Theoretical survey of the photochemical deracemization mechanism of chiral allene 3-(3,3-dimethyl-1-buten-1-ylidene)-2-piperidinone

Abstract

The question of how a racemate can be selectively converted into a single enantiomer is a challenging and fundamental scientific issue of chemistry. Recently, an unprecedented catalytic enantioselective photochemical transformation that enables the deracemization of chiral allene 3-(3,3-dimethyl-1-buten-1-ylidene)-2-piperidinone (rac-1a, 1a/ent-1a = 50/50, 0% ee) and their analogues was realized via a light-driven process in the presence of photosensitizer thioxanthone 2 by Bach's group (A. Hölzl-Hobmeier, A. Bauer, A. V. Silva, S. M. Huber, C. Bannwarth and T. Bach, Catalytic deracemization of chiral allenes by sensitized excitation with visible light, Nature, 2018, 564, 240–243). However, the deracemization mechanism has not been explored deeply either in experiment or in theory. In this work, the photosensitized deracemization mechanism was theoretically investigated by using a density functional theory (DFT) method. The calculation results show that, firstly, photosensitizer 2 (S0 state) is vertically excited to its S2 state under irradiation at the wavelength of λ = 420 nm. Then, a rapid internal conversion of S2 → S1 would occur near the crossing point during the relaxation of the S2 state. Consequently, the T1 state of photosensitizer 2 can be easily formed from its S1 state via an intersystem crossing. After that, a Dexter-type energy transfer from the T1 of photosensitizer 2 to the S0 state of ent-1a would take place, resulting in the S0 state of photosensitizer 2 and T1 of ent-1a. Finally, the T1 of ent-1a can cross the triplet potential surface and convert to a transition state between ent-1a and 1a of the ground state S0, and ultimately form 1a/ent-1a (50/50).