Enamine pyramidality

Evidently, in the crystalline state, BENA can adopt one of two conformations (A or B) presented in Fig. 3.3. Nevertheless, the nitrogen atom is characterized by a high degree of pyramidalization in both conformations, and the C-N bond is substantially elongated compared to similar bond in classical enamines (see, e.g., (417)). It should also be noted that the C,C double bond in BENA is substantially shortened compared to the analogous bond in standard enamines. 

Fig. 8 Correlation between the twist angle, t, and the pyramidalization at nitrogen, yN, for anilines (O) and naphthalenes (+), enamines ( ) and amides (A). Data points for amides and thioamides are not shown. Reprinted with permission from Gilli et ul. (1986). Copyright 1986 American Chemical Society.

In the conformational behaviour of enamines a kind of demon seems to rule the extent of pyramidalization at nitrogen and of rotational twist around the C—N bond as shown graphically234 in Figure 6. 

FIGURE 6. Pyramidalization and rotational distortion around the C—N bond of enamines. (Reproduced with permission from Reference 234)... 

In the case of pyramidally planar A N-dimethyl vinylamine 133 of type 117, the HOMO lies at —8.01 eV and the calculated splitting between nl and n2 is 3.76 eV. Both values still do not match the experimentally observed range of values presented in Table 18, but calculated tertiary vinylamine values are shifted towards the experimental values of tertiary enamines. Especially, the calculated value for the second IP from n2 is calculated to be too high. For the rotated form of types 120 and 121 the calculated splitting is now 0.49 and 0.21 eV, less than in the case of 115 mentioned above. 

FIGURE 20. Scatter plot of pyramidality coefficient PN,% versus the absolute value of PD1 for the sample of 474 enamine fragments... 

TABLE 4. Selected statistical data for the absolute values of angular out-of-plane parameter, xNi (deg), and twist parameters, tni C2 and tc2 C3 (deg), together with pyramidality coefficients P (%) for the N1, C2 and C3 atoms of the enamine grouping both for a sample containing enamines with unrestricted substituents (size 474) and with carbon as the first atom in each of the RJ-R5 substituents (sample size 178)... 

FIGURE 3. Enamine nitrogen pyramidality. 0 is L RNR, co is the angle between the extended C—N bond and L RNR bisector. (Reprinted from Ref. 6, p. 16 by courtesy of Marcel Dekker, Inc.). 

An extensive study of the crystal structures of enamines derived from pyrrolidine, piperidine and morpholine (l)14 led to the result that the amino group in unhindered enamines (R1 = R2 = H) assumes the shape of a rather flat pyramid with the nearest... 

The electronic spectroscopic data collected both on the model enamines and on the PDC-bound ones leave little doubt that all of these enamines are planar, highly conjugated structures. Therefore, invoking a pyramidal C2a atom in the enamine intermediates no longer offers a valid hypothesis, at least on PDC, and likely not on other ThDP-dependent enzymes either. 

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