Enamines electronic interaction

The structure of the protonated enamines has been investigated by infrared spectroscopy. On protonation there is a characteristic shift of the band in the double-bond stretching region to higher frequencies by 20 to 50 cm with an increased intensity of absorption (6,13,14a). Protonated enamines also show absorption in the ultraviolet at 220-225 m/x due to the iminium structure (14b). This confirms structure 5 for these protonated enamines, because a compound having structure 4 would be expected to have only end absorption as the electrons on nitrogen would not be available for interaction with the n electrons of the double bond. 

The course of alkylation is also influenced by the steric arrangement of the enamine. 1-Pyrrolidino-l-cycloheptene gave approximately equal quantities of the C- and N-alkylated products in dioxane, while 1-pyrroli-dino-l-cyclooctene, and 1-pyrrolidino-l-cyclononene afforded N-alkylated products exclusively under similar eonditions (29). The reason for N alkylation in the eight- and nine-membered ring compounds is to be found in the conformation of these rings, which prevents full interaction of the unshared electrons on nitrogen with the n eleetrons of the double bond. 

Mesomerism involving polarized and nonpolarized contributing enamine forms influences the enamine s spectral properties and chemical reactivity. For mesomerism to be present, a planar arrangement is required for the three atoms of enamine grouping and the five atoms immediately bound to this system. If this condition is not fulfilled, full interaction of the tt electrons of the double bond with the free electron pair on the nitrogen atom is impossible. Enamines in which mesomerism is inhibited do not show the properties characteristic of enamines, and only the mutual electrostatic interaction of the double bond and lone electron pair of the nitrogen atom can be observed. Such steric hindrance of mesomerism occurs mainly in polycyclic systems. 

The simplest examples of this type of compound are enamines derived from the quinuclidine skeleton (67). The formulation of enamines of qflmuclidine in a inesomeric form would violate Bredt s rule. Actually, the ultraviolet spectrum of 2,3-benzoquinuclidine shows that there exists no interaction of aromatic ring tt electrons and the nitrogen-free electron pair (160,169). The overlap of the olefinic tt orbital and the lone pair orbital on nitrogen is precluded. 

Only a few papers on the formation of compounds with small rings have been published. One example is the [2 + 2]-cycloaddition of electron-rich enamines to Schiff bases under high pressure (1.4 GPa) (87JOC365). The reaction leads to substituted azetidines (1). Four-membered ring heterocycles, thietane derivatives (4), are formed by interaction of sulfene (2) with enamines (3) (86CB257 93JOC3429). 

We illustrate this by two examples. Reaction of the 6-phenyl-1,2,4-triazine (407) with the enamine (406) follows orientation A since secondary orbital interactions between the n-electrons of the amino group and the 77-electrons of the phenyl ring are possible. The dihydropyridine (408) can eliminate the amine to form the cyclopenta[c]pyridine (409) since the cis orientated proton at C-3 can shift to the nitrogen to form the 1,4-dihy-dropyridine (410) from which the amine is eliminated. 

Even stronger polarizations of double bonds in alkenes are induced by electron withdrawing substituents, as present in enol ethers, enones, and enamines (Sections 4.6.2, 4.7, and 4.9.2). Deshielding of C-7 in norbornadiene (75.5 ppm, Table 4.12) is understood as arising from interaction of antibonding n orbitals at the olefinic carbon atoms with o orbitals of the bridgehead bonds [214, 216]. Spiroconjugation in spiro[4.4]nonatetraene is interpreted similarly [242]. 

The diazoacetonitrile-imine reaction may be considered complimentary to azide addition to cinnamonitriles because in the latter case only triazoline thermolysis products result.284 The reversed order of reactivity of the diazoacetonitrile to that of diazomethane implies an electrophilic attack on the imine and is explained in terms of a LUMOdi MC lonit[ile-HOMOin,int controlled interaction. Thus electron-rich enamines, which do not react with diazoalkanes, may be expected to react with electron-poor diazo compounds. 

A tautomeric equilibrium between enamine and methylene imine forms has been demonstrated to exist in 2-(3,4-dihydro-3-oxo-2(l//)-quinoxalinylidene)-Al-phenylacetamides and 3,4-dihydro-3-oxo-/V-phenyl-2-quinoxaline acetamides when these are in DMSO solution either in the absence or presence of TFA [95JHC671]. The reduction potentials of some pyrazines and their benzo-fused analogs have been summarized as part of an EPR study of the electron transfer interaction between nitrogen heterocycles and n-B N+BHf [95JOM123]. 

Fig. 2.5 Electronic and steric interactions in the approach of the electrophilic heteroatom (Het) to the nucleophilic carbon atom in the chiral enamine intermediate.

Theoretical analysis of this [4% + 27r]-cycloaddition reaction by consideration of frontier-orbital interactions between the electron-rich olefin (highest occupied molecular orbital, HOMO) and the electron-poor 5-nitropyrimidine (LUMO) has shown that the FMO perturbation theory correctly predicts an exclusive regiospecific addition of the enamine to N-l and C-4 of the pyrimidine ring (86JOC4070). 

Although proton chemical shifts are influenced significantly by factors other than electron density, they also reflect the polarization of the enamine framework and the degree of n,n interaction. Thus, the chemical shifts of the vinylic protons are modulated by the same factors discussed for the chemical shifts of the corresponding olefinic carbons, such as amine component, steric and electronic effects of the substituents and ring size effects. In particular, the chemical shift of the proton(s) at C(2) is lowered by increasing njt interaction, in parallel with what has been observed for < C(2). No general correlation exists between the chemical shifts of both nuclei probably as a consequence of their different sensitivity to steric, electronic and, particularly, anisotropic effects of the substituents. Nevertheless, for sets of structurally related compounds, reasonable linear correlations can be found between <5C(2) and <5H(2) (see below). Since the XH-NMR data available for enamines are more abundant than those for 13C and 15N, more complete structural information can be obtained for wider sets of compounds. 

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