ISSN:
0030-493X
Keywords:
Chemistry
;
Analytical Chemistry and Spectroscopy
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Tertiary α-carbomethoxy-α,α-dimethyl-methyl cations a have been generated by electron impact induced fragmentation from the appropriately α-substituted methyl isobutyrates 1-4. The destabilized carbenium ions a can be distinguished from their more stable isomers protonated methyl methacrylate c and protonated methyl crotonate d by MIKE and CA spectra. The loss of I - and Br· from the molecular ions of 1 and 2, respectively, predominantly gives rise to the destabilized ions a, whereas loss of Cl· from [3]+ · results in a mixture of ions a and c. The loss of CH3· from [4]+· favours skeletal rearrangement leading to ions d. The characteristic reactions of the destabilized ions a are the loss of CO and elimination of methanol. The loss of CO is associated by a very large KER and non-statistical kinetic energy release (T50 = 920 meV). Specific deuterium labelling experiments indicate that the α-carbomethoxy-α,α-dimethyl-methyl cations a rearrange via a 1,4-H shift into the carbonyl protonated methyl methacrylate c and eventually into the alkyl-O protonated methyl methacrylate before the loss of methanol. The hydrogen rearrangements exhibit a deuterium isotope effect indicating substantial energy barriers between the [C5H9O2]+ isomers. Thus the destabilized carbenium ion a exists as a kinetically stable species within a potential energy well.
Additional Material:
4 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/oms.1210240607
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