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

• Published in: European journal of organic chemistry 2017-04, Vol.2017 (15), p.2164-2169
• Main Authors: Martinez‐Haya, Rebeca, Miranda, Miguel A., Marin, M. Luisa
• Format: Article
• Language: English
• Subjects: Absorption spectra; Acetophenone; Amines; Anions; Benzyl bromide; Bromides; Electron transfer; Hydrogen; Light irradiation; Oxidation; Photocatalysis; Photophysics; Quenching; Reactive intermediates; Redox chemistry; Reduction (metal working); Riboflavin; Thermodynamics; Time‐resolved spectroscopy; Toluene; Viability
• ISSN: 1434-193X; 1099-0690
• DOI: 10.1002/ejoc.201601494

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 α‐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 (1RF* and 3RF*) was analyzed by time‐resolved techniques. The quenching of 1RF* 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 1RF* 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. The reductive dehalogenation of organic bromides has been achieved by using riboflavin (RF) as photocatalyst under visible‐light irradiation. The viability of the different competing processes on the timescale of the corresponding RF excited states and RF radical anion were evaluated by time‐resolved techniques. Thus, the key role of the RF radical anion has been proven.