Ruthenium-Catalyzed Addition of Terminal Alkynes to Alkynylstannanes with Migration of the Stannyl Group

Abstract

Abstract In the presence of a catalytic amount of a ruthenium complex coordinated with PBu3, alkynylstannanes (R1C≡CSnBu3) were found to accept the addition of terminal alkynes (R2C≡CH) with a 1,2-shift of the stannyl group to give (E)- and (Z)-1-tributylstannylbut-1-en-3-ynes (Bu3SnC(R1)=CHC≡CR2). Various combinations of substrates having an aromatic and/or aliphatic substituent can be used, and the stereochemical outcome depends largely on the character of these substituents. The reaction of aliphatic terminal alkynes proceeds stereoselectively, and substituent R1 on the alkynylstannanes determines the configuration: E for R1 = alkyl and Z for R1 = aryl. In contrast, the reaction of arylacetylenes gave a mixture of stereoisomers irrespective of the character of substituent R1 on the alkynylstannane. Ruthenium–β-stannylvinylidene complexes generated from a ruthenium complex and an alkynylstannane with migration of the stannyl group are likely to be key intermediates, which accept addition of the C–H bond of terminal alkynes to give the corresponding stannylenynes. DFT calculation clearly shows that the 1,2-shift of the stannyl group on formation of ruthenium–β-stannylvinylidene complexes is more facile than the corresponding 1,2-hydrogen shift of the coordinating terminal alkynes. The effect of the substituents on the stereoselectivity also is discussed based on the calculation.