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