Carbocation rearrangements of trimethylsilyl adducts of saturated acyclic C 5–C 7 ketones in the gas phase

Abstract

Metastable ion decompositions of TMS + adducts of all the saturated, acyclic C 5–C 7 ketones and of selected ketone conjugate acid ions are compared. The proportion of ions that result from rearrangement of the carbon skeleton tends to increase with the size of the starting ketone. Parent ions derived from α-branched ketones can be subdivided into pairs that rearrange and decompose via common intermediates. In addition to pathways outlined by previous workers, the present study delineates the involvement of ion–neutral complexes and also presents evidence for 1,4-hydride shift. Inclusion of this latter mechanism accentuates the parallelism between the rearrangements of gaseous TMS + adducts and those of protonated ketones in solution. Loss of (CH 3) 3SiOH from TMS + adducts occurs primarily via 1,2-shift followed by 1,3-elimination, just as water loss takes place from ketones in superacid solution. In most cases the product from this elimination is an allylic cation, but 1,4-hydride shift appears more likely to produce a cyclopropylcarbinyl ion. Density functional calculations give relative energies of pertinent intermediates, products, and transition states for cationic rearrangements.