The problem of competition between concerted and stepwise diradical mechanisms in 1,3-dipolar cycloadditions was addressed by studying the reaction between nitrone and ethene with DFT (R(U)B3LYP/6-31G) and post HF methods. According to calculations this reaction should take place via the concerted cycloaddition path. The stepwise process is a viable but not competitive alternative. The R(U)B3LYP/6-31G study was extended to the reaction of the same 1, 3-dipole with cyclobutadiene and benzocyclobutadiene. The very reactive antiaromatic cyclobutadiene has an electronic structure that is particularly disposed to promote stepwise diradical pathways. Calculations suggest that its reaction with nitrone represents a borderline case in which the stepwise process can compete with the concerted one on similar footing. Attenuation of the antiaromatic character of the dipolarophile, i.e., on passing from cyclobutadiene to benzocyclobutadiene, causes the concerted 1,3-dipolar cycloaddition to become once again prevalent over the two-step path. Thus, our results suggest that, in 1,3-dipolar cycloadditions that involve normal dipolarophiles, the concerted path (Huisgen's mechanism) should clearly overwhelm its stepwise diradical (Firestone's mechanism) counterpart.

Structures and energetics of reactants and transition structures of cycloadditions of allene (A), monofluoroallene (MFA), and 1,1-difluoroallene (DFA) with diazomethane, formonitrile oxide, cyclopentadiene, and furane have been investigated with the use of ab initio molecular orbital calculations. Structure optimizations were performed with both RHF/3-21G and RHF/6-31G* calculations but were limited to RHF/3-21G for the reactions of cyclopentadiene and furane. For these reactions as well as for the reactions involving DFA electronic energies

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The oxygen insertion into C-H bonds (of methane, isobutane, and acetone) by dioxiranes (parent dioxirane and dimethyldioxirane) to give alcohols was studied with the DFT theory, using both restricted and unrestricted B3LYP methods, and 6-31G(d) and 6-311+G(d,p) basis sets to evaluate the feasibility of stepwise mechanisms and their competition with the concerted counterpart. Confirming previous results by other authors, we have located, with the RB3LYP method, concerted TSs in which the oxygen bound to be inserted interacts very strongly with the hydrogen atom and very weakly with the carbon atom of the C-H bond. These TSs nicely explain all the experimental observations (e.g., configuration retention at the chiral centers), but all of them exhibit an RHF --> UHF wave function instability that preclude considering them as genuine transition structures. We also were able to characterize, with UB3LYP methods, two alternative two-step processes that can lead to final products (alcohol + carbonyl compound) via singlet radical pair intermediates. For the first step of both processes we located genuine diradicaloid TSs, namely, TSs rad,coll and TSs rad,perp, that have stable wave functions. In TSs rad,coll the alkane C-H bond tends to be collinear with the breaking O(1)- - -O(2) bond while in TSs rad,perp the alkane C-H bond is almost perpendicular to the O(1)- - -O(2) bond. The first step, of both processes, can represent an example of a "molecule induced homolysis" reaction: collision between alkane and dioxirane brings about the homolytic cleavage of the dioxirane O-O bond and the hydrogen abstraction follows afterward to produce the diradicaloid TS that then falls down to a singlet radical pair. This hypothesis was fully confirmed by IRC analysis in the case of TSs rad,coll. The possible pathways that lead from the intermediate radical pair to final products are discussed as well as the hypothesis that the radical collinear TSs may collapse directly to products in a "one-step nonconcerted" process. However, diradical mechanisms cannot explain the experimental data as satisfactorily as the concerted pathway does. As for computational predictions about competition of diradical vs concerted mechanisms, they strongly depend (i) on the alkane C-H type, (ii) on whether gas phase or solution is considered, and (iii) on the basis set used for calculations. In short, the concerted TS benefits, with respect to the corresponding diradicaloid TSs, of alkyl substitution at the C-H center, solvation effects, and basis set extension. Actually, in the case of DMD reactions with methane and acetone, the diradicaloid TSs are always (both in gas phase and in solution and with both the basis sets used) strongly favored over their concerted counterpart. In the case of DMD reaction with isobutane tertiary C-H bond the large favor for the diradicaloid TSs over the concerted TS, predicted in gas phase by the B3LYP/6-31G(d) method, progressively decreases as a result of basis set extension and introduction of solvent effects: the higher th...

Structures and energetics of reactants and transition structures of the cycloadditions of diazomethane (DZM) and formonitrile oxide (FNO) with ethene (ET), propene (PR), acrylonitrile (ACN), and methyl vinyl ether (MVE) have been investigated with the use of ab initio molecular orbital calculations. The reaction of acetonitrile oxide (MNO) with acrylonitrile has been also included for comparisons. Structure optimizations were performed at the RHF/6-31G(d) and density functional B3LYP/6-31G(d) levels of approximation. Single-point electronic energies were computed up to the MP4SDTQ/6-31G(d) level. Kinetic contributions to activation enthalpies and entropies were computed at the RHF/6-31G(d) level. Transition structures of ethene cycloadditions (prototype reactions) were also checked with the MP2/6-31G(d) approximation. Solvent effects were introduced both at a semiempirical level (AMSOL) and at an ab initio level using the Pisa model (interlocking spheres) and the IPCM procedure (isodensity surface polarized continuum model). Electronic activation energies are found to be very sensitive to the treatment of electron correlation and failed to converge to values unaffected by further theoretical improvements: indeed, the inclusion of full fourth-order correlation (MP4) decreases the activation energies by 5-10 kcal/mol with respect to the preceding level of correlation (MP3). Anyway, activation free enthalpies and entropies of the reactions under study appear to be close to the experimental values available for this class of reactions. Still in agreement with experimental observations is the effect of solvent polarity on the reaction rates. Theoretical regioselectivity is less sensitive to the level of calculation, although the inclusion of electron correlation, both with the Moeller-Plesset technique and the use of the density functional theory, is able to reverse the regiochemical predictions obtained with RHF energies for the reactions of nitrile oxides with acrylonitrile. This explains why the frontier orbital theory, which is based on uncorrelated HF-wave functions, cannot arrive at the correct prediction of the regiochemistry in these cases. Calculated solvent effects appear to influence the regiochemistry of 1,3-dipolar cycloaddition, but in general, they reinforce the prediction obtained in vacuo.