Isomerization Tautomerization Rearrangments

The hydroniumm ion-catalysed vinyl alcohol-acetaldehyde isomerization has been investigated627 via ab initio MO calculations, and the results have supported the stepwise mechanism shown in Scheme 125. A theoretical study of the tautomeric rearrangements in mono- and di-chalcogenide analogues of formic acid, [HC(X)YH ... 

Oxazol-5(2H)-one, 2-benzylidene-4-methyl-tautomerism, 6, 186 Oxazol-5(2ff)-one, 2-methylene-isomerization, 6, 226 Oxazol-5(2H)-one, 2-trifluoromethyl-acylation, 6, 201 Oxazol-5(4ff)-one, 4-allyl-thermal rearrangements, 6, 199 Oxazol-5(4H)-one, 4(arylmethylene)-Friedel-Crafts reactions, 6, 205 geometrical isomerism, 6, 185 Oxazol-5(4ff)-one, 4-benzylidene-2-phenyl-configuration, 6, 185 photorearrangement, 6, 201 Oxazol-5(4ff)-one, 4-benzyl-2-methyl-Friedel-Crafts reactions, 6, 205 Oxazol-5(4ff)-one, 4-methylene-in amino acid synthesis, 6, 203 Oxazol-5(4ff) -one. 2-trifluoromethyl-hydrolysis, 6, 206 Oxazolones... 

A mixture of ethyl 3,5-dimethyl-l/f-thieno[2,3-c]-1,2-diazepine-l-carboxylate (6) (10-15%) and the isomeric ethyl 2,5-dimethyl-3//-thieno[2,3-t/]-l, 3-diazepine-3-carboxylate (8) (50-60%) is obtained when a solution of the thienopyridine A-iminc 4 in benzene is irradiated with a 400-W high-pressure mercury lamp for 2-3 hours at 20rC. It has been suggested that product 6 is formed from the intermediate 5. The isomeric intermediate 7 undergoes two consecutive walk rearrangements, followed by valence tautomerization, to give the major product 8.168... 

Recently, the hydroxy derivatives of furan, thiophene, and selenophene have been studied with regard to their physical properties and reactions. These compounds are tautomeric and if the oxygen function is placed in the 2-position they exist as unsaturated lactones and undergo carbon-carbon rearrangement, whereas the 3-hydroxy derivatives form oxo-enol tautomeric systems. By NMR the structures of the different tautomeric forms have been determined as well as the position of the tautomeric equilibrium and the rate of isomerization. 

The cydoaddition of different 1,3-dipoles such as azides [331, 341] and diazoalkanes [342-344] to acceptor-substituted allenes was thoroughly investigated early and has been summarized in a comprehensive review by Broggini and Zecchi [345], The primary products of the 1,3-dipolar cycloadditions often undergo subsequent fast rearrangements, for example tautomerism to yield aromatic compounds. For instance, the five-membered heterocycles 359, generated regioselectively from allenes 357 and diazoalkanes 358, isomerize to the pyrazoles 360 (Scheme 7.50) [331]. 

REARRANGEMENT (Organic Chemistry). These are reactions involving the transfer of an atom or group from one part of the molecule to another. Tautomcrism is a special case of rearrangements in which the two forms are in dynamic equilibrium. See Tautomerism in entry on Isomerism. When such reactions take place, the establishment of structural formulas becomes complex. Some of the better known rearrangements include ... 

The classical Claisen rearrangement is the first and slow step of the isomerization of allyl aryl ethers to orlho-a ly lated phenols (Figure 14.46). A cyclohexadienone A is formed in the actual rearrangement step, which is a [3,3]-sigmatropic rearrangement. Three valence electron pairs are shifted simultaneously in this step. Cyclohexadienone A, a nonaromatic compound, cannot be isolated and tautomerizes immediately to the aromatic and consequently more stable phenol B. 

Fig. 2-33. Alkaline peeling reaction of cellulose (R = cellulose chain). 1 — 2, Isomerization 2 — 3, 2,3-enediol formation 3 — 4, j3-alkoxy elimination 4 —> 5, tautomerization 5 — 6, benzilic acid rearrangement leading to glucoisosaccharinic acid.

The alkaline hydrolysis of [NP(OMe)2]3 4 gives the monohydroxy derivatives, N3P3(OMe)5(OH) and N4P4(0Me)70H which exist in the tautomeric oxo-form (38). The rearrangement of octamethoxycyclo-tetraphosphazene, N4P4(0Me)8, in the presence of methyl iodide yields two isomeric phosphazanes the more abundant isomer has the 2-trans-4-cis-6-trans-8 structure (13). 

Tautomerism Special case of structural isomerism in which two isomers are directly interconvertible, the reversibility of the change is due to mobility of a group on atom, which can move from one position to another in the molecule, often with the rearrangement of a double bond. 

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