Normal and inverse electron demand Diels–Alder cycloaddition of protonated and methylated carbonyl compounds in the gas phase

Abstract

AbstractProtonated and methylated quinones, α,β‐unsaturated ketones and aldehydes and saturated ketones all react to form cycloaddition products with butadiene. The reagent ions are generated by chemical ionization (CI) and react at nominally zero kinetic energy with butadiene in an r.f.‐only quadrupole of a pentaquadrupole mass spectrometer. In selected cases, the product ions were characterized by sequential product ion dissociation (triple stage mass spectrometry [MS3]). The activated dicarbonyl ions, such as protonated quinone and protonated 4‐cyclopentene‐1,3‐dione, are more reactive than the protonated α,β‐unsaturated carbonyl compounds and the protonated saturated ketones. The methylated ions are less reactive than their protonated analogs. MS3 spectra of the quinone and α,β‐unsaturated carbonyl adducts and ab initio calculations of product ion stability are interpreted as indicating Diels–Alder cycloaddition at the carbon–carbon double bond. Benzoquinones and the α,β‐unsaturated ketones are also good dienophiles in solution. The differences in reactivity between these two groups of reactant ions, between the protonated and methylated ions and between individual members of each of these groups are ascribed to differences between the HOMO and LUMO orbital energies (ΔE) of the diene and reactant ion, respectively. The correlations observed between the cycloaddition reactivity and the energy gap indicate that normal Diels–Alder reactions occur for the quinones and α,β‐unsaturated ions. Correlations between ion–molecule reactivity and the HOMO–LUMO energy gaps also extend to the protonated saturated ketones, where MS3 studies confirm that cycloaddition occurs at the carbon–oxygen double bond. In all cases, when the proton affinity of the conjugate base of the dienophile is close to that of the diene, proton transfer between the diene and the dienophile becomes a major competitive process; this in turn decreases the cycloaddition yield. Gas‐phase inverse electron demand Diels–Alder reactions are studied using methylated 2‐butenone as diene and several neutral alkenes as dienophile. Higher reactivity is achieved with electron‐donating alkenes as dienophiles, in agreement with observations made in solution chemistry on inverse electron demand Diels–Alder reactions.