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

Abstract

AbstractΕniminium 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).

Intramolecular [2+2] Photocycloaddition of Cyclic Enones: Selectivity Control by Lewis Acids and Mechanistic Implications

“…R f =0.58 (pentane/Et 2 O 1:1) [KMnO 4 ]; [ α ] D 25 =+155 ( c= 1.1 in CH 2 Cl 2 ); 1 H NMR (500 MHz, CDCl 3 , 25 °C, TMS): δ =1.06 (s, 3 H, Me‐3a), 1.29–1.36 (m, 1 H, H H ‐ 3), 1.36–1.42 (m, 1 H, H H ‐ 1), 1.45 (ddd, 2 J =13.7 Hz, 3 J 1 =9.1 Hz, 3 J 2 =4.2 Hz, 1 H, H H ‐ 8), 1.57–1.63 (m, 1 H, H H ‐3), 1.71–1.83 (m, 4 H, H H ‐1, H‐2, H H ‐8), 1.86 (ddd, 2 J =12.7 Hz, 3 J =7.2 Hz, 4 J =1.4 Hz, 1 H, H H ‐ 4), 1.88–1.98 (m, 2 H, H‐7), 2.01 (dd, 2 J =12.7 Hz, 3 J =11.0 Hz, 1 H, H H ‐4), 2.21–2.28 (m, 1 H, H H ‐ 6), 2.37–2.44 ppm (m, 2 H, H‐4a, H H ‐6); 13 C NMR (126 MHz, CDCl 3 , 27 °C, TMS): δ =22.0 (t, C‐7), 22.9 (q, Me‐3a), 23.9 (t, C‐2), 29.5 (t, C‐8), 35.1 (t, C‐4), 40.0 (t, C‐6), 41.7 (t, C‐1), 42.0 (t, C‐3), 44.6 (s, C‐3a), 45.3 (d, C‐4a), 51.3 (s, C‐8a), 216.5 ppm (s, C‐5); Chiral GC: τ R (minor)=131.9 min, τ R (major)=136.7 min, [60 °C (1 min), 100 °C (30 °C min −1 ), 100 °C (157 min), 135 °C (3 °C min −1 ), 200 °C (20 °C min −1 ), 200 °C (3 min)], Cyclosil‐B. The analytical data obtained matched those reported in the literature …”

“…td, 2 J ≈ 3 J 1 =12.2 Hz, 3 J 2 =7.0 Hz, 1 H, H H ‐ 1), 1.62 (virt. ddt, 2 J =12.7 Hz, 3 J =6.2 Hz, 4 J 1 ≈ 4 J 2 =1.5 Hz, 1 H, H H ‐3), 1.69–1.84 (m, 3 H, H‐2, H H ‐ 4), 1.91 (d, 2 J =14.2 Hz, 1 H, H H ‐8), 1.93–1.99 (m, 1 H, H H ‐1), 2.02 (dd, 2 J =12.5 Hz, 3 J =11.2 Hz, 1 H, H H ‐4), 2.12 (ddd, 2 J =16.1 Hz, 4 J 1 =2.5 Hz, 4 J 2 =1.3 Hz, 1 H, H H ‐ 6), 2.21 (dd, 2 J =16.1 Hz, 4 J =0.8 Hz, 1 H, H H ‐6), 2.36 ppm (ddd, 3 J 1 =11.2 Hz, 3 J 2 =7.7 Hz, 4 J =1.3 Hz, 1 H, H‐4a); 13 C NMR (126 MHz, CDCl 3 , 27 °C, TMS): δ =24.3 (q, Me‐3a), 24.3 (t, C‐2), 28.0 (q, Me‐7), 31.8 (q, Me‐7), 33.9 (s, C‐7), 34.8 (t, C‐4), 40.8 (t, C‐3), 42.8 (t, C‐8), 43.7 (d, C‐4a), 44.0 (t, C‐1), 45.4 (s, C‐3a), 50.0 (s, C‐8a), 52.7 (t, C‐6), 216.2 ppm (s, C‐5); Chiral GC: τ R (major)=94.3 min, τ R (minor)=95.0 min, [60 °C (0.5 min), 70 °C (10 °C min −1 ), 114 °C (0.4 °C min −1 ), 200 °C (10 °C min −1 ), 200 °C (3 min)], Lipodex E. The analytical data obtained matched those reported in the literature …”

“…After purification by column chromatography (silica, P/Et 2 O = 5:1), ketone 6f ( The analytical data obtained matched those reported in the literature. [49] Photocycloaddition product 6g: Racemic:F ollowing GP1, enone 5g (38.5 mg, 200 mmol, 1.00 equiv) was irradiated in dichloromethane (10 mL) for 5.5 h. After purification by column chromatography (silica, P/Et 2 O = 6:1), ap roduct mixture was obtained, which contains inseparable impurities. To facilitate purification, the mixture was submitted to ozonolysis which was conducted at À78 8C in dichloromethane (3 mL).…”

“…After purification by column chromatography (silica, P/Et 2 O = 6:1), ketone 6i ( 34.8 (t, C-4), 40.8 (t, C-3), 42.8 (t, C-8), 43.7 (d, C-4a), 44.0 (t, C-1), 45.4 (s, C-3a), 50.0 (s, C-8a), 52.7 (t, C-6), 216.2 ppm (s, C-5);C hiral GC: t R (major) = 94.3 min, t R (minor) = 95.0 min, [60 8C( 0.5 min), 70 8C( 10 8Cmin À1 ), 114 8C( 0.4 8Cmin À1 ), 200 8C( 10 8Cmin À1 ), 200 8C( 3min)],L ipodex E. The analytical data obtained matched those reported in the literature. [49]…”

Chromophoraktivierung von α,β‐ungesättigten Carbonylverbindungen und ihre Anwendung in enantioselektiven Photoreaktionen

“…Allerdings ist das ISC von S 1 nach T 1 fürI miniumionen ineffizient, [25] und die bekannte Photochemie spielt sich auf der Singulett-Hyperfläche ab.I nu nserem Arbeitskreis wurde untersucht, ob Iminiumionen eine Chromophoraktivierung ermçglichen, die darauf beruht, dass der Tr iplettzustand eines a,b-ungesättigten Iminiumions (Eniminiumions) tiefer liegt als jener der entsprechenden Carbonylverbindung.D ie Tr iplettenergie des Enons 61 konnte durch Phosphoreszenzspektren bestimmt werden und betrug in Einklang mit analogen Verbindungen [66] 289 kJ mol À1 . 253 kJ mol À1 noch eine intramolekulare [2+ +2]-Photocycloaddition des Substrats 62 ermçglichten, solche mit E T (sens) 245 kJ mol À1 aber nicht mehr,l ieß sich die Tr iplettenergie E T ' abschätzen: [60] Sie ist etwa 40 kJ mol À1 niedriger als die der Carbonylverbindung (Schema 29). 253 kJ mol À1 noch eine intramolekulare [2+ +2]-Photocycloaddition des Substrats 62 ermçglichten, solche mit E T (sens) 245 kJ mol À1 aber nicht mehr,l ieß sich die Tr iplettenergie E T ' abschätzen: [60] Sie ist etwa 40 kJ mol À1 niedriger als die der Carbonylverbindung (Schema 29).…”