Cobalt-catalyzed ring expansion/ring opening of oxetanes using phosphine oxides as promoters under hydroformylation conditions

•Phosphine oxides activate the Co2(CO)8 catalyst in the transformations of oxetanes.•The reactions occur in the syngas atmosphere under mild conditions in green solvents.•Oxetanes undergo either carbonylative ring expansion or reductive ring opening.•The products are γ-lactones or primary alcohols depending on the substrate structure.