PubMed Health⌕ Search

Biomedical subjects

Xiping Su

Publications and source records attributed to Xiping Su.

4 recordsLinked to original sources
PubMed2006

Chiral phosphoramide-catalyzed aldol additions of ketone trichlorosilyl enolates. Mechanistic aspects.

The mechanism of the catalytic, enantioselective addition of trichlorosilyl enolates to aldehydes has been investigated. Kinetic studies using ReactIR and rapid injection NMR (RINMR) spectroscopy have confirmed the simultaneous operation of dual mechanistic pathways involving either one or two phosphoramides bound to a siliconium ion organizational center. This mechanistic dichotomy was initially postulated on the basis of catalyst loading studies and nonlinear effects studies. This duality explains the difference in reactivity and stereoselectivity of various classes of phosphoramides. Determination of Arrhenius activation parameters revealed that aldol addition occurs through the reversible albeit unfavorable formation of an activated complex, and natural-abundance 13C NMR kinetic isotope effect (KIE) studies have determined that the turnover limiting step is the aldol addition. A thorough examination of a range of phosphoramides has established empirical structure-activity selectivity relationships. In addition, the effects of catalyst loading, rate of addition, solvents, and additives have been studied and together allow the formulation of a unified mechanistic picture for the aldol addition.

Journal Article↗
PubMed2006

Chiral phosphoramide-catalyzed enantioselective addition of allylic trichlorosilanes to aldehydes. Preparative and mechanistic studies with monodentate phosphorus-based amides.

The addition of allylic trichlorosilanes to benzaldehyde promoted by chiral phosphoramides to give the enantioenriched homoallylic alcohol has been investigated. In a survey of Lewis bases as activators for the addition of allyltrichlorosilane to benzaldehyde, phosphorus-based amides have been found to be the most effective promoters. To achieve asymmetric induction, chiral phosphoric triamides derived from chiral diamines have been developed and applied in the allylation reaction albeit with modest enantioselectivities. The addition of 2-butenylsilanes was highly diastereoselective, suggesting a closed, chair-like transition structure. A detailed mechanistic study has been carried out to probe into the origin of activation. From a combination of nonlinear effects and kinetics studies, the reaction was found to likely involve two phosphoramides in both the rate and stereochemistry determining steps. These studies provided the background for the development of highly selective and reactive catalysts.

Aldehydes↗
PubMed1999

Synthesis of Phosphoramides for the Lewis Base-Catalyzed Allylation and Aldol Addition Reactions.

Both chiral and achiral phosphoramides of diverse structure were prepared from diamines by the coupling to phosphorus(V) or phosphorus(III) reagents. Several enantiopure 1,2-diphenyl-1,2-ethanediamine analogues have been prepared by the reductive coupling of the corresponding N-silylimine with NbCl(4)(THF)(2) and subsequent resolution by the formation of diastereomeric menthyl carbamates. (S,S)-N,N'-Di-(1-naphthyl)-1,2-diphenyl-1,2-ethanediamine 15 was prepared by the arylation of (S,S)-1,2-diphenyl-1,2-ethanediamine with naphthyl iodide.

Journal Article↗