60th Annual Report on Research 2015

Many powerful synthetic transformations proceed via organic radical intermediates. Recently, new strategies for accessing reactive organic radicals via visible light-induced electron transfer have allowed a variety of synthetic transformations to be achieved and improved under mild reaction conditions, circumventing harsh redox reagents or dangerous UV light. Our group is interested in 1) understanding the mechanistic details of photoredox catalysis and 2) developing new photosensitizers based for use in photoredox catalysis.

Over the last year, we have explored the application of copper bisphenanthroline complexes as photoredox catalysts. The metal-to-ligand charge transfer excited states of these complexes are potent reductants. In 1987, Kern and Sauvage reported the use of [Cu(dap)]⁺ (dap = 2,9-bis(p-anisyl)-1,10-phenanthroline) to catalyze the reductive dimerization of nitrobenzenes. The bulky dap ligand hinders excited-state distortion, thus preventing excited-state quenching via exciplex formation and increasing the quantum yield of the photosensitizer. Recently, Reiser reported the use of [Cu(dap)]⁺ as a photoredox catalyst for the atom transfer radical addition (ATRA) of organic halides to alkenes. A mechanism based on the oxidative quenching of [Cu(dap)]⁺* was proposed (Figure 1). Interested in the details of the reaction mechanism, we have utilized transient absorption spectroscopy to probe reaction intermediates and determine kinetics for elementary reaction steps. Our initial studies focused on the copper bisphenanthroline catalyzed ATRA of CBr₄ to styrene under visible light irradiation, as reported by Reiser.

We have determined that both  [Cu(dpp)]⁺ (dpp = 4,7-diphenyl-1,10-phenanthroline) and [Cu(dsbtmp)]⁺ (dsbtmp = 2,9-bis(sec-butyl)-3,4,7,8-tetramethyl-phenanthroline) promote the ATRA of CBr₄ to styrene with comparable conversion yields to the reported [Cu(dap)]⁺ system. Photoluminescence quenching experiments were utilized to evaluate the quenching of these copper photosensitizers by CBr₄.Second order rate constants of 4.4 x 10⁸ M^-1 s^-1 and 4.0 x 10⁹ M^-1 s^-1 for [Cu(dpp)]⁺ and [Cu(dsbtmp)]⁺, respectively, were determined from Stern-Volmer analyses (Figure 2). The difference in rate constants is consistent with difference in E°'(Cu^2+/+*) for the two photosensitizers.

We confirmed that the presence of styrene does not influence the excited state electron transfer from the copper photosensitizer to CBr₄. However, the presence of styrene influences the recovery kinetics of the Cu⁺ bleach, indicating that the Cu²⁺ species formed can oxidize a reaction intermediate. Further, we have observed the net loss of the Cu⁺ photosensitizer during the course of irradiation. UV-Vis absorption and photoluminescence spectra also reveal the appearance of new species. In our ongoing work, we are investigating the identity of these reaction byproducts of intermediates and their role in the reaction cycle.

These experiments are revealing new details of photoredox catalysts. We anticipate they will help us identify means by which visible-light driven catalytic reactions can be optimized and substrate scopes expanded.  

Figure 1. Proposed mechanism of the Cu bisphenanthroline catalyzed ATRA reaction of CBr₄ to styrene. Adapted from Reiser and coworkers, Chem. Eur. J., 2012, 18, 7336–7340.

Figure 2. First order rate constants determined from excited-state quenching of [Cu(dsbtmp)₂]⁺ in the presence of CBr₄. The linear relationship between k_obs and [CBr₄] afforded the second order rate constant for electron transfer, k_q.