Zinc tetrafluoroborate catalyzed Mannich-type reaction of aldimines and silyl enol ethers in aqueous medium
Brindaban C. Ranu*, Sampak Samanta and Sankar K. Guchhait
*Department of Organic Chemistry, Indian Association for the Cultivation of
Science, Jadavpur, Calcutta 700 032, India, Email: ocbr
mahendra.iacs.res.in
B. C. Ranu, S. Samanta, S. K. Guchhait, Tetrahedron, 2002, 58, 983-988.
DOI: 10.1016/S0040-4020(01)01177-2
Abstract
The use of zinc tetrafluoroborate as a catalyst in Mannich-type addition of silyl enol ethers to aldimines in aqueous THF provides β-amino ketones or β-amino esters in high yields. A one-pot reaction of an aldehyde, amine and silyl enolate also works well.
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Details
The document discusses the use of zinc tetrafluoroborate as a catalyst in Mannich-type reactions involving aldimines and silyl enol ethers in an aqueous medium. This method efficiently produces β-amino ketones or β-amino esters in high yields. The classical Mannich reaction, which is crucial for synthesizing β-amino carbonyl compounds, often requires harsh conditions and long reaction times. The introduction of zinc tetrafluoroborate offers a more environmentally friendly and efficient alternative, avoiding the use of toxic and hazardous Lewis acids and chlorinated solvents.
The experimental procedure involves stirring an imine with a silyl enol ether in aqueous THF at room temperature with a catalytic amount of zinc tetrafluoroborate. The reaction is quick, typically taking 10-40 minutes, and yields high-purity products after extraction and purification. The method works well with various aromatic aldimines and silyl enol ethers of acyclic and cyclic ketones, showing good diastereoselectivity.
Additionally, a one-pot three-component condensation of aldehydes, amines, and silyl enol ethers is also effective, though with slightly lower yields compared to the two-component reaction. The procedure is noted for its operational simplicity, cost-effectiveness, and general applicability, making it a valuable method in synthetic organic chemistry.
Key Words
Mannich reaction, β-Amino Ketones
ID: J72-Y2002-1010
