Iridium-catalyzed conversion of alcohols into amides via oximes

[reaction: see text] The iridium catalyst [Ir(Cp*)Cl2]2 is effective for the rearrangement of oximes to furnish amides. The reaction has been combined with catalytic transfer hydrogenation between an alcohol and alkene to allow the conversion of alcohols into amides in a one-pot process.     

Development of more labile low electron count Co(I) sources: mild, catalytic functionalization of activated alkanes using a [(Cp*Co)2-μ-(η4:η4-arene)] complex

Catalytic transfer dehydrogenation of silyl protected amines, requiring sp(3) C-H bond activation, is mediated by a bridging arene complex of the type [(Cp*Co)(2)-μ-(η(4):η(4)-arene)] under mild conditions. Mechanistic and qualitative rate studies establish the compound as a more reactive Co(I) source when compared to other known Cp*Co(I) 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.     

sp2 C-H activation of dimethyl fumarate by a [(Cp*Co)2-μ-(η4 : η4-toluene)] complex

The well-defined oxidative addition of the vinylic sp(2) C-H bond of dimethyl fumarate is mediated by the cobalt triple decker complex [(Cp*Co)(2)-μ-(η(4) : η(4)-toluene)] (1) at ambient temperature, affording the dinuclear, bridging cobalt hydride, fumaryl compound (2). The C-H activation product has been characterized by mass spectrometry, NMR spectroscopy, and X-ray crystallography. Computational studies of 2 support asymmetric bonding interactions between the two metal centres and the bridging hydride/fumaryl fragments. Monitoring the reaction of dimethyl fumarate with 1 by (1)H NMR spectroscopy allows observation of intermediate [Cp*Co(MeO(2)CCH=CHCO(2)Me)](n) (n = 1 or 2) (3). Addition of 4 equivalents of dimethyl fumarate to 1 results in rapid formation of the bis(ligand) adduct Cp*Co(η(2)-MeO(2)CCH=CHCO(2)Me)(2) (5). Reversibility of the C-H activation was probed by reaction of additional dimethyl fumarate with 2, suggesting ligand induced reductive elimination is possible under ambient conditions. Reaction between 2 and strong σ or π ligands, such as PMe(3) or CO, affords the corresponding Cp*Co(η(2)-MeO(2)CCH=CHCO(2)Me)(L) (L = PMe(3) (7); L = CO (8)) complexes when heated, demonstrating the ability of 2 to undergo two electron redox processes. Further evidence for reversible C-H activation is provided by the isomerization of dimethyl maleate to the corresponding fumarate using 2, suggesting the complex can serve as a source of Co(I) under the appropriate catalytic conditions.     

Metal telluride clusters composed of niobocene carbonyl, telluride, and cobalt carbonyl units: syntheses, structures, and reactivity

Abstract: The reaction of [Cp#2NbTe2H] (1#; Cp# = Cp* (C5Me5) or Cp(x) (C5Me4Et)) with two equivalents of [Co2(CO)8] gives a series of cobalt carbonyl telluride clusters that contain different types of niobocene carbonyl fragments. At 0 degrees C, [Cp#2NbTe2CO3(CO)7] (2#) and [Co4Te2(CO)10] (3) are formed which disappear at higher temperatures: in boiling toluene a mixture of [cat2][Co9Te6(CO)8] (5#) (cat= [Cp#2Nb(CO)2]+) and [cat2][Co11Te7(CO)10] (6#) is formed along with [cat][Co(CO)4] (4#). Complexes 6# transform into [cat][Co11Te7(CO)10] (7#) upon interaction with HPF6 or wet SiO2. The molecular structures of 2(Cp(x)), 4(Cp(x)), 5(Cp*), 6(Cp*) and 7(Cp*) have been determined by X-ray crystallography. The structure of the neutral 2(Cp(x)) consists of a [Co3(CO)6Te2] bipyramid which is connected to a [(C5Me4Et)2Nb(CO)] fragment through a mu4-Te bridge. The ionic structures of 4(Cp(x)), 5(Cp*), 6(Cp*) and 7(Cp*) each contain one (4, 7) or two (5, 6) [Cp#2Nb(CO)2]+ cations. Apart from 4, the anionic counterparts each contain an interstitial Co atom and are hexacapped cubic cluster anions [Co9Te6(CO)8]2- (5) or heptacapped pentagonal prismatic cluster anions [Co11Te7(CO)10]n- (n=2: [6]2- , n=1: [7]-), respectively. Electrochemical studies established a reversible electron transfer between the anionic clusters [Co11,Te7(CO)10]- and [Co11Te7(CO)10]2in 6# and 7# and provided evidence for the existence of species containing [Co11Te7(CO),0] and [Co11Te7(CO)0]3-. The electronic structures of the new clusters and their relative stabilities are examined by means of DFT calculations.     

Organometallic dithiolene complexes of the group 8-10 metals: reactivities, structures and electrochemical behavior

Research progress in the organometallic dithiolene complexes such as [Cp(or Cp*)M(dithiolene)] (M = Co, Rh, Ir, Ni), [(C(6)R(6))Ru(dithiolene)] and [(C(4)R(4))Pt(dithiolene)] complexes during the past decade is described and the reactivities, structures and electrochemical behavior are summarized in this paper. The five-membered metalladithiolene ring (MS(2)C(2)) undergoes addition reactions to the M[double bond, length as m-dash]S bond to form 18-electron adducts by an imido, alkylidene, alkene or norbornene group and also undergoes dimerizations on the basis of the unsaturation in the ring. The aromaticity of the ring causes substitution reactions on the dithiolene carbon by a C-centered radical, S-centered radical or succinimide group when the ring has a C-H bond. Furthermore a dithiolene-dithiolene homo-coupling reaction by an acid or dithiolene-aryl cross-coupling occurs based on the aromaticity in the ring. Dissociations of the 18-electron adducts are observed by those thermolyses, photolyses, electrochemical redox reactions and other chemical reactions with tertiary phosphorus compounds. One representative example of them is the imido adduct dissociation with PR(3) under heating toward the intramolecular imido migration to a Cp ligand. Since all products are rearomatized by those adduct dissociations, it is concluded that the 'coexistence of aromaticity and unsaturation' in the metallacycle mediates the diverse chemical reactions.     

Comparison of the Reactivities and Selectivities of Group 9 [Cp*MIII] Catalysts in C−H Functionalization Reactions

Pentamethylcyclopentadienyl (Cp*)‐based Group 9 metal (Co, Rh, or Ir) catalysts have emerged as powerful tools for C?H functionalization reactions. Whilst a diverse range of organic transformations have been developed by using [Cp*M^III] catalysts, they have often exhibited orthogonal reactivities and varied selectivities that depended on the choice of the central metal atom. An understanding of the physicochemical properties of the metals, as well as of their reaction mechanisms, has led to significant expansion of the synthetic scope of C?H functionalization reactions. This Focus Review summarizes and discusses the comparative catalytic reactivities and selectivities of the [Cp*M^III] catalysts, with an emphasis on metal‐dependent pathway‐switching by considering the mechanistic rationale.     

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.     

Homogeneous catalysis with methane. A strategy for the hydromethylation of olefins based on the nondegenerate exchange of alkyl groups and sigma-bond metathesis at scandium

The scandium alkyl Cp*(2)ScCH(2)CMe(3) (2) was synthesized by the addition of a pentane solution of LiCH(2)CMe(3) to Cp*(2)ScCl at low temperature. Compound 2 reacts with the C-H bonds of hydrocarbons including methane, benzene, and cyclopropane to yield the corresponding hydrocarbyl complex and CMe(4). Kinetic studies revealed that the metalation of methane proceeds exclusively via a second-order pathway described by the rate law: rate = k[2][CH(4)] (k = 4.1(3) x 10(-4) M(-1)s(-1) at 26 degrees C). The primary inter- and intramolecular kinetic isotope effects (k(H)/k(D) = 10.2 (CH(4) vs CD(4)) and k(H)/k(D) = 5.2(1) (CH(2)D(2)), respectively) are consistent with a linear transfer of hydrogen from methane to the neopentyl ligand in the transition state. Activation parameters indicate that the transformation involves a highly ordered transition state (DeltaS++ = -36(1) eu) and a modest enthalpic barrier (DeltaH++ = 11.4(1) kcal/mol). High selectivity toward methane activation suggested the participation of this chemistry in a catalytic hydromethylation, which was observed in the slow, Cp*(2)ScMe-catalyzed addition of methane across the double bond of propene to form isobutane.     

Facile amine formation by intermolecular catalytic amidation of carbon-hydrogen bonds

A simple copper-based catalytic system has been developed for the carbon-hydrogen amidation reaction. The copper-homoscorpionate complex Tp(Br3)Cu(NCMe) catalyzes the transfer of the nitrene unit NTs (Ts = p-toluenesulfonyl) and its subsequent insertion into the sp(3) C-H bonds of alkyl aromatic and cyclic ethers or the sp(2) C-H bonds of benzene using PhI=NTs as the nitrene source, affording the corresponding trisubstitued NR(1)HTs amines in moderate to high yields. The use of the environmentally friendly chloramine-T has also proven effective, with the advantage that sodium chloride is formed as the only byproduct. A tandem, one-pot consecutive nitrene-carbene insertion system has been developed to yield amino acid derivatives.     

水相不对称转移氢化*

手性醇和胺是重要的精细化学品,不对称转移氢化是获得这类手性化合物有效、实用的途径之一。在众多的催化剂中,Noyori等发展的手性二胺与过渡金属钌TsDPEN-Ru（TsDPEN = 1,2-二苯基乙二胺）络合物是最有效的催化剂。近年来,随着化学家对绿色化学的日益重视,水作为绿色溶剂被广泛地用作为不对称催化转移氢化的反应介质,具有很高的反应活性、对映选择性和化学选择性。本文综述近年来应用未经修饰和修饰的手性二胺配体与过渡金属钌[(cymene)RuCl₂]₂、铑[(Cp*)RhCl₂]₂和铱[(Cp*)IrCl_2]2的络合物催化的水相中酮、亚胺和活化烯烃的不对称转移氢化的研究进展。     

The clusters [Ma(ECp*)b] (M=Pd, Pt; E=Al, Ga, In): structures, fluxionality, and ligand exchange reactions

The synthesis and structural characterization of the novel homoleptic cluster complexes [Pd2(GaCp*)2(mu2-GaCp*)3] (1c), [Pd3(GaCp*)4(mu2-GaCp*)4] (2b) and [Pd3(AlCp*)2(mu2-AlCp*)2(mu3-AlCp*)2] (3) (Cp*=C5Me5) are presented. Furthermore, ligand exchange reactions of these cluster complexes are explored. In contrast to the electronically and sterically saturated complexes [M(ECp*)4] (M=Ni, Pd, Pt), the new unsaturated analogues [M(a)(ER)b] (E=Al, Ga, In) react with a variety of typical ligands (Cp*Al, CO, phosphines, isonitriles) to give new di- and tri-substituted compounds like [Pt2(GaCp*)2(mu2-AlCp*)3] (1d), [PdPt(GaCp*)(PPh3)(mu2-GaCp*)3] (4b), or [Pd3(PPh3)3(mu2-InCp*)(mu3-InCp*)2] (8). The trends of the reactivity of [M(a)(ER)b] as well as their fluxional behavior in solution has been elucidated by NMR spectroscopy, resulting in a mechanistic rationale for the ligand exchange reactions as well as the fluxional processes.     

Activation of carbon-hydrogen bonds via 1,2-addition across M-X (X = OH or NH(2)) bonds of d(6) transition metals as a potential key step in hydrocarbon functionalization: a computational study

Recent reports of 1,2-addition of C-H bonds across Ru-X (X = amido, hydroxo) bonds of TpRu(PMe3)X fragments {Tp = hydridotris(pyrazolyl)borate} suggest opportunities for the development of new catalytic cycles for hydrocarbon functionalization. In order to enhance understanding of these transformations, computational examinations of the efficacy of model d6 transition metal complexes of the form [(Tab)M(PH3)2X]q (Tab = tris-azo-borate; X = OH, NH2; q = -1 to +2; M = TcI, Re(I), Ru(II), Co(III), Ir(III), Ni(IV), Pt(IV)) for the activation of benzene C-H bonds, as well as the potential for their incorporation into catalytic functionalization cycles, are presented. For the benzene C-H activation reaction steps, kite-shaped transition states were located and found to have relatively little metal-hydrogen interaction. The C-H activation process is best described as a metal-mediated proton transfer in which the metal center and ligand X function as an activating electrophile and intramolecular base, respectively. While the metal plays a primary role in controlling the kinetics and thermodynamics of the reaction coordinate for C-H activation/functionalization, the ligand X also influences the energetics. On the basis of three thermodynamic criteria characterizing salient energetic aspects of the proposed catalytic cycle and the detailed computational studies reported herein, late transition metal complexes (e.g., Pt, Co, etc.) in the d6 electron configuration {especially the TabCo(PH3)2(OH)+ complex and related Co(III) systems} are predicted to be the most promising for further catalyst investigation.     

Ambient-pressure hydrogenation of ketones and aldehydes by a metal-ligand bifunctional catalyst [Cp*Ir(2,2′-bpyO)(H2O)] without using base

An efficient catalytic system for hydrogenation of ketones and aldehydes using a Cp*Ir complex [Cp*Ir(2,2′-bpyO)(H₂O)] bearing a bipyridine-based functional ligand as catalyst has been developed. A wide variety of secondary and primary alcohols were synthesized by the catalyzed hydrogenation of ketones and aldehydes under facile atmospheric-pressure without a base. The catalyst also displays an excellent chemoselectivity towards other carbonyl functionalities and unsaturated motifs. This catalytic system exhibits high activity for hydrogenation of ketones and aldehydes with H₂ gas.     

Immobilization of chiral Rh catalyst on glass and application to asymmetric transfer hydrogenation of aryl ketones in aqueous media

A chiral catalyst, Cp＊RhTsDPEN （Cp＊ = pentamethyl cyclopentadiene, TsDPEN = substitutive phenylsulfonyl-l,2-diphenylethylenediamine）, was synthesized and immobilized at the surface of glass. The immobilized catalyst exhibited good catalytic efficiency for asymmetric transfer hydrogenation of aromatic ketones in water with HCOONa as hydrogen source.     

Mechanism of alkane transfer-dehydrogenation catalyzed by a pincer-ligated iridium complex

The mechanism of (PCP)Ir-catalyzed transfer-dehydrogenation has been elucidated for the prototypical substrate/acceptor couple, COA/TBE, at 55 degrees C (COA = cyclooctane; TBE = tert-butylethylene). The catalytic cycle may be viewed as the sum of two reactions: (i) hydrogenation of TBE by (PCP)IrH2 and C-H addition of a second mole of TBE to give (PCP)IrH(tert-butylvinyl), and (ii) dehydrogenation of COA by (PCP)IrH(tert-butylvinyl) to give (PCP)IrH2, COE, and TBE. These two stoichiometric reactions have been observed independently and their kinetics determined. The overall catalysis has also been monitored in situ, and (PCP)IrH2 and (PCP)IrH(tert-butylvinyl) have been observed as the resting states; the ratio of these two complexes is found to be proportional to [TBE]2. Based upon the proportionality constant thus obtained and the catalytic rate as a function of [TBE] (which reaches a maximum at ca. 0.3 M), the respective rate constants for the hydrogenation and dehydrogenation segments can be obtained. Good agreement is found between the rates independently obtained from stoichiometric and catalytic runs. Within the overall TBE-hydrogenation reaction, labeling experiments indicate that the rate-determining step is the reductive elimination of TBA (2,2-dimethylbutane) from (PCP)IrH(tert-butylethyl) (which is formed via insertion of TBE into an Ir-H bond of (PCP)IrH2). Based upon considerations of microscopic reversibility, it can be further inferred that the rate-determining step for the alkane dehydrogenations is C-H addition (and not beta-H elimination).     

Preparation, structure, and ethylene polymerization behavior of half-sandwich picolyl-functionalized carborane iridium, ruthenium, and rhodium complexes

The synthesis of half-sandwich transition-metal complexes containing the Cab(N) and Cab(N,S) chelate ligands (HCab(N) = HC2B10H10CH2C5H4N (1), LiCab(N,S) = LiSC2B10H10CH2C5H4N (4)) is described. Compounds 1 and 4 were treated with chloride-bridged dimers [{Ir(Cp*)Cl2}2] (Cp* = eta5-C5Me5), [{Ru(p-cymene)Cl2}2] and [{Rh(Cp*)Cl2}2] to give half-sandwich complexes [Ir(Cp*)Cl(Cab(N))] (2), [Ru(p-cymene)Cl(Cab(N))] (3), and [Rh(Cp*)Cl(Cab(N,S))] (5), respectively. Addition reaction of LiCab(S) (Cab(S) = SC2(H)B10H10) to the rhodium complex 5 yields [Rh(Cp*)(Cab(S))(Cab(N,S))] (6). All the complexes were characterized by IR and NMR spectroscopy, and by elemental analysis. In addition, X-ray structure analyses were performed on complexes 2, 3, 5, and 6, in which the potential C,N- and N,S-chelate ligands were found to coordinate in a bidentate mode. The carborane complex 2 shows catalytic activities up to 3.7x10(5) g PE mol(-1) Ir h(-1) for the polymerization of ethylene in the presence of methylaluminoxane (MAO) as cocatalyst. The polymer obtained from this homogeneous catalytic reaction has a spherical morphology. Catalytic activities and the molecular weight of polyethylene have been investigated for various reaction conditions.     

Facile and selective aliphatic C-H bond activation at ambient temperatures initiated by Cp*W(NO)(CH2CMe3)(eta(3)-CH2CHCHMe)

Thermolysis of Cp*W(NO)(CH2CMe3)(eta(3)-CH2CHCHMe) (1) at ambient temperatures leads to the loss of neopentane and the formation of the eta(2)-diene intermediate, Cp*W(NO)(eta(2)-CH2=CHCH=CH2) (A), which has been isolated as its 18e PMe3 adduct. In the presence of linear alkanes, A effects C-H activations of the hydrocarbons exclusively at their terminal carbons and forms 18e Cp*W(NO)(n-alkyl)(eta(3)-CH2CHCHMe) complexes. Similarly, treatments of 1 with methylcyclohexane, chloropentane, diethyl ether, and triethylamine all lead to the corresponding terminal C-H activation products. Furthermore, a judicious choice of solvents permits the C-H activation of gaseous hydrocarbons (i.e., propane, ethane, and methane) at ambient temperatures under moderately elevated pressures. However, reactions between intermediate A and cyclohexene, acetone, 3-pentanone, and 2-butyne lead to coupling between the eta(2)-diene ligand and the site of unsaturation on the organic molecule. For example, Cp*W(NO)(eta(3),eta(1)-CH2CHCHCH2C(CH2CH3)2O) is formed exclusively in 3-pentanone. When the site of unsaturation is sufficiently sterically hindered, as in the case of 2,3-dimethyl-2-butene, C-H activation again becomes dominant, and so the C-H activation product, Cp*W(NO)(eta(1)-CH2CMe=CMe2)(eta(3)-CH2CHCHMe), is formed exclusively from the alkene and 1. All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them have been established by X-ray crystallographic analyses. Finally, the newly formed alkyl ligands may be liberated from the tungsten centers in the product complexes by treatment with iodine. Thus, exposure of a CDCl3 solution of the n-pentyl allyl complex, Cp*W(NO)(n-C5H11)(eta(3)-CH2CHCHMe), to I2 at -60 degrees C produces n-C5H11I in moderate yields.     

Transfer hydrogenation of ketones catalyzed by 2,6‐bis(triazinyl)pyridine ruthenium complexes: The influence of alkyl arms

The transfer hydrogenation of ketones catalyzed by transition metal complexes has attracted much attention. A series of ruthenium(II) complexes bearing 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridine ligands (R-BTPs) were synthesized and characterized by NMR analysis and X-ray diffraction. These ruthenium(II) complexes were applied in the transfer hydrogenation of ketones. Their different catalytic activity were attributed to the alkyl arms on the 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridine. As the length of the alkyl arms rising, the catalytic activities of the complex catalysts decreased. By means of 0.4 mol % catalyst RuCl₂(PPh₃)(3-methylbutyl-BTP) in refluxing 2-propanol, a variety of ketones were reduced to their corresponding alcohols with >95% conversion over a period of 3 h. © 2019 Elsevier Science. All rights reserved.