Reversible Addition Reactions

Aldehydes and ketones undergo reversible addition reactions with alcohols. The product of addition of one mole of alcohol to an aldehyde or ketone is referred to as a hemiacetal or hemiketal, respectively. Dehydration followed by addition of a second molecule of alcohol gives an acetal or ketal. This second phase of the process can be catalyzed only by acids, since a necessary step is elimination of hydroxide (as water) from the tetrahedral intermediate. There is no low-energy mechanism for base assistance of this... 

Ring-chain tautomerism via intramolecular reversible addition reactions to the C=0 group 95AHC(64)251. 

Incorporation of lsO into the ketone occurs hardly at all under these conditions, i.e. at pH 7, but in the presence of a trace of acid or base it occurs [via the hydrate (13)] very rapidly indeed. The fact that a carbonyl compound is hydrated will not influence nucleophilic additions that are irreversible it may, however, influence the position of equilibrium in reversible addition reactions, and also the reaction rate, as... 

Recent Developments in Ring-Chain Tautomerism L Intramolecular Reversible Addition Reactions to the C = O Group ... 

This chapter is the first part of a review that updates an earlier work by R. E. Valters and W. Flitsch, Ring-Chain Tautomerism (A. R. Katritzky, ed.). Plenum Press, New York and London, 1985. The second part of this review, to be published in a subsequent volume of Advances in Heterocyclic Chemistry, includes data (1982-1993) for intramolecular reversible addition reactions to C = N, C=N, C = C, and C=C groups. 

Ring-chain tautomeric interconversions proceeding by intramolecular reversible addition reactions to the C=0 group (Scheme 1) have been well studied, particularly with respect to the 3- and 4-oxocarboxylic acids containing five- or six-membered rings, respectively. Relatively few new investigations have appeared in the literature. 

It has been shown by thermodynamic calculations (89TH1) that, under equal structural conditions, the ratio of the tautomeric equilibrium constants for the reversible addition reaction of the SH group and that for the OH group should be 10 in favor of the sulfur addition product. A similar result (>10 ) was estimated (90T6545) from a comparison of the stability of the 1,3-thiazolidine ring with that of the 1,3-oxazolidine ring. 

Nucleophiles and leaving groups reversible addition reactions... 

It can be also very important for determining the number of isomers of higher fullerenes of regioisomers of exohedral fullerene adducts and for carrying out mechanistic investigations on the escape of the endohedral guest and reversible addition reactions [125]. The Xe NMR spectrum ofXe C5Q has also been reported [126]. 

The electrophilic reactions of NP with SH- and several SR- have been studied and reviewed (28). The nature of the reversible addition reactions [Eq. (5)] are reasonably well understood for the thiolates. A kinetic study including some bioinorganically relevant nucleophiles (cysteine, glutathione) was performed by using stopped-flow and T-jump techniques (77). The rate constants for the forward and reverse processes in Eq. (5) were in the range 103-104 M-1 s-1 and 101 K)3 s-1 at 25 °C, respectively. 

Hug GL, Bonifacic M, Asmus K-D, Armstrong DA (2000a) Fast decarboxylation of aliphatic amino adds induced by 4-carboxybenzophenone triplets in aqueous solutions. A nanosecond laser flash photolysis study. J Phys Chem B 104 6674-6682 Hug GL, Carmichael I, Fessenden RW (2000b) Direct EPR observation of the aminomethyl radical during the radiolysis of glycine. J Chem Soc Perkin Trans 2 907-908 Hunter EPL, DesrosiersMF, Simic MG (1989) The effect of oxygen, antioxidants and superoxide radical on tyrosine phenoxyl radical dimerization. Free Rad Biol Med 6 581-585 Ito O (1992) Flash photolysis study for reversible addition reactions of thiyl radicals with olefins and acetylenes. Trends Phys Chem 3 245-266... 

Ito O (1995) Flash photolysis study on reversible addition reactions of thiyl radicals. Res Chem In-termed 21 69-93... 

Valters, R. E., Ffilop, F., Korbonits, D., Recent Developments in Ring-Chain Tautomerism I. Intramolecular Reversible Addition Reaction of the C = 0 Group,... 

III. Intramolecular Reversible Addition Reactions to Other Groups. 54... 

The mechanisms of reactions of benzylmagnesium halides can be either homolytic or concerted. As with allylic Grignard reagents, also reversible addition reactions with benzylic Grignard reagents and carbonyl compounds have been reported. In general, yields of normal alcohols are low, and such alcohols are obtained in much better yields by use of dibenzylcadmium [66] or benzyllithium [67]. However, in 1980, it was demonstrated that, by far, the best and quickest results for the synthesis of these normal addition reaction products are obtained by applying benzylic halides in the one-step Li-Barbier reaction under ultrasonic irradiation [68,69]. 

A detailed study of trichloroacetonitrile addition to 2,3,4,6-tetra-O-benzyl-D-glucose showed that from the 1-oxide the -trichloroacetimidate is formed preferentially or even exclusively in a very rapid and reversible addition reaction (Scheme 16). However, this product anomerizes in a slow, base-catalyzed reaction via retroreaction, 1-oxide anomerization and renewed addition) practically completely to the a-trichloroacetimidate, with the electron-withdrawing 1-substituent in an axial position as favored by the thermodynamically efficient anomeric effect. Thus with different bases, for instance K2CO3 and... 

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