INTRAMOLECULAR 1,2-ALKYL SHIFTS IN UNSYMMETRIC DIALKOXYCARBENES STUDIED BY VERY-LOW VAPOR-PRESSURE (VLVP) PYROLYSIS MASS-SPECTROMETRY

Methoxy-(2,2,2-trifluoroethoxy)carbene radical cations, CH3O-C-OCH2CF3 .+, 1(.+), are cleanly generated by the dissociative electron ionizati on (EI) of 2,2,2-trifluoroethoxy)-Delta(3)-1,3,4-oxadiazoline I. Neutr alization-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 mu s time scale o f the experiment. Very low vapour pressure (VLVP) pyrolysis - mass spe ctrometry of I in conjunction with (multiple) collision experiments sh ows 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 corr esponding ialkoxy-5,5-dimethyl-Delta(3)-1,3,4-oxadiazolines. The pyrol ytically generated carbene 2 behaves analogously to 1 and completely i somerizes 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 mig ratory aptitude are explained in terms of a transition state model sim ilar to that of a 1,2-H shift in carbenes, with development of negativ e charge in the migrating group. The greater migratory aptitude of CF3 CH2, as compared to CH3 and CH3CH2, is attributed to the stabilization of negative charge in the transition state by strongly electron-withd rawing beta-fluorines whereas the differences in migratory aptitude be tween the alkyl groups in 3 and 4 are largely due to the greater polar izability of isopropyl and ethyl groups, as compared to the methyl gro up.