Erlangen 2022 – scientific programme

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MO: Fachverband Molekülphysik

MO 11: Photochemistry I

MO 11.4: Talk

Wednesday, March 16, 2022, 11:15–11:30, MO-H7

Mechanistic Studies on a Deracemization Reaction via a Triplet 1,3-Diradical Induced by Energy Transfer from a Chiral Sensitizer — •Roger Jan Kutta¹, Xinyao Li², Christian Jandl², Andreas Bauer², Thorsten Bach², and Patrick Nuernberger¹ — ¹Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Germany — ²Lehrstuhl für Organische Chemie I, Technische Universität München, Germany

The photochemical deracemization[1] of spiro[cyclopropane-1,3’-indolin]-2’-ones (spirocyclopropyl oxindoles) was investigated by time-resolved spectroscopies and computational approaches. The corresponding 2,2-dichloro compound is configurationally labile upon direct irradiation and when sensitized by excited achiral thioxanthen-9-one. In the latter reaction the triplet 1,3-diradical intermediate is generated via triplet energy transfer from the photosensitizer.

Deracemization is achieved by using a chiral thioxanthone photosensitizer with a lactam hydrogen bonding site. Here, three factors co-act favorably for high enantioselectivity: i) a factor 3 differing binding constants to the chiral thioxanthone for the two enantiomers. ii) unequal molecular distances in the complexes, presumably lead to differing energy transfer efficiencies. iii) the 1,3-diradical lifetime exceeds the complex lifetime, facilitating a racemic deactivation back to the ground state.[2]

[1] Nature 2018, 564, 240-243.

[2] Angew. Chem. Int. Ed. 2020, 59, 21640-21647.