Photocatalysis with Reversibly Coordinating Cerium Complexes
Introduction. Photoinduced ligand to metal charge transfer (LMCT) is well known and observed in many coordination compounds; text book examples are Cr(III) aquo and hydroxido complexes or titanium alkoxides. However, only recently, applications of visible light driven cerium LMCT reactions in organic synthesis were reported. The reversible coordination of a cerium (III) or (IV) salt to alcohols followed by photoinduced LMCT yields alkoxy radicals under mild reaction conditions. The alkoxy radicals were used for intramolecular C–H activation by hydrogen atom abstraction (HAT) and the resulting sp3 alkyl radical was reacted with alkenes. The high reactivity of alkoxy radicals in HAT allowed even the intermolecular C–H activation of methane gas using cerium complexes in alcohol solution. The reported applications of cerium photocatalysis of the last five years have clearly demonstrated the synthetic potential of the cheap and earth abundant metal ion in light driven transformations. The key step in reactions with alcohols is an LMCT process. However, so far only chlorido cerium complexes that are kinetically labile have been used, which hampers a rational catalyst optimization. In addition, the roles of additives and the widely used radical reaction partner dibenzylazodicarboxylate in the mechanism of the reaction remain unclear. Schelter et al. established a cerium complex structure – property relationship for guanidinate−amide and guanidinate−aryloxide Ce(III) complexes. However, these complexes have found only few synthetic applications so far.
Summary of the project. We aim in this project to develop better-defined LMCT cerium photocatalysts and apply the LMCT principle to new transformations, such as selective intra- and intermolecular C–H functionalizations in compounds bearing hydroxyl groups, oximes or hydroxamic acids. Moreover, to achieve selectivity in intermolecular C–H abstraction on oligohydroxy compounds selective activation is proposed, which results in weakening and preferred abstraction of C–H bonds by photocatalytically generated alkoxy radicals. The effect of Lewis-acid assembly on specific C–H bonds is observable in NMR spectroscopy allowing the prediction of assembly controlled photocatalytic C–H activation from spectroscopic data.
V. R. Yatham, P. Belotti, B. König, ´Decarboxylative hydrazination of unactivated carboxylic acids by cerium photocatalysis´ Chem. Commun. 2019, 55, 3489-3492.
J. Schwarz, B. König, ´Visible-light mediated C–C bond cleavage of 1,2-diols to carbonyls by cerium-photocatalysis´ Chem. Commun. 2019, 55, 486-488.
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