Intramolecular 1,2-alkyl shifts in unsymmetric dialkoxycarbenes studied by very low vapour pressure (VLVP) pyrolysis - mass spectrometry

Article abstract of DOI:10.1139/v96-059

Methoxy-(2,2,2-trifluoroethoxy)carbene radical cations, CH₃O-C-OCH₂CF₃^·+, 1^·+, are cleanly generated by the dissociative electron ionization (EI) of 2-methoxy-5,5-dimethyl-2-(2, 2,2-trifluoroethoxy)-Δ³-1,3,4-oxadiazoline I. Neutralization-reionization (NR) mass spectrometry of the neutral carbene 1, generated by one-electron reduction of 1^·+, shows no recovery ion signal and thus 1 is not a viable species within the μs time scale of the experiment. Very low vapour pressure (VLVP) pyrolysis - mass spectrometry of I in conjunction with (multiple) collision experiments shows that 1 completely isomerizes, via a 1,2-trifluoroethyl shift, into methyl 3,3,3-trifluoropropionate, CF₃CH₂C(=O)OCH₃, 1a. This technique was also used to study the related dialkoxycarbenes C₂H₅O-C-OCH₂CF₃, 2, CH₃O-C-OC₂H₅, 3, and CH₃O-C-OCH(CH₃)₂, 4, generated from the corresponding 2,2-dialkoxy-5,5-dimethyl-Δ³-1,3,4-oxadiazolines. The pyrolytically generated carbene 2 behaves analogously to 1 and completely isomerizes to ethyl 3,3,3-trifluoropropionate, 2a. The neutral carbenes 3 and 4 undergo only a partial isomerization via 1,2-alkyl shifts in which the ethyl and isopropyl groups show a slightly greater migratory aptitude, respectively, than the methyl group. The differences in migratory aptitude are explained in terms of a transition state model similar to that of a 1,2-H shift in carbenes, with development of negative charge in the migrating group. The greater migratory aptitude of CF₃CH₂, as compared to CH₃ and CH₃CH₂, is attributed to the stabilization of negative charge in the transition state by strongly electron-withdrawing β-fluorines whereas the differences in migratory aptitude between the alkyl groups in 3 and 4 are largely due to the greater polarizability of isopropyl and ethyl groups, as compared to the methyl group.

Full text of DOI:10.1139/v96-059

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Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.