Jacques Fossey

An abinitio study of protonation and alkylation of aminopyridine

Abstract STO-3G abinitio calculations show that the first site of protonation in 2-, 3- and 4-aminopyridines is the cyclic nitrogen, whatever the position of the amino group. There is an excellent linear relationship between the calculated proton affinities and the experimental pKa values (r = 0.995). The protonation site is also confirmed by π→π* transition energy variations as a function of position of the amino-group due to protonation; these effects are connected with the MO structures. Basicity and reactivity in alkylation reveal lone pair localization.

Fragment interaction analysis in the framework of ab-initio UHF-MO computations : Part IV. The second row effect in β-substituted ethyl radicals

Abstract The conformational preference in β-substituted ethyl free-radicals has been analysed using a quantitative perturbational MO (PMO) analysis performed in the framework of an ab-initio UHF-MO treatment. The second-row effect, i.e. the increase of the rotaional barrier around the Cα-Cβ bond when the β substituent belongs to the second-row has been analysed in details. It has been found that the hyperconjugation is mainly responsible for this effect. In the 2-chloro-ethyl radical the delocalization of the single electron of the SOMO into the antibonding σC-Cl orbital represents the principal contribution to the second-row effect. On the other hand, in the 2-mercapto, 2-phosphino and 2-silyl-ethyl radicals this effect is mainly due to the electron delocalization from the bonding σC-X (X=SH, PH2, SiH3) into the SOMO. It has also been found that in all cases the contribution associated with the steric effects and the homoconjugation p-d are small and the one associated with the homoconjugation p-p is practically negligible.

Theoretical study of free-radical migrations

Free-radical migrations have been investigated by ab initio molecular orbital theory to account for the facts that 1,2-migrations are observed for Cl-atoms, aryl or vinyl groups, whereas for H atoms only 1,5- and 1,6-migrations have been seen in contrast to the easy 1,2-hydride shifts in cations. The optimum geometry and energy models of the transition states of 1,2- and 1,5-migrations, and of their corresponding initial states have been determined using the Gaussian 70 STO-3G RHF method. The calculated activation energies ΔE are in agreement with experimental observations. The main features of the 1,2-migration reactions are (i) ΔE is more important for elements of the first and second row of the periodic classification than for those belonging to the third row; (ii) protonation strongly reduces ΔE. For both 1,2- and 1,5-migrations a correlation exists between ΔE and the energetic change of the frontier orbital, Δϵsomo: the reactions are under frontier orbital control.

Studies of Radical Alternating Copolymerization. I. Quantitative Determination of the Relative Reactivities between Free Comonomers and a Complex between Them; A Theoretical Treatment in Terms of Frontier Molecular Orbital Interactions. Application to Radical Maleic Anhydride-Vinyl Acetate Alternating Copolymerization

Abstract A simple quantitative determination of the ratio β1 of rate constants kij corresponding to the addition of a complex or a free monomer on a same growing chain in alternating copolymerization was determined. The application of this method to the case of alternating maleic anhydride (A)-vinyl acetate (D) copolymerization give for β1 = kAC/kAD and for β2 = kDC/kDA (C refers to the complex) the respective values of 7 and 0. A complex was more reactive than A but less reactive than D toward a growing chain. In order to explain these reactivities, a frontier molecular orbital treatment was proposed. A good qualitative correlation with experimental results was obtained.

Fragment Interaction Analysis in the Framework of ab initio UHF MO Computations. I. Conformational Preference in the Ethyl Radical

In this paper we describe a procedure which allows to obtain estimates of the energy effects associated with the orbital interactions occurring between the component fragments of a radical species in the framework of ab initio Unrestricted Hartree-Fock computations. This procedure is used here to analyze the factors which control the conformational isomerism of the ethyl radical.

Peracids and free radicals: A theoretical and experimental approach

Organic peracids behave as a source of alkyl radicals and serve as a good model for studying their chemistry. The OH transfer by alkyl radicals and the stereoelectronic control of the reaction were investigated in detail. Inter- and the intramolecular hydrogen shifts by alkyl radicals were also studied. These results were used in order to undertake selective hydroxylation of hydrocarbons. Finally we turned our attention to the reactivity of electrophilic radicals towards peracids.

Decomposition of peroxydecanoic acid using BCHPC [di(4-tert-butylcyclohexyl) peroxydicarbonate] under argon atmosphere

Using BCHPC as a source of alkoxycarbonyloxyl radical at moderate temperature in benzene or cyclohexane and in the absence of O2(under argon), the peroxydecanoic acid RCO3H is transformed into nonan-1-ol ROH in good yield. A mechanism, implying the acylperoxyl radical RCO3˙ as an intermediate, is proposed.

Interpretation of the reactivity of benzyl free radical towards peroxyacids in terms of orbital interactions. Competition between energy gap control and overlap control

The factors which control the reactivity of alkyl free radicals R˙ in reaction (i) are studied. The reactivity [graphic omitted] R′→ R – OH + R′CO˙2(i), of R˙ in (i) depends on the key orbital interaction between the SOMO of the radical and the LUMO of the peroxyacid. This interaction involves two contributions: (i) the energy gap SOMO–LUMO and (ii) the overlap SOMO–LUMO. In reaction (i) the main factor is overlap control which depends on spin delocalisation in the radical R˙. This proves that reaction (i) does not involve electron transfer. The energy gap control, which depends on the nucleophilic character of R˙, is only observed when the first factor is constant along a series of R˙.