Two iron complexes, cis-(CO) 4 Fe[C(O)C(O)CH 3 ](CO 2 CH 3 ) (1a) and cis-(CO) 4 Fe[C(O)CO 2 -CH 3 ][C(O)CH 3 ] ( 3), are reported. Each complex can be considered as a γ-keto ester with a metal atom inserted into the chain linking its two organic groups. 1a is the only one found to evolve thermally, similarly to organic γ-keto esters, into the methoxy metallalactone (CO) 4 FeC(O)C(CH 3 )(OCH 3 )OC(O) (2a). This reaction is not reversible and does not require any acid or alkaline catalysis. The process does not result from a spontaneous dissociation of the methoxycarbonyl ligand of 1a but from an associative intra or intermolecular mechanism. A similar process can also be induced by pronucleophile reagents, and it affords a series of substituted metallalactones: (CO) 4 FeC(O)C(CH 3 )(Nu)OC(O) (Nu ) OCH 3 (2a), OC 2 H 5 (2b), SC 2 H 5 (2c), P(C 6 H 5 ) 2 (2d), P(C 6 H 11 ) 2 (2e)). Complex 2e has been characterized structurally. These substituted lactones are formed by nucleophilic attack of the reagent at the pyruvyl β-carbonyl followed by carbonylate attack on the alkoxycarbonyl ligand.

Anionic nucleophilic reagents Nu -) CH 3 O -, C 2 H 5 O -, t-BuO -, CH 3 S -, and P(C 6 H 5 ) 2were found to react with (CO) 4 Fe(CO 2 CH 3 )[C(O)C(O)CH 3 ] to afford two isomers of anionic trifunctionalized metallalactones, {(CO) 3 Fe[C(O)C(CH 3 )(Nu)OC(O)](CO 2 CH 3 )} -, whose formation resulted from an addition of the nucleophile to the β carbonyl of the pyruvoyl ligand. In this reaction, the metallacycle formation was found to occur by a further addition of the oxygen of the same β carbonyl on a terminal carbonyl. In acidic medium, these anionic complexes evolved into neutral metallalactones (CO) 4 Fe[C(O)C(CH 3 )(Nu)OC(O)] already obtained by performing the same reaction with pronucleophiles (Nu-H type reagents). Achievement of (CO) 4 Fe[C(O)C(dCH 2 )OC(O)] when the reaction was performed with t-BuOcould result from an intermediate formation of the enolate form of the pyruvoyl ligand [Fe]-C(O)-C(dCH 2 )-Ounder basic conditions.

International audienceThe reaction of carbanions with the pyruvoyl-substituted iron complex [(CO)4Fe(CO2CH3){C(O)C(O)CH3}] (1) affords the anionic trifunctionalized metallalactones [(CO)3Fe{C(O)C(CH3)(CRR′R″)OC4(O)(Fe–C4)}(CO2CH3)]– (3), whose formation results from the addition of the nucleophile to the β carbonyl of the pyruvoyl moiety, followed by attack of the oxygen of this β carbonyl on a terminal carbonyl ligand. These anionic lactones react, at low temperature, with HCl to give rise to the neutral lactones [(CO)4Fe{C(O)C(CH3)(CRR′R″)OC4(O)(Fe–C4)}] (2), which were previously obtained by addition of NuH nucleophiles to 1. Complex 3(3), whose lactonic ring formation has been performed using the diethyl malonate anion (R = R′ = CO2C2H5; R″ = H), and the dimethyl-substituted neutral lactone 2(1) (R = R′ = R″ = H) have been characterized by X-ray diffraction studie