Chiral induction in photochemical reactions - 15.: Detection of stereoelectronic effects by temperature dependent measurements of the diastereoselectivity in the photosensitized [2+2]-cycloaddition

• Published in: Tetrahedron Vol. 50; no. 38; pp. 11167 - 11186
• Main Authors: Hoffmann, Norbert, Buschmann, Helmut, Raabe, Gerhard, Scharf, Hans-Dieter
• Format: Journal Article
• Language: English
• Published: OXFORD Elsevier Ltd 1994; Elsevier
• Subjects: Chemistry; Chemistry, Organic; Physical Sciences; Science & Technology; SELECTIVITY PRINCIPLE; ORGANIC-SYNTHESIS; CYCLOADDITION REACTIONS; ALPHA,BETA-UNSATURATED KETONES; TRIPLET BIRADICALS; INTRAMOLECULAR 2+2 PHOTOCYCLOADDITION; CARBOHYDRATE RADICALS; PATERNO-BUCHI REACTION; CONJUGATED ENONES; VINYL ETHERS
• ISSN: 0040-4020; 1464-5416
• DOI: 10.1016/S0040-4020(01)89419-9

The mechanism of the diastereoselection in the photosensitized[2 + 2]-cycloaddition reaction of ethylene to 5-alkoxy-2(5 H)-furanones is investigated. The dependence of product ratio on the reaction temperature as well as on structural features of the enone system is measured. The structure dependent activation parameter differences (ΔΔ H ≠,ΔΔ S ≠) obtained by this method (modified Eyring plot) are used as a tool to characterize the stereoelectronic factors, which are responsible for the diastereoselection. In detail these factors involve: pyramidalization of the β-carbon, homoanomeric effect, and the steric requirements of the alkoxy substituent in the relaxed ( 3ππ *)-excited furanones. The mechanism of the diasteroselection in the corresponding reaction is investigated by determination of the dependence of the stereoselectivity on the temperature and on R abd R′.