Neutral Ti complexes as catalysts for the hydroamination of alkynes and alkenes:: Do the labile ligands change the catalytic activity?

A detailed comparison between three four-coordinate Ti complexes featuring the general bidentate ligand [eta(5)-(C5H4)-SiMe2-NtBu](2-) and two ligands X (NMe2, Me, Cl) as catalyst precursors (I-III) for the intermolecular hydroamination of alkynes and the intramolecular hydroamination of alkenes is presented. The results strongly suggest that the catalytically active species are only identical for reactions performed with the bis(dimethylamido) complex I or the dimethyl complex II, Under the reaction conditions, the labile ligands X are proteolytically removed by the reacting amine to form catalytically active imido or amido complexes, together with dimethylamine or methane. Although both catalyst precursors can be used successfully for many substrate combinations, the preparative and kinetic studies clearly indicate that dimethylamine, which is formed from the bis(dimethylamido) catalyst precursor I and the reacting amine, is able to convert the catalytically active imido or amido complexes back into the catalyst precursor and therefore inhibits the reactions. As a consequence, the bis(dimethylamido) catalyst precursor I shows a poorer catalytic performance than the corresponding dimethyl complex II. Additionally, it is shown that the dichloro complex III is only a suitable catalyst precursor for selected hydroamination reactions. Corresponding reactions that are more difficult to achieve - such as reactions of diarylalkynes or amino alkenes - do not work efficiently with this complex. A possible explanation for this observation is the finding that the dichloro catalyst precursor III is obviously converted into a different catalytically active species. This can happen if the [eta(5)-(C5H4)-SiMe2-NtBu]-ligand system of the catalyst precursor is being destroyed and removed from the Ti center under the reaction conditions. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).