Flash photolysis of 4-diazoisochroman-3-one in aqueous solution. Hydration of the carbene produced by loss of nitrogen and ketonization of the enol hydration product

The 2-Oxocyclobutanecarboxylic Acid Keto−Enol System in Aqueous Solution: A Remarkable Acid-Strengthening Effect of the Cyclobutane Ring

“…The primary isotope effect here is also accompanied by a secondary effect, but now the secondary effect is in the normal direction ( k H / k D > 1) . The result is an overall effect that is quite large, similar to the strong effect observed here, and isotope effects of this magnitude have in fact been found on the ketonization of other enols effected by carbon protonation through proton transfer from water. 22b, …”

“…This diagonal section is followed by another, quite short uncatalyzed portion, which could be due either to carbon protonation of the carboxylate form of the substrate by H 2 O or to protonation of the doubly ionized but still minor carboxylate−enolate form by H + ; in the latter case H + is produced in a prior equilibrium but is used up in the rate-determining step, again leading to [H + ] cancellation and a profile section independent of [H + ]. It is sometimes possible to rule out the first of these interpretations because such a mechanism gives rate constants too similar in value for reactions catalyzed by acids as different in strength as H + and H 2 O . It is likewise sometimes possible to rule out the second of these interpretations because that gives an impossibly large value to the rate constant for reaction catalyzed by H + . , Neither of these situations, however, applies here (vide infra), and a choice between the two alternatives cannot be made on the basis of the available information.…”

Keto−Enol/Enolate Equilibria in the Isochroman-4-one System. Effect of a β-Oxygen Substituent

“…The striking difference in magnitude between these two isotope effects is, in fact, a hallmark of enol ketonization reactions. Similarly large isotope effects on have been measured for the carbon protonation by H 2 O of the enolate ions of isobutyrophenone ( k H / k D = 7.5), mandelic acid ( k H / k D = 6.9), methyl mandelate ( k H / k D = 7.7), and 4-hydroxyisochroman-3-one ( k H / k D = 7.8) …”

7 Reaction mechanisms. Part (i) Polar reactions

“…Kinetic and thermodynamic studies of the formation and subsequent reactions of the intermediates produced by laser flash photolysis of 47 in aqueous solution and X-ray Scheme 8 crystallographic determination of the structure of the final product proved that the reaction followed mechanism A (Scheme 11) and not B (Scheme 11), which was contrary to what was inferred from earlier studies on similar systems. 44 Acidity constants for cations 48 (R = H, Me) and 49 (R = H) generated upon laser flash photolysis of corresponding aryl azides NR 2 -C 6 H 4 -C 6 H 4 -N 3 or upon twoelectron oxidation of corresponding aryl amines NR 2 -C 6 H 4 -C 6 H 4 -NH 2 in aqueous solution were reported. 45 The dications 48 (R = H, Me) were shown to be 100-fold less reactive towards addition of water than the monocation forms 49 (R = H, Me).…”