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. 2022 Aug 22;27(16):5342.
doi: 10.3390/molecules27165342.

Photocatalytic Isomerization of (E)-Anethole to (Z)-Anethole

Marvin Korff  1   2 , Tiffany O Paulisch  1   3 , Frank Glorius  1   3 , Nikos L Doltsinis  4 , Bernhard Wünsch  1   2
Affiliations

Photocatalytic Isomerization of (E)-Anethole to (Z)-Anethole

Marvin Korff et al. Molecules. .

Abstract

Natural product (E)-anethole was isomerized to (Z)-anethole in a photocatalytic reaction. For this purpose, a self-designed cheap photoreactor was constructed. Among 11 photosensitizers (organo and metal complex compounds), Ir(p-tBu-ppy)3 led to the highest conversion. Triplet energies of (E)- and (Z)-anethole were predicted theoretically by DFT calculations to support the selection of appropriate photosensitizers. A catalyst loading of 0.1 mol% gave up to 90% conversion in gram scale. Further additives were not required and mild irradiation with light of 400 nm overnight was sufficient. As a proof of concept, (E)- and (Z)-anethole were dihydroxylated diastereoselectively to obtain diastereomerically pure like- and unlike-configured diols, respectively.

Keywords: (E/Z)-isomerization; DFT calculations; calculation of triplet energy; catalysis; diastereoselective dihydroxylation; green chemistry; natural product; photocatalysis; photosensitizer.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Energy transfer mechanism from a photosensitizer to a substrate in the photocatalyzed (E)-/(Z)-isomerization. (A): (a) The photosensitizer is excited by irradiation of light from the singlet ground state S0 into the S1 state; (b) the sensitizer relaxes from higher vibrational/rotational S1 states to the lowest S1 state; (c) ISC from S1 state to T1 state; (d) upon collision of the photosensitizer with the substrate, energy from the catalyst is transferred to the substrate in a coupled transition. (B) “Dexter Energy Transfer”: upon collision, an electron (sphere) in the T1 state of the sensitizer is transferred in the T1 state of the substrate, while an electron of the S0 state of the substrate is transferred to the S0 state of the sensitizer, yielding a relaxed sensitizer and an excited substrate.
Figure 2
Figure 2
(A) Mechanism of (E)- to (Z)-isomerization of anethole. By energy transfer of the excited photosensitizer (PS*), (E)-anethole is excited to the T1 state and reaches the global energy minimum by rotation about the former C=C double bond. Intersystem crossing (ISC) leads to the global maximum of the S0 state. Relaxation can now occur either to (E)- or (Z)-anethole. The backwards reaction from (Z)- to (E)-anethole occurs analogously. (B) The ratio of (E)-anethole: (Z)-anethole in the photostationary state is dependent on the ratio of spectral overlap (J) of photosensitizer with (E)-anethole (green area + green/orange striped area) to overlap of photosensitizer with (Z)-anethole (green/orange striped area).
Figure 3
Figure 3
Examples of approved drugs with a 1,2-disubstituted phenylalkane motif (orange).
Scheme 1
Scheme 1
Diastereoselective Upjohn dihydroxylation of (E)-anethole and (Z)-anethole yielding like- and unlike-configured diols like-2 and unlike-2.
See this image and copyright information in PMC

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