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 degrees C) revealed that lithium p-fluorophenolate and the lithium enolates of cyclohexanone, cyclopentanone and 4-fluoroacetophenone have tetrameric structures in THF/Et(2)O and THF/Et(2)O-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)(2)Li(-) and (RO)(3)Li(2-). Better characterized solutions of these lithiates were prepared by addition of phosphazenium enolates (using P4-(t)Bu base) to the lithium enolate in 1:1 ratio to form triple ion (RO)(2)Li(-) P4H(+), or 2:1 ratio to form the higher lithiate (RO)(3)Li(2-) (P4H(+))(2)) (quadruple ions). The E isomer of 5F-Li is also dimeric in 3:2 THF/Et(2)O 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.
Kolonko
Kristopher J
KJ
Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, USA.
Biddle
Margaret M
MM
Guzei
Ilia A
IA
Reich
Hans J
HJ