Neutral group-IV metal catalysts for the intramolecular hydroamination of Alkenes

A detailed comparison of the group-IV metal catalysts Ti(NMe2)(4), Ind(2)TiMe(2), Ind(2)ZrMe(2) and Ind(2)HfMe(2) in the intramolecular hydroamination of amino alkenes is presented. Among these catalysts, the benchmark catalyst Ti(NMe2)(4) is the most active in the formation of pyrrolidines. A comparison between Ind(2)TiMe(2), Ind(2)ZrMe(2) and Ind(2)HfMe(2) suggests that in the synthesis of pyrrolidines, Zr complexes show the highest catalytic activity of the group-IV metal catalysts. Although Ind(2)TiMe(2)- and the Ind(2)ZrMe(2)- catalyzed formation of a pyrrolidine is first-order in the concentration of the substrate, the corresponding Ti(NMe2)(4)-catalyzed cyclization is second-order in the concentration of the substrate. The results obtained for the formation of piperidines catalyzed by Ti(NMe2)(4), Ind(2)TiMe(2), Ind(2)ZrMe(2) and Ind(2)HfMe(2) suggest that for these reactions, Ti catalysts show increased catalytic activity compared with the corresponding Zr catalysts. Unfortunately, the formation of aminocyclopentane side-products by C-H activation processes is a severe drawback of the Ti catalysts. The corresponding side-products are not formed in Ind(2)ZrMe(2)- and Ind(2)HfMe(2)-catalyzed reactions. However, the former catalyst gives better yields of the desired piperidine products. In contrast to the results obtained for the synthesis of pyrrolidines, the formation of a piperidine is zero-order in the concentration of the substrate for the indenyl catalysts Ind(2)TiMe(2) and Ind(2)ZrMe(2), and first-order for the homoleptic catalyst Ti(NMe2)(4). Interestingly, Ind(2)TiMe(2) is able to catalyze a slow hydroamination of an N-methylated amino alkene, whereas the homoleptic complex Ti(NMe2)(4) as well as Ind(2)ZrMe(2) and Ind(2)HfMe(2) do not catalyze the same reaction. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).