[3,3]‐Sigmatropic Rearrangement Step in the Gold‐Catalyzed Cyclization of Allyl‐(ortho‐alkinylphenyl)methyl Ethers

Abstract

The gold-catalyzed conversion of allyl-(ortho-alkynylphenyl)methyl ethers was investigated, and allylated isochromenes were obtained. An optimization of the catalysis conditions with respect to different phosphane and carbene ligands on gold, different counterions, and different solvents was conducted. Subsequently, the scope and limitations of this reaction were investigated with 21 substrates. The mechanistic studies show an allylic inversion, as supported by NMR data and an X-ray crystal structure analysis, as well as an intermolecular reaction, as determined by crossover experiments. There is no competition of protodeauration even in the presence of water. All these observations differ from other related conversions and clearly indicate product formation by a [3,3]sigmatropic rearrangement in the step forming the new C-C bond.

On the Accessible Reaction Channels of Vinyl Gold(I) Species: π‐ and σ‐Pathways

“…The chair‐like transition state 7‐TS leads to 8 . Notably, structure 8 does not resemble IV (Figure a) from the initially proposed catalytic cycle . Instead, the final product (with the olefin π‐coordinated to gold) is reached.…”

“…As shown below, they serve as extremes, and thus are optimal to identify the nature of changes in bonding. Our efforts initiated with the study of the Au‐catalysed cyclisation and subsequent [3,3]‐sigmatropic rearrangement of allyl‐( o ‐alkynylphenyl)methyl ethers . For the Au‐catalysed cyclisation and [3,3]‐sigmatropic rearrangement we computed the reaction path using the TPSS functional in combination with the def2‐TZVPP basis set and Grimme's dispersion correction D3 using Becke–Johnson damping .…”

“…The choice of CCSD(T)‐F12 calculations was made for two reasons: 1) the lack of a consensus on an appropriate DFT functional to use for Au‐catalysed reactions; and 2) the overall challenges that pericyclic reactions pose, even with post‐Hartree–Fock methods (for full computational details and details of DLPNO‐CCSD(T) calculations, see the Supporting Information). In the report of the experimental study, a catalytic cycle (Figure a) was proposed, consisting of an initial coordination of the cationic Au‐complex to the alkyne moiety ( I to II ), cyclisation through nucleophilic attack of the oxygen lone pair ( II to III ), the [3,3]‐sigmatropic rearrangement ( III to IV ) and finally the release of the substrate from IV , which closes the catalytic cycle. We set out to probe the viability of this proposal using a truncated model system where the phenyl substituent of the alkene was replaced with a hydrogen atom.…”

Au-Catalyzed Synthesis and Functionalization of Heterocycles

“…The reaction of dienynes 239 under gold catalysis gave functionalised pyrroles or furans 240 (Scheme 61). Structures such as indoles [204], isoindoles [205], isoxazoles [206] and isochromenes [207] have been obtained with this methodology (Scheme 62). Proton loss and protodemetalation would lead to the final products.…”

“…Enantiomerically enriched propargylic alcohols underwent the substitution with complete racemisation in agreement with the formation of a carbocation intermediate. For example, methoxycarbonyl afforded anti-selective alkylation (207), whereas diethoxyphosphonyl group gave the corresponding syn product. Following Campagne's report, Chan and co-workers described the related allylic alkylation of heterocycles and other electron-rich arenes with allylic alcohols (197) catalysed by AuCl 3 [121].…”