Paterno-Buchi 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 1 and 2,limits the quantum yields for both photochemical processes in Table 4. The biradical nature of the distonic adduct in Scheme 3 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-Buchi coupling (by direct excitation of DB) can proceed via the same sequence of reactive intermediates.

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Journal of the American Chemical Society