Neutral Group‐IV Metal Catalysts for the Intramolecular Hydroamination of Alkenes

A detailed comparison of the group‐IV metal catalysts Ti(NMe 2 ) 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(NMe 2 ) 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(NMe 2 ) 4 ‐catalyzed cyclization is second‐order in the concentration of the substrate. The results obtained for the formation of piperidines catalyzed by Ti(NMe 2 ) 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(NMe 2 ) 4 . Interestingly, Ind 2 TiMe 2 is able to catalyze a slow hydroamination of an N ‐methylated amino alkene, whereas the homoleptic complex Ti(NMe 2 ) 4 as well as Ind 2 ZrMe 2 and Ind 2 HfMe 2 do not catalyze the same reaction. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)