Gas-Phase Broensted versus Lewis Acid-Base Reactions of 6,6-Dimethylfulvene. A Sensitive Probe of the Electronic Structures of Organic Anions

Article abstract of DOI:10.1021/ja00217a002

The gas-phase reactions of a series of negative ions with 6,6-dimethylfulvene (DMFU) have been investigated in a flowing afterglow apparatus.Products are formed from competing Broensted and Lewis acid-base reactions wherein the negative ion abstracts a proton from one of the acidic methyl groups, and/or adds to the exocyclic double bond to yield a substituted cyclopentadienide ion.Localized anions such as hydroxide, alkoxides, trifluoromethide, and thiolates react exclusively by proton abstraction, while delocalized ions such as allyl and benzyl yield mainly adduct with somewhat reduced efficiencies.The observed product distributions areindependent of the total pressure in the reactor.Enolate ions show a wide variation in reactivity with DMFU: acyclic aldehyde and ketone enolates react exclusively by proton abstraction, while amide and ester enolates produce mainly adduct, despite their greater thermodynamic basicities.A pronounced ring-size effect is also found with cyclic ketone enolates wherein the larger ring ions such as cyclohexanone enolate show only proton transfer, whereas the small-ring ions such as cyclobutanone enolate show both addition and proton transfer.It is shown that yields for the Broensted and Lewis acid-base reaction products are determined kinetically and that charge delocalization in the negative ion reactant substantially reduces its kinetic basicity.The results forthe ambident enolate ions are shown to be largely determined by the proton affinity differences between the carbon and oxygen ends, i.e., the lower the proton affinity at oxygen, the greater the preference for adduct formation with DMFU through carbon.A detailed mechanism and energy profile is proposed for the DMFU reactions which is based on the results for over 30 different negative ions.Discussion of the relative nucleophilicities and kinetic basicities of these ions is also presented.