title: Phosphine-catalyzed annulations of azomethine imines: allene-dependent [3 + 2], [3 + 3], [4 + 3], and [3 + 2 + 3] pathways.
abstract: In this paper we describe the phosphine-catalyzed [3 + 2], [3 + 3], [4 + 3], and [3 + 2 + 3] annulations of azomethine imines and allenoates. These processes mark the first use of azomethine imines in nucleophilic phosphine catalysis, producing dinitrogen-fused heterocycles, including tetrahydropyrazolo-pyrazolones, -pyridazinones, -diazepinones, and -diazocinones. Counting the two different reaction modes in the [3 + 3] cyclizations, there are five distinct reaction pathways-the choice of which depends on the structure and chemical properties of the allenoate. All reactions are operationally simple and proceed smoothly under mild reaction conditions, affording a broad range of 1,2-dinitrogen-containing heterocycles in moderate to excellent yields. A zwitterionic intermediate formed from a phosphine and two molecules of ethyl 2,3-butadienoate acted as a 1,5-dipole in the annulations of azomethine imines, leading to the [3 + 2 + 3] tetrahydropyrazolo-diazocinone products. The incorporation of two molecules of an allenoate into an eight-membered-ring product represents a new application of this versatile class of molecules in nucleophilic phosphine catalysis. The salient features of this protocol--the facile access to a diverse range of nitrogen-containing heterocycles and the simple preparation of azomethine imine substrates--suggest that it might find extensive applications in heterocycle synthesis.
authors.affiliation: Department of Applied Chemistry, China Agricultural University, Beijing 100193, PR China.
authors.forename: Risong
authors.initials: R
authors.lastname: Na
authors.forename: Chengfeng
authors.initials: C
authors.lastname: Jing
authors.forename: Qihai
authors.initials: Q
authors.lastname: Xu
authors.forename: Hui
authors.initials: H
authors.lastname: Jiang
authors.forename: Xi
authors.initials: X
authors.lastname: Wu
authors.forename: Jiayan
authors.initials: J
authors.lastname: Shi
authors.forename: Jiangchun
authors.initials: J
authors.lastname: Zhong
authors.forename: Min
authors.initials: M
authors.lastname: Wang
authors.forename: Diego
authors.initials: D
authors.lastname: Benitez
authors.forename: Ekaterina
authors.initials: E
authors.lastname: Tkatchouk
authors.forename: William A
authors.initials: WA
authors.lastname: Goddard
authors.forename: Hongchao
authors.initials: H
authors.lastname: Guo
authors.forename: Ohyun
authors.initials: O
authors.lastname: Kwon
mesh_terms: D000466:Alkadienes / Q000737:chemistry; D001391:Azo Compounds / Q000737:chemistry*; D002384:Catalysis; D007097:Imines / Q000737:chemistry*; D010720:Phosphines / Q000737:chemistry*; D013882:Thiosemicarbazones / Q000737:chemistry*
chemical_list: D000466:Alkadienes; D001391:Azo Compounds; D007097:Imines; D010720:Phosphines; D013882:Thiosemicarbazones; C512188:azomethine; C025947:propadiene; C044646:phosphine