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DOI: 10.1002/cctc.201100199
Catalytic Properties of Low Oxidation State Iron Complexes in Cross-Coupling Reactions: Anthracene Iron(�I) Complexes as CompetentCatalysts
Katharina Weber,[a] Eva-Maria Schnçckelborg,[a] and Robert Wolf*[a, b]
Transition-metal catalyzed C�C cross-coupling reactions are in-dispensable tools in modern organic synthesis.[1] Highly effi-cient synthetic protocols have been developed for nickel andpalladium-catalyzed cross couplings, and the catalytic mecha-nisms are generally well understood.[2] Owing to some inherentdisadvantages of nickel and palladium catalysis, for example,the toxicity, restricted availability, and high price of thesemetals, interest in iron-catalyzed cross couplings (Scheme 1)has been growing steadily.[3–10] Iron compounds are ideallysuited for sustainable catalysis, because iron is a cheap, abun-dant, non-toxic, and environmentally benign element.[11] How-ever, mechanistic investigations are frequently complicated bythe high lability and the paramagnetic nature of the organo-iron intermediates.
The first iron-catalyzed cross couplings of alkyl Grignardreagents with alkenyl bromides were reported by Kochi et al.as early as 1971.[3] Following this pioneering work, the scopewas greatly extended by the groups of Cahiez, F�rstner, Naka-mura, Hayashi, Jacobi von Wangelin, and others.[4–11] Various ef-ficient protocols have been developed for cross couplings in-volving alkyl, aryl, alkenyl, and acyl electrophiles, but themechanisms of these reactions remain largely unclear.[12] Openquestions relate to the oxidation state of iron in the catalytical-ly active species, the specific role of the magnesium cationsand the influence of various co-ligands and additives on thecatalytic activity.
Kochi originally suggested an FeI/FeIII catalytic cycle(Scheme 2) for cross couplings of alkenyl halides with aryl andalkyl Grignard reagents.[3e] This mechanism essentially corre-sponds to the “canonical” mechanism for transition metal-cata-
lyzed cross couplings. Various other mechanistic scenarioshave been proposed, which include a wide range of possiblecatalytic intermediates with oxidation states from �II to +IVfor iron. One important mechanistic proposal is the Fe�II/Fe0
cycle postulated by F�rstner et al. for the coupling of aryl hal-ides with alkylmagnesium reagents, which is depicted inScheme 2. Inter-metallic iron Grignard compounds of composi-tion [Fe(MgX)2]n are crucial catalytic intermediates in thiscycle.[13]
Few mechanistic studies of iron-catalyzed cross-coupling re-actions have been reported to date.[3, 12] Investigations of well-defined iron complexes can give new mechanistic insights.[14]
Therefore, we decided to investigate the catalytic properties oflow-valent iron complexes in a systematic fashion. Here wereport that low-oxidation state anthracene iron(�I) complexesare competent catalysts for cross couplings of aryl, alkyl, andalkenyl electrophiles. Furthermore, our results suggest that theoxidation state of iron in the catalyst precursor is not a deci-sive factor, but the presence of a labile ligand environment iscrucial for high catalytic activity.
To assess the influence of different oxidation states andligand environments, we investigated the catalytic behavior ofcomplexes 1–8 displayed in Figure 1. These structurally well-defined and well-characterized complexes show iron atoms inthe formal oxidation states �I to +II, and diverse ligands onthe metal center.[15, 16] Complexes 1 and 2 feature iron in theformal oxidation state �I and can be considered as sources ofthe “Fe�” anion, owing to the presence of labile ligands thatare readily substituted in stoichiometric reactions.[15a, b, 17] Com-plex 1 features two anthracene ligands, whereas one anthra-cene ligand and one 1,5-cyclooctadiene ligand are present incomplex 2. Complex 3 was prepared by reacting complex 1
Scheme 1. General scheme of iron-catalyzed cross couplings betweenorganic electrophiles R�X and organomagnesium reagents R’MgX.
Scheme 2. FeI/FeIII catalytic cycle for iron-catalyzed cross couplings accord-ing to Kochi et al. (left) and Fe0/Fe�II mechanism proposed by F�rstner et al.(right).
[a] K. Weber, E.-M. Schnçckelborg, Prof. Dr. R. WolfInstitute of Inorganic and Analytical ChemistryUniversity of M�nster, Corrensstr. 30, 48149 M�nster (Germany)Fax: (+ 49) 251-8336660E-mail : [email protected]
[b] Prof. Dr. R. WolfInstitute of Inorganic ChemistryUniversity of Regensburg, 93040 Regensburg (Germany)Fax: (+ 49) 251-8336660E-mail : [email protected]
Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cctc.201100199.
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with diphenylacetylene and displays in h6-coordinated hexa-phenylbenzene molecule next to the h2-coordinated alkyne.[15b]
Complex 4 is structurally related to 2. The structure of 4 dis-plays an h6-coordinated toluene molecule, whereas complex 2shows an h4-coordinated anthracene ligand.[15c] Complexes 5and 6 display h4-coordinated naphthalene units and stericallyencumbering pentamethylcyclopentadienyl (Cp*) ligands.[15d,e]
The salient structural feature of the “iron Grignard” complex 7is the presence of an iron magnesium bond.[15f] Such covalentFe�Mg bonds have been suggested to be important in theproposed [Fe(MgX)2]n intermediates in iron-catalyzed crosscouplings (Scheme 2).[12a] The iron(II) bromide 8 is related tocomplex 7 and was included in this study for comparison.High catalytic activities have been reported for the iron(�II)ethene complex 9, which displays two short Fe�Li bonds.[12a]
First, we studied the reaction of cyclohexyl bromide 10 withphenyl magnesium bromide as an example for the cross cou-pling of an alkyl electrophile with an aryl Grignard reagent.The results are shown in Table 1. Using bis(anthracene)ferrate1, we isolated cross-coupling product 11 in moderate to goodyields (Table 1, entries 1–3). Some homocoupling product 12was also formed. No cross coupling was observed when weused phenyl lithium as the nucleophile, which shows that thepresence of magnesium cations is essential for catalysis(entry 4). The heteroleptic anthracene complex 2 behaved simi-larly as 1 (entry 5), but complexes 3–8 gave only low yields ofthe desired cross-coupling product 11 (entries 6–11). Signifi-cant amounts of the homocoupling product 12 were formedin most cases. For comparison, we also studied the behavior ofan [Fe(acac)3]/TMEDA mixture (acac: acetylacetonate; TMEDA:N,N,N’,N’-tetramethylethylene-1,2-diamine). As previously re-ported, this catalyst system gave the desired cross-couplingproduct 11 in excellent yield (entry 12).[6c] Homocoupling prod-uct 12 was not observed in this case. Interestingly, the iron(II)complex [FeBr2(dme)] (DME: 1,2-dimethoxyethane) turned outto be an equally efficient precatalyst as [Fe(acac)3] (entry 13).
The presence of TMEDA is essential to achieve high catalyticefficiencies for both [Fe(acac)3] and [FeBr2(dme)]. In contrast,the addition of TMEDA to the iron(�I) precursor 1 did nothave a dramatic effect on the product distribution. Indeed, weobtained a slightly better yield of 11 in the absence of TMEDA(entry 3).
Subsequently, we tested the catalytic behavior of com-pounds 1–8 in the coupling of aryl chloride 13 with n-octylmagnesium bromide. The results are listed in Table 2. In thepresence of the co-solvent N-methyl pyrrolidone (NMP), preca-talysts 1 and 2 gave high yields (up to 87 %) of the desiredcross-coupling product 14 (Table 2, entries 1 and 2). Moderateyields of 14 were also isolated in the absence of NMP (entries 3and 4), which shows that this co-solvent is not essential to ini-tiate catalysis with complexes 1 and 2. Complex 3 also featuresa formal Fe�I center, but did not initiate any cross coupling(Table 2, entry 5). Low yields of cross-coupling product 14 werealso isolated with precatalysts 4 and 5. Only traces of 14 weredetected when we employed the cyclopentadienyl-substitutedcomplexes 6–8 (entries 6–10). In agreement with previous re-ports, cross-coupling product 14 was obtained in high yieldwith [Fe(acac)3] .[6a, 13a,b] However, the addition of NMP is essen-tial and the Grignard reagent must be added slowly in smallportions (entries 11 and 12). Low yields of 14 were obtainedwith other additives (anthracene and TMEDA, entries 13 and14). In contrast to the coupling of alkyl halide 10, the iron(II)complex [FeBr2(dme)] did not show any catalytic activity forthe coupling of 13 (entry 15).
Having identified complex 1 as a highly effective precatalystfor the coupling of aryl chloride 13, we subsequently investi-
Figure 1. Iron complexes investigated as precatalysts in cross-couplingreactions.
Table 1. Comparison of the catalytic competence of [Fe(acac)3] withstructurally-defined iron complexes 1–9 in the cross coupling of an arylmagnesium halide with an alkyl bromide.[a]
Entry Iron precatalyst Reaction time[h]
Yield of 11[%]
Yield of 12[%]
1 1 24 63 242[b] 1 2 58 263[c] 1 16 73 154[d] 1 16 0[e] 205 2 2 53 246 3 16 0[e] <5[e]
7 4 16 6 <5[e]
8 5 16 16 309 6 16 0[e] <5[e]
10 7 16 25 1011 8 16 7 912 [Fe(acac)3] 1 93 <5[e]
13 [FeBr2(dme)] 2 94 <5[e]
14[f] 9 0.12 94 –
[a] 5 mol % Iron precatalyst, excess TMEDA unless otherwise stated, 0 8Cto RT [b] Reaction run at RT. [c] No TMEDA. [d] Using PhLi, no TMEDA.[e] determined by GC. [f] Ref. [6c], yield of the homocoupling product notdetermined.
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gated the substrate scope of this catalyst (Table 3). First, westudied the influence of the alkyl magnesium reagent. n-Octylmagnesium bromide with aryl chloride 13 gave an excellentyield of cross-coupling product 14 (Table 2, entry 1), but onlytraces of cross coupling were observed when we used methylmagnesium bromide or phenyl magnesium bromide (Table 3,entries 1 and 2). Second, we modified the aryl electrophile.Moderate yields of cross-coupling product could be isolatedfrom the reaction of n-octyl magnesium bromide with thepara-trifluoromethyl substituted bromide 15 and iodide 16(entries 3 and 5) only in the presence of NMP (entry 4). A mod-erate yield of cross-coupling product was also obtained withthe methyl-substituted triflate 17, but reactions with themethyl and methoxy-substituted haloarenes 18–20 (entries 7–9) and benzyl chloride 21 were unsuccessful (entries 7–10). Thecoupling of n-octyl magnesium bromide with alkenyl bromide22 gave the desired cross-coupling product in high yield(entry 11). In summary, cross couplings proceed efficiently forelectron-deficient chloro-substituted arenes and an alkenylhalide. Yields are lower for more electron rich arene electro-philes and for electrophiles with more reactive leaving groups.
The most pertinent result of the present study is the goodcatalytic performance of the anthracene iron(�I) complexes 1and 2 both in the coupling of cyclohexyl bromide 10 withPhMgBr (Table 1) and the coupling of aryl chloride 13 with n-octyl magnesium bromide (Table 2). Complexes 1 and 2 givesimilarly high yields of cross-coupling product as the conven-tional iron(III) precatalysts [Fe(acac)3] , and the previously ex-plored iron(�II) complex 9 (Table 1, entry 14, and Table 2,entry 16).[6, 12a,b] The reactivity pattern of complex 1 also(Table 3) resembles that of [Fe(acac)3] and complex 9. Consider-
ing the similar catalytic behavior of these quite different cata-lyst precursors, it seems likely that the reactions follow acommon mechanism that is independent of the specific preca-talyst used.
The oxidation levels of iron in the active precatalysts rangefrom +III in [Fe(acac)3] to �I in complexes 1 and 2, and even�II in complex 9. Clearly, the iron oxidation state in the preca-talyst has little significance for its catalytic performance. How-ever, a common feature of all active precatalysts is the highlylabile coordination environment of iron, which could be neces-sary to create enough free coordination sites for substratebinding during the catalytic cycle. The catalytically active com-plexes 1, 2, and 9 exclusively have labile anthracene andalkene ligands. In contrast, compounds 3–8, where some ofthe ligands bind more tightly to the metal center, hardly showany catalytic activity.
Unlike previously established catalysts, complexes 1 and 2are also catalytically active in the absence of additives such asNMP or TMEDA. Significantly, the formation of cross-couplingproduct 14 was also observed with [Fe(acac)3] when anthra-cene was added to the catalytic mixture (Table 2, entry 13).This might indicate that the anthracene partly remains coordi-
Table 2. Comparison of the catalytic competence of structurally-definediron complexes 1–9 in the cross coupling of n-octyl magnesium bromidewith an aryl chloride.[a]
Entry Iron precatalyst Additive Reaction time[h]
Yield[%]
1 1 NMP 16 872 2 NMP 16 753 1 – 16 514 2 – 16 525 3 NMP 16 0[b]
6 4 NMP 24 67 5 NMP 24 118 6 NMP 16 0[b]
9 7 NMP 16 <5[b]
10 8 NMP 16 <5[b]
11 [Fe(acac)3] – 16 0[b]
12 [Fe(acac)3] NMP 1 9213 [Fe(acac)3] anthracene 16 914 [Fe(acac)3] TMEDA 1 21[b]
15 [FeBr2(dme)] NMP 16 0[b]
16[c] 9 NMP 0.12 85
[a] 5 mol % Iron precatalyst, stepwise addition of the Grignard reagent atRT. [b] Yield determined by GC. [c] Ref. [13a] , using n-C6H14MgBr,
Table 3. Substrate scope of iron(�I) precatalyst 1 in cross couplings ofalkyl and aryl magnesium bromides R�MgBr with aryl halides, aryl triflatesand an alkenyl bromide.[a]
Entry Ar�X R�MgBr Yield ofAr�R [%]
1 MeMgBr 0[b]
2 PhMgBr <5[b]
3 (n-C8H17)MgBr 43
4 (n-C8H17)MgBr 0[b,c]
5 (n-C8H17)MgBr 37
6 (n-C8H17)MgBr 37
7 (n-C8H17)MgBr 0[b]
8 (n-C8H17)MgBr 7
9 (n-C8H17)MgBr 0[b]
10 (n-C8H17)MgBr 5
11[c] (n-C8H17)MgBr 85
[a] All reactions were carried out in THF/NMP with 5 mol % 1 as precata-lyst. [b] Determined by GC. [c] No NMP.
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nated during the catalytic cycle and stabilizes the active spe-cies. Furthermore, it is noteworthy that some homocouplingside product 12 is always formed with precatalyst 1 during thecoupling of cyclohexyl bromide with PhMgBr, regardless of thepresence of TMEDA (Table 1, entries 1–4). In fact, a higher yieldof cross-coupling product 11 was isolated in the absence ofTMEDA, and the formation of homocoupling product 12 wassignificantly reduced. In contrast, TMEDA is essential to achievegood selectivities with [Fe(acac)3] . Nagashima et al. suggestedthat the addition of TMEDA effectively suppresses homocou-pling because of the formation of adducts [FeR2(tmeda)] .[12] Co-ordination of anthracene in complexes 1 and 2 may possiblyprevent the formation of such adducts, leading to a differentcatalytic pathway.[18]
A further significant observation is the strong dependenceof the coupling of aryl chloride 13 on the alkyl magnesium re-agent. Although n-octyl magnesium bromide gave a goodyield of the desired cross-coupling product 14 (Table 2), nocross coupling was observed with methyl or phenyl magnesi-um bromide (Table 3, entries 1 and 2). This reactivity difference,which appears to be a general feature of iron-catalyzed crosscouplings, may be attributed to the different reductive path-ways of these Grignard reagents.[6, 12a,b] Homocoupling of alkylor aryl substituents is one major reductive pathway for the re-duction of iron(III) species, for example, [Fe(acac)3] , to iron(II)complexes (Scheme 3 a). The iron(II) species are not reducedfurther easily via the homocoupling pathway. If no b-hydro-gens are present, for example, in PhMgBr, the catalysts there-fore remain at the iron(II) level, which is sufficient to catalyzethe cross coupling of cyclohexyl bromide 10 and related alkylhalides. However, the iron(II) intermediates formed in this caseare not reactive enough to engage with electron deficient arylhalides such as 13.
Grignard reagents that contain b-hydrogen atoms, such asn-octyl magnesium bromide, may follow a different and morepowerful reductive pathway by b-hydride elimination and theformation of alkanes and alkene. Highly reactive, low-valentiron intermediates may be formed in this manner which can in-itiate catalysis with electron deficient aryl halides such as 13(Scheme 3 b). It has been suggested that these catalytically
active intermediates might correspond to intermetallic iron(�II)species [Fe(MgX)2]n.[12a] The high catalytic activity of complex 9(Tables 1 and 2) apparently supports the suggested Fe�II/Fe0
mechanism shown in Scheme 2.[12a] The structure of the di-anionic iron(�II) ethene complex 9 (Figure 1) features two Fe�Li bonds and thus mimics the proposed [Fe(MgX)2]n clusters toa certain extent. However, the anthracene iron(�I) complexes1 and 2 are similarly competent precatalysts as 9, while the“iron Grignard reagent” 7, which has a covalent Fe-Mg bond,
hardly shows any catalytic activity (Tables 1 and 2). The pres-ence of metal-metal bonds in the catalyst precursors obviouslydoes not influence the catalytic behavior, and it thus remainsdoubtful whether iron(�II) species [Fe(MgX)2]n are true catalyticintermediates in iron-catalyzed cross couplings.[19]
Well-defined, low-valent anthracene iron(�I) complexes 1and 2 are competent precatalysts of iron-catalyzed cross-cou-pling reactions. The other organoiron complexes that we inves-tigated show little to no activity. The oxidation state of iron inthe catalyst precursors has little bearing on the catalytic perfor-mance, but a labile coordination environment seems crucialfor high catalytic activities. Although this study has shed someadditional light on the requirements for generating an activecross coupling catalyst, the nature of the catalytic intermedi-ates remains poorly understood. In future work, we aim to ad-dress this question in more detail, and we will further explorethe potential of low-valent polyarene iron complexes incatalysis.
Experimental Section
All reactions were performed under an atmosphere of dry argonand using dry solvents.
General procedure for the cross coupling of cyclohexylbromide (10) with phenylmagnesium bromide (Table 1)
In a glove box, a 25 mL round-bottom Schlenk flask was chargedwith the precatalyst (0.10 mmol, 5 mol %). The flask was sealedwith a septum and THF (2 mL) was added. The flask was cooled to0 8C, and cyclohexyl bromide (0.25 mL, 2.0 mmol, 1.0 equiv) wasadded. A mixture of N,N,N’,N’-tetramethylethylene-1,2-diamine(0.36 mL, 1.2 equiv) and a phenylmagnesium bromide solution (1 m
in THF, 2.4 mL, 1.2 equiv) was prepared in a Schlenk flask and wasadded dropwise over a period of 20 min to the cooled solution.The mixture was quenched with HCl (1 m, 10 mL), extracted withEt2O (3 � 5 mL) and dried over Na2SO4. The solvent was removed invacuo and the residue was purified by flash chromatography onsilica gel using n-pentane as eluent. The cross-coupling productcyclohexylbenzene 11 eluted as the first fraction. The homocou-pling product biphenyl 12 eluted as the second fraction.
General procedure for the cross coupling of n-octyl magne-sium bromide with an aryl electrophile (Tables 2 and 3)
In a glove box, a 25 mL round-bottom Schlenk-flask was chargedwith the precatalyst (0.12 mmol, 5 mol %). The flask was sealedwith a septum and the precatalyst was dissolved in THF (7 mL).4-Chlorobenzotrifluoride 13 (0.33 mL, 2.4 mmol, 1.0 equiv) and N-methylpyrrolidone (0.78 mL, 8.2 mmol, 3.4 equiv) were added. Asolution of n-octyl magnesium bromide (2 m in Et2O, 2.8 mL,1.4 mmol, 1.2 equiv) was added stepwise in portions of 0.2 mLevery 5 min at room temperature. The reaction mixture turnedblack after the addition of the Grignard reagent, and a dark rub-bery precipitate formed. The reaction mixture was quenched withHCl (1 m, 10 mL), extracted with Et2O (3 � 5 mL) and dried overNa2SO4. The solvent was removed in vacuo, and the residue waspurified by flash chromatography on silica gel using n-pentane aseluent.
Scheme 3. Formation of distinct catalytic intermediates in the cross couplingof Grignard reagents with and without b-hydrogen atoms.
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The spectroscopic data of the isolated products are given in theSupporting Information.
Acknowledgements
Financial support from the Deutsche Forschungsgemeinschaftand the Fonds der Chemischen Industrie is gratefully acknowl-edged. We thank Prof. Ulrich Zenneck and Dr. Jan J. Weigand forthe joint preparation of complex 4, and Prof. Werner Uhl for hisgenerous support.
Keywords: catalysis · cross couplings · iron · reactionmechanisms
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[10] Direct Cross Couplings: a) W. M. Czaplik, M. Mayer, A. Jacobi von Wange-lin, Angew. Chem. 2009, 121, 616; Angew. Chem. Int. Ed. 2009, 48, 607;b) W. M. Czaplik, M. Mayer, A. Jacobi von Wangelin, ChemCatChem2011, 3, 135; c) W. M. Czaplik, M. Mayer, S. Grupe, A. Jacobi von Wange-lin, Pure Appl. Chem. 2010, 82, 1545.
[11] a) S. Enthaler, K. Junge, M. Beller, Angew. Chem. 2008, 120, 3363 – 3367;Angew. Chem. Int. Ed. 2008, 47, 3317 – 3321; b) Iron Catalysis in OrganicSynthesis (Ed. : B. Plietker), Wiley-VCH, Weinheim, 2008.
[12] a) A. F�rstner, R. Martin, H. Krause, G. Seidel, G. Goddard, C. W. Leh-mann, J. Am. Chem. Soc. 2008, 130, 8773; b) J. Kleimark, A. Hedstrçm, P.-F. Larsson, C. Johansson, P.-O. Norrby, ChemCatChem 2009, 1, 152; c) D.Noda, Y. Sunada, T. Hatakeyama, M. Nakamura, H. Nagashima, J. Am.Chem. Soc. 2009, 131, 6078; d) T. Hatakeyama, S. Hashimoto, K. Ishizuka,M. Nakamura, J. Am. Chem. Soc. 2009, 131, 11949.
[13] a) L. E. Aleandri, B. Bogdanovic, P. Bons, C. Duerr, A. Gaidies, T. Hartwig,S. C. Huckett, M. Lagarden, U. Wilczok, R. A. Brand, Chem. Mater. 1995,7, 1153; b) B. Bogdanovic, M. Schwickardi, Angew. Chem. 2000, 112,4788; Angew. Chem. Int. Ed. 2000, 39, 4610.
[14] O. Garc�a-MancheÇo, Angew. Chem. 2011, 123, 2264; Angew. Chem. Int.Ed. 2011, 50, 2216.
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[16] A recent complementary study by F�rstner et al. investigated the cata-lytic behavior of the ethene iron(�II) complex 9 and closely relatedalkene iron complexes.[12a]
[17] a) R. Wolf, J. C. Slootweg, A. W. Ehlers, F. Hartl, B. de Bruin, M. Lutz, A. L.Spek, K. Lammertsma, Angew. Chem. 2009, 121, 3150; Angew. Chem. Int.Ed. 2009, 48, 3104; b) R. Wolf, A. W. Ehlers, M. M. Khusniyarov, F. Hartl, B.de Bruin, G. J. Long, F. Grandjean, F. M. Schappacher, R. Pçttgen, J. C.Slootweg, M. Lutz, A. L. Spek, K. Lammertsma, Chem. Eur. J. 2010, 16,14322.
[18] Previous studies suggest that radical pathways might be important iniron-catalyzed cross couplings.[6f, 12c] In this context, it should be notedthat the anthracene ligands of complexes 1 and 2 have significant radi-cal character, which could trigger single-electron transfer processes.Radical processes could also be involved in the formation of the catalyt-ically active species from the catalyst precursors. Although we did not
1576 www.chemcatchem.org � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2011, 3, 1572 – 1577
observe the formation of by-products originating from radical inter-mediates in significant quantities, the formation of short-lived radicalintermediates thus cannot be excluded at present.
[19] An elegant, recent study by Norrby et al. also questions the proposedFe�II/Fe0 cycle.[12b]
Received: June 16, 2011Published online on August 9, 2011
ChemCatChem 2011, 3, 1572 – 1577 � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemcatchem.org 1577
