Structure of enamines

In 1954 Stork et al. (i) reported that the alkylation of the pyrrolidine enamine of cyclohexanone (5) with methyl iodide followed by acid hydro-I ysis led to the monoalkylated ketone. It was thus obvious that the enamine (7) derived by the loss of proton from the intermediate methylated iminium cation (6) failed to undergo any further alkylation. 

The pyrrolidine enamine of 2-methylcyclohexanone (7) was in fact found to be quite inert toward further alkylation and was shown to consist only of the trisubstituted isomer (4) on the basis of the NMR spectral data. The 

2-phenylcyclohexanone failed to show any styrene-type absorption in the ultraviolet, which would have been exhibited by the tetrasubstituted isomer. 

This type of allylic interaction between the equatorial methyl group and the methylene group adjacent to the nitrogen atom has been recently generali2ed by Johnson and Malhotra (7,7a) as the strain between substituents R and R in the cyclohexenyl-type system shown, as in (11), the dihedral angle between the substituents being 40°. The magnitude of 

That the methyl group in the pyrrolidine enamine of 2-methylcyclo-hexanone (7) is in fact axial was demonstrated by Johnson and Whitehead (8). They found that careful hydrolysis of the pyrrolidine enamine of the conformationally more stable system, i.e., 2-methyI-4-t-butylcyclohexanone (13), led to a 1 4 mixture of cis and trans isomers of the ketone (14 and 15), showing that the methyl group in the enamine is largely in the axial orientation. 

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The above mentioned mesomerism between a polarized and a nonpolarized structure of enamines possessing a tertiary nitrogen atom is reflected in their physical properties and chemical reactivity. For... 

If this interaction is not possible, the shift does not occur.37 The ultraviolet spectra have been studied in connection with the structure of aliphatic 3 and cyclic enamines,1, 28,37, 43 cis-trans isomerism about their double bond,27 and the structures of enamines with additional functional groups.17, 20, 31,32 Ultraviolet spectral data have been used also as a general method to determine the structure of alkaloids and in the study of some reaction mechanisms. 

Summary of experimentally observed variations in molecular structures of enamines... 

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... 

Enamines have been recognized in organic chemistry as useful synthetic reagents since the early reports from Stork s laboratory1. At almost the same time similar chemical moieties were being implicated in biochemical systems. Because of their intrinsic instability in water, the biochemical enamines exist primarily as intermediates, although, some well-known coenzymes that participate in oxidation-reduction reactions also incorporate enamine structures in one of their oxidation states. The electronic structure of enamines involves two extreme resonance contributions as shown in equation 1. 

The structure of enamines, on first inspection, would seem straightforward, and, indeed, most elementary texts present a simplistic picture in which the nitrogen is fully s/i -hybridized with complete p-orbi-tal overlap between nitrogen and the carbon-carbon ir-bond system (1). 

Figure 2.1 (a) Calculations on possible enamine intermediates (b) X-ray structure of enamine... 

Sharif, S., Powell, D.R., Schagen, D., Steiner, T.M., Toney, M.D., Fogle, E., and Limbach, H.H. (2008) X-ray crystallographic structures of enamine and amine Schiff bases of pyridoxal and its 1 1 hydrogen bonded complexes with benzoic acid derivatives evidence for... 

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