Reaction of di-t-butoxyethyne with Fe₂(CO)₉: X-ray crystal structure of (tetra-t-butoxycyclopentadienone)tricarbonyliron(0) and an improved formal synthesis of hydrocroconic acid and the croconate dianion

Di-t-butoxyethyne (1a) reacts with [Fe₂(CO)₉] affording (tetra-t-butoxycyclopentadienone)-tricarbonyliron(0) (5a), the crystal structure of which has been determined by X-ray diffraction. Treatment of (5a) with trimethylamine oxide affords tetra-t-butoxycyclopentadienone (8), which had been previously converted into hydrocroconic acid (2) and the croconate dianion (3). On the other hand, (5a) with trifluoroacetic acid affords (tetrahydroxycyclopentadienone)tricarbonyliron(0) (5b), which is converted into the croconate dianion (3) by oxidation with molecular oxygen in basic medium.