Reactivities of Side-Chains

Reactives of Side-Chains of Monocyclic Thiophens. - The rate constants for the esterification of some 3-, 4-, and 5-substituted thiophen-2-carboxylic acids and of some 2- and 4-substituted thiophen-3-carboxylic acids with diazodiphenylmethane in methanol solution have been measured, and linear correlations gave information about the transmission of substituent effects. The rates of alkaline hydrolysis of ethyl thiophen-2-carboxylate in ethanol-water and DMSO-water media have been measured and compared with those of other heterocyclic esters. The kinetics of iodination of 2-acetylthiophen in methanol-water, using different carboxylate buffers, have been studied.Basicity constants have been measured for j3-(2-thienyl)-acrylamides and compared with those of the corresponding benzene and furan derivatives. The acidity constants of ( )-a-phenyl-j3-(2-thienyl)-acrylic acids and analogous furan-, selenophen-, and pyridine-substituted compounds have been measured, and have been rationalized by an equation involving separate contributions of polar, conjugative, and steric effects of the heterocycles.  

Buscemi, G. Consiglio, and D. Spinelli, J. HeterocycL Chem., 1981, 18, 

Alberghina, M. E. Amato, S. Fisichella, and S. Occhipinti, Gazz. Chim. Ital., 1981, 

Maccarone, A. Mamo,G. Perrini, and M. Torre,/. HeterocycL Chem., 1981,18,735. 

Electrochemical hydrodimerization of thiophen-2-carbaldehyde via the anion radicals led only to a 10% yield of the pinacol the same amount of 

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A very large number of furoxan derivatives have been synthesised, taking advantage of the reactivity of side-chain functional groups in the ring [6, 9,11]. Here, the principal procedures used to synthesise the furoxan system will only be discussed briefly [6-8, 10] (Scheme 6.3). 

Reactivity of side-chains and fused benzene rings 161... 

Curiously enough, variation of A in Eq. (4) and of the equivalent ratios in Eqs. (1) and (2) is quite clear-cut for passage of substituent effects across different aromatic and heteroaromatic systems, as will be emphasized in this review subsequently, but its deviation from unity in the case of reactivities of side chains of varying geometry is much less certain.8-10 Alternative sets of a, and aR values to those of Taft have been offered by Exner.11... 

The potential for interaction of the heteroatom (electron donation) with positive charge on a side-chain, especially at an a-position, has a number of effects amongst the most important is the enhanced reactivity of side-chain derivatives carrying leaving groups. Similarly, carbonyl groups attached to five-membered heterocycles have somewhat reduced reactivity, as implied by the resonance contributor shown. 

Reactivities of Side-Chains of Monocyclic Thiophens. - The protonation behaviour of some 3-, 4-, and 5-substituted thiophene-carboxamides in aqueous sulphuric acid solution has been studied by the u.v. technique. + values were determined by and Bunnett-Olsen methods using multivariate analysis, mission of electronic effects was obtained... 

Reactivities of Side-chains of Monocyclic Thiophens.—The protonation of various furan- and thiophen-carboxamides in aqueous sulphur acid solution has been investigated by u.v. spectroscopy. The values of pKbh that were calculated using the acidity function and the Bunnett-Olsen linear free-energy relationship indicate the lower basicity of furan- and thiophen-2-carboxamides with respect to that of benzamides and of the 3-derivatives. The kinetics of isomerization of cw-l-(2-thienyl)-2-phenylacrylonitrile, as well as of its 2-furyl and 2-selenienyl analogues, have been studied in a solution of decahydronaph-thalene, with methanesulphonic acid and potassium t-butoxide as catalysts, and the mechanism has been discussed. ... 

Reactivity of side chains 4.6.1 Reactivity of methyl groups... 

It was also reported [225] that photo-reactivity of side-chain liquid crystalline polymers can align liquid crystals both in a parallel mode or perpendicularly, depending on the degree of the photoreaction of the polymers. Presumably, this particular polymer can multiphoto-ahgn the liquid crystal pattern without a change of the direction of the linearly polarized UV light. The chemical structure of such an aligning polymer is depicted as follows ... 

I. Reactivity of Side Chains 1. Toward Electrophilic Reagents... 

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