Electronic conjugative effects

TABLE 6.21 Ti-Electron conjugation effect on the group contribution of phenylene (Yg and Ym)... 

Theoretical approaches have been applied to understand the features that are responsible for the stability of imidazole (in)-2-ylidine. Density Functional theory (DFT) was applied in this respect. Studies examining the relationship between structure and reactivity have shown that thermodynamic stabilization (electronic conjugation effect) usually plays an important role in stabilizing the singlet state and it can become less reactive to such an extent that it can be isolated under ambient conditions. 

A silicon atom might be expected to release electrons inductively, but because of empty 7-orbitals shows the overall character ( + 7 —717). Nitration of trimethylsilylbenzene with nitric acid in acetic anhydride at —10 to o °C gives 25-5,39-8,30-2 and 6-8 %, respectively, of 0-, m-, and /)-nitro-trimethylsilylbenzene and nitrobenzene, with a rate of reaction relative to that of benzene of about 1-5. The figures give no indication of an important conjugative effect. 

Methacryhc acid and its ester derivatives are Ctfjy -unsaturated carbonyl compounds and exhibit the reactivity typical of this class of compounds, ie, Michael and Michael-type conjugate addition reactions and a variety of cycloaddition and related reactions. Although less reactive than the corresponding acrylates as the result of the electron-donating effect and the steric hindrance of the a-methyl group, methacrylates readily undergo a wide variety of reactions and are valuable intermediates in many synthetic procedures. 

NMR measurements on 2,3-fluorophenylthiirene 1-oxides and 1,1-dioxides show that electron withdrawing conjugative effects are greater for the sulf one than for the sulfoxide and that both are less conjugated than cyclopropenones <79JA390). 

Later there was an attempt by ab initio calculation to fit the electron structure of diazirine into the Walsh model of cyclopropane (69MI50800). According to these SCF-LCAO-MO calculations three MOs add to the description of the lone electron pairs, all of which also contribute to some extent to ring bonding. As to strain, 7r-character and conjugative effect, the term pseudo-rr-character was used. 

Azulene does have an appreciable dipole moment (0.8 The essentially single-bond nature of the shared bond indicates, however, that the conjugation is principally around the periphery of the molecule. Several MO calculations have been applied to azulene. At the MNDO and STO-3G levels, structures with considerable bond alternation are found as the minimum-energy structures. Calculations which include electron correlation effects give a delocalized n system as the minimum-energy structure. ... 

Electron-accepting conjugative effects of sulfinyl and sulfonio groups may be seen in a few experimental observations. For example, it was reported twenty years ago that p-iodophenyl sulfoxide reacted readily with alkali hydroxide whereas the m-isomer did not react under the same conditions36. More recent quantitative data on the electron-accepting effects of these tri- and tetra-coordinated sulfur-containing groups can be found in the quantitative data on reactivities, listed in Table 6. 

The electron-accepting conjugative effect of the sulfonio group is far greater than that of the sulfonyl group and even exceeds that of the nitro group, as can be observed from the... 

With one more N-B bond, the cyclic conjugation is discontinuous in 1,3,2,4-diaza-diborine. The donors and acceptors are alternately disposed along the cyclic chain. Electrons cannot effectively delocalize in a cyclic manner, but between the adjacent donor-acceptor pairs in a non-cyclic manner. The diazadiborine is not predicted to be aromatic. 

The effects of cyclic 6n electron conjugation have been found in the optimized geometries of pentazole 17 [102] and hexazine 18 [97], The N=N bond is longer than the isolated double bond in NH=NH. The N-N single bond in the tetrazadiene moiety is shorter than the single bond in NH NH. The bond lengths in 18 are nearly intermediate between those in NH NH and NH=NH. The aromatic character of pentazoles was supported by the effect of electron donating substituents on the thermodynamic and kinetic stabilization [103],... 

Conjugation of the 7t-electrons of the carbon-carbon double bond with the LUMO sulfur 3d-orbitals would be expected to stabilize the Hiickel 4n -I- 2 (n = 0) array of n-electrons in the thiirene dioxide system. No wonder, therefore, that the successful synthesis of the first member in this series (e.g. 19b) has initiated and stimulated several studies , the main objective of which was to determine whether or not thiirene dioxides should be considered to be aromatic (or pseudo-aromatic ) and/or to what extent conjugation effects, which require some sort of n-d bonding in the conjugatively unsaturated sulfones, are operative within these systems. The fact that the sulfur-oxygen bond lengths in thiirene dioxides were found to be similar to those of other 802-containing compounds, does not corroborate a Hiickel-type jr-delocalization... 

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