Acylium ions react with neutral isoprene and other 1,3-dienes in the gas phase to form covalently bound adducts by polar [4 + 2+] Diels-Alder cycloaddition. This general reaction was previously unknown either in the gas phase or as the corresponding solution reaction, but the mechanism is supported by ab initio calculations and experimental studies. Proton transfer competes with cycloaddition, as does fragmentation of both the acylium ion and the nascent cycloaddition product. Structural information on the ion/molecule product is obtained by performing multiple-stage (MS3) experiments using a pentaquadrupole mass spectrometer. Cycloreversion dominates the fragmentation behavior of these adducts. Both alkyl- and aryl-substituted acylium ions, as well as the thioacetyl cation, undergo facile cycloaddition. The α,β-unsaturated acylium ions (13 and 17) undergo a novel cycloaddition reaction at the double bond, the positively charged C≡O+ acting as an activating group. Substitution at the C=C double bond (14-16) can change the regioselectivity redirecting cycloaddition to the C≡O+ bond, as shown by MS3 experiments which display cycloreversion only when addition is on the carbonyl.115209226923
