Electrophilic fluorination of cationic Pt-aryl complexes

Mechanism of the Aryl-F Bond-Forming Step from Bi(V) Fluorides

In this article, we describe a combined experimental and theoretical mechanistic investigation of the C(sp2)-F bond formation from neutral and cationic high-valent organobismuth(V) fluorides, featuring a dianionic bis-aryl sulfoximine ligand. An exhaustive assessment of the substitution pattern in the ligand, the sulfoximine, and the reactive aryl on neutral triarylbismuth(V) difluorides revealed that formation of dimeric structures in solution promotes facile Ar-F bond formation. Noteworthy, theoretical modeling of reductive elimination from neutral bismuth(V) difluorides agrees with the experimentally determined kinetic and thermodynamic parameters. Moreover, the addition of external fluoride sources leads to inactive octahedral anionic Bi(V) trifluoride salts, which decelerate reductive elimination. On the other hand, a parallel analysis for cationic bismuthonium fluorides revealed the crucial role of tetrafluoroborate anion as fluoride source. Both experimental and theoretical analyses conclude that C-F bond formation occurs through a low-energy five-membered transition-state pathway, where the F anion is delivered to a C(sp2) center, from a BF4 anion, reminiscent of the Balz-Schiemann reaction. The knowledge gathered throughout the investigation permitted a rational assessment of the key parameters of several ligands, identifying the simple sulfone-based ligand family as an improved system for the stoichiometric and catalytic fluorination of arylboronic acid derivatives.

Phosphorescent biaryl platinum(IV) complexes obtained through double metalation of dibenzoiodolium ions

The first series of neutral, tris-chelate, phosphorecent Pt(IV) complexes is reported, which combine two cyclometalated 2-arylpyridine ligands and a dimetalated biaryl. The introduction of biaryl ligands is achieved under mild conditions through the oxidative addition of dibenzoiodolium ions to Pt(II) precursors to give Pt(IV) intermediates with a singly metalated 2-(2-iodoaryl)aryl ligand, followed by the reductive metalation of the C-I bond. The modulation of emission characteristics via derivatization of both types of ligands is demonstrated.

Four- and five-coordinate nickel(ii) complexes bearing new diphosphine-phosphonite and triphosphine-phosphite ligands: catalysts for N-alkylation of amines

The reaction of Ph2PCH2OH with PhPCl2 and PCl3 in the presence of Et3N afforded new phosphonite compounds PhP(OCH2PPh2)21 and P(OCH2PPh2)32, respectively. The reaction between 1 and [NiCl2(DME)] in dichloromethane gave the five-coordinate complex [NiCl2(1-κ3 P,P,P)] 3. Conversely, 1 reacts with [NiCl2(DME)] in the presence of NH4PF6 in dichloromethane to yield the four coordinate ionic complex [NiCl(1-κ3 P,P,P)][PF6] 4. The reactions between 1, [NiCl2(DME)] and KPF6 in the presence of RNC (R = Xylyl, t Bu and iPr) in dichloromethane yielded the five coordinate monocationic [NiCl(1-κ3 P,P,P)(RNC)][PF6] (R = Xylyl) and dicationic [Ni(1-κ3 P,P,P)(RNC)2][PF6]2 (R = t Bu and iPr) complexes, respectively. The analogous reaction of 2 with [NiCl2(DME)] in the presence of KPF6 gave complex [NiCl(2-κ4 P,P,P,P)][PF6], 8. The structures of all complexes were determined by single crystal X-ray diffraction studies and supported by spectroscopic methods. To demonstrate their catalytic application, N-alkylation reactions between primary aryl amines, benzyl and 4-methoxy benzyl alcohols were found to proceed smoothly in the presence of 2.5 mol% of complexes bearing ligand 1 and <0.5 mmol of KOBu t in toluene at 140 °C. The C-N coupled products were formed in very good yields. Its substrate scope includes sterically encumbered, heterocyclic amines and aliphatic alcohol.

Fluorination Reactions at a Platinum Carbene Complex: Reaction Routes to SF3 , S(=O)F and Fluorido Complexes

The electron-rich Pt complex [Pt(IMes)2 ] (IMes: [1,3-bis(2,4,6-trimethylphenyl)-2-imidazolinylidine]) can be used as precursor for the syntheses of a variety of fluorido ligand containing compounds. The sulfur fluoride SF4 undergoes a rapid oxidative addition at Pt0 to yield trans-[Pt(F)(SF3 )(IMes)2 ]. A photolytic reaction of SF6 at [Pt(IMes)2 ] in the presence of IMes gave the fluorido complexes trans-[Pt(F)2 (IMes)2 ] and trans-[Pt(F)(SF3 )(IMes)2 ] along with trans-[Pt(F)(SOF)(IMes)2 ] and trans-[Pt(F)(IMes')(IMes)] (IMes': cyclometalated IMes ligand), the latter being products produced by reaction with adventitious water. trans-[Pt(F)(SOF)(IMes)2 ] and trans-[Pt(F)2 (IMes)2 ] were synthesized independently by treatment of [Pt(IMes)2 ] with SOF2 or XeF2 . A reaction of [Pt(IMes)2 ] with a HF source gave trans-[Pt(H)(F)(IMes)2 ], and an intermediate bifluorido complex trans-[Pt(H)(FHF)(IMes)2 ] was identified. Compound trans-[Pt(H)(F)(IMes)2 ] converts in the presence of CsF into trans-[Pt(F)(IMes')(IMes)].

Strategy for Selective Csp2-F and Csp2-Csp2 Formations from Organoplatinum Complexes

By changing the parameters of fluorination reaction of bisaryl-platinum(II) complexes, each possible competitive pathway of Ar-Ar and Ar-F formation can be selectively controlled. It was discovered that steric hindrance, type of fluorinating reagent, and temperature of reaction are determinants for Ar-F vs Ar-Ar bond formation pathway from bisaryl-fluoro-platinum(IV) complexes. The combination of bulky ligands such as mesityl with Selectfluor at RT leads to Ar-F bond formation in the presence of possible Ar-Ar formation.

A C^N Cycloplatinated(II) Fluoride Complex: Photophysical Studies and Csp3-F Bond Formation

This work reports the synthesis and characterization of a new C^N-based cycloplatinated(II) fluoride complex, [Pt(ppy)(PPh₃)F] (2; ppy = 2-phenylpyridinate), involving a Pt-F bond. The new complex is highly luminescent in the green area with a high quantum yield of 94.6% at 77 K. A comparison study of the heavier halogen derivatives reveals a descending emission quantum yield order of F > Cl > Br > I. Time-dependent density functional theory calculations ascribe the decreased emission efficiency to the decreasing trend of an intraligand (IL) transition from F to I, which accounts for the major radiative pathway. In addition, 2 is capable of the fluorinating alkyl halides, leading to C_sp³-F bond formation at room temperature.

Nickel(II/IV) Manifold Enables Room-Temperature C(sp3)-H Functionalization

This Article demonstrates a mild oxidatively induced C(sp³)-H activation at a high-valent Ni center. In contrast with most C(sp³)-H activation reactions at Ni^II, the transformation proceeds at room temperature and generates an isolable Ni^IV σ-alkyl complex. Density functional theory studies show two plausible mechanisms for this C-H activation process involving triflate-assisted C-H cleavage at either a Ni^IV or a Ni^III intermediate. The former pathway is modestly favored over the latter (by ∼3 kcal/mol). The Ni^IV σ-alkyl product of C-H cleavage reacts with a variety of nucleophiles to form C(sp³)-X bonds (X = halide, oxygen, nitrogen, sulfur, or carbon). These stoichiometric transformations can be coupled using N-fluoro-2,4,6-trimethylpyridinium triflate as a terminal oxidant in conjunction with chloride as a nucleophile to achieve a proof-of-principle Ni^II/IV-catalyzed C(sp³)-H functionalization reaction.

Aryl-Fluoride Bond-Forming Reductive Elimination from Nickel(IV) Centers

The treatment of pyridine- and pyrazole-ligated Ni^II σ-aryl complexes with Selectfluor results in C(sp²)-F bond formation under mild conditions. With appropriate design of supporting ligands, diamagnetic Ni^IV σ-aryl fluoride intermediates can be detected spectroscopically and/or isolated during these transformations. These studies demonstrate for the first time that Ni^IV σ-aryl fluoride complexes participate in challenging C(sp²)-F bond-forming reductive elimination to yield aryl fluoride products.

Impact of Oxidation State on Reactivity and Selectivity Differences between Nickel(III) and Nickel(IV) Alkyl Complexes

Described is a systematic comparison of factors impacting the relative rates and selectivities of C(sp³ )-C and C(sp³ )-O bond-forming reactions at high-valent Ni as a function of oxidation state. Two Ni complexes are compared: a cationic octahedral Ni^IV complex ligated by tris(pyrazolyl)borate and a cationic octahedral Ni^III complex ligated by tris(pyrazolyl)methane. Key features of reactivity/selectivity are revealed: 1) C(sp³ )-C(sp² ) bond-forming reductive elimination occurs from both centers, but the Ni^III complex reacts up to 300-fold faster than the Ni^IV , depending on the reaction conditions. The relative reactivity is proposed to derive from ligand dissociation kinetics, which vary as a function of oxidation state and the presence/absence of visible light. 2) Upon the addition of acetate (AcO^- ), the Ni^IV complex exclusively undergoes C(sp³ )-OAc bond formation, while the Ni^III analogue forms the C(sp³ )-C(sp² ) coupled product selectively. This difference is rationalized based on the electrophilicity of the respective M-C(sp³ ) bonds, and thus their relative reactivity towards outer-sphere S_N 2-type bond-forming reactions.

The Versatile Behavior of Platinum Alkyne Complexes towards XeF2 : Formation of Fluorovinyl and Fluorido Complexes

Reactions of platinum(0) tolane complexes, bearing a chelating ligand with P and N donor atoms, with the electrophilic fluorinating agent XeF₂ give facile access to platinum(II) β-fluorovinyl fluorido complexes. A series of new platinum(II) β-fluorovinyl complexes have been synthesized and were structurally characterized. Further oxidation with XeF₂ led to ortho-metalated platinum(IV) fluorido compounds. Additional reactions of platinum(0) tolane complexes, bearing a chelating P,P donor ligand, with XeF₂ led to a variety of fluorido and fluorovinyl complexes.

Oxidatively Induced C-H Activation at High Valent Nickel

This communication describes a series of oxidatively induced intramolecular arene C-H activation reactions of Ni^II model complexes to yield Ni^IV σ-aryl products. These reactions proceed within 10 min at room temperature, which represents among the mildest conditions reported for C-H cleavage at a Ni center. A combination of density functional theory and preliminary experimental mechanistic studies implicate a pathway involving initial 2e^- oxidation of the Ni^II starting materials by the F⁺ transfer reagent N-fluoro-2,4,6-trimethylpyridinium triflate followed by triflate-assisted C-H cleavage at Ni^IV to yield the products.

Experimental and Computational Assessment of Reactivity and Mechanism in C(sp(3))-N Bond-Forming Reductive Elimination from Palladium(IV)

This report describes a combined experimental and computational investigation of the mechanism of C(sp(3))-N bond-forming reductive elimination from sulfonamide-ligated Pd(IV) complexes. After an initial experimental assessment of reactivity, we used ZStruct, a computational combinatorial reaction finding method, to analyze a large number of multistep mechanisms for this process. This study reveals two facile isomerization pathways connecting the experimentally observed Pd(IV) isomers, along with two competing SN2 pathways for C(sp(3))-N coupling. One of these pathways involves an unanticipated oxygen-nitrogen exchange of the sulfonamide ligand prior to an inner-sphere SN2-type reductive elimination. The calculated ΔG(⧧) values for isomerization and reductive elimination with a series of sulfonamide derivatives are in good agreement with experimental data. Furthermore, the simulations predict relative reaction rates with different sulfonamides, which is successful only after considering competition between the proposed operating mechanisms. Overall, this work shows that the combination of experimental studies and new computational tools can provide fundamental mechanistic insights into complex organometallic reaction pathways.

Reactivity of platinum alkyne complexes towards N-fluorobenzenesulfonimide: formation of platinum compounds bearing a β-fluorovinyl ligand

The platinum(0) alkyne complexes [Pt(L)(η(2)-PhC[triple bond, length as m-dash]CPh)] 1-4 were synthesized by reactions of [Pt(cod)2] with diphenylacetylene and a phosphine ligand precursor (1: L = dcpe, 2: L = xantphos, 3: L = κ(2)-(P,N)-iPr2PC3H6NMe2, 4: L = κ(2)-(P,N)-iPr2PC2H4NMe2). Treatment of 1 or 4 with NFSI gave the complexes [Pt(F){N(SO2Ph)2}(dcpe)] (5) and [Pt(PhC[double bond, length as m-dash]CFPh){N(SO2Ph)2}{κ(2)-(P,N)-iPr2PC2H4NMe2}] (8), whereas the reactivity of 2 and 3 towards NFSI led to product mixtures. The compounds [Pt(F){N(SO2Ph)2}(xantphos)] (6a) as well as [Pt(PhC[double bond, length as m-dash]CFPh){N(SO2Ph)2}{κ(2)-(P,N)-iPr2PC2H4NMe2}] (7a) and [Pt(PhC[double bond, length as m-dash]CFPh)(F){κ(2)-(P,N)-iPr2PC2H4NMe2}] (7b) were clearly identified. Ligand exchange reactions at 8 resulted in the formation of the β-fluorovinyl platinum(ii) complexes [Pt(PhC[double bond, length as m-dash]CFPh){OC(O)CF3}{κ(2)-(P,N)-iPr2PC2H4NMe2}] (9), [Pt(PhC[double bond, length as m-dash]CFPh)(FHF){κ(2)-(P,N)-iPr2PC2H4NMe2}] (10) and [Pt(PhC[double bond, length as m-dash]CFPh)(F){κ(2)-(P,N)-iPr2PC2H4NMe2}] (11). Treatment of 8 with dihydrogen yielded the fluorinated olefin (Z)-(1-fluoroethene-1,2-diyl)dibenzene and [Pt{N(SO2Ph)2}(H){κ(2)-(P,N)-iPr2PC2H4NMe2}] (12).

Promoting C-C Bond Coupling of Benzyne and Methyl Ligands in Electron-Deficient (triphos)Pt-CH3+ Complexes

In situ generated benzyne reacts at room temperature with (triphos)Pt–CH₃⁺ to form a five-coordinate π-complex (2) that is isolable and stable in solution. Thermolysis of 2 at 60 °C generates (triphos)Pt(o-tolyl)⁺ (3), which is the product of formal migratory insertion of CH₃^– onto the coordinated benzyne. The reaction of 2 with the acid Ph₂NH₂⁺ yields toluene at room temperature over the course of 8 h, while the same reaction with 3 only proceeds to 40% conversion over 2 days. These data indicate that the protonolysis of 2 does not proceed by CH₃ migration onto benzyne to form 3 followed by protodemetalation. Instead, the data suggest either that protonation of 2 is first and is followed by H migration to yield a Pt^IVPh(Me) dication or that this latter species is generated by direct protonolysis of coordinated benzyne prior to reductive elimination of toluene.

Selective aryl-fluoride reductive elimination from a platinum(IV) complex

A difluoro(mesityl)platinum(IV) complex underwent highly selective reductive elimination of 2-fluoromesitylene upon heating in toluene. Kinetic analysis and DFT calculations suggest that the CF coupling involves a five-coordinate Pt(IV) transient intermediate resulting from the rate-limiting dissociation of the pyridine ligand.

Electrophilic halogenation-reductive elimination chemistry of organopalladium and -platinum complexes

CONSPECTUS: Transition metal-catalyzed organic transformations often reveal competing reaction pathways. Determining the factors that control the selectivity of such reactions is of extreme importance for the design of reliable synthetic protocols. Herein, we present the account of our studies over the past decade aimed at understanding the selectivity of reductive elimination chemistry of organotransition metal complexes under electrophilic halogenation conditions. Much of our effort has focused on finding the conditions for selective formation of carbon (aryl)-halogen bonds in the presence of competing C-C reductive elimination alternatives. In most cases, the latter was the thermodynamically preferred pathway; however, we found that the reactions could be diverted toward the formation of aryl-iodine and aryl-bromine bonds under kinetic conditions. Of particular importance was to maintain the complex geometry that prohibits C-C elimination while allowing for the elimination of carbon-halogen bonds. This was achieved by employing sterically rigid diphosphine ligands which prevented isomerization within a series of Pt(IV) complexes. It was also important to understand that the neutral M(IV) products often observed or isolated in the oxidative addition reactions are not necessarily the intermediates in the reductive elimination chemistry as it generally takes place from unsaturated species formed en route to relatively stable M(IV) complexes. While aryl-halide reductive elimination for heavier halogens can be competitive with aryl-aryl coupling in diaryl M(IV) complexes, the latter reaction always prevails over aryl-fluoride bond formation. Even when one of the aryl groups is a part of a rigid cyclometalated ligand C-C coupling is still the dominant reaction pathway. However, when one of the aryl groups is replaced with a phenolate donor aryl-F bond formation becomes preferred over C-O bond elimination. During our studies, other interesting reactions have been discovered. For example, the fluorination of the C(sp(3))-H bond can be very selective and compete favorably with C-C coupling. Also, in electron-poor complexes, metal oxidation can have higher energy than oxidation of the coordinated iodo ligand resulting in I-F elimination instead of the formation of aryl-I bond. Overall, electrophilic fluorination can lead to often very selective elimination reactions giving new C-C, C-I, C-F, or I-F bonds, with this selectivity dependent on the metal center, supporting ligands, complex geometry, and electrophilic fluorine source. Together with the many reports on the halogenation of organometallic compounds that appeared in recent years, our results contribute to understanding the requirements for selective transformations under electrophilic conditions and design of new synthetic methods for making organohalogen compounds.

Late-stage fluorination: fancy novelty or useful tool?

Charming fluorine: This Essay examines the recent surge in late-stage fluorination reactions and outlines challenges that need to be overcome to increase the impact of modern fluorination methods on the synthesis of complex organofluorine compounds. It is outlined how an improved understanding of the bonding interactions of fluoride could lead to a new class of mild fluorinating reagents and a range of functional-group-tolerant reactions.

Anion-Dependent Switch in C-X Reductive Elimination Diastereoselectivity

Reaction of a complex Pt organometallic species with electrophilic halogen sources in the presence of X- ligands changes the mechanism of reductive elimination from a concerted reductive coupling type to an SN2 type reductive elimination. In the absence of the added X- ligand the reductive elimination is stereoretentive; in its presence, the process is stereoinvertive. This selectivity hinges on the reactivity of a key five-coordinate Pt(IV) intermediate with the X- ligand.

Unprecedented 1,3-migration of the aryl ligand in metallacyclic aryl α-naphthyl Pt(IV) difluorides to produce β-arylnaphthyl Pt(II) complexes

Electrophilic fluorination of aryl α-naphthyl Pt(II) complexes leads to an unprecedented 1,3-migration of the aryl ligand to the β-position of the naphthyl group. The reaction proceeds via the initial oxidative addition of two fluoro ligands to the Pt center followed by C(sp(2))-C(sp(2)) coupling and aryl migration.

Introduction of fluorine and fluorine-containing functional groups

Over the past decade, the most significant, conceptual advances in the field of fluorination were enabled most prominently by organo- and transition-metal catalysis. The most challenging transformation remains the formation of the parent C-F bond, primarily as a consequence of the high hydration energy of fluoride, strong metal-fluorine bonds, and highly polarized bonds to fluorine. Most fluorination reactions still lack generality, predictability, and cost-efficiency. Despite all current limitations, modern fluorination methods have made fluorinated molecules more readily available than ever before and have begun to have an impact on research areas that do not require large amounts of material, such as drug discovery and positron emission tomography. This Review gives a brief summary of conventional fluorination reactions, including those reactions that introduce fluorinated functional groups, and focuses on modern developments in the field.

Copper-catalyzed, directing group-assisted fluorination of arene and heteroarene C-H bonds

We have developed a method for direct, copper-catalyzed, auxiliary-assisted fluorination of β-sp(2) C-H bonds of benzoic acid derivatives and γ-sp(2) C-H bonds of α,α-disubstituted benzylamine derivatives. The reaction employs a CuI catalyst, a AgF fluoride source, and DMF, pyridine, or DMPU solvent at moderately elevated temperatures. Selective mono- or difluorination can be achieved by simply changing reaction conditions. The method shows excellent functional group tolerance and provides a straightforward way for the preparation of ortho-fluorinated benzoic acids.

Catalytic enantioselective cyclization and C3-fluorination of polyenes

(Xylyl-phanephos)Pt(2+) in combination with XeF(2) mediates the consecutive diastereoselective cation-olefin cyclization/fluorination of polyene substrates. Isolated yields were typically in the 60-69% range while enantioselectivities reached as high as 87%. The data are consistent with a stereoretentive fluorination of a P(2)Pt-alkyl cation intermediate.