Intramolecular hydrogen bonding transition state

Factor b above is discussed in Sections II, B, 1 II, B, 4 and II, C. A hydrogen-bonded structure such as 221 can account for the facile reaction of 5-bromouracil or for the unique, so-called hydrolyzability of carboxymethylthio-azines (237). The latter may also react via the intramolecular mechanism indicated in 136. The hydrogen-bonded transition state 238 seems a reasonable explanation of the fact that 3,4,6- and 3,4,5-trichloropyridazines react with glacial acetic acid selectively to give 3-pyridazinones while other nucleophiles (alkoxides, hydrazine, ammonia, or sulfanilamide anion) react at the 4- and 5-positions. In this connection, 4-amino-3,5-dichloro-pyridazine in liquid hydrazine gives (95°, 3hr, 60%yield)the isomer-... 

Scheme 5. Stabilization of dinuclear species in aqueous solution by intramolecular hydrogen-bond formation (VI) is well established. Stabilization of the mononuclear species in aqueous solution by intermolecular hydrogen bonds (IV) may be important in some systems. Interconversion between mononuclear and dinuclear species may occur via non-hydrogen-bonded and hydrogen-bonded transition states, respectively, as schematically shown in II and V. Dashed lines denote hydrogen bonds dotted lines denote bond making and bond breaking.

A six-membered hydrogen-bonded transition state apparently enables AcOH-catalysed nucleophilic addition of benzylic C of 2-methyl azaarenes to aldehydes. Tertiary enamides and enecarbamates undergo nucleophilic intramolecular C p2 addition to an iV-CHjCHjCHO group to form 4-hydroxytetrahydropyridine derivatives the enantioselective reaction is promoted by a BINOL-Ti complex. 0... 

The Gibbs free energy reaction profiles in Fig. 16 have been calculated from the results in Table 16 and the mechanism in (30) and refer to reaction in a 1 1 2-methylphenol buffer at buffer concentrations of 0.001 and 0.1 moldm" (Fig. 16(a) and (b), respectively). TS(1) is the transition state for opening of the intramolecular hydrogen bond and TS(2) is the transition... 

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