Transmission of polar substituent effects in the adamantane ring system as monitored by 19F NMR: hyperconjugation as a stereoinductive factor

Abstract

A limited series of (E)- and (Z)-5-substituted adamant-2-yl fluorides 8 and 9, respectively, has been synthesized and characterized, and the 19 F chemical shifts have been measured in several solvents. A wide range of 19 F substituent chemical shifts (SCS, ppm) are obtained for the former system (ca. 9.1 (c-C 6 H 12 ), 9.6 (CDCl 3 ), and 11.8 (HFIP)) compared to the latter (ca. 0.6 (c-C 6 H 12 ), 0.7 (CDCl 3 ), and 2.3 (HFIP)). Factorization of the 19 F SCS into polar field (ρ F σ F ) and residual contributions ( 19 F SCS-ρ F σ F ) reveals the predominance of the latter solvent-independent component for the E fluorides (8). Comparison of 8 with a similar dissection of the 19 F SCS of 4-substituted bicyclo[2.2.]oct-1-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 π-facial diastereoselection in 2,5-disubstituted adamantanes is discussed