Caterina Arnone

Kinetics of the reactions of 2-bromo-3,5-dinitrothiophen with ortho-substituted anilines in methanol. An application of the Fujita–Nishioka equation

The rates of anilino-debromination of 2-bromo-3,5-dinitrothiophen (I) by some ortho-substituted anilines (II; X = OMe, Me, Et, Cl, Br, and F) have been measured in methanol. The kinetic results have been treated according to the multiparameter analysis of ortho-effects proposed by Fujita and Nishioka. This treatment of data has also been applied to acid–base equilibria of anilinium ions. The susceptibility constants obtained in the two series of reactions (respectively ρ–2.96 and –2.90, δ 1.31 and 0.56, f–1.83 and –1.70) have been explained by taking into account the structure of the transition states involved.

A study of the conformation of some 3,3′-bithienyl and biselenienyl derivatives by measurements of twisting powers in nematic liquid crystals and of dipole moments

Abstract The conformation of chiral 2,2′-dinitro-3,3′-bithienyl-4,4′-dicarboxylic acid ( Ia ), its dimethylester ( Ib ), and analogous selenium derivatives Ic and Id was investigated by means of induced cholesteric mesophases. A parallel investigation on the diester ( Ib ) and on other model 3,3′-bithienyls was carried out with dipole moment measurements. The conclusions reached with the two independent methods for derivative Ib are in excellent agreement and indicate, if a single conformation is assumed, a preferred cisoid conformation with a dihedral angle somewhat smaller than 90°. Alternatively, if the existence of both cisoid and transoid conformations having supplementary dihedral angles is assumed, the data indicate a higher population for the cisoid form.

An analysis of 13C nuclear magnetic resonance substituent chemical shifts in 3-substituted thiophene-2-carboxylic and 2-substituted benzoic acids by linear free energy relationships

The 13C n.m.r. chemical shifts of the title compounds and of the corresponding anions have been measured in methanol. The observed substituent chemical shifts have been analysed by means of cross-correlations and of single and dual substituent parameter linear free energy relationships. Different results have been obtained for thiophene and benzene derivatives using DSP analysis: for the first ring system the results have been rationalized in terms of separate resonance and inductive contributions of substituent effects (pointing out the occurrence of an apparent reverse polar effect); in contrast the same treatment of the data for the benzene system gave poor results, confirming once again the different proximity effects in six-and five-membered ring derivatives.

Kinetic study of the reactions of some 2-L-3-Y-5-nitrothiophenes with primary and secondary amines in benzene

The kinetics of the reactions of some 2-L-3-Y-5-nitrothiophenes (L = Br, OC6H5, OC6H4NO2-p YCONH2, CO2Me) with n-butylamine, benzylamine, pyrrolidine and piperidine in benzene at 293.15 K have been studied with the aim of obtaining and insight into the role played by the ortho-like substituent (Y) in determining the occurrence and the form of base catalysis. The results obtained have also allowed to gain important information about the interactions between the substituents, the thiophene ring and the reaction centre.

An analysis of 13C nuclear magnetic resonance substituent chemical shifts in 4- and 5-substituted thiophene-2-carboxylic acids by linear free energy relationships

The 13C n.m.r. chemical shifts of some 4- and 5-substituted thiophene-2-carboxylic acids and of the corresponding anions have been measured in methanol. The observed substituent chemical shifts have been compared with the calculated values and then analysed according to single and dual substituent parameter linear free energy relationships. The results have been rationalized in terms of separate resonance and inductive contributions of substituent effects. The resonance term has shown that no conjugation occurs between the ring and the carboxy group.

Linear free energy relationships in the thiophen series. Part 2. The kinetics of the reactions of some 2-bromo-3-nitro-5-X-thiophens with substituted anilines in methanol

The rate constants for the reactions of 2-bromo-3-nitro-5-X-thiophens (X = H, Br, CONH2, CO2Me, Ac, SO2Me, CN, or NO2) with some substituted anilines YC6H4NH2(Y =p-OMe, p-Me, H, m-Cl, or p-CO2Me) have been measured in methanol at various temperatures. The sensitivity parameters, ρ(Y) and β(X), obtained from Hammett and Bronsted correlations are practically independent of the reactivity of the starting system. The relevance of these results to the reactivity–selectivity principle is discussed.

A 13C n.m.r. study of 5-cyano-, 5-methoxycarbonyl-, 5-carbamoyl-, and 5-acetyl-3-nitro-2-X-thiophenes: substituent effects and their relation to the charge distribution in corresponding 2,2-dimethoxy Meisenheimer adducts

A 13C n.m.r. study in (CD3)2SO has been carried out on 5-cyano-(5), 5-methoxycarbonyl-(6), 5-carbamoyl-(7), and 5-acetyl-3-nitro-2-X-thiophenes (8) in order to investigate the 2-X-substituent effect on the C(α) chemical shifts of the different 5-probes. The results obtained show that, unlike the case of the acetyl group, the α-carbon chemical shifts of the cyano, methoxycarbonyl, and carbamoyl groups are not appreciably affected by through-conjugation with the 2-X-substituents, π-polarization being the more important outcome of the substituent effect on the probe group. The analysis of both the C(5) and C(α) chemical-shift variations in the series (5)–(8), by means of a gradual modification of the electron-releasing power of the substituents, reveals a trend which has been interpreted as a useful indicator of the electronic effects in play on the distribution of the π-electron densities in the corresponding Meisenheimer adducts (1)–(4).

Catalysis in aromatic nucleophilic substitution. Part 7. Kinetics of the reactions of some 5-substituted 2-methoxy-3-nitrothiophenes with piperidine in benzene

The kinetics of the reactions of some 2-methoxy-3-nitro-5-X-thiophenes (Ia–g; X = H, CONH2, CO2Me, Ac, SO2Me, CN, or NO2) with piperidine and with n-butylamine in benzene have been measured in the range 20–40 °C. The reactions with piperidine are catalysed by piperidine, being third-order overall (second-order in amine). The electronic effects of the 5-substituent on the ‘catalytic’ constants are shown to be most consistent with the SB-GA mechanism of base catalysis in benzene.

The protonation of some 5-substituted 3-nitro-2-NN-dimethylaminothiophens

The title reaction has been studied in aqueous perchloric acid. The experimental pKBH+ values obtained according to the Hammett, Bunnett and Olsen, and Yates and Cox methods give good Hammett correlations with ‘thiophen’σ– values. A comparison with the susceptibility constants measured for some series of NN-dimethylanilines has been made and the influence of the hyper-ortho relation in determining the pKBH+ values of the title compounds has been discussed.

Catalysis in aromatic nucleophilic substitution. Part 6. Reactions of 2-methoxy-3-nitrothiophen with cyclic secondary amines in methanol and benzene

The kinetics of the reactions of 2-methoxy-3-nitrothiophen (I) with pyrrolidine in methanol and benzene and with perhydroazepine in benzene have been studied as a function of amine concentration. The pyrrolidino-substitution of (I) in methanol is second-order overall whereas the corresponding piperidino-substitution is base-catalysed. The reactions of (I) with pyrrolidine, piperidine, and perhydroazepine in benzene are all catalysed by the nucleophile and by diazabicyclo[2.2.2]octane but the pattern of catalysis depends on the ring size of the cyclic amine. The data are most consistent with the specific base–general acid mechanism for base catalysis. The remarkable difference in behaviour between pyrrolidine and piperidine is suggested to stem from conformational effects concerning the amino-moieties in the transition state for the general acid-catalysed departure of methoxy-group.