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5.1. Catalytic Generation of "Silylene"

Clean generation of silylene-palladium species is possible by use of silylboranes bearing a dialkylamino group on the silicon atom. The reaction mechanism may involve oxidative addition of the Si-B bond to palladium(0) followed by b-elimination of the amino groups on the silicon atom with boryl group on the palladium atom. The "silylene-palaldium species" give 2,4-disubsituted siloles regioselectively in the reaction with terminal alkynes. The reaction of aminosilylborane with 1,3-dienes affords [4+1] cyclization products stereospecifically with respect to the stereochemistry of the starting 1,3-dienes. The stereospecificity is characteristic to the present reaction system in comparison to known reactions of "free silylene" species generated thermally or photochemically.

silylene.jpg
 

5.2. Bis-Silylation

Since silicon is quite abundant in the earth's crust, its utilization in organic synthesis, material science, and medicinal science is highly desirable. We have developed new catalytic silylation reactions on the basis of activation of the silicon-containing σ-bond by transition metal catalysts. For instance, intramolecular bis-silylations have been developed on the basis of our finding on new palladium catalysts, i.e., palladium-isocyanide catalysts [Ref .1], for the activation of the Si-Si bond. The intramolecular reactions enabled the synthesis of enantioenriched allylsilanes [Ref. 2], allenylsilanes [Ref. 3] and polyols [Ref. 4] in stereoselective fashions.

avenaciolide synthesis via bis-silylation80.jpg
 
allylsilane synthesis via bis-silylation80.jpg
 
allenylsilane synthesis via bis-silylation80.jpg
 

References

5.3. New Iminium Ion generators for Amination Reactions

In the course of our study on the synthetic application of cyanoboranes, we found interesting reactivity of bis(dialkylamino)cyanoboranes toward carbonyl compounds. In 1:1 reaction of the cyanoboranes with aldehydes at room temperature, α-dialkylamino nitriles were obtained in high yields [Ref. 1].

aminocyanation80.jpg

We designed aminoboranes bearing bulky diisopropyl groups on the boron atom. The aminoboranes served as a universal iminium ion generator, allowing use of free sec-amines in the Mannich-type reaction as the source of the amino group through facile amino group exchange on the boron atom [Ref. 2].

Mannich-type reaction80.jpg

The reaction protocol could be extended to other amination reactions, such as reductive amination [Ref. 3] and Ugi reactions [Ref. 4].

reductive amination80.jpg
 
Ugi reaction80.jpg

More recently, we found that diphenylborinic acid derivatives serve as “catalytic” iminium ion generators in the Mannich-type reactions [Ref. 5].

catalytic iminium ion generator80.jpg
 

References

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Last-modified: 2014-08-22 (金) 18:39:44 (1183d)