Five ketones R1COCH2R2 (1a−e) were enolizedin tetrahydrofuran solvent employing lithium arylamideswith different electron-withdrawing and -donating substituents on the phenyl ring (4a−e). Enolate selectivity isunaffected by a moderate electron-releasing or -withdrawinggroup, but significantly enhanced by strong electron-withdrawing substituents to yield predominantly Z-enolate.Outstanding selectivity was achieved with lithium trichloroanilide (5) and lithium diphenylamide (6). The results arerationalized in terms of electronic effects on the tightnessof the transition states.