Furans by Propargyl Claisen Reaction

the authors tested some variations of the metal catalyst the entries with the copper triad metals are listed in Table 12.8. Copper(I) iodide was inactive (entry 1), silver tetrafluoroborate and AUCI3 both delivered the allene without cycloisomerization within 24 h (entries 2 and 4), and potassium tetrachloroaurate was inactive (entry 3). Without activation, phosphanegold(I) chlorides were inactive (entry 5), but after removal of the chloro ligand by silver salts, excellent yields were obtained in short reaction times of 40 min (entries 6-8). 

TABLE 12.8. Cationic Gold Complexes are Crucial for the Rearrangement 

TABLE 12.9. Silver Delivers Only Poor Results in the Tandem Sequence 

while the classical Cul (Table 12.10, entry 1) and AgSbFg (entry 2) deliver some product, the gold(I) complex that was tested does not deliver any (entry 3). Thus, for the oxidation state + 1, silver is the optimum, but it cannot compete with AuC13, which delivered superior yields (entries 4 and 5), with temperatures slightly above room temperature preferred. 

TABLE 12.10. Gold Outperforms the Other Catalysts, but it has to Be Gold(III) 

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