Iron Pincer Complexes as Catalysts and Intermediates in Alkyl–Aryl Kumada Coupling Reactions

Abstract

Abstract. Iron-catalyzed alkyl-aryl Kumada coupling has developed into an efficient synthetic method, yet its mechanism remains vague. Here, we apply a bisoxazolinylphenylamido pincer ligand (Bopa) to stabilize the catalytically active Fe center, resulting in isolation and characterization of well-defined iron complexes whose catalytic roles are probed and confirmed. Reactivity studies of the iron complexes identifies an Fe(II) "ate" complex, [Fe(Bopa-Ph)(Ph) 2 ] -, as the active species for the oxidative addition of alkyl halide. Experiments using radicalprobe substrates and DFT computations reveal a bimetallic and radical mechanism for the oxidative addition. The kinetics of the coupling of an alkyl iodide with PhMgCl indicates that formation of the "ate" complex, rather than oxidative addition, is the turnover determining step. This work provides insights in iron-catalyzed cross coupling reactions of alkyl halides.3

Iron-Catalyzed Cross-Coupling: Mechanistic Insight for Rational Applications in Synthesis

“…59,60 Unlike TMEDA, the Bopa pincer ligands employed by the Hu group (Scheme 9) at least partly bound to an iron(II) center to afford a catalytically active species, which the authors assigned as Ph 2 Fe(Bopa) -. 66 Several protocols for iron-catalyzed cross-coupling make use of phosphines as ligands. The Neidig group characterized iron(I) and iron(II) complexes of a number of bisphosphines relevant in this context.…”

“…The results of several radical clock and radical trap experiments are consistent with the generally accepted involvement of radicals in iron-catalyzed cross-coupling reactions of alkyl halides (also see section 3.2). 21,23,48,66,69,71 However, most of these experiments have been criticized for not being unambiguous in the case of organometallic systems. 5f Strong support for the suggested mechanism comes from the work of the Neidig group, who found the iron(II) complexes PhFeBr(SciOPP) and Ph 2 Fe(SciOPP) to react with bromocycloheptane at catalytically relevant rates.…”

A DFT Study on FeI/FeII/FeIII Mechanism of the Cross-Coupling between Haloalkane and Aryl Grignard Reagent Catalyzed by Iron-SciOPP Complexes

“…There is a consensus that the haloalkane substrates undergo halogen abstraction by an (organo)iron species to generate the corresponding alkyl radical intermediates. A variety of radical mechanisms have been discussed thus far by Hu [ 28 ], Norrby [ 29 ], Bedford [ 30 ], Tonzetich [ 31 ], Fürstner [ 22 ], Neidig [ 32 , 33 ], and us [ 19 , 34 ]. Our group and the Neidig group have independently crystalized iron-bisphosphine complexes, such as Fe II X 2 (SciOPP), Fe II XAr(SciOPP), Fe II Ar 2 (SciOPP), where X = halide and Ar = Ph or Mes [ 32 , 33 , 34 ].…”

Synthesis and crystal structures of tetrameric [2-(4,4-dimethyl-2-oxazolin-2-yl)anilido]sodium and tris[2-(4,4-dimethyl-2-oxazolin-2-yl)anilido]ytterbium(III)

“…There are 68 structure in the CSD (version 5.39, update of November, 2018;Groom et al, 2016) containing a substituted anilido-oxazoline ligand (R-L1 À ) coordinated to a transition or main group metal. [COZFIH (Coeffard et al, 2009); DEJHIK (Cabeza et al, 2006); EDEBOG (Niwa & Nakada, 2012); EFICON (Bian et al, 2014); FONYAI (Mikami et al, 1999); GIWYES (Chen et al, 2014); GUTTOF (Inagaki et al, 2010); ISEWAG (Abbina et al, 2016); LUNGOS (Bauer et al, 2015a); LUNHAF (Bauer et al, 2015a); MALVUS, MALWAZ and MALWED (Kieltsch et al, 2010); MICTID, MICTOJ, MICTUP, MICVAX and MICVEB (Lu et al, 2013); MUQNAN and MUQNER (Wan et al, 2002); NANFEP, NANFIT, NANFOZ, NANFUF, NANGAM, NANGEQ, NANGIU and NANGOA (Peng & Chen, 2011); OCIHOX, OCIHUD and OCIJAL (Cabaleiro et al, 2001); PUDKUV, PUDLAC, PUDLEG and PUDLIK (Chen et al, 2009a); QIFFES and QIFFIW (Abbina & Du, 2012); RAKTAA (McKeon et al, 2011); RAMFIW, RAMFOC, RAMFUI, RAMGAP and RAMGET ; RAMKEY (Huang et al, 2017); ROGWAM (Nakada & Inoue, 2007); SELVIQ (Nixon & Ward, 2012); SUYQOS, SUYQUY, SUYRAF and SUYREJ (Castro et al, 2001); TIMLIL and TIMLOR (Chen et al, 2007); VOZZOB, VOZZUH and VUBBAX (Bauer et al, 2015b); VUQZAK (O'Reilly et al, 2015); WUGQOF, WUGQUL, WUGRAS and WUGREW (Chen et al, 2009b); XIGYEU (Wu et al, 2018); XOQVEG and XOQVIK (He et al, 2014); XOYVOW, XOYVUC, XOYWAJ, XOYWEN and XOYWIR (Castro et al, 2002)]. In contrast, there is only one structure of an unsubstituted anilido-oxazoline ligand (H-L1 À ) coordinated to a transition metal (Cabeza et al, 2006) and there are no structures of this type with a lanthanide metal.…”