Visible Light-Mediated Photochemical Reactions of 2-(2′-Alkenyloxy)cycloalk-2-enones

Abstract

The title compounds were prepared, and their reactivity was studied upon sensitized irradiation at λ = 420 nm. Thioxanthen-9-one was employed as the sensitizer at a loading of 10 mol % in small-scale reactions and of 2.5 mol % on a larger scale. Cyclohex-2-enones substituted by a 2′-propenyloxy, 2′-butenyloxy, 2′-pentenyloxy, or 2′-methyl-2′-propenyloxy group in the 2-position gave the products of an intramolecular [2 + 2] photocycloadditon. The reaction proceeded with high regioselectivity (crossed product) and perfect diastereoselectivity (nine examples, 34–99% yield). If the olefin in the tether was trisubstituted (3′-methyl-2′-butenyloxy), no cycloaddition was observed. Rather, a cyclization with subsequent hydrogen abstraction occurred (three examples, 65–86% yield). The results are consistent with a reaction course via a triplet enone intermediate and the formation of a 1,4-diradical by an initial cyclization. The analogous cyclopent-2-enones were less prone to an intramolecular reaction. Instead, decomposition or intermolecular [2 + 2] photocycloaddition reactions prevailed. In the latter event, two main products were identified (three examples, 30–43% yield), resulting either from a head-to-head [2 + 2]-photodimerization or from a twofold [2 + 2] photocycloaddition of the enone to the olefin. The latter reaction sequence generated pentacyclic products with a central [1,5]dioxocane ring. The structure assignment of the two product types was corroborated by a single-crystal X-ray analysis.

Enantioselective crossed intramolecular [2+2] photocycloaddition reactions mediated by a chiral chelating Lewis acid

“… 18 2-(Allyloxy)cyclohex-2-enone (4a) served as the test substrate which undergoes a clean crossed [2+2] photocycloaddition at λ = 420 nm to product rac -5a under sensitizing conditions. 12 If irradiated under the same conditions, there was no background reaction as verified in the present set of experiments by irradiation with a light emitting diode (LED) at λ = 425 nm. A first screen in different solvents ( Table 1 , entries 1–4) with a catalyst loading of 2 mol% gave a promising enantioselectivity (ee = enantiomeric excess) in dichloromethane (entry 4) at ambient temperature.…”

“…The compounds were readily available from the respective 1,2-diketones or α,β-epoxyketones (see the ESI† for details). 12,20 Scheme 2 summarizes the reactions which delivered the desired bridged tricyclic products 5 . We found substitution at the internal carbon atom of the tethered olefin (R 1 ) and at its terminal carbon atom (R 2 ) compatible with the reaction conditions.…”

“… 11 We found the reaction to be possible upon visible light irradiation ( λ = 420 nm) in the presence of thioxanthen-9-one (TXT) as the sensitizer and we studied the influence of substituents and of the enone ring size. 12 The oxygen substituent attached to the carbon atom in α-position to the carbonyl group represents a remarkable and distinct feature of the substrates. Since the formation of five-membered chelates is well established with bidentate Lewis acids, 13 we wondered whether it would be possible to employ a chiral Lewis acid for an enantioselective reaction of 2-(alkenyloxy)cyclohex-2-enones.…”

“…Limitations of the reactions are intrinsic to the photocycloaddition itself but not to the catalytic conditions. In previous work, 12 we had already seen that cyclopent-2-enones were not suitable for the reaction nor was it possible to obtain cyclobutanes from substrates with a two-fold substitution at the terminal carbon atom of the tethered olefin. A cyclohex-2-enone with a two-fold substitution at carbon atom C4 had in the racemic series already given a poor yield (38%) and failed to give any products under the catalytic conditions.…”

Recent Visible Light and Metal Free Strategies in [2+2] and [4+2] Photocycloadditions

“…A year later, thioxanthen‐9‐one (TX) was also investigated by Bach as triplet photosensitizer for the [2+2] photocycloaddition of a family of cyclohexenones 40 bearing a 2’‐propenyloxy, 2’‐butenyloxy, 2’‐pentenyloxy or 2’‐methyl‐2’‐propenyloxy group in the 2‐position [38] . Being these shorter tethers, crossed products 41 , rather than straight ones, were achieved in all cases with good yields and high regio‐ and diastereoselectivity.…”