3-Substituted-1-(Trimethylsilylmethyl)cyclobutyl Cations: Stereochemistry of Solvent Capture of β-Trimethylsilyl Carbocations

Abstract

3-tert-Butyl-or phenyl-substituted-1-(trimethylsilylmethyl)cyclobutyl trifluoroacetates react in methanol via β-trimethylsilyl carbocationic intermediates formed from loss of trifluoroacetate ion. These cationic intermediates react to give a significant amount of methyl ether substitution products along with the expected alkene elimination products. Different methyl ether substitution products are formed from isomeric trifluoroacetates, and complete retention of configuration of the ether group relative to the starting trifluoroacetates is observed. The proposed origin of this retention is the involvement of two different β-trimethylsilyl carbocation intermediates that form while utilizing backside participation of the trimethylsilyl group. Computational studies have located three classical 1-(trimethylsilylmethyl)cyclobutyl carbocation energy minima. Two of these cations have a significant barrier (13−16 kcal/mol) to interconversion. It is proposed that methanol can capture each of these cations from different sides, leading to isomeric methyl ether products. A third energy minimum, formed by cyclobutane ring inversion, is thought to be unimportant in determining the stereochemistry of the methyl ether substitution products.

Substituent Effects on the Equilibria between Cyclopropylcarbinyl, Bicyclobutonium, Homoallyl, and Cyclobutyl Cations

“…The CB structure for these non-π donating groups are always higher in energy than the corresponding BCB by anywhere from 1.5 (R = Cl) to 15.8 (R = NO 2 ) kcal/mol (See Figure S2 for all values). Thus, it is likely that interconversion between BCB stereoisomers through a CB cation is possible in cases where this barrier is low enough, as shown recently by Creary . To our surprise, there are few substituents that allow both the CPC and BCB structures to be minima on the potential energy surface (PES).…”

“…Thus, it is likely that interconversion between BCB stereoisomers through a CB cation is possible in cases where this barrier is low enough, as shown recently by Creary. 67 To our surprise, there are few substituents that allow both the CPC and BCB structures to be minima on the potential energy surface (PES). These include H, SiR 3 , Cl, and the strong EWGs CO 2 CH 3 , CF 3 , CN, and NO 2 (blue points in Figure 3A, representative IRC in Figure 3C), and the free energy difference between the structures is at most ±10 kcal/mol.…”