Tautomeric mixture equilibrium position

E. Influence of External Factors on the Equilibrium Position of a Tautomeric Mixture... 

A3-Pyrrolines are obtained, as the major product, by the reduction of pyrroles by zinc in hydrochloric acid via the initial protonation of the ring at the 2-position. The minor products are the A1- and A2-isomers, which result from the reduction of the 3-protonated species. As shown in Scheme 54 (Section 3.05.1.5), there is a tautomeric equilibrium between the A1- and A2-pyrrolines, which is catalyzed by the acidic medium, but there is no evidence of a tautomeric equilibrium between these isomers and the A3-pyrrolines. The tautomeric mixture of the A1- and A2-pyrrolines can inter-react to yield the 3-(2-pyrrolidinyl)-A -pyrroline and any attempt to isolate the pure A -pyrroline results in the... 

The Beirut process as a method for preparing QDO and PDO is sometimes inconvenient. One of the main problems has been evidenced when substituted Bfxs were used as a reagent for the Beirut reaction. In this case, a mixture of positional isomers, in general non-separable by ordinary chromatographic techniques, of QDO or PDO have been obtained. This finding is the result of the well-known tautomerism that affects the Bfx reactants at room temperature (see example in Scheme 2) [41,42], In general, the tautomeric equilibrium energy barrier is very low at room temperature and therefore... 

Spectroscopic techniques which do not influence the position of the equilibrium can more certainly be used to ascertain the composition of the equilibrium tautomeric mixture. The presence of significant amounts of... 

Such dearomatization of the arene ligand activates it toward an electrophilic addition. Thus, osmium(ll) was used as a dearomatization agent for the direct 10/3-alkylation of /3-estradiol 70 (equation 29). When the tautomeric mixture 71 72 was placed in acidic methanol and reprecipitated, a 3 1 equilibrium ratio of the phenolic 71 and dienone 72 tautomers was observed . This intermolecular Michael addition to the C(10) position of the aromatic steroid was unprecedented. 

Tautomeric equilibria, elucidation using basicity measurements, 327 Tautomeric mixture composition of, 318 equilibrium position of, 318 influence of external factors on composition of, 318... 

Quinolizine exists as a mixture of up to three species, namely the 2H-, 4H- and 9a/7-tautomers (17 and 18, respectively). This tautomeric equilibrium has been extensively studied in the case of tetramethyl quinolizine-1,2,3,4-tetracarboxylate, where, as previously mentioned, the 4/7-tautomer 17 has been shown to be thermodynamically more stable than the 9/7-species 18 through ab initio Hartree-Fock calculations. An interesting feature of these compounds is that the rate of the interconversion of the 9a//-tautomer 18 into the more stable 4//-cornpound 17 depends on the position of substituents in ring B <2003JST651>. 

The solvent, temperature, and substituent (R and R1) -dependent ring-chain tautomerism of 2-pyridylaminomethylenemalononitriles (167 168) was studied (86JOC2988). 2-Pyridylaminomethylenemalo-nonitriles were prepared in the reaction of 2-aminopyridines and 2-(l-ethoxyalkylidene)malononitrile in ethanol at room temperature or in a melt at 120°C, and in a one-pot reaction, starting from 2-aminopyridines, a triethyl orthoester, and malononitrile at 110°C for 10 minutes. The content of the equilibrium mixture of 2-pyridylaminomethylenemalononitrile 167 (R = R1 = H) is shown in Table X. Elevation of the temperature increased the proportion of the chain tautomer. The ratio between the two tautomeric forms 167 and 168 is influenced primarily by the steric properties of substituents R and R1 in positions 6(6) and 3(9), respectively,... 

A special case of reversible ionization of a hexacoordinate silicon complex has been described as a novel tautomeric equilibrium.41 It differs from the formation of siliconium-ion salts in that the positive charge resides on nitrogen, in a dimethylammonium cation, and not on silicon. The transsilylation of lg with 12 in equimolar concentrations leads to the pentacoordinate zwitterionic complex 13 (Eq. (10), Section II.B.5). However, when the molar ratio was 2 1, respectively, an equilibrium mixture of tautomers (58, 59) was obtained, as shown in Eq. (21). The same mixture was also obtained when a second mole-equivalent of lg was added to 13. 

---