Regioselective Addition of Tris(dialkylamino) Phosphines to [Fe₂(CO)₆(μ-PPh₂){μ-η¹:η²-(H)CCCH₂}]:  Novel P−C Coupling Reactions and Unusual Hydrocarbyl Rearrangements

Nucleophilic addition of tris(dialkylamino) phosphines, P(NR₂)₃ (R = Me or Et, ⁿPr), to [Fe₂(CO)₆(μ-PPh₂){μ-η¹:η²-(H)C_αC_βC_γH₂}] (1) affords the dimetallacyclopentene derivatives [Fe₂(CO)₆(μ-PPh₂)(μ-η¹:η¹-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-η¹:η¹-(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-η¹:η²-(H)C_αC_βC_γH₂}] gave the dimetallacyclopentenes 2a−c, 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-η¹:η²-(H)C_αC_βC_γH₂}] undergoes a clean and quantitative acid-promoted rearrangement to the σ−η-acetylide-bridged isomer [Fe₂(CO)₆(μ-PPh₂){μ-η¹:η²-C⋮CH₃}] (5). ¹H NMR and deuterium labeling studies suggest that this isomerization occurs via initial protonation at C_γ 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-η¹:η²-C⋮CCH₃}], and not from nucleophilic addition followed by hydrogen migration. In refluxing toluene, the dimetallacyclopentenes [Fe₂(CO)₆(μ-PPh₂)(μ-η¹:η¹-HCC{P(NR₂)₃}CH₂)] slowly decarbonylate to give [Fe₂(CO)₅(μ-PPh₂)(μ-η¹:η³-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-η¹:η²-{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-η¹:η¹-HCC{PPh₃}CH₂)] (8) to its σ−η-alkenyl counterpart [Fe₂(CO)₅(μ-PPh₂){μ-η¹:η²-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.