Switch from palladium-catalyzed cycloisomerization/dimerization of terminal allenyl ketones to preferential formation of monomers by a 5-palladatricyclo[4.1.0.0(2,4)]heptane catalyst: Synthesis of furans from substrates incompatible with the commonly used silver catalysts

By making the choice of either PdCl2(MeCN)(2) or PTH rac-4 as catalyst, the allenyl ketones 1 could be preferentially cycloisomerized/dimerized to either 2,4-disubstituted furans 3 or preferentially cycloisomerized to the monosubstituted furans 2. Since the PTH catalyst tolerates functional groups like terminal alkynes, alpha-halogen ketones, and alkyl halides that inhibit the silver catalysis, the latter method is an important extension of Marshall's Ag-I-catalyzed isomerization of 1 to 2. Some of these latter reactions also showed exciting chemoselectivities, e.g, with allenyl ketones, such as 1c and 1d, which also possess a 1,6-enyne substructure, no enyne-cyclization was observed. This is also the first reported example of catalysis by a PTH.