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3.6.13.1.1.6 Homocoupling as a Side Reaction in Cyclizations Catalyzed by Gold(III)

DOI: 10.1055/sos-SD-103-00038

Hopkinson, M. N.; Gouverneur, V.Science of Synthesis Knowledge Updates, (20112106.

The ability of gold to selectively activate carbon–carbon multiple bonds such as alkynes and allenes toward nucleophiles has been widely exploited in numerous synthetically useful addition and cyclization reactions.[‌4‌] These transformations are thought to proceed via initial attack of the nucleophile onto the gold-activated multiple bond, generating an organogold intermediate. This species most commonly undergoes protodeauration to give the hydrofunctionalization product and regenerate the catalyst. For gold(III)-catalyzed processes, suitably substituted organogold(III) intermediates can undergo competitive reductive elimination to deliver coupled products. Reacting 1-propadienylcyclohexanol with gold(III) chloride (5 mol%) in acetonitrile results in the formation of dimer 12 resulting from a cyclization–homocoupling in 10% yield, along with 47% of the protodeaurated dihydrofuran 11 (Scheme 6).[‌44‌] The observed 10% yield of 12 is consistent with the theoretical maximum homocoupling yield based on a 5 mol% loading of gold(III) chloride. Reactions of this type are potentially powerful as they combine the well-established reactivity of gold as a soft π-acid with oxidative coupling in a cascade process. The formation of 12 from 1-propadienylcyclohexanol can be explained by the intermediates shown in Scheme 6. A 5-endo-trig cyclization of the allenyl alcohol onto the gold-activated allene (oxyauration) leads to the organogold(III) complex 13. This species can either undergo protodeauration to afford the major dihydrofuran product 11 or generate the diorganogold(III) complex 14 upon transmetalation with 13 or oxyauration of 1-propadienylcyclohexanol. Reductive elimination from 14 delivers dimer 12 and gold(I).

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