Solution Structures of Lithium Enolates of Cyclopentanone, Cyclohexanone, Acetophenones, and Benzyl Ketones. Triple Ions and Higher Lithiate Complexes

Multinuclear NMR spectroscopic studies at low temperature (−110 to −150 °C) revealed that lithium p-fluorophenolate and the lithium enolates of cyclohexanone, cyclopentanone and 4-fluoroacetophenone have tetrameric structures in THF/Et₂O and THF/Et₂O−HMPA by study of the effects of the addition of HMPA. The Z and E isomers of the lithium enolate of 1,3-bis-(4-fluorophenyl)-2-propanone (5F-Li) show divergent behavior. The Z isomer is completely dimeric in pure diethyl ether, and mostly dimeric in 3:2 THF/ether, where monomer could be detected in small amounts. TMTAN and PMDTA convert Z-5F-Li to a monomeric amine complex, and HMPA converts it partially to monomers, and partially to lithiate species (RO)₂Li⁻ and (RO)₃Li²⁻. Better characterized solutions of these lithiates were prepared by addition of phosphazenium enolates (using P4-^tBu base) to the lithium enolate in 1:1 ratio to form triple ion (RO)₂Li⁻ P4H⁺, or 2:1 ratio to form the higher lithiate (RO)₃Li²⁻ (P4H⁺)₂) (quadruple ions). The E isomer of 5F-Li is also dimeric in 3:2 THF/Et₂O solution, but is not detectably converted to monomer either by PMDTA or HMPA. In contrast to Z-5F-Li, the E isomer is tetrameric in diethyl ether even in the presence of excess HMPA. Thus for the two isomers of 5F six different enolate structures were characterized: tetramer, dimer, CIP-monomer, SIP-monomer, triple ion, and quadruple ion.