Theoretical study of the orientation rules in photonucleophilic aromatic substitutions.

The photonucleophilic aromatic substitution reactions of nitrobenzene derivatives were studied by ab initio and Density Functional Theory methods. The photohydrolysis is shown to proceed via an addition-elimination mechanism with two intermediates, except in the case of a chlorine leaving group. Depending on the substituents, the addition step, the elimination step, or the radiationless transition is the rate-determining process. The solvent effect on the SN2 Ar* reactions was evaluated by a continuum model. Next, the regioselectivity of the addition step is investigated within the framework of the so-called spin-polarized conceptual density functional theory. It is shown that the preference observed for the meta or para (with respect to the NO2 group) pathways in the addition step can be predicted by using the spin-polarized Fukui functions applied for the prereactive pi-complex.

Spin-polarized conceptual density functional theory study of the regioselectivity in ring closures of radicals.

The regioselectivity of ring-forming radical reactions is investigated within the framework of the so-called spin-polarized conceptual density functional theory. Two different types of cyclizations were studied. First, a series of model reactions of alkyl- and acyl-substituted radicals were investigated. Next, attention was focused on the radical cascade cyclizations of N-alkenyl-2-aziridinylmethyl radicals (a three-step mechanism). In both of these reactions, the approaching radical (carbon or nitrogen centered) adds to a carbon-carbon double bond within the same molecule to form a radical ring compound. In this process, the number of electrons is changing from a local point of view (a charge transfer occurs from one part of the molecule to another one) at constant global spin number Ns (both the reactant and the product ring compound are in the doublet state). It is shown that the experimentally observed regioselectivities for these ring-closure steps can be predicted using the spin-polarized Fukui functions for radical attack, f0NN(r).

Mechanism of the ring-chain rearrangement in phosphiranes: hydrogen versus halogen migration.

Ab initio quantum chemical calculations including HF, MP2, CCSD(T), CASSCF(10,10)/CASPT2, and B3LYP methods with the 6-31G(d,p) basis set were used to probe the mechanism of the ring-chain rearrangement of halogeno-phosphiranes. It is confirmed that the lowest energy interconversion between C-halogenated-(X)-phosphiranes and vinylphosphines, with X = H, F, Cl, and Br, is a one-step process in which the C-P bond cleavage and X-sigmatropic migration from C to P occur in a concerted manner in a single transition structure. The migration of a hydrogen from CH(H) is slightly favored over that of CX(H), and thus, the cleavage of the C(X)-P bond is preferred. The energy barrier for the whole process involving hydrogen migration in the parent phosphirane is calculated to be about 45 +/- 5 kcal/mol. The migratory aptitude of the atoms X in the uncomplexed species is found as follows: H > Br > Cl > F, either in the gaseous phase or in aqueous and DMSO solutions. The solvation enthalpies that were estimated using a polarizable continuum model (PCM) are rather small and do not modify the relative ordering of the energy barriers. Such a trend is at variance with recent experimental findings on metal-phosphinidene complexes in which only halogen migration was observed. This might arise from a peculiar effect of the metal fragments W(CO)(5) used in the experimental studies to stabilize the phosphorus species that induce a quite different mechanism. Calculations of the (31)P chemical shifts using the GIAO/B3LYP/6-311+G(d,p) method show a remarkable correlation between the delta(31)P(X) chemical shifts of X-phosphiranes and those of X-phosphines (XCH(2)PH(2)), suggesting that the large beta substituent effect is not inherent to the small rings.

The mechanism of 1,2-addition of disilene and silene. 1. Water and alcohol addition.

The mechanism of 1,2-addition reactions of water, methanol, and trifluoromethanol to Si=Si, Si=C, and C=C bonds has been investigated by ab initio quantum chemical methods. Geometries and relative energies of the stationary points and all the transition states were determined using the MP2/6-311++G(d,p), B3LYP/6-311++G(d,p), and CBS-Q levels of theory. The investigated reactions can be characterized by two main thermodynamical profiles. The type in which the reagent molecule attacks a carbon atom is moderately exothermic, with a high activation barrier. The second type, in which water or alcohol attacks a silicon, is strongly exothermic, with a small activation energy. At the early stage of all the reactions, a weakly bonded initial complex is found which determines the further mechanism of the reaction. On the basis of the HOMO, LUMO, and Laplacian of electron distribution of disilene and silene, several mechanisms have been assumed, depending on the substrate (disilene, substituted disilene, silene, or ethene) and the reagent (water, methanol, or trifluoromethanol). The reaction diagrams and proposed mechanisms explain the experimentally found regioselectivity and diastereoselectivity well.