Gianlorenzo Marino

Photoelectron spectra of the α-substituted derivatives of furan, thiophen, selenophen, and tellurophen. A comparative study of the molecular orbital energies

Vertical ionization energies of the two highest molecular orbitals and of orbitals mainly localized on the substituent of 31 α-substituted derivatives of furan, thiophen, selenophen, and tellurophen are discussed. The substituent effects confirm the reversal in the energy sequence of the two highest occupied π-MOs of tellurophen with respect to other five-membered heteroaromatic congeners and permit the assignment of the ionization energy values of the and π2, π3 MOs in selenophen. Assignments for some of the bands other than the first two it :he photoelectron spectra of tellurophen and selenophen are proposed. The effect of the ring on the orbitals mainly localized on the substituent is briefly discussed.

A comparative study of the aromatic character of furan, thiophen, selenophen, and tellurophen

The ground state aromaticities of furan, thiophen, selenophen, and telurophen have been compared using seven different criteria: the n.m.r. dilution shift, the difference in chemical shifts of the β- and α-protons, the effect of a 2-methyl substituent on the ring proton chemical shifts, the diamagnetic susceptibility exaltation, the sum of the bond orders, the Julg parameter, and the mesomeric dipole moment. The agreement among the results obtained using the various approaches is remarkably good. The following order of decreasing aromaticity has been establised: benzene > thiophen > selenophen > tellurophen > furan.

Relative reactivities and activation parameters for the Vilsmeier–Haack formylation of five-membered heteroaromatic compounds containing the Group VI elements

Rate constants and activation parameters are determined for the formylation by the HCONMe2–COCl2 complex in CHCl3, of furan, thiophen, selenophen, and tellurophen. ΔS‡ Values are identical within experimental error, and the reaction is enthalpy controlled. The activation entropies are discussed in terms of the position of the transition states along the reaction co-ordinate, whereas ΔH‡ values are correlated with ‘empirical resonance energies’ and ΔEloc. The results indicate that the transition states for all four compounds lie in a similar position along the reaction co-ordinate and that the differences in ground state energy play a fundamental role in determining the relative reactivities of the α-positions.

The mechanism of the Vilsmeier–Haack reaction. Part II. A kinetic study of the formylation of thiophen derivatives with dimethylformamide and phosphorus oxychloride or carbonyl chloride in 1,2-dichloroethane

The kinetic order of the reaction of thiophen derivatives with dimethylformamide and phosphorus oxychloride in 1,2-dichloroethane depends on the reactivity of the substrate. Reactions of relatively inert substrates (such as thiophen and the methylthiophens) follows third-order kinetics, first-order in each reagent. On the other hand reactions of very labile substrates (such as 2-methoxythiophen) follow second-order kinetics, the rates being independent of the concentration and the nature of the substrate. The observed kinetics are consistent with a mechanism involving an equilibrium leading to an electrophilic complex, which then attacks the heterocyclic substrate. According to the reactivity of the substrate, the rate-determining step is either the attack of the complex on the aromatic compound or the formation of the complex. The reactions with the complex formed from dimethylformamide and carbonyl chloride follow pure second-order kinetics, first-order in substrate and first-order in complex.

The mechanism of the Vilsmeier–Haack reaction. Part III. Structural and solvent effects

New kinetic data on the Vilsmeier–Haack reaction of heterocyclic compounds are reported which permit the conclusions that (i) the reaction is very selective, as shown by a ρ value of –7·3 for the formylation of thiophen derivatives in chloroform, (ii) the rate of substitution is only affected to a small extent by the polarity of the solvent, and (iii) the rate of substitution is highly dependent on the nature of amide. The reaction with NN-dimethylacetamide–carbonyl chloride complex proceeds ca. 5 × 103 times slower than the reaction with corresponding NN-dimethylformamide complex.

Nucleophilic heteroaromatic substitution—XXIV: Kinetics of piperidinodechlorination of 2- and 6- alkyl-4-chloropyrimidines in ethanol and toluene—evidence for a steric hindrance to solvation of the aza-group

Abstract The rate constants for the reaction of 2- and 6-alkyl-4-chloropyrimidines with piperidine in toluene and ethanol have been determined at 30.0°. The reactivity ratio k Me /k t-Bu increases considerably in passing from the reaction of 6-alkyl-4-chloropyrimidines in toluene (1.62) to the reaction of 2-alkyl-4-chloropyrimidines in ethanol (17.3). This remarkable increase (over a factor of ten) is ascribed to a steric hindrance to solvation of the aza -groups caused by the bulkier substituent.

Kinetics and mechanism of N-substitution of indoles and carbazoles in Vilsmeier–Haack acetylation

The rate constants of acetylation of several pyrroles, indoles, and carbazoles by the Vilsmeier–Haack reagent (NN-dimethylacetamide–carbonyl chloride) have been measured in 1.2-dichloroethane at 25 °C. Most substrates undergo N-substitution and the data strongly support a mechanism involving rate determining direct attack on the nitrogen atom. The order of susceptibility to the electrophile for positions 1–3 of the indole nucleus is C-3 > N-1 > C-2. Differences in behaviour towards N-substitution of pyrrole, indole, and carbazole are discussed in terms of loss of resonance energy in the transition states.

Linear free energy relationships for electrophilic substitutions at α- and β-positions of thiophen

Isomer distributions have been determined for several electrophilic substitutions of thiophen: bromination by Br2 and Br+, chlorination, tin tetrachloride, or iodine-catalysed acetylation by acetic anhydride, trifluoroacetylation, reaction with acetyl trifluoroacetate, and Vilsmeier formylation. The α : β ratios vary from 100 to over 1000, according to the ‘selectivity’ of the electrophilic agent. The results obtained together with other data from the literature permit us to test the applicability of linear free energy treatments to electrophilic substitutions at the α- and β-positions of thiophen. Plots of log αf and log βf against ρ for nine reactions are linear; from the slopes values of σα+=–0·79 and σβ+=–0·52 are obtained. The linearity obtained argues for a similarity in the character of the transition states for substitution at thiophen and benzene rings.

Heteroaromatic rings as substituents. Part IV. The ‘non-constancy’ of the σp+ constants of the 2-thienyl and 2-furyl groups

The σm+ and σp+ constants for the 2-thienyl group have been calculated from the rates of solvolysis of 1-(2-thienylphenyl)ethyl acetates in aqueous 30% ethanol. A comparison with the values previously determined from similar reactions shows that, whereas the σm+ value is constant, the σp+ value varies within wide limits. The σ+ constants for the 2-thienyl and 2-furyl groups have also been calculated from the carbonyl stretching frequencies of thienyl- and furyl-substituted acetophenones by applying the Traylor and Ware correlation. The possible reasons for such ‘non-constancy’ of the σp+ values for these ‘aromatic’ substituents are discussed.

Electrophilic substitutions in five-membered heteroaromatic systems. Part XII. A quantitative study on the reactivities of the α- and β-positions of benzofuran and benzothiophen in electrophilic substitutions

The isomer distributions and the rates relative to those of the parent heterocyclic compounds for several electrophilic substitutions of benzo[b]furan and benzo[b]thiophen have been determined. Surprisingly, although the orientation of the substitution in the two bicyclic systems is different, the effect caused by ‘annelation’ on the reactivity of the α- and β-positions is substantially similar in the two rings: the reactivity of the α-position is always decreased by a similar factor and the reactivity of the β-position is (with two exceptions) increased, in both systems.