Conjugative effect—

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. 

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. 

Other substituted systems, however, might be planar due to conjugation effects with acceptor substituents, as has been found in an X-ray structural analysis of 1,4-dioxocin-6-carboxylic acid chloride the eight-membered ring is practically planar with a coplanar arrangement of the substituent.9... 

Both have the same steric and conjugative effects, but the polar effect is largely absent in the acid-catalyzed reactions. Subtraction of the logarithms of the two relative rates therefore leaves just the polar effect. This is written... 

The structural features and the spectroscopic characteristics of the thiirene dioxide system (22) are of special theoretical interest since, on the basis of analogy with cyclopropenone (23), it is a possible nonbenzenoid aromatic system with all the physical and chemical implications involved. Aromatic and/or conjugative effects, if any, require transmission through the d-orbitals of the sulfur atom. 

The less strain energy inherent in the four-membered ring sulfoxides and sulfones, their less distorted geometries and the lack of potential aromatic -type conjugation effects make the comparison of their physical and chemical properties with other cyclic and... 

In view of the limited capacity of the sulfur atom in the sulfoxide and sulfone functional groups to transmit conjugative effects due to the insulating effect of the LUMO sulfur d-orbitals45,46,56, the application of the UV technique even in the case of the cyclic vinyl sulfones (e.g. thiete dioxides 6b) cannot be expected to find extensive use. UV spectra of substituted thiete dioxides in which an extended conjugated system (e.g. 194) exists in the molecule, did provide useful information for structure elucidation231. However, the extent... 

The existence or nonexistence of conjugative effects involving the sulfone group in thiophene dioxides (a problem analogous to that in thiirene oxide and dioxide systems2,1 ) has been the subject of many studies resulting, nonetheless, in no unequivocal conclusion280. 

Yukawa and coworkers (1972)84 determined a0 values from the rate constants for alkaline hydrolysis of m- and p-substituted-benzyl benzoates in 70% (v/v) aqueous acetone at 25 °C. ap° values for SOMe and S02Me were found to be 0.573 and 0.749 respectively. These were compared with 0.564 and 0.721, respectively, for values determined from the rate constants of alkaline hydrolysis of substituted ethyl benzoates in 85% aqueous ethanol. From these values there is no evidence for any — R cross-conjugative effect of SOMe as a substituent in the benzoate moiety, which is eliminated when it is in the benzyl. However, both the values for SOMe are substantially higher than most of the ap values for SOMe which we have surveyed previously. For S02Me the order ap° > if significant,... 

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... 

Data are also available with a-acetylenic aliphatic sulphones, which involve only two steps i.e., saturation of the triple bond without subsequent cleavage of the Caliphalic—S bond, since it is not reactive. However, the introduction of an aromatic ring to the S02 group does not lead, contrary to what is observed with enones, to a potential shift toward less reducing potential values. Thus, the aromatic moiety introduced apparently does not bring any additional conjugation effect but even seems to decrease the activation of the unsaturated bond, as shown by data in Tables 6 and 7 where most of the potentials refer to the same saturated calomel electrode under similar experimental conditions. 

Many micro-channel blocks have a high thermal conductivity and conjugate effects may become important. In this chapter we considered analytical and experi-... 

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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