90196-91-1Relevant articles and documents
N-Ammonium Ylide Mediators for Electrochemical C-H Oxidation
Saito, Masato,Kawamata, Yu,Meanwell, Michael,Navratil, Rafael,Chiodi, Debora,Carlson, Ethan,Hu, Pengfei,Chen, Longrui,Udyavara, Sagar,Kingston, Cian,Tanwar, Mayank,Tyagi, Sameer,McKillican, Bruce P.,Gichinga, Moses G.,Schmidt, Michael A.,Eastgate, Martin D.,Lamberto, Massimiliano,He, Chi,Tang, Tianhua,Malapit, Christian A.,Sigman, Matthew S.,Minteer, Shelley D.,Neurock, Matthew,Baran, Phil S.
, p. 7859 - 7867 (2021/05/26)
The site-specific oxidation of strong C(sp3)-H bonds is of uncontested utility in organic synthesis. From simplifying access to metabolites and late-stage diversification of lead compounds to truncating retrosynthetic plans, there is a growing need for new reagents and methods for achieving such a transformation in both academic and industrial circles. One main drawback of current chemical reagents is the lack of diversity with regard to structure and reactivity that prevents a combinatorial approach for rapid screening to be employed. In that regard, directed evolution still holds the greatest promise for achieving complex C-H oxidations in a variety of complex settings. Herein we present a rationally designed platform that provides a step toward this challenge using N-ammonium ylides as electrochemically driven oxidants for site-specific, chemoselective C(sp3)-H oxidation. By taking a first-principles approach guided by computation, these new mediators were identified and rapidly expanded into a library using ubiquitous building blocks and trivial synthesis techniques. The ylide-based approach to C-H oxidation exhibits tunable selectivity that is often exclusive to this class of oxidants and can be applied to real-world problems in the agricultural and pharmaceutical sectors.
Toward an understanding of the high enantioselectivity in the osmium-catalyzed asymmetric dihydroxylation (AD). 1. Kinetics
Kolb, Hartmuth C.,Andersson, Pher G.,Sharpless, K. Barry
, p. 1278 - 1291 (2007/10/02)
A systematic study of the relationship between ligand structure and saturation rate constants (kc) in the amine-catalyzed osmylation of terminal olefins was carried out with the aim of learning more about the interactions between the reactants (i.e. OsO4, the ligand, and the olefin) and of establishing the origin of the large rate accelerations observed with cinchona alkaloid ligands. The results reveal that the saturation rate constants are influenced principally by the nature of the O9 substituent of the cinchona analogs studied, especially if aromatic substrates are used. Additionally, the binding constants (Keq) for OsO4 and the test ligands were measured, and the observed trends show that Keq can be regarded as an approximate measure of the steric hindrance in the vicinity of the ligand-binding site. Interestingly, the binding constants and the saturation rate constants kc are not correlated, indicating that the observed rate variations are apparently not caused by variations in ground-state energy due to steric interactions. Rather, the rate data can be interpreted in terms of a relative stabilization of the transition state of the reaction in the case of 'fast' ligands. A transition-state stabilization may result from stacking of the olefin and ligand substituents, and this theory is consistent with the fact that flat aromatic substrates give much higher rate constants than aliphatic ones. Further support for this theory was obtained from solvent effect and Hammett studies as well as from X-ray data on osmium glycolate complexes. Phthalazine ligand 1 gives exceptionally high rate constants with aromatic substrates, an effect which can be attributed to the presence of a 'binding pocket', set up by the phthalazine and methoxyquinoline moieties of the ligand, which enables especially good transition-state stabilization for aromatic olefins within the pocket. The enantioselectivity trends were found to parallel the rate trends; therefore, our results allow us to draw conclusions with regard to the mode of chirality transfer in the reaction, leading to a revised mnemonic device.
