Jaika Dörfler

Aminopyridinato Titanium Catalysts for the Hydroaminoalkylation of Alkenes and Styrenes

As the formed a-alkylated amines are of great industrial importance it is not surprising that a number of hydroaminoalkylation catalysts have already been identified. Successful hydroaminoalkylation reactions of alkenes can be achieved in the presence of ruthenium, iridium, Group 5 metals, zirconium, or titanium catalysts but the use of ruthenium and iridium catalysts is limited to amine substrates possessing a directing 2-pyridinyl substituent bound to the nitrogen atom of the amine. While in the presence of Group 5 metal catalysts or [Ind2TiMe2] (Ind = h -indenyl), the hydroaminoalkylation of 1-alkenes, such as 1-octene (2), with secondary amines, such as N-methylaniline (1), always gives the branched product 3a exclusively (Scheme 1), the corresponding [Ind2TiMe2]-catalyzed reaction of styrene (4) also leads to the formation of the linear product 5b as a side product. Unfortunately, successful [Ind2TiMe2]-catalyzed hydroaminoalkylation reactions could only be achieved with N-methylanilines and in this context, it must also be mentioned that by using Group 5 metal catalysts only a single additional example of a successful hydroaminomethylation of a styrene has been reported. In this case, the branched product was again the exclusive product of the reaction. Interestingly, linear side products could also be obtained from hydroaminoalkylation reactions of 1-alkenes performed with [Ti(NMe2)4] [6b] or [TiBn4] [6c] (Bn = benzyl) as the catalysts. However, to our knowledge, up to now, no early transition-metal-catalyzed hydroaminoalkylation of an alkene or a styrene that leads to the formation of the industrially important linear product as the major product of the reaction has been reported. In addition, hydroaminoalkylation reactions of styrenes with dialkylamines or Nalkylanilines possessing alkyl groups larger than a methyl group have never been achieved. To expand the substrate scope of the hydroaminoalkylation of alkenes, we recently performed a number of corresponding transformations with N-methylated aminoheteroaromatics as substrates in the presence of the precatalyst [Ti(NMe2)4]. During these unsuccessful attempts we observed that the addition of 2-(methylamino)pyridine (2-MeAP-H) to a bright yellow solution of the alkene and the precatalyst [Ti(NMe2)4] in toluene results in a significant color change to dark red which suggests a fast formation of titanium complexes with 2-aminopyridinato ligands, such as [(2MeAP)2Ti(NMe2)2] or [(2-MeAP)Ti(NMe2)3] (Scheme 2). In this context, it must be noted that Kempe has already synthesized the complex [(2-MeAP)2Ti(NMe2)2] by amine Scheme 1. [Ind2TiMe2]-catalyzed hydroaminoalkylation. [6d]

Efficient Access to Titanaaziridines by CH Activation of N‐Methylanilines at Ambient Temperature

Titanaaziridines or η(2)-imine titanium complexes are considered key intermediates of the titanium-catalyzed hydroaminoalkylation of alkenes. Herein, we present an efficient synthetic route to this class of compounds, starting from N-methylanilines and a bis(η(5):η(1)-pentafulvene)titanium complex. Consecutive reactions on the η(2)-methyleneaniline complexes, characterized for the first time, prove a high chemical versatility. In particular, hydroaminoalkylation products were found in reactions of the three-membered titanacycles with alkenes. For the first time, all the intermediates of the hydroaminoalkylation of alkenes were isolated and characterized.

A 2,6‐Bis(phenylamino)pyridinato Titanium Catalyst for the Highly Regioselective Hydroaminoalkylation of Styrenes and 1,3‐Butadienes

The C-C bond forming catalytic hydroaminoalkylation of terminal alkenes, 1,3-dienes, or styrenes allows a direct and highly atom efficient (100 %) synthesis of amines which can result in the formation of two regioisomers, the linear and the branched product. We present a new titanium catalyst with 2,6-bis(phenylamino)pyridinato ligands for intermolecular hydroaminoalkylation reactions of styrenes and 1-phenyl-1,3-butadienes that delivers the corresponding linear hydroaminoalkylation products with excellent regioselectivities.