As part of our continuing interest in the connection between chemical reactivity and the HSAB principle, both globally and locally, we explored via 3-21 G calculations on local and global softness the statement made by Gazquez and Mendez. Their statement, claiming that two chemical species should react through atoms showing equal softness, has been precised to rationalize the regioselectivity of Normal Electron Demand Diels−Alder reactions between terminally monosubstituted 1,3-butadienes and monosubstituted ethenes. In the presence of this substitution pattern, a head-to-head mode of cyclization is largely experimentally observed, also known as “ortho rule”. The closest values of the condensed local softness were found for the unsubstituted termini, thereby suggesting that these cycloadditions would proceed through an asynchronous pathway, as supported by transition state geometries reported by Houk et al.

The coordination behavior of three ester-functionalized monoorganotin trichlorides of the type ω-(trichlorostannyl)alkyl acetate, CH3COO(CH2) n SnCl3 (n = 3−5), has been investigated by multinuclear solid- and solution-state NMR, as well as AM1 quantum-chemical calculations. The data reveal basically the existence of a fast equilibrium between two species involved respectively in an intramolecular and intermolecular donor-acceptor interaction between the ester function and the tin atom. In the n = 3 case, the species involving the intramolecular interaction is by far the dominant one, with a coordination from the alkoxy oxygen atom to the tin atom, resulting in an interaction of the type −CH2O(Ac)→SnCl3 (Ac = CH3CO−). In contrast, for the n = 4 and 5 cases, the species involving the intermolecular interaction prevails, resulting in (cyclo)dimeric or oligomeric species displaying coordinations from the carbonyl oxygen atom to the tin atom of the type −CH2OC(CH3)O→SnCl3.

C-Alkylation and O-alkylation of ethyl acetoacetate by a series of alkylating agents is investigated, in the gas phase as well as in the solvent, within a local viewpoint of the hard and soft acids and bases (HSAB) principle at the Hartree−Fock level using a 3-21+G* basis set. The Gázquez and Méndez formula for calculating the interaction energy is used to reveal the influence of the alkylating agent softness on the two possible orientations in the ethyl acetoacetate alkylation. O-Alkylation was found to be favored by a hard alkylating agent, whereas C-alkylation becomes less and less disfavored upon increasing softness.