On the mechanism of Lewis base catalyzed aldol addition reactions: kinetic and spectroscopic investigations using rapid-injection NMR
| Autor: | Peter J. Yao, Brian M. Eklov, Scott E. Denmark, Martin D. Eastgate |
|---|---|
| Rok vydání: | 2009 |
| Předmět: |
Magnetic Resonance Spectroscopy
Time Factors Silylation Ketene Biochemistry Catalysis Injections Substrate Specificity chemistry.chemical_compound Colloid and Surface Chemistry Aldol reaction Chlorides Computational chemistry Organic chemistry Phosphoric Acids Lewis acids and bases Aldehydes Acetal Silicon Compounds Stereoisomerism General Chemistry Hempa Amides Lewis acid catalysis Kinetics chemistry Phosphoramides Selectivity |
| Zdroj: | Journal of the American Chemical Society. 131(33) |
| ISSN: | 1520-5126 |
| Popis: | The mechanistic foundations of the Lewis base catalyzed aldol addition reactions have been investigated. From a combination of low-temperature spectroscopic studies ((29)Si and (31)P NMR) and kinetic analyses using a rapid-injection NMR apparatus (RINMR), a correlation of the ground states and transition structures for the aldolization reactions has been formulated. The aldol addition of the tert-butylsilyl ketene acetal of tert-butyl propanoate with 1-naphthaldehyde is efficiently catalyzed by a combination of silicon tetrachloride and chiral phosphoramide Lewis bases. The rates and selectivities of the aldol additions are highly dependent on the structure of the Lewis bases: bisphosphoramides give the highest rate and selectivity, whereas a related monophosphoramide reacts slowly and with low selectivity. The monophosphoramide shows no nonlinear behavior. All of the additions show a first-order kinetic dependence on silyl ketene acetal and 1-naphthaldehyde and a zeroth-order dependence on silicon tetrachloride. The kinetic order in catalyst is structure dependent and is either half-, two-thirds-, or first-order. All of the phosphoramides are saturated with silicon tetrachloride in some form, and the resting-state species are mixtures of monomeric and dimeric, pentacoordinate cationic, or hexacoordinate neutral complexes. These data allow the formulation of a unified mechanistic scheme based on the postulate of a common reactive intermediate for all catalysts. |
| Databáze: | OpenAIRE |
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