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Regioselectivity and Competition of the Paternò–Büchi Reaction and Triplet–Triplet Energy Transfer between Triplet Benzophenones and Pyrimidines: Control by Triplet Energy Levels
Author(s) -
Liu XiuLing,
Wang JianBo,
Tong Yao,
Song QinHua
Publication year - 2013
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201300958
Subject(s) - oxetane , chemistry , regioselectivity , quenching (fluorescence) , ketone , photochemistry , benzophenone , pyrimidine , triplet state , stereochemistry , fluorescence , molecule , organic chemistry , physics , quantum mechanics , catalysis
The photochemical reaction of a pyrimidine and a ketone occurs either as a Paternò–Büchi (PB) reaction or as energy transfer (ET) from the triplet ketone to the pyrimidine. It is rare for the two types of reactions to occur concurrently, and their competitive mechanism remains unknown. In this work, two classes of products, regioisomeric oxetane(s) ( 2 , 3 ) from a PB reaction and three isomeric dimers of 5‐fluoro‐1,3‐dimethyl uracil (FDMU) ( 4 – 6 ) from a photosensitized dimerization of FDMU, are obtained through the UV irradiation of FDMU with various benzophenones (BPs). The ratio of the two products (oxetanes to dimers) reveals that the two competitive reactions depend strongly on the triplet energy levels ( E T ) of the BPs. The BPs with higher E T values lead to higher proportions of dimers, whereas those with lower E T values give higher proportions of oxetane(s), with the generation of just two regioisomeric oxetanes for the BP with the lowest E T of the eight BPs investigated. The ratio of the two oxetanes ( 2 : 3 ) decreases with the BP E T value. The competitive mechanism for the two types of photochemical reactions is demonstrated through quenching experiments and investigation of temperature effects. Kinetic analysis shows that the rate constants of the two [2+2] photocycloadditions are comparable. Furthermore, in combination with the results of previous studies, we have gained insight into the dependence of the photochemical type and the regioselectivity in the PB reaction on the triplet energy gaps (Δ E ) between the pyrimidines and ketones. For ketones with higher E T values than the pyrimidines, the photochemical reaction is a photosensitized dimerization of the pyrimidine. In the opposite case, a PB reaction occurs, and the lower the E T of the ketones, the lower the ratio of oxetanes ( 2 : 3 ). When the E T of values of the ketones are close to those of the pyrimidines, the two reactions occur concurrently, and the higher the E T of the ketones, the higher the proportion of the dimers. The ratio of oxetanes ( 2 : 3 ) decreases with the E T value of the BPs.