Mechanisms of the Cu(I)-Catalyzed Intermolecular Photocycloaddition Reaction Revealed by Optical and X-ray Transient Absorption Spectroscopies

Abstract

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/or steric interactions. We have used a combination of optical and X-ray transient absorption spectroscopies to elucidate the mechanism of the Cu(I)-catalyzed intermolecular photocycloaddition reaction using norbornene and cyclohexene as model substrates. We find that for norbornene the reaction proceeds through an initial metal-to-ligand charge transfer (MLCT) state that persists for 18 ns before the metal returns to the monovalent oxidation state. The Cu K-edge spectrum continues to evolve until ∼5 μs and then remains unchanged for the 50 μs duration of the measurement, reflecting product formation and ligand dissociation. We hypothesize that the MLCT transition and reverse electron transfer serve to sensitize the triplet excited state of one of the norbornene ligands, which then dimerizes with the other to give the product. For the case of cyclohexene, however, we do not observe a charge transfer state following photoexcitation and instead find evidence for an increase in the metal−ligand bond strength that persists for several ns before product formation occurs. This is consistent with a mechanism in which ligand photoisomerization is the initial step, which was first proposed by Salomon and Kochi in 1974 to explain the stereoselectivity of the reaction. Our investigation reveals how this photocatalytic reaction may be directed along strikingly disparate trajectories by only very minor changes to the structure of the substrate.

Intermolecular 2+2 Imine-Olefin Photocycloadditions Enabled by Cu(I)-Alkene MLCT

“…4). This is supported by previous DFT calculations which concluded an MLCT of idealized cationic Cu(I)-ethylene compounds was responsible for cyclobutane forming 2 + 2 photocycloadditions 35 and recent optical and X-ray transient absorption spectroscopic insights of Cu(I)mediated photodimerization of norbornene 36 . The selective imine C = N capture of this excited state affords the cycloadduct azetidine.…”

Spectroscopic Techniques to Unravel Mechanistic Details in Light‐Induced Transformations and Photoredox Catalysis

“…Moreover, as the atomic ionization energy (energy required to remove an electron from an atom) depends on the atomic number, XTAS allows the specific analysis of an element, which renders XTAS a powerful method to identify electronic configurational changes at specific metal centers induced by visible-light excitation. [33] This powerful technique allowed to study the excited state structure of light-absorbing molecules [34] as well as to decipher the mechanisms and kinetics of photoinduced CO 2 reduction, [35] water oxidation [36] and reduction [37] and organic synthetic [38] processes.…”

Aqueous Amine-Tolerant [2+2] Photocycloadditions of Unactivated Olefins

“…We then focused on applying our chemistry to pharmacologically relevant scaffolds (Scheme 1B). Since numerous medications are based on the tropane skeleton, 23 we thought it apt to synthesize a novel cyclobutane analogue (12). This was accomplished by nucleophilic allylation of nitrone 4 followed by reoxidation to nitrone 10 and intramolecular [3+2] cycloaddition upon heating to furnish 11.…”

“…For a productive [2+2] reaction, it has been proposed that (1) Cu(I) coordinates to the reacting olefins in the ground state, and (2) this complex undergoes metal-toligand charge transfer upon irradiation. 12 Both of these tasks seem highly improbable for a Cu(II) species. While it is unclear how the Cu(II) is reduced in this aqueous reaction, it is important to note that Cu(II)(OTf) 2 is known to work for standard Kochi−Salomon reactions in dry organic solvents.…”

“…Although we cannot exclude a Cu­(II)-catalyzed process, we presume that the cycloaddition initiates from an in situ formed Cu­(I) species. For a productive [2+2] reaction, it has been proposed that (1) Cu­(I) coordinates to the reacting olefins in the ground state, and (2) this complex undergoes metal-to-ligand charge transfer upon irradiation . Both of these tasks seem highly improbable for a Cu­(II) species.…”