Stable simple enols. Resolution of chiral 1-[9′-(2′-fluoroanthryl)]-2,2-dimesitylethenol. A different racemization mechanism for the enol and its acetate

Article abstract of DOI:10.1021/jo020497q

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 ΔG_e^?, ΔH_e^?, and ΔS_e^? values of 26.2, 27.6 kcal mol^-1, and 4.3 eu, respectively, as expected for a rotational ββ′-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 ββ′-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.