TY - JOUR
T1 - Substituent Effects on the Equilibria between Cyclopropylcarbinyl, Bicyclobutonium, Homoallyl, and Cyclobutyl Cations
AU - Larmore, Sean P.
AU - Champagne, Pier Alexandre
N1 - Publisher Copyright:
© 2024 American Chemical Society.
PY - 2024/6/7
Y1 - 2024/6/7
N2 - 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.
AB - 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.
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U2 - 10.1021/acs.joc.4c00810
DO - 10.1021/acs.joc.4c00810
M3 - Article
AN - SCOPUS:85193487122
SN - 0022-3263
VL - 89
SP - 8146
EP - 8156
JO - Journal of Organic Chemistry
JF - Journal of Organic Chemistry
IS - 11
ER -