Oxiranones: α-lactones or zwitterions? Insights from calculated electron density distribution analysis

Abstract

Electron density distributions for oxiranone and hydroxyoxiranone have been analysed in vacuo p)] and in water [SCI-PCM/MP2/6-31+G(d,p)//HF/6-31+G(d,p)] and compared with those for cyclopropane, cyclopropanone, and oxirane. Oxiranone possesses a ring critical point in vacuo, and may be considered as an α-lactone with considerable ionic character in the endocyclic C α -O n bond. In water, oxiranone has neither a ring critical point nor a bond critical point for C α -O n , and may be considered as a zwitterion, whose carboxylate group has a net charge of Ϫ0.63. Geometrically, however, the molecule still possesses an acute-angled three-membered ring with a C α CO n angle of only 69Њ. Electronically, hydroxyoxiranone is acyclic and zwitterionic even in vacuo, but geometrically it still looks like an α-lactone.

Theoretical characterization of the chemical bonds of some three-membered ring compounds through QTAIM theory

“…Based on the foregoing discussion, the O 2 -C 3 bond may be considered as a weak single covalent bond. These results do not agree with those reported by Ruggiero et al [22,23]. These researchers considered that this bond is essentially of ionic character.…”

“…Ruggiero et al through the topological analysis of the electron charge density analyzed oxiranone and hydroxyoxiranone and compared them with cyclopropane, cyclopropanone and oxirane [22,23]. They concluded that oxiranone may be considered as an a-lactone that has considerable ionic character in the endocyclic C a -O (C 1 -O 2 ) bond (see Scheme 1 for the atom numbering).…”

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

“…The 1,2‐hydrogen shift mechanism shown in Eqn 13b is facilitated by the high degree of polarization expected for the α‐lactone (Eqn 14), and by the fact that the β‐cationic carboxylate (β(+)–CO 2 ) that would result from hydride transfer is unstable with respect to decarboxylation in the gas phase. The coupling of hydrogen migration and leaving‐group loss is analogous to what has been proposed for the Schmidt reaction, and has been observed previously in reactions such as the elimination of protonated ethers and acetates …”

“…[85] There has long been a discussion regarding the electronic structure of α-lactones, and whether they are best considered to be cyclic, canonical structures [56,[58][59][60] or zwitterionic, [55] although those discussions generally relate to the structure in solution. The calculated structures of gas-phase α-lactones have exceptionally long (~1.55 Å) and weak [87] C α -O bonds, and even in the gas phase, there is considerable ionic character. [87] α-Lactones are highly reactive, and react rapidly by polymerization, by nucleophilic addition, or by decomposition.…”

Synthesis of a Donor‐Stabilized Silacyclopropan‐1‐one

“…[19] As suggested by the unusual length observed in the X-ray crystal structure, the bond density of the endocyclic C1ÀC2 s bond is highly deformed towards the Si1 atom (the BCP is located inside the three-membered ring), in contrast to the Si À C bonds, for which the BCP is found outside the ring, as is typical for strained s bonds in three-membered rings ( Figure 2). [20] These results strongly suggest an electron delocalization of the CÀC s bond towards the silicon atom. Furthermore, as a result of this delocalization, the electron density at the BCP (C1ÀC2) is significantly lower (0.164 e au À3 ) than that of classical C À C single bonds (0.262 e au À3 ).…”