Dramatic effect of aggregation on rates and thermodynamics of stereoisomerization of magnesium enolates.
Thermodynamic stereocontrol of the (hexamethyldisilazide)magnesium enolates of propiophenone in THF is reported. The overall stereoselectivity proves to be very sensitive to concentration, since dimeric species with bridging enolates show no stereoselectivity while monomeric enolates show a very strong thermodynamic preference for the Z enolate. Kinetically, interconversion among aggregates is remarkably slow, whereas stereoisomerization of the monomer, even in the absence of a proton source such as ketone or amine, is remarkably fast. Furthermore, stereoisomerization takes place in the absence of a proton source or excess ketone. These observations contrast with accepted views of these fundamentally important processes and have implications for understanding the identity and reactivity of metal enolates.
Hurley
Erin R
ER
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556-5670.
He
Xuyang
X
Brown
Seth N
SN
Henderson
Kenneth W
KW
D000438:Alcohols / Q000737:chemistry*; D005663:Furans / Q000737:chemistry; D007700:Kinetics; D008274:Magnesium / Q000737:chemistry*; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000737:chemistry*; D011427:Propiophenones / Q000737:chemistry; D013237:Stereoisomerism; D013816:Thermodynamics; D013997:Time Factors
D000438:Alcohols; D005663:Furans; D009942:Organometallic Compounds; D011427:Propiophenones; C018674:tetrahydrofuran; D008274:Magnesium