Sequential Addition Reactions of Lithium Acetylides and Grignard Reagents to Selenoiminium Salts Leading to 2-Propynyl Tertiary Amines Bearing a Tetrasubstituted Carbon Center

Selenoiminium salts generated in situ from selenoamides and MeOTf were reacted sequentially with lithium acetylides and Grignard reagents to give 2-propynyl tertiary amines bearing a tetrasubstituted carbon center. The lithium acetylides used were generated from (trimethylsilyl)acetylene, phenylacetylene, and 1-hexyne. Among them, that obtained from (trimethylsilyl)acetylene most effectively gave the corresponding products in higher yields. As Grignard reagents, alkyl-, vinyl-, allyl-, and benzylmagnesium halides gave the products with high efficiency, whereas the reaction of arylmagnesium halides did not proceed as well. A variety of 4-penteneselenoamides were synthesized by four-component coupling reactions of terminal acetylenes, elemental selenium, allylic bromides, and secondary amines, and subjected to the sequential addition reactions to give 5-amino-1,6-enynes. The diastereoselectivity of the reaction was found to depend on the substituents at the α-position of selenoamides and Grignard reagents. In the reaction of selenoiminium salts with excess Grignard reagents, amines that incorporated two molecules of Grignard reagents were formed in good yields.