Reactivity parameters thiophene

There are several systematic nuclear magnetic resonance studies of the interaction between the substituents and the protons and ring atoms of five-membered heterocycles. In some 2-substituted furans, thiophenes, selenophenes, and tellurophenes there is a linear correlation between the electronegativity of the chalcogen and several of the NMR parameters.28 As there also is a good correlation between the shifts of the corresponding protons and carbons in the four heterocycles, the shifts of unknown selenophene and tellurophene derivatives can be predicted when those of thiophene are known. This is of special interest for the tellurophene derivatives, since they are difficult to synthesize. In the selenophene series, where a representative set of substituents can be introduced in the 2- as well as in the 3-position, the correlation between the H and 13C shifts and the reactivity parameters according to Swain and Lupton s two-parameter equation... 

Relative rates. The order of reactivities at position 2 is pyrrole > furan > tellurophene > selenophene > thiophene. Where data are available for both 2- and 3-positions, the following order is seen 2-furan > 2-thiophene > 3-furan > 3-thiophene. Reactivity parameters (+) for the 2- and 3-positions of thiophene for reactions of varying -values (ranging from 0.66 to 12.0) have been established. 

Rate coefficients and kinetic parameters for iododeboronation were determined for the benzene- and thiophene-boronic acids, and the results are given in Table 256. The relative reactivities derived from this work correlated well with those obtained in a number of other electrophilic substitutions572, which is perhaps surprising in view of the large variation in the entropies of activation. These differences were explained by Brown et al.132 in terms of the transition state for the phenyl compound occurring earlier along the reaction coordinate than for the... 

The reactivities of isomeric thienothiophenes calculated in n -electron approximation by the PPP method, and those calculated considering all valence electrons, show reasonable agreement. It should be noted, however, that the choice of parameters in PPP calculations is somewhat arbitrary, especially for heavy atoms (e.g., sulfur). This may lead to a discrepancy between theoretical (in 7r-electron approximation) and experimental estimation of reactivities. For example, Clark applied the semiempirical method PPP SCF MO to calculate the reactivities of different positions in thienothiophenes 1—3, thiophene, and naphthalene from the localization energy values and found the following order of decreasing reactivity for electrophilic substitution thieno[3,4-b]-thiophene (3) > thieno[2,3-Z>]thiophene (I) > thieno [3,2-b]thiophene... 

Internal plasticizing demands a chemical relationship between the components which constitute the product. Therefore, good effects can be expected from copolymers of styrene and isobutylene, ethylene, or diolefins like butadiene or isoprene. Internal plasticizing of PVC can be effected by copolymerizing vinyl chloride with acrylates of higher alcohols or maleates and fumarates. The important ABS products are internal copolymers of butadiene, styrene, and acrylonitrile. The hardness of the unipolymers of styrene and acrylonitrile can be modified by butadiene which, as a unipolymer, gives soft, rubberlike products. As the copolymerization parameters of most monomers are known, it is relatively easy to choose the most suitable partner for the copolymerization. When the product of the r—values is l, there is an ideal copolymerization, because the relative reactivity of both monomers toward the radicals is the same. Styrene/butadiene, styrene/vinyl thiophene, and... 

C(8a)—C(9) are elongated. The common bond C(3a)—C(9a) is also lengthened. A comparison of the 7r-electron densities reveals that all three procedures show almost the same trends. The calculated 7r-electron densities for thieno[2,3-6]quinoline (378) and thieno[3,4-6]quinoIine (379) do not differ significantly from the corresponding thienopyridines provided that the same set of parameters is used. Calculated reactivity indices indicate that electrophilic substitution reactions should occur predominantly in the thiophene unit. 

It is evident that localization energies are excellent for predicting the results here (and are totally within a given molecule) whereas tt densities are very poor. Localization energies also correctly predict (using the same parameters) the reactivity order in thiophene, the thienothiophenes (Section 5) and the dithienothiophenes (Section 6). By contrast for benzo-[6)thiophene (Section 2), only tt densities correctly predict the overall order whereas neither method works for dibenzothiophene (Section 3). Other TT-density calculations for the dithienobenzenes [69ZN(B)12] also fail. 

The scope of this reaction is limited to electron-rich arenes and heteroarenes such as thiophenes, pyrroles, furans, indoles, and alkoxybenzenes as nucleophilic partners, corresponding to a Mayr ir-nucleophilicily parameter N>-1 [75-78], Electron-neutral to electron-deficient iodo(hetero)arenes are suitable electrophilic partners. Aryl halides or pseudohalides that are less reactive towards oxidative addition (Br, Cl, OTf) are not sufficiently reactive partners in this reaction. The reactivity of sterically hindered and/or ortho substituted iodoarenes has not been demonstrated. However biaryls bearing one ortho substituent of relatively small steric demand (e.g., from methoxybenzene or /V-mcthylindole) have been prepared. 

As concerns the factors leading to the observed reactivity scale, it is our opinion that the relative ground-state energies play a more important role than the relative stabilities of the intermediate carbocations. From a qualitative point of view, it is observed that the more aromatic is the starting molecule, the smaller is the rate of substitution. The order of reactivity is in fact the reverse of the order of the ground-state aromaticities, as determined by several different approaches thiophene > selenophene > tellurophene > furan (see Section II,C). A quantitative confirmation of this hypothesis has been obtained by analysis of the activation parameters for the formylation reaction.57... 

The resonance energy for thiophene as calculated by Dewar is found to be 6.5 kcal mol and shows a decrease in aromaticity in the order benzene > thiophene > pyrrole > furan. This is in agreement with other criteria such as chemical reactivity, etc. The HSRE is found to be 4.5 kcal mol and the order is benzene > pyrrole > thiophene > furan. Furthermore, the resonance energies obtained by this method are generally much lower than those by Dewar. Obviously, the parameters are defined much more clearly in the Dewar system. 

Finally, we will concentrate on the chemical reactivity of silyl derivatives of thiophene. The oxidative polymerization of various silyl monomers lead to polythiophene. The evaluation of this new polymerization reaction implies a precise characterization of the produced conjugated materials. Knowledge and the control of the pertinent parameters which direct the properties of the conjugated systems are essential. Also required is the development of methods which allow a precise characterization of the samples. The role of vibrational infrared and Raman spectroscopy is of fundamental importance in this field. Optical spectroscopy is one of the few tools for unravelling the structure of these materials and understanding their properties. First, new criteria based on infrared, Raman and photoluminescence spectroscopy which allow precise estimates of the conjugation properties will be reported. Then the synthesis and characterization of polythiophene samples arising from the oxidative polymerization of silyl thiophene will be presented. 

Theoretical Studies and Physical Properties.—Quantum chemical calculations on the reactivity of thienothiophens have been carried out. " The e.s.r. spectra of the radical anions of carbonyl, nitro, and cyano derivatives of the two [b]-fused thiophens have been studied. The proton chemical shifts of 2-substituted thieno[2,3-b]thiophens have been correlated with the two-parameter equation of Swain and Lupton. " ... 

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