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[(NHC)Au I ]‐Catalyzed Formation of Conjugated Enones and Enals: An Experimental and Computational Study
Author(s) -
Marion Nicolas,
Carlqvist Peter,
Gealageas Ronan,
de Frémont Pierre,
Maseras Feliu,
Nolan Steven P.
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200700134
Subject(s) - chemistry , cationic polymerization , catalysis , carbene , medicinal chemistry , indene , conjugated system , intramolecular force , stereoselectivity , organic chemistry , polymer
Abstract The [(NHC)Au I ]‐catalyzed (NHC = N‐heterocyclic carbene) formation of α,β‐unsaturated carbonyl compounds (enones and enals) from propargylic acetates is described. The reactions occur at 60 °C in 8 h in the presence of an equimolar mixture of [(NHC)AuCl] and AgSbF 6 and produce conjugated enones and enals in high yields. Optimization studies revealed that the reaction is sensitive to the solvent, the NHC, and, to a lesser extent, to the silver salt employed, leading to the use of [(I t Bu)AuCl]/AgSbF 6 in THF as an efficient catalytic system. This transformation proved to have a broad scope, enabling the stereoselective formation of ( E )‐enones and ‐enals with great structural diversity. The effect of substitution at the propargylic and acetylenic positions has been investigated, as well as the effect of aryl substitution on the formation of cinnamyl ketones. The presence or absence of water in the reaction mixture was found to be crucial. From the same phenylpropargyl acetates, anhydrous conditions led to the formation of indene compounds via a tandem [3,3] sigmatropic rearrangement/intramolecular hydroarylation process, whereas simply adding water to the reaction mixture produced enone derivatives cleanly. Several mechanistic hypotheses, including the hydrolysis of an allenol ester intermediate and S N 2′ addition of water, were examined to gain an insight into this transformation. Mechanistic investigations and computational studies support [(NHC)AuOH], produced in situ from [(NHC)AuSbF 6 ] and H 2 O, instead of cationic [(NHC)AuSbF 6 ] as the catalytically active species. Based on DFT calculations performed at the B3LYP level of theory, a full catalytic cycle featuring an unprecedented transfer of the OH moiety bound to the gold center to the CC bond leading to the formation of a gold–allenolate is proposed.