Hubert Figeys

Studies in NMR spectroscopy: The helicenes and lower benzologues☆☆☆★

Abstract This paper reports the results of a comparative study of the NMR spectra of phenanthrene to nonahelicene (I-VII, Fig. 1). The four spin systems present in these hydrocarbons (end rings) have been calculated and the corresponding protons (A, B, C, D) have been assigned on the basis of the coupling constants. These assignments are fully confirmed by the study of substituted derivatives, epi cross-ring couplings and specific solvent effects. The repercussion of the progressive overlap of benzene rings on the NMR spectra is discussed.

Proton N.M.R.-spectroscopic studies of substituted aromatic compounds

Para-hydrogen chemical shifts in a series of monosubstituted benzene derivatives have been shown to depend linearly on the π-electron density variations on the adjacent carbon atom calculated by a non-empirical Huckel LCAO method. Attention is drawn to the fact that important modifications of ring-current intensity occur when a substituent is introduced in the benzene molecule.

Quantum-chemical studies on the aromaticity of conjugated systems—II : Aromatic and anti-aromatic annulenes: The (4n + 2)-π electron rule☆

Abstract The LCAO-BETA method described in Part I of this series21 has been applied to the calculation of the heats of atomization of annulenes up to C30H30 and to a number of even linear polyenes. It is shown that, in the compounds of the second type, the heats of atomization are additive functions of “single” and “double” CC bond energies. The results obtained were used to investigate the aromaticity of the annulenes according to Breslows and Dewars definition. Huckels rule is verified, the (4n + 2)-π electron systems showing increased stability with respect to the “localised” model compound; the (4n)-π electron annulenes on the other hand are anti-aromatic.

Olefinic and acetylenic compounds—II : New routes to dibenzobarrelenes☆

Abstract Two new high-yield syntheses of dibenzobarrelene have been developed and applied to the case of 2-substituted derivatives; 2-chloro- and 2-cyanodibenzobarrelene have been synthesized.

An INDO-LMO and 13C NMR spectroscopic study of the C-H bonds in tetracyclo[3.2.0.02,7.04,6]heptane (quadricyclane) and tricyclo[3.1.0.02,4]hexane

Abstract Using an INDO approximation to Von Niessens charge density localization method, the percent s character of the C atom hybrid orbitals engaged in C-H bonds is computed for a series of open-chain and cyclic saturated and unsaturated hydrocarbons. A semi-empirical linear relationship is found between these quantities and the experimental nuclear spin-spin coupling constants J CH (standard deviation 4·98 Hz). The experimental values of J CH were measured for quadricyclane ( 1 ) and tricyclo[3.1.0.0 2,4 ]hexane ( 2 ) and were shown to be in fair agreement with the theoretical predictions. An analogous relationship is also established between the percent s character of the involved C atom hybrid orbital and the C-H bond distance, the standard deviation being 0·0040A. This relation is used in order to predict the different C-H bond lengths in the vibrational ground state of 1 and 2 , for which equal CH distances were assumed in the analysis of electron diffraction experiments. The results indicate that molecular geometries can be refined with the use of semi-empirical correlations of the above mentioned type.

Rearrangements of N-heteroaromatic-2-vinyl-aziridines. Synthesis of isothiazolo-, pyrido- and thieno-azepines

Abstract N-heteroaromatic-2-vinyl-aziridines have been synthesized and thermally isomerized to the corresponding isothiazolo-, pyrido- and thienoazepine derivatives. The ease of rearrangement is strongly dependent on the nature (π-deficient or -rich) of the aromatic heterocycle.

Experimental and theoretical study of the induced paramagnetic ring-current in the 4-membered ring of biphenylene and related hydrocarbons

Abstract Ring current intensities and proton chemical shifts have been calculated for a series of condensed aromatic hydrocarbons including biphenylenes. The calculations show that an induced paramagnetic ring current occurs in the 4-membered ring of each of the biphenylenes studied and this effect accounts for the observed spectra. PMR measurements on 1-mono-, 2-mono- and 2,3,6,7-tetra-deuteriated biphenylene prove that the chemical shifts of the 1- and 2-protons are at δCDCl3 6.60 and 6.70 respectively and not vice versa as assumed by previous authors.

Diels-alder reactions with inverse electron demand. II. The reaction of benzamidine with π-deficient heteroaromatic compounds.

Abstract Benzamidine (I) reacts with several s-tetrazines to give as-triazines in moderate to good yield, together with some s-triazines; separate experiments have shown that these are due to the fact that (1) also undergoes Diels-Alder- reactions with as- and even with s-triazines.

Diels-Alder Reactions with Inverse Electron Demand -III- New Synthesis of Amino-Pyridazines; Evidence for an Acetamidine - Ketene - NN - Acetal Tautomerism

Abstract As part of our investigations in the field of the so-called “Diels-Alder reactions with inverse electron demand” involving various π-deficient heteroaromatic compounds as the diene1, we describe herein the reaction of N,N′,N′-trisubstituted acetamidines with several s-tetrazines. The ability of open-chain CN multiple bonds to act as dienophiles in this type of reaction has indeed untill recently largely escaped investigation1,2.

Quantum-chemical studies on the aromaticity of conjugated systems : Re-evaluation of the “thermodynamical” value of β and calculation of the heat of atomization of polycyclic aromatic hydrocarbons by the improved huckel-method

Abstract The total π-energy and σ-compression energy of a series of benzenoid aromatic hydrocarbons have been calculated by the iterative β-procedure developed recently.13 By regression analysis of the relation between the experimental and calculated heats of atomization ΔH°f, we have obtained the following values Eσ (Csp2-H) = 4·314 eV Eσ (Csp2-Csp2) = 3·726 eV, for R(Csp2-Csp2) = 1·517 A βCCπ = 1·292 eV, for RCC = 1·40 A The method was then used to calculate Δ°f-values for a series of open-chain as well as cyclic, alternant and non-alternant, fully conjugated hydrocarbons; agreement with experimental values is very good.