Palladate Precatalysts for the Formation of C–N and C–C Bonds

Di(pyridin-2-yl)amino-substituted 1,10-phenanthrolines and their Ru(II)-Pd(II) dinuclear complexes: synthesis, characterization and application in Cu-free Sonogashira reaction

Dinuclear complexes bearing Ru(II) photoactive centers are of interest for the development of efficient dual catalysts for many photocatalyzed reactions. Ditopic polypyridine ligands, bis(pyridin-2-yl)amino-1,10-phenanthrolines, containing an additional coordination site (bis(pyridin-2-yl)amine, dpa) at positions 3, 4 or 5 of the 1,10-phenanthroline core (Phen-3NPy2, Phen-4NPy2 and Phen-5NPy2) were synthesized. They were used as bridging ligands to obtain dinuclear complexes [(bpy)2Ru(Phen-NPy2)PdCl2](PF6)2 (Ru(Phen-NPy2)Pd) in good yields via stepwise complexation. In these complexes Ru(II) is coordinated to 1,10-phenanthroline, while Pd(II) is bound to the dpa chelating moiety, as established by NMR spectroscopy and X-ray single crystal analysis. The influence of the position of dpa in the phenanthroline ring on the structural, optical and electrochemical properties of Ru(Phen-NPy2)Pd complexes was studied. The complexes exhibit photoluminescence in argon-saturated MeCN solution with maxima in the range of 615-625 nm, with emission quantum yields ranging from 0.11 to 0.15 for Ru(Phen-NPy2) complexes and from 0.018 to 0.026 for dinuclear Ru(Phen-NPy2)Pd complexes. All the complexes absorb visible light in the range of 370-470 nm with high extinction coefficients and can be considered useful as photocatalysts. The Ru2+/3+ potential in Ru(Phen-NPy2)Pd complexes showed no significant dependence on the dpa position, while the Pd2+/0 reduction potential was significantly lower for Ru(Phen-3NPy2)Pd and Ru(Phen-4NPy2)Pd, than for Ru(Phen-5NPy2)Pd (-0.57 V and -0.72 V vs. Ag/AgCl, KCl(sat.), respectively). The complexes were used as photoactivated precatalysts in Cu-free Sonogashira coupling under blue LEDs (12 W) irradiation. The reaction proceeded roughly three times faster when Ru(Phen-4NPy2)Pd and Ru(Phen-3NPy2)Pd were used as catalyst precursors compared to the mixed catalytic system Ru(bpy)3(PF6)2/(RNPy2)PdCl2.

Room-Temperature CuI-Catalyzed N-Arylation of Cyclopropylamine

A general and efficient CuI/N-carbazolyl-1H-pyrrole-2-carbohydrazide catalyst system was developed for the N-arylation of cyclopropylamine using aryl bromides at room temperature. Herein, 5 mol % CuI and 5 mol % of the ligand were used to synthesize N-aryl cyclopropylamines in moderate to excellent yields. This protocol was scaled up to produce the desired product at gram levels and has been generalized for C-N coupling between aryl bromides and amines at room temperature.

Highly efficient α-arylation of aryl ketones with aryl chlorides by using bulky imidazolylidene-ligated oxazoline palladacycles

α-Aryl derivatives of carbonyl compounds are important building blocks. Herein, we presented an efficient catalytic system for the α-arylation of aryl ketones with inactive aryl chlorides by firstly using N,N'-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr)-ligated chiral oxazoline palladacycles, and tolerated a wide range of substrates at low catalyst loadings, leading to the desired products in good to excellent yields.

A Second-Generation Palladacycle Architecture Bearing a N-Heterocyclic Carbene and Its Catalytic Behavior in Buchwald–Hartwig Amination Catalysis

Palladacyclic architectures have been shown as versatile motifs in cross-coupling reactions. NHC-ligated palladacycles possessing unique electronic and steric properties have helped to stabilize the catalytically active species and provide additional control over reaction selectivity. Here, we report on a synthetic protocol leading to palladacycle complexes using a mild base and an environmentally desirable solvent, with a focus on complexes bearing backbone-substituted N-heterocyclic carbene ligands. The readily accessible complexes exhibit high catalytic activity in the Buchwald–Hartwig amination. This is achieved using low catalyst loading and mild reaction conditions in a green solvent.

Realizing 1,1-Dehydration of Secondary Alcohols to Carbenes: Pyrrolidin-2-ols as a Source of Cyclic (Alkyl)(Amino)Carbenes

Herein we report secondary pyrrolidin-2-ols as a source of cyclic (alkyl)(amino)carbenes (CAAC) for the synthesis of CAAC-CuI -complexes and cyclic thiones when reacted with CuI -salts and elemental sulfur, respectively, under reductive elimination of water from the carbon(IV)-center. This result demonstrates a convenient and facile access to CAAC-based CuI -salts, which are well known catalysts for different organic transformations. It further establishes secondary alcohols to be a viable source of carbenes-realizing after 185 years Dumas' dream who tried to prepare the parent carbene (CH2 ) by 1,1-dehydration of methanol. Addressed is also the reactivity of water towards CAACs, which proceeds through an oxidative addition of the O-H bond to the carbon(II)-center. This emphasizes the ability of carbon-compounds to mimic the reactivity of transition-metal complexes: reversible oxidative addition and reductive elimination of the O-H bond to/from the C(II)/C(IV)-centre.

Synthesis and catalytic activity of palladium complexes bearing N-heterocyclic carbenes (NHCs) and 1,4,7-triaza-9-phosphatricyclo[5.3.2.1]tridecane (CAP) ligands

The synthesis and characterization of novel palladium complexes bearing N-heterocyclic carbenes (NHCs) and 1,4,7-triaza-9-phosphatricyclo[5.3.2.1]tridecane (CAP) are reported. These organometallic complexes can be easily obtained using two different synthetic strategies that involve either the substitution of the pyridine ligand from trans-[Pd(NHC)(Py)Cl2] or by simple addition of the CAP ligand to dimeric species [Pd(NHC)Cl2]2. The mixed NHC/CAP complexes were tested as pre-catalysts in the Buchwald-Hartwig aryl amination coupling, showing good catalytic activity, especially in the case of cis-[Pd(IPr)(CAP)Cl2].

N-Heterocyclic carbene complexes enabling the α-arylation of carbonyl compounds

The considerable importance of α-arylated carbonyl compounds, which are widely used as final products or as key intermediates in the pharmaceutical industry, has prompted numerous research groups to develop efficient synthetic strategies for their preparation in recent decades. In this context, the α-arylation of carbonyl compounds catalyzed by transition-metal complexes have been particularly helpful in constructing this motif. As illustrated in this contribution, tremendous advances have taken place using palladium- and nickel-NHC (NHC = N-heterocyclic carbene) complexes as pre-catalysts for the arylation of a wide range of ketones, aldehydes, esters and amides with electron-rich, electron-neutral, electron-poor, and sterically hindered aryl halides or pseudo-halides. Despite significant progress, especially in asymmetric α-arylations promoted by chiral NHC ligands, there are numerous challenges which have and continue to encourage further studies on this topic. Some of these are presented in this report.

The “weak base route” leading to transition metal–N-heterocyclic carbene complexes

N-Heterocyclic carbenes (NHCs) are nowadays ubiquitous in organometallic chemistry and catalysis. A simple synthetic route to these is presented.

Different effects of metal-NHC bond cleavage on the Pd/NHC and Ni/NHC catalyzed α-arylation of ketones with aryl halides

Fundamental differences in the behavior of Pd/NHC and Ni/NHC catalytic systems in ketones α-arylation were elucidated and exploited.

Benzyloxycalix[8]arene supported Pd–NHC cinnamyl complexes for Buchwald–Hartwig C–N cross-couplings

The synthesis of a Pd–NHC cinnamyl-complex supported on a calix[8]arene and its use in Buchwald–Hartwig amination is reported. Thanks to the support, the products were isolated with low levels of residual palladium, in some cases below standards.

A Mechanistically and Operationally Simple Route to Metal-N-Heterocyclic Carbene (NHC) Complexes

We have been puzzled by the involvement of weak organic and inorganic bases in the synthesis of metal-N-heterocyclic carbene (NHC) complexes. Such bases are insufficiently strong to permit the presumed required deprotonation of the azolium salt (the carbene precursor) prior to metal binding. Experimental and computational studies provide support for a base-assisted concerted process that does not require free NHC formation. The synthetic protocol was found applicable to a number of transition-metal- and main-group-centered NHC compounds and could become the synthetic route of choice to form M-NHC bonds.

Sterically enriched bulky 1,3-bis(N,N′-aralkyl)benzimidazolium based Pd-PEPPSI complexes for Buchwald–Hartwig amination reactions

A simple and efficient synthesis of a series of unexisting Pd-PEPPSI complexes is summarized. These complexes are exploited for their high catalytic activity towards Buchwald–Hartwig amination.

Anticancer Indole-Based Chalcones: A Structural and Theoretical Analysis

The crystal structures of five new chalcones derived from N-ethyl-3-acetylindole with different substituents were investigated: (E)-3-(4-bromophenyl)-1-(1-ethyl-1H-indol-3-yl)prop-2-en-1-one (3a); (E)-3-(3-bromophenyl)-1-(1-ethyl-1H-indol-3-yl)prop-2-en-1-one (3b); (E)-1-(1-ethyl-1H-indol-3-yl)-3-(4-methoxyphenyl)prop-2-en-1-one (3c); (E)-1-(1-ethyl-1H-indol-3-yl)-3-mesitylprop-2-en-1-one (3d); and (E)-1-(1-ethyl-1H-indol-3-yl)-3-(furan-2-yl)prop-2-en-1-one (3e). The molecular packing of the studied compounds is controlled mainly by C–H⋅⋅⋅O hydrogen bonds, C–H⋅⋅⋅π interactions, and π···π stacking interactions, which were quantitatively analyzed using Hirshfeld topology analysis. Using density functional theory (DFT) calculations, the order of polarity (3b ˂ 3d ˂ 3e ˂ 3a ˂ 3c) was determined. Several chemical reactivity indices such as the ionization potential (I), electron affinity (A), chemical potential (μ), hardness (η), electrophilicity (ω) and nucleophilicity (N) indices were calculated, and these properties are discussed and compared. In addition, the antiproliferative activity of the five new chalcones was studied.