Dimethylcyclodecyl cations. Evidence for .mu.-hydrido bridging

Abstract

The carbocations 1,5-and 1,6-dimethylcyclododecyl are postulated to have a p-hydrido-bridged structure. This assignment is based on the observation of a single very high-field hydrogen in the 'H NMR, anomalously low coupling constants involving this hydrogen, and very small isotope perturbation shifts using Saunders' criterion for equilibrating and resonance structural distinction. Both positional and conformational isomerization is possible in these bridged ions, but no evidence for cis-trans isomerization was found. The p-hydrido bridging is best treated as an "open" three-center, two-electron resonance system involving a single potential minimum for the bridged hydrogen. The bridging hydrogen is not electrophilic, and the bridged ions cannot be prepared by protonating the central bond of decalins nor can the p-hydrogen be removed as a proton. Numerous theoretical calculations have been performed and correlated with the experimental data.We have recently reported the direct observation of a series of secondary medium-ring cycloalkyl cations for which a p-hydrido-bridged structure 1 (R, = R2 = H ) was m \[I I R2 1, m = n = 3; m = 3, n = 4; m = 3, n = 5; m =4, n = 5Although these secondary cation investigations are very relevant in terms of previously observed transannular reactivity in medium rings: they are not ideal species for looking in detail at the evidence for this p-hydrido bridging. The corresponding ditertiary analogues, e&, R1 = Rz = CH3, have two major advantages: (1) they are thermally more stable, and (2) the proposed p-hydrido bridge can be fixed geometrically simply by positioning the two alkyl groups in a known relationship.In this work, we report a detailed investigation of the dimethylcyclodecyl ~y s t e m .~

A quantum-chemical study of hydride transfer from alkanes to carbenium cations in the gas phase: A comparison with liquid- and solid-acid catalytic systems

“…The gas-phase hydride-transfer reactions from alkanes to carbenium cations have been studied by mass-spectrometry, e.g. 27,32,33 Carbocations in the gas phase, unlike those in the above mentioned solid-and liquid-catalytic systems, should be considered as practically free. Therefore, a comparison of the hydride transfer reactions from alkanes to carbenium ions in the gas phase with those in other media would be quite useful for better understanding of solvation e †ects.…”

Reactions of cyclohexane derivatives in superacids

“…A 1.5 m long stainless steel column of 2 mm internal diameter, packed with 60-80 mesh Celite coated with 10% Reoplex 100, was used analytically. (6).-Triphenylphosphine (20 g) in tetrahydrofuran (THF) (50 cm3) was added to N-chlorosuccinimide (13 g) in THF (200 cm3) cooled in a salt-ice bath. Menthol (12.5 g), obtained from Aldrich, was slowly added to the cooled slurry, then stirred for 4 h. The solvent was then removed under reduced pressure and the products extracted with pentane.…”

MO Calculations Involving μ-Hydrido Cation Intermediates Relevant to the Heptane to Toluene Dehydrocyclization Reaction

“…However, after preparing the 1,5-dimethylcyclodecyl cation 14, and comparing this with the isomeric 1,6-isomer 5, it was realized that a l,5^-H-bridged (10) structure was a better fit of various NMR parameters than those expected for the secondary l,6^-H-bridged cyclodecyl cation 1, particularly the chemical shift position of the μ-Η (14,15). Support for this conclusion was provided by several further studies: (1) the 1,5 and 1,6-dimethylcyclodecyl cations 14 and 5 (both bridged structures) underwent equilibration in superacid solution at -84 °C to give a 2.8/1 mixture favoring the 1,5-structure.…”