The [2 + 2] photocycloaddition provides a simple, single-step route to cyclobutane moieties that would otherwise be disfavored or impossible due to ring strain and steric interactions. This approach has been used in many classic syntheses, including those of cubane and caryophyllene. While the copper-catalyzed route broadens the scope of this reaction by providing a means of transforming substrates that lack visible or near-UV absorption bands, it also brings its own set of limitations. To tackle these shortcomings, we have begun an effort to elucidate the mechanism of the photocycloaddition process via copper K-edge transient absorption spectroscopy, with the copper-catalyzed photodimerization of norbornene serving as a model system. By tracking the evolution of the copper K-edge spectrum across six decades in time following photoexcitation, we successfully identified three key reaction intermediates that appear on the picosecond, nanosecond, and microsecond timescales. At 100 ps, the XANES difference spectrum is consistent with a divalent copper intermediate, thus ruling out the previously favored LMCT-initiated photocupration mechanism. Within 150 ns, the copper returns to the monovalent oxidation state, but the spectrum continues to evolve until 1 μs, at which point it remains unchanged for the duration of the measurement. This final spectrum reflects a long-lived Cu(I) intermediate with a coordination environment distinct from that of the starting bis(norbornene) complex. We are also concurrently investigating intramolecular copper-catalyzed photocycloaddition reactions, which necessarily proceed from a ligand geometry distinct from that in the intermolecular case. Our goal is to provide clear pictures of synthetically significant photocatalytic reaction mechanisms to allow chemists to design next-generation catalysts capable of driving these reactions for a wide range of previously inaccessible substrates.

Mechanistic insights into the copper-catalyzed photocycloaddition reaction by optical and X-ray transient absorption spectroscopy

ACS Fall 2020 Virtual Meeting

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