Secondary amines hydroaminoalkylation

Scheme 17 Proposed catalytic cycle for the hydroaminoalkylation of secondary amines with tantalum precatalysts...

Scheme 19 intramolecular hydroaminoalkylation of unactivated olefins with secondary amines catalyzed by mono- and bis(amidate) tantalum ami do complexes... 

Neutral group 4 metal complexes appear to possess a relatively broad scope for catalytic hydroaminations. They have been employed for the intramolecular hydroamination of alkynes [2], allenes [3], and alkenes [4] as well as the inter-molecular hydroaminations of alkynes [5] and allenes [6]. Primary aryl- and alkylamines readily react, but secondary amines have posed a greater challenge for this type of transformation with neutral catalysts [7]. For the reactions of the latter, cationic Zr and Ti complexes have been employed in intramolecular cyclizations of aminoalkenes [8]. Very recent work suggests that substrates that are difficult to hydroaminate may favor hydroaminoalkylations instead (Scheme 13.2) [9]. 

Niobium and tantalum The intermolecular hydroaminoalkylation of unactivated alkenes RCH=CH2 and styrenes with secondary amines ArNHMe to produce amines (229) has been reported to be catalysed by the tantalum and niobium binaphtholate complexes with <98% ee. The reaction has been found to be first order in the amine... 

An interesting observation in these investigations is the fact that intermolecular hydroaminoalkylation catalysts are not viable for application in intramolecular reactions. Furthermore, there are no catalysts that are effective for the intramolecular hydroaminoalkylation of secondary aminoalkenes and all high yielding examples of intramolecular hydroaminoalkylation are restricted to primary aminoalkenes. Meanwhile, intermolecular hydroaminoalkylation with primary amines has not yet been reported. 

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