Among numerous examples of transition-metal-mediated cyclization to obtain medium-sized ring compounds, [1] intramolecular hydroacylation provides the most promising way to prepare cyclopentanones from 4-pentenal through the CΓHbond activation of an aldehyde. [2, 3] However, its application has been limited to the synthesis of five-membered rings with a few exceptions, [3] because the competing decarbonylation of the acyl metal hydride intermediate prevails during the formation of the larger ring. In the course of our studies into chelation-assisted CΓH-and CΓC-bond activation, [4] allylic amine 1 a, which bears a coordination site, was devised and used as a masked form of formaldehyde in the hydroacylation of 1-alkenes to synthesize dialkyl ketones. [4d] We envisaged a cyclization of dienes 2 [5Β±7] with 1 a to furnish cycloalkanones 3 with various sizes, since decarbonylation cannot occur in the reaction of 1 a (Scheme 1). Herein we present a facile synthesis of various cycloalkanones from the reaction of allylic amines with dienes through chelation-assisted CΓHand CΓC-bond activation.
The reactions of 1 a with various dienes (2) are summarized in Table 1. For example, when 1 a was allowed to react with 1,4-pentadiene (2 a) in the presence of [{(C 8 H 14 ) 2 RhCl} 2 ] (4, 5 mol %) and PCy 3 (5, 15 mol %) at 150 8C for 2 h, cyclohexanone (3 a) and 2-methylcyclopentanone (3 b) were obtained in 87 % and 13 % yield, respectively, after hydrolysis (Table 1, entry 1). Furthermore, the reaction with 1,5-hexadiene (2 b) provided cycloheptanone (3 c), 2-methylcyclohexanone (3 d), and 2-ethylcyclopentanone (3 e) in a ratio of 38:40:22 (Table 1, entry 2). [8] The mechanism for this reaction is depicted in Scheme 2. Aldimine 6 a is generated by Rh-catalyzed isomerization of 1 a. Subsequent intermolecular hydroiminoacylation [9] of 2 b COMMUNICATIONS