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 synthetic transformations forming cyclopropylcarbinyl, cyclobutyl, or homoallyl products. It has recently been recognized that such cations, when generated from chiral electrophiles, are themselves chiral and can react with nucleophiles stereospecifically. However, the CPC and BCB cations are in equilibrium with each other and with other related structures such as the cyclobutyl (CB) and homoallyl (HA) cations, from which stereospecificity is not guaranteed. Currently, the effect of substitution on the composition of cation mixtures containing CPC/BCB/CB/HA cations is not understood, precluding the prediction and control of the major products generated from such cations. Using Density Functional Theory and DLPNO-CCSD(T) calculations, we have studied the electronic and steric effects on the equilibria between mono- and polysubstituted C4H7+ cations. Our results indicate that electron-donating groups at the C1 position favor CPC structures, while BCB/CB structures are favored for the C2 position and HA structures for the C3/C4 positions. Electron-withdrawing groups yield shallower potential energy surfaces where many related structures are energetically accessible. Strong Hammett correlations (σ+) are observed for the various substituent effects, which appear to be additive in nature. In addition, BCB cations with more substituents are energetically destabilized compared to CPC cations, except with donating substituents at the C2 position. This work provides a predictive model for the major structures observed in mixtures of CPC/BCB/CB/HA cations, for given substituent patterns.