Reaction Paths of the [2 + 2] Cycloaddition of XCY Molecules (X, Y = S or O or CH₂). Ab Initio Study

Abstract

The reaction paths of [2+2] cycloaddition of the X=C=Y cumulenes were modeled at the MP2/aug-cc-pVDZ level. Cycloadditions of allene and CO2, CS2, or OCS lead in part to the same four-membered products as dimerizations of either ketene or thioketene or addition of ketene and thioketene, respectively. All the reactions studied are concerted and mostly asynchronous. The majority of the allene cycloadditions studied are endoergic and proceed with much higher activation barriers than do the alternative (thio)ketene additions. In comparison with the energy of the substrates, the four-membered cycles incorporating S-atoms are stabilized more than the analogous structures with O-atoms built into the rings. There are also some products that are thermodynamically disfavored, yet seem to be obtainable thanks to a relatively low barrier of the reaction. The AIM analysis of the electron density distribution in the transition state structures allowed distinguishing pericyclic from pseudopericyclic and nonplanar-pseudopericyclic types of reaction.

Reactivity and Selectivity of Boron-Substituted Alkenes in the Diels–Alder Reaction with Cyclopentadiene. A Study of the Electron Charge Density and Its Laplacian

“…See text and Figure 1 for an explanation of symbols and identification of atoms.In all the TSs two new bcps corresponding to the forming C 1 -C 6 and C 2 -C 3 σ-bonds and a ring critical point associated with the six-membered ring were found which agree with the characteristic topological pattern of the TSs with [4 + 2] character (Figure 1) [22][23]…”

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 It has been previously demonstrated that the ellipticity profile evaluated at the bcp corresponding to a forming or breaking bond along the reaction coordinate constitutes a good indicator to distinguish pericyclic and pseudopericyclic reactions. [22][23][24][25][26] In addition, for the DA reactions of the isoprene and boron-substituted dienophiles, the ellipticity evaluated at the forming bond next to the boron atom was found to be a useful parameter to differentiate the pathways associated with [4 + 3] or [4 + 2] TSs and to distinguish between endo and exo modes of addition. 31 The ellipticity profiles evaluated at the C 1 -C 6 bcps for both channels of selected dienophiles are shown in Figure 6.…”

Competing mechanisms for the reaction of dichloropropynylborane with 2-tert-butylbutadiene. Diels–Alder reaction versus alkynylboration

“…24,25,[28][29][30] To further elucidate the electron density redistribution during the two competing processes and in an attempt to explain how the zwitterionic six-membered ring rearranges to the seven-membered ring structure leading to the DA cycloadduct from AB product, we carried out a topological analysis of charge density along the IRC coordinate associated with TSC-m and TSD-m. We analyzed the variations of several topological properties (the charge density ρ b , its Laplacian ∇ 2 ρ b and the ellypticity ε) at the bond critical point (bcp) in some bonds along the reaction path as well as the delocalization index (DI). 24,25,[28][29][30] To further elucidate the electron density redistribution during the two competing processes and in an attempt to explain how the zwitterionic six-membered ring rearranges to the seven-membered ring structure leading to the DA cycloadduct from AB product, we carried out a topological analysis of charge density along the IRC coordinate associated with TSC-m and TSD-m. We analyzed the variations of several topological properties (the charge density ρ b , its Laplacian ∇ 2 ρ b and the ellypticity ε) at the bond critical point (bcp) in some bonds along the reaction path as well as the delocalization index (DI).…”

“…However, the formation of the meta enyne as the major product at short reaction times in dichlorometane observed with highly substituted systems and why the cycloadduct became the major product when the reaction was left for longer could not be completely rationalized. The analysis based on QTAIM along the reaction paths connecting the stationary points has been successfully applied to rationalize the mechanism of chemical reactions, such as the DA reactions of unsaturated organoboron dienophiles, 25 the dimerization of cyclopentadiene, 26 the Cope rearrangement of 1,5hexadiene, 27 pericyclic and pseudopericyclic reactions, [28][29][30][31][32][33][34] among others. This study indicated that in the initial stage of the reaction the σ-overlap between the 2p atomic orbitals is more stabilizing for the C-B secondary interaction than for the C-C counterpart, favoring the [4 + 3] pathway.…”

Addition Reactions: Cycloaddition

“…27 An ab initio study of the 2 + 2-cycloadditions of allene to isocyanic acid and ketene to vinylimine found the reactions to be concerted and mostly asynchronous. 28,29 The diastereoselective 2 + 2-cycloaddition of dichloroketene with a chiral enol ether (26) produced the cyclobutanone (27), which leads to a key intermediate (28) in de the total synthesis of the natural alkaloid (−)-Swainsonine (29) (Scheme 8). 30 (32), which readily underwent Cope rearrangement to substituted 5,6-dihydro-1H -azocinones (33) in high yields (Scheme 9).…”