Addition of Organolithium Nucleophiles to the Diiron Allenyl Complex [Fe₂(CO)₆(μ-PPh₂){μ-η¹:η²_α,β-(H)C_αC_βC_γH₂}]:  Synthesis and Characterization of Organodiiron-Coordinated β,γ-Unsaturated Ketones

Abstract

The binuclear allenyl complex [Fe2(CO)6(μ-PPh2){μ-η1:η2 α,β-(H)CαCβCγH2}] (1) has been prepared, and its reactivity with organolithium nucleophiles is described. Prop-2-yne bromide reacts with [Fe2(CO)7(μ-PPh2)]-Na+, via an SN2 mechanism, to give [Fe2(CO)6(μ-PPh2){μ-η1:η2 α,β-(H)CαCβCγH2}], the first example of a phosphido-bridged allenyl complex. The molecular structure of [Fe2(CO)6(μ-PPh2){μ-η1:η2-(H)CαCβCγH2}] (1) was determined by single-crystal X-ray diffraction and shows that the allenyl ligand is coordinated through Cα−Cβ. Variable-temperature 1H and 13C NMR studies reveal a high-energy exchange process that equilibrates the diastereotopic allenyl protons, presumably via a zwitterionic intermediate, as well as two independent trigonal rotations that act to exchange the carbonyl ligands on each unique Fe(CO)3 group. Complex 1 reacts with organolithium reagents (RLi; R = Me, nBu, Ph, C4H3S), via allenyl−carbonyl−nucleophile coupling, to afford the binuclear β,γ-unsaturated ketones [Fe2(CO)5{P(OMe)3}(μ-PPh2)(μ-η1:η2-{RC(O)CH2}CCH2)] (R = Me, 3a; nBu, 3b; Ph, 3c; C4H3S, 3d), and a single-crystal X-ray structure determination of 3a was undertaken to confirm the connectivity of the hydrocarbyl ligand. The most likely mechanism for the formation of 3a − d involves nucleophilic attack of R- at CO to give an acylate intermediate followed by migration of RCO to Cα of the allenyl and protonation of the resulting enolate to give the unstable alkenyl complexes [Fe2(CO)5(μ-PPh2)(μ-η1(C):η1(C):η2(C)-{RC(O)CH2}CCH2)] (R = Me, 2a; Bu, 2b; Ph, 2c; C4H3S, 2d). Finally, substitution of the metal-coordinated ester carbonyl in 2a − d with trimethylphosphite affords 3a − d as stable crystalline products.

Regioselective Addition of Tris(dialkylamino) Phosphines to [Fe2(CO)6(μ-PPh2){μ-η1:η2-(H)CCCH2}]: Novel P−C Coupling Reactions and Unusual Hydrocarbyl Rearrangements

“…The sum of the angles at C(4) (359.73°) and C(2) (359.68°) is consistent with sp 2 hybridization. The structure of 7b is closely similar to the β,γ-unsaturated carbonyl compounds [Fe 2 (CO) 5 (μ-PPh 2 )(μ-η 1 :η 2 -{NuC(O)CH 2 }CCH 2 )] (Nu = OR, NHR, alkyl). ,10b, The most notable difference between 7b and these β,γ-unsaturated carbonyl-bridged dimers is the five-membered metallacycle, which in [Fe 2 (CO) 5 (μ-PPh 2 )(μ-η 1 :η 2 -{NuC(O)CH 2 }CCH 2 )] is best described in terms of a saturated FeC 3 O chelate ring formed by coordination of the ester carbonyl, whereas the C−C and C−O bond length pattern in 7b indicates significant delocalization within the metallacycle. The bonding within the metallacycles of 7a − c and 10 is also supported by the values of ν(CO) for the bridging hydrocarbyl ligands.…”

“…Isomerization of [Fe 2 (CO) 5 ( μ -PPh 2 ) ( μ - η 1 : η 2 -C(H)C{P(NR 2 ) 3 }CH 2 )] to [Fe 2 (CO) 5 ( μ -PPh 2 ) { μ - η 1 : η 2 -{P(NR 2 ) 3 C(O)CHC  CH 2 }]. The isomerization of 2b , c into 7b , c is intriguing since the latter closely resembles the β,γ-unsaturated esters (Nu = OR), amides (Nu = NRH), and ketones (Nu = R) [Fe 2 (CO) 5 (μ-PPh 2 )(μ-η 1 :η 2 -{NuC(O)CH 2 }CCH 2 )] isolated from the reaction between 1 and the corresponding alcohol, amine, and alkyllithium reagent, respectively (eq 3). − One possible mechanism for the conversion of 1 into the β,γ-unsaturated ketones was suggested to involve nucleophilic attack of R - at CO, to give an acylate intermediate, migration of the RCO group to C α of the allenyl, followed by protonation (Scheme ). Given the structural similarity between 7b and these β,γ-unsaturated carbonyl derivatives, it is tempting to suggest that 7b forms via a similar pathway, i.e.…”

“…While the reactivity of mononuclear cationic transition metal allenyl complexes has been well documented, dominated largely by the generation of central carbon-substituted allyl derivatives via regiospecific addition of nucleophiles to the central carbon atom, the reaction chemistry of binuclear allenyl complexes is far more diverse. For instance, examples of nucleophilic addition to the central and both terminal allenylic carbon atoms are commonplace and, more recently, reports of nucleophile−carbonyl−allenyl coupling involving alkyllithium reagents, amines, and alcohols, a diphosphine-promoted migratory insertion−elimination, and a host of obscure ligand coupling reactions …”

CC Bond Formation in Diiron Complexes

“…Conversely, bridging allenyl can undergo nucleophilic addition to both central and terminal allenyl carbons. Doherty also found interesting nucleophile–carbonyl–allenyl coupling involving alkyllithium reagents,94a amines94f, 95 and alcohols 96. One example illustrating a nucleophilic attack at CO with migration to the allenyl α‐C is shown in Scheme .…”

“…A further example of CC double bond formation at the α‐C position is observed for the vinyliminium complexes [Fe 2 {μ‐η 1 :η 3 ‐C(R′)CHCN(Me)(R)}(μ‐CO)(CO)(Cp) 2 ][SO 3 CF 3 ] (R=Me, CH 2 Ph, R′=COOMe, Me, 141 ) upon treatment with NaH without other reagents able to replace the α‐CH. Reductive coupling occurs with formation of the tetrairon complexes 142 (Scheme ) 94a…”