Development of new eco-aware asymmetric catalytic processes; Photocatalyzed oxidation with water as the unique oxygen atom source. – Water Act

The development of dyads to perform photocatalytic reactions requires the sound selection of both main partners involved in the process: i) A light-absorbing photosensitizing Ru(bpy)3-like fragment known to an efficient chromophore and ii) a catalytic reaction center (Ru(tpy)(bpy)-OH2) known to achieve organic substrate oxidation through its ruthenium-oxo form.

Two differents dyads were reported by our group. Both were capable of using light to activate water molecule in order to perform selective sulfide oxygenation into sulfoxide via an oxygen atom transfer from H2O to the substrate. On the basis of electrochemical and photophysical studies, a proton-electron transfer process yielding to an oxidant Ru(IV)=O species was proposed.

An optimisation of the photocatalytic efficiency of the catalyst has to be performed. This involves the design of a more stable photocatalyst as well as more efficient electron-transfers from the catalytic subunit to the sacrificial electron transfer via the photosensitizer.

O. Hamelin, P. Guillo, F. Loiseau, M. F. Boissonnet, S. Ménage, Inorg . Chem. 2011, 50, 7952

In the last decades, as a consequence of the inevitable end of fossil energy resources associated with a high demand on sustainable chemistry, attempts to develop “green solutions” have emerged all over the word. As a consequence, tremendous efforts were made to take advantage of the exceptional photophysical properties of ruthenium polypyridyl complexes with the final objective to convert solar energy into chemical energy. In our research program aiming to develop new polypyridyl ruthenium-based catalysts for oxidation, we are interested in the combination of a photosensitizer and a catalytic fragment within the same complex to achieve catalytic light-driven oxidation. As far as we know, only one paper, published in 2009, reported the application of such a system for alcohol oxidation (Rocha et al. Angew. Chem. Int. Ed. 2009, 48, 9672). However, on the contrary to the alcohol oxidation which requires “only” a proton-coupled electron transfer (PCET), sulfides, alkenes and alkanes oxidation requires also an oxygen atom transfer. Due to the lack in such a field, we have tackled a new eco-aware catalytic system able to perform the oxidation of sulfides via an oxygen atom transfer from H2O to the substrate. This approach avoids the use of classical (and sometimes relatively toxic and/or hazardous) oxidants such as peroxides and peracids. In this proposal, we envisage to introduce a third properties of octahedral ruthenium complexes, still poorly exploited, that is to say their particular kind of chirality (Chirality Delta/Lambda) in order to develop new “eco-aware” methods for asymmetric catalytic light-driven oxidation using water as the unique oxygen atom source.
The originality of the proposed project relies on four main points:
i) Water, as an abundant and non-toxic molecule, will be used as the unique oxygene atom source,
ii) Whereas all the metal-dependant asymmetric catalyses involve chiral organic ligands, in this project, the metal will be the unique stereogenic center (Chirality Delta/Lambda). This will be our priority. However, the use of chiral organic ligands will also be explored.
iii) The photosensitizer, the chiral inducer and the catalytic center will be associated within a unique catalyst.
iv) Solar (light) energy will be converted into chemical energy.

Consequently, this project involves the synthesis of chiral dyads in which one ruthenium center will be the catalytic center and the second one will be at the same time the photosensitizer and the chiral inducer. The reactivity of the catalysts will be tuned by appropriate structural and electronic modifications achieved on the ligands determined thanks to photophysical studies performed by the group of Pr. Frédérique Loiseau, Partner 2.