Unusual induced isotope effects in the reaction of 2-pentanone with dialkylamide bases. Evidence on the nature of the reactive base species

Abstract

The treatment of 2-pentanone and 2-pentanone-1,1,1-d 3 with various bases (3 mol/mol of ketone) in tetrahydrofuran (THF) or THF-hexamethylphosphoric triamide (HMPA) (3 mol/mol of base) gave enolate mixtures which were treated with trimethylsilyl chloride (TMSCl) to give trimethylsilyl enol ethers (80-100% yields, mostly >90%) for analysis by GC. The bases were lithium diisopropylamide (LDA), lithium hexamethyldisilazide (LHMDS), and lithium tetramethylpiperidide (LTMP). Isotope effects for attack at methyl, calculated from product ratios, showed a variety of temperature dependences: normal (A aH /A aD =0.6-0.8, all reactions in THF-HMPA) to temperature independent (k H /k D and A aH /A aD =2.8-2.9, LDA in THF) to inverse (A aH /A aD =12.3, LTMP in THF)