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 PDCl 2 (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, α‐halogen ketones, and alkyl halides that inhibit the silver catalysis, the latter method is an important extension of Marshall's Ag 1 ‐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 also possess a 1,6‐enyne substructure, no enynecyclization was observed. This is also the first reported example of catalysis by a PTH.