Tetrabenzyltitanium: An Improved Catalyst for the Activation of sp3 C-H Bonds Adjacent to Nitrogen Atoms

A detailed study of intra- and intermolecular hydroaminoalkylation reactions by activation of C-H bonds adjacent to nitrogen atoms performed with a number of different titanium-based catalysts and the identification of tetrabenzyltitanium ([TiBn4], Bn=benzyl) as a preferred catalyst for practical synthetic hydroaminoalkylation procedures is described. In the presence of 5 mol% of this catalyst, geminally disubstituted and unsubstituted primary 1-amino-6-heptenes undergo selective cyclizations to 1-amino-2-methylcyclohexanes. In comparison to [Ti(NMe2)(4)], improved yields are obtained with [TiBn4] for most of the substrates. Only in the case of the strongly Thorpe-Ingold-activated 1-amino-2,2-diphenyl-6-heptene is a better yield obtained with [Ti(NMe2)(4)] especially when the reaction is performed at 145 degrees C. Whereas secondary N-aryl aminoalkenes undergo intramolecular hydroaminoalkylation with poor efficiency, the corresponding intermolecular reactions of N-aryl amines with alkenes can be achieved in the presence of 10 mol% of [Ti(NMe2)(4)] or [TiBn4]. With two exceptions, [TiBn4] again turned out to be catalytically more active than [Ti(NMe2)(4)]. A number of reactions which can not be achieved at all with [Ti(NMe2)(4)] take place in the presence of [TiBn4], including the first successful transition metal-catalyzed hydroaminoalkylation of styrene by a C-H activation process.