Does double hyperconjugation play a role in 1-bicyclo[2.2.2]octyl cation ? The through-space effect of polar substituents

Nature of the Electronic Factor Governing Diastereofacial Selectivity in Some Reactions of Rigid Saturated Model Substrates

“…The well-defined through-bond contributions to the 19 F SCS of 14 and 23 (see Tables and , respectively) is a significant result since it demonstrates unambiguously that besides electrostatic field effects long-range polar σ-inductive effects can be transmitted efficiently by a conjugative effect in saturated systems. These effects have been difficult to define unambiguously with reactivity probes, particularly with the usual basis set of σ-electron-withdrawing groups, because of the camouflaging envelope of the dominant field mechanism. 48a-c,49b, However, because it is generally accepted that hyperconjugative interactions are more pronounced in excited or electron-deficient species such as carbocations than in the neutral ground state, the aforementioned 19 F chemical shift studies of 14 and 23 strongly suggested that double hyperconjugation should be an important mode of cation stabilization in the bicyclo[2.2.2]octyl and adamantyl systems ( 25 and 26 ( E -cation), respectively) as illustrated by canonical structures 27 and 28 for the former and 29 and 30 for the latter. The metalloidal substituents (MMe 3 ; M = Si, Ge, and Sn) seemed ideal for this purpose since they are good σ-electron donors but exert negligible field influences (σ F ≈ 0; see Tables and ).…”

Through-bond interactions of .beta.-carbonyl and .beta.-imine lone pairs with a cationic 2p orbital. Quantum calculations on bicyclo[2.2.2]oct-1-yl cation and derivatives

“…For the 3-oxo and 3-(£)-imino derivatives (13 and 15), the corresponding limiting structures B" are predicted to contribute more than B' as it was found that the ffC(2),C(3) bond lengthenings are larger than those of trC( 5),C(6) (Table IV). Interestingly, this differential effect is significantly reduced in the case of the 3-(Z) -iminobicyclo[2.2.2]oct-l-yl cation and reversed for the 3-methylidene ( 19), 3-(dicyanomethylidene (21), 3-cyano- (23), and 3-nitrobicyclo [2,2,2]oct-l-yl cations (25)! Thus, as proposed above on the basis of the relative stabilities of 2-and 3-substituted bicyclo[2.2.2]oct-l-yl cations, the n(CO) and n(C=NH) electron pairs in 13 and 15, respectively, intervene in the stabilization of the corresponding limiting structures B", i.e.…”