Comprehensive computational study of decamethyldizincocene formation. 2. Reaction of KH/ZnCl2 with decamethylzincocene.

Computational methods were used to study the experimental finding that forming decamethyldizincocene is more efficient when using a reducing agent (e.g., KH) and ZnCl2 as opposed to a sole ZnR2 reagant. The results show that the methyl groups of decamethylzincocene have an indirect effect on the reaction. When zincocene is used as a reactant, the reaction with KH favors the route that results in the formation of the zincate, K(+)[Zn(η(1)-C5H5)3](-). However, the path of formation for the zincate K(+)[Zn(η(1)-C5Me5)3](-) is simply not favorable kinetically or thermodynamically, so the formation of decamethyldizincocene is the only option when decamethylzincocene is used.

Comprehensive computational study of decamethyldizincocene formation. 1. Reaction of ZnR2 reagents with decamethylzincocene.

Computational methods were used to study the surprising 2004 synthesis of decamethyldizincocene, Zn2(η(5)-C5Me5)2, which was the first molecule to have a direct, unbridged bond between two first-row transition metals. The computational results show that the methyl groups of decamethylzincocene, Zn(η(5)-C5Me5)(η(1)-C5Me5), affect the transition-state stability of its reaction with ZnEt2 (or ZnPh2) through steric hindrance, and this could possibly allow a counter-reaction, the homolytic dissociation of Zn(η(5)-C5Me5)(η(1)-C5Me5) into Zn(η(5)-C5Me5)(•) and (η(1)-C5Me5)(•), to occur, and because no such steric hindrance occurs when zincocene, Zn(η(5)-C5H5)(η(1)-C5H5), is used as a reactant, its dissociation never occurs regardless of what ZnR2 reagent is used.

Oxidized forms of a tripyrrane: alpha-tripyrrinone, beta-tripyrrinone and a C2 symmetric hexapyrrole.

alpha- and beta-tripyrrinone isomers (), and a C(2) symmetric hexapyrrole () were isolated from the oxidation of meso-perfluorophenyl tripyrrane (and meso-2,6-dichlorophenyl tripyrrane ) with DDQ under aerobic conditions, and the structure of was determined by X-ray crystallographic analysis.

A brief computational study of decamethyldizincocene formation via diethylzinc and decamethylzincocene.

We have used density functional theory and ab initio methods to study different mechanistic possibilities for the formation of decamethyldizincocene from the reaction between decamethylzincocene and diethylzinc. Our results suggest that decamethyldizincocene could form from the combination of two pentamethylcyclopentadienylzinc radicals. More importantly, our data show that homolytic dissociation of decamethylzincocene into pentamethylcyclopentadienylzinc and pentamethylcyclopentadienyl radicals is 6.7 kcal/mol less costly than the analogous dissociation of zincocene. If such an energy difference is coupled with the fact that the activation barrier to form the half-sandwich product pentamethylcyclopentadienylethylzinc is 11.8 kcal/mol more costly than to form cyclopentadienylethylzinc, we can rationalize why dizincocene does not form experimentally.

Competing isomerizations: a combined experimental/theoretical study of phenylpentenone isomerism.

The possible competition of Z/E versus hydrogen-shift isomerization in (E)-5-phenyl-3-penten-2-one (E-1) and (E)-5-phenyl-4-penten-2-one (E-2) was studied, both experimentally and theoretically. Iodine-catalyzed isomerization experiments and computational modeling studies show that the equilibrated system consists predominantly of E-1 and E-2, with E-2 in moderate excess, and with no detectable amounts of the Z (cis) diastereoisomers. Density functional theory (DFT) calculations corroborated the free energy difference (Delta(r) and Delta(r) were -0.7 and -1.1 kcal mol(-1), respectively), and computations of Boltzmann-weighted (1)H NMR spectra were found to be useful in confirming the assignment of the isomers. The relevance of this equilibrium to earlier work on double-bond stabilization is discussed.

Mechanism of cis/trans equilibration of alkenes via iodine catalysis.

Iodine is commonly used to speed the equilibration of Wittig cis/trans alkene products. This study uses computational chemistry to study the catalyzed isomerization mechanism in detail for seven different examples of 1,2-disubstituted alkenes. We find that the iodo intermediates of the conventional three-step reaction path are weakly stable, bound by less than 7 kJ mol(-1) in five cases and nonexistent in the other two. These variations in relative stability appear to be closely related to the degree of conjugation interruption in the alkene upon attachment of iodine. The rate-determining reaction barrier always occurs in the middle step, the internal rotation of the iodo intermediate, and the variations in the barrier heights are dictated by varying levels of steric hindrance in the seven cases. Regiospecificity of I-atom addition and noticeable hyperconjugative effects are discussed. Comparisons between various theoretical approximations are performed to demonstrate the great difficulty in obtaining accurate results for iodine-atom bond-forming and bond-breaking energies.