Understanding the role of an easy-to-prepare aldimine–alkyne carboamination catalyst, [Ti(NMe2)3(NHMe2)][B(C6F5)4]

A series of reactivity studies of the carboamination pre-catalyst [Ti(NMe2)3(NHMe2)][B(C6F5)4] as well as the preparation of other catalysts are reported in this work. Treatment of [Ti(NMe2)3(NHMe2)][B(C6F5)4] with the aldimines Ar′NCHtol (Ar′ = 2,6-Me2C6H3, tol = 4-MeC6H4), and depending on the reaction conditions, results in isolation of [Me2NCHR′][B(C6F5)4] (1) or (Me2N)2CHtol, as well as the asymmetric titanium dimer [(Me2N)2(HNMe2)Ti(μ2-N[2,6-Me2C6H3])2Ti(NHMe2)(NMe2)][B(C6F5)4] (2). Protonation of CpTi(NMe2)3 and Cp∗Ti(NMe2)3 results in isolation of the salts, [CpTi(NMe2)2(NHMe2)][B(C6F5)4] (3) and [Cp∗Ti(NMe2)2(NHMe2)][B(C6F5)4] (4), respectively. Treatment of compounds 3 or 4 with H2N[2,6-iPr2C6H3] results in formation of the imido salts [CpTi(N[2,6-iPr2C6H3])(NHMe2)2][B(C6F5)4] (5) (58% yield) or [Cp∗Ti(N[2,6-iPr2C6H3])(NHMe2)2][B(C6F5)4] (6). When Ti(NMe2)4 is treated with [Et3Si][B(C6F5)4], the salt [Ti(NMe2)3(N[SiEt3]Me2)][B(C6F5)4] (7) is obtained, and treatment of the latter with [2,6-iPr2C6H3]NCHtol produces the imine adduct [Ti(NMe2)3(κ1-[2,6-iPr2C6H3]NCHtol)][B(C6F5)4] (8). The carboamination catalytic activity of complexes 2–7 was investigated and compared to [Ti(NMe2)3(NHMe2)][B(C6F5)4]. Likewise, a proposed mechanism to the active carboamination catalyst stemming from [Ti(NMe2)3(NHMe2)][B(C6F5)4] is described.

A series of reactivity studies of the aldimine/alkyne carboamination pre-catalyst [Ti(NMe2)3(NHMe2)][B(C6F5)4] as well as the preparation of close analogues are reported. In this work we demonstrate that the imide source derives from the aldimine, while the amine is necessary for substrate binding. Variances of one amide for Cp– or Cp∗–, as well as changes in the nature of activator, result in formation of less active carboamination catalysts.Figure optionsDownload as PowerPoint slideResearch highlights
► Without a proton, the titanium precursor is not an active carboamination catalyst.
► Other electrophiles are not as good activators as a proton.
► The source of the imide ligand derives from the aldimine NAr group.
► The dimethylamide ligands are non-innocent and the carboamination cycle is not atom-economical.
► Replacing one dimethylamide ligand with Cp or Cp∗ lowers the efficiency of the catalyst.