Silver-Catalyzed Synthesis of 2,3-Dihydro-pyrroles

Significance

Wender and Strand report the synthesis of 2,3-dihydropyrroles 3 via a formal [3+2] cycloaddition between aziridines 1 and unactivated acetylenes 2 under either Lewis or Brønsted acid catalysis. The silver-catalyzed process is tolerant of a wide range of substrates, including electron-rich and electron-poor acetylenes, and exhibits excellent levels of regioselectivity (>20:1 in favor of C2 R⁵ aryl products). The reaction is not limited to aryl acetylenes, with vinyl and alkyl groups also being tolerated, although Sc(OTf)₃ was required as catalyst to effect the reaction of the latter substrate. Competition studies showed that electron-rich acetylenes undergo the reaction faster than electron-poor ones, indicating the capture of a cationic intermediate in the product-forming step.