Aromaticity reactions with conservation

Scheme 3.2-3. Reactions with conservation of 2e aromaticity. It is the sigma skeleton which is changed, namely from classical in 2a via partially non-classical in 3a and classical in 4b to partially non-classical again in 3b. 4b has a...

The equilibrium constant for the reaction of the electron transfer from the anion radical salts of aromatic compounds (with the usual isotope content) to neutral molecules of the same compounds containing heavier isotopes is less than unity (entries 1-10 in Table 2-1). This means that for heavier compounds (enriched with neutrons), the electron affinity is smaller. This difference is conserved at different temperatures and reaction mediums (including those favorable to the destruction of ionic pairs—in HMPA and in THF containing 18-crown-6). 

Certain heterocycles, e.g. pyridines or quinolines, bearing of an electron-withdrawing group such as oxazoline, undergo the Michael-type nucleophilic 1,4-addition accompanied with loss of aromaticity to give the new C-C bond. Thus formed dihydropyridine or benzodihydropyridine can be oxidatively aromatized with conservation of chirality, primary induced by an influence of chiral oxazoline moiety. In this manner, Meyers and coworkers [27] described the Michael-type addition of 1-naphthyllithium (609) to the oxazoline 610 at low temperature to form 611 in 90% yield. The latter was oxidatively aromatized to the naphthylquinoline 612 in 87% yield with 88 12 ratio of two diastereomers. Diastereoselectivity in this reaction remained on the same level as obtained by the nucleophilic addition of 609 to 610 indicating the virtually complete conservation of chirahty, from sp -type in the compound 611 to the axially chiral compound 612, Scheme 11. 

Although Otto Diels and Kurt Alder won the 1950 Nobel Prize in Chemistry for the Diels-Alder reaction, almost 20 years later R. Hoffmann and R. B. Woodward gave the explanation of this reaction. They published a classical textbook, The Conservation of Orbital Symmetry. K. Fukui (the co-recipient with R. Hoffmann of the 1981 Nobel Prize in Chemistry) gave the Frontier molecular orbital (FMO) theory, which also explains pericyclic reactions. Both theories allow us to predict the conditions under which a pericyclic reaction will occur and what the stereochemical outcome will be. Between these two fundamental approaches to pericyclic reactions, the FMO approach is simpler because it is based on a pictorial approach. Another method similar to the FMO approach of analyzing pericyclic reactions is the transition state aromaticity approach. 

From the systematic investigation of the Park and Jew group, several highly efficient and practical polymeric cinchona PTCs were developed (Scheme 6.6). Interestingly, polymeric catalysts with a specific direction of attachment between aromatic linkers (e.g., benzene or naphthalene) and each cinchona unit were found to be effective in the asymmetric alkylation of 4b. The phenyl-based polymeric PTCs with the meta-relationship between cinchona units such as 14, 15, and 18 showed their high catalytic efficiencies. Furthermore, the 2,7-dimethylnaphthalene moiety as in 16 and 17 was ultimately found to be the ideal spacer for dimeric cinchona PTC for this asymmetric alkylation. For example, with 5 mol% of 16, the benzylation of 4b was completed within a short reaction time of 30 min at 0 ° C, affording (S)-5a in 95% yield with 97% ee. Almost optically pure (>99% ee) (S)-5a was obtained at lower reaction temperature (—40 °C) with 16, and moreover, even with a smaller quantity (1 mol%), its high catalytic efficiency in terms of both reactivity and enantioselectivity was well conserved. 

---