Regioselective addition of tert-BuN equivalent to C to the alpha carbon atom of the allenyl ligand in [Fe-2(CO)(6)(mu-PPh2){mu-eta(1):eta(2)(alpha,beta)-(H)C-alpha=C-beta=C gamma H2}]: Formation of [Fe-2(CO)(6)(mu-PPh2){mu-eta(1):eta(1)-(tert-BuN equivalent to C)C=CCH3}] and [Fe-2(CO)(6)(mu-PPh2)(mu-eta(1):eta(2)-{tert-BuNHC(O)CH2}C=CH2)] via competitive 1,3-hydrogen migration and hydrolysis of the reactive allene-bridged intermediate [Fe-2(CO)(6)(mu-PPh2){mu-eta(1):eta(1)-(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=CgammaH2}] results in nu cleophilic addition to C-alpha to give the mu-eta(1):eta(1)-parallel to-alk yne [Fe-2(CO)(6)-(mu-PPh2){mu-eta(1):eta(1)-(tert-BuN=C)C=CCH3}] (2) and th e beta,gamma-unsaturated amide [Fe-2(CO)(6)(mu-PPh2)(mu-eta(1):eta(2)-{tert -BuNHC(O)CH2}C=CH2)] (3). Compounds 2 and 3 are proposed to form via initia l nucleophilic attack at C-alpha to give [Fe-2(CO)(6)(mu-PPh2){mu-eta(1):et a(1)-(tert-BuNC)HC=C=CH2}], an unstable zwitterionic allene-bridged interme diate which subsequently undergoes either a 1,3-hydrogen migration to give 2 or hydrolysis by extraneous water to give the beta,gamma-unsaturated amid e 3. An alternative pathway involving initial nucleophilic attack at C-beta to give the metallacyclopentene intermediate [Fe-2(CO)(6)(mu-PPh2){mu-eta( 1):eta(1)-HC=C(tert-BuNC)CH2}], followed by 1,3-hydrogen migration and C-be ta to C-alpha tert-BuN=C migration has been considered. Isotope labeling ex periments using [Fe-2(CO)(6)(mu-PPh2){mu-eta(1):eta(alpha,beta)(2)-(D)-C-al pha=C-beta=CgammaH2}] (1-d(1)) are consistent with a large primary kinetic isotope effect for the transfer of hydrogen between C-alpha and C-gamma. Ad dition of excess isopropylamine to a hexane solution of 2 gave the amidiniu m-substituted mu-eta(1):eta(1)-parallel alkyne derivative [Fe-2(CO)(6)(mu-P Ph2){mu-eta(1):eta(1)-C(tert-BuHNC)(NHPri)C=CCH3}] (4), in near quantitativ e yield via addition of N-H across the C-N multiple bond. Chloroform soluti ons of 3 smoothly decarbonylate over several days to afford [Fe-2(CO)(5)(mu -PPh2)(mu-eta(1)(C):eta(1)(O):eta(2)(C)-{tert-BuNHC(O)CH2}C=CH2)] (5), whic h contains a five-membered metallacycle by virtue of coordination of the am ide carbonyl oxygen atom. The single-crystal X-ray structures of 2, 4, and 5 are reported.