Gold‐Catalyzed, Intramolecular, Oxygen‐Transfer Reactions of 2‐Alkynyl‐1,5‐diketones or 2‐Alkynyl‐5‐ketoesters: Scope, Expansion, and Mechanistic Investigations on a New [4+2] Cycloaddition

Abstract

The gold-catalyzed intramolecular oxygen-transfer reactions of 2-alkynyl-1,5-diketones or 2-alkynyl-5-ketoesters-obtained from tetra-n-butylammonium fluoride mediated Michael addition of activated allenes to electron-deficient olefins-furnished cyclopentenyl ketones under very mild conditions. These reactions proceeded much easier and faster than similar reactions reported in literature, and the corresponding products were obtained in very good yields. Mechanistic investigations on the cycloisomerization were carried out by means of both (18) O isotopic experiments and quantum chemical calculations. The results from both, the designed isotopic experiments and theoretical calculations, satisfactorily supported the novel proposed intramolecular [4+2] cycloaddition of a gold-containing furanium intermediate to a carbonyl group, instead of the previous well-accepted [2+2] pathway.

The Outer‐Sphere Mechanism of Nitrene Transfer onto Gold(I) Alkyne Complexes

“…[13][14][15][16][17] It is well known that a-oxo metal carbene/carbenoids are powerful intermediates in organict ransformationss uch as CÀHa ctivation, ylide formation,a nd cyclopropanation reactions. [18,19] However,aclassic approacht ot hese compounds involves diazo carbonyl compounds,w hich requirec omplicated manipulation.S ignificant efforth as therefore been directed toward the development of novel and safer synthetic methods to generate, for instance, a-oxo gold carbenes by using oxidant reagents that promote intra- [20][21][22][23] or intermolecular [24,25] oxygen transfer to alkynes, which thus avoids the undesired diazo compounds. [26][27][28] Access to difficult-to-obtain and synthetically versatile aimino carbenes was recently realized by using an approach that mimics that of oxo derivatives.…”

“…However, a classic approach to these compounds involves diazo carbonyl compounds, which require complicated manipulation. Significant effort has therefore been directed toward the development of novel and safer synthetic methods to generate, for instance, α‐oxo gold carbenes by using oxidant reagents that promote intra‐ or intermolecular oxygen transfer to alkynes, which thus avoids the undesired diazo compounds …”