Stable simple enols. Resolution of chiral 1-[9'-(2'-fluoroanthryl)]-2,2-dimesitylethenol. A different racemization mechanism for the enol and its acetate.
Chiral 1-[9'-(2'-methoxyanthryl)]-2,2-dimesitylethenol (2), 1-[9'-(2'-fluoroanthryl)]-2,2-dimesitylethenol (3), and 1-[9'-(2'-fluoroanthryl)]-2,2- dimesitylvinyl acetate (4) were synthesized and their DNMR behavior in C6D5NO2 was studied. 3 and 4 were resolved on an amylose tris(3,5-dimethylphenylcarbamate) HPLC column to their enantiomers. Acetate 4 racemizes slowly in solution with DeltaG(e)(++), DeltaH(e)(++), and DeltaS(e)(++) values of 26.2, 27.6 kcal mol(-)(1), and 4.3 eu, respectively, as expected for a rotational betabeta'-2-ring flip process in a vinyl propeller and the racemization is unaffected by added TFA, Et3N, and EtOD. Although 3 racemizes almost 350 times faster, the racemization is catalyzed by TFA and shows bell-shape catalysis by Et3N and a KIE in a partially deuteriated solvent. From this and the DNMR data, it is concluded that 3 does not racemize via a rotational betabeta'-2-ring flip. Five nonflip routes are discussed for the racemization of 3, and it is concluded that only the one initiated by protonation at C1 does not contradict the experimental data. By analogy with the E/Z isomerization of the structurally related 2-(m-methoxymesityl)-1,2-dimesitylethenol 17, it is suggested that in the absence of added catalyst one or more enol molecule(s) catalyze the enantiomerization of another one. Only partial resolution was achieved for 2 and from the similarity of its behavior with that of 3, it is suggested that it racemizes by the same mechanism.
Rochlin
Elimelech
E
Department of Organic Chemistry, The Hebrew University, Jerusalem 91904, Israel.
Rappoport
Zvi
Z