Henkel-like Decarboxylation Produces Mononuclear Arylalkalis, [4-(CH₃)₃N(C₆H₄)M]⁺ which Hydrolyse and Undergo Surprising S_N2 Reactions Between Alkali Hydroxides and (CH₃)₃NC₆H₅⁺

Benzoate substituted with a cationic quaternary ammonium group at the para-position, [4-(CH₃)₃N(C₆H₄)CO₂], forms mononuclear alkali metal ion complexes, [4-(CH₃)₃N(C₆H₄)CO₂M]⁺ (M=Li, Na, K, Rb and Cs), which are observed by electrospray-ionisation mass spectrometry. When mass selected and subjected to collision-induced dissociation each of these complexes undergoes decarboxylation to produce the mononuclear arylalkali complex cations, [4-(CH₃)₃N(C₆H₄)M]⁺, which subsequently react with water via hydrolysis to yield the quaternary ammonium cation, (CH₃)₃NC₆H₅⁺. The unexpected product ions, [M((CH₃)₂NC₆H₅)]⁺, [M(CH₃OH)]⁺, and M⁺, which are associated with the hydrolysis pathway, suggest an unusual reaction occurring within the exit channel ion-molecule complex, [(CH₃)₃NC₆H₅+MOH]⁺. DFT calculations were used to examine the: decarboxylation reaction of [4-(CH₃)₃N(C₆H₄)CO₂M]⁺, which readily proceeds via a four-centred transition state; the hydrolysis and S_N2 reactions accounting for formation of products in the exit channel, which are connected via a roaming mechanism in which the metal hydroxide moiety migrates from the para-hydrogen of (CH₃)₃NC₆H₅⁺ to one of the methyl groups.