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Coordinatore RAVELLI Davide
Titolo Organic Synthesis via Visible Light Photocatalytic Hydrogen Transfer
Università/ente UniversitÓ degli Studi di PAVIA
Codice Progetto RBSI145Y9R
Settore principale ERC PE5 - Synthetic Chemistry and Materials: Materials synthesis, structure-properties relations, functional and advanced materials, molecular architecture, organic chemistry
Parole chiave ORGANIC SYNTHESIS
PHOTOCATALYSIS
GREEN CHEMISTRY
FREE RADICALS
HYDROGEN ATOM TRANSFER
Durata 36 mesi
Finanziamento concesso 423.500 €
Abstract

The generation of highly reactive intermediates under mild conditions, in accordance with the general principles of Green Chemistry, is currently a major goal in organic synthesis. Photochemistry is a convenient approach, where the absorption of a photon leads to the introduction of a large amount of energy selectively into the absorbing molecule and leaves no waste to get rid of after the process, thus offering a smart way to generate active intermediates under controlled conditions. Photocatalysis, where a catalyst operating only in the excited state is responsible for the activation of the reagent through a chemical step, allows the extension of the method to transparent compounds. These reactions mostly involve transfer of an electron (ET; dubbed as "photoredox catalysis") or of an atom (usually a hydrogen; Hydrogen Atom Transfer - HAT) and are typically carried out by using UV-light emitting lamps which increase both the cost and environmental footprint of the processes compared with the direct utilization of incident sunlight. In recent years, however, several processes activated by visible light have been developed. These are based on an ET step to induce mono-electronic reduction or oxidation, while homolytic hydrogen atom transfer has been considered only in a couple of cases, due to the lack of suitable visible light photocatalysts able to cleave strong R-H bonds, despite the great synthetic potential of this mechanism (high regio- and stereo-selectivity).

Aim of the present project is to devise suitable photocatalysts able to perform HAT reactions under mild conditions induced by visible light irradiation and to develop synthetic strategies exploiting their activation mode.

Visible light absorbing inorganic (PolyOxoMetalates, POMs) and organic (aromatic ketones) photocatalysts will be investigated. A preliminary theoretical simulation (using the TD-DFT approach) will be aimed at finding the best POMs. Then, different structures (Lindqvist, decatungstate-like, Keggin and Wells-Dawson) based on V, Nb, Mo and W metal-centres (also including heteropolyanions) will be prepared, characterized and investigated in model photocatalytic reactions. Commercially or purposely functionalized (with sulfonic acid groups) aromatic ketones (anthraquinone, fluorenone, (thio)xanthone) will be likewise tested. The introduction of an acid functionality will allow their easy removal from the reaction mixture by simple aqueous extraction.

The obtained photocatalysts will be then used in the activation of (increasingly strong) C-H bonds (in aldehydes, oxygenated derivatives, amides and hydrocarbons) under visible light irradiation. The desired radicals will be then trapped by electron-poor olefins for the formation of valuable C-C bonds. A series of radical substitution reactions will be next tested. In detail, the reactions of photogenerated radicals with isonitriles (for cyanation reactions), with Selectfluor reagents (for fluorination reactions), with aromatics (for ipso-substitution reactions) will be pursued. Later, a variety of sulfonylated derivatives of general formula ArSO2-X (with X = CN, N3, F, CF3, alkynyl, ...), where the good radicofugal properties of the Ar-SO2· moiety will allow for the introduction of the X group in a position previously occupied by a hydrogen, will be carried out. In addition, heteroatom-centred radicals will be generated by cleavage of a X-H bond from silanes (containing at least one Si-H bond) and (carbene)-boranes (with at least one B-H bond) and exploited in conjugate radical addition reactions with electron-poor olefins.

Finally, the performance of the devised photocatalytic systems will be optimized by using flow conditions in view of a possible scale-up and the overall environmental performance of the synthetic procedures will be tracked and improved, having recourse to suitable Green Metrics tools.