Lithium hexamethyldisilazide-mediated enolizations: influence of triethylamine on E/Z selectivities and enolate reactivities.
Lithium hexamethyldisilazide (LiHMDS) in triethylamine (Et 3N)/toluene is shown to enolize acyclic ketones and esters rapidly and with high E/ Z selectivity. Mechanistic studies reveal a dimer-based mechanism consistent with previous studies of LiHMDS/Et 3N. E/ Z equilibration occurs when <2.0 equiv of LiHMDS are used. Studies of the aldol condensation and Ireland-Claisen rearrangement of the resulting Et 3N-solvated enolates show higher and often complementary diastereoselectivities when compared with analogous reactions in THF. The Et 3N-solvated enolates also display a marked (20-fold) acceleration of the Ireland-Claisen rearrangement with evidence of autocatalysis. A possible importance of amine-solvated enolates is 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
D002384:Catalysis; D019281:Dimerization; D005021:Ethylamines / Q000737:chemistry*; D005663:Furans / Q000737:chemistry; D007659:Ketones / Q000737:chemistry*; D018020:Lithium Compounds / Q000737:chemistry*; D012821:Silanes / Q000737:chemistry*; D013237:Stereoisomerism
D005021:Ethylamines; D005663:Furans; D007659:Ketones; D018020:Lithium Compounds; D012821:Silanes; C442341:lithium hexamethyldisilazide; C016162:triethylamine