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

The cyclopropylcarbinyl (CPC) and bicyclobutonium (BCB) structures of the CH cation have been proposed as intermediates in various reactions forming cyclopropylcarbinyl, cyclobutyl, or homoallyl products. While these cations can react with nucleophiles stereospecifically, in each system there are usually multiple BCB/CPC cations in equilibrium with related cyclobutyl (CB) and homoallyl (HA) cations, from which stereospecificity is jeopardized. Using density functional theory (DFT) and DLPNO-CCSD(T) calculations, we studied the electronic and steric effects on the equilibria between mono- and polysubstituted CH cations.

Cyclopropylcarbinyl-to-Homoallyl Carbocation Equilibria Influence the Stereospecificity in the Nucleophilic Substitution of Cyclopropylcarbinols.

The synthesis of quaternary homoallylic halides and trichloroacetates from cyclopropylcarbinols, as reported by Marek ( , , 5543-5548), is one of the few reported examples of stereospecific nucleophilic substitution involving chiral bridged carbocations. However, for the phenyl-substituted substrates, poor specificity is observed and mixtures of diastereomers are obtained. To understand the nature of the intermediates involved and explain the loss of specificity for certain substrates, we have performed a computational investigation of the reaction mechanism using ωB97X-D optimizations and DLPNO-CCSD(T) energy refinements.