Evidence for Triplet Sensitization in the Visible‐Light‐Induced [2+2] Photocycloaddition of Eniminium Ions

• Published in: Angewandte Chemie International Edition Vol. 57; no. 3; pp. 827 - 831
• Main Authors: Hörmann, Fabian M., Chung, Tim S., Rodriguez, Elsa, Jakob, Matthias, Bach, Thorsten
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 15-01-2018; John Wiley and Sons Inc
• Edition: International ed. in English
• Subjects: Carbonyl compounds; Carbonyls; Catalysis; Catalysts; Communication; Communications; cycloaddition; Enantiomers; enantioselectivity; Energy transfer; enones; homogenous catalysis; Intermediates; Iridium; Irradiation; photochemistry; Ruthenium; enantioselectivity; homogenous catalysis; cycloaddition; photochemistry; enones
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201710441

Εniminium ions were prepared from the corresponding α,β‐unsaturated carbonyl compounds (enones and enals), and were found to be promoted to their respective triplet states by energy transfer. The photoexcited intermediates underwent intra‐ or intermolecular [2+2] photocycloaddition in good yields (50–78 %) upon irradiation at λ=433 nm or λ=457 nm. Iridium or ruthenium complexes with a sufficiently high triplet energy were identified as efficient catalysts (2.5 mol % catalyst loading) for the reaction. The intermolecular [2+2] photocycloaddition of an eniminium ion derived from a chiral secondary amine proceeded with high enantioselectivity (88 % ee).
The energy is the limit: The [2+2] photocycloaddition of iminium ions 1 is catalyzed by iridium or ruthenium complexes and proceeds via triplet energy transfer. The intermediate iminium ions 2 are hydrolyzed to the respective carbonyl compounds such as aldehyde 3, which was formed in 78 % yield from a chiral eniminium ion precursor.