Henkel-like Decarboxylation Produces Mononuclear Arylalkalis, [4-(CH3)3N(C6H4)M]+ which Hydrolyse and Undergo Surprising SN2 Reactions Between Alkali Hydroxides and (CH3)3NC6H5+

Benzoate substituted with a cationic quaternary ammonium group at the para-position, [4-(CH3)(3)N(C6H4)CO2], forms mononuclear alkali metal ion complexes, [4-(CH3)(3)N(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)(3)N(C6H4)M](+), which subsequently react with water via hydrolysis to yield the quaternary ammonium cation, (CH3)(3)NC6H5+. 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 SN2 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.