Titanium hydroaminationcatalysts bearing a 2-aminopyrrolinato spectator ligand: monitoring the individual reaction steps

Abstract

A series of new titanium half sandwich complexes, containing a 2-aminopyrrolinato ligand {N^XylN}⁻ as the ancillary ligand, have been prepared and are shown to be pre-catalysts for the hydroamination of alkynes. The coordination of {N^XylN}⁻ to titanium was achieved by reaction of [Cp*TiMe₃] with the protioligand N^XylNH giving [Cp*Ti(N^XylN)(Me)₂] (1). Upon reaction of complex 1 with an excess of tert-butylamine, the imido complex [Cp*Ti(N^XylN)(N^tBu)(NH₂^tBu)] (2) was formed. The latter provided the preparative entry to the synthesis of a range of N-aryl substituted imido complexes. Imido ligand exchange with 2,6-dimethylaniline, 2,4,6-trimethylaniline as well as 2,6-diisopropylaniline gave the corresponding arylimido complexes 3–5 in clean reactions. Reaction of the titanium imido complex [Cp*Ti(N^XylN)(N^tBu)(NH₂^tBu)] 2 with terminal arylacetylenes, such as phenylacetylene and tolylacetylene, led to C–H activation and the formation of alkynyl/amido complexes, whereas the arylimido complexes 3 and 5 cleanly underwent {2 + 2} cycloaddition, giving the azatitanacyclobutene derivatives. A single-crystal X-ray structure analysis of the azatitanacyclobutene [Cp*Ti(N^XylN){κ²N(2,6-C₆H₃Me₂)CTolCH}] (11) provided the first crystallographically characterized Markovnikov cycloaddition product of an imidotitanium complex with a terminal alkyne. The mechanistic aspects of the hydromanination of alkynes with the new Ti half sandwich complexes were studied and established a reversible {2 + 2} cycloaddition step and the cleavage of the metallacyclic intermediate as the rate determining step in the catalytic cycle. The titanium half sandwich imido complexes were found to be active catalysts for the inter- and intramolecular hydroamination of a broad range of alkynes and ω-aminoalkynes.