An Update on Recent Stoichiometric and Catalytic CâF Bond

Review pubs.acs.org/Organometallics

An Update on Recent Stoichiometric and Catalytic C−F Bond Cleavage Reactions by Lanthanide and Group 4 Transition-Metal Complexes Marcus Klahn and Uwe Rosenthal* Leibniz-Institut für Katalyse e. V. an der Universität Rostock, Albert-Einstein-Straße 29a, D-18059 Rostock, Germany ABSTRACT: In organometallic chemistry numerous complexes are known to cleave activated and nonactivated C−F bonds of organic compounds. These reactions are either stoichiometric or catalytic and offer different possibilities for a wide range of synthetic and catalytic applications. This review attempts to summarize a selection of most important developments made since 1997 for C−F bond cleavages by lanthanide and group 4 transition-metal complexes.

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INTRODUCTION The activation of molecules with one or more C−F bonds, socalled fluorocarbons, is still a challenging task for organometallic chemistry, leading to great efforts in this field of research. This significance was pointed out by highlights contributing to this topic.1 For elucidation several points have to be taken into account to reasonably explain the intricate activation of C−F bonds. This bond is the strongest single bond between a carbon atom and another element. The high bond energy results from the small size and the high electronegativity of the fluorine atom.1a Additional factors include the limited ability of the carbon atom to increase its coordination number, as well as the lack of any acidity and basicity. Fluorinated groups have been widely introduced in organic molecules, leading to remarkable changes in their physical properties, chemical reactivity, and physiological activity.2 Not only has this induced a constantly increasing number of fluorine-containing pharmaceutical drugs and agrochemicals but also other applications are broadly used, such as coatings, coolants, and artificial blood. However, from an environmentally concerned point of view the advantages of these compounds are becoming their biggest drawback. For example, combustion products of fluorinated compounds are greenhouse gases and accumulate in the upper atmosphere, therefore contributing to the global warming effect.3 Taking these facts into account, the necessity for a chemical conversion of fluorocarbons is beyond all question. The most promising way to render the fluorocarbons harmless is to transform the C−F bond into a C−H bond in the proximity of coordination compounds. Such complexes include both early and late transition metals; the latter are also capable of mediating C−C coupling reactions.4 However, this review will focus exclusively © 2012 American Chemical Society

on recent stoichiometric and catalytic conversions using lanthanide and group 4 transition-metal complexes since the review published by Crabtree et al. in 1997.1b

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STOICHIOMETRIC C−F BOND ACTIVATIONS Group 3 and Lanthanides. The example for group 3 compounds, especially lanthanide complexes, includes the cooperative experimental and computational studies of Andersen, Eisenstein, Maron, and co-workers of the hydrogen for fluorine exchange by cerium complexes.5 The reactions of monomeric Cp′2CeH (Cp′ = 1,3,4-(Me3C)3C5H2) with hexafluorobenzene and pentafluorobenzene were studied first within this context.5a The conversion of Cp′2CeH with hexafluorobenzene in C6D6 yielded Cp′2CeF, Cp′2Ce(C6F5), and H2 (Scheme 1a). Scheme 1

Cp′2Ce(C6F5) decomposed slowly upon standing at room temperature for several days to give Cp′2CeF and tetrafluorSpecial Issue: Fluorine in Organometallic Chemistry Received: October 14, 2011 Published: January 9, 2012 1235

dx.doi.org/10.1021/om200978a | Organometallics 2012, 31, 1235−1244

Organometallics

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The fluoroalkyl complex Cp′2CeCF3 was not observed in the course of the reaction. Therefore, it was generated in an alternative attempt by reacting the hydride Cp′2CeH with Me3SiCF3. Unfortunately, the obtained products were only Cp′2CeF, tri-tert-butylbenzenes, tri-tert-butylfluorobenzenes, and Me3SiH, but this reaction proved the assumed pathway. In a different approach the metallacycle Cp′[(Me3C)3C5H2C(Me2)CH2]Ce5a was also reacted with CH3F, CH2F2, and CHF3. The resulting fluoroalkyls, Cp′2CeCH2F, Cp′2CeCHF2, and Cp′2CeCF3, were deduced by their decomposition products, which were Cp′2CeF, tri-tert-butylbenzenes, and tritert-butylfluorobenzenes, respectively. DFT calculations on the C−H and C−F bond activation pathways showed that the C− H activation by Cp2CeH proceeds with a low barrier. It was also revealed that carbene ejection and trapping by H2 is the rate-determining step. The barrier of the rate-determining step is raised as the number of fluorines increases, while that of the C−H activation path is lowered as the number of fluorines increases, which parallels the acidity. These factors contribute to the explanations for the different reaction outcomes, particularly with regard to the reaction with CHF3 producing no methane. The benzyl complex Cp′2CeCH2Ph (Cp′ = 1,2,4-tri-tertbutylcyclopentadienyl) and its reactions with haloforms CH3X (X = F, Cl, Br, I) were discussed in a further report.5c The outcomes of these conversions were Cp′2CeX and CH3CH2Ph. Due to the equilibrium between the benzyl complex and the metallacycle and toluene,5a the reaction got more complex, because the metallacycle itself reacted with CH3X to give Cp′[(Me3C)2(EtMeC)C5H2]CeX. Labeling studies showed that the methyl group of the methyl halide was transferred intact to the benzyl group to give ethylbenzene. The reaction mechanism, according to DFT calculations on Cp2CeCH2Ph and CH3F, does not include a four-center transition state, as would occur for σ-bond metathesis. Thus, a lower barrier process involving a haptotropic shift of the Cp2Ce fragment is found. This results in a transition state with the para carbon atom of the benzene ring attached to the Cp2Ce fragment. The CH2 fragment of the benzyl group is capable of performing a nucleophilic attack at the CH3F which is activated by coordination to the metal ion. Therefore, the mechanism can be classified as an associative interchange process. Deacon et al. reported on an ytterbocene dimethoxyethane complex, which is capable of cleaving a C−F bond in perfluorodecalin and perfluoro(methylcyclohexane).6a In both cases, a nine-coordinate, fluoride-bridged dimer was obtained (Scheme 3). The fluorocarbons acted as an oxidant promoting the formation of YbIII from YbII. Later, the same authors extended this reaction to ytterbium complexes with aryloxo ligands of the type Yb(OAr)2(THF)3 (OAr = OC6H5-2,6-tBu2-4-R; R = H, Me, tBu) to give [Yb(OAr)2F(THF)]2.6b In addition to the change of the ligand at the ytterbium center, it was also shown that the solvent has a crucial influence on the reaction outcome. The reaction of

obenzyne, which was trapped by a [2 + 4] cycloaddition with C6D6 (Scheme 1b). Therefore, Cp′2Ce(C6F5) was regarded as a reaction intermediate. A similar reaction with pentafluorobenzene gave a slightly different product mixture, namely Cp′2CeF, H2, and the intermediates Cp′2Ce(C6F5), Cp′2Ce(p-C6F4H), and Cp′2Ce(C6F4)CeCp′2. After 7 days at room temperature the intermediate cerium compounds formed Cp′2CeF and [2 + 4] cycloadducts of C 6D6 with tetrafluorobenzyne and trifluorobenzyne as well. Similar reactions were carried out with the metallacycle Cp′[(Me3C)3C5H2C(Me2)CH2]Ce, which could be obtained quantitatively from the benzyl complex Cp′2Ce(CH2Ph) after release of toluene at elevated temperatures. The metallacycle is prone to C−H bond activation, which was proved by the reaction with C6F5H leading to Cp′2Ce(C6F5). Additionally, the authors used DFT calculations to explore the possible reaction pathways. It turned out that the hydrogen−fluorine exchange is a key step, which leads to the conversion of Cp′2CeH and C6F6 into Cp′2CeF and C6F5H. A preliminary η1-F-C6F5 interaction with the cerium hydride is important for the reaction. A subsequent σ-bond metathesis results in the migration of the fluoride in the ortho position to the initial Ce−F interaction toward the hydride. This process has a relatively low activation energy and gives Cp′2CeF and C6F5H. The observed dihydrogen and the intermediate Cp′2Ce(C6F5) are formed by the reaction of C6F5H with Cp′2CeH. The decomposition of the aryl complex Cp′2Ce(C6F5) into Cp′2CeF and C6F4, which is trapped by one solvent molecule, has a high barrier that is responsible for the slow conversion. The research interest in the cerium hydride complexes Cp′2CeH (Cp′ = 1,2,4-tri-tert-butylcyclopentadienyl) led also to the reactions with fluoromethanes.5b The Cp′2CeH reacted instantaneously with CH3F, but more slowly with CH2F2, to give Cp′2CeF and CH4 in both cases. The fluorine atoms were exchanged completely for hydrogen. The reaction of the cerium hydride proceeded very slowly with CHF3 and not at all with CF4. In contrast to the case for CH3F and CH2F2, the formation of methane was not observed but Cp′2CeF, H2, and 1,2,4- and 1,3,5-tri-tert-butylbenzene, respectively, were found after workup of the reaction mixture. It was postulated that the substituted benzenes were generated from trapping of the carbene moiety “CF2” by the ligand Cp′ after α-fluoride abstraction from Cp′2Ce(CF3) (Scheme 2). Scheme 2

Scheme 3

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Group 4 Metals. Compounds with group 4 metals furnish the largest number of examples within this review. The three metals titanium, zirconium, and hafnium are discussed separately. The Hessen group showed the formation of the cationic TiIII complex [Cp*2Ti][BPh4] and its reactivity toward C−F bonds.9 The positively charged titanium center readily coordinated an additional donor molecule such as THF and even fluorobenzene. The latter gave an adduct with a η1 coordination of the fluorobenzene via the fluorine atom. However, this binding mode is weak, because a displacement readily occurred in the presence of diethyl ether or THF. The compound [Cp*2Ti(η1-FC6H5)][BPh4] is stable in fluorobenzene at room temperature for several days, showing no C−F bond cleavage. In contrast, the addition of α,α,α-trifluorotoluene to the aforementioned complex led to the formation of Cp*2TiF2 and 1,2-diphenyl-1,1,2,2-tetrafluoroethane. This finding was transferred to other anions containing fluorine atoms such as [B(C6F5)4]− and [B{3,5-(CF3)2C6H3}4]−. Tetrakis(pentafluorophenyl)borate showed no C−F bond activation by the cationic titanium center. Nevertheless, similar to the reaction with trifluorotoluene, the addition of [PhNMe2H][B{3,5-(CF3)2C6H3}4] to Cp*2TiMe gave the difluoride Cp*2TiF2 as a result of C−F activation at the boranate anion (Scheme 6). The displacement of fluoride by a dimethylamino group at (pentafluorophenyl)cyclopentadiene or 3-(pentafluorophenyl)indene mediated by Ti(NMe2)4 was observed by Deck and coworkers.10 The reaction started via the deprotonation step of the cyclopentadiene ring and subsequent protonation of one amide, giving dimethylamine and the intermediate CpRTi(NMe2)3 with CpR = (pentafluorophenyl)cyclopentadienyl or 3-(pentafluorophenyl)indenyl (Scheme 7). Thus, an interaction between the titanium and the o-fluoride was observed in NMR spectroscopic experiments. This could serve as an explanation for the selectivity for the ortho substitution by a nucleophilic C−F activation via the titanium center to give either 3-[2(dimethylamino)trifluorophenyl]indene or 1-[2(dimethylamino)trifluorophenyl]cyclopentadiene. Furthermore, the second o-fluoro substituent could be also exchanged for the dimethylamine group to yield 3-[2,6-bis(dimethylamino)trifluorophenyl]indene and 1-[2,6-bis(dimethylamino)trifluorophenyl]cyclopentadiene. The defluorinated compounds were isolated and characterized after hydrolytic workup of the reaction mixture. In 2005 Beckhaus et al. reported on C−F activations at pentafluoropyridine, 2-fluoropyridine, and cyanuric fluoride by free titanocene fragments, generated from Cp 2 Ti(η 2 Me3SiC2SiMe3), and for comparison on the reactions with their fully hydrogenated analogues.11 The general outcomes were dinuclear complexes with either hydride or fluoride as bridging ligands (Scheme 8). The molecular structure showed

Cp 2 Yb(DME) with perfluorodecalin or perfluoro(methylcyclohexane) in DME instead of THF gave the trimeric complex [Cp2YbF]3. A similar reaction of (MeCp)2Yb(THF) with perfluorodecalin in DME led to the isolation of a novel tetrameric bis(methylcyclopentadienyl)ytterbium fluoride without any coordinated solvent, whereas in THF (MeCp)2YbF(THF) was obtained. Unfortunately, detailed analysis of the post-reaction fluorocarbons could be only obtained from perfluoro(methylcyclohexane). An intramolecular C−F bond activation was observed at the ligand N,N-dialkyl-N′-2,3,5,6-tetrafluorophenylethane-1,2-diamine (alkyl = methyl, ethyl) by Deacon, Junk, and co-workers.7 The THF-containing complex [Yb(L)2(THF)2] degraded due to its thermal instability over several weeks at room temperature in toluene, yielding tetranuclear ytterbium complexes of the type [Yb4F6(L)6] together with defluorinated ligands (Scheme 4). The C−F activation occurred at the ortho Scheme 4

position of the aryl ring, due to an interaction of the ytterbium center with one o-fluorine atom, which was corroborated by the molecular structure of the stable [Yb(L)2(DME)], as well as the chemical shift separation between the o- and m-fluorine atoms. The key step is a single electron transfer giving YbIII−F and an organic radical, which was hydrolyzed for GC analysis, also revealing multiple defluorination products. Junk et al. reported on the use of Hg(C6F5)2 as oxidizing agent for the formation of trivalent formamidinate lanthanide complexes, which resulted in an abstraction of one fluoride atom of the pentafluorophenyl group.8a The reaction proceeded via coordination of two amidinate ligands and binding of one pentafluorophenyl ring; while the bulkiness of the amidinates governed the outcome, smaller ligands only promoted the formation of tris(formamidinato)lanthanide(III) complexes.8b The sterically controlled C−F activation is shown in Scheme 5, which is possible with several lanthanides. Even the fate of the defluorinated product was elucidated by isolation. It is formed by a concomitant insertion of the eliminated tetrafluorobenzyne into the Ln−O bond of the coordinated THF molecule and reaction with unreacted amidinate. Scheme 5

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Scheme 6

Scheme 7

Scheme 8

Scheme 9

temperature with the in situ generated [(PNP)TiCtBu] to form the C−H activation products (PNP)Ti(CHtBu)(ArF) depending on the educt as regioisomers, but at least in ortho position to one fluorine atom. The thermal treatment (>100 °C) of these complexes resulted in the formation of (PNP)Ti(CHtBu)F by β-fluoride elimination and release of benzyne. However, the benzyne trapping was only cleanly achieved by adding durene, giving the Diels−Alder cycloaddition product 5,6,7,8-tetrafluoro-1,4-dihydro-2,3,9,10-tetramethyl-1,4-ethenonaphthalene (Scheme 10). Scheme 10

two hydride-bridged titanium(III) centers in the case of pyridine, which was attacked at the 2-position, and for 2fluoropyridine the fluoride-bridged compound was formed exclusively. This underlined the preference for C−F cleavage over C−H activation. Even the reaction with pentafluoropyridine occurred selectively at the 2-position, giving the fluoride bridge. This is worth mentioning, because these results are in contrast to other results (vide infra) of the analogous zirconocenes “Cp2Zr” with fluorinated pyridines, leading to a preferred C−H activation and in the case of the pentafluoropyridine to C−F bond cleavage exclusively at the 4-position.20 Mindiola and co-workers used the complex (PNP)Ti(CHtBu)(CH2tBu) (PNP = bis(2-(diisopropylphosphino)-4methylphenyl)amide) for 1,2-addition reactions of arenes.12a The active species [(PNP)TiCtBu] was formed in situ by elimination of neopentane. Because of the ability of this complex to also cleave C−H and C−O bonds, the C−F activations were only realized when none of these bonds were present in the substrate molecule. These transformations generated a mixture of two rotamers, which equilibrated in solution and at elevated temperatures (Scheme 9). The reaction with perfluorotoluene selectively produced the para-activation product. Further investigations on the reactivity of (PNP)Ti(CHtBu)(CH2tBu) toward C−F bonds were reported by the same group.12b,c Several fluorinated benzenes, starting from singly up to 5-fold fluorinated substitution, were reacted at room

The fluoride abstraction was accelerated when other fluorides were adjoining to the leaving fluoride. Also, the modified complex (PNP)Ti(CHSiMe3)(CH2SiMe3), releasing tetramethylsilane, was tested in the reactions mentioned above, but the C−H activation step had to be performed at elevated temperatures (>60 °C) instead of room temperature and for longer reaction times. The outcome of this reaction was the C−H activation product together with decomposition products, which resulted from the thermal instability of (PNP)Ti(CHSiMe 3 )(CH2SiMe3). However, the C−F bond cleavage reaction also took place at temperatures higher than 100 °C over 72 h to yield (PNP)Ti(CHSiMe3)F and benzyne. Furthermore, the reactive species [(PNP)TiCXMe3] (X = C, Si) were able to defluorinate 1-fluorohexane to form 1-hexene and (PNP)Ti(CHXMe3)F, respectively, but unfortunately without detection of intermediates such as [(PNP)Ti(CHXMe3)(2-C6H12F)]. Nonetheless, the hydrodefluorination of the aliphatic and aromatic fluorocarbons proceeded with the same mechanism: an initial 1,2-C−H bond addition to the titanium alkylidyne and a subsequent thermally induced fluoride abstraction. In 1238


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addition, the possibility of a catalytic cycle was discussed to hydrodefluorinate fluorobenzene. The regeneration of the precursor (PNP)Ti(CHXCMe3)(CH2XMe3) turned out to be the challenging step, although (PNP)Ti(CHXMe3)F gave (PNP)Ti(CHXCMe3)(CH2XMe3) by treatment with Li(CH2XMe3) in good yields. However, Li(CH2XMe3) and other transmetalation agents reacted primarily with fluorobenzene under the conditions for C−F bond cleavage. Therefore, a catalytic system could not be established. The first example for zirconium compounds is the unexpected formation of [(C6F5)5ZrF2]3−, which was observed by the Girolami group when reacting ZrCl4 with an excess of C6F5Li in the presence of tetramethylethylenediamine (Scheme

Scheme 13

tetrafluorobenzyne generation and the other for radical chain formation. Therefore, investigations with radical initiators and inhibitors revealed a significant effect on the reaction rate for the formation of polyfluoroarenes, giving evidence for a radical mechanism. Moreover, Jones et al. described the activation of monofluorinated aliphatic hydrocarbons in contrast to that of monofluoroarenes in a subsequent article.15c Here, the monomeric bis(pentamethylcyclopentadienyl)zirconocene dihydride complex Cp*2ZrH2 was reacted with 1-fluorohexane in deuterated cyclohexane at 23 °C to quantitatively yield Cp*2ZrHF and hexane after 2 days. Subsequent addition of a second equiv of 1-fluorohexane and heating at 120 °C for 10 days led to Cp*2ZrF2 and hexane in quantitative amounts. Similar reactions with either 1-fluorocyclohexane or 1fluoroadamantane were also performed, but elevated temperatures, an H2 atmosphere (for stabilizing the Cp*2ZrH2), and longer reaction times were required, giving a mixture of Cp*2ZrHF and Cp*2ZrF2. Further investigations stated a decreasing reactivity order from primary to tertiary C−F bonds. The course of action was explained by a radical chain mechanism involving a Cp*2ZrIIIH species, which was further corroborated by adding radical inhibitors and initiators affecting the reaction rate. Moreover, the reaction with (fluoromethyl)cyclopropane provided good evidence for the generation of alkyl radicals by producing Cp*2Zr(n-butyl)H (Scheme 14). In contrast, the reaction outcome of monofluoroarenes, like that of 1-fluoronaphthalene, with Cp*2ZrH2 gave Cp*2ZrHF and hydrogenated arenes, caused by a hydridic attack on the aromatic ring and a succeeding fluoride abstraction. Thus, a different pathway is responsible for the fluoride abstraction at aromatic C−F bonds, because Cp*2Zr(C6H5)F was formed after a preliminary o-C−H activation and an ensuing β-fluoride abstraction to give a benzyne complex which inserted into the Zr−H bond. In conclusion, the activation of aliphatic C−F bonds occurs via a radical mechanism, whereas a nucleophilic displacement mechanism takes place at aromatic C−F bonds. Later, this work was extended to C−F bond activations at perfluorinated arenes, such as hexafluorobenzene, pentafluorobenzene, perfluorotoluene, perfluoronaphthalene, and perfluorobiphenyl.15d Reactions with Cp*2ZrH2 and excess hexafluorobenzene gave Cp*2ZrHF, Cp*2Zr(C6F5)H, and C6F5H in a 2:1:1 ratio, together with small amounts of Cp*2ZrF2. Additionally, with an excess of pentafluorobenzene Cp*2ZrHF, Cp*2Zr(C6F5)H, Cp*2Zr(o-C6F4)H, p-C6F4H2, and o-C6F4H2 were obtained, with o-C−F activation being favored. The nature of the products for the other substrates was the same as in the hexafluorobenzene case, though the product distribution was different. The investigation of the mechanism ruled out a radical pathway, since the addition of radical

Scheme 11

11).13 The exact mechanism of the C−F bond cleavage at the pentafluorophenyl group as well as the nature of the degradation products could not be uncovered. However, it was thought to be similar to the formation of Cp2TiF(C6F5) from Cp2Ti(C6F5)2.14 Fundamental pioneering work in C−F bond activation and cleavage was reported by Jones et al. Preliminary studies showed the role of zirconocene hydride complexes such as [Cp2ZrH2]2 and Cp3ZrH in C−F activations.15a The dimeric zirconocene dihydride formed with hexafluorobenzene at room temperature Cp2Zr(C6F5)F, Cp2ZrF2, C6F5H, and H2. Performing the same reaction with Cp3ZrH as the hydride source gave a mixture of CpH, Cp2Zr(C6F5)F, C6F5H, Cp2ZrF2, Cp4Zr, and Cp3ZrF. Kinetic investigations revealed that the reaction of [Cp2ZrH2]2 follows a zero-order kinetic model, whereas in the case of Cp3ZrH it shows a first-order rate in the zirconocene hydride and hexafluorobenzene. Nevertheless, the concentration of C6F6 had no influence on the composition and ratio of the reaction outcome at all. In addition, [Cp2ZrH2]2 was reacted with pentafluorobenzene, giving a mixture of Cp2Zr(pC6F4H)F, Cp2ZrF2, C6F4H2, and H2 (Scheme 12). In detail, an initial association of the hexafluorobenzene and the zirconium center via a fluorine atom occurred, thus leading to loss of H2 or CpH and the actual C−F bond activation to form a Zr−F bond. Alternatively, the Zr−F and C−H bonds could be formed by σ-bond metathesis between the zirconium hydride and hexafluorobenzene. Additionally, the thermal decomposition of Cp2Zr(C6F5)2 in THF to produce Cp2Zr(C6F5)F and tetrafluorobenzyne was shown by the same authors.15b The tetrafluorobenzyne reacted with the solvent THF-d8 to give several products. Moreover, the presence of either furan or durene also yielded Cp2Zr(C6F5)F and Diels−Alder products of the dienes with tetrafluorobenzene. Performing the reaction with hexafluorobenzene gave linear chains of perfluoroarenes, e.g. perfluorobiphenyl, together with Cp2Zr(C6F5)F (Scheme 13). These observations indicated two independent mechanisms: one for Scheme 12

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Scheme 14

initiators and inhibitors did not affect the reaction rate or the product distribution. It was assumed that a nucleophilic attack of a hydride on hexafluorobenzene with subsequent fluoride elimination led to the formation of C6F5H and 1 equiv of Cp*2ZrHF. The second equivalent of Cp*2ZrHF and Cp*2Zr(C6F5)F was produced by a different, less elucidative pathway. As the initial step, one hydride was transferred from the zirconium center to one pentamethylcyclopentadienyl ring, followed by an attack on the fluorinated aryl ring, resulting in a zwitterionic intermediate. The second step involved fluoride ion abstraction and deprotonation of the diene ligand, generating Cp*2Zr(C6F5)H and HF. Subsequently, the HF could produce Cp*2ZrHF and H2 after reaction with additional Cp*2ZrH2. As an alternative mechanism the homolytic cleavage of a C−F bond by a hydrogen-depleted dimer such as [Cp*2ZrH]2 was also considered for the formation of Cp*2Zr(C6F5)H.15e Thereafter, the Cp*2ZrH2-mediated C−F activation chemistry was extended to fluorinated olefins by Eisenstein, Jones, and co-workers.15f Mixing Cp*2ZrH2 and perfluoropropene at −40 °C resulted in the formation of Cp*2ZrHF and (E)CF3CFCHF together with a small amount of Cp*2Zr(CH2CH2CH3)H and traces of undefined fluorinated species. This showed the selectivity of the fluorine−hydrogen exchange at the terminal sp2-C atom. The addition of a second equivalent of Cp*2ZrH2 caused only an increasing amount of Cp*2Zr(CH2CH2CH3)H. In this respect, performing the reaction with 8 equiv of Cp*2ZrH2 and perfluoropropene at room temperature led to a complete hydrodefluorination, giving a mixture of Cp*2ZrHF, Cp*2Zr(CH2CH2CH3)H, and a small amount of Cp*2ZrF2. Additionally, the Cp*2Zr(CH2CH2CH3)H would release n-propane to regenerate 1 equiv of the starting dihydride complex Cp*2ZrH2 under a hydrogen atmosphere (Scheme 15). DFT investigations of the reaction mechanism

of attack at the allylic C−F bonds by insertion of the olefin and subsequent β-fluoride elimination. The different mechanisms could be also observed experimentally, as Cp*2ZrHF solely reacted with the vinylic C−F bonds of perfluorocyclobutene to yield Cp*2ZrF2 and 3,3,4,4-tetrafluorocyclobutene but not with the allylic C−F bonds (Scheme 16). Performing the reactions

Scheme 15

The outcome is governed by the nature of the additional substituent X, i.e. its ability to act as a leaving group. In the case of the tosylate, the β,β-difluoro product was obtained solely, whereas the carbamate yielded both compounds. Only ethers as substituents gave the desired formation of (E)-1-fluorovinylzirconocene as a result of selective β-fluoride abstraction. The presence of a ligand Y at the zirconium center was assumed by NMR spectroscopic investigations concerning the generation of the fluorovinylzirconocene by observing three major doublets, which disappeared after acidic workup. This is the first example of vinylic C−F bond activation with a low-valent zirconocene thus achieving a stereoselective formation of (E)-1-fluorovinylzirconocene. Caulton et al. showed that the reaction of vinyl fluoride with Cp2ZrHCl to give Cp2ZrFCl and C2H4 as primary products proceeded similarly to the C−O cleavage of vinyl ethers.17 In a second step the liberated ethylene could react with additional Cp2ZrHCl to give Cp2Zr(CH2CH3)Cl (Scheme 18). In order to generalize the observed reactivity of fluorinesubstituted olefins, CH2CF2 was reacted with Cp2ZrHCl to yield CH2CHF, Cp2Zr(CH2CH3)Cl, and CH2CH2. Again, the products formed indicate that the first step gives Cp2ZrFCl and, in this case, vinyl fluoride, which can then react with a second

Scheme 16

with an excess of Cp*2ZrH2 and under an H2 atmosphere gave fully hydrodefluorinated cyclobutane and cyclopentane in moderate yields. Ichikawa and Minami reported on the activation of vinylic C−F bonds with low-valent zirconocene using the obtained species as a precursor for palladium-catalyzed cross-coupling reactions.16 The difluoroolefin reacts with in situ formed zirconocene, prepared from zirconocene dichloride and nbutyllithium, to give either the desired 1-fluorovinylzirconocene or the substituent X elimination product (Scheme 17). Scheme 17

supported a pathway with an internal insertion of the olefin and subsequent β-fluoride elimination. In order to gain additional insight into the mechanisms of the Cp*2ZrH2-promoted C−F bond activation, Jones et al. investigated vinylic C−F activation at perfluorinated cycloalkenes and corroborated their results with DFT calculations.15g The reaction of Cp*2ZrH2 with perfluorinated cyclic olefins produced Cp*2ZrHF and hydrodefluorinated products under mild conditions. The first C−F bond activation step occurred at the vinylic carbons of the cycloolefin on exchanging fluorine for hydrogen (H/F σ-bond metathesis). The second step consisted 1240


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Scheme 18

Scheme 19

Scheme 20

Scheme 21

imidozirconocene complex of the type rac-(ebthi)Zr(NHCMe3)Me (ebthi = 1,2-ethylene-1,1′-bis(η5-tetrahydroindenyl)).19 In order to prove the proposed intermediate [rac(ebthi)Zr(=NCMe3)], the addition of pyridine resulted in the formation of rac-(ebthi)Zr(NCMe3)(py). When this reaction was repeated with the less electron-donating ligand pentafluoropyridine, o-C−F activation was observed to give rac(ebthi)Zr{-N(CMe3)-o-NC6F4}(F), which also had been characterized by X-ray analysis (Scheme 20). Our group reported on the reaction of zirconocene bis(trimethysilyl)acetylene complexes of the type Cp′2Zr(L)(η2-Me3SiC2SiMe3) (Cp′2 = Cp2 with L = THF, pyridine; Cp′2 = rac-ebthi, no L) with a series of fluorinated pyridines and the competition between C−H and C−F bond activation, which could be readily compared with the findings of Beckhaus et al.11 regarding the influence of the metals titanium and zirconium.18 The zirconocene complexes Cp2Zr(L)(η2-Me3SiC2SiMe3) (L = THF, pyridine) and the ansa-bridged complex rac-(ebthi)Zr(η2Me3SiC2SiMe3) both reacted with 2,3,5,6-tetrafluoropyridine under C−H activation to give the 4-substituted pyridyl complexes with agostic alkenyl groups Cp′2Zr(4-C5NF4)[−C-

equiv of Cp2ZrHCl. Due to missing intermediates DFT calculations were performed to unravel the reaction mechanism. A σ-bond metathesis mechanism is unfavorable compared to a Zr−H addition across the CC bond, which is more likely to occur. This leads to a defined regiochemistry where the fluorine is located at the β-carbon atom of the fluoroethyl ligand. Subsequently, the β-fluoride elimination results in the formation of Cp2ZrFCl and CH2CH2. In addition, there is no η2-olefin intermediate of either ethylene or vinyl fluoride found on the reaction path by DFT studies. A photoassisted approach was presented by Berg et al., showing an intramolecular C−F activation at a zirconium center (Scheme 19).18 The key reaction step is the photolytic zirconium−benzyl bond cleavage at (C6F5NCH2CH2OCH2−)2Zr(CH2Ph)2 in toluene solution, resulting in dibenzyl formation and C−F activation at the 2position of one of the pentafluorophenyl groups of the tetradentate ligand. The mechanism for the formation of the two dinuclear complexes could not be revealed. Bergman and his group observed a C−F activation while investigating C−H bond activation reactions with an 1241


Organometallics

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(SiMe3)=CH(SiMe3)] (Cp′2 = Cp2, rac-ebthi). Similarly, the conversion of 2,3,4,6-tetrafluoropyridine yielded after C−H cleavage the tetrafluoropyridyl complex with an agostic alkenyl group; in the case of the ebthi ligand two isomers were observed (Scheme 21). Moreover, the reaction of the zirconocene complex Cp2Zr(py)(η2-Me3SiC2SiMe3) with 3-chloro-2,4,5,6-tetrafluoropyridine resulted after dissociation of pyridine and bis(trimethylsilyl)acetylene in an oxidative addition of the C−Cl bond to the zirconocene center to form Cp2Zr(3-C5NF4)Cl.20d In addition, mixing of Cp2Zr(py)(η2-Me3SiC2SiMe3) and pentafluoropyridine gave a C−F activation at the 4-position instead of the expected attack at the 2-position in analogy to the titanocene reactions (Scheme 22).11 These findings underline

showed two other compounds, an ion pair resulting from a hydride transfer to the boron and a C−F activation product, in which a Zr−C bond was formed in an ortho position of one pentafluorophenyl ring and a hydride as bridging unit between zirconium and boron. However, the ion pair species could be regarded as an intermediate, because it could be reacted either to rac-(ebthi)ZrF2 by thermolysis or to the other compounds by addition of B(C6F5)3. This finding is a rare example of a nucleophilic aromatic substitution on B(C6F5)3. Together with the reaction of rac-(ebthi)ZrF2 with iBu2AlH to form [rac(ebthi)Zr(μ-H)H]2 (vide infra), which is active in ethylene polymerization, the shown C−F activation has an impact on the understanding of possible deactivation pathways of such catalysts. Whereas migration of a pentafluorophenyl group from boron to zirconium gives an catalytically inactive complex, the rac-(ebthi)ZrF2 can be reactivated by the organoaluminum hydride compound.24 Schrock and co-workers investigated the pyridyldiamines [(2,6-X2C6H3NHCH2)2C(CH3)(2-C5H4N)] (X = Cl, F) and their related zirconium and hafnium complexes for 1-hexene polymerization.22 Within this study they found a C−F bond activation at the difluorophenyl moiety of the ligand (Scheme 24). In order to prepare [ArF2Npy]Hf(NMe2)2, Hf(NMe2)4 was reacted with the free ligand [ArF2Npy]H2, leading to unexpected compounds. These substances are characterized by the exchange of one or two o-fluorines of the 2,6difluorophenyl rings by dimethylamino groups. However, the observations were corroborated by the molecular structures obtained from X-ray diffraction analysis. Nevertheless, the fluorides bound to the hafnium center could be exchanged for chlorides by employing Me3SiCl. Unfortunately, the 1-hexene polymerization with the defluorination products as catalysts failed under the general conditions. Hafnium complex mediated C−F bond activations of aromatic, vinylic, and aliphatic fluorocarbons were described by Jones et al.23 The reactivity pattern of Cp*2HfH2 is similar to that of Cp*2ZrH2, giving Cp*2HfHF and Cp*2HfF2. However, to observe a reaction, elevated temperatures and longer reaction times were necessary. Nevertheless, for a number of substrates no conversion could be detected at all.

Scheme 22

the different reactivities of titanium and zirconium, which was demonstrated by either the preferences toward C−H and C−F activation or the different regioselectivities at pentafluoropyridine. Upon investigation of deactivation pathways of ethylene polymerization catalyst systems containing zirconocene complexes and tris(pentafluorophenyl)borane, it was found that C− F activation occurred at a C6F5 ring of B(C6F5)3.21 Heating [rac-(ebthi)Zr(μ-H)H]2 with B(C6F5)3 to 90 °C for 14 days yielded in a nearly quantitative formation of rac-(ebthi)ZrF2 and a small amount of a byproduct bearing a Zr-bound C6F5 group and a B(C6F5)2 group at the 2-position of one cyclopentadienyl ring (Scheme 23). Detailed investigations Scheme 23

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CATALYTIC C−F BOND ACTIVATIONS In contrast to stoichiometric C−F bond activation, the amount of catalytic reactions is very small in number. Pioneering work in this area of research was reported by Jones et al., including also a possibility of regenerating the active complex.15 A catalytic system for the room-temperature hydrodefluorination of pentafluoropyridine was presented by our group in 2006, which included the conversion of Zr−F by Al−H to give Al−F and Zr−H, whereby the latter reacted as the catalytically active species with transformation of C−F to C−H under regeneration of Zr−F.24 The formation of the Al−F bond as the driving force promoted the regeneration of the active zirconocene hydride species. The zirconocene difluoride complexes rac-(ebthi)ZrF2 and Cp2ZrF 2 were used as precatalysts and were transformed by diisobutylaluminium hydride to the active zirconocene dihydride catalysts of the type [rac-(ebthi)Zr(μ-H)H]2 and [Cp2Zr(μ-H)H]2 (Scheme 25). The results of catalytic reactions showed a turnover number of 67 after 24 h at room temperature, the hitherto known best performance for this hydrodefluorination reaction.25 The findings revealed the influence of the ligand type in the 1242


Organometallics

Review

Scheme 24

Scheme 25

Scheme 27.

can be seen that the chemist's analytical toolbox has been significantly improved over the last few years. Therefore, it is possible to obtain new research results which would have been neglected some years ago due to a lack in the available methodologies.

zirconocene complexes; higher yields were obtained with the ebthi ligand in comparison to those for the unsubstituted Cp ligand. Additionally, no significant influence could be noticed, whether the catalytic hydrodefluorination was started with zirconocene difluorides or dihydrides. This proved the idea of zirconocene dihydrides as catalytically active species. However, the hydrodefluorination of nonactivated C−F bonds could not be achieved. The corresponding chiral menthyl-substituted complexes (η5-menthyl-C5H4)2MF2 (M = Ti, Zr, Hf) could have a potential for C−F activation of prochiral fluorinated substrates to possibly give chiral hydrodefluorination products.26 Kühnel and Lentz reported on the catalytic hydrodefluorination of hexafluoropropene by Cp2TiF2 as precatalyst and a silane as the hydrogen source (Scheme 26).27 The reaction

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AUTHOR INFORMATION

Corresponding Author

*Tel: ++49-381-1281-176. Fax: ++49-381-1281-51176. E-mail: [email protected].

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ACKNOWLEDGMENTS We thank Dr. Torsten Beweries for fruitful discussions on this paper. Financial support by the DFG and the BMBF is gratefully acknowledged.

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DEDICATION This work is dedicated to Eluvathingal D. Jemmis on the occasion of his 60th birthday.

Scheme 26

REFERENCES

(1) (a) Perutz, R. N. Science 2008, 321, 1168. (b) Burdeniuc, J.; Jedlicka, B.; Crabtree, R. H. Chem. Ber./Recl. 1997, 130, 145. (c) Meier, G.; Braun, T. Angew. Chem. 2009, 121, 1575; Angew. Chem., Int. Ed. 2009, 48, 1546. (d) Amii, H.; Uneyama, K. Chem. Rev. 2009, 109, 2119. (e) Clot, E.; Eisenstein, O.; Jasim, N.; Macgregor, S. A.; McGrady, J. E.; Perutz, R. N. Acc. Chem. Res. 2011, 44, 333. (f) Braun, T.; Perutz, R. N. In Comprehensive Organometallic Chemistry III; Crabtree., R. H., Mingos, D. M. P., Eds.; Elsevier: Oxford, U.K., 2007; Chapter 1.26. (2) (a) Hiyama, T., Ed. Organofluorine Compounds Chemistry and Applications; Springer: New York, 2000. (b) Smart, B. E. In Organofluorine Chemistry: Principles and Commercial Applications; Banks, R. E., Smart, B. E., Tatlow, J. C., Eds.; Plenum Press: New York, 1994; Chapter 3, p 57. (3) Shine, K. P.; Sturges, W. T. Science 2007, 315, 1804. (4) Sun, A. D.; Love, J. A. Dalton Trans. 2010, 39, 10362. (5) (a) Maron, L.; Werkema, E. L.; Perrin, L.; Eisenstein, O.; Andersen, R. A. J. Am. Chem. Soc. 2005, 127, 279. (b) Werkema, E. L.; Messines, E.; Perrin, L.; Maron, L.; Eisenstein, O.; Andersen, R. A. J. Am. Chem. Soc. 2005, 127, 7781. (c) Werkema, E. L.; Andersen, R. A.; Maron, L.; Eisenstein, O. Dalton Trans. 2010, 39, 6648.

performance gave considerable turnover frequencies of up to 26 min−1 and turnover numbers of up to 125. Even 1,1,3,3,3pentafluoropropene and 3,3,3-trifluoropropene were hydrodefluorinated to the corresponding tetrafluoropropene and difluoropropene, respectively (Scheme 26). In principle the reaction follows a similar mechanism of the hydrodefluorination of pentafluoropyridine (Scheme 25) by formation of an in situ titanium hydride species (Scheme 27). In summary, over the past decade numerous examples of early transition metal complex mediated C−F bond activationseither on purpose or found accidentallyhave been reported. Some findings led to the establishment of catalytic systems or were one step behind to achieve this goal. Several contributions offer helpful insights into possible mechanisms and reaction pathways to get a better understanding of the essential tools for a successful C−F activation. Furthermore, it 1243


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