Enamino-imino tautomerism

Shin was unable to observe enamino-imino tautomerism in the case of dehydrovaline compounds (253, 373). E. Ohler and U. Schmidt (287, 350) were able to convert a-iminocarboxylic acid esters into a-enaminocarboxylic acid esters (42) by preparing the hydrochlorides of the former, which slowly rearranged into the hydrochlorides of the latter. This reaction is the simplest means of preparing a-enaminocarboxyhc acid esters, as the a-iminocarboxylic acid esters obtained by N-chlorina-tion/dehydrochlorination of amino acid esters can be rearranged directly, without intermediate purification. 

As pointed out in the introduction, if one of the substituents on the nitrogen atom is a hydrogen atom, tautomeric equilibrium between enamino and imino forms strongly favors the latter form 18,140,141). According to physiochemical measurements, the occurrence of simply substituted /1 -pyrrolines and zl -piperideines is very improbable. The formulation of this type of compound with a double bond in the position (used mainly by early authors) was of formal meaning only, having no experimental evidence (142-144). 

Tautomeric studies of equilibria between enamino and methylene imino form have been reported for several heterocycles. Tautomerism of 1,2,4-tri-azino[4,3- ]quinoxalin-5-ones was deduced to be solvent dependent the enamino form 170b is predominant in DMSO-dg ( H NMR), whereas 170a is the major tautomer in the solid state (IR in nujol) (90JHC691 95H2057) (cf. Section III,B). 

In the case of primary (and secondary60) jff-keto enamines, the enamino keto form 30a or 31a is stabilized by its push-pull conjugated mesomeric form 30b or 31b and by an intramolecular hydrogen bridge which dominates over the tautomeric imino enol form 30c or 31c (equation 3). Similar results are reported for jft-thioketone enamines61. 

Figure 3.36 Imino/enamino tautomerism and coordination as an anionic chelate ligand.

The same ring-chain tautomerism is exhibited by the pair 146 147. The cyclic form 147 exists as the imino tautomer, as aromatic stability would be disrupted by the o-quinoidal structural requirement of the enamino tautomer 148. ... 

Ultrafast studies on tautomerism concentrate on compounds that can exhibit hydrogen transfer in the electronically excited state. Hydrogen transfer is a very typical reaction for the interconversion between different tautomeric forms. It converts enol to keto, amino to imino, imino to enamino, and lactim to lactam forms, to name some examples. For time-resolved experiments, excited-state intramolecular proton transfer (ESIPT) is particularly well suited since a short laser pulse in the visible or ultraviolet (UV) spectral region can trigger this process by promoting the molecule into the electronically excited state and initiating the transfer in this way [3]. The vast majority of experiments on tautomerism with ultrafast time resolution are therefore done on compounds exhibiting ESIPT. 

The imino-enamino tautomerism of P-imino sulfoxides was studied, showing in all instances the imino form [Ar-S(=0)-CH2-C(R )=NR] as the predominant tautomer, independent of substitution [54]. 

A route to a-enaminocarboxylic acid esters based on N-chlorination/ dehydrochlorination of amino acid esters has been described by H. Poisel and U. Schmidt 312, 313). N-Chlorination of BOC-amino acid esters followed by dehydrochlorination with methoxide affords a-methoxy-a-BOC-amino acid esters (41), which, on treatment with HCl, undergo elimination of methanol and cleavage of the protective group in a single step. The free enamino esters (42) are liberated from their hydrochlorides by treatment with ammonia or triethylamine. NMR spectroscopic studies showed that (42a) and (42c) exist exclusively in the enamino tautomeric form, while (42b) contains about 40% imino tautomer. 

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