Ene imines

The results presented above indicate that the previously unknown head-to-tail polymerization is the major reaction product of the iminium methide species. To investigate the generality of this reaction, we next studied a neutral ene-imine species shown in Scheme 7.9.48 As illustrated in this scheme, the generation of this reactive species requires quinone reduction followed by elimination of acetic acid. The ene-imine is structurally related to the methyleneindolenine reactive species that is a metabolic oxidation product of 3-methylindole (Scheme 7.9).57 59... 

The toxicity of 3-methylindole has been attributed to methyleneindolenine trapping of nitrogen and sulfur nucleophiles.57 60-62 Likewise, the ene-imine shown in Scheme 7.9 readily reacted with hydroquinone nucleophiles, resulting in head-to-tail products. Shown in Fig. 7.6 is the 13C-NMR spectrum of a 13C-labeled ene-imine generated by reductive activation. The presence of the methylene center of the ene-imine is apparent at 98 ppm, along with starting material at 58 ppm and an internal redox reaction product at 18 ppm. Thus, the reactive ene-imine actually builds up in solution and can be used as a synthetic reagent. 

Reductive activation of the quinone shown in Scheme 7.9 and incubation in methanol afforded a complex mixture of products consisting mainly of head-to-tail coupling at C-5 or C-7 (Scheme 7.10). Minor reactions involve transfer of H2 from the hydroquinone to the ene-imine (internal redox reaction) and methanol trapping. The structures of the dimers and trimers in Scheme 7.10 were derived from H-NMR,... 

SCHEME 7.9 Ene-imine formation by reductive activation and by metabolic activation. The 13C labels are designated with asterisks ( ). 

C-NMR, COSY, HMQC (heteronuclear multiple quantum coherence), and HMBC (heteronuclear multiple bond correlation).48 Furthermore, the structure of trimer was confirmed by X-ray crystallography.48 The incorporation of 13C into the indole 3a position proved valuable in these structural determinations and in documenting the ene-imine intermediate. For example, the presence of a trimer was readily determined from its 13C-NMR spectrum (Fig. 7.7). 

The head-to-tail-coupling reactions described above are potentially useful in the design of dynamic combinatorial libraries. Features of these reactions include the rapid and reversible formation of carbon-carbon bonds, multifunctional ene-imine building blocks, and formation of stereo centers upon ene-imine linkage. Support for template-directed synthesis utilizing ene-imine building blocks is the formation of a poly ene-imine species that could recognize 3 -GGA-5 sequences of DNA.48 It is noteworthy that some polyene-imines are helical and could form a triple helix with DNA. 

SCHEME 7.13 Probing the DNA reaction products of the ene-imine and quinone methide. 

The discoveries made possible by the use of 13C labels are outlined below. A new ene-imine carbon-carbon bond coupling reaction was discovered that afforded... 

Figure 7.13 (A) Hydraamphiphile and (B) polystyrene-poly(propy ene imine) diblock structures...

Fig. 13. Schematic representation of a scandium triflate cross-linked fourth generation poly(propyl-ene imine) dendrimer (DAB).

Simultaneously, Woodward and co-workers (21) confirmed structure X for the alkaloid by chemical methods. They first recorded the caly-canine synthesis already mentioned. The intermediate tetraaminodi-aldehyde, first postulated by Robinson and Teuber (10), was also the basis for their structural speculations. The mercuric acetate oxidation product (dehydrocalycanthine) of Marion and Manske (13), formed by loss of two hydrogens, was smoothly converted by the action of alcoholic alkali into methylamine, and the resulting amide alcohol was written as XI. It was concluded that dehydrocalycanthine is an ene-imine, the relevant portion of the molecule being shown as XII. Other structures derivable from the tetraaminodialdehyde would more probably generate on amidine, and this would be impossible with X because of steric strain at a bridgehead double bond. 

Since it seems clear that it is the combination of acid functions and free water in the electrolytes that leads to the chemical instability of WO3, the use of anhydrous protonic polymers may be attractive for this kind of application. Recently, anhydrous polymers have been synthesized and studied. However, only a few studies are reported on the performances of such complete ECDs. Table 38.2 presents the values of conductivity, at room temperature, of some of these polymers. These materials are promising for an electrochromic cell with WO3 but their conductivity has to be improved to be of the order of 10" (Q.cmj Either ammonium salts or acids have been added to polymers such as poly(ethylene oxide) (PEO), polyvinylpirolydone (PVP), poly(ethylene imine) (PEI), poly(vinylalcohol) (PVA), poly(acrylic acid) (PAA), branched poly(ethyl-ene imine) (BPEI), poly(acrylamide) (Paam) to obtain anhydrous protonic conductors. 

On the basis of IR and NMR spectral data it was shown that of the three possible tautomeric forms (bis-keto-imine, bis-enol-imine, bis-keto-enamine), quinoxaline 208a adopts the bis-keto-enamine form. Of the six possible tautomeric forms (every two of keto-imine/keto-enamine, hydroxyl-ene-imine/keto-ene-amine, keto-enamine/enol-imine forms) in quinoxaline 210 the hydroxyl-ene-imine/keto-ene-amine form is adopted (Waring et al. 2002). The structures of compounds 208a and 210 were also confirmed by X-ray analysis and deduced from theoretical calculations of the possible limiting stractures (Fig. 2.19) (Waring et al. 2002). 

---