Lithium hexamethyldisilazide-mediated enolizations: influence of chelating ligands and hydrocarbon cosolvents on the rates and mechanisms.
Enolizations of 2-methylcyclohexanone by lithium hexamethyldisilazide (LiHMDS) in the presence of three chelating ligands--trans-N,N,N',N'-tetramethylcyclohexanediamine, N,N,N',N'-tetramethylethylenediamine, and dimethoxyethane--reveal an approximate 40-fold range of rates. NMR spectroscopic analyses and rate studies reveal isostructural transition structures based on monomeric LiHMDS for the diamines. Rate studies of LiHMDS/dimethoxyethane-mediated enolizations implicate a substantial number of monomer- and dimer-based mechanisms. The rate laws vary for the three ligands because of ligand-dependent structural differences in both the reactants and the transition structures. The importance of LiHMDS-ketone complexes and the role of hydrocarbon cosolvents are discussed.
Godenschwager
Peter F
PF
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University Ithaca, New York 14853-1301, USA.
Collum
David B
DB
D002614:Chelating Agents / Q000737:chemistry; D003512:Cyclohexanones / Q000737:chemistry*; D053138:Cyclohexenes / Q000737:chemistry*; D003514:Cyclohexylamines / Q000737:chemistry; D007700:Kinetics; D008024:Ligands; D018020:Lithium Compounds / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D012821:Silanes / Q000737:chemistry*; D013055:Spectrophotometry, Infrared
D002614:Chelating Agents; D003512:Cyclohexanones; D053138:Cyclohexenes; D003514:Cyclohexylamines; D008024:Ligands; D018020:Lithium Compounds; D012821:Silanes; C442341:lithium hexamethyldisilazide