Stereochemical studies of the photochemical cycloaddition reaction of alkenes with N-benzoylindole and N-carboethoxyindole; evidence for biradical intermediacy

Abstract

. Can. J. Chem. 69, 1171 (1991).The stereochemistries of the 2+2 cycloaddition products obtained from the photochemical addition reaction between N-benzoylindole or N-carboethoxyindole and the alkenes cyclopentene, cyclohexene, cycloheptene, cis-and trans-2-butene, and cis-and trans-4-octene are examined. The structures of the products are shown to be consistent with a photo-cycloaddition mechanism involving the intermediacy of triplet 1,4-biradical species. The quantum yields of adduct formation between N-benzoylindole and both cis-and trans-octene were measured as a function of alkene concentration. The results suggest that cis-octene reacts with the indole derivative's triplet excited state with a rate constant of (1.7 + 0.3) X lo7 M-' s-' . The results are also consistent with the immediate products of this reaction being 1,4-biradicals, 98% of which revert to the ground state indole derivative and alkene, and only 2% of which proceed to cycloadduct. In marked contrast, the same treatment suggests that trans-octene reacts with the triplet excited state of N-benzoylindole with a rate constant estimated to be in the range of 1 X lo6 and 6 X 105M-I s-', and it appears that the 1,4-biradicals formed revert much less efficiently to the starting materials; it is estimated that between 67 and 100% of the 1,4-biradicals proceed to cycloadducts. In the reaction with cis-octene biradical reversion leads to the formation of trar~s-octene ("Schenk isomerization"); the quantum yield of this process is determined to be 0.074 * 0.004, which may imply that approximately 75% of the biradicals collapse to cis-alkene and 25% collapse to the trans isomer.Key words: indole, photocycloaddition, 1,4-biradicals.DAVID J. HASTINGS and ALAN C. WEEDON. Can. J. Chem. 69, 1171 (1991).On a ttudiC les stCrCochimies des produits de cycloaddition 2 + 2 obtenus par rCaction d'addition photochimique entre le N-benzoylindole ou le N-carboCthoxyindole et les alcknes suivants : cyclopentene, cyclohexkne, cycloheptkne, cis-et transbut-2-Cne et cis-et trar~s-oct-4-kne. On dkmontre que les structures des produits correspondent B celles attendues sur la base d'un mecanisme de photo-cycloaddition impliquant des espkces 1,4-biradicalaires triplets comme intermkdiaries. On a dCterminC les rendements quantiques pour la formation des adduits entre le N-benzoylindole et les cis-et trans-octknes en fonction de la concentration d'alckne. Les rksultats suggkrent que le cis-octkne rtagit avec I'Ctat excitC triplet du dCrivt de l'indole avec une constante de vitesse de (1,7 * 0,3) X lo7 M-' s-'. Les rCsultats sont aussi en accord avec I'existence de 1,4-biradicaux comme produits immkdiats de cette rCaction dont 98% retourneraient aux Ctats fondamentaux de l'alckne et du dCrivC de l'indole alors que seulement 2% conduiraient au produit de cycloadduction. Par ailleurs, un traitement similaire suggkre que le trans-octkne rCagit avec l'ttat excite triplet du N-benzoylindole avec une vitesse de rCaction qui se situerait entre 1 x lo6 et 6 X lo5 M -I s-I alors que le...

Arene–Alkene Cycloaddition

“…Nonbenzenoid aromatics of course also have a rich photochemistry, but they tend to behave more like regular dienes or olefins and have been deliberately omitted here. Typically, furans react either as the olefinic partner with excited benzenes (and these examples will be cited in the relevant section) or undergo other photoprocesses such as the Paternò–Büchi reaction. − With pyrones and pyridones, furans tend to react in [4 + 4] cycloadditions. − Indoles, on the other hand, react with alkenes in [2 + 2] − or Diels–Alder cycloadditions …”

Non‐Heme Iron Complexes for Stereoselective Oxidation: Tuning of the Selectivity in Dihydroxylation Using Different Solvents

“…General Information: Commercially available chemicals were used without further purification unless noted otherwise. H 2 18 O 2 (90% 18 O‐enriched, 2% solution in H 2 16 O) and H 2 18 O (95% 18 O‐enriched) was obtained from ICON Services Inc. N3Py33 and cis ‐4‐octene53 were prepared according to literature procedures.…”

Organophosphane-Catalyzed Construction of Functionalized 2-Ylideneoxindoles via Direct β-Acylation

“…Following the previously reported procedure, 12 the reaction mixture of indole (1.0 equiv), sodium hydroxide (3.0 equiv), and tetrabutylammonium bromide (1.5 mol%.) in dry CH2Cl2 and cooled it to 0 °C.…”

“…By following the previously reported procedure, 12 a mixture of indole (1.0 equiv), sodium hydroxide (3.0 equiv), and tetrabutylammonium bromide (1.5 mol%) in anhydrous CH 2 Cl 2 was cooled to 0 °C. Benzoyl chloride (2.0 equiv) was then added dropwise and the mixture was stirred for 15 min at the same temperature.…”