Activation of sp3 C-H bonds with cobalt(I): catalytic synthesis of enamines

C-H bond activation has been extensively studied with (Cp*)M(L)n (M = Ir, Rh), but cobalt, the third member of this triad, has not previously been shown to activate sp3 C-H bonds. Further, practical functionalization of the metal alkyl products of oxidative addition has not been fully explored. Toward these ends, we have developed catalytic dehydrogenation of alkyl amines with a Co(I) catalyst. Amine substrates are protected with vinyl silanes, followed by catalytic transfer hydrogenation, to yield a broad range of stable protected enamines and 1,2-diheteroatom-substituted alkenes, including several unprecedented heterocycles. (Cp*)Co(VTMS)2 catalyzes transfer hydrogenation under surprisingly mild conditions with high chemo-, regio-, and diastereoselectivity, while tolerating additional functionality.     

Spectral and structural characterization of amidate-bridged platinum-thallium complexes with strong metal-metal bonds

The reactions of [Pt(NH3)2(NHCOtBu)2] and TlX3 (X = NO3-, Cl-, CF3CO2-) yielded dinuclear [{Pt(ONO2)(NH3)2(NHCOtBu)}Tl(ONO2)2(MeOH)] (2) and trinuclear complexes [{PtX(RNH2)2(NHCOtBu)2}2Tl]+ [X = NO3- (3), Cl- (5), CF3CO2- (6)], which were spectroscopically and structurally characterized. Strong Pt-Tl interaction in the complexes in solutions was indicated by both 195Pt and 205Tl NMR spectra, which exhibit very large one-bond spin-spin coupling constants between the heteronuclei (1J(PtTl)), 146.8 and 88.84 kHz for 2 and 3, respectively. Both the X-ray photoelectron spectra and the 195Pt chemical shifts reveal that the complexes have Pt centers whose oxidation states are close to that of Pt(III). Characterization of these complexes by X-ray diffraction analysis confirms that the Pt and Tl atoms are held together by very short Pt-Tl bonds and are supported by the bridging amidate ligands. The Pt-Tl bonds are shorter than 2.6 Angstrom, indicating a strong metal-metal attraction between these two metals. Compound 2 was found to activate the C-H bond of acetone to yield a platinum(IV) acetonate complex. This reactivity corresponds to the property of Pt(III) complexes. Density functional theory calculations were able to reproduce the large magnitude of the metal-metal spin-spin coupling constants. The couplings are sensitive to the computational model because of a delicate balance of metal 6s contributions in the frontier orbitals. The computational analysis reveals the role of the axial ligands in the magnitude of the coupling constants.     

Mechanistic studies of ethylene hydrophenylation catalyzed by bipyridyl Pt(II) complexes

Cationic platinum(II) complexes [((t)bpy)Pt(Ph)(L)](+) [(t)bpy =4,4'-di-tert-butyl-2,2'-bipyridyl; L = THF, NC(5)F(5), or NCMe] catalyze the hydrophenylation of ethylene to generate ethylbenzene and isomers of diethylbenzene. Using ethylene as the limiting reagent, an 89% yield of alkyl arene products is achieved after 4 h at 120 °C. Catalyst efficiency for ethylene hydrophenylation is diminished only slightly under aerobic conditions. Mechanistic studies support a reaction pathway that involves ethylene coordination to Pt(II), insertion of ethylene into the Pt-phenyl bond, and subsequent metal-mediated benzene C-H activation. Studies of stoichiometric benzene (C(6)H(6) or C(6)D(6)) C-H/C-D activation by [((t)bpy)Pt(Ph-d(n))(THF)](+) (n = 0 or 5) indicate a k(H)/k(D) = 1.4(1), while comparative rates of ethylene hydrophenylation using C(6)H(6) and C(6)D(6) reveal k(H)/k(D) = 1.8(4) for the overall catalytic reaction. DFT calculations suggest that the transition state for benzene C-H activation is the highest energy species along the catalytic cycle. In CD(2)Cl(2), [((t)bpy)Pt(Ph)(THF)][BAr'(4)] [Ar' = 3,5-bis(trifluoromethyl)phenyl] reacts with ethylene to generate [((t)bpy)Pt(CH(2)CH(2)Ph)(η(2)-C(2)H(4))][BAr'(4)] with k(obs) = 1.05(4) × 10(-3) s(-1) (23 °C, [C(2)H(4)] = 0.10(1) M). In the catalytic hydrophenylation of ethylene, substantial amounts of diethylbenzenes are produced, and experimental studies suggest that the selectivity for the monoalkylated arene is diminished due to a second aromatic C-H activation competing with ethylbenzene dissociation.     

Heterolytic activation of H-X (X = H, Si, B, and C) bonds: an experimental and theoretical investigation

The highly electrophilic, coordinatively unsaturated, 16-electron [Ru(P(OH)3)(dppe)2][OTf]2 (dppe = Ph2PCH2CH2PPh2) complex 1 activates the H-H, the Si-H, and the B-H bonds, in H2(g), EtMe2SiH and Et3SiH, and H3B.L (L = PMe3, PPh3), respectively, in a heterolytic fashion. The heterolysis of H2 involves an eta2-H2 complex (observable at low temperatures), whereas the computations indicate that those of the Si-H and the B-H bonds proceed through unobserved eta1-species. The common ruthenium-containing product in these reactions is trans-[Ru(H)(P(OH)3)(dppe)2][OTf], 2. The [Ru(P(OH)3)(dppe)2][OTf]2 complex is unique with regard to activating the H-H, the Si-H, and the B-H bonds in a heterolytic manner. These reactions and the heterolytic activation of the C-H bond in methane by the model complex [Ru(POH)3)(H2PCH2CH2PH2)2][Cl][OTf], 4, have been investigated using computational methods as well, at the B3LYP/LANL2DZ level. While the model complex activates the H-H, the Si-H, and the B-H bonds in H2, SiH4, and H3B.L (L = PMe3, PPh3), respectively, with a low barrier, activation of the C-H bond in CH4 involves a transition state of 57.5 kcal/mol high in energy. The inability of the ruthenium complex to activate CH4 is due to the undue stretching of the C-H bond needed at the transition state, in comparison to the other substrates.     

Countercations direct one- or two-electron oxidation of an Al(III) complex and Al(III)-oxo intermediates activate C-H bonds

Hydrogen abstraction by aluminum(III)-oxo intermediates via reaction pathways reminiscent of late transition metal chemistry has been observed. Oxidation of M(+)[(IP(2-))(2)Al](-) (IP = iminopyridine, M = Na, Bu(4)N) yielded [Na(THF)(DME)][(IP(-))(IP(2-))Al(OH)] (3) or [(IP(-))(2)Al(OH)] (4), via O-atom transfer and subsequent C-H activation or proton abstraction, respectively.     

可溶过渡金属体系对饱和烃C-H键的活化与官能化

本文综述了饱和烃C-H键活化的新进展以及利用可溶过渡金属多氢化物对饱和烃C-H键进行活化、选择性催化官能化。     

Activation of C-H / H-H bonds by rhodium(II) porphyrin bimetalloradicals

Reactivity, kinetic, and thermodynamic studies are reported for reactions of a rhodium(II) bimetalloradical with H(2), and with the methyl C-H bonds for a series of substrates CH(3)R (R = H, CH(3), OH, C(6)H(5)) using a m-xylyl diether tethered diporphyrin ligand. Bimolecular substrate reactions involving the intramolecular use of two metalloradical centers and preorganization of the four-centered transition state (M*...X...Y*...M) result in large rate enhancements as compared to termolecular reactions of monometalloradicals. Activation parameters and deuterium kinetic isotope effects for the substrate reactions are reported. The C-H bond reactions become less thermodynamically favorable as the substrate steric requirements increase, and the activation free energy (DeltaG++) decreases regularly as DeltaG degrees becomes more favorable. An absolute Rh-H bond dissociation enthalpy of 61.1 +/- 0.4 kcal mol(-1) is directly determined, and the derived Rh-CH(2)R BDE values increase regularly with the increase in the C-H BDE.     

Catalytic coupling of haloolefins with anilides

A strategy in which C-H activation reactions promoted by Pd(II) have been combined with beta-heteroatom elimination to create a catalytic cycle achieving the arylation of haloacrylates is reported. The catalytic cycle can be subdivided into four parts: (1) C-H activation; (2) the functionalization step, migratory insertion of the olefin into a metal-carbon bond; (3) beta-heteroatom elimination; and (4) exchange of metal halide (if X = halogen) for a less coordinating anion. In this catalytic cycle, the oxidation state of the metal does not change, and an oxidant is not required. The method is more functional group tolerant compared with the existing alkene-arene coupling methods based on electrophilic C-H activation.     

Enzymatic functionalization of carbon-hydrogen bonds

The development of new catalytic methods to functionalize carbon-hydrogen (C-H) bonds continues to progress at a rapid pace due to the significant economic and environmental benefits of these transformations over traditional synthetic methods. In nature, enzymes catalyze regio- and stereoselective C-H bond functionalization using transformations ranging from hydroxylation to hydroalkylation under ambient reaction conditions. The efficiency of these enzymes relative to analogous chemical processes has led to their increased use as biocatalysts in preparative and industrial applications. Furthermore, unlike small molecule catalysts, enzymes can be systematically optimized via directed evolution for a particular application and can be expressed in vivo to augment the biosynthetic capability of living organisms. While a variety of technical challenges must still be overcome for practical application of many enzymes for C-H bond functionalization, continued research on natural enzymes and on novel artificial metalloenzymes will lead to improved synthetic processes for efficient synthesis of complex molecules. In this critical review, we discuss the most prevalent mechanistic strategies used by enzymes to functionalize non-acidic C-H bonds, the application and evolution of these enzymes for chemical synthesis, and a number of potential biosynthetic capabilities uniquely enabled by these powerful catalysts (110 references).     

Density functional study of the sigma and pi bond activation at the Pd=X (X = Sn, Si, C) bonds of the (H2PC2H4PH2)Pd=XH2 complexes. Is the bond cleavage homolytic or heterolytic?

The mechanism for the activation of the sigma bonds, the O-H of H2O, C-H of CH4, and the H-H of H2, and the pi bonds, the C[triple bond]C of C2H2, C=C of C2H4, and the C=O of HCHO, at the Pd=X (X = Sn, Si, C) bonds of the model complexes (H2PC2H4PH2)Pd=XH2 5 has been theoretically investigated using a density functional method (B3LYP). The reaction is significantly affected by the electronic nature of the Pd=X bond, and the mechanism is changed depending on the atom X. The activation of the O-H bond with the lone pair electron is heterolytic at the Pd=X (X = Sn, Si) bonds, while it is homolytic at the Pd=C bond. The C-H and H-H bonds without the lone pair electron are also heterolytically activated at the Pd=X bonds independent of the atom X, where the hydrogen is extracted as a proton by the Pd atom in the case of X = Sn, Si and by the C atom in the case of X=C because the nucleophile is switched between the Pd and X atoms depending on the atom X. In contrast, the pi bond activation of C[triple bond]C and C=C at the Pd=Sn bond proceeds homolytically, and is accompanied by the rotation of the (H2PC2H4PH2)Pd group around the Pd-Sn axis to successfully complete the reaction by both the electron donation from the pi orbital to Sn p orbital and the back-donation from the Pd dpi orbital to the pi orbital. On the other hand, the activation of the C=O pi bond with the lone pair electron at the Pd=Sn bond has two reaction pathways: one is homolytic with the rotation of the (H2PC2H4PH2)Pd group and the other is heterolytic without the rotation. The role of the ligands controlling the activation mechanism, which is heterolytic or homolytic, is discussed.     

Synthesis and characterization of amidate bridged dimeric and oligomeric platinum complexes having short platinum-platinum interactions

A number of pivalamidate bridged dinuclear [PtII2(RNH2)4(NHCOtBu)2]2+, [PtIII2LL (RNH2)4(NHCOtBu)2]n+ (2RNH2 = 2NH3, 1,2-ethylenediamine, 1,2-diaminocyclohexane; L, L' = NO3-, H2O, or ketonate), trinuclear [{PtII(dap)(NHCOtBu)2}2PdIII]3+ (dap = 1,2-diaminopropane), tetranuclear [{PtII2(NH3)2(DACH)(NHCOtBu)2}2]4+ (DACH = 1,2-diaminocyclohexane), pentanuclear [{Pt2(C5H7O)(NH3)2Cl2(NHCOtBu)2}2PtCl4], and hexanuclear [Pt2(NH3)2(en)(NHCOtBu)2Pt(NO2)4]2 platinum complexes containing Pt(II)-Pt(II), Pt(II)-Pt(III), Pt(II)-Pd(III), and Pt(III)-Pt(III) interactions have been prepared and structurally characterized. The Pt-Pt interactions are characteristic of covalent, dative, or orbital symmetric Pt-Pt bonds. The dimeric Pt(III) complexes are able to activate C-H bonds of ketones to afford ketonate platinum(III) complexes. The Pt-Pt bonds are either doubly amidate-bridged or ligand unsupported. Their distances are 2.99-3.22 A for Pt(II)-Pt(II), 2.59-2.72 A for Pt(III)-Pt(III), 2.98 A for Pt(II)-Pt(III), and 2.66 A for Pt(II)-Pd(III) bonds depending on the oxidation states of the two metals and the ancillary ligands.     

Catalytic addition of alkyne C-H, amine N-H, and phosphine P-H bonds to carbodiimides: an efficient route to propiolamidines, guanidines, and phosphaguanidines

Various metal complexes (e.g., lanthanides, early transition metals, and alkali metals) can serve as catalyst precursors for the catalytic addition of alkyne C-H, amine N-H, and phosphine P-H bonds to carbodiimides, to give a new family of propiolamidines, guanidines, and phosphaguanidines, some of which were difficult to prepare previously. The catalytic reaction proceeds generally through nucleophilic addition of an M-ER (E=CR(1)R(2), NR(1), PR(1)) bond, which is formed by an acid-base reaction between a catalyst precursor and a RE-H bond, to a carbodiimide compound, followed by protonolysis of the resultant amidinate-(phospha)guanidinate species "{R'NC(ER)NR'}M" with RE-H.     

Carbon-oxygen bond formation via organometallic Baeyer-Villiger transformations: a computational study on the impact of metal identity

Metal-mediated formation of C-O bonds is an important transformation that can occur by a variety of mechanisms. Recent studies suggest that oxygen-atom insertion into metal-hydrocarbyl bonds in a reaction that resembles the Baeyer-Villiger transformation is a viable process. In an effort to identify promising new systems, this study is designed to assess the impact of metal identity on such O-atom insertions for the reaction [(bpy)(x)M(Me)(OOH)](n) → [(bpy)(x)M(OMe)(OH)](n) (x = 1 or 2; bpy = 2,2'-bipyridyl; n is varied to maintain the d-electron count at d(6) or d(8)). Six d(8)-square-planar complexes (M = Pt(II), Pd(II), Ni(II), Ir(I), Rh(I), and Co(I)) and eight d(6)-octahedral systems (M = Ir(III), Rh(III), Co(III), Fe(II) Ru(II), Os(II), Mn(I), and Tc(I)) are studied. Using density functional theory calculations, the structures and energies of ground-state and transition-state species are elucidated. This study shows clear trends in calculated ΔG(++)'s for the O-atom insertions. The organometallic Baeyer-Villiger insertions are favored by lower coordination numbers (x = 1 versus x = 2), earlier transition metals, and first-row (3d) transition metals.     

Comparative reactivity of TpRu(L)(NCMe)Ph (L = CO or PMe3): impact of ancillary ligand l on activation of carbon-hydrogen bonds including catalytic hydroarylation and hydrovinylation/oligomerization of ethylene

Complexes of the type TpRu(L)(NCMe)R [L = CO or PMe3; R = Ph or Me; Tp = hydridotris(pyrazolyl)borate] initiate C-H activation of benzene. Kinetic studies, isotopic labeling, and other experimental evidence suggest that the mechanism of benzene C-H activation involves reversible dissociation of acetonitrile, reversible benzene coordination, and rate-determining C-H activation of coordinated benzene. TpRu(PMe3)(NCMe)Ph initiates C-D activation of C6D6 at rates that are approximately 2-3 times more rapid than that for TpRu(CO)(NCMe)Ph (depending on substrate concentration); however, the catalytic hydrophenylation of ethylene using TpRu(PMe3)(NCMe)Ph is substantially less efficient than catalysis with TpRu(CO)(NCMe)Ph. For TpRu(PMe3)(NCMe)Ph, C-H activation of ethylene, to ultimately produce TpRu(PMe3)(eta3-C4H7), is found to kinetically compete with catalytic ethylene hydrophenylation. In THF solutions containing ethylene, TpRu(PMe3)(NCMe)Ph and TpRu(CO)(NCMe)Ph separately convert to TpRu(L)(eta3-C4H7) (L = PMe3 or CO, respectively) via initial Ru-mediated ethylene C-H activation. Heating mesitylene solutions of TpRu(L)(eta3-C4H7) under ethylene pressure results in the catalytic production of butenes (i.e., ethylene hydrovinylation) and hexenes.     

Activation of arene C[bond]H bonds by a cationic hafnium silyl complex possessing an alpha-agostic Si[bond]H interaction

The reaction of Cp2Hf(SiMes2H)Me (1) with B(C6F5)3 produces zwitterionic Cp2Hf(eta2-SiHMes2)(mu-Me)B(C6F5)3 (2), which is stable for >8 h at -40 degrees C in toluene-d8. Spectroscopic data provide evidence for an unusual alpha-agostic Si-H interaction in 2. At room temperature, 2 reacts with the C-H bonds of aromatic hydrocarbons such as benzene and toluene to produce Cp2Hf(Ph)(mu-Me)B(C6F5)3 (3), isomers of Cp2Hf(C6H4Me)(mu-Me)B(C6F5)3 (4-6), and Cp2Hf(CH2Ph)(mu-Me)B(C6F5)3 (7), respectively. The reaction involving benzene is first-order in both 2 and benzene; rate = k[2][C6H6]. Mechanistic data including activation parameters (DeltaH = 19(1) kcal/mol; DeltaS = -17(3) eu), a large primary isotope effect of 6.9(7), and the experimentally determined rate law are consistent with a mechanism involving a concerted transition state for C-H bond activation.     

Stoichiometric and catalytic activation of P-H and P-P bonds

The abilities of transition metal species to activate P-H and P-P bonds are emerging. Such investigations provide novel M-P species, as well as stoichiometric and catalytic routes to P(III) compounds. The application of organometallic approaches and methodologies to phosphorus chemistry is providing emerging, stoichiometric and catalytic routes to phosphorus compounds and materials. This tutorial review surveys recent advances, with a focus on the activation of P-H and P-P bonds. The isolation of novel M-P species provides insight, while stoichiometric and catalytic reactivity expands the arsenal of synthetic strategies leading to P(III) compounds.     

A first principles study of oxygen reduction reaction on a Pt(111) surface modified by a subsurface transition metal M (M = Ni, Co, or Fe)

We have performed first-principle density functional theory calculations to investigate how a subsurface transition metal M (M = Ni, Co, or Fe) affects the energetics and mechanisms of oxygen reduction reaction (ORR) on the outermost Pt mono-surface layer of Pt/M(111) surfaces. In this work, we found that the subsurface Ni, Co, and Fe could down-shift the d-band center of the Pt surface layer and thus weaken the binding of chemical species to the Pt/M(111) surface. Moreover, the subsurface Ni, Co, and Fe could modify the heat of reaction and activation energy of various elementary reactions of ORR on these Pt/M(111) surfaces. Our DFT results revealed that, due to the influence of the subsurface Ni, Co, and Fe, ORR would adopt a hydrogen peroxide dissociation mechanism with an activation energy of 0.15 eV on Pt/Ni(111), 0.17 eV on Pt/Co(111), and 0.16 eV on Pt/Fe(111) surface, respectively, for their rate-determining O2 protonation reaction. In contrast, ORR would follow a peroxyl dissociation mechanism on a pure Pt(111) surface with an activation energy of 0.79 eV for its rate-determining O protonation reaction. Thus, our theoretical study explained why the subsurface Ni, Co, and Fe could lead to multi-fold enhancement in catalytic activity for ORR on the Pt mono-surface layer of Pt/M(111) surfaces.     

Synthesis of neutral (Pd(II), Pt(II)), cationic (Pd(II)), and water-induced anionic (Pd(II)) complexes containing new mesocyclic thioether-aminophosphonite ligands and their application in the Suzuki cross-coupling reaction

Mesocyclic thioether-aminophosphonite ligands, {-OC10H6(mu-S)C10H6O-}PNC4H8O (2a, 4-(dinaphtho[2,1-d:1',2'-g][1,3,6,2]dioxathiaphosphocin-4-yl)morpholine) and {-OC10H6(mu-S)C10H6O-}PNC4H8NCH3 (2b, 1-(dinaphtho[2,1-d:1',2'-g][1,3,6,2]dioxathiaphosphocin-4-yl)-4-methylpiperazine) are obtained by reacting {-OC10H6(mu-S)C10H6O-}PCl (1) with corresponding nucleophiles. The ligands 2a and 2b react with (PhCN)2PdCl2 or M(COD)Cl2 (M = Pd(II) or Pt(II)) to afford P-coordinated cis-complexes, [{(-OC10H6(mu-S)C10H6O-)PNC4H8X-kappaP}2MCl2] (3a, M = Pd(II), X = O; 3b, M = Pd(II), X = NMe; 4a, M = Pt(II), X = O; 4b, M = Pt(II), X = NMe). Compounds 2a and 2b, upon treatment with [Pd(eta3-C3H5)Cl]2 in the presence of AgOTf, produce the P,S-chelated cationic complexes, [{(-OC10H6(mu-S)C10H6O-)PNC4H8X-kappaP,kappaS}Pd(eta3-C3H5)](CF3SO3) (5a, X = O and 5b, X = NMe). Treatment of 2a and 2b with (PhCN)2PdCl2 in the presence of trace amount of H2O affords P,S-chelated anionic complexes, [{(-OC10H6(mu-S)C10H6O-)P(O)-kappaP,kappaS}PdCl2](H2NC4H8X) (6a, X = O and 6b, X = NMe), via P-N bond cleavage. The crystal structures of compounds 1, 2a, 2b, 4a, and 6a are reported. Compound 6a is a rare example of crystallographically characterized anionic transition metal complex containing a thioether-phosphonate ligand. Most of these palladium complexes proved to be very active catalysts for the Suzuki-Miyaura reaction with excellent turnover number ((TON), up to 9.2 x 10(4) using complex 6a as a catalyst).     

Evidence for the net addition of arene C-H bonds across a Ru(II)-hydroxide bond

TpRu(PMe3)2(OH) (1) reacts with C6D6 to initiate H/D exchange between the hydroxide ligand and the deuterated benzene. In addition, complex 1 catalyzes H/D exchange between H2O and C6D6. Mechanistic and computational studies suggest that a likely reaction pathway for the H/D exchange involves loss of PMe3 to produce {TpRu(PMe3)(OH)}, followed by the net addition of a benzene C-H(D) bond across the Ru-OH bond to form the putative complex TpRu(PMe3)(OH2)(Ph).     

Ru催化的C-H键的活化反应

概述了Ru催化的碳氢键的活化反应，包括C-H/烯烃，C-H/炔径和C-H/CO/烯烃 偶联反应，加氢酰化反应，硅化反应。