Unique reactivity of an α-ketiminopyridine ligand with metal–alkyls: Synthesis and ROP of ε-caprolactone

Abstract

The reaction of an α-ketimininopyridine ligand 2-((Me)CN(C₆H₃-2,6-iPr₂))-6-(OMe)C₅H₃N (L¹) with metal–alkyls, such as MeLi, Et₂Zn, Me₃Al and Me₂AlCl, was studied. The reaction of L¹ with MeLi led exclusively to the formation of [2-((H₂C)C–N(C₆H₃-2,6-iPr₂))-6-(OMe)C₅H₃N]Li·(THF)₂ (1·(THF)₂) with a deprotonated ketimine methyl group as a product of methane elimination. The treatment of L¹ with Et₂Zn provided, at the first stage, only complex [2-((Me)CN(C₆H₃-2,6-iPr₂))-6-(OMe)C₅H₃N]ZnEt₂ (2). The nonstability of 2 led to ethane elimination along with the deprotonation of a ketimine methyl group, which further yielded [2-((H₂C)C–N(C₆H₃-2,6-iPr₂))-6-(OMe)C₅H₃N]ZnEt (3). In contrast, Me₃Al reacted with L¹ in a carboalumination fashion and [2-((Me)₂C–N(C₆H₃-2,6-iPr₂))-6-(OMe)C₅H₃N]AlMe₂ (4) was isolated. In the case of Me₂AlCl, an ionic species {[2-((Me)CN(C₆H₃-2,6-iPr₂))-6-(OMe)C₅H₃N]AlMe₂}⁺ {Me₂AlCl₂}⁻ (5) was formed as a result of a spontaneous dissociation of Me₂AlCl initiated by the ligand L¹. Since compounds 2–5 contain a metal–alkyl fragment, they were used as pre-catalysts in ROP of ε-caprolactone, as well as compound 1·(THF)₂.