A comparative study of solid state and solution aldol addition reactions of the lithium enolate of methyl 3,3-dimethylbutanoate: stereochemistry and mechanistic implications

Abstract

We report the first systematic study of the solid state aldol addition reactions of the lithium enolate of methyl 3,3-dimethylbutanoate with various aliphatic or aromatic, achiral or chiral, solid or liquid aldehydes, in comparison with the same reactions performed in tetrahydrofuran (THF) solution. The diastereoselection of the solid state reactions in all the cases was found to be essentially identical to that of the solution reactions. At low temperatures (< −20δC), both the solid state and the solution reactions are kinetically selective with the syn:anti ratios of approximately 50:50, except for the reactions involving sterically small aldehydes. Decreasing the bulkiness of the aldehydes from pivalaldehyde to acetaldehyde resulted in changes of the syn:anti ratio from 42:58 to 90:10. At higher temperatures, the stereoselection of both the solid state and the solution reactions is thermodynamically controlled, giving predominately anti isomers. The solid state reactions generally afforded low yields and low conversions of the aldehydes. Increasing the reaction time or temperature had little effect on the yields of the solid state reactions. The stereochemical outcomes can be interpreted satisfactorily using the Zimmerman-Traxler-Dubois transition state models. The results suggest that the same reacting species and transition states are involved in both the solid state and the solution reactions under our experimental conditions.