Direct α-Vinylidenation of Aldehydes and Subsequent Cascade: Gold and Amine Catalysts Work Synergistically

Carbonyl-substituted allenes are highly important synthetic intermediates for a number of heterocycles and strained-ring systems. However, chemistry of allenyl aldehydes has not been explored as extensively as their ketone, ester, or amide analogues because of a lack of general synthetic methods. Described herein is the first direct -vinylidenation of aldehydes and an -vinylidenation/-functionalization cascade to access tri- and tetrasubstituted allenyl aldehydes using a combination of a gold catalyst and an secondary amine. The reactive enamine intermediate of an aldehyde reacts with the gold-activated hypervalent silylethynyl benziodoxolone to selectively generate the corresponding trisubstituted allenyl aldehyde. The allenyl aldehyde can further react with another equivalent of the alkynylation reagent or other electrophiles to afford tetrasubstituted allenes bearing an aldehyde group, an acetylene, and a halogen functionality. This method enables rapid access to polysubstituted furans from aldehydes.