Paterno−Büchi Coupling of (Diaryl)acetylenes and Quinone via Photoinduced Electron Transfer

Photoinduced coupling of an acetylene with a quinone in two wavelength regions (λ_DB and λ_CT) can be regioselective to yield a single quinone methide adduct when various diarylacetylenes (DA) and 2,6-dichlorobenzoquinone (DB) are used. Thus, the direct photoexcitation of DB at λ_DB = 355 nm or the specific activation of the 1:1 electron donor−acceptor complex [DA,DB] at λ_CT = 532 nm both lead to the transient ion-radical pair [DA^•+,DB^•-], which is established by time-resolved (ps,ns) spectroscopy. Competition between back electron transfer (k_BET) and ion-radical pair collapse (k_C) to the distonic adduct DA-DB, as described in Schemes and , limits the quantum yields for both photochemical processes in Table . The biradical nature of the distonic adduct in Scheme accommodates the various facets of acetylene reactivity and unique regioselectivity to yield the same quinone methide by both actinic processes. In a more general context, the electron-transfer mechanism established by the charge-transfer excitation of [DA,DB] provides compelling evidence that the Paterno−Büchi coupling (by direct excitation of DB) can proceed via the same sequence of reactive intermediates.