GAS-PHASE HETEROAROMATIC SUBSTITUTION .8. ELECTROPHILIC ATTACK OF ETHYL CATION ON PYRROLE, N-METHYLPYRROLE, FURAN, AND THIOPHENE

Et cation, obtained in the dil. gas state, together with CH5+, from the γ-radiolysis of methane, has been allowed to react with pyrrole, N-methylpyrrole, furan, and thiophene, in the pressure range 50-760 Torr and in the presence of variable concns. of a gaseous base (NMe3). The mechanism of the substitution and of the subsequent isomerization of the relevant ionic intermediates is discussed, and the intrinsic positional selectivity of the C2H5+ ions is evaluated. Gas-phase C2H5+ ion attack on pyrroles is characterized by a significant positional selectivity toward those substrate positions with the highest net neg. charge (N:α:β = 13%:10%:77% for pyrrole; α:β = 19%:81% for N-methylpyrrole). Interaction of C2H5+ with the furan center having the max. value of the neg. charge, i.e. the O atom, favors occurrence of substitution (α:β = 57%:43%). Thiophene displays no significant positional discrimination (α:β = 54%:46%). Gas-phase attack of C2H5+ on simple five-membered heteroaroms. is mainly governed by electrostatic interactions established within the encounter pair. This characterizes gaseous C2H5+ as a very hard electrophile, rather than a borderline acid, as expected on the grounds of the alkyl cation hardness scale. This deviation is explained in terms of the bridged geometry for C2H5+ and its effect on the LUMO energy level of the ion

ChemInform Abstract: Gas-Phase Heteroaromatic Substitution. Part 13. A Quantitative Application of the Curve-Crossing Reactivity Model to Heteroaromatic Substitution

Gas- hase kinetic data concerning the electrophilic attack of free, unsolvated cations, such as CH3+, C2HS+i,- C3H7+, embodied into Klopman's charge and frontier orbital control reactivity model. The origin of the deviations from a linear correlation between the positional selectivity of some of the above electrophiles and their SCF STO-3G calculated LUMO energy has been investigated. The influence of the computational level used for estimating the LUMO eigenvalues of the ionic reactants has been assessed by increasing the basis set dimensions from the STO-3G to the 6-31G*. Similar positional selectivity vs LUMO energy correlations have been obtained at both computational levels, revealing no significant difference at the two levels of theory employed. Direct evaluation of the activation parameters governing the gas-phase electrophilic attack on pyrrole by some representative ionic reactants, such as i-C3H7+,t -C4H9+C, F3+, and (CH3)2F+, has been derived from their Arrhenius plots, measured within the temperature interval 30-140 OC. The curve-crossing reactivity model has been quantitatively applied to interpret the experimental kinetic results. A satisfying linear relationship between the intramolecular selectivity of the electrophiles considered and their G values has been obtained, the G parameter being a function of the vertical ionization potential IP, of the heteroarene and of the vertical electron affinity EA, of the electrophile. The influence of the G parameter in determining the nature of the transition states and the relative height of the activation barriers involved in the attack of the ionic electrophile on the a and 0 carbons of pyrroles has been discussed and compared with the effects of Klopman's HOMO (donor)-LUMO (acceptor) energy gap on the corresponding positional selectivity. A close relationship between the G parameter and the HOMO-LUMO gap, both reflecting the properties of the unperturbed donor-acceptor pair, has been verified

GAS-PHASE HETEROAROMATIC SUBSTITUTION .13. A QUANTITATIVE APPLICATION OF THE CURVE-CROSSING REACTIVITY MODEL TO HETEROAROMATIC SUBSTITUTION

Gas- hase kinetic data concerning the electrophilic attack of free, unsolvated cations, such as CH3+, C2HS+i,- C3H7+, embodied into Klopman's charge and frontier orbital control reactivity model. The origin of the deviations from a linear correlation between the positional selectivity of some of the above electrophiles and their SCF STO-3G calculated LUMO energy has been investigated. The influence of the computational level used for estimating the LUMO eigenvalues of the ionic reactants has been assessed by increasing the basis set dimensions from the STO-3G to the 6-31G*. Similar positional selectivity vs LUMO energy correlations have been obtained at both computational levels, revealing no significant difference at the two levels of theory employed. Direct evaluation of the activation parameters governing the gas-phase electrophilic attack on pyrrole by some representative ionic reactants, such as i-C3H7+,t -C4H9+C, F3+, and (CH3)2F+, has been derived from their Arrhenius plots, measured within the temperature interval 30-140 OC. The curve-crossing reactivity model has been quantitatively applied to interpret the experimental kinetic results. A satisfying linear relationship between the intramolecular selectivity of the electrophiles considered and their G values has been obtained, the G parameter being a function of the vertical ionization potential IP, of the heteroarene and of the vertical electron affinity EA, of the electrophile. The influence of the G parameter in determining the nature of the transition states and the relative height of the activation barriers involved in the attack of the ionic electrophile on the a and 0 carbons of pyrroles has been discussed and compared with the effects of Klopman's HOMO (donor)-LUMO (acceptor) energy gap on the corresponding positional selectivity. A close relationship between the G parameter and the HOMO-LUMO gap, both reflecting the properties of the unperturbed donor-acceptor pair, has been verified

Solid state deposition of chiral amphiphilic porphyrin derivatives on glass surface

Herein, we present a straightforward method to achieve optically active films based on porphyrin derivatives. The introduction of an aminoacid functionality on the porphyrin platform confers to the macrocycle both the amphiphilic and chiral character exploited for its solvent-promoted self-aggregation leading to the formation of chiral supramolecular architectures. These ordered suprastructures have the propensity to spontaneously layer as solid films on glass surfaces. The deposited material has been characterized by means of UV-visible, fluorescence emission, circular dichroism spectroscopy and AFM. The reported studies show once more how the stereochemical information stored on a single porphyrin framework can induce the formation of supramolecular chiral architectures, in solution, as well as in solid state. Furthermore, slight modifications on the porphyrin skeleton can influence the aggregation process and the structural features of the final assemblies, leading to solid surfaces featuring different morphologies. These combined aspects can be of great importance for the achievement of solid state chemical sensors with stereoselective properties