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.

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The influence of halogenating Si in zeolite type model systems (SiH3-n F n −OH−AlH3 and SiH3-n Cl n −OH−AlH3 with n =0, 1, 2, and 3) and silanols (SiH3-n F n −OH and SiH3-n Cl n −OH with n =0, 1, 2, and 3) was investigated by calculating different properties of the hydroxyl group (IR frequencies, integrated IR intensities) and comparing these with gas-phase acidities, ΔG acid. The trends in these quantities are explained in terms of the competition between electronegativity and softness as a function of the systems considered. The results of this competition are reflected in the electronic charges on the hydrogen and oxygen in the hydroxyl group and compared to previously reported trends for halogenated silanols.

Raman spectroscopy was used to investigate the kinetics of the free-radical polymerization of methyl methacrylate in toluene, initiated with azobisisobutyronitrile at 70OC. The decrease in the intensity of the 1636 cm-' C=C stretching band compared with a solvent band as an internal standard allows an accurate determination of the rate of conversion of monomers to polymeric chains. A careful quantitative study of the evolution of the C -0 stretch band profile, interpreted in terms of intra-chain dipoldipole interactions, gives a similar dependence for the conversion. ~~~~~

The coordination behavior of ω-trichlorostannyl alcohols of the type HO(CH2) n SnCl3 (n = 3−5) has been investigated by solid-state 13C and 117Sn NMR, by 1H, 13C, 119Sn, and 17O as well as gradient-assisted 2D 1H−119Sn HMQC and 119Sn EXSY NMR spectroscopy in CD2Cl2 and acetone-d 6 solutions, by X-ray diffraction for the C5 (n = 5) alcohol, and AM1 quantum mechanical calculations. The crystal structure of the C5 alcohol reveals a polymeric structure that arises from significant intermolecular HO → Sn interactions of 2.356(6) Å. The tin atom is thus five-coordinate and exists in a distorted trigonal bipyramidal geometry with the oxygen and one of the chloride atoms defining the axial positions. The polymeric nature of the C5 alcohol explains its insolubility, unlike the C3 and C4 alcohols, in CD2Cl2 solution. In this solvent, the C3 and C4 alcohols display almost exclusively intramolecular HO → Sn coordination, resulting in five- and six-membered ring structures, respectively. In the C5 alcohol, the solid-state intermolecular HO → Sn interaction is too strong for CD2Cl2 to break up the polymer. The acetone-d 6 NMR data reveal a complex coordination behavior combining five- and six-coordinated species in fast equilibrium in which HO → Sn and (CD3)2CO → Sn interactions are evidenced. This behavior is accompanied by very slow hydrolysis, observed in acetone but not in dichloromethane, ascribed to limited slow acetone autocondensation.