Ligand Exchange on and Allylic C–H Activation by Iron(0) Fragments: π-Complexes, Allyliron Species, and Metallacycles

摘要

The complexes [(dippp)Fe(C_(2)H_(4))_(2)] ( 2 ) and [CpFe(C_(2)H_(4))_(2)][Li·(tmeda)] ( 5 ) both contain a formally zerovalent iron center but exhibit markedly different catalytic properties. Whereas 5 is able to induce a broad range of cycloisomerization and cycloaddition reactions, 2 is so far basically limited to cyclotrimerizations of alkynes and nitriles. Investigations into the behaviors of both complex vis-à-vis unsaturated substrates provided insights into the likely origins of this distinct behavior. Thus, ordinary terminal or internal alkenes were found not to replace the ligated ethylene units in 2 , whereas the stronger π-acceptor ligands 1,5-cyclooctadiene, 2-norbornene, and tolane afforded the corresponding π-complexes 8 , 9 , 10 , and 13 . A cyclopropene derivative engaged in oxidative cyclization with formation of the corresponding metallacycle 12 . Allyl-9-BBN or alkenyl-9-BBN derivatives succumbed to allylic C–H activation with formation of the unorthodox allyliron complexes 25 and 27 featuring a bridging hydride ligand between the iron and the boron atoms. Along the same line, 1,3-dienes bind well to 2 but undergo spontaneous activation if allylic C–H bonds are present; the resulting hydride is transferred to a residual ethylene ligand, as manifest in the formation of the cyclopentadienyl ethyl complex 22 . The same elementary steps surface in a remarkable reaction cascade comprising two consecutive C–H activation reactions and a stereoselective C–C bond formation, which ultimately provides the substituted cyclohexadienyl complexes 20 and 23 . In contrast, the heterobimetallic complex 5 neither induces allylic C–H activation nor binds 1,3-butadiene under conditions where it proved catalytically active. The targeted butadiene complex 34 had to be made by an indirect route and is distinguished by a noteworthy “flyover” constitution. Therefore, we conclude that the known cycloaddition and cycloisomerization reactions catalyzed by 5 do not