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

The cyclopropylcarbinyl (CPC) and bicyclobutonium (BCB) structures of the C₄H₇⁺ 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 C₄H₇⁺ cations. We find that the shapes of the potential energy surfaces (PESs) vary significantly, with structures that are minima for a given substituent becoming high-energy transition structures for another. Electron-donating groups at C1, C2, or C3/C4 positions favor CPC, BCB/CB, and HA structures, respectively. Electron-withdrawing groups yield shallower PESs where multiple related structures are energetically accessible. Strong Hammett correlations (σ⁺) are observed for the substituent effects, which appear to be additive. In addition, BCB cations with more substituents are energetically destabilized compared to CPC cations, except with donating substituents at the C2 position. This work allows predictions of the major structures expected in mixtures of CPC/BCB/CB/HA cations for given substituent patterns, and of the major products produced from such cations.