Regioselective Addition of tert-BuN⋮C to the α Carbon Atom of the Allenyl Ligand in [Fe₂(CO)₆(μ-PPh₂){μ-η¹:η²_α,β-(H)C_αC_βC_γH₂}]:  Formation of [Fe₂(CO)₆(μ-PPh₂){μ-η¹:η¹-(tert-BuN⋮C)CCCH₃}] and [Fe₂(CO)₆(μ-PPh₂)(μ-η¹:η²-{tert-BuNHC(O)CH₂}CCH₂)] via Competitive 1,3-Hydrogen Migration and Hydrolysis of the Reactive Allene-Bridged Intermediate [Fe₂(CO)₆(μ-PPh₂){μ-η¹:η¹-(tert-BuNC)HCCCH₂}]

Abstract

Addition of tert-BuN⋮C to the σ−η-allenyl complex [Fe2(CO)6(μ-PPh2){μ-η1:η2α,β-(H)CαCβCγH2}] results in nucleophilic addition to Cα to give the μ-η1:η1-||-alkyne [Fe2(CO)6(μ-PPh2){μ-η1:η1-(tert-BuN⋮C)CCCH3}] (2) and the β,γ-unsaturated amide [Fe2(CO)6(μ-PPh2)(μ-η1:η2-{tert-BuNHC(O)CH2}CCH2)] (3). Compounds 2 and 3 are proposed to form via initial nucleophilic attack at Cα to give [Fe2(CO)6(μ-PPh2){μ-η1:η1-(tert-BuNC)HCCCH2}], an unstable zwitterionic allene-bridged intermediate which subsequently undergoes either a 1,3-hydrogen migration to give 2 or hydrolysis by extraneous water to give the β,γ-unsaturated amide 3. An alternative pathway involving initial nucleophilic attack at Cβ to give the metallacyclopentene intermediate [Fe2(CO)6(μ-PPh2){μ-η1:η1-HCC(tert-BuNC)CH2}], followed by 1,3-hydrogen migration and Cβ to Cα tert-BuN⋮C migration has been considered. Isotope labeling experiments using [Fe2(CO)6(μ-PPh2){μ-η1:η2α,β-(D)CαCβCγH2}] (1-d1) are consistent with a large primary kinetic isotope effect for the transfer of hydrogen between Cα and Cγ. Addition of excess isopropylamine to a hexane solution of 2 gave the amidinium-substituted μ-η1:η1-parallel alkyne derivative [Fe2(CO)6(μ-PPh2){μ-η1:η1-C(tert-BuHNC)(NHPri)CCCH3}] (4), in near quantitative yield via addition of N−H across the C−N multiple bond. Chloroform solutions of 3 smoothly decarbonylate over several days to afford [Fe2(CO)5(μ-PPh2)(μ-η1(C):η1(O):η2(C)-{tert-BuNHC(O)CH2}CCH2)] (5), which contains a five-membered metallacycle by virtue of coordination of the amide carbonyl oxygen atom. The single-crystal X-ray structures of 2, 4, and 5 are reported.