Arene Activation through Iminium Ions: Product Diversity from Intramolecular Photocycloaddition Reactions

Abstract

While 2-alk-ω-enyloxy-sustituted benzaldehydes do not display any photochemical reactivity at the arene core, the respective iminium perchlorates were found to undergo efficient reactions either upon direct irradiation (λ = 366 nm) or under sensitizing conditions (λ = 420 nm, 2.5 mol% thioxanthen-9-one). Three pathways were found: (a) Most commonly, the reaction led to benzoxacyclic products in which the olefin in the tether underwent a formal, yet unprecedented carboformylation (13 examples, 44-99 % yield). The cascade process occurred with high diastereoselectivity and was found to be stereoconvergent. (b) If a substituent resides in the 3-position of the benzene ring, a meta photocycloaddition was observed which produced tetracyclic skeletons with five stereogenic centers in excellent regioand diastereoselectivity (2 examples, 58-79 % yield). (c) If the tether was internally substituted at the alkene, an arene photocycloaddition was avoided and an azetidine was formed in an aza Paternò-Büchi reaction (2 examples, 95-98 % yield).

Energy transfer photocatalysis: exciting modes of reactivity

“…The authors majorly focussed on the formation of benzoxepanes 22 that are produced via arene-alkene photocycloaddition followed by multiple rearrangement steps. 79 A combination of the previous two concepts has also been reported by the same group. 80…”

“…The authors majorly focussed on the formation of benzoxepanes 22 that are produced via arene-alkene photocycloaddition followed by multiple rearrangement steps. 79 A combination of the previous two concepts has also been reported by the same group. 80 Although there are numerous exciting reports on the intermolecular version of EnT-mediated photocycloaddition, two case studies from Schindler's and Yoon's groups introduced diversity in the selection of photosensitizable substrates (Scheme 3C).…”

Rejuvenation of dearomative cycloaddition reactions via visible light energy transfer catalysis

“…6 ). 61 Olefin-tethered aryl iminium ions 6.1 produced the [2 + 2] cycloadducts 6.2 in the presence of 2.5 mol% thioxanthone (TXT, PS-8, E T = 63.1 kcal mol −1 ) under 420 nm blue light irradiation for 18 h. Cycloadducts 6.2 undergo iminium ion-triggered rearrangement to produce cyclized products 6.3. The rearranged products 6.4 were isolated after treating the reaction mixture with aq.…”

Chiral Lewis acid catalysis in a visible light-triggered cycloaddition/rearrangement cascade

“…1 In recent years, several groups have illustrated the power of photochemically initiated cascade reactions (Scheme 1, top). 3–6 Hoffmann and co-workers provided the first example for an intramolecular cascade of a naphthalene with a tethered resorcinyl moiety. The transformation of substrate 1 to rac - 2 can be understood as an intramolecular ortho photocycloaddition followed by an acid-catalysed ring opening/rearrangement.…”

“… 5 The three examples provide a glimpse on the structural diversity that cascade reactions can offer when combined with photochemistry. 6 A remarkable feature of these transformations is the skeletal rearrangement that follows the initial ortho photocycloaddition. Although mechanistically complex, the predictable creation of three-dimensional structures from arene precursors in these cascade reactions is a concise and attractive pathway to achieve structural product diversity.…”