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Regioselective Addition of Tris(dialkylamino) Phosphines to [Fe2(CO)6(μ-PPh2){μ-η12-(H)CCCH2}]:  Novel P−C Coupling Reactions and Unusual Hydrocarbyl Rearrangements

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journal contribution
posted on 1999-01-26, 00:00 authored by Simon Doherty, Mark Waugh, Tom H. Scanlan, Mark R. J. Elsegood, William Clegg
Nucleophilic addition of tris(dialkylamino) phosphines, P(NR2)3 (R = Me or Et, nPr), to [Fe2(CO)6(μ-PPh2){μ-η12-(H)CαCβCγH2}] (1) affords the dimetallacyclopentene derivatives [Fe2(CO)6(μ-PPh2)(μ-η11-HCC{P(NR2)3}CH2)] (R = Me, 2a; R = Et, 2b; R = nPr, 2c) or a mixture of the vinylidene- and dimetallacyclobutene-bridged complexes [Fe2(CO)6(μ-PPh2)(μ-η1-CC(CH3){P(NMe2)3})] (3a) and [Fe2(CO)6(μ-PPh2)(μ-η11-(CH3)CC{P(NMe2)3})] (4a), respectively, depending upon the reaction conditions. For instance, addition of P(NR2)3 to an ether solution of [Fe2(CO)6(μ-PPh2){μ-η12-(H)CαCβCγH2}] gave the dimetallacyclopentenes 2ac, whereas pretreatment of a solution of the allenyl starting material with HBF4 prior to the addition of P(NR2)3 gave the vinylidene- and dimetallacyclobutene-bridged products, which co-crystallized as a 67:33 mixture, as determined by single-crystal X-ray crystallography and 1H NMR spectroscopy. We have subsequently shown that the σ−η-allenyl complex [Fe2(CO)6(μ-PPh2){μ-η12-(H)CαCβCγH2}] undergoes a clean and quantitative acid-promoted rearrangement to the σ−η-acetylide-bridged isomer [Fe2(CO)6(μ-PPh2){μ-η12-C⋮CH3}] (5). 1H NMR and deuterium labeling studies suggest that this isomerization occurs via initial protonation at Cγ to afford a kinetic intermediate which rapidly rearranges to its thermodynamically more stable propyne-bridged counterpart followed by deprotonation. Clearly, the vinylidene and dimetallacyclobutene products isolated from the reaction between 1 and tris(dialkylamino) phosphine in the presence of acid arise from nucleophilic addition to the α- and β-carbon atoms of the acetylide bridge in [Fe2(CO)6(μ-PPh2){μ-η12-C⋮CCH3}], and not from nucleophilic addition followed by hydrogen migration. In refluxing toluene, the dimetallacyclopentenes [Fe2(CO)6(μ-PPh2)(μ-η11-HCC{P(NR2)3}CH2)] slowly decarbonylate to give [Fe2(CO)5(μ-PPh2)(μ-η13-C(H)C{P(NR2)3}CH2)] (R = Me, 6a; R = Et, 6b; R = nPr, 6c) bridged by a σ−η3-coordinated vinyl carbene. In the case of R = Et and nPr a competing isomerization also affords the highly unusual zwitterionic α-phosphonium-alkoxide-functionalized σ−σ-alkenyl complex [Fe2(CO)5(μ-PPh2){μ-η12-{P(NR2)3}C(O)CHCCH2}] (R = Et, 7b; R = nPr, 7c), via a P(NR2)3−carbonyl−allenyl coupling sequence. In contrast, isomerization of dimetallacyclopentene [Fe2(CO)6(μ-PPh2)(μ-η11-HCC{PPh3}CH2)] (8) to its σ−η-alkenyl counterpart [Fe2(CO)5(μ-PPh2){μ-η12-PPh3C(O)CHCCH2}] (9) is essentially complete within 1 h at room temperature with no evidence for the formation of the corresponding vinyl carbene. Thermolysis of a toluene solution of 8 in the presence of excess P(NEt2)3 results in exclusive formation of 7b, whereas at room temperature phosphine substitution affords 2b, via PPh3−P(NEt2)3 exchange. The isomerization of 8 to 9 and 2b,c to 7b,c appears to involve a dissociative equilibrium between the kinetic regioisomeric intermediate dimetallacyclopentene and 1, nucleophilic attack of phosphine at a carbonyl ligand of 1 to give a zwitterionic acylate intermediate, followed by acyl−allenyl coupling to afford the thermodynamically favored zwitterionic σ−η-alkenyl derivative. Qualitatively, the rate of isomerization increases as the steric bulk of the phosphine increases, in the order P(NMe2)3 < P(NEt2)3 ≈ P(NnPr2)3 < PPh3. The single-crystal X-ray structures of 2a, 3a, 4a, 6b, 7b, 8, and 9 are reported.