Aromatic hydroxy acids, the compounds of large industrial importance, can be
prepared in the Kolbe-Schmitt reaction, i.e. a carboxylation reaction of
alkali metal phenoxides (MOPh) and naphthoxides (MONaph). On the basis of the
experimental results two contradictory reaction mechanisms have been
proposed: the one of direct carboxylation, and the other involving initial
formation of the MOPh-CO2 or MONaph-CO2 complex. Previous theoretical
investigations of the carboxylation reaction of sodium 2-naphthoxide,
performed by means of the B3LYP method, confirmed the initial formation of
the NaONaph-CO2 complex, and showed that the carbon of the CO2 moiety
performs an electrophilic attack at C1 of the ring, leading to the formation
of sodium 2-hydroxy-1-naphthoate (E1). Surprisingly, transition states for
possible electrophilic attacks at C3 and C6 were not revealed, and the
formation of other two products (E3 and E6) was explained by a number of
consecutive rearrangements. In addition, this mechanism includes a reaction
step with rather high activation energy. Since more sophisticated functionals
are today available, the aim of this work is to reinvestigate the mechanism
of the Kolbe-Schmitt reaction of NaONaph in all three positions (1, 3, and
6). Our investigations with the M062X method demonstrated that CO2 and
NaONaph can spontaneously build two complexes: B (the one previously
reported) and C. While B cannot be further transformed to yield the reaction
products, the CO2 moiety in C takes perfect position for electrophilic
attacks at all three sites of the ring. These attacks are realized via the
transition states TS1, which lead to the formation of the new C-C bonds, and
corresponding intermediates D. In the next, bimolecular reaction step two D
intermediates exchange the protons adjacent to the CO2 groups. These
intermolecular reaction steps require significantly lower activation energies
in comparison to the intramolecular proton shift from C to O. The
carboxylation reaction in the position 6 is both kinetically and
thermodynamically unfavourable, whereas the pathways in the positions 1 and 3
are competitive. Pathway 1 requires the lowest activation energies, but E3 is
significantly more stable than other two products. In accord with these
findings are the experimental results which show that, at very low
temperature (293 K) only E1 is formed at low yield, whereas the yields of E3
and E6 increase with the increasing temperature. Since the Kolbe-Schmitt
reaction is experimentally performed at relatively high temperatures (around
500 K), the main product is thermodynamically most stable E6.
Carboxylation of sodium 2-naphthoxide. Reinvestigation of the mechanism by means of a hybrid meta density functional theory method
