Title
Enhanced asymmetric photocycloaddition of anthracene tethered to maleate versus fumarate through non-fluorescent exciplex intermediate
Author
Makoto Ichikawa, Yoshihisa Inoue, Tadashi Mori
Year
2016
Journal
Journal of Photochemistry and Photobiology A: Chemistry
Abstract
The asymmetric photocycloaddition of anthracene-tethered maleate with a small (S)-1-methylpropyl auxiliary at the peripheral position (1Z) underwent much faster than that of the corresponding fumarate (1E). The absolute configuration of the newly created chiral center in the major product was ambiguously assigned as (4S) by X-ray crystallographic as well as CD spectral studies. The facile E–Z photoisomerization was simultaneously observed for both 1Z and 1E, again much faster for 1Z. The concurrent electron transfer works as a quenching process and the fumarate acted as a better quencher than maleate in both intramolecular and intermolecular systems. The fluorescence quenching and quantum yield investigations also supported such facile electron transfer from excited-state anthracene to acceptor in the singlet manifold. Observed diastereoselectivities in the photocycloaddition of 1Z and 1E were moderate but substantial (5–12% de), given the fact that the chiral auxiliary is introduced at the remote position from the reaction center (4-bond separation). The non-fluorescent exciplex, probably in a triplet manifold derived from the singlet exciplex, plays a crucial role in this efficient cyclization process, affording a biradical intermediate, that can either dissociate or undergo second bond formation to afford a diastereomeric trans adduct 2. The diastereoselectivity was higher in less polar solvents, suggesting the polar nature of the exciplex.
Instrument
FP-8500
Keywords
Fluorescence, Quantum yield, Organic chemistry