Cyclization of the Pyruvyl(methoxycarbonyl)tetracarbonyliron Complex cis-(CO)₄Fe[C(O)C(O)CH₃](CO₂CH₃) Induced by Pronucleophile Reagents

Abstract

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.

Ring Closure of Alkoxycarbonyl(tetracarbonyl)pyruvoyliron Complexes into Metallalactones Induced by Nucleophilic Attack of Carbanions

“…The coordination polyhedron at the metal center is a distorted octahedron built of three terminal carbonyl ligands, an alkoxycarbonyl, and the chelating carbonylated ligand. As observed for neutral metallalactones [7,10,12,13] and for five-membered iron metallacycles, [17] the bite angle of the chelate C(7)-Fe-C (6) [18] Therefore, its opposite angle on the metal [C(1)-Fe-C(2)] displays a larger value than that expected for an octahedral geometry with values close to 90°. The Fe-terminal carbonyl distances [1.79(1)-1.90(1) Å] are similar to those in the homologous trifunctionalized iron monoanion [9] or to the neutral complex [(CO) 4 Fe(CO 2 tBu) 2 ] [18] but are longer than the Fe- (8) 114.0(9) 112.1(9) 112.5(8) O7-C7-C8 115.0 (9) 117 (1) 118 (1) 117(1) O8-C8-C7 107.6 (8) 108.6(9) 110(1) 110 (1) CϵO distance observed in a monofunctionalized iron anion.…”

“…[7,10,13] As shown by the deviations of O(8) (+0.13 and +0.18 Å) and C(8) (+0.11 and +0.20 Å) from the plane defined by C(6)-Fe-C(7), the metallalactone ring of 3(3) is not rigorously planar. It is worth noting that C(8) and O (8) [7,10,13] ) and a shortening of the Fe-C (6) This crystallographic work confirms without any doubt the structure attributed to this series of anionic trifunctionalized metallalactones 3. However, a clear relation between the nature of the nucleophile and the trans/cis ratio observed for the different complexes 3 (from 100 % to 50 % of the trans isomer) has not been established.…”

“…This lack of reactivity is not too surprising as 1 only reacts slowly with CH 3 OH, C 2 H 5 SH, HP(C 6 H 5 ) 2 , or HP(C 6 H 11 ) 2 . [7] On the other hand, as shown by IR monitoring, 1 reacts very rapidly with carbanionic reagents such as MeLi, nBuLi, NaCH(CO 2 C 2 H 5 ) 2 , NaCH(CϵN) 2 , NaCH[C(O)CH 3 ] 2 , NaCH[C(O)CH 3 ]-(CO 2 CH 3 ), NaCH(CO 2 C 2 H 5 )(CϵN), NaC(CH 3 )(CO 2 -C 2 H 5 ) 2 , or NaC(CH 2 CH=CH 2 )(CO 2 C 2 H 5 ) 2 to afford instantaneously, even at -70°C, new complexes displaying a set of three ν(CϵO) bands between 2084 and 1970 cm -1 . These frequencies are similar to those observed for the product of the reaction between 1 and RO -, RS -, or R 2 P -anions, [13] and are intermediate between the ν(CϵO) of the monofunctionalized anion [Fe(CO) 4 (CO 2 R)]…”

“…When performed in the presence of an excess of pronucleophilic reagents (Nu-H), the reaction affords Nu-sub-stituted metallalactones (Scheme 1, b) which are formed, under these conditions, by an addition of the nucleophile to the β carbonyl of the pyruvoyl followed by metallalactone ring formation due to attack of the oxygen of this β carbonyl on the alkoxycarbonyl ligand and elimination of the methoxy group of this latter ligand. [7] To the best of our knowledge, prior to our study, only one example of nucleophilic addition at a carbonyl carbon of a ligand displaying a double C(O) chain had been described in the literature. In this paper, hydride was reported to attack the β carbonyl of a ligand of [(CO) 5 Mn{C(O)C(O)C 6 H 5 )}] to afford an anionic metallalactone by cyclization of the phenylglyoxyl with a terminal carbonyl.…”

“…Indeed, addition of nucleophiles at C α of this type of ligand has been proposed to account for the formation of phenylpyruvic esters by dicarbonylation of organic halides catalyzed by [Co(CO) 4 ] -. [6] The very few nucleophilic additions performed on cationic (Fe [7] Pt [8] ) or neutral complexes (Fe [9] ) displaying an M-C(O)-C(O)-R chain have been shown to occur at a terminal carbonyl li-gand of the molecule. We recently observed the original behavior of the complex cis-[(CO) 4 Fe{C(O)C(O)CH 3 }-(CO 2 CH 3 )] (1), which afforded its cyclic isomer 2 quantitatively, at 15°C, in a reaction involving the formation of a metallalactone ring by addition of the oxygen of the β carbonyl of the pyruvoyl to the alkoxycarbonyl ligand and a migration of the methoxy group of this latter ligand to the β carbon of the pyruvoyl (Scheme 1, a).…”