Metal-Free Photocatalytic Reductive Dehalogenation Using Visible-Light: A Time-Resolved Mechanistic Study

This is the peer reviewed version of the following article: Martínez-Haya, Rebeca, Miranda Alonso, Miguel Ángel, Marín García, Mª Luisa. (2017). Metal-Free Photocatalytic Reductive Dehalogenation Using Visible-Light: A Time-Resolved Mechanistic Study.European Journal of Organic Chemistry, 15, 2164-2169, which has been published in final form at http://doi.org/10.1002/ejoc.201601494. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving[EN] The reductive dehalogenation of organic bromides has been achieved in the presence of riboflavin (RF) as photocatalyst under visible-light irradiation. Specifically, benzyl bromide (2) and -bromoacetophenone (3) were quantitatively converted into toluene and acetophenone, respectively, by using amines as electron donors and iPrOH as hydrogen donor, whereas bromobenzene (1) did not react. The thermodynamics of the reduction of the radical anion of RF were evaluated by using the redox potentials of the species involved: The reaction was found to be thermodynamically exergonic for 2 and 3, but not expected to occur for bromobenzene (1). The viability of the different competing processes on the timescales of the corresponding singlet and triplet RF excited states ((RF)-R-1* and (RF)-R-3*) was analyzed by time-resolved techniques. The quenching of (RF)-R-1* by amines was very efficient, and comparison of the transient absorption spectra recorded in the absence and presence of amines additionally confirmed the efficient redox process between (RF)-R-1* and the amines. Moreover, RF- was quenched by bromides 2 and 3, but not by 1. Thus, a deeper understanding of the overall mechanism of the photocatalytic reductive reaction has been achieved, and the key role of the radical anion of the photocatalyst has been demonstrated.R. M.-H. acknowledges generous support from the Spanish Government (grant SEV-2012-0267). We also thank for support from the Ministerio de Economia y Competitividat (Project CTQ2012-38754-C03-03), the Generalitat Valenciana (Prometeo Program) and the VLC/Campus.Martínez-Haya, R.; Miranda Alonso, MÁ.; Marín García, ML. (2017). Metal-Free Photocatalytic Reductive Dehalogenation Using Visible-Light: A Time-Resolved Mechanistic Study. European Journal of Organic Chemistry. (15):2164-2169. https://doi.org/10.1002/ejoc.201601494S216421691

A highly reducing metal-free photoredox catalyst: design and application in radical dehalogenations

Here we report the use of 10-phenylphenothiazine (PTH) as an inexpensive, highly reducing metal-free photocatalyst for the reduction of carbon-halogen bonds via the trapping of carbon-centered radical intermediates with a mild hydrogen atom donor. Dehalogenations were carried out on various substrates with excellent yields at room temperature in the presence of air

Strategies for Next-Generation Functional Polymer Synthesis using Light

The development of new synthetic platforms for the preparation of small molecule and polymeric materials represents a research area of tremendous value for modern day society. Progress in chemical synthesis has enabled innumerable advances in areas ranging from biodegradable plastics to new pharmaceuticals and drug delivery technologies. Importantly, there has been a concerted effort to ensure that many of these novel synthetic processes are realized using “green” methodologies. This includes using non-toxic reagents, readily available solvents and abundant stimuli such as visible light. In recent years, there has been a resurgence of the use of light in chemical synthesis as an enabling platform to unlock exceedingly mild chemical pathways otherwise not accessible using more conventional thermally driven processes. In the context of macromolecular synthesis, light mediated controlled radical polymerization has emerged as a powerful strategy, offering an added dimension of control over reaction kinetics, macromolecular sequence and overall material composition. Herein, I would like to showcase the implementation of light in a few cutting-edge areas of polymer synthesis, including spatial patterning of surface-tethered polymer brushes, facile polymer chain-end modification, easy access to functional semi-fluorinated and degradable polymers, and the synthesis of multifunctional, temperature tunable materials. The aforementioned topics all rely on the simplicity of external regulation using light, and provide proof-of-concept utility that will create both synthetic and application-driven opportunities across the broader chemical sciences

Strategies for Next-Generation Functional Polymer Synthesis using Light

The development of new synthetic platforms for the preparation of small molecule and polymeric materials represents a research area of tremendous value for modern day society. Progress in chemical synthesis has enabled innumerable advances in areas ranging from biodegradable plastics to new pharmaceuticals and drug delivery technologies. Importantly, there has been a concerted effort to ensure that many of these novel synthetic processes are realized using “green” methodologies. This includes using non-toxic reagents, readily available solvents and abundant stimuli such as visible light. In recent years, there has been a resurgence of the use of light in chemical synthesis as an enabling platform to unlock exceedingly mild chemical pathways otherwise not accessible using more conventional thermally driven processes. In the context of macromolecular synthesis, light mediated controlled radical polymerization has emerged as a powerful strategy, offering an added dimension of control over reaction kinetics, macromolecular sequence and overall material composition. Herein, I would like to showcase the implementation of light in a few cutting-edge areas of polymer synthesis, including spatial patterning of surface-tethered polymer brushes, facile polymer chain-end modification, easy access to functional semi-fluorinated and degradable polymers, and the synthesis of multifunctional, temperature tunable materials. The aforementioned topics all rely on the simplicity of external regulation using light, and provide proof-of-concept utility that will create both synthetic and application-driven opportunities across the broader chemical sciences