Catalytic 1,3-H Atom Shift of a Terminal Benzylic Alkyne by Iron and Alkali Metal Silylamides – Switching between Allene and Internal Alkyne

Abstract

Herein the examination of the transformation of alkynes by low-coordinate iron silylamides is presented. An anionic linear iron(I) silylamides ([Fe(N{Dipp}SiR3)2]−; Dipp=2,6-di-iso-propylphenyl, R=Me) acts as precatalyst for the cyclotrimerization of diphenyl acetylene but is unable to transform internal alkynes with aliphatic substituents or terminal alkynes accordingly. For a benzylic, terminal alkyne, however, the 1,3-H-shift to the internal alkyne is observed and proceeds via phenyl allene. Using 10 mol% of the iron complex, the terminal alkene is selectively transformed to phenyl allene within minutes, and further fully transformed to the internal alkyne within 24 h. The transformation is retraced on a stoichiometric level and leads to the isolation of a π-alkyne complex with a shifted triple bond. Further, the anionic, trigonal iron(II) silylamide [Fe(NR2)3]− also mediates the catalytic conversion of the terminal alkyne but is restricted to allene formation. Overall, a deprotonation/reprotonation mechanism is assumed for these transformations. This was ultimately proven by using potassium hexamethyldisilazanide KNR2, which is an even more active catalyst for the complete triple bond shift.