Oxonium ions trifluoromethyl

Using the same experimental approach, a family of enantiomerically pure oxonium ions, i.e., O-protonated 1-aryl-l-methoxyethanes (aryl = 4-methylphenyl ((5 )-49) 4-chlorophenyl ((5)-50) 3-(a,a,a-trifluoromethyl)phenyl ((5)-51) 4-(a,a,a-trifluoromethyl)phenyl ((S)-52) 1,2,3,4,5- pentafluorophenyl ((/f)-53)) and 1-phenyl-l-methoxy-2,2,2-trifluoroethane ((l )-54), has been generated in the gas phase by (CH3)2Cl -methylation of the corresponding l-arylethanols. ° Some information on their reaction dynamics was obtained from a detailed kinetic study of their inversion of configuration and dissociation. Figs. 23 and 24 report respectively the Arrhenius plots of and fc iss for all the selected alcohols, together with (/f)-40) of Scheme 23. The relevant linear curves obey the equations reported in Tables 23 and 24, respectively. The corresponding activation parameters were calculated from the transition-state theory. 

Since this review was written a wealth of kinetic data on THF poly-menzations initiated by esters of trifluoromethyl sulphonic acid, fluoro-sulphonic acid and other super acids has appeared [164—168]. It has correctly been pointed out [167, 169] that secondary oxonium ions should not be observable separately under the experimental conditions of Pruckmayr and Wu [96] and that some of their NMR assignments are wrong. Instead, it has been verified that the observed spectrum corresponds to a very solvent sensitive equilibrium between a macro-ester and a macro-ion as proposed earlier by Smith and Hubin [98], viz. 

In vivo stability of the trifluoromethyl analogue 13a of artemether is anticipated to be increased in two ways first, the trifluoromethyl group could decrease the oxidation of the methoxy group [30, 34-37] second, the intermediate oxonium ion 14 should be much more difficult to generate in acidic medium [30],... 

Taking into account the lack of chemoselectivity and stereoselectivity of these silver ion-mediated reactions, it is reasonable to postulate an SNl-type mechanism for this process, leading to the formation of oxonium ion 14, despite the electron-withdrawing effect of the CF3 group (see Scheme 6.8). The stereochemical outcome should be ascribed to the steric or electronic preference of methanol addition to two possible diastereo-faces of the trifluoromethyl oxonium ion 14, in which the interactions are probably different from those in nonfluorinated DHA derivatives. In order to disfavor the formation of oxonium 14, the reaction was performed without silver salt in MeOH. While the reaction rate did not decrease significantly, only 23% of glycal 18 was obtained. Furthermore, the... 

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