Intramolecular Cycloadditions of Photogenerated Azaxylylenes: An Experimental and Theoretical Study

Abstract

The mechanism of intramolecular cycloadditions of azaxylylenes photogenerated via excited-state intramolecular proton transfer (ESIPT) in aromatic o-amido ketones and aldehydes bearing unsaturated functionalities was studied experimentally and computationally. In time-correlated single-photon counting experiments, no relation was found between lifetimes of singlet species and the nature of the amide pendant, either unsaturated furanpropanamide, capable of photocyclization, or the acetamide control. Steady-state emission for amido-tetralone derivatives showed comparable dual emission bands, but bromo substitution decreased the intensity of the ESIPT band. The most reactive derivatives of amidobenzaldehydes were virtually lacking the ESIPT band. The quantum yield of cycloaddition is decreased in the presence of triplet quenchers, O2 or trans-piperylene, and improved with heavy atom substitution in the aromatic ring, providing further evidence for the initial mechanistic hypothesis in which the fast singlet-state ESIPT is accompanied by the ISC in the tautomer (azaxylylene), which undergoes stepwise addition to the tethered unsaturated pendants.

Chirality and Excited State Proton Transfer: From Sensing to Asymmetric Synthesis

“…Therefore, DCM was selected as the optimum solvent because it allowed for cooling below 0 °C. Due to the high energy absorption of the oxazolines group (<330 nm), a triplet sensitizer was used to allow for 365 nm excitation, sensitized generation of the aza‐o‐xylylene triplet excited state, and subsequent reaction from the triplet manifold . From the optimization studies, the highest ee (90 %) was obtained for the [4+4] product of 45a in DCM at −78 °C with 10 mol % of the triplet sensitizer, 4,4′‐dimethoxybenzophenone, with a 51 % isolated yield.…”

Studies in organic and physical photochemistry – an interdisciplinary approach

“…In contrast to the 23 [4+2] cycloaddition, a concerted [4+4] cycloaddition at the singlet ground state is forbidden according to the Woodward-Hoffmann rules 53. In order to get a deeper insight into the mechanism, intensive photo-physical and theoretical studies have been carried out 54. Based on the results the following mechanism has been suggested (Scheme 13).…”

Photoassisted Diversity-Oriented Synthesis: Accessing 2,6-Epoxyazocane (Oxamorphan) Cores

“…20 According to our recent experimental and theoretical mechanistic study, 21 it is likely that the intramolecular cycloaddition of azaxylylenes photogenerated via ESIPT occurs in a stepwise manner in the triplet manifold, thus offering a rationale for the formation of both [4 + 4] and [4 + 2] cycloadducts. From our prior work we infer that the ratio of [4 + 4] to [4 + 2] products is affected by the length of the tether between the azaxylylene precursor and the nature of unsaturated pendants, with longer linkers often favoring [4 + 4] products.…”