The 1-(trimethylsilyl)bicyclobutonium ion: NMR spectroscopy, isotope effects, and quantum chemical ab initio calculations of a new hypercoordinated carbocation

Article abstract of DOI:10.1021/ja00127a011

The 1-(trimethylsilyl)bicyclobutonium ion is generated from (1′-(trimethylsilyl)cyclopropyl)methanol by reaction with SbF₅. The NMR spectroscopic data in SO₂ClF/SO₂F₂ solution at -128°C are in accord with a bridged puckered bicyclobutonium structure undergoing a 3-fold rapid degenerate rearrangement that renders the two β- and one γ-methylene groups equivalent, leading to one averaged ¹³C-NMR signal for the CH₂ groups at 48.9 ppm. Conformational ring inversion is slow so that two separate signals for the three averaged endo-CH₂ (4.04 ppm) and three averaged exo-CH₂ hydrogens (3.24 ppm) are observed. The deuterium equilibrium isotope effects for exo- and endo-CHD-labeled cations are different in sign and magnitude and are rationalized by different endo- and exo-C-H bond force constants at the pentacoordinated carbon. NMR chemical shift calculations for the 1-silylbicyclobutonium ion and the (1′-silylcyclopropyl)methyl cation were performed with the GIAO-SCF and GIAO-MP2 methods. The experimental shifts are satisfactorily reproduced by GIAO-MP2//tzp/dz calculated shifts for the 1-silylbicyclobutonium structure. The good agreement between theory and experiment supports a fully degenerate set of interconverting 1-(trimethylsilyl)bicyclobutonium ions and excludes contributions from other isomers to the observed equilibrium process. The geometric and electronic properties of the 1-(trimethylsilyl)bicyclobutonium cation are intermediate between those of the parent bicyclobutonium ion and those of the methyl-substituted analogue.