Pyrroles rates

Recently [89IZV(ip)] the effect of the cycloalkanone oxime ring size on their pyrrolization rate under the action of acetylene in the KOH/DMSO system at 86°C and atmospheric pressure has been examined (Scheme 19). 

Detailed ICR analysis of the complex reaction pattern induced by the attack of the ions from biacetyl (acetyl, biacetyl, and triacetyl ions) on pyrrole has allowed the demonstration of single and double resonance effects on the abundance of the product ions (72JA6862 73IJM195). Interestingly, triacetyl cations have been found to be more reactive than biacetyl ions toward pyrrole (rate constants 11 versus 4.3 x 10 ° cm molecule sec ), whereas acetyl cations appear unreactive. Furthermore, the rate of the gas-phase reaction of biacetyl cations with pyrrole is only four times as fast as with benzene, in striking contrast with the enormous reactivity difference of the two aromatic substrates measured for a number of electrophilic reactants in solution. 

Early qualitative observations (see Section 1.1.2) that pyrrolization rate considerably increases on going from LiOH to NaOH and further to the KOH and the LiOH/CsF systems are confirmed. In the presence of LiOH and NaOH/CsF systems, the vinylation is suppressed and selectivity of 4,5,6,7-tetrahydroindole formation rises. 

The one-electron reduction of thiazole in aqueous solution has been studied by the technique of pulse radiolysis and kinetic absorption spectrophotometry (514). The acetone ketyl radical (CH ljCOH and the solvated electron e were used as one-electron reducing agents. The reaction rate constant of with thiazole determined at pH 8.0 is fe = 2.1 X 10 mole sec in agreement with 2.5 x 10 mole sec" , the value given by the National Bureau of Standards (513). It is considerably higher than that for thiophene (6.5 x 10" mole" sec" ) (513) and pyrrole (6.0 X10 mole sec ) (513). The reaction rate constant of acetone ketyl radical with thiazolium ion determined at pH 0.8 is lc = 6.2=10 mole sec" . Relatively strong transient absorption spectra are observed from these one-electron reactions they show (nm) and e... 

Endo adducts are usually favored by iateractions between the double bonds of the diene and the carbonyl groups of the dienophile. As was mentioned ia the section on alkylation, the reaction of pyrrole compounds and maleic anhydride results ia a substitution at the 2-position of the pyrrole ring (34,44). Thiophene [110-02-1] forms a cycloaddition adduct with maleic anhydride but only under severe pressures and around 100°C (45). Addition of electron-withdrawiag substituents about the double bond of maleic anhydride increases rates of cycloaddition. Both a-(carbomethoxy)maleic anhydride [69327-00-0] and a-(phenylsulfonyl) maleic anhydride [120789-76-6] react with 1,3-dienes, styrenes, and vinyl ethers much faster than tetracyanoethylene [670-54-2] (46). 

The reactivity sequence furan > tellurophene > selenophene > thiophene is thus the same for all three reactions and is in the reverse order of the aromaticities of the ring systems assessed by a number of different criteria. The relative rate for the trifluoroacetylation of pyrrole is 5.3 x lo . It is interesting to note that AT-methylpyrrole is approximately twice as reactive to trifluoroacetylation as pyrrole itself. The enhanced reactivity of pyrrole compared with the other monocyclic systems is also demonstrated by the relative rates of bromination of the 2-methoxycarbonyl derivatives, which gave the reactivity sequence pyrrole>furan > selenophene > thiophene, and by the rate data on the reaction of the iron tricarbonyl-complexed carbocation [C6H7Fe(CO)3] (35) with a further selection of heteroaromatic substrates (Scheme 5). The comparative rates of reaction from this substitution were 2-methylindole == AT-methylindole>indole > pyrrole > furan > thiophene (73CC540). 

Table 7 also indicates that the rate enhancements for a 3- and 5-methyl group vary significantly among 1,2-azoles. The difference between the increments in log units for a 3-and 5-methyl group, which should vary directly with bond fixation in the ground state, is larger for isoxazole (1.4) than for pyrazole (0.7) and for isothiazole (0.2). This indicates that the aromaticity increases in the same order and contributes the first quantitative evidence that the 1,2-azoles follow the same aromaticity order as furan < pyrrole < thiophene. 

By introducing reasonable values (about 2 for nitrogen, 4 for oxygen) for the electron affinity parameter relative to carbon, 8, and for the induced electron affinity for adjacent atoms (32/8i = Vio), we have shown that the calculated permanent charge distributions for pyridine, toluene, phenyltrimethylammonium ion, nitrobenzene, benzoic acid, benzaldehyde, acetophenone, benzo-nitrile, furan, thiophene, pyrrole, aniline, and phenol can be satisfactorily correlated qualitatively with the observed positions and rates of substitution. For naphthalene and the halogen benzenes this calculation does not lead to results... 

Meanwhile, the R-R coupling (see Sect. 2.2) has evidently found general acceptance as the main reaction path for the electropolymerization of conducting polymers The ionic character of the coupling species explains why polar additives such as anions or solvents with high permittivity accelerate the rate of polymerization and function as catalysts. Thus, electropolymerization of pyrrole is catalyzed in CHjCN by bromide ions or in aqueous solution by 4,5-dihydro-1,3-benzenedisulfonic acid The electrocatalytic influence of water has been known since the work... 

Atkinson R, SM Aschmann, AM Winer, WPL Carter (1985b) Rate constants for the gas-phase reactions of NOj radicals with furan, thiophene, and pyrrole at 295 1 K and atmospheric pressure. Environ Sci Technol 19 87-90. 

Similarly, pyrrole, indole, and tetrahydrocarbazole [285] as well as diarylamines give the corresponding N-vinyl compounds [286]. Several improvements of yields and reaction rates were observed by conducting the reactions in the presence of additives [287, 288]. The vinylation of imidazole and benzimidazole was reported to be catalyzed by KOH in the presence of zinc or cadmium salts [289]. The above reactions were reviewed in 1965 [290]. 

A large number of different pyrrole-based polymers have now been electrochemically synthesised, using a variety of conditions, and these are summarised in Table 2, although it should be noted that the size of this field and its rate of growth mean that it is impossible to make such a table completely comprehensive, and that reports of related new materials, particularly of copolymers incorporating pyrrole are continually appearing in the literature. Water-soluble polypyrroles have also recently been reported [246],... 

The rate of eh reaction with ethylene is low, 106 M-1s-1. An electron-donating group adjacent to NH2 or OH makes the rate very low. Similarly, an electron-accepting group enhances the rate as in the case of pyrrole, vinyl alcohol, or ethylene derivatives, where some reactions proceed at diffusion-controlled rates. 

Figure J.73 Cyclic voltammograms ofa 20nm-(hick poly pyrrole on Pt in (etracthylammonium tetrafluoroboralc/CHjCN solulion. The scan rates arc as shown. From Diaz t>r at. (1981).

Figure 3.81 Typical cyclic voltammograms of a poly pyrrole film on Pt in Nrsatu rated 1 M NaClOj. The voltammograms were collected immediately after holding the film at -0.6 V vs, SCE for 5 min and after cycling for 5 min. The scan rate was 100 mV s "1 and the film thickness 84 nm. Reprinted from Electrochimica Acta, 36, P.A, Christensen and A. Hamnett, In situ Spectroscopic Investigations of the Growth, Electrochemical Cycling and Overoxidation of Polypyrrole in Aqueous Solution , pp. 1263-1286(1991), with kind permission from Pergamon Press Ltd., Headington Hill Hall, Oxford OX3 0BW, UK.

Electrophilic substitution in furan, thiophene, selenophene and pyrrole has, up to 1970, been comprehensively reviewed by Marino.66 Italian workers have determined the relative reactivities of selenophene and thiophene as well67 relative rates are given in Table I. Including furan, the order of reactivity is furan > selenophene > thiophene. 

However, the rate of substitution of pyrrole is too high and that of benzene too low to be followed by standard techniques, and consequently a kinetic study was limited to furan, thiophene, selenophene, and tellurophene. Activation entropies are constant for all four members of the series, indicating that the arrangement of the atoms around the reaction center is similar, i.e., the transition states of all four rings occur at similar positions along the reaction coordinate. The relative rates for the formylation are thus controlled by the activation enthalpies. At 30UC relative rates are furan (107), thiophene (1), selenophene (3.64), and tellurophene (36.8).68... 

Individual substitutions may not necessarily be true electrophilic aromatic substitution reactions. Usually it is assumed that they are, however, and with this assumption the furan nucleus can be compared with others. For tri-fluoroacetylation by trifluoroacetic anhydride at 75 C relative rates have been established, by means of competition experiments 149 thiophene, 1 selenophene, 6.5 furan, 1.4 x 102 2-methylfuran, 1.2 x 105 pyrrole, 5.3 x 107. While nitrogen is usually a better source of electrons for an incoming electrophile (as in pyrrole versus furan) there are exceptions. For example, the enamine 63 reacts with Eschenmoser s salt at the 5-position and not at the enamine grouping.150 Also amusing is an attempted Fischer indole synthesis in which a furan ring is near the reaction site and diverted the reaction into a pyrazole synthesis.151... 

The formation of inter- and intrapolymer complexes has also been shown to affect the polymerization kinetics. For example, Ferguson and Shah (1968) investigated the influence of intrapolymer complexation on the kinetics of AA in the presence of copolymer matrices composed of either A-vinylpyrrolidone and acrylamide or A--vi nyl pyrrol idone and styrene. The polymerization rate reaches a maximum in the vicinity of AA to VP ratio equal to one for the VP/AAm matrix. This maximum in the polymerization rate is most pronounced in the presence of copolymer with the highest content of VP. When the hydrophilic acrylamide is replaced with the more hydrophobic styrene monomer in the copolymer matrix, the observed maximum in AA polymerization rate occurred at a lower than equimolar ratio of AA to VP. The hydrophilic groups of VP were interacting with the hydrophobic nucleus consisting of the styrene units in the VP/St copolymer, and were thus unable to participate in the formation of the complex unlike in the case of VP/AAm copolymer matrix. 

The heteroaromatic compounds like furans, pyrroles or thiophenes cannot be generally used as dienes in Diels-Alder syntheses, because at the higher temperature required for the addition of less reactive dienophiles, the equilibrium is on the side of the starting materials due to the unfavorable T AS term comparable to the benzenoid aromatic compounds as mentioned. High pressure again shows the two effects already discussed the shift of the equilibrium toward the products and the enhancement of the rate of reaction which allows the temperature of reaction to be lowered. One... 

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