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

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.