An ONIOM study on the enantioselective Diels–Alder reaction catalyzed by SiO2 – Immobilized chiral oxazaborolidinium cation

The mechanism and enantioselectivity of the Diels–Alder reaction between 2-methyl-1,3-butadiene and 2,3-dimethyl-1,4-benzoquinone catalyzed by SiO2-supported chiral cationic oxazaborolidinium catalyst were investigated by the hybrid ONIOM(B3LYP/6-31G(d):UFF) method. The amorphous silica bulk was obtained by classical molecular dynamics using the melt-quench method and the heterogeneous surface was created by cleaving the bulk structure. The resultant amorphous silica surface was found to closely resemble silicas thermally treated at 673 K. The organocatalytic Diels–Alder reaction on the silica surface was observed to proceed via a concerted but highly asynchronous mechanism. The preferred transition state was found to correspond to an endo arrangement of the diene and the catalyst-coordinated benzoquinone, and the enantioselectivity was calculated to be 86.5% at 298 K. The mechanism and enantioselectivity of the studied reaction were generally comparable to those observed for the homogeneous catalyst.

Graphical abstractThe Diels–Alder reaction between 2-methyl-1,3-butadiene and 2,3-dimethyl-1,4-benzoquinone catalyzed by SiO2-supported chiral cationic oxazaborolidinium catalyst is likely to proceed via a concerted asynchronous mechanism with an endo transition state.Figure optionsDownload full-size imageDownload high-quality image (83 K)Download as PowerPoint slideResearch highlights▶ SiO2 surface is obtained by melt-quench method and resembles silica calcined at 673 K. ▶ Diels–Alder reaction on SiO2 surface shows concerted highly asynchronous mechanism. ▶ The mechanism and enantioselectivity are comparable to homogeneous catalyst.