摘要:
The rate–pH profiles for hydrolysis of the title compounds 1
show four distinct regions: kA,
kB and kC for rate plateaux
corresponding to cationic, neutral and anionic species, plus
kD for attack of OH- on the anion.
At the ends of the pH scale the reaction is much slower than for model
amides of comparable pKlg, due to exceptional charge
dispersal in reactant and leaving group. The plateau rates
kB and kC are due to hydrolysis
by water, not to some kinetically equivalent process, and are much
faster than model calculations would predict. This is traced to
intramolecular general base catalysis via solvent bridges, and
leads to remarkable rate enhancements in aqueous alcohols. The
considerable, and quite independent, variations in
kB, kC and acid
pKa with only alkyl substitution in the amide moiety
points to a dominant effect of conformation which has been explored
using a number of techniques, notably octanol–water partitioning,
and appears best rationalised in terms of Taft’s ‘steric
hindrance of motions’ or Tillett’s ‘entropic
strain’. The overall picture for the effect of pH is of
successively increasing C–O bond formation in the transition
state along the sequence
kA → kB
→ kC but with C–N bond
breaking quite out of line and largely dependent on conformational
factors.Given pKcat < 0, the
presence of effective intramolecular general base catalysis in
kB is unexpected. We explain this as being
due to a unique feature of 1 whereby catalyst and leaving group are part
of the same conjugated structure, leading to
pKcat → pK
lg as C–N bond-breaking proceeds. Further
light on kB comes from the
ring-N-methylated analogue 3d, which cannot form the
intramolecular hydrogen bond found elsewhere and whose otherwise similar
rate–pH profile shows an anomalous ‘apparent
pKa’ that can be explained as a
consequence of this.