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Abstract:
The complexes [(dippp)Fe(C2H4)2] (2) and
[CpFe(C2H4)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 commence at a 1,3-diene motif but require an enyne entity as starter unit.
