Substituent-dependent [4+2] or [2+2] cycloadditions of phenylallenyl phosphine oxides with arynes.

A [4+2] cycloaddition strategy to assemble phenanthren-9-yldiphenylphosphine oxides is reported. This reaction relies on the strategic use of readily available phenylallenyl phosphine oxides as dienes to participate in [4+2] cycloaddition with arynes. Notably, benzo[ b ][1,4]oxaphosphinin-4-iums can be controllably synthesized by simply tuning the substituents in the phosphine oxide unit through a [2+2] cycloaddition cascade.