Inter- and intramolecular activation of aromatic C-H bonds by diphosphine and hydrido-bridged dinuclear iridium complexes

Reactions of [(Cp*Ir)2(mu-dmpm)(mu-H)2]2+ (1) with NaOtBu in aromatic solvent at room temperature give [(Cp*Ir)(H)(mu-dmpm)(mu-H)(Cp*Ir)(Ar)]+ [Ar = Ph (3), p-Tol (4a), m-Tol (4b), 2-furanyl (5a), 3-furanyl (5b)] via intermolecular aromatic C-H activation. Treatment of [(Cp*Ir)2(mu-dppm)(mu-H)2]2+ (2) with base (Et2NH) results in intramolecular C-H activation of the phenyl group in the dppm ligand to give [(Cp*Ir)(H){mu-PPh(C6H4)CH2PPh2}(mu-H)(Cp*Ir)]+ (6). The structures of 3, 5a, and 6 have been determined by X-ray diffraction methods.     

C-H activation on a diphosphine and hydrido-bridged diiridium complex: generation and detection of an active IrII-IrII species [(Cp*Ir)2(micro-dmpm)(micro-H)]+

Reaction of [Cp*Ir(micro-H)](2) (5) (Cp* = eta(5)-C(5)Me(5)) with bis(dimethylphosphino)methane (dmpm) gives a new neutral diiridium complex [(Cp*Ir)(2)(micro-dmpm)(micro-H)(2)] (3). Treatment of 3 with methyl triflate at -30 degrees C results in the formation of [(Cp*Ir)(H)(micro-dmpm)(micro-H)(Me)(IrCp*)][OTf] (6). Warming a solution of above 0 degrees C brings about predominant generation of 32e(-) Ir(II)-Ir(II) species [(Cp*Ir)(micro-dmpm)(micro-H)(IrCp*)][OTf] (7). Further heating of the solution of 7 up to 30 degrees C for 14 h leads to quantitative formation of a new complex [(Cp*Ir)(H)(micro-Me(2)PCH(2)PMeCH(2))(micro-H)(IrCp*)][OTf] (8), which is formed by intramolecular oxidative addition of the methyl C-H bond of the dmpm ligand. Intermolecular C-H bond activation reactions with 7 are also examined. Reactions of 7 with aromatic molecules (benzene, toluene, furan, and pyridine) at room temperature result in the smooth sp(2) C-H activation to give [(Cp*Ir)(H)(micro-dmpm)(micro-H)(Ar)(IrCp*)][OTf] (Ar = Ph (9); Ar = m-Tol (10a) or p-Tol (10b); Ar = 2-Fur (11)) and [(Cp*Ir)(H)(micro-dmpm)(micro-C(5)H(4)N)(H)(IrCp*)][OTf] (12), respectively. Complex also reacts with cyclopentene at 0 degrees C to give [(Cp*Ir)(H)(micro-dmpm)(micro-H)(1-cyclopentenyl)(IrCp*)][OTf] (13). Structures of 3, 8 and 12 have been confirmed by X-ray analysis.     

Bimetallic-induced tail-to-tail dimerization and C-H activation of methyl acrylate

An organometallic complex resulting from tail-to-tail dimerization and C-H activation of methyl acrylate (MA), [Mo(CO2Cp(eta 3-(MeO2C)CH[symbol: see text]CH[symbol: see text]CHCH2(CO2Me)] 2, has been fully characterized from the reaction of the heterobimetallic complex [Cp*Ni=Mo(mu-CO)(CO)2Cp] with MA and an exclusively eta 3-allyl bonding mode of the coupled ligand was established for the first time by X-ray diffraction; formation of 2 is accompanied by that of the mu 3-alkylidyne-capped cluster [NiMo2(mu 3-CCH2CO2Me)(CO)4Cp*Cp2] 3 which results from a double C-H activation of the CH2 group of MA; none of these reactions occur with the corresponding homodinuclear complexes.     

Variable coordination of amine functionalised N-heterocyclic carbene ligands to Ru, Rh and Ir: C-H and N-H activation and catalytic transfer hydrogenation

Chelating amine and amido complexes of late transition metals are highly valuable bifunctional catalysts in organic synthesis, but complexes of bidentate amine-NHC and amido-NHC ligands are scarce. Hence, we report the reactions of a secondary-amine functionalised imidazolium salt 2a and a primary-amine functionalised imidazolium salt 2b with [(p-cymene)RuCl(2)](2) and [Cp*MCl(2)](2) (M = Rh, Ir). Treating 2a with [Cp*MCl(2)](2) and NaOAc gave the cyclometallated compounds Cp*M(C,C)I (M = Rh, 3; M = Ir, 4), resulting from aromatic C-H activation. In contrast, treating 2b with [(p-cymene)RuCl(2)](2), Ag(2)O and KI gave the amine-NHC complex [(p-cymene)Ru(C,NH(2))I]I (5). The reaction of 2b with [Cp*MCl(2)](2) (M = Rh, Ir), NaO(t)Bu and KI gave the amine-NHC complex [Cp*Rh(NH(2))I]I (6) or the amido-NHC complex Cp*Ir(C,NH)I (7); both protonation states of the Ir complex could be accessed: treating 7 with trifluoroacetic acid gave the amine-NHC complex [Cp*Ir(C,NH(2))I][CF(3)CO(2)] (8). These are the first primary amine- or amido-NHC complexes of Rh and Ir. Solid-state structures of the complexes 3-8 have been determined by single crystal X-ray diffraction. Complexes 5, 6 and 7 are pre-catalysts for the catalytic transfer hydrogenation of acetophenone to 1-phenylethanol, with ruthenium complex 5 demonstrating especially high reactivity.     

Catalytic asymmetric benzylic C-H activation by means of carbenoid-induced C-H insertions

Tetrakis[N-[4-dodecylphenyl)sulfonyl]-(S)-prolinate]dirhodium [Rh(2)(S-DOSP)(4)]-catalyzed decomposition of methyl aryldiazoacetates in the presence of substituted ethylbenzenes results in benzylic C-H activation by means of a rhodium-carbenoid-induced C-H insertion. A Hammet study showed that positive charge buildup occurred on the benzylic carbon in the transition state of the C-H activation step. C-H activation of toluene and isopropylbenzene is possible, but a competing double cyclopropanation occurs with these substrates. The C-H activation is highly regioselective and enantioselective, and in certain cases, moderate diastereoselectivity is also possible.     

Key mechanistic features of enantioselective C-H bond activation reactions catalyzed by [(chiral mono-N-protected amino acid)-Pd(II)] complexes

Monoprotected chiral amino acids have recently been established as a class of ligand scaffolds for effecting Pd-catalyzed enantioselective C-H bond activation reactions. However, to elucidate the mechanistic details and controlling factors of these reactions, more comprehensive studies are needed. In this work we report computational investigations into the key mechanistic features of enantioselective C-H bond activation reactions catalyzed by a [chiral (mono-N-protected amino acid)-Pd(II)] complex. Structural analysis points to a C-H insertion intermediate in which the nitrogen atom of the ligand is bound as a neutral σ-donor. The formation of this C-H insertion intermediate could, in principle, proceed via a "direct C-H cleavage" or via "initial N-H bond cleavage followed by C-H cleavage". The computational studies presented herein show that the pathway initiated by N-H bond cleavage is more kinetically favorable. It is shown that the first step of the reaction is the N-H bond cleavage by the coordinated acetate group (OAc). In the next stage, the weakly coordinated OAc(-) (the second acetate group) activates the ortho-C-H bond of the substrate and transfers the H-atom from the C-atom to the bound N-atom of the ligand. As a result, a new Pd-C bond is formed and the carbamate is converted from X-type to L-type ligand. The absolute configuration of the products that are predicted on the basis of the calculated energies of the transition states matches the experimental data. The calculated enantioselectivity is also comparable with the experimental result. On the basis of these data, the origin of the enantioselectivity can be largely attributed to steric repulsions in the transition states.     

Theoretical analysis of the mechanism of palladium(II) acetate-catalyzed oxidative Heck coupling of electron-deficient arenes with alkenes: effects of the pyridine-type ancillary ligand and origins of the meta-regioselectivity

A systematic theoretical study is carried out on the mechanism for Pd(II)-catalyzed oxidative cross-coupling between electron-deficient arenes and alkenes. Two types of reaction pathways involving either a sequence of initial arene C-H activation followed by alkene activation, or the reverse sequence of initial alkene C-H activation followed by arene activation are evaluated. Several types of C-H activation mechanisms are discussed including oxidative addition, σ-bond metathesis, concerted metalation/deprotonation, and Heck-type alkene insertion. It is proposed that the most favored reaction pathway should involve an initial concerted metalation/deprotonation step for arene C-H activation by (L)Pd(OAc)(2) (L denotes pyridine type ancillary ligand) to generate a (L)(HOAc)Pd(II)-aryl intermediate, followed by substitution of the ancillary pyridine ligand by alkene substrate and direct insertion of alkene double bond into Pd(II)-aryl bond. The rate- and regio-determining step of the catalytic cycle is concerted metalation/deprotonation of arene C-H bond featuring a six-membered ring transition state. Other mechanism alternatives possess much higher activation barriers, and thus are kinetically less competitive. Possible competing homocoupling pathways have also been shown to be kinetically unfavorable. On the basis of the proposed reaction pathway, the regioselectivity predicted for a number of monosubstituted benzenes is in excellent agreement with experimental observations, thus, lending further support for our proposed mechanism. Additionally, the origins of the regioselectivity of C-H bond activation is elucidated to be caused by a major steric repulsion effect of the ancillary pyridine type ligand with ligands on palladium center and a minor electronic effect of the preinstalled substituent on the benzene ring on the cleaving C-H bond. This would finally lead to the formation of a mixture of meta and para C-H activation products with meta products dominating while no ortho products were detected. Finally, the multiple roles of the ancillary pyridine type ligand have been discussed. These insights are valuable for our understanding and further development of more efficient and selective transition metal-catalyzed oxidative C-H/C-H coupling reactions.     

C-H activations at iridium(I) square-planar complexes promoted by a fifth ligand

In the presence of ligands such as acetonitrile, ethylene, or propylene, the Ir(I) complex [Ir(1,2,5,6-eta-C8H12)(NCMe)(PMe3)]BF4 (1) transforms into the Ir(III) derivatives [Ir(1-kappa-4,5,6-eta-C8H12)(NCMe)(L)(PMe3)]BF4 (L = NCMe, 2; eta2-C2H4, 3; eta2-C3H6, 4), respectively, through a sequence of C-H oxidative addition and insertion elementary steps. The rate of this transformation depends on the nature of L and, in the case of NCMe, the pseudo-first-order rate constants display a dependence upon ligand concentration suggesting the formation of five-coordinate reaction intermediates. A similar reaction between 1 and vinyl acetate affords the Ir(III) complex [Ir(1-kappa-4,5,6-eta-C8H12){kappa-O-eta2-OC(Me)OC2H3}(PMe3)]BF4 (7) via the isolable five-coordinate Ir(I) compound [Ir(1,2,5,6-eta-C8H12){kappa-O-eta2-OC(Me)OC2H3}(PMe3)]BF4 (6). DFT (B3LYP) calculations in model complexes show that reactions initiated by acetonitrile or ethylene five-coordinate adducts involve C-H oxidative addition transition states of lower energy than that found in the absence of these ligands. Key species in these ligand-assisted transformations are the distorted (nonsquare-planar) intermediates preceding the intramolecular C-H oxidative addition step, which are generated after release of one cyclooctadiene double bond from the five-coordinate species. The feasibility of this mechanism is also investigated for complexes [IrCl(L)(PiPr3)2] (L = eta2-C2H4, 27; eta2-C3H6, 28). In the presence of NCMe, these complexes afford the C-H activation products [IrClH(CH=CHR)(NCMe)(PiPr3)2] (R = H, 29; Me, 30) via the common cyclometalated intermediate [IrClH{kappa-P,C-P(iPr)2CH(CH3)CH2}(NCMe)(PiPr3)] (31). The most effective C-H oxidative addition mechanism seems to involve three-coordinate intermediates generated by photochemical release of the alkene ligand. However, in the absence of light, the reaction rates display dependences upon NCMe concentration again indicating the intermediacy of five-coordinate acetonitrile adducts.     

Intramolecular C-H activation reactions of molybdenacyclobutanes

Molybdenacyclobutane complexes can be prepared by reaction of Cp*Mo(NO)(=CHCMe3) (formed spontaneously by loss of neopentane from Cp*Mo(NO)(CH2CMe3)2 (1) under ambient conditions) with cyclic olefins, including cyclopentene, cycloheptene, and cyclooctene. The cyclopentene metallacyclobutane orients the metallacycle bridgehead protons cis to each other. A trans arrangement is observed for the cyclooctene congener, and both cis and trans orientations occur for the cycloheptene species. The reaction of 1 with cyclohexene initially forms a metallacyclobutane that then undergoes an allylic C-H activation to form an allyl-alkyl-coupled product with the concomitant loss of H2. The analogous allyl-alkyl structure is also observed as the thermodynamic product for the reactions of cycloheptene and cyclooctene with 1. The cyclooctene allyl-alkyl compound (5C) can be converted to an eta2-diene complex by heating with pyridine. Alternatively, heating of 5C in the presence of a variety of olefins displaces the diene ligand that can then be isolated in its unbound state.     

Theoretical study of reaction pathways for the rhodium phosphine-catalysed borylation of C-H bonds with pinacolborane

The reaction mechanism of the rhodium-phosphine catalysed borylation of methyl-substituted arenes using pinacolborane (HBpin) has been investigated theoretically using DFT calculations at the B3PW91 level. Factors affecting selectivity for benzylic vs. aromatic C-H bond activation have been examined. It was found that [Rh(PR3)2(H)] is the active species which oxidatively adds the C-H bond leading to an eta3-benzyl complex which is the key to determining the unusual benzylic regioselectivity observed experimentally for this catalyst system. Subsequent reaction with HBpin leads to a [Rh(PR3)2(eta3-benzyl)(H)(Bpin)] complex from which B-C reductive elimination provides product and regenerates the catalyst. The electrophilic nature of the boryl ligand assists in the reductive elimination process. In contrast to Ir(L)2(boryl)3-based catalysts, for which Ir(III)-Ir(V) cycles have been proposed, the Rh(I)-Rh(III) cycle is operating with the system addressed herein.     

C-C versus C-H activation and versus agostic C-C interaction controlled by electron density at the metal center

Based on the PCN ligand 2, a remarkable degree of control over C-C versus C-H bond activation and versus formation of an agostic C-C complex was demonstrated by choice of cationic [Rh(CO)(n)(C(2)H(4))(2-n)] (n=0, 1, 2) precursors. Whereas reaction of 2 with [Rh(C(2)H(4))(2)(solv)(n)]BF(4) results in exclusive C-C bond activation to yield product 5, reaction with the dicarbonyl precursor [Rh(CO)(2)(solv)(n)]BF(4) leads to formation of the C-H activated complex 9. The latter process is promoted by intramolecular deprotonation of the C-H bond by the hemilabile amine arm of the PCN ligand. The mixed monocarbonyl monoethylene Rh species [Rh(CO)(C(2)H(4))]BF(4) reacts with the PCN ligand 2 to give an agostic complex 7. The C-C activated complex 5 is easily converted to the C-H activated one (9) by reaction with CO; the reaction proceeds by a unique sequence of 1,2-metal-to-carbon methyl shift, agostic interaction, and C-H activation processes. Similarly, the C-C agostic complex 7 is converted to the same C-H activated product 9 by treatment with CO.     

An efficient low-temperature route to polycyclic isoquinoline salt synthesis via C-H activation with [Cp*MCl2]2 (M = Rh, Ir)

Bi-, tri-, and tetracyclic isoquinoline salts were readily synthesized in excellent yields at room temperature from readily available starting materials after three reaction steps. Aromatic C-H activation was first promoted by sodium acetate with [Cp*MCl2]2 (M = Rh, Ir) at room temperature to form cyclometalated compounds. Dimethylacetylenedicarboxylate was then found to insert into the metal-carbon bonds of the cyclometalated compounds. Finally, the insertion compounds underwent oxidative coupling to form the desired isoquinoline salts and regenerate [Cp*MCl2]2. All of the intermediate compounds following C-H activation, alkyne insertion, and oxidative coupling were fully characterized, including the determination of X-ray structures in several cases, and the results shed light on the overall mechanism. Moreover, it was possible to synthesize the isoquinoline salts from readily available starting materials using one-pot procedures; thus, this work provides a novel, efficient method for metal-mediated synthesis of heterocycles.     

Room temperature benzene C-H activation by a new [PSiP]Ir pincer complex

The synthesis and reactivity of coordinatively unsaturated Rh and Ir complexes supported by the new bis(phosphino)silyl pincer ligand [kappa(3)-(2-Cy(2)PC(6)H(4))(2)SiMe](-) ([Cy-PSiP](-)) are reported, including the first example of facile, room temperature intermolecular arene C-H bond activation mediated by a silyl pincer complex.     

Mechanistic analysis of iridium(III) catalyzed direct C-H arylations: a DFT study

In a recent experimental work the Ir complex [Ir(cod)(py)(PCy(3))](PF(6)) (that is, Crabtree's catalyst) has been shown to catalyze the C-H arylation of electron-rich heteroarenes with iodoarenes using Ag(2)CO(3) as base. For this process, an electrophilic metalation mechanism, (S(E)Ar) has been proposed as operative mechanism rather than the concerted metalation-deprotonation (CMD) mechanism, widely implicated in Pd-catalyzed arylation reactions. Herein we have investigated the C-H activation step for several (hetero)arenes catalyzed by a Ir(III) catalyst and compared the data obtained with the results for the Pd(II)-catalyzed C-H bond activation. The calculations demonstrate that, similar to Pd(II)-catalyzed reactions, the Ir(III)-catalyzed direct C-H arylation occurs through the CMD pathway which accounts for the experimentally observed regioselectivity. The transition states for Ir(III)-catalyzed direct C-H arylation feature stronger metal-C((arene)) interactions than those for Pd(II)-catalyzed C-H arylation. The calculations also demonstrate that ligands with low trans effect may decrease the activation barrier of the C-H bond cleavage.     

Novel intramolecular C-H bond activation in an iridium dppm complex

Reaction of TpIr(C(2)H(4))(2) (Tp = tris-pyrazolylborate) with various chelating phosphine ligands has been explored. Reaction with bis-diphenylphosphinoethane leads to complete displacement of the Tp ligand. With bis-diphenylphosphinomethane, an intramolecular proton transfer from the methylene bridge to the iridium center occurs to give an iridium hydride complex formally resulting from oxidative C-H bond activation. Reaction with 2,2-bis(diphenylphosphino)propane (dppip) affords an Ir(I) complex formulated as kappa(2)-TpIr(dppip). Protonation of this Ir(I) complex gives a six coordinate Ir(III) hydride species.     

C-H bond activation of hydrocarbons by an imidozirconocene complex

Monomeric imidozirconocene complexes of the type Cp2(L)Zr=NCMe3 (Cp = cyclopentadienyl, L = Lewis base) have been shown to activate the carbon-hydrogen bonds of benzene, but not the C-H bonds of saturated hydrocarbons. To our knowledge, this singularly important class of C-H activation reactions has heretofore not been observed in imidometallocene systems. The M=NR bond formed on heating the racemic ethylenebis(tetrahydro)indenyl methyl tert-butyl amide complex, however, cleanly and quantitatively activates a wide range of n-alkane, alkene, and arene C-H bonds. Mechanistic experiments support the proposal of intramolecular elimination of methane followed by a concerted addition of the hydrocarbon C-H bond. Products formed by activation of sp2 C-H bonds are generally more thermodynamically stable than those formed by activation of sp3 C-H bonds, and those resulting from reaction at primary C-H bonds are preferred over secondary sp3 C-H activation products. There is also evidence that thermodynamic selectivity among C-H bonds is sterically rather than electronically controlled.     

Ultrafast UV pump/IR probe studies of C-H activation in linear,cyclic, and aryl hydrocarbons

The photochemical C-H activation reactions of eta(3)-TpRh(CO)(2) (Tp = HB-Pz(3), Pz = 3,5-dimethylpyrazolyl) and CpRh(CO)(2) (Cp = C(5)H(5)) have been studied in a series of linear, cyclic, and aromatic hydrocarbon solvents on a femtosecond to microsecond time scale. These results have revealed that the structure of the hydrocarbon substrate affects the final C-H bond activation step, which is in accordance with the known preference of bond activation toward primary C-H sites. In the case of aromatic C-H activation, the reaction is divided into parallel channels involving sigma- and pi-solvated intermediates. Results for the analogous CpRh(CO)(2) molecule have shown that the coordination of the cyclopentadienyl ligand does not play a direct role in the dynamics of the reaction, in contrast to the C-H activation mechanism observed in eta(3)-TpRh(CO)(2) studies.     

Generating and dimerizing the transient 16-electron phosphinidene complex [Cp*Ir=PAr]: a theoretical and experimental study

The properties of the 16-electron phosphinidene complex [CpRIr=PR] were investigated experimentally and theoretically. Density functional theory calculations show a preferred bent geometry for the model complex [CpIr=PH], in contrast to the linear structure of [CpIr=NH]. Dimerization to give [[CpIr=PH]2] and ligand addition to afford [Cp(L)Ir=PH] (L=PH3, CO) were calculated to give compounds that were energetically highly favorable, but which differed from the related imido complexes. Transient 16-electron phosphinidene complex [Cp*Ir=PAr] could not be detected experimentally. Dehydrohalogenation of [Cp*IrCl2(PH2Ar)] in CH2Cl2 at low temperatures resulted in the novel fused-ring systems 17 (Ar=Mes*) and 20 (Ar=Mes), with dimeric [[Cp*Ir=PAr]2] being the likely intermediate. Intramolecular C-H bond activation induced by steric factors is considered to be the driving force for the irreversible formation of 17 and 20. ONIOM calculations suggest this arises because of the large steric congestion in [[Cp*Ir=PAr]2], which forces it toward a more reactive planar structure that is apt to rearrange.     

Sequential intermolecular aminopalladation/ortho-arene C-H activation reactions of N-phenylpropiolamides with phthalimide

A novel palladium-catalyzed intermolecular aminopalladation/C-H activation method for selectively synthesizing (E)-(2-oxindolin-3-ylidene)phthalimides has been developed. In the presence of Pd(OAc)2 and PhI(OAc)2, alkynes were difunctionalized with a phthalimide and an arene sp2 C-H bond to selectively synthesize (E)-(2-oxoindolin-3-ylidene)phthalimides, which products are of great potential pharmaceutical value products in many major therapeutic areas, such as oncology, inflammation, neurology, immunology, and endocrinology. To the best of our knowledge, the reaction serves as the first example of intermolecular aminopalladation/C-H activation reactions of alkynes.     

Palladium-catalyzed intramolecular C-H activation/C-C bond formation: a straightforward synthesis of phenanthridines

The palladium-catalyzed intramolecular C-H activation/C-C cross-coupling has been developed for a straightforward and efficient synthesis of phenanthridines. With Pd(OAc)(2) (4 mol %) as the catalyst, PCy(3) (8 mol %) as the ligand, and Cs(2)CO(3) as the base, this protocol was applied to synthesize a small library of phenanthridine derivatives in good yields in THF.