Mechanism of alkyl migration from oxygen to metal in iron-manganese ethoxycarbyne complexes. Induction of postcleavage intermolecular ethyl exchange by hydride bridging of mononuclear iron species

Article abstract of DOI:10.1021/om00009a008

The iron-manganese ethoxycarbyne Cp(CO)Fe(μ-COCH₂CH₃)(μ-CO)Mn(CO)MeCp (2a) undergoes thermal decomposition at 65°C to give MeCpMn(CO)₃ and, in the presence of PPh₂Me, CpFe(CO)(PPh₂Me)CH₂CH₃ (4a). The reaction is first order in carbyne and zero order in phosphine and exhibits a kinetic deuterium isotope effect k_H/k_D = 2.0 ± 0.1. Crossover experiments between 2a and its bis MeCp, CD₂CD₃ analog (2b-d₅) or between 2a-d₅ and 2b result in scrambling of the alkyl label between products 4a and 4a-d₅ and their MeCp analogs 4b and 4b-d₅. A crossover experiment between 2a and 2b-¹³C labeled specifically at the ethoxy methylene carbon gave complete ¹³C exchange in the methylene carbons of 4a,b but no ¹³C exchange in the starting materials after 50% conversion and no ¹³C scrambling into the methyl position of 4a,b. Ethylene does not affect the rate of decomposition of 2b or 2b-d₅, does not affect ethyl scrambling, and is not incorporated into 4b during the decomposition of 2b-d₅, ruling out ethylene rather than ethyl exchange. The course of the reaction between 2a and 2b-d₅ is not altered by the additives 2,6-di-tert-butyl-4-methoxyphenol, galvinoxyl, or thiophenol, ruling out involvement of radicals. Control crossover experiments between 2a-d₅ and 4b and between 2b-d₅ and 4a give no exchange. However, the iron hydride RC₅H₄Fe(CO)(PPh₂Me)H (R = H, 5a; R = Me, 5b) forms by β-elimination from RCpFe(CO)CH₂CH₃ followed by PPh₂Me displacement of ethylene, and alkyl exchange occurs when 2b and 5a are combined under the reaction conditions but not when 4b and 5a are combined. A mechanism for ethyl exchange is described in which catalytic amounts of 5a, 5b, and deuterated analogs 5a-d₁ and 5b-d₁ intercept the 16-electron carbyne decomposition intermediates CpFe(CO)CH₂CH₃, MeCpFe(CO)CH₂CH₃, and their C₂D₅ analogs to give hydride-bridged species. Concomitant migration of the ethyl group and the PPh₂Me ligand between the two iron atoms in these bridged species followed by cleavage to regenerate the hydride and 16-electron intermediate completes the exchange event. Slower trapping of the 16-electron intermediates by PPh₂Me irreversibly removes them from the catalytic cycle, giving products 4a,b, 4a-d₅, and 4b-d₅.