TRANSMISSION OF POLAR SUBSTITUENT EFFECTS IN THE ADAMANTANE RING-SYSTEM AS MONITORED BY F-19 NMR - HYPERCONJUGATION AS A STEREOINDUCTIVE FACTOR

A limited series of (E)- and (Z)-5-substituted adamant-2-yl fluorides 8 and 9, respectively, has been synthesized and characterized, and the F-19 chemical shifts have been measured in several solvents. A wide range of F-19 substituent chemical shifts (SCS, ppm) are obtained for the former system (ca. 9.1 (c-C6H12), 9.6 (CDCl3), and 11.8 (HFIP)) compared to the latter (ca. 0.6 (c-C6H12), 0.7 (CDCl3), and 2.3 (HFIP)). Factorization of the F-19 SCS into polar field (rho-F-sigma-F) and residual contributions (F-19 SCS - rho-F-sigma-F) reveals the predominance of the latter solvent-independent component for the E fluorides (8). Comparison of 8 with a similar dissection of the F-19 SCS of 4-substituted bicyclo[2.2.2]oct-1-yl fluorides 2 strongly suggests that the origin of the large residual contributions for 8 is "through-three-bond" electron delocalization or double hyperconjugation. The importance of this long-range electronic mechanism as a factor governing pi-facial diastereoselection in 2,5-disubstituted adamantanes (1) is discussed. In particular, the assertion that the p-anilino substituent is an electron donor group at remote probe or reaction sites in saturated systems is addressed.