Push-pull delocalizations

Let us first try to obtain a reactant-like perspective on the IRC pathway. We can employ the CHOOSE keyHst (Section 5.5) to specify a reactant-Uke bond pattern, and thereby continue to follow the progress of the NBO push-pull delocalizations (10.11 and 10.12) whose values are plotted at the left of... 

Figure 10.12 Leading product-type (hydroxymethylene) push-pull delocalizations (10.8 and 10.19) (and estimates) in TS geometry for formaldehyde isomerization (cf. Fig. 10.11).

Figure 10.13 Similar to Fig. 10.12, for reactant-type (formaldehyde) push-pull delocalizations. 

The chemical reactions of Sections 10.1-10.3, although elementary by the standard (TS energetic) criterion, involve multiple NBO push-pull delocalizations. Such reactions appear mechanistically complex from adiabatic NBO perspective, typically leading to high activation barrier and formal forbidden character. We now wish to... 

Thiophene-1-oxide and 1 -substituted thiophenium salts present reduced aromaticity.144 A variety of aromaticity criteria were used in order to assess which of the 1,1-dioxide isomers of thiophene, thiazole, isothiazole, and thiadiazole was the most delocalized (Scheme 46).145 The relative aromaticity of those molecules is determined by the proximity of the nitrogen atoms to the sulfur, which actually accounts for its ability to participate in a push-pull system with the oxygen atoms of the sulfone moiety. The relative aromaticity decreases in the series isothiazole-1,1-dioxide (97) > thiazole-1,1 -dioxide (98) > thiophene-1-dioxide (99) then, one has the series 1,2,5 -thiadiazole-1,1 -dioxide (100) > 1, 2,4-thiadiaz-ole-1,1-dioxide (101) > 1,2,3-thiadiazole-1,1 -dioxide (102) > 1,3,4-thiadiazole-l,1-dioxide (103) in the order of decreasing aromaticity. As 1,2,5-thiadiazole-1,1-dioxide (100) was not synthesized, the approximations used extrapolations of data obtained for its 3,4-dimethyl-substituted analogue 104 (Scheme 46). 

In the ground state, aminomethylenemalonates possess an essentially planar geometry, which maximizes the electron delocalization in the molecules. In the heteropolar transition state, the plane of the groups R3 and NR R2 and the plane of the two carbonyl groups occupy orthogonal positions. More details of the dynamic and static stereochemistry of push-pull ethylenes, as in compounds 1 and 2, are discussed in two excellent reviews (73TS295 83TS83). 

The low C=C barriers in push-pull ethylenes compared to the 6S.S kcal/ mol in ethylene show that die effects of delocalization on the tr-electron energy in the transition state must be much greater than the effects in the ground state— that is, the important substituent effects on the barriers must occur in the transition state. Besides, an effect that improves delocalization in the ground state would be barrier raising, if it were not accompanied by an at least equal stabilization of the transition state. 

The second chapter, by Jan Sandstrom, deals with stereochemical features of push-pull ethylenes. The focus is on rotational barriers, which span a large range of values. The ease of twisting is partly a matter of electron delocalization and partly a matter of steric and solvent effects. Electronic structure and such related items as dipole moments and photoelectron spectra for these systems are discussed. The chapter also deals with the structure and chiroptical properties of twisted ethylenes that do not have push-pull effects, such as frans-cyclooctene. 

Important classes of enamines are those having electron-withdrawing substituents, R2 and/or R3, at C(2), particularly when the substituent(s) act(s) by a +R effect, as in aminoenones and nitroenamines. The stronger and more extensive electron delocalization and the concomitant changes in bond orders of the resulting push-pull systems is well reflected in the NMR spectra and have been extensively investigated in this way. 

However, push-pull ethylenes and polyenes are also of interest from a stereochemical point of view due to the observation of hindered rotation of donor and acceptor groups with considerable barriers, and also of low barriers to rotation about the carbon-carbon double bond. It may be appropriate here to lay out briefly, with 3-dimethylaminoacrolein as an example, the principles on which a discussion of the C1—N, C2-acceptor and C1=C2 barriers may be based. The most primitive approach considers only the electron delocalization in the ground state. The electron distribution is described by superposition of two limiting structures, the nonpolar A and the dipolar B (Scheme 1). 

Donor/acceptor-substituted phosphole 22 exhibits classical properties, namely the phosphorus atom has a pyramidal geometry and the aromatic character of the heterole is similar to that of cyclopentadiene <2000JOC2631>. Due to the push-pull substitution pattern, significant delocalization of the endocyclic 7t-electron density over the entire system... 

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