Understanding the mechanism of base-assisted decomposition of (N-halo),N-alkylalcoholamines.
The base-assisted decomposition of (N-X),N-methylethanolamine (X = Cl, Br) takes place mainly through two concurrent processes: a fragmentation and an intramolecular elimination. The global process follows second order kinetics, first order relative to both (N-X),N-methylethanolamine and base. Interaction of the base with the ionizable hydroxylic hydrogen triggers the reaction. The intramolecular elimination pathway leads to formaldehyde and 2-aminoethanol as reaction products via base-assisted proton transfer from the methyl to the partially unprotonated hydroxylic oxygen, with loss of halide. Meanwhile, the fragmentation pathway leads to methylamine and two equivalents of formaldehyde via bimolecular base-promoted concerted breakage of the molecule into formaldehyde, halide ion and N-methylmethanimine. Kinetic evidences allow a crude estimation of the concertedness and characterization of the transition structure for both processes, which are slightly asynchronous, the proton transfer to the base taking place ahead of the rest of the molecular events. The degree of asynchroneity increases as the bases become weaker. Electronic structure calculations, at the B3LYP/6-31++G** level, on the fragmentation pathway support the proposed mechanism.
Andrés
Juan
J
Departament de Ciències Experimentals, Universitat Jaume I, Campus de Riu Sec, E-12071, Castelló, Spain. andres@exp.uji.es
Armesto
Xosé L
XL
Canle
Moisés
M
García
M Victoria
MV
Ramos
Daniel R
DR
Santaballa
J Arturo
JA