Abstract
The use of high-valent antimony–oxo porphyrins as visible-light photocatalysts operating via direct hydrogen atom transfer has been demonstrated. Computational analysis indicates that the triplet excited state of these complexes shows an oxyl radical behavior, while the SbV center remains in a high-valent oxidation state, serving uniquely to carry the oxo moiety and activate the coordinated ligands. This porphyrin-based system has been exploited upon irradiation to catalyze C–H to C–C bond conversion via the addition of hydrogen donors (ethers and aldehydes) onto Michael acceptors in a redox-neutral fashion without the need of any external oxidant. Laser flash photolysis experiments confirmed that the triplet excited state of the photocatalyst triggers the desired C–H cleavage.
Highlights
The development of eco-sustainable synthetic methodologies has recently received a fundamental contribution by visiblelight photocatalysis.[1]
Inspired by the chameleonic behavior of Eosin Y, we reasoned that other classes of catalysts designed for single electron transfer (SET) or energy transfer (EnT) might work as visible light absorbing PCHAT as well
We investigated the structures of I+ and oxo-I in the gas phase, and the corresponding optimized structures are reported in the Supporting Information, clearly showing the presence of a significantly shorter Sb−O bond in oxo-I (Sb O: 1.84 Å vs Sb−OH: 2.00 Å), indicative of a double bond character
Summary
The development of eco-sustainable synthetic methodologies has recently received a fundamental contribution by visiblelight photocatalysis.[1]. The approach is remarkably different from that of iron- and manganese−oxo porphyrins known to promote hydroxylation and halogenation reactions in alkanes via a thermal HAT step.[35] The latter behavior may be attributed to the presence of a partially filled d shell in the metal center, endowing the complex with a radicaloid character These complexes are characterized by several low-lying, close-in-energy electronic states that mix together, giving rise to the so-called “redox mesomerism” phenomenon.[36] these metal−oxo porphyrins trigger a thermal homolytic C−H cleavage, and the resulting C-centered radical can be used for C−X bond formation through a rebound mechanism,[35] with the concomitant reduction of the complex (the oxidation state at M goes from n+ to (n − 2)+). This peculiar characteristic makes these complexes unsuitable for other types of chemistry, such as the redox-neutral process presented
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