ELECTROSTATIC VS HYPERCONJUGATIVE EFFECTS AS STEREOINDUCTIVE FACTORS IN THE ADAMANTANE RING-SYSTEM

A series of 5-substituted (X) adamant-2-yl derivatives 1 (Y = O and CH2) have been synthesized and their C-13 NMR spectra measured. The carbonyl and ethylenic C-13 substituent chemical shifts (SCS) are shown to be proportional to substituent field effects (sigma(F)). By use of the polar field susceptibility parameter (rho(F)) for the former shifts, polar field parameters (Delta sigma(F) values) have been calculated for a series of p-SC6H4 substituents. II-Facial diastereoselectivities for the reduction (NaBH4) and methylation (MeLi) of para-substituted (S) phenyl derivatives of 1 (Y = O; X = p-SC6H4) and, as well, for the hydrochlorination of similarly substituted alkenes 1 (Y = CH2; X = p-SC6H4) have been determined and correlated successfully against polar field parameters (Delta sigma(F) values). The slopes of these plots (log(10)[Z]/[E] vs Delta sigma(F)) provide polar-field susceptibility parameters (rho(FS)) which have been deployed to calculate appropriate diastereoselectivities of the aforementioned reactions for 5-substituted (X) derivatives of 1 (Y = O and CH2) in general. A comparison of these calculated values with observed diastereoselectivities for a wide range of substituents reveals that direct electrostatic field interactions play a dominant role in governing the phenomena for nucleophilic additions of the ketones 1 (Y = O) and that it is unnecessary to invoke Cieplak's transition-state hyperconjugative model. This appears to be also the case for electrophilic additions to 1 (Y = CH2) not involving final nucleophilic capture of an intermediate cation. However, hyperconjugation clearly impinges significantly on II-facial diastereoselectivity for those reactions mediated by 2-adamantyl cations. New results for methyl-substituted derivatives of 1 (Y = O and CH2, X = CH3) reinforces the sigma-electron-withdrawing character of this substituent with respect to gamma- and delta-interactions in the neutral ground-state and cationic species.