Tautomerisation

When naphthyl amines e.g. 23) are used in the Bucherer carbazole synthesis, they are converted by the catalytic action of aqueous bisulfite into tetralonesulfonic acid derivative 13 by the Bucherer reaction. Addition of NaHSOs gives an enamine, which tautomerises to the imine 24 24 is hydrolysed to keto form 13 and subsequent Bucherer carbazole synthesis follows to afford the benzocarbazole product 20. ... 

Structural analogues of the /]4-vinylketene E were isolated by Wulff, Rudler and Moser [15]. The enaminoketene complex 11 was obtained from an intramolecular reaction of the chromium pentacarbonyl carbene complex 10. The silyl vinylketene 13 was isolated from the reaction of the methoxy(phenyl)-carbene chromium complex 1 and a silyl-substituted phenylacetylene 12, and -in contrast to alkene carbene complex 7 - gave the benzannulation product 14 after heating to 165 °C in acetonitrile (Scheme 6). The last step of the benzannulation reaction is the tautomerisation of the /]4-cyclohexadienone F to afford the phenol product G. The existence of such an intermediate and its capacity to undergo a subsequent step was validated by Wulff, who synthesised an... 

For the indene derivatives M two different reaction pathways have been discussed so far, starting from the ( )-metallatriene D. A strongly coordinating solvent may induce an electrocyclic ring closure yielding the metallacyclohexa-diene K, and the indene product is obtained after tautomerisation and reductive... 

The oxidations by Cu(II) and Ag(I) are zero-order in oxidant and completely analogous to that by Hg(II) (p. 334). They involve a slow, acid-catalysed tautomerisation of H2POOH to HP(OH)2 followed by a rapid attack on the latter by the oxidant. 

Hydroxyenones have also been used in catalytic amide formation, although 1,2,4-triazole is required as a co-catalyst. Assumed protonation of the Breslow intermediate and tautomerisation generates an acylazolium intermediate, which is trapped by triazole, releasing the NHC and generating the acyltriazole 60 that is the active acylating agent for the amine (Scheme 12.11) [16]. 

Ye and co-workers have shown that NHC 67 can catalyse the aza-Morita-Bay-lis-Hillman reaction of enones 66 and N-tosyl imines 63, presumably via initial NHC conjugate addition to the enone to generate an azolium enolate 68 [18]. A related conjugate addition approach has been exploited by Fu and co-workers, with tautomerisation of the initial enolate 72 derived from NHC conjugate addition to 70 giving 73, with subsequent cyclisation resulting in the umpolung of Michael acceptors (Scheme 12.13) [19]. 

A range of a-functionalised aldehydes have been used to generate acylazolium species via the corresponding enol intermediate. For example, addition of an NHC to an a-halo aldehyde 84 presumably generates the Breslow species 86, with elimination of HX to afford the enol 87. Subsequent in situ tautomerisation generates... 

The ring nitrogens react with electrophiles to afford either 1,3,4-thiadiazolium salts or l,3,4-thiadiazol-2(37/)-ones depending on the tautomerisability of the substituents at the C-2 or C-5 positions. While N-alkylation is the most common electrophilic reaction of 1,3,4-thiadiazoles, reactions with acyl and cyanogen halides as well as Mannich salts have also been reported. 

The transfer of a proton between an acidic and a basic group within the same molecule is often more complex than the process shown in (1). The proton may be transferred along hydrogen-bonded solvent molecules between the acidic and basic groups if these are too remote to permit formation of an intramolecular hydrogen bond. Alternatively, two inter-molecular proton transfers with an external acid or base may be necessary. Tautomerisation of oxygen and nitrogen acids and bases (3) will be described in Section 6. The reactions are usually quite rapid and fast reaction... 

Various nmr techniques have been used to investigate the intramolecular double proton transfer which occurs in the tautomerisation of meso-tetra-phenylporphyrin (40) (Limbach et al., 1982). The reaction has been studied (Storm and Teklu, 1972) by observation of the nmr signals due to the protons... 

The tautomerisation of the purine bases adenine and guanine and of the pyrimidine bases thymine, cytosine, and uracil has important implications in molecular biology, and the occurrence of rare tautomeric forms of these bases has been suggested as a possible cause of spontaneous mutagenesis (Lowdin, 1965 Pullman and Pullman, 1971 Kwiatowski and Pullman, 1975). Three of the most likely tautomers for cytosine are shown in [87]—[89], together with the less likely imino forms [90] and [91] (Scanlan and Hillier,... 

In DNA, base pairing occurs between guanine and cytosine in its most stable form [87], as in [92]. However, cytosine in its imino form [91] is able to form a hydrogen bond to adenine as in [93], and such a base pairing could lead to the formation of mutations if it continued in DNA replication. The kinetics of the tautomerisation of purine and pyrimidine bases has been... 

The tautomerisation of the N(9)H- and N(7)H-isomers of adenine (93) has been studied in aqueous solution using the temperature-jump technique with Joule heating (Dreyfus et al., 1975). The equilibrium constant K = [N(7)H]/[N(9)H] has a value of 0.28 at 20°C. The reciprocal relaxation time... 

For this mechanism, values of kr =k 2 = 1.3 x 10lodm3mol-1s-1 are calculated from the experimental value for k in (94), and this means that the proton-transfer steps in (96) are diffusion-controlled in the thermodynamically favourable directions. The hydroxide ion catalysed tautomerisation... 

A non-dissociative mechanism for water-catalysed tautomerisation of uracil monoanion and of imidazole has been suggested on the basis of nmr line broadening measurements (Chang and Grunwald, 1976). However, doubts have been expressed about these conclusions (Bensaude et al., 1978). The mechanisms deduced from nmr measurements are shown in (108) and (109) and involve water behaving as a bifunctional catalyst. Rate coefficients... 

An attempt has been made to predict the general requirements for the operation of a non-dissociative mechanism of tautomerisation between oxygen and nitrogen centres (Bensaude et al., 1977). The proximity of the two centres between which the proton is transferred is obviously of critical importance but more experimental results are required before a clear picture will emerge. 

---