Gold‐Catalyzed Oxidative Cyclization of 1,5‐Enynes Using External Oxidants

Abstract

Pd-catalyzed oxidative cyclizations of 1,6-enynes have found useful applications in organic synthesis, [1] but such reactions with Au and Pt catalysis remain largely unexplored. [2] Goldcatalyzed cycloisomerizations of 1,5-and 1,6-enynes provide uncommon and useful carbocyclic frameworks. [3] In the presence of organic oxidants, most enynes fail to produce oxidative cyclization products because oxidations of hypothetical gold-carbenoid intermediates are difficult. [4,5] Herein, we report two new oxidative cyclizations of 1,5-enynes via 5endo-dig and 5-exo-dig cyclizations, respectively; both reactions are implemented with Au I and 8-methylquinoline Noxide. The success of such reactions relies on the prior oxidations of enyne [6] form a-carbonyl carbenoids A and B, followed by their intramolecular cyclizations (Scheme 1). Terminal alkynes favor the oxidation at the C2 alkynyl carbon atom and aminoalkynes prefer the C1 carbon atom.

ChemInform Abstract: Gold‐Catalyzed Oxidative Cyclizations of cis‐3‐En‐1‐ynes To Form Cyclopentenone Derivatives.

“…[1,2] Our group and the groups of others have reported the catalytic cycloisomerizations of cis-3-en-1-ynes to form cyclopentadiene derivatives using ruthenium or platinum catalysts, respectively (Scheme 1 a); [5] the corresponding oxidative cyclization remains undocumented. We enhanced the yields of 4 a to 83 % and 77 % using [IPrAuCl]/AgNTf 2 and [IPrAuCl]/ AgSbF 6 , respectively (entries [4][5]. Notably, the mechanistic transformation of this C À H activation is proven to proceed through a noncarbene route, thus excluding the intermediacy of the a-carbonyl carbene A.…”

1,3-Dien-5-ynes: Versatile Building Blocks for the Synthesis of Carbo- and Heterocycles

“…These reactions afford 3-aminocarbonyl-substituted indenes in high yields and with complete selectivity when the external olefin of the dienyne is unsubstituted; otherwise, moderate yields are obtained due to the formation of appreciable amounts of acyclic adducts. 167 Moreover, this reaction is restricted to aminoethynyl styrenes, as substrates unsubstituted or alkyl-substituted at the alkyne exclusively produce benzocyclopentenones (see Scheme 94, eq. 1, 3).…”

Gold‐Catalyzed Oxidative Cyclizations on 1,4‐Enynes: Evidence for a γ‐Substituent Effect on Wagner–Meerwein Rearrangements

“…The object of this work is to clarify if an anti or syn activation occurs in carbocations with a gold substituent in the g-position, as in carbocations F or F'. [7][8]. For LAuNTf 2 , the reactions proceeded less efficiently in dichloromethane and CH 3 CN, giving cyclopentenone 2 a in 62 % and 55 % yield, respectively (Table 1, entries 9-10).…”

“…Shown in Scheme 1 is our strategy to illustrate the gsubstituent effect; the key reaction involves a gold-catalyzed oxidative cyclization of 1,4-enyne 3. [7,8] The initially formed aoxo gold carbene E [9] is expected to have its olefin p electrons parallel to the positive + Au = C p orbital to achieve a throughspace interaction. We envisage that this spatial arrangement will undergo a facile alkene/carbene coupling [10] to give 2-oxocyclopent-1-yl cation F or F', through either a disrotation or conrotation route.…”