Tautomerism with imines

Dihydroazoles can exist in at least three forms (cf. Section 4.01.1.3), which in the absence of substituents are tautomeric with each other. The forms in which there is no hydrogen on at least one ring nitrogen normally predominate because imines are generally more stable than vinylamines in aliphatic chemistry. Thus for dihydropyrazoles the stability order is A" (hydrazone) (288) > A (azo) (289) >A (enehydrazine) (290). 

The intramolecular Heck reaction presented in Scheme 8 is also interesting and worthy of comment. Rawal s potentially general strategy for the stereocontrolled synthesis of the Strychnos alkaloids is predicated on the palladium-mediated intramolecular Heck reaction. In a concise synthesis of ( )-dehydrotubifoline [( )-40],22 Rawal et al. accomplished the conversion of compound 36 to the natural product under the conditions of Jeffery.23 In this ring-forming reaction, the a-alkenylpalladium(n) complex formed in the initial oxidative addition step engages the proximate cyclohexene double bond in a Heck cyclization, affording enamine 39 after syn /2-hydride elimination. The latter substance is a participant in a tautomeric equilibrium with imine ( )-40, which happens to be shifted substantially in favor of ( )-40. 

Unlike the parent system, 5-methyl-5//-dibenz[c,e]azepine (1, R1 = Me R2 = H) on treatment with lithium diisopropyl amide fails to yield the tautomeric phenanthridine-imine (see Section 3.2.1.5.4.2.), but forms the 5-carbanion, which on quenching with deuterium oxide furnishes 5-methyl-[5-2H,]-5//-dibenz[e,e]azepine (l).83 5,7-Diphenyl-5//-dibenz[r,e]azepine (1. R1 = R2 = Ph) behaves similarly. In contrast, however, 5,7-dimethyl-5//-dibcnz[c,e]azepine (1, R1 = R2 = Me) yields theazaallyl anion 3, which on addition of deuterium oxide deuterates regiospecifically at the 7-methyl group to give derivative 4. 

Chemical evidence for the aromatic character of (232) is provided by its stability to acid and base. No tautomerization to imine forms such as (223) is detectable by NMR, indicating a lower limit of perhaps 84 kJ mol-1 for the stabilization energy. Finally, the 1,4-dimethyl derivative (230d) forms a charge transfer complex with trinitrobenzene (Amax 488). 

To suppress enamine-derived side products, we explored addition of benzotriazole (BtH) to the reaction mixture. The premise behind these experiments was the ability of BtH to form stable adducts with imines,23,24 thereby blocking tautomerization of 19 to 20 through in situ formation of the benzotriazolyl derivative 21. It was hoped that subsequent hydride displacement of the Bt moiety would afford the desired mono alkylated products 23. Indeed, analytical high-performance liquid chromatography (HPLC) revealed a remarkable improvement in terms of product purity, especially for reactions carried out at room temperature, with the desired secondary anilines 23 being essentially the only products detected. In... 

Associated with this scheme is, however, the observation that if a base happens to be in one of its rare tautomeric forms, imine for... 

A detailed investigation by Pfau and Ughetto-Monfrin45 of the reaction of N-isopropylidenecyclohexylamine (26) with methyl acrylate (28) demonstrates convincingly the imine-enamine (27) tautomerism. All the products 29, 31-34 which were separated and characterized result from the enamine tautomer (Scheme 1). Hence, although the imine tautomer is the exclusive form based on spectroscopic observations, tautomerization between imine and enamine does exist and is sufficiently rapid to give products from the latter. 

Esters of pyridazinyl-3-acetic acid 60 can be prepared when, for example, 3-methoxypyridazine I-oxide reacts with methyl )3-aminocrotonate in the presence of benzoyl chloride to give 59. Since )8-aminocrotonic esters are tautomeric with the imines of acetoacetic esters, mild acid hydrolysis converts them into the corresponding acetoacetic ester and then to ester 60 (78JHCI425). 

As in the furanol series, there is a problem of tautomerism with fura-namines. In general, aminofurans can exist to some extent as imines acid hydrolysis frequently eliminates ammonia. Meinwald and his colleagues have studied such 2-aminofuran hydrolyses and found them to give not the expected butenolides but dimers.215... 

By analogy with the pyridine AT-imines (Section II,A,3) the intermediate 49 can be assumed for the valence tautomerization AT-imine triazene. The equilibria of Scheme 4 are demonstrated by the formation of carbazoles in the thermal decomposition of benzocinnoline Ar-imines. 

Replacement of an hydroxyl group in resorcinol, most probably through tautomerism and imine formation, has been effected by heating it in an autoclave with 1-aminobutane and a small amount of phosphoric acid at 200°C under pressure (13 bar) for 8 hours. Only the monobutylamino substitution product was obtained but by phase transfer catalysis on the reaction product, with a benzyttrimethylammonium salt, (formed in situ from a surfactant and potassium iodide), sodium hydroxide solution and 1-bromobutane for 20 hours at 60-80°C, 3-dibutylaminphenol was produced in 64% yield (ref. 106). 

A detailed study of the scope of the amidine Diels-Alder reaction of 1,3,5-triazines has been carried out. The thermal reaction of amidines with symmetrical 1,3,5-triazines proceeds with in situ amidine to 1,1-diaminoethene tautomerism, [4+2] cycloaddition, loss of ammonia with imine generation, imine to enamine tautomerism, then retro-Diels-Alder loss of ethyl cyanoformate to yield substituted 4-aminopyrimidines. 

A detailed mechanistic study of the chemical and electrochemical oxidative dehydrogenation of the iron(II) complex 14 has been reported. The key intermediate is an iron(II) amido complex which is tautomeric with an iron(II) amido radical species it is this species which undergoes electron loss to give the iron(II) complex of a C-protonated imine (Scheme 6). The reaction of dicyclohexylcar-bodiimide, RN=C=NR, with ReClg results in nucleophilic attack of halide upon the diimide to yield complexes of RNHCC1=NR. The complex... 

The degradation of polyacrylonitrile deuteriated in the alpha position has been followed by Fourier transform i.r. spectroscopy. The results are consistent with imine-enamine tautomerism followed by oxidation to give pyridone structures. A variety of thermo-analytical techniques has been employed to study thermal and thermo-oxidative reactions and it is concluded from these that a carboxylate moiety is responsible for initiating cyclization in an oxidizing atmosphere. The role of oxygen is said to be three-fold, creation of the carboxylate initiator sites, dehydrogenation, and crosslinking. 

The equilibrium position for the three tautomers (C-H tautomer 5, N-H tautomer 6, and 0-H tautomer 7) is dependent on the pyrazolone substituents and the solvent used. Tautomerism with substituent participation is observed in oxo heterocycles as well as for the corresponding thiones and imines. 

Analogous compounds with a secondary amino group (a,j8-unsaturated secondary amines) can, in principle, exist in either the form of imines (6) or the tautomeric form of enamines (7). As they practically occur and react in the former structure, it is more convenient to use the group designation imines. ... 

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