REARRANGEMENT AND CYCLIZATION IN THE IONIZATION OF THE 4-CHLORO-3-METHYLBUTANOYL CATION

The second ionization of the 4‐chloro‐3‐methylbutanoyl ion forms a primary alkyl acyl dication, as a tight ion pair. Methyl and hydrogen shifts occur to comparable extents indicating that the relative stability of the product (sec‐ or tert‐carbocation) does not influence the energy barrier for the shift. The product of methyl shift (1,3‐sec‐alkyl acyl dication) loses the proton closest to the counterion in the tight ion pair and forms the pent‐3‐enoyl cation. Protonation–deprotonation of the latter, followed by internal acylation, gives the protonated cyclopent‐2‐enone. The dication resulting from hydrogen shift loses a proton from C‐2 and gives the 3‐methylbut‐2‐enoyl cation. Copyright © 1990 John Wiley & Sons Ltd.