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

Abstract Benzoate substituted with a cationic quaternary ammonium group at the para ‐position, [4‐(CH 3 ) 3 N(C 6 H 4 )CO 2 ], forms mononuclear alkali metal ion complexes, [4‐(CH 3 ) 3 N(C 6 H 4 )CO 2 M] + (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‐(CH 3 ) 3 N(C 6 H 4 )M] + , which subsequently react with water via hydrolysis to yield the quaternary ammonium cation, (CH 3 ) 3 NC 6 H 5 + . The unexpected product ions, [M((CH 3 ) 2 NC 6 H 5 )] + , [M(CH 3 OH)] + , and M + , which are associated with the hydrolysis pathway, suggest an unusual reaction occurring within the exit channel ion‐molecule complex, [(CH 3 ) 3 NC 6 H 5 +MOH] + . DFT calculations were used to examine the: decarboxylation reaction of [4‐(CH 3 ) 3 N(C 6 H 4 )CO 2 M] + , which readily proceeds via a four‐centred transition state; the hydrolysis and S N 2 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 (CH 3 ) 3 NC 6 H 5 + to one of the methyl groups.