Nr. Cullum et al., THE AMINOLYSIS AND HYDROLYSIS OF N-(4,6-DIPHENOXY-1,3,5-TRIAZIN-2-YL)SUBSTITUTED PYRIDINIUM SALTS - CONCERTED DISPLACEMENT MECHANISM, Perkin transactions. 2, (8), 1996, pp. 1559-1563
Reaction of the title pyridinium salts in aqueous buffers containing a
mines involves nucleophilic displacement of the pyridine by the amine
nitrogen in addition to background hydrolysis. The second-order rate c
onstants for attack of a series of unhindered primary amines on the (4
,6-diphenoxy-1,3,5-triazin-2-yl)pyridinium cation obey a Bronsted-like
equation with slope beta(nuc) = 0.68. Six-membered-ring cyclic second
ary amines have a similar Bronsted selectivity but are approximately 1
0-fold more reactive than primary amines of similar pK(a). The reactiv
ity of ethanolamine against the title pyridinium salts obeys a Bronste
d-like equation with beta(1g) = -0.47. In aminolysis of the triazinyl
pyridinium salts the changes in bond fission and bond formation signal
led by the polar substituent effects give Leffler alpha(1g) and alpha(
nuc) values of 0.38 and 0.54, respectively, which indicate strong coup
ling between the bonding changes consistent with a concerted process f
or the displacement reaction. The adduct in the putative stepwise mech
anism is considered too reactive to possess a significant barrier for
it to exist as a discrete molecule.