Gold Redox Catalytic Cycles for the Oxidative Coupling of Alkynes

Abstract

Au­(I)/Au­(III) catalytic cycles are catalytically competent to perform the oxidative coupling of alkynes in the homogeneous phase at room temperature and without any protecting atmosphere. Selectfluor as oxidant, wet acetonitrile as solvent, and sodium carbonate as base are the reagents of choice. Both aromatic and alkyl alkynes can be coupled, and mechanistic studies reveal that at least two gold species having different oxidation states are implicated in the key step of the coupling.

Electrochemical monitoring of the oxidative coupling of alkynes catalyzed by triphenylphosphine gold complexes

“…The possibility of performing the electrochemically driven homocoupling reaction under our experimental conditions using O 2 as the oxidant was confirmed by electrolysis experiments at -1.5 V where the FTIR band at 2240 cm -1 , characteristic of the terminal alkyne, disappears progressively (see Fig. 3c,d), the entire spectra retaining all other alkyne features in agreement with homocoupling features [20]. The process would be electrochemically driven by the generation of O 2 · -in the process C ox , followed by the formation of a superoxide-Au(I) adduct, as described for copper-superoxide adducts [29,30] Scheme 1 summarizes a tentative scheme for the overall electrochemical pathway.…”

“…This voltammetric response can be rationalized by assuming that selectfluor reacts quantitatively with the Parent Au(I) complex to form a Au(III) fluorinated complex which is reduced, stepwise, to Au(I) and Au(0). The variation of the voltammetric parameters for peak C 2 and its anodic counterparts A 2 * and A 2 ** with the potential scan rate suggests that the C 2 reduction process can be described in terms of a essentially reversible reduction preceded by a fast chemical reaction (CE mechanism) which can be represented, prior chemical reactivity data [20] as: . 2c).…”

“…To reproduce conditions used in 'chemical' experiments [20], the solvent contained 1-5 mM water. Chemicals were prepared as already described [20]. Similar responses were obtained for complexes 2 and 3.…”

“…In a prior report, we have described the catalytic effect of a series of PPh 3 -Au(I) complexes on the oxidative homocoupling of terminal alkyl and aryl alkynes using selectfluor as oxidant and Na 2 CO 3 as base in non-dried acetonitrile [20]. Monitoring the reaction by in-situ 1 H and 31 P-NMR spectroscopy and with the support of preliminary electrochemical data [20], it was suggested that the reaction can be achieved with gold catalysts at room temperature and indicated that both oxidation states (I) and (III) could be involved. The presence of a base allows to neutralize the two protons liberated from the terminal alkyne and to form the corresponding final salt of BF 4 -or F -.…”

Synthesis of 3,4,5-Trisubstituted Isoxazoles through Gold-Catalyzed Cascade Cyclization-Oxidative Alkynylation and Cyclization-Fluorination of 2-Alkynone O-Methyloximes

“…Initial coordination of the gold(I) catalyst A to the terminal alkyne 3a would form a π-coordinated alkyne adduct B, which transforms into the σ- This mechanism has been well documented recently. 24 On the other hand, the coordination of gold(III) intermediate D to the alkyne of Omethyloxime 1a and subsequent cyclization would lead to the gold(III) complex E bearing both isoxazole and alkyne substituents; E readily decomposes to yield cross-coupled product 4a and the cationic gold(I) catalyst A after reductive elimination. In the other possible mechanistic pathway, the gold(I)-catalyzed cyclization of O-methyloxime 1a may occur before the formation of σ-acetylide gold(I) complex C. The gold(I) complex G, which is formed after the demethylation of the cyclized gold(I) complex F, may give the protodeauration product 5.…”