Ruthenium-catalyzed aromatization of enediynes via highly regioselective nucleophilic additions on a pi-alkyne functionality. A useful method for the synthesis of functionalized benzene derivatives.
TpRu(PPh3)(CH3CN)2 PF6 (10 mol %) catalyst effected the nucleophilic addition of water, alcohols, aniline, acetylacetone, pyrroles, and dimethyl malonate to unfunctionalized enediynes under suitable conditions (100 degrees C, 12-24 h) and gave functionalized benzene products in good yields. In this novel cyclization, nucleophiles very regioselectively attack the internal C1' alkyne carbon of enediynes to give benzene derivatives as a single regioisomer. Experiments with methoxy substituents exclude the possible involvement of naphthyl cations as reaction intermediates in the cyclization of (o-ethynylphenyl) alkynes. Deuterium-labeling experiments indicate that the catalytically active species is ruthenium-pi-alkyne rather than ruthenium-vinylidene species. This hypothesis is further confirmed by the aromatization of o-(2'-iodoethynyl)phenyl alkynes with alcohols. We propose a nucleophilic addition/insertion mechanism for this nucleophilic aromatization on the basis of a series of experiments.
Odedra
Arjan
A
Department of Chemistry, National Tsing-Hua University, Hsinchu, Taiwan, ROC.
Wu
Chang-Jung
CJ
Pratap
Taduri Bhanu
TB
Huang
Chun-Wei
CW
Ran
Ying-Fen
YF
Liu
Rai-Shung
RS