NOVEL [3-CYCLOADDITION OF THE IONIZED CARBONYL YLIDE +CH2OCH2-CENTER-DOT WITH CARBONYL-COMPOUNDS IN THE GAS-PHASE(2] 1,3)

For the first time [3 + 2] 1,3-cycloaddition of an ionized carbonyl yl ide has been observed in gas phase ion-molecule reactions of (+CH2OCH2 .) (1) with several carbonyl compounds. The reaction, which competes w ith electrophilic addition that leads to net CH2.+ transfer, occurs ac ross the C=O double bond of acetaldehyde and several acyclic ketones y ielding ionized 4,3-dialkyl-1,3-dioxolanes as unstable cycloadducts. R apid dissociation of the nascent cycloadducts by loss of a 4-alkyl sub stituent as a radical leads to the observed products, that is cyclic 3 -alkyl-1,3-dioxolanylium ions. Cycloaddition of 1 with cyclic ketones yields bicyclic spiro adducts, which also undergo rapid dissociation. Cyclobutanone yields ionized 1,3-dioxaspiro[4,3]octane, which dissocia tes exclusively by neutral ethene loss to ionized 4-methylene-1,3-diox olane. Ionized 1,3-dioxaspiro[4,4]nonane is formed in reactions with c yclopentanone, and its rapid dissociation by loss of C3H6 and C2H5. yi elds the ionized 4-methylene-1,3-dioxolanylium and the 4-ethenyl-1,3-d ioxolanylium product ions, respectively. A systematic study of this no vel reaction and characterization of the product ions carried out via pentaquadrupole (QqQqQ) multiple stage (MS-(1) and MS3) mass spectrome tric experiments provide experimental evidence for the cycloaddition m echanism. The dissociation chemistry observed for the cycloaddition pr oducts correlate well with their proposed structures and was compared to that of both isomeric and reference ions. Ab initio MP2/6-31G(d,p)/ /HF/6-31G(d,p) + ZPE potential energy surface diagrams for the reactio ns of 1 with acetone, fluoroacetone, and 1,1,1-trifluoroncetone suppor t the operation of the two competitive reaction pathways, that is CH2. + transfer and [3 + 2] 1,3-cyclonddition/dissociation, and show that t he cycloaddition process is favored by electron-withdrawing substituen ts.