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.