of the electronic flexibility of the PYA ligand, while enolate coordination was identified as the turnover-limiting step.

An expedient, mild and aqueous method for Suzuki-Miyaura diversification of (hetero)aryl halides or (poly)chlorinated pharmaceuticals

The development of mild, aqueous conditions for the cross-coupling of highly functionalized (hetero)aryl chlorides or bromides is attractive, enabling their functionalization and diversification. Herein, we report a general method for Suzuki–Miyaura cross-coupling at 37 °C in aqueous media in the presence of air. We demonstrate application of this general methodology for derivatisation of (poly)chlorinated, medicinally active compounds and halogenated amino acids. The approach holds the potential to be a useful tool for late-stage functionalization or analogue generation.

Simple, aqueous and direct cross-coupling of diverse and complex (hetero)aromatic halides and active pharmaceutical agents.

Halogenases: a palette of emerging opportunities for synthetic biology-synthetic chemistry and C-H functionalisation

The enzymatic generation of carbon-halogen bonds is a powerful strategy used by both nature and synthetic chemists to tune the bioactivity, bioavailability and reactivity of compounds, opening up the opportunity for selective C-H functionalisation. Genes encoding halogenase enzymes have recently been shown to transcend all kingdoms of life. These enzymes install halogen atoms into aromatic and less activated aliphatic substrates, achieving selectivities that are often challenging to accomplish using synthetic methodologies. Significant advances in both halogenase discovery and engineering have provided a toolbox of enzymes, enabling the ready use of these catalysts in biotransformations, synthetic biology, and in combination with chemical catalysis to enable late stage C-H functionalisation. With a focus on substrate scope, this review outlines the mechanisms employed by the major classes of halogenases, while in parallel, it highlights key advances in the utilisation of the combination of enzymatic halogenation and chemical catalysis for C-H activation and diversification.

Different effects of metal-NHC bond cleavage on the Pd/NHC and Ni/NHC catalyzed α-arylation of ketones with aryl halides

Fundamental differences in the behavior of Pd/NHC and Ni/NHC catalytic systems in ketones α-arylation were elucidated and exploited.

Late-Stage Diversification of Tryptophan-Derived Biomolecules

Pd-mediated reactions have emerged as a powerful tool for the site-selective and bioorthogonal late-stage diversification of amino acids, peptides and related compounds. Indole moieties of tryptophan derivatives are susceptible to C2 H-activation, whereas halogenated aromatic amino acids such as halophenylalanines or halotryptophans provide a broad spectrum of different functionalisations. The compatibility of transition-metal-catalysed cross-couplings with functional groups in peptides, other biologically active compounds and even proteins has been demonstrated. This Review primarily compiles the application of different cross-coupling reactions to modify halotryptophans, halotryptophan containing peptides or halogenated, biologically active compounds derived from tryptophan. Modern approaches use regio- and stereoselective biocatalytic strategies to generate halotryptophans and derivatives on a preparative scale. The combination of bio- and chemocatalysis in cascade reactions is given by the biocompatibility and bioorthogonality of Pd-mediated reactions.

Coupling of amides with ketones via C–N/C–H bond cleavage: a mild synthesis of 1,3-diketones

Amides react with enolizable ketones to give 1,3-diketones via C–N cleavage of amides and deprotonation of ketones.

Zn(II)-Catalyzed Addition of Aromatic/Heteroaromatic C(sp2)-H to Azoalkenes: A Polarity-Reversed Arylation of Carbonyl Compounds

An umpolung α-(hetero)arylation strategy that involves the Michael-type reaction between electron-rich (hetero)aromatic substrates and azoalkenes is developed. The reaction proceeds under very mild conditions at room temperature and in the presence of inexpensive, nontoxic ZnCl₂ catalyst to provide access to otherwise inaccessible hydrazone structures. Subsequent hydrolysis of these latter to ketones as well as other valuable synthetic transformations to a variety of heterocyclic scaffolds demonstrate the usefulness of this protocol.