Mechanism of Anionic [3 + 2] Cycloadditions. An ab Initio Computational Study on the Cycloaddition of Allyl-, 2-Borylallyl-, and 2-Azaallyllithium to Ethylene

Abstract

The anionic [3 + 2] cycloaddition of allyl anions or allyllithium compounds to double or triple bonds is an elegant route both to carbocyclic and to heterocyclic five-membered rings. The mechanism of such reactions has not yet been established conclusively. In this computational study, the concerted 4πs + 2πs mechanism, expected on the basis of the Woodward−Hoffmann rules, is found to be less favorable than two-step pathways for the cycloadditions of ethylene to the allyl, 2-borylallyl, and 2-azaallyl anions and their lithiated counterparts at Becke3LYP/6-311+G** and MP2(fc)/6-31+G* levels of theory. Except for allyllithium, the 4πs + 2πs cycloadditions (in C s symmetry) are not concerted, since only second-order saddle points, rather than true transition structures, are involved. The anisotropy of the reactant polarizabilities is responsible. Instead, two-step cycloaddition pathways are followed by all three model systems. In accord with experimental experience, 2-borylallyl and 2-azaallyl compounds are found to undergo this type of reaction more readily than the unsubstituted allyl anion or allyllithium. The second, ring-closing step is facilitated by the anion-stabilizing effect of nitrogen and the boryl substituent.

Mechanism Switch in Mannich‐Type Reactions: ELF and NCI Topological Analyses of the Reaction between Nitrones and Lithium Enolates

“…At the intermediate IN1a a clear non‐covalent interaction is observed between C6 and O2 (blue surface) in agreement with a relatively short distance of 2.70 Å and despite the formation of the second C6–O2 bond does not have started, as mentioned above. This observation is in agreement with that made for anionic stepwise (3+2) cycloadditions by Schelyer and co‐workers who considered this sort of interaction as strictly electrostatic 41. More recently, we have also observed the same type of interactions in the stepwise cycloaddition between nitrone ylides and alkenes 20b.…”

Metal-free activation of C–C multiple bonds through halide ion pairs: Diels–Alder reactions with subsequent aromatization

“…26 Schleyer et al, however, pointed out that for polarisable starting products a two-step pathway essentially involving a zwitterionic intermediate was energetically more favourable than the Woodward-Hoffmann route. 27 In 1993, Reetz and Gansauer found that a heterogenous dispersion on Lithium perchlorate was able to catalyse Diels-Alder reactions and regioselective 1,3-Claisen rearrangements, 28 a system which was already quite closely related to the present observations. Meanwhile, there is collected theoretical and experimental evidence that at least Diels-Alder reactions with more polarizable reactants progress along a two-step mechanism involving a charged or zwitterionic intermediate (e.g., see Refs.…”

Revealing Stepwise Mechanisms in Dipolar Cycloaddition Reactions: Computational Study of the Reaction between Nitrones and Isocyanates

“…The dihedral angles illustrated in Figure 3 Electron-deficient N2 and the electron-rich N4 (in TS1a-S) or O6 (in TS3a-S) are alligned in the transition structures, enabling a stabilizing electrostatic interaction between electrondeficient N2 and either electron-rich N4 or O6 atoms (distance N2−N4: 2.68 Å in TS1a-S; distance N2−O6: 2.66 Å in TS3a-S). That sort of electrostatic interactions had been predicted by von Schleyer and co-workers for anionic stepwise (3 + 2) cycloadditions 38 and observed by us in the addition of enolates to nitrones 18c and the cycloaddition of nitrone ylides with alkenes. 39 This first step of the reaction is, actually, a nucleophilic addition to the central carbon of the isocyanate.…”