Effect of ring substituents

The quatemization of the nitrogen atom of the thiazole ring (the Menschutkin s reaction) by alkyl halide or methyl tosylate can be used to measure the reactivity of this atom and thus to evaluate steric and electronic effects of ring substituents. 

The effects of ring substituents and counterions on the preparation and reac-tivity of the Af-fluoropyridinium salts have been reported in detail [74, 75, 76]. 

The effects of ring substituents on the rate are consistent with a unimolecular rate-determining cleavage. " ... 

Table 3.17. The Effect of Ring Substituents on Type 11 Cleavage...

Type I cleavage of ketones, 75 Type II cleavage of ketones effect of 7-substituents, 122-124 effect of ring substituents, 124 of 5 -(+)-4-methyl-l-phenyl-1- hexanone, 118... 

The very small p- and m-values observed for the fast bromination of a-methoxystyrenes deserve comment since they are the smallest found for this electrophilic addition. The rates, almost but not quite diffusion-controlled, are amongst the highest. The sensitivity to polar effects of ring substituents is very attenuated but still significant that to resonance is nil. These unusually low p-values for a reaction leading to a benzylic carbocation are accompanied by a very small sensitivity to the solvent. All these data support a very early transition state for this olefin series. Accordingly, for the still more reactive acetophenone enols, the bromination of which is diffusion-controlled, the usual sensitivity to substituents is annulled. 

The effect of ring substituents on the rate constants, deuterium kinetic isotope effects and Arrhenius parameters for ene-additions of acetone to 1,1-diphenylsilane have been explained in terms of a mechanism involving fast, reversible formation of a zwitterionic silene-ketone complex, followed by a rate-limiting proton transfer between the a-carbonyl and silenic carbon. A study of the thermal and Lewis acid-catalysed intramolecular ene reactions of allenylsilanes with a variety of... 

Stockdale, M. and Selwyn, M.J. Effects of rings substituents on the activity of phenols as inhibitors and nnconplers of mitochondrial respiration, Eur. J. Biochem., 21 565-574, 1971. 

The effects of ring substituents on electrophilic heteroaromatic substitution follow predictable general trends, i.e. electron-releasing substituents, especially hydroxy (6) and amino... 

Scheme 32 Reproduced from Davoli, P. Forni, A. Moretti, I. Prati, F. Torre, G. On the effect of ring substituents in the carbonyiation of aziridines. Tetrahedron 2001,57, 1801-1812, with permission from Eisevier.

In all cases the boron goes to the side of the double bond that has more hydrogens, whether the substituents are aryl or alkyl.35B Thus the reaction of 43 with BH3 gives 98% 44 and only 2% of the other product. This actually follows Markovnikov s rule, since boron is more positive than hydrogen. However, the regioselectivity is caused mostly by steric factors, though electronic factors also play a part. Studies of the effect of ring substituents on rates and on the direction of attack in hydroboration of substituted styrenes showed that the attack by boron has electrophilic character.359 When both sides of the double bond are... 

Figure 8.6 Effect of ring substituents on the pof benzoic acid, phenyl acetic acid, and phenol.

Vasudevan, D., and A. T. Stone, Adsorption of catechols, 2-aminophenols, and 1,2-phenylenediamines at the metal (hydr)oxide-water interface Effects of ring substituents on the adsorption onto Ti02 , Environ. Sci. Technol., 30, 1604-1613 (1996). 

Sehlotho N, Nyokong T (2006) Effects of ring substituents on electrocatalytic activity of manganese phthalocyanines towards the reduction of molecular oxygen. J Electroanal Chem... 

J. Ring Substituents. One has to bear in mind that two main effects of ring substituents on the Norrish-Yang reaction of aromatic ketones exist. 

Thiazoles readily undergo AT-alkylation by alkyl halides or tosylates (Menshutkin reaction). The sensitivity of this SN2 quaternization reaction to the molecular environment of the nitrogen atom has been used to evaluate, in a quantitative way, steric and electronic effects of ring substituents. The electronic effect of alkyl substituents (unperturbed by any steric effect) may be evaluated from the rate constants for the reaction of 5-alkylthiazoles with methyl iodide (in nitrobenzene at 30 °C) Table 19 shows that introducing a methyl group at the 5-position corresponds to an acceleration of the rate by a factor of 2 but that each addition of a methyl ramification to the 5-alkyl group enhances the rate only by a factor of 1.1. The data in Table 19 fit well with a Hammett-Taft equation (3) ... 

Reaction of 1,2,3-thiadiazoles with electrophiles has not been extensively studied, so effects of ring substituents have not been tabulated. Reaction of 1,2,3-thiadiazoles with acyl halides or metal ions has not even been reported. 

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