This communication details the copper catalyzed ring expansion of vinyl aziridines to 3-pyrrolines. Broad substrate scope (24 examples) using tosyl and phthalimide protected vinyl aziridine substrates is observed. Cu(hfacac) 2 was determined to be superior to all other catalysts tested.3-Pyrrolines (2,5-dihydropyrroles), while important in their own right, are synthetically versatile heterocycles which allow ready access to pyrrolidines and pyrroles. 1 Collectively these ring systems occur in countless natural products and pharmaceuticals. 2 Not surprisingly, a number of different synthetic approaches toward pyrrolines have been developed but the most popular are Birch reduction of pyrroles, 3 [3+2] cyclization, 4 and ring closing metathesis. 5 Despite these and other creative approaches, a requisite exists for a simple and selective method which allows access to structurally complex 3-pyrrolines. Herein, we demonstrate that vinyl aziridines can be efficiently converted to 3-pyrrolines using commercially available copper(II) catalysts.Analogous to the vinyl cyclopropane rearrangement, 6 the thermolysis of vinyl aziridines to 3-pyrrolines was first observed in the late 1960's. Vinyl aziridines are most easily synthesized by one of three methods: the addition of a nitrene to a diene, the addition of an allylic ylide to an imine or the cyclization of unsaturated amino alcohols. 11 Each method has limitations often directly related to the nitrogen protecting group. This paper focuses on the p-toluenesulfonyl (Ts) and phthalimido (Phth) protecting groups. Tosyl aziridines are easily accessed by employing an aziridination protocol developed by Sharpless 12 or by catalytic decomposition of N-phenyliodinanes. 13 Although phthalimide substituted aziridines are less commonly used, they offer an attractive option based on their ease of synthesis utilizing a stabilized singlet nitrene and broader substrate scope. 14 Vinyl aziridine 1 was chosen as a model substrate due to the growing interest in dehydroprolines. 15 Based on previous work by our group on the catalytic ring expansion of vinyl oxiranes and vinyl thiiranes, 16 copper catalysts were tested ( Table 1). The more electrophilic copper(II) hexafluoroacetylacetonate (entries 14-18) proved far superior to other commercially available and synthetic catalysts. Interestingly, the reaction proceeds faster and with a higher yield when the Cu(hfacac) 2 hydrate is dried prior to use. Optimized reaction conditions were found to be 150 °C, 0.1 M [substrate], and 5 mol % catalyst loading. Table 2 displays the results of applying these optimized conditions to a variety of phthalimide and tosyl protected vinyl aziridines. These products can be readily deprotected to give the N-H pyrrolines. 17 Simple substrates (Entries 1-11) demonstrate that all substitution patterns are well tolerated resulting in excellent yields. Entries 4, 5, and 8 display an important synthetic aspect of this rearrangement, as each 3-pyrroline can originate from two regioisomeric vinyl aziridine prec...

Pyrrole derivatives R 0120 Lewis Acid Catalyzed [1,3]-Sigmatropic Rearrangement of Vinyl Aziridines. -The reaction of various mono-, disubstituted and fused aziridines allows an easy access to 3-pyrrolines and complex bicyclic systems. High functional group tolerance is shown. -(BRICHACEK, M.; LEE, D.; NJARDARSON*, J. T.; Org. Lett. 10 (2008) 21, 5023-5026; Dep. Chem. Chem. Biol., Cornell Univ., Ithaca, NY 14853, USA; Eng.) -R. Steudel 12-126