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Effect of microheterogeneity of CF 3 CH 2 OH–H 2 O mixed solvent on reactions of carbocations from 1,2,2,3‐tetramethyl‐1,2‐dihydroquinoline
Author(s) -
Nekipelova Tatiana D.,
Lygo Olga N.,
Khodot Evgenii N.,
Kurkovskaya Lidiya N.
Publication year - 2013
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.3135
Subject(s) - carbocation , chemistry , solvent , photodissociation , steric effects , reaction rate constant , photochemistry , solvent effects , stereochemistry , organic chemistry , kinetics , physics , quantum mechanics
Carbocations are key intermediates in many important organic reactions. The remarkable effect of the solvent composition on the kinetic parameters of the carbocation decay and product composition was found in the photolysis of 1,2,2,3‐tetramethyl‐1,2‐dihydroquinoline ( 1 ) in 2,2,2‐trifluoroethanol (TFE)–H 2 O mixtures. The rate constant of the intermediate carbocation decay has a maximum, and the activation energy is minimal in the TFE–H 2 O mixture 3 : 7 ( v / v ). In the steady‐state photolysis , products of oligomerization of 1 with n up to 8 and their adducts with TFE and H 2 O were identified at this solvent composition. The results were rationalized in terms of TFE clustering in aqueous mixtures, with the maximum of cluster formation at 30 vol % TFE. The clusters form a pseudo‐phase, in which the molecules of 1 are concentrated and the carbocations are generated. TFE, H 2 O and 1 compete in the combination reaction with the photogenerated carbocation to afford the products. This effect was not observed for 1,2,2,4‐tetramethyl‐1,2‐dihydroquinoline ( 2 ), the isomer of 1 , due to steric hindrance at C(4) carbon atom of the heterocycle, the active site of the intermediate carbocation, which makes impossible for the carbocation from 2 to react further with 2 . Thus, the kinetic parameters and the product composition in the photolysis of 1 in TFE–H 2 O mixtures reflect the changes in the microstructure of the binary solvent. Copyright © 2013 John Wiley & Sons, Ltd.