Regioselective addition of tert-BuNC to the α carbon atom of the allenyl ligand in [Fe2(CO)6(μ-PPh2){μ-η1:η2α,β-(H)Cα=Cβ=CγH2}]:: Formation of [Fe2(CO)6(μ-PPh2){μ-η1:η1-(tert-BuNC)C=CCH3}] and [Fe2(CO)6(μ-PPh2)(μ-η1:η2-{tert-BuNHC(O)CH2}C=CH2)] via competitive 1,3-hydrogen migration and hydrolysis of the reactive allene-bridged intermediate [Fe2(CO)6(μ-PPh2){μ-η1:η1-(tert-BuNC)HC=C=CH2}]

Addition of tert-BuN=C to the sigma-eta-allenyl complex [Fe(2)(CO)(6)(mu-PPh(2)){mu-eta(1):eta(alpha,beta)(2)-(H)-C(alpha)=C(beta)=C(gamma)H(2)}] results in nucleophilic addition to C(alpha) to give the mu-eta(1):eta(1)-parallel to-alkyne [Fe(2)(CO)(6)-(mu-PPh(2)){mu-eta(1):eta(1)-(tert-BuN=C)C=CCH(3)}] (2) and the beta,gamma-unsaturated amide [Fe(2)(CO)(6)(mu-PPh(2))(mu-eta(1):eta(2)-{tert-BuNHC(O)CH(2)}C=CH(2))] (3). Compounds 2 and 3 are proposed to form via initial nucleophilic attack at C(alpha) to give [Fe(2)(CO)(6)(mu-PPh(2)){mu-eta(1):eta(1)-(tert-BuNC)HC=C=CH(2)}], 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 beta,gamma-unsaturated amide 3. An alternative pathway involving initial nucleophilic attack at C(beta) to give the metallacyclopentene intermediate [Fe(2)(CO)(6)(mu-PPh(2)){mu-eta(1):eta(1)-HC=C(tert-BuNC)CH(2)}], followed by 1,3-hydrogen migration and C(beta) to C(alpha) tert-BuN=C migration has been considered. Isotope labeling experiments using [Fe(2)(CO)(6)(mu-PPh(2)){mu-eta(1):eta(alpha,beta)(2)-(D)-C(alpha)=C(beta)=C(gamma)H(2)}] (1-d(1)) are consistent with a large primary kinetic isotope effect for the transfer of hydrogen between C(alpha) and C(gamma). Addition of excess isopropylamine to a hexane solution of 2 gave the amidinium-substituted mu-eta(1):eta(1)-parallel alkyne derivative [Fe(2)(CO)(6)(mu-PPh(2)){mu-eta(1):eta(1)-C(tert-BuHNC)(NHPr(i))C=CCH(3)}] (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 [Fe(2)(CO)(5)(mu-PPh(2))(mu-eta(1)(C):eta(1)(O):eta(2)(C)-{tert-BuNHC(O)CH(2)}C=CH(2))] (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.