On the mechanism of Lewis base catalyzed aldol addition reactions: kinetic and spectroscopic investigations using rapid-injection NMR.
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.
Denmark
Scott E
SE
Roger Adams Laboratory, Department of Chemistry, University of Illinois, Urbana, Illinois 61801, USA. sdenmark@illinois.edu
Eklov
Brian M
BM
Yao
Peter J
PJ
Eastgate
Martin D
MD
D000447:Aldehydes / Q000737:chemistry*; D000577:Amides / Q000737:chemistry*; D002384:Catalysis; D002712:Chlorides / Q000737:chemistry; D006492:Hempa / Q000737:chemistry; D007267:Injections; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D063088:Phosphoramides; D010756:Phosphoric Acids / Q000737:chemistry*; D017655:Silicon Compounds / Q000737:chemistry; D013237:Stereoisomerism; D013379:Substrate Specificity; D013997:Time Factors
D000447:Aldehydes; D000577:Amides; D002712:Chlorides; D063088:Phosphoramides; D010756:Phosphoric Acids; D017655:Silicon Compounds; C005438:phosphoramide; C116609:3-hydroxybutanal; C039676:silicon tetrachloride; D006492:Hempa