Structural and Solvent Effects on the Mechanism of the Thermal Decarboxylation of 2-Oxetanones. A Limiting Case between Concerted and Stepwise Pathways in Pericyclic Reactions

Abstract

The thermal decarboxylation of 2-oxetanones (β-lactones) to yield alkenes has been studied using ab initio SCF-MO computational methods. Solvent effects have also been taken into account. The reaction is predicted to be concerted in the gas phase, the corresponding transition structures having non-Woodward−Hoffmann topologies. A set of 10 substituted 2-oxetanones has been studied, and it has been found that donating groups (methyl, vinyl, phenyl) at C4 facilitate the [2+2] cycloreversion. The saddle points which connect substrates and products are predicted to be highly polar in nature, and, therefore, even moderately polar solvents such as o-dichlorobenzene accelerate the reaction. trans-2-Oxetanones are predicted to react faster than their cis isomers if solvent effects are taking into account. Self-consistent reaction field methods based on multipole expansions of the free energy of solvation tend to overestimate the stability of zwitterionic intermediates at the Hartree−Fock level. This results in stepwise mechanisms in several cases, although higher levels of theory predict again concerted mechanisms.

Comparison of the reactivity of β-thiolactones and β-lactones toward ring-opening by thiols and amines

“…In view of the fact that thiolates are well-known to open β-lactones by S N 2 attack at the β-position, this remarkable reaction might be viewed as an intramolecular S N 2-like process involving a bicyclo[2.2.0]hexane-like transition state made possible by the relatively long C-S bonds, both existing and incipient (Scheme 3, path a). Alternatively, taking note of the widely accepted stepwise zwitterionic intermediate model for the extrusion of CO 2 from β-lactones, [38][39][40][41][42][43][44][45][46][47][48] it may be considered as arising from trapping of a transient zwitterionic form of the lactone (Scheme 3, path b). While the elucidation of the mechanism of formation of 13 from 4 must await further work, the formation of 13 serves to underline the preference of β-lactones toward nucleophilic ring opening at the β-position by soft nucleophiles [49][50][51][52][53][54][55][56] even though the nucleophile is ideally placed for attack on the carbonyl centre.…”

Mass spectrometric study of the decomposition pathways of canonical amino acids and α‐lactones in the gas phase

“…). Considering that decarboxylation of β‐lactones results directly in the formation of the olefin and occurs with moderate energy barriers, we explored the possibility of a mechanism involving the dyotropic shift to the β‐lactone followed by decarboxylation (Eqn 13a). However, the search for the transition state for α‐lactone to β‐lactone rearrangement did not give a structure for a dyotropic process, as shown in 10a, but instead gave a structure that consists of 1,2‐hydrogen shift without assistance of the carboxylate anion, as shown in Eqn 13b.…”

Determination ofsyn/anti Isomerism in DCNQI Derivatives by 2D Exchange Spectroscopy: Theoretical Underpinning

“…The electrostatic contribution of the solution energy was estimated by means of the Onsager method, [42] denoted as L1A1. [43] The Onsager reaction field method has been implemented [44] in the GAUSSIAN 94 series of programs.…”