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Rhodium‐Catalyzed Oxidative Cycloaddition of N ‐ tert ‐Butoxycarbonylhydrazones with Alkynes for the Synthesis of Functionalized Pyrroles via C( sp 3 )–H Bond Functionalization
Author(s) -
Chan ChunMing,
Zhou Zhongyuan,
Yu WingYiu
Publication year - 2016
Publication title -
advanced synthesis and catalysis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.541
H-Index - 155
eISSN - 1615-4169
pISSN - 1615-4150
DOI - 10.1002/adsc.201600900
Subject(s) - chemistry , regioselectivity , cycloaddition , pyrrole , rhodium , tautomer , catalysis , medicinal chemistry , isoquinoline , alkyl , bond cleavage , oxidative coupling of methane , oxidizing agent , stereochemistry , organic chemistry
A rhodium(III)‐catalyzed cycloaddition of N ‐ tert ‐butoxycarbonylhydrazones with internal alkynes was developed. The reaction features a regioselective α‐imino alkyl C( sp 3 )−H bond functionalization resulting in selective formation of highly functionalized NH‐free pyrroles. Our studies showed that utilizing the N ‐ tert ‐butoxycarbonyl ( N ‐Boc) as the oxidizing directing group is critical for achieving the observed pyrrole formation versus the isoquinoline formation. To account for the pyrrole formation, we hypothesized that a prior tautomerization of the N ‐Boc‐hydrazones to enamines should occur, followed by regioselective C( sp 2 )–H cleavage to form a putative five‐membered rhodacycle. Subsequent coupling of the rhodacycle with the alkynes would afford the pyrrole products.