Thienyl groups, substituent effects

The acid cleavage of the aryl— silicon bond (desilylation), which provides a measure of the reactivity of the aromatic carbon of the bond, has been applied to 2- and 3-thienyl trimethylsilane, It was found that the 2-isomer reacted only 43.5 times faster than the 3-isomer and 5000 times faster than the phenyl compound at 50,2°C in acetic acid containing aqueous sulfuric acid. The results so far are consistent with the relative reactivities of thiophene upon detritia-tion if a linear free-energy relationship between the substituent effect in detritiation and desilylation is assumed, as the p-methyl group activates about 240 (200-300) times in detritiation with aqueous sulfuric acid and about 18 times in desilylation. A direct experimental comparison of the difference between benzene and thiophene in detritiation has not been carried out, but it may be mentioned that even in 80.7% sulfuric acid, benzene is detritiated about 600 times slower than 2-tritiothiophene. The aforementioned consideration makes it probable that under similar conditions the ratio of the rates of detritiation of thiophene and benzene is larger than in the desilylation. A still larger difference in reactivity between the 2-position of thiophene and benzene has been found for acetoxymercuration which... 

Also as noted above any substituents present have little effect upon such oxidations. In 2,2 -methylenedifuran (118) the rings are attacked simultaneously giving a tetramethoxy derivative.297 Even the bulk of the fert-butyl group has little effect.298 The only marked substituent effect is that exerted by an aromatic (benzene, thiophene, furan) residue which, if directly attached at the 2-position, promotes elimination instead of the addition of another methoxy group. The net process then becomes one of arylation, as when 2-(2-thienyl)furan (119) is oxidized to 120.298 There are reports that acetyl and carboxy groups can be ejected during oxidation, but that ester groups are usually retained.287... 

Scheme 31 Structural formulas of a series of mero-substituted ZnPs appended with p-carboxyphenyl and thienyl substituents. Adapted with permission from Ambre R, Chen K-B, Yao C-F, Luo L, Diau EW-G, Hung C-H (2012) Effects of porphyrinic mero-substituents on the photovoltaic performance of dye-sensitized solar cells number and position of p-carboxyphenyl and thienyl groups on zinc porphyrins. J Phys Chem C 2012 116(22) 11907-11916. Copyright (2012) American Chemical Society...

In many cases, substituents linked to a pyrrole, furan or thiophene ring show similar reactivity to those linked to a benzenoid nucleus. This generalization is not true for amino or hydroxyl groups. Hydroxy compounds exist largely, or entirely, in an alternative nonaromatic tautomeric form. Derivatives of this type show little resemblance in their reactions to anilines or phenols. Thienyl- and especially pyrryl- and furyl-methyl halides show enhanced reactivity compared with benzyl halides because the halogen is made more labile by electron release of the type shown below. Hydroxymethyl and aminomethyl groups on heteroaromatic nuclei are activated to nucleophilic attack by a similar effect. 

Thienyl tellurolates also remove several different substituents such as acetoxy, mesyloxy, phenylthio and 2-thienyltelluro groups, as well as effect the reductive dehalo-genation of bromoacetanilide, and of a-haloacids (such as a-bromophenylacetic acid, a-bromo-l-naphthylacetic acid and a-chlorodiphenylacetic acid). ... 

Italian workers have reported extensive studies on the evaluation of a values for thienyl and furyl groups. These are collected in Table XII. The am, Om, and am values reveal the electron-withdrawing inductive effect of the rings, while as expected from the previous discussion of this section, these substituents fall into that small category whose op, oj>, and Op values are all different, revealing an ability to both accept and donate electrons by resonance under the appropriate incentives. 

The cycloaddition of diazomethane to SchifT bases from heterocyclic aldehydes and anilines provides a useful route to heterocyclic substituted triazolines. Unlike olefins bearing heterocyclic substituents, the heterocyclic imines can be obtained readily by reaction of the appropriate aldehyde and amine thus the diazomethane-imine addition has greater scope than the olefin-azide reaction. NMR spectroscopic studies of the orientation of addition are in accord with previously reported mechanistic considerations (see, e.g., Scheme 93).329 In addition to the influence of the N-aryl group, the electron-withdrawing power of the heterocyclic substituent on the Schiff-base carbon also has a substantial effect on imine reactivity, in the order 2-quinolyl 2-, 3-, or 4-pyridyl > phenyl > 2-thienyl as 2-furyl.329... 

IR spectra there is absorption in the region 1540-1620 cm-1, characteristic of the chelated cis-enol bands characteristic of a free carbonyl group are absent. The effect of substituents on the enolization of /3-di-ketones (in acetone) was studied by NMR spectroscopy. In the cu-acyl-2-acetoselenophene (20) and a> -acyl-3-acetoselenophene(21) series, the acyl groups COR increase the enolization in the following order of R methyl < a-furyl < a-thienyl < trifluoromethyl < phenyl < a-pyridyl. Thus, /S-diketones containing aroyl or heteroaroyl radicals together with selenienyl exist completely as the cis-enols.121... 

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