Gold(I)‐Catalyzed Divergence in the Preparation of Bicyclic Enol Esters: From Exclusively [3C+2C]‐Cycloaddition Reactions to Exclusive Formation of Vinylcyclopropanes

Abstract

With the use of benzonitrile-stabilized Au(I) catalyst [Au(IPr)(NCPh)]SbF(6) (Ic; IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), a spectrum of reactivity is observed for propargyl ester 4a with cyclic vinyl ethers, ranging from exclusively [3C+2C] cycloaddition reactions to exclusively cyclopropanation depending only on the structure of the substrate. Some initially formed cyclopropanation products rearrange into the corresponding formally [3C+2C] cycloaddition products after treatment with fresh Au(I) complex at 80 °C. Vinylcyclopropanes formed from dihydrofuran and dihydropyran resisted such rearrangement, even in the presence of fresh Au(I) catalyst at elevated temperature. This study addresses an important mechanistic question concerning whether the five-membered-ring products were produced by a direct [3C+2C] cycloaddition reaction or by a sequential cyclopropanation/ring-expansion reaction. A dual pathway is proposed for the Au(I)-catalyzed reactions between propargyl esters and cyclic vinyl ethers. The different behavior among vinyl cyclic ethers is attributed to the difference in the polarization of the π bond. Highly polarized bonds appear to undergo the cycloaddition reaction whereas less polar π-bonds produce cyclopropanes.

Gold catalysis: up to six new bonds by a domino [3+2]/[2+1]/[2+1] cycloaddition

“…[2][3][4] Au(I) complexes are especially active as catalysts in many cyclization and cycloaddition reactions because of the unique ability of gold complexes to activate carbon-carbon p-systems. 5 11 In our recent study, 11 some of the cyclopropanation products such as 5 were isolated although no similar product was observed with 1-methoxycyclohexene as the substrate (Scheme 1). The vinyl cyclopropane 5 was easily converted to the corresponding cyclopentene derivative (6, Scheme 2) with the Au(I) catalyst 2 (IPrAu + NCPhSbF 6 À ).…”

“…From the results of this and previous studies on goldcatalyzed reactions between propargyl esters and enol ether or enol esters, 11,15 product distribution depends mainly on the structure of the enols. Enol ethers may lead to formal [3+2] cycloaddition products, but enol esters produce primarily cyclopropanation products.…”

“…In the absence of the Au(I) catalyst, vinyl cyclopropane 5 was resistant to change even at high temperatures up to 110°C. 11 We proposed a likely two step mechanism for the Au(I)-catalyzed ring expansion reaction. First, the initial coordination of the enolate ester double bond of 5 to the cationic Au(I) center leads to the opening of the cyclopropane ring and the concurrent formation of an oxycarbenium ion (C in Scheme 2).…”

“…The other products include the previously identified cyclopropanation products 5 and 12c and the initial [3+2] product 6, (Scheme 5). 11 The stereochemistry of the domino [3+2]/[2+1] cycloaddition reaction product was determined by two-dimensional NMR spectroscopy including NOESY. The NOE effect between the vinyl proton of the enolate and the two cyclopropane protons for compound 10 is illustrated in Figure 1.…”

Can hydrogen-bonding donors abstract chloride from LAu(I)Cl complexes: a computational study

“…We had a first-hand experience when we performed the reactions shown in Scheme 1. 11 When propargylic ester and vinyl ether were treated with ImesAuCl in the presence of AgSbF 6 , no desired product was isolated.…”