Electrophilic Fluorination of Group 10 Organometallic Complexes

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Electrophilic Fluorination of Group 10 Organometallic Complexes: Chemistry beyond Oxidative Addition Arkadi Vigalok* School of Chemistry, The Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel ABSTRACT: Utilization of electrophilic fluorinating reagents in latetransition-metal-mediated transformations is a rapidly growing area of research. Oxidation of organo-palladium or -platinum compounds with such reagents leads to complexes in a high oxidation state that can undergo reductive elimination reactions. Thus, electrophilic fluorination can be directed toward the selective formation of C
C or C
X bonds under conditions often complementary to those used in the reactions assisted by metals in low oxidation states. The nature of the ligands and complex geometry can influence the outcome of the reductive elimination step and can be particularly important in bondforming processes.

1. INTRODUCTION Unlike the more abundant organometallic platinum group complexes with heavier halides (Cl, Br, and I), molecular fluoro complexes were largely unexplored until the late 1990s.1,2 Unsurprisingly, the major pathways toward these compounds involved a nucleophilic exchange reaction with an appropriate source of the fluoride ion.1,3
6 Completion in such reactions is usually achieved by the removal of the leaving anion from the reaction mixture, typically as a silver(I) halide (eq 1). Significantly less attention was paid to the electrophilic fluorinations of organometallic group 10 complexes. Although such reactions are well-established for the synthesis of the high-oxidation-state transition-metal fluorides,7
10 the presence of a metal
carbon bond can potentially trigger new types of reactivity, beyond the formation of the oxidative addition product (Scheme 1). In addition to giving the neutral M(IV) species (path a), the initially formed unsaturated intermediate can undergo C
F elimination (path b) or isomerization followed by either C
F or C
Y elimination (path c). In this paper, we present an overview of the reactivity of the high-oxidation-state organometallic group 10 metal complexes formed under the electrophilic fluorination conditions.11 Ln M
X þ AgF f Ln M
F þ AgX

ð1Þ

2. OXIDATIVE FLUORINATION—EARLY STUDIES Elemental fluorine, halogen fluorides, and fluoroxenon derivatives were used to prepare several M(IV) complexes with multiple fluoro ligands (eqs 2 and 3).12
16 The inorganic materials were comprised largely of metal salts of the MF62
anion and were characterized by X-ray crystallography and 19F NMR spectroscopy. The crystal structure of the same anion, but r 2011 American Chemical Society

with XeF5+ as the counterion, was also reported.17 More recently, electrophilic fluorination was employed to prepare stable M(IV) alkyl complexes.18,19 M 0x MX4 þ ½F2  f M 0x MF6 þ X2

ð2Þ

M 0x MX6 þ ½F2  f M 0x MF6 þ X2

ð3Þ

0

M ¼ alkali metal; x ¼ 2 M 0 ¼ alkali-earth metal; x ¼ 1 Due to the lack of robust low-valent carbonyl precursors of the group 10 metal complexes, no electrophilic fluorination of complexes containing the metal
CO bond were reported. Such a reaction has been thoroughly studied with the neighboring group 9 metal carbonyl complexes.20
22 Oxidative addition of the electrophilic fluorine to these complexes became the first mechanistic studies of the kind in the late-transition-metal series (eq 4).22 It was proposed, in a similar system, that the initial trans-oxidative addition reaction takes place, in an analogy to the SN2-type mechanism at a metal center (Scheme 2). In the case of the coordinating acetonitrile, it was possible to observe and spectroscopically characterize the solvent adduct with the fluoro ligand in the axial position. In the noncoordinating CH2Cl2, only the product of the cis oxidative addition of two fluoro ligands was observed. The electrophilic fluorination of Ir(I) carbonyls also became the first reaction in a late-transition-metal series where the initial oxidative addition of the fluoro ligand led to the formation of another bond in the metal coordination sphere—CO insertion into the M
F bond—giving the fluoroacyl complexes Received: June 13, 2011 Published: August 16, 2011 4802

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Scheme 1. Possible Reactivity Pathways in the Electrophilic Fluorination of Organometallic Complexes

Scheme 2. Fluorination of Ir(I) Complexes with XeF2

Scheme 3. Fluorination of a Carbonyl Ligand in the Ir Coordination Sphere

Scheme 5. Electrophilic Fluorination/C
C Coupling in the Synthesis of M(II) Difluorides

Scheme 4. Reactivity of the Coordinated Fluoroacyl Group

(Scheme 3).22 More basic phosphine ligands facilitated the overall reaction, supporting the intermediacy of the oxidative addition step. In addition, the fluoroacyl group reacted with Lewis acids, such as BF3, regenerating the carbonyl ligand along with the BF4
anion. The reaction could be reversed by adding a Lewis base (Scheme 4). trans-½IrðCOÞXðPEt3 Þ2  þ XeF2 f ½IrðCOÞF2 XðPEt3 Þ2  ð4Þ

3. CARBON
CARBON BOND FORMATION In 2003, our group reported the first example of an electrophilic fluorination of organometallic Pd(II) and Pt(II) complexes.23 Dialkyl and diaryl complexes bearing bidentate diphosphine ligands reacted selectively with XeF2 under very mild conditions, giving the corresponding difluoro complexes as products of the C
C reductive elimination (Scheme 5). These previously unknown complexes were isolated in quantitative yields by simple precipitation from their CH2Cl2 solutions by added pentane. Although no intermediates were observed during the reaction even at low temperatures, it is likely that the oxidative addition to form a M(IV) species takes place in the first step. The viability of the oxidative addition step in the reaction of (R3P)2M(R)2 with XeF2 was established with the platinum complexes bearing monodentate phosphine ligands. In this reaction, quantitative 4803

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Organometallics Scheme 6. Preparation of Pt(IV) Difluoro Diaryl Complexes with Monodentante Phosphine Ligands

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Scheme 8. Different Reactivity Pathways for the Bi- And Monodentate Pt(II) Complexes in Electrophilic Iodination

Scheme 7. Preparation of Pt(IV) Difluoro Diaryl Complexes with Bidentante Phosphine Ligands

formation of thermally stable Pt(IV) difluoro complexes was observed (Scheme 6).24 The stability of these complexes toward C
C elimination was attributed to the high energy barrier for phosphine ligand dissociation. Such dissociation creates a free coordination site, which was found to be essential for the C
C coupling in this and similar Pt(IV) systems. It is worth noting that the phosphine ligand in the corresponding dichloro complexes was significantly more labile, making them capable of eliminating a biphenyl molecule under mild conditions. The reaction was inhibited by an added phosphine ligand. Considering the steric and electronic similarities between the complexes of the mono- and bidentate phosphine ligands, the difference in their reactivity toward XeF2 was noteworthy. Using Br2 as the reagent, followed by halide exchange with AgF, it was possible to isolate the Pt(IV) difluoro complexes of the bidentate ligands, testifying for their stability at room temperature (Scheme 7).25 Thus, it is highly likely that the C
C reductive elimination in Scheme 5 (M = Pt) takes place at the stages prior to the formation of a stable Pt(IV) difluoride. The initially formed cationic Pt(IV) intermediate I will have the fluoro ligand at the axial position of a square pyramidal structure. Its consequent isomerization to a more stable structure with the aryl group in the axial position, capable of C
C reductive elimination, depends on the rigidity of the phosphine framework, with the monodentate phosphines being significantly more reactive. In support of this notion, the monodentate diphosphine complexes of Pt(II) give stable Pt(IV) oxidative addition products upon reaction with I2 (Scheme 8a), while the corresponding complexes bearing bidentate phosphine ligands undergo facile Ar
I elimination (Scheme 8b). In both cases, the initial formation of the cationic Pt(IV) intermediates was established.26,27 Carbon
carbon reductive elimination from a transient Pd(IV) center was postulated by Michael et al. in the N-fluorobenzenesulfonimide (NFSI)-promoted competitive catalytic carboamidation

and aromatic solvent activation/C
C bond formation sequence.28 The competing transformations at the Pd(IV) species, formed in situ, are shown in Scheme 9. The role of the fluoro ligand in the proposed sequence was to assist the aromatic C
H bond activation to provide the aryl alkyl complex of Pd(IV).29 Closely related catalytic formation of C
N bonds under the electrophilic fluorination conditions has also recently appeared in the literature.30
32 Due to the extremely high reactivity of intermediate Pd(IV) complexes bearing phosphine ligands in C
C reductive elimination (Scheme 5), they could not be observed even at low temperatures. Unambiguous support for the participation of Pd(IV) intermediates in the C
C bond formation, triggered by the addition of “F+” to a Pd(II) precursor, was provided by Sanford and co-workers, who isolated the oxidative addition products of the reaction of L2Pd(Aryl)CF3 (L2 = bidentate nitrogen ligand) with a fluoropyridinium salt (Scheme 10).33,34 The stable Pd(IV) products were fully characterized, including by X-ray structure analysis. Although the geometry of the isolated Pd(IV) complex shows the aryl group in the axial position with the F and CF3 ligands in the same plane with the dinitrogenbased chelate, this arrangement is likely the result of rapid isomerization of the transient cationic intermediate that has the fluoro ligand in an axial position. Such isomerization, which requires the presence of an empty coordination site,35
38 is expected to be facile in the presence of the labile triflate group. The dissociation of the triflate ligand was proposed to take place prior to the Ar
CF3 reductive elimination step.34 By varying the nature of the chelating ligand, Sanford and co-workers were able to greatly accelerate the C
C reductive elimination reaction. For example, replacing the bipyridine-type ligand with tetramethylethylenediamine (tmeda) allowed the reaction to take place at room temperature instead of at 80 °C.

4. CARBON
HALOGEN BOND FORMATION Considering the importance of organofluorine compounds in the pharmaceutical and agrochemical industries,39 there is much current interest in the metal-mediated synthesis of these compounds.40
43 Electrophilic fluorination was successfully employed in the formation of C
F bonds in catalytic and stoichiometric transformations mediated by group 10 metals. The Sanford group reported the first catalytic fluorination of C
H bonds in the presence of a directing ligand.44 The reaction 4804

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Scheme 9. Proposed Competing C
H Activation/C
C Coupling and Hydroamidation Pathways Triggered by Electrophilic Fluorination

Scheme 10. Aryl
CF3 Coupling from Isolated Pd(IV) Fluoro Complexes

Scheme 11. Catalytic Fluorination of Phenylpyridine Derivatives

utilized a fluoropyridinium salt as a source of “F+” and Pd(OAc)2 as a catalyst (Scheme 11). Employment of triflamide-protected benzylamines instead of phenylpyridine derivatives, reported by Yu et al., allowed for the convenient access to various o-fluoro aromatic compounds via the postmodification of the amide directing group (Scheme 12).45 More recently, the Yu group reported the extension of catalytic electrophilic C
H fluorination to derivatives of benzoic acid.46 Using a (polyfluorinated)aryl amide auxiliary, the authors obtained the monofluorinated products with very high selectivity. Interestingly, replacing CH3CN as a solvent with PhCF3 led to the selective formation of difluorinated products. In both cases, addition of N-methyl-2-pyrrolidinone (NMP) improved the reaction yields (Scheme 13). The amide group could be hydrolyzed under base-catalyzed conditions, giving the mono- or difluorinated benzoic acid derivatives in high yields. Although the formation of Pd(IV) intermediates was postulated, the isolation of the Pd(IV) fluoro complexes by Ritter and Furuya was crucial in confirming their role in the formation of C
F bonds.47 The complexes were prepared by treating the cyclometalated Pd(II) benzo[h]quinolyl precursors with Selectfluor. Similar Pd(II) complexes with simple aryl ligands reacted with Selectfluor to give fluoroarenes without isolation of potential Pd(IV) intermediates.48 In addition to the cyclometalated

aryl group, the coordinating sulfonamide group also provided the stabilization of the Pd(IV) fluoro complexes and allowed for their unambiguous characterization. Alternatively, a pyridine molecule could be attached in place of the sulfonamide SdO ligand. Relative positioning of the ligands in the cationic Pd(IV) complexes was established by multinuclear 2D NMR experiments, which confirmed the mutual cis positioning of the fluoro ligand and aromatic carbon atom of the benzo[h]quinolyl group. Heating a series of these Pd(IV) compounds resulted in the selective reductive elimination of the C
F bond (Scheme 14). The extensive mechanistic studies suggested that the dissociation of the weakly coordinated ligand, oxygen atom of the sulfonamide moiety or pyridine, precedes the concerted three-centered C
F reductive elimination reaction, a notion supported by DFT calculations.49 The difluoro Pd(IV) complex, prepared using XeF2 as the source of electrophilic fluorine, was characterized by X-ray crystallography (Scheme 14) but was significantly less reactive due to the absence of a good leaving group. The formation of an unsaturated Pd(IV) intermediate was also proposed by our group to explain the competitive formation of Ar
I and Ar
F bonds in the reaction between (P
P)Pd(Ar)I and an N-fluoro-2,4,6-collidinium salt (Scheme 15).50 Ar
I elimination was the dominant pathway (ca. 90%) in the reaction. Using XeF2 instead of an “N
F+” reagent resulted in the quantitative formation of the C
I reductive elimination products (Scheme 16). The reaction scope also included the formation of C6F5
I, which is extremely unusual, considering the strong coordination of the pentafluorophenyl group to latetransition-metal centers. While the direct oxidative addition to a Pd(II) complex seems to be a likely first step in the observed sequence of transformation, we cannot discount the electrophilic attack at the coordinated iodine ligand. Such an attack, which has 4805

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Scheme 12. Catalytic Fluorination of Triflamide Derivatives

Scheme 13. Catalytic Selective Fluorination of Benzoic Acid Derivatives

Scheme 14. First Example of the Directly Observed C
F Elimination from a M(IV) Center

Scheme 15. Competitive Formation of C
F and C
I

precedence in synthetic organic chemistry,51 would lead to the formation of I
F capable of oxidative addition to the Pd(II) center (Scheme 17). The oxidative addition step would place the iodo ligand in the axial position, suitable for aryl iodide reductive elimination.

By adding XeF2 to a Pd(II) aryl fluoro complex, Sanford and Ball prepared a Pd(IV) complex bearing two fluoro ligands and one FHF ligand at the metal center (Scheme 18).52 The complex could be isolated and characterized by X-ray analysis. Interestingly, although its thermal decomposition only led to the 4806

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C
F elimination to produce the fluoroarene (Scheme 18). As added N-bromosuccinimide also led to C
F reductive elimination, the role of the oxidant most likely involves the activation of the HF2
ligand in the metal coordination sphere. To suppress the extremely rapid C
C elimination (Scheme 5), we prepared a series of Pt(II) diaryl complexes with one of the aryl groups being a part of a five-membered chelate ring. Their reaction with XeF2 afforded the oxidative addition products with two fluoro ligands in a position trans to the aryl groups (Scheme 19).53 The removal of one of the fluorides with Me3Si-OTf led to C
C coupling followed by ligand rotation and cyclometalation

formation of the diaryl, heating in the presence of an electrophilic fluorination reagent, “N
F” compounds or XeF2, resulted in Scheme 16. Selective Aryl Iodide Elimination under Electrophilic Fluorination Conditions

Scheme 17. Plausible Reaction Pathways in Aryl Iodide Elimination

Scheme 18. Aryl Fluoride Synthesis via the Pd(IV) Intermediate

Scheme 19. Electrophilic Fluorination of Cyclometalated Pt(II) Aryl Complexes

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Scheme 20. Selective Benzylic Ortho C
H Fluorination of a Pt(II) Mesityl Complex

at the other side of the ligand and HF elimination. The same product can be accessed directly by reacting the Pt(II) precursors with the N-fluoro-2,4,6-collidinium salt, in which case an unsaturated Pt(IV) intermediate was observed in solution (Scheme 19). Unexpectedly, the reaction between the mesityl Pt(II) complex and XeF2 afforded the product of selective monofluorination at one of the ortho positions of the mesityl group (Scheme 20). The fluorination of the C
H bond was confirmed by multinuclear NMR analysis as well as X-ray crystallography. The crystal structure of the PPh3 complex showed no interaction between the metal center and benzylic fluorine atom, and the chemical shift of this fluorine atom in the 19F NMR spectrum appears at a position very close to that of benzyl fluoride (
206.7 ppm). However, it is possible that such an interaction exists in the complex having a weakly bound pyridine ligand, as the chemical shift of the benzylic fluorine appears about 10 ppm downfield in comparison with the signal for the PPh3 analogue. This reaction represents the only example of C
H fluorination at the platinum center. Its mechanism likely involves the initial oxidative addition of two fluoro ligands to the Pt(II) center, followed by fluorine-assisted C
H activation. The final product is obtained either via C
F reductive elimination or SN2-type attack by a fluoride anion at the benzylic carbon ligand (Scheme 21). After this paper was submitted, Gagne et al. reported evidence for the formation of an analogue of the proposed Pt(IV) cyclometalated fluoro complex.54 The complex could be observed at low temperature as a byproduct of C(alkyl)
F bond formation under electrophilic fluorination conditions. The authors investigated a series of Pt(II) alkyl complexes of a pincer triphos ligand in their reactions with various sources of “F+”. Xenon difluoride was found to be a particularly useful reagent for the formation of alkyl fluorides, giving the products within minutes at 0 °C (Scheme 22). While the reactions worked well with the benzyl and sterically crowded carbocyclic ligands, simple alkyl groups gave significantly poorer yields of alkyl fluorides, with the β-H elimination being the major competing pathway. On the basis of the stereochemistry of the alkyl group in the product, which retains its configuration, and low-temperature experiments, the authors proposed the oxidative addition/reductive elimination sequence for the formation of the C(alkyl)
F bond. Interestingly, treating (Ph2PCH2CH2Ph2)Pt(Cyclohexyl)Cl with XeF2 gave a mixture of fluoro- and chlorocyclohexane, indicating the competing C-Halide elimination pathways in this platinum system.

Scheme 21. Proposed Mechanism for the Benzylic Ortho C
H Fluorination

Scheme 22. Formation of C
F Bonds in the Electrophilic Fluorination of Pt(II) Alkyl Complexes

4. SUMMARY In the past few years, the electrophilic fluorination of organometallic complexes of Pd(II) and Pt(II) has received a significant deal of attention. Unlike the case for early-transition-metal chemistry, in these complexes the initial oxidative addition of the fluoro ligand gives M(IV) species capable of selective reductive elimination chemistry. The use of electrophilic fluorine allowed for a mechanistic understanding of the C
C, C
F, and C
I reductive elimination steps in high-oxidation-state complexes and their development for various catalytic applications. From a mechanistic point of view, the oxidative addition step puts the fluoro ligand in the axial position of a square-pyramidal 4808

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Organometallics M(IV) complex, the consequent isomerization to a more stable intermediate often ensuing. Thus, this isomerization often determines the outcome of the reductive elimination step and must be considered in the design of catalytic processes that use the source of electrophilic fluorine. While several catalytic transformations leading to the formation of C
C and C
F bonds have been reported, it is likely that the potential synthetic applications of electrophilic fluorinating reagents in organometallic chemistry is only now unraveling. For example, a particularly interesting development would be the intermolecular selective catalytic fluorination of unactivated C
H bonds under electrophilic conditions.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ BIOGRAPHY

Arkadi Vigalok was born in 1970 in Kazan, Russia. He obtained his M.Sc. degree from Kazan State University in 1992. In 1994, he started his Ph.D. studies in the group of Prof. David Milstein at the Weizmann Institute of Science, Rehovot, Israel, where he received his formal organometallic education. For his Ph.D. work, he received the Schmidt Prize for the best Ph.D. thesis in chemistry in Israel. After graduation, he joined the group of Prof. Timothy Swager at MIT, as a Fulbright postdoctoral fellow, where he worked on metal-containing conducting polymers. Since 2002, he has been on the faculty of the School of Chemistry at Tel Aviv University. Last year, he received the Israel Chemical Society award for excellent young scientists. His research interests include the formation of carbon
halogen bonds, particularly carbon
fluorine bonds, mediated by highoxidation-state group 10 metal complexes. He also has interests in supramolecular chemistry and organic synthesis in aqueous media.

’ ACKNOWLEDGMENT I thank two generations of my students, who performed all the experimental work. I also thank my collaborators: Profs. Israel Goldberg (X-ray crystallography, Tel Aviv University) and Andrei N. Vedernikov (DFT studies, University of Maryland). Financial support received throughout the years from the Israel Science Foundation, US-Israel Binational Science Foundation, Germany-Israel Science Foundation, and Tel Aviv University is gratefully acknowledged.

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