DOSY NMR for monitoring self aggregation of bifunctional organocatalysts: increasing enantioselectivity with decreasing catalyst concentration.
In this report, we demonstrate that self-aggregation is an intrinsic problem of bifunctional organocatalysts, especially in the case when the substrates do not have functional groups which are able to bind strongly with catalyst. Due to their self-association phenomena, the enantioselectivity of bifunctional catalysts dramatically decreases with increasing catalyst concentration or decreasing temperature. Thus, when the substrate concentration is kept constant, the enantioselectivity of bifunctional catalysts dramatically increases with decreasing catalyst loading. The ee values obtained at different catalyst concentrations are fairly consistent with the diffusion coefficients (D) of the catalysts, strongly indicating that their degree of self-association plays a crucial role in determining their enantioselectivity.
Jang
Hyeong Bin
HB
Department of Chemistry, Sungkyunkwan University, 300 Cheoncheon, Jangan, Suwon, Gyeonggi 440-746, Korea.
Rho
Ho Sik
HS
Oh
Joong Suk
JS
Nam
Eun Hye
EH
Park
Sang Eun
SE
Bae
Han Yong
HY
Song
Choong Eui
CE
D002384:Catalysis; D004058:Diffusion; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D015394:Molecular Structure; D055601:Organic Chemistry Phenomena; D013237:Stereoisomerism