The Wacker reaction is the oxidation of olefins to ketones and typically requires expensive and scarce palladium catalysts in the presence of an additional copper co-catalyst under harsh conditions (acidic media, high pressure of air/dioxygen, elevated temperatures). Such a transformation is relevant for industry, as shown by the synthesis of acetaldehyde from ethylene as well as for fine-chemicals, because of the versatility of a carbonyl group placed at specific positions. In this regard, many contributions have focused on controlling the chemo-and regioselectivity of the olefin oxidation by means of well-defined palladium catalysts under different sets of reaction conditions. However, the development of Wacker-type processes that avoid the use of palladium catalysts has just emerged in the last few years, thereby paving the way for the generation of more sustainable procedures, including milder reaction conditions and green chemistry technologies. In this Minireview, we discuss the development of new catalytic processes that utilize more benign catalysts and sustainable reaction conditions.
Selective iridium-catalyzed C–H bond borylations of unbiased or directing-group-free substrates typically occur under long reaction times and mild temperatures in order to avoid unselective processes including catalyst deactivation. Herein, we describe a supramolecular approach that enables the C–H bond borylation of challenging pyiridines and imidazoles in very short reaction times (up to 2 h) with a negligible incubation period for catalyst activation. The catalyst is based on a highly rigid zinc–porphyrin substrate-recognition site in the secondary coordination sphere and a triazolopyridine chelating fragment attached to the first coordination sphere at iridium. The borylation occurs at the C–H bond from the substrate located at four chemical bonds apart from the molecular recognition site with the selectivity being exclusively imposed by the distance between the active site and the molecular recognition site regardless of the nature of the N,N-chelating fragment coordinating to iridium as further supported by density functional theory (DFT) calculations. Additional studies (control experiments, nuclear magnetic resonance, and single-crystal X-ray diffraction) unraveled key catalyst deactivation pathways in which up to three different partners (water, methoxide ligands from the iridium precursor, and the triazolopyridine fragment) compete with the N-heterocycle substrate for binding to the molecular recognition site of the supramolecular catalyst. This fundamental understanding made possible the identification of a supramolecular catalyst featuring a 4-methyl substitution pattern in the first coordination sphere at iridium that provides a suitable balance of steric and electronic effects in both primary and secondary coordination spheres, thereby bypassing the manifold catalyst deactivation pathways. DFT calculations further indicated the importance of noncovalent interactions beyond the molecular recognition site on the stabilization of the different intermediates and transition sates.
A straightforward protocol to acylate oxindoles using methyl and phenyl esters mediated by LiHMDS and KOtBu respectively via the mixed Claisen condensation under mild reaction conditions.
The Wacker reaction is the oxidation of olefins to ketones and typically requires expensive and scarce palladium catalysts in the presence of an additional copper co-catalyst under harsh conditions (acidic media, high pressure of air/dioxygen, elevated temperatures). Such a transformation is relevant for industry, as shown by the synthesis of acetaldehyde from ethylene as well as for fine-chemicals, because of the versatility of a carbonyl group placed at specific positions. In this regard, many contributions have focused on controlling the chemo-and regioselectivity of the olefin oxidation by means of well-defined palladium catalysts under different sets of reaction conditions. However, the development of Wacker-type processes that avoid the use of palladium catalysts has just emerged in the last few years, thereby paving the way for the generation of more sustainable procedures, including milder reaction conditions and green chemistry technologies. In this Minireview, we discuss the development of new catalytic processes that utilize more benign catalysts and sustainable reaction conditions.
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