Gold(I) complexes bearing a PNP-type pincer ligand: photophysical properties and catalytic investigations.

The synthesis and characterization of two dinuclear and five tetranuclear gold(I) complexes bearing the 2,6-bis(diphenylphosphinomethyl)pyridine diphosphane ligand (DPPMPY) are herein reported. The reaction between the dinuclear complexes, DPPMPY(AuCl)2 (1) or DPPMPY(AuBr)2 (2), with 1 or 2 equivalents of Ag salts yielded five tetranuclear gold(I) complexes, DPPMPY2Au4X2 (3-7), differing in the terminal ancillary ligands (X = Cl, Br, acetonitrile) and the counter ions (SbF6- or BF4-). The structures of complexes 1, 2, 3, and 5 were confirmed by single-crystal X-ray diffraction studies. The Au⋯Au distances found in complexes 3 and 5 are in the range of aurophilic interactions and the arrangement of the Au atoms varies from a linear arrangement in complex 3 to a zigzag arrangement in complex 5. The photophysical characterization of the compounds was performed both in solution and in the solid state. Very high emission quantum yields were observed for the acetonitrile complexes 4 and 6 in the solid state. The use of this family of gold(I) complexes as catalysts for lactone synthesis via oxidative heteroarylation of alkenes was investigated and yields up to ca. 65% were obtained. Dicationic halide complexes 3 and 5 showed a slight enhancement of the yield of the catalytic reaction, indicating that there is no influence of the counter ion employed on the reaction outcome. Luminescence techniques have been also used to follow the progress of the catalytic reaction.

Mechanistic Studies on the Synthesis of Pyrrolidines and Piperidines via Copper-Catalyzed Intramolecular C-H Amination.

We have recently developed a method for the synthesis of pyrrolidines and piperidines via intramolecular C-H amination of N-fluoride amides using [Tp x CuL] complexes as precatalysts [Tp x = tris(pyrazolyl)borate ligand and L = THF or CH3CN]. Herein, we report mechanistic studies on this transformation, which includes the isolation and structural characterization of a fluorinated copper(II) complex, [(TpiPr2OH)CuF] [TpiPr = hydrotris(3,5-diisopropylpyrazolyl)borate], pertinent to the mechanistic pathway. The effects of the nature of the Tp x ligand in the copper catalyst as well as of the halide in the N-X amides employed as reactants have been investigated both from experimental and computational perspectives.

Two Copper-Carbenes from One Diazo Compound.

Many transition-metal complexes MLn decompose diazo compounds N2═CR1R2 generating metal-carbenes LnM═CR1R2 which transfer the carbene group to other substrates, constituting an important tool in organic synthesis. All previous reports have shown that the CR1R2 fragment at the metal-carbene remains intact from the parent diazo compound. Herein we report the detection and isolation of a monosubstituted copper carbene where the CR1R2 ligand has undergone a modification from the initial diazo reagent. When TpMsCu(THF) (TpMs = hydrotris(3-mesityl)pyrazolylborate ligand) was reacted with N,N-diethyl diazoacetamide [N2═C(H)(CONEt2)], the stable copper carbene TpMsCu═C(H)(NEt2) was isolated, resulting from a decarbonylation process, with carbon monoxide being trapped as TpMsCu(CO). The simultaneous observation of products derived from the intramolecular carbene insertion reaction into C-H bonds demonstrates that the expected TpMsCu═C(H)(CONEt2) complex is also formed. Experimental data, DFT calculations, and microkinetic models allow us to propose that the latter undergoes CO loss en route to the former.

Copper-catalysed radical reactions of alkenes, alkynes and cyclopropanes with N-F reagents.

The mild generation of nitrogen-centred radicals from N-F reagents has become a convenient synthetic tool. This methodology provides access to the aminative difunctionalisation of alkenes and alkynes and the radical ring-opening of cyclopropanes, among other similar transformations. This review article aims to provide an overview of recent developments of such processes involving radical reactions and N-F reagents using copper-based catalysts.

Group 11 tris(pyrazolyl)methane complexes: structural features and catalytic applications.

Tris(pyrazolyl)methane ligands (Tpmx) have been for years a step behind their highly popular boron-anionic analogues, the tris(pyrazolyl)borate ligands (Tpx). However, in the last decade the development of new members of this family of ligands has boosted a number of contributions albeit their use in coordination chemistry. This fact has also triggered the application of metal-Tpmx complexes as catalysts for a range of organic transformations, particularly with group 11 metals. The main structural features of complexes containing the TpmxM (M = Cu, Ag, Au) unit and their success as catalysts in a variety of reactions under homogeneous or heterogeneous conditions are presented.

Copper-Catalyzed N-F Bond Activation for Uniform Intramolecular C-H Amination Yielding Pyrrolidines and Piperidines.

The dual function of the N-F bond as an effective oxidant and subsequent nitrogen source in intramolecular aliphatic C-H functionalization reactions is explored. Copper catalysis is demonstrated to exercise full regio- and chemoselectivity control, which opens new synthetic avenues to nitrogenated heterocycles with predictable ring sizes. For the first time, a uniform catalysis manifold has been identified for the construction of both pyrrolidine and piperidine cores. The individual steps of this new copper oxidation catalysis were elucidated by control experiments and computational studies, clarifying the singularity of the N-F function and characterizing the catalytic cycle to be based on a copper(I/II) manifold.

The Elusive Palladium-Diazo Adduct Captured: Synthesis, Isolation and Structural Characterization of [(ArNHC-PPh2 )Pd(η2 -N2 C(Ph)CO2 Et)].

The first example of a diazo palladium adduct is reported. The complexes [(ArNHC-PPh2 )M(η2 -N2 C(Ph)CO2 Et)] (M=Ni, 3; M=Pd, 4; ArNHC-PPh2 =3-(2,6-diisopropylphenyl)-1-[(diphenylphosphino)ethyl]imidazol-2-ylidene) were prepared by ligand exchange with styrene-coordinated precursors [(ArNHC-PPh2 )M(styrene)] (M=Ni, 1; M=Pd, 2). Complex 4 was fully characterized, including X-ray analyses; this constitutes the first example of a diazo adduct compound with palladium, thereby closing the gap between Groups 8 and 10 regarding this type of compounds.

Triazolylidene-Iridium Complexes with a Pendant Pyridyl Group for Cooperative Metal-Ligand Induced Catalytic Dehydrogenation of Amines.

Two iridium(III) complexes containing a C,N-bidentate pyridyl-triazolylidene ligand were prepared that are structurally very similar but differ in their pendant substituent. Whereas complex 1 contains a non-coordinating pyridyl unit, complex 2 has a phenyl group on the triazolylidene substituent. The presence of the basic pyridyl unit has distinct effects on the catalytic activity of the complex in the oxidative dehydrogenation of benzylic amines, inducing generally higher rates, higher selectivity towards formation of imines versus secondary amines, and notable quantities of tertiary amines when compared to the phenyl-functionalized analogue. The role of the pyridyl functionality has been elucidated from a set of stoichiometric experiments, which demonstrate hydrogen bonding between the pendant pyridyl unit and the amine protons of the substrate. Such Npyr ⋅⋅⋅H-N interactions are demonstrated by X-ray diffraction analysis, 1 H NMR, and IR spectroscopy, and suggest a pathway of substrate bond-activation that involves concerted substrate binding through the Lewis acidic iridium center and the Lewis basic pyridyl site appended to the triazolylidene ligand, in agreement with ligand-metal cooperative substrate activation.

Copper-Carbene Intermediates in the Copper-Catalyzed Functionalization of O-H Bonds.

Copper-carbene [Tp(x)Cu=C(Ph)(CO2Et)] and copper-diazo adducts [Tp(x)Cu{η(1)-N2C(Ph)(CO2Et)}] have been detected and characterized in the context of the catalytic functionalization of O-H bonds through carbene insertion by using N2=C(Ph)(CO2Et) as the carbene source. These are the first examples of these type of complexes in which the copper center bears a tridentate ligand and displays a tetrahedral geometry. The relevance of these complexes in the catalytic cycle has been assessed by NMR spectroscopy, and kinetic studies have demonstrated that the N-bound diazo adduct is a dormant species and is not en route to the formation of the copper-carbene intermediate.

Phototransformation of benzimidazole and thiabendazole inside cucurbit[8]uril.

The phototransformation of benzimidazole (BZ) and of the benzimidazole pesticide thiabendazole (TBZ) was investigated in aqueous solution in the absence and presence of the supramolecular host cucurbit[8]uril (CB8). ESI-MS and NMR reveal that both compounds form stable 1 : 2 host-guest complexes with CB8 (BZ2@CB8, TBZ2@CB8). The phototransformation of free BZ leads to dehydrodimerization, while for TBZ the photoreactivity leads to BZ, benzimidazole-2-carboximide and 2-acetylbenzimidazole. Inside CB8, BZ undergoes photohydrolysis to form 2-aminoformanilide, while for TBZ2@CB8 additional photoproducts were observed which are pH dependent. At pH 1.2 photolysis of TBZ2@CB8 leads to new red-shifted photoproducts with extended π conjugation.

Regioselective allene hydrosilylation catalyzed by N-heterocyclic carbene complexes of nickel and palladium.

Regioselective methods for allene hydrosilylation have been developed, with regioselectivity being governed primarily by the choice of metal. Alkenylsilanes are produced via nickel catalysis with larger N-heterocyclic carbene (NHC) ligands, and allylsilanes are produced via palladium catalysis with smaller NHC ligands. These complementary methods allow either regioisomeric product to be obtained with exceptional regiocontrol.

Cu(I)-catalyzed atom transfer radical cyclization of trichloroacetamides tethered to electron-deficient, -neutral, and -rich alkenes: synthesis of polyfunctionalized 2-azabicyclo[3.3.1]nonanes.

A novel synthetic entry to 2-azabicyclo[3.3.1]nonanes based on a copper(I)-catalyzed intramolecular coupling of amino-tethered trichloroacetamides and unsaturated nitriles, esters and alkenes, as well as enol acetates, is described. A study of the reaction conditions and the scope of the process is reported.

Hydrotris(3-mesitylpyrazolyl)borato-copper(I) alkyne complexes: synthesis, structural characterization and rationalization of their activities as alkyne cyclopropenation catalysts.

The use of the bulky hydrotris(3-mesitylpyrazolyl)borate anionic ligand has allowed the synthesis of stable Tp(Ms)Cu(alkyne) complexes (alkyne = 1-hexyne, 1, phenylacetylene, 2, and ethyl propiolate, 3). The spectroscopic and structural features of these compounds and their relative reactivity have been examined, indicating the existence of a low π back-bonding from the copper(I) centre to the alkyne. Ligand exchange experiments have shown that terminal alkyne adducts are more stable than internal alkyne analogues. In good accordance with this, the previously reported alkyne cyclopropenation reaction catalysed by the Tp(x)Cu complexes can be rationalized and correlated with their relative stability.

Stable N-heterocyclic carbene (NHC)-palladium(0) complexes as active catalysts for olefin cyclopropanation reactions with ethyl diazoacetate.

The Pd(0) complexes [(NHC)PdL(n)] (NHC=N-heterocyclic carbene ligand; L=styrene for n=2 or PR(3) for n=1) efficiently catalyse olefin cyclopropanation by using ethyl diazoacetate (EDA) as the carbene source with activities that improve on previously described catalytic systems based on this metal. Mechanistic studies have shown that all of these catalyst precursors deliver the same catalytic species in solution, that is, [(IPr)Pd(sty)], a 14e(-) unsaturated intermediate that further reacts with EDA to afford [(IPr)Pd(=CHCO(2)Et)(sty)], from which the cyclopropane is formed.

Mechanistic and computational studies of the atom transfer radical addition of CCl4 to styrene catalyzed by copper homoscorpionate complexes.

Experimental as well as theoretical studies have been carried out with the aim of elucidating the mechanism of the atom transfer radical addition (ATRA) of styrene and carbon tetrachloride with a Tp(x)Cu(NCMe) complex as the catalyst precursor (Tp(x) = hydrotrispyrazolyl-borate ligand). The studies shown herein demonstrate the effect of different variables in the kinetic behavior. A mechanistic proposal consistent with theoretical and experimental data is presented.

An efficient, selective, and reducing agent-free copper catalyst for the atom-transfer radical addition of halo compounds to activated olefins.

Efficient and selective ATRA reactions of CCl(4), CBr(4), TsCl (Ts = tosyl), or Cl(3)CCO(2)Et with activated olefins (styrene, methyl methacrylate, n-butyl methacrylate, tert-butyl methacrylate) using the Tp(tBu)Cu(NCMe) complex as a catalyst have been achieved in the absence of any reductant and with low catalyst loadings.

Rediscovering copper-based catalysts for intramolecular carbon-hydrogen bond functionalization by carbene insertion.

A series of Tp(x)Cu complexes (Tp(x) = hydrotrispyrazolylborate ligand) have been tested as catalysts for the decomposition of several diazoacetates and N,N-disubstituted diazoacetamides and the subsequent formation of lactones and lactams, respectively. The complexes containing the ligands Tp(Br3) or Tp(Ms) have provided activities and selectivities for these transformations comparable with or, in some cases, better than the well-known rhodium catalyst Rh(2)(OAc)(4).

Synthesis, characterization, and reactivity of ruthenium diene/diamine complexes including catalytic hydrogenation of ketones.

Thermal reactions between [RuCl2(diene)]n (diene = 2,5-norbornadiene, nbd; 1,5-cyclooctadiene, cod) with an excess of N,N,N',N'-tetramethylethylene diamine (tmeda) afforded derivatives [RuCl2(diene)(tmeda)] (diene = nbd, 1; cod, 2) as a mixture of cis and trans isomers. When thermolysis was performed under H2 mixtures of hydride species [RuCl(H)(diene)(tmeda)] (diene = nbd, 3; cod, 4) and the bis-tmeda adduct trans-[RuCl2(tmeda)2] (5) were obtained in different ratios depending upon the reaction conditions and reaction times. Heating polymeric Ru(II) precursors in toluene in the presence of a 5-fold excess of the bulkier N,N,N',N'-tetraethylethylene diamine (teeda) resulted in a rare diamine dealkylation process with formation of trans-[RuCl2(nbd)(Et2NCH2CH2NHEt)] (6) and trans-[RuCl2(cod)(EtHNCH2CH2NHEt)] (7) in high yields. The presence of N-H functionalities in the coordinated diamine ligands of 6 and 7 was unambiguously established by single-crystal X-ray diffraction studies. The dealkylation process of the teeda ligand seems to proceed intramolecularly as shown by solution NMR studies performed with the soluble Ru(II) precursors trans-[RuCl2(amine)2(diene)] (diene = nbd, amine = morpholine, 9; diene = cod, amine = Et2NH, 10). The above complexes [RuCl2(diene)(diamine)] have been tested as precatalysts in the hydrogenation of ketones both for transfer as well as direct hydrogenation, the latter route being the most effective.

Copper-homoscorpionate complexes as active catalysts for atom transfer radical addition to olefins.

Cu(I) complexes containing trispyrazolylborate ligands efficiently catalyze the atom transfer radical addition (ATRA) of polyhalogenated alkanes to various olefins under mild conditions. The catalytic activity is enhanced when bulky and electron donating Tpx ligands are employed. Kinetic data have allowed the proposal of a mechanistic interpretation that includes a Cu(II) pentacoordinated species that regulates the catalytic cycle.

The carbene insertion methodology for the catalytic functionalization of unreactive hydrocarbons: no classical C-H activation, but efficient C-H functionalization.

This contribution intends to highlight the use of the metal-catalyzed functionalization of unreactive carbon-hydrogen bonds by the carbene insertion methodology, that employs diazo compounds as the carbene source.