Phosphine-Triggered Selectivity Switch in Silver-Catalyzed o-Alkynylbenzohydroxamic Acid Cycloisomerizations

A silver-catalyzed cycloisomerization reaction of a series of o-alkynylbenzohydroxamic acids is reported. Several 5-exo-dig and 6-endo-dig modes of cydization were observed with the nitrogen or oxygen atoms of the amide group acting as nucleophiles. The selectivity was strongly dependent on the silver salt used and on the presence of triphenylphosphine as an additive. Indeed, while the use of Ag2O at room temperature allowed the isolation of isobenzofuran-1-one oximes (7 compounds, 48-92% yield), [Ag(Im)](n) with the concomitant addition of 2 equiv of PPh3 led to a switch in selectivity and to a family of isoindolin-1-ones (10 compounds, 59-87%).