Electronic effects transmission

Although Noyce and Fike have recently found for the solvolysis of 2-thiazolyl-ethyl chlorides analogous modality of substituent electronic effect transmission from position 2 toward position 5 and from position 5 toward position 2(60). a more general conclusion indicates that the... 

However, not all of the vinyloxyphosphazene monomers will undergo radical polymerization) those with amino substituents are unreac-tive. The i C nmr data indicate that these species electronically resemble vinyl ethers (which do not undergo radical polymerization) whereas the reactive derivatives resemble vinyl acetate. These data demonstrate an excellent example of electronic effect transmission in cyclophosphazene systems. 

The information concerning the character and degree of the substituent electronic effect transmission from C-2 to C-5(6) and in the opposite direction can readily be obtained from the correlation equation (3.2) of C-5(6) chemical shifts of 2-substituted benzimidazoles and C-2 chemical shifts of the 5(6)- isomers (Scheme 3.12) [220, 689, 691],... 

However, few parameters of this kind have so far been determined experimentally. Therefore, in heteroaromatic chemistry the quantitative investigation of reactivity remains a needed area of research. Also, being in a large measure of a formalized character, the equations obtained require a detailed analysis, which should reveal the peculiarities of transmission of electronic effects in various organic families, and allow understanding of the role played by the heteroatom in electronic effect transmission and the details of the subtle structure of interacting substituents. In this task, there is still an acute problem in quantifying the effect of solvent on the reactivity of heterocyclic compounds of different classes. 

The transmission of electronic effects from the substituent to the reaction site. 

Calculated on the assumption that no transmission of electronic effects occurs through the indole nitrogen atom. 

On the basis of a naive analogy with cyclopropenones, the ground-state aromatic stabilization of which has been recently reconfirmed, some kind of aromaticity can, in principle, be assigned to these systems when Z = SO or S02, assuming a possibility for transmission of electronic effects via -conjugation. 

Most studies of the transmission of electronic effects through —SO— and —S02— have involved examining the influence of substituents on the pKa values of carboxylic acids and phenols of the types XC6H4SO YC6H4C02H or XC6H4SO C6H4OH where Y = CH2 or CH=CH. There have been a few relevant spectroscopic studies. 

Konovalov and colleagues145 studied the chemical shifts of the CH2 protons for a series of derivatives of arylsulfonylacetic acids and /i-disulfones in DMSO or CDClj. The transmission of electronic effects through the S02 bridge was detected and attributed to the possibility of participation of the 3d orbitals of sulfur in the formation of the S=0 bond and to the existence of dn-pn conjugation . The paper contains various references to related (mainly Soviet) work. 

Finally, although strictly outside the scope of this chapter (and book), we mention a study of the transmission of electronic effects in iV-alkylidene-sulfenamides, -sulfinamides and -sulfonamides, XC6H4SO N=CHC6H4OH-2, through measuring the PMR chemical shifts of the o-OH148. The p values lie in the order S < SO > S02 (cf. above) which is attributed to a complex interplay of factors. 

Charton s at values for SOPh and S02Ph are based on the pKa values for phenylsulfinyl- and phenylsulfonyl-acetic acids137, which we have already discussed in connection with the transmission of electronic effects by SO and S02 (Section III.C). These groups have also been the subject of a detailed study by Hogeveen and Montanari172, who measured the pKa values of some 3-phenylthio-, 3-phenylsulfinyl- and... 

A recent H-NMR study of imidazole bonding in synthetic hemichromes led La Mar to the conclusion that 7r-bonding is important for these systems, and that imidazole acts primarily as a 7r-acceptor (158). However, our evidence on the osmochrome/ osmichrome system lends support to the idea that the jr-donor function of Him should not be overlooked, and more experiments are needed to understand the hemichrome systems. The difficulty encountered in their theoretical treatment is due to the fact that their optical spectra are of the mixed hyper/hypso type (50). Thus there will be a variety of transmission paths for electronic effects which remain to be illuminated. The present article may serve as an intuitive guide and a challenge to further research. 

The metal, in acting as a collection point to bring reactants together, is likely at the very least to promote enhanced rates of reaction by operation of the neighboring group effect. If, in addition, the transmission of electronic effects of the metal (Secs. 6.2-6.5) also occurs, as is usually the case, then large overall rate enhancements may be encountered. 

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