Developing less polluting and more selective processes has become one of the most important objectives in the field of organic synthesis. For this reason, photocatalysis has grown exponentially as one of the most promising tools to achieve such a task.[1] In this work, two different photocatalytic approaches have been carried out. The development of new platinum(II) complexes as photocatalysts in different organic transformations and the discovery of a new process to synthesize tetrahydroquinolines.
On one hand, the synthesis of different photocatalytic platinum(II) complexes is presented and their versatility as photocatalysts in different organic synthetic reactions was demonstrated.[2] Platinum(II) complex with a sulphur donor atom ligand was found to have higher activity than the analogous with an oxygen donor atom. We studied such a Pt(II) complexes in the oxidation of boronic acids to form alcohols, the cross-dehydrogenative coupling of tetrahydroisoquinolines and the enantioselective alkylation of aldehydes. In addition, mechanistic studies were performed by theoretical calculations regarding the structure of the different platinum(II) complexes. Hence, the Pt(II) complex with the sulphur ligand presents a larger ability to couple the bright S1 state with the T1 one, which would explain its higher activity in comparison with the O-bonded Pt(II) complex.
On the other hand, the well-known photocatalyst Ir(ppy)3 was used to develop a new photocatalytic method for the synthesis of tetrahydroquinolines by intramolecular radical cyclization.[3] We carried out the reaction synthesizing tetrahydroquinolines with different substituents at the nitrogen group and the aromatic ring, obtaining complete selectivity towards the 6-exo-trig form. Furthermore, the methodology allowed to perform the cyclization without protecting the amino group, which is a limiting factor in the five-membered cyclization reaction previously reported. In addition, this type of reactions requires low concentrations, therefore, their development on a larger batch scale in a photocatalytic fashion is complicated. For this reason, we expanded this methodology within flow conditions, which allowed shortening the reaction time and being able to synthesize the tetrahydroquinolines on a larger scale.
References:
1. C. K. Prier, D. A. Rankic, D. W. C. MacMillan. Chem. Rev. 2013, 113, 7, 5322-5363. 10.1021/cr300503r.
2. A. Casado-Sánchez, M. Uygur, D. González-Muñoz, F. Aguilar-Galindo, J.L. Nova-Fernández, J. Arranz-Plaza, S. Díaz-Tendero, S. Cabrera, O. García Mancheño, J. Alemán. J. Org. Chem. 2019, 84, 6437-6447. 10.1021/acs.joc.9b00520.
3. D. González-Muñoz, J.L. Nova-Fernández, A. Martinelli, G. Pascual-Coca, S. Cabrera, J. Alemán. Eur. J. Org. Chem. 2020, 5995-5999. 10.1002/ejoc.202001018.