Broad-Scope Amination of Aryl Sulfamates Catalyzed by a Palladium Phosphine Complex.

Among phenol-derived electrophiles, aryl sulfamates are attractive substrates since they can be employed as directing groups for C-H functionalization prior to catalysis. However, their use in C-N coupling is limited only to Ni catalysis. Here, we describe a Pd-based catalyst with a broad scope for the amination of aryl sulfamates. We show that the N-methyl-2-aminobiphenyl palladacycle supported by the PCyp2ArXyl2 ligand (Cyp = cyclopentyl; ArXyl2 = 2,6-bis(2,6-dimethylphenyl)phenyl) efficiently catalyzes the C-N coupling of aryl sulfamates with a variety of nitrogen nucleophiles, including anilines, primary and secondary alkyl amines, heteroaryl amines, N-heterocycles, and primary amides. DFT calculations support that the oxidative addition of the aryl sulfamate is the rate-determining step. The C-N coupling takes place through a cationic pathway in the polar protic medium.

Synthesis and characterization of enantiopure chiral NH2/SO palladium complexes.

A series of enantiopure chiral NH2/SO palladium complexes have been synthesised with high yields by treating the corresponding tert-butylsulfinamide/sulfoxide derivatives with Pd(CH3CN)2Cl2. The enantiopure chiral ligands were prepared by stereoselective addition of tert-butyl or phenyl methylsulfinyl carbanions to different tert-butylsulfinylimines. In all cases, coordination occurs with concomitant desulfinylation. X-ray studies of the Pd complexes showed a higher trans influence of the phenylsulfinyl group in comparison to that of the tert-butylsulfinyl group. Furthermore, we have obtained and characterised two possible palladium amine/sulfonyl complexes, epimers at sulfur, resulting from N-desulfinylation and coordination of palladium with both oxygens of the prochiral sulfonyl group. The catalytic activity and enantioselectivity of the new Pd(II) complexes of acetylated amine/tert-butyl- and phenylsulfoxides in the carboxylated cyclopropanes arylation reaction has been studied, obtaining the best results with the phenylsulfoxide ligand 25(SC,SS) that produced the final arylated product in a 93 :7 enantiomeric ratio.

Important Role of NH-Carbazole in Aryl Amination Reactions Catalyzed by 2-Aminobiphenyl Palladacycles.

2-Aminobiphenyl palladacycles are among the most successful precatalysts for Pd-catalyzed cross-coupling reactions, including aryl amination. However, the role of NH-carbazole, a byproduct of precatalyst activation, remains poorly understood. Herein, the mechanism of the aryl amination reactions catalyzed by a cationic 2-aminobiphenyl palladacycle supported by a terphenyl phosphine ligand, PCyp2ArXyl2 (Cyp = cyclopentyl; ArXyl2 = 2,6-bis(2,6-dimethylphenyl)phenyl), P1, has been thoroughly investigated. Combining computational and experimental studies, we found that the Pd(II) oxidative addition intermediate reacts with NH-carbazole in the presence of the base (NaO t Bu) to yield a stable aryl carbazolyl Pd(II) complex. This species functions as the catalyst resting state, providing the amount of monoligated LPd(0) species required for catalysis and minimizing Pd decomposition. In the case of a reaction with aniline, an equilibrium between the carbazolyl complex and the on-cycle anilido analogue is established, which allows for a fast reaction at room temperature. In contrast, heating is required in a reaction with alkylamines, whose deprotonation involves coordination to the Pd center. A microkinetic model was built combining computational and experimental data to validate the mechanistic proposals. In conclusion, our study shows that despite the rate reduction observed in some reactions by the formation of the aryl carbazolyl Pd(II) complex, this species reduces catalyst decomposition and could be considered an alternative precatalyst in cross-coupling reactions.

LPdCl2(amine) complexes supported by terphenyl phosphanes: applications in aryl amination reactions.

Despite the excellent catalytic properties display by NHC-Pd-PEPPSI complexes in cross-coupling, phosphane analogs have been barely screened. In this work, we report the synthesis and characterization of a series of LPdCl2(amine) complexes bearing dialkylterphenyl phosphanes (PR2Ar') and pyridine or morpholine ligands. The novel compounds have been tested as precatalysts in aryl amination reactions. The complex [(PCyp2ArXyl2)PdCl2(morpholine)] shows the best catalytic activity allowing the room-temperature coupling of aryl bromides and chlorides with aniline.

A highly constrained cis-dihydride platinum complex trapped by cooperative gold/platinum dihydrogen activation.

A series of Au(I)/Pt(0) combinations that behave as bimetallic frustrated Lewis pairs activates dihydrogen in a cooperative manner. The steric bulk of the terphenyl phosphines that stabilize both fragments allows for the isolation of a rather unique and highly distorted cis-type dihydride platinum(II) structure.

Ni(II) Precatalysts Enable Thioetherification of (Hetero)Aryl Halides and Tosylates and Tandem C-S/C-N Couplings.

Ni-catalyzed C-S cross-coupling reactions have received less attention compared with other C-heteroatom couplings. Most reported examples comprise the thioetherification of most reactive aryl iodides with aromatic thiols. The use of C-O electrophiles in this context is almost uncharted. Here, we describe that preformed Ni(II) precatalysts of the type NiCl(allyl)(PMe2 Ar') (Ar'=terphenyl group) efficiently couple a wide range of (hetero)aryl halides, including challenging aryl chlorides, with a variety of aromatic and aliphatic thiols. Aryl and alkenyl tosylates are also well tolerated, demonstrating, for the first time, to be competent electrophilic partners in Ni-catalyzed C-S bond formation. The chemoselective functionalization of the C-I bond in the presence of a C-Cl bond allows for designing site-selective tandem C-S/C-N couplings. The formation of the two C-heteroatom bonds takes place in a single operation and represents a rare example of dual electrophile/nucleophile chemoselective process.

Zero-valent ML2 complexes of group 10 metals supported by terphenyl phosphanes.

Bulky terphenyl phosphane ligands PMe2Ar' (Ar' = terphenyl group) facilitate the isolation of zero-valent bis-phosphane complexes of nickel, palladium and platinum. The former show coordination numbers greater than two in the solid state due to the existence of Ni-Carene interactions with the terphenyl fragment.

Dinuclear Cu(I) Halides with Terphenyl Phosphines: Synthesis, Photophysical Studies, and Catalytic Applications in CuAAC Reactions.

Several dinuclear terphenyl phosphine copper(I) halide complexes of composition [CuX(PR2Ar')]2 (X = Cl, Br, I; R = hydrocarbyl, Ar' = 2,6-diarylterphenyl radical), 1-5, have been isolated from the reaction of CuX with 1 equiv of the phosphine ligand. Most of them have been characterized by X-ray diffraction studies in the solid state, thus allowing comparative discussions of different structural parameters, namely, Cu···Cu and Cu···Aryl separations, conformations adopted by coordinated phosphines, and planarity of the Cu2X2 cores. Centrosymmetric complexes [CuI(PMe2ArXyl2)]2, 1c, and [CuI(PEt2ArMes2)]2, 3c, despite their similar structures, show very distinct photoluminescence (PL) in powder form at room temperature. The photophysical behavior of these compounds in liquid solution, solid-solid Zeonex solution and powder samples at room temperature and 77 K have been investigated and supported by DFT calculation. Identification of vibronic coupling modes, done by group theory calculations and the technique of projection operators, shows that the manifestation of these modes is conditioned by crystal packing. Complexes [CuI(PMe2ArXyl2)]2, 1c, and [CuI(PEt2ArMes2)]2, 3c, display remarkable activity in copper-catalyzed azide-alkyne cycloaddition reactions involving preformed and in situ-made azides. Reactions are performed in H2O, under aerobic conditions, with low catalyst loadings and tolerate the use of iodoalkynes as substrates.

Dialkylterphenyl Phosphine-Based Palladium Precatalysts for Efficient Aryl Amination of N-Nucleophiles.

A series of 2-aminobiphenyl palladacycles supported by dialkylterphenyl phosphines, PR2 Ar' (R=Me, Et, iPr, Cyp (cyclopentyl), Ar'=ArDipp2 , ArXyl2f , Dipp (2,6-C6H3-(2,6-C6H3-(CHMe2)2)2), Xyl=xylyl) have been prepared and structurally characterized. Neutral palladacycles were obtained with less bulky terphenyl phosphines (i.e., Me and Et substituents) whereas the largest phosphines provided cationic palladacycles in which the phosphines adopted a bidentate hemilabile k1 -P,η1 -Carene coordination mode. The influence of the ligand structure on the catalytic performance of these Pd precatalysts was evaluated in aryl amination reactions. Cationic complexes bearing the phosphines PiPr2 ArXyl2 and PCyp2 ArXyl2 were the most active of the series. These precatalysts have demonstrated a high versatility and efficiency in the coupling of a variety of nitrogen nucleophiles, including secondary amines, alkyl amines, anilines, and indoles, with electronically deactivated and ortho-substituted aryl chlorides at low catalyst loadings (0.25-0.75 mol % Pd) and without excess ligand.

Palladium-mediated intramolecular dearomatization of ligated dialkylterphenyl phosphines.

Aryl-Pd(ii) chlorides stabilized by dialkylterphenyl phosphine ligands undergo a thermal isomerization process, leading to the formation of allyl-Pd(ii)-chloride species. The transformation involves the intramolecular functionalization of a C-H bond of the terphenyl group mediated by the Pd(ii) center.

Synthesis, Structure and Nickel Carbonyl Complexes of Dialkylterphenyl Phosphines.

The experimental and computational characterization of a series of dialkylterphenyl phosphines, PR2 Ar' is described. The new P-donors comprise five compounds of general formula PR2 Ar Dtbp 2 (R=Me, Et, iPr, c-C5 H9 and c-C6 H11 ); Ar Dtbp 2 = 2,6-C6 H3 -(3,5-C6 H3 -(CMe3 )2 )2 ), and another five PR2 Ar' phosphines containing the bulky alkyl groups iPr, c-C5 H9 or c-C6 H11 , in combination with Ar'=Ar Xyl 2 , Ar Xyl ' 2 , or Ar Ph 2 (L1-L10). Steric and electronic parameters have been determined computationally and from IR and X-ray data obtained for the phosphines and for some derivatives, including tricarbonyl and dicarbonyl nickel complexes, Ni(CO)3 (PR2 Ar') and Ni(CO)2 (PR2 Ar'). In the solid state, the free phosphines PR2 Ar' adopt one of the three possible structures formally related by rotation around the Cipso -P bond. Details on their relative energies and on the influence of the free phosphine structure on its coordination chemistry towards Ni(CO)n (n = 2, 3) fragments has been obtained by experimental and computational methods.

Ni-Catalyzed Amination Reactions: An Overview.

Nitrogen-containing organic compounds are valuable in many fields of science and industry. The most reliable method for the construction of C(sp(2) )-N bonds is undoubtedly palladium-catalyzed amination. In spite of the great achievements made in this area, the use of expensive Pd-based catalysts constitutes an important limitation for large-scale applications. Since nickel is the least expensive and most abundant among the group 10 metals, the interest in Ni-based catalysts for processes typically catalyzed by palladium has grown considerably over the last few years. Herein, we revise the development of Ni-catalyzed amination reactions, emphasizing the most relevant and recent advances in the field.

Copper-catalysed azide-alkyne cycloadditions (CuAAC): an update.

The reactions of organic azides and alkynes catalysed by copper species represent the prototypical examples of click chemistry. The so-called CuAAC reaction (copper-catalysed azide-alkyne cycloaddition), discovered in 2002, has been expanded since then to become an excellent tool in organic synthesis. In this contribution the recent results described in the literature since 2010 are reviewed, classified according to the nature of the catalyst precursor: copper(I) or copper(II) salts or complexes, metallic or nano-particulated copper and several solid-supported copper systems.

1,2,3-Triazoles from carbonyl azides and alkynes: filling the gap.

Electron deficient azides are challenging substrates in CuAAC reactions. Particularly, when N-carbonyl azides are applied the formation of N-carbonyl triazoles has not yet been observed. We report herein the first example of this class of reaction, with a copper-based system that efficiently enables the synthesis of N-carbamoyl 1,2,3-triazoles by [3+2] cycloaddition of N-carbamoyl azides and alkynes.

Synthesis, structural characterization, reactivity, and catalytic properties of copper(I) complexes with a series of tetradentate tripodal tris(pyrazolylmethyl)amine ligands.

Novel tris(pyrazolylmethyl)amine ligands Tpa(Me3), Tpa*(,Br), and Tpa(Br3) have been synthesized and structurally characterized. The coordination chemistries of these three new tetradentate tripodal ligands and the already known Tpa and Tpa* have been explored using different copper(I) salts as starting materials. Cationic copper(I) complexes [Tpa(x)Cu]PF6 (1-4) have been isolated from the reaction of [Cu(NCMe)4]PF6 and 1 equiv of the ligand. Complexes 2 (Tpa(x) = Tpa*) and 3 (Tpa(x) = Tpa(Me3)) have been characterized by X-ray studies. The former is a 1D helical coordination polymer, and the latter is a tetranuclear helicate. In both structures, the Tpa(x) ligand adopts a µ(2):κ(2):κ(1)-coordination mode. However, in solution, all of the four complexes form fluxional species. When CuI is used as the copper(I) source, neutral compounds 5-8 have been obtained. Complexes 6-8 exhibit a 1:1 metal-to-ligand ratio, whereas 5 presents 2:1 stoichiometry. Its solid-state structure has been determined by X-ray diffraction, revealing its 3D polymeric nature. The polymer is composed by the assembly of [Tpa2Cu4I4] units, in which Cu4I4 presents a step-stair structure. The Tpa ligands bridge the Cu4I4 clusters, adopting also a µ(2):κ(2):κ(1)-coordination mode. As observed for the cationic derivatives, the NMR spectra of 5-8 show the equivalence of the three pyrazolyl arms of the ligands in these complexes. The reactivities of cationic copper(I) derivatives 1-4 with PPh3 and CO have been explored. In all cases, 1:1 adducts [Tpa(x)CuL]PF6 [L = PPh3 (9-11), CO (12-15)] have been isolated. The crystal structure of [Tpa*Cu(PPh3)]PF6 (9) has been obtained, showing that the coordination geometry around copper(I) is trigonal-pyramidal with the apical position occupied by the tertiary amine N atom. The Tpa* ligand binds the Cu center to three of its four N atoms, with one pyrazolyl arm remaining uncoordinated. In solution, the carbonyl adducts 13-15 exist as a mixture of two isomers; the four- and five-coordinate species can be distinguished by means of their IR νCO stretching bands. Finally, the catalytic activities of complexes 1-4 have been demonstrated in carbene- and nitrene-transfer reactions.

Reaction of alkynes and azides: not triazoles through copper-acetylides but oxazoles through copper-nitrene intermediates.

Well-defined copper(I) complexes of composition [Tpm*(,Br) Cu(NCMe)]BF4 (Tpm*(,Br) =tris(3,5-dimethyl-4-bromo-pyrazolyl)methane) or [Tpa(*) Cu]PF6 (Tpa(*) =tris(3,5-dimethyl-pyrazolylmethyl)amine) catalyze the formation of 2,5-disubstituted oxazoles from carbonyl azides and terminal alkynes in a direct manner. This process represents a novel procedure for the synthesis of this valuable heterocycle from readily available starting materials, leading exclusively to the 2,5-isomer, attesting to a completely regioselective transformation. Experimental evidence and computational studies have allowed the proposal of a reaction mechanism based on the initial formation of a copper-acyl nitrene species, in contrast to the well-known mechanism for the copper-catalyzed alkyne and azide cycloaddition reactions (CuAAC) that is triggered by the formation of a copper-acetylide complex.

Silver-catalyzed silicon-hydrogen bond functionalization by carbene insertion.

The catalytic functionalization of silicon-hydrogen bonds by means of the insertion of carbene units :CHCO(2)Et from ethyl diazoacetate (EDA) has been achieved using a silver-based catalyst, constituting the first example of this metal to promote this transformation. Competition experiments have revealed that the relative reactivity of substituted silanes depends on the bond dissociation energy of the Si-H bond (tertiary > secondary > primary for ethyl substituted). In the presence of bulky substituents such order reverts to secondary > primary ≈ tertiary (for phenyl substituted). Screening with other diazo compounds has shown that N(2)C(Ph)CO(2)Et displays similar reactivity to that of EDA, whereas other N(2)C(R)CO(2)Et (R = Me, CO(2)Et) gave lower conversions.

Kumada-Tamao-Corriu coupling of heteroaromatic chlorides and aryl ethers catalyzed by (IPr)Ni(allyl)Cl.

The complex (IPr)Ni(allyl)Cl (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolidene) catalyzes the cross-coupling reactions of heteroaromatic chlorides with aryl Grignard reagents. Catalyst loadings as low as 0.1 mol % have been used to afford the products in excellent yields. This nickel-based catalytic system also promotes the activation of the C(Ar)-O bond of anisoles in the Kumada-Tamao-Corriu reaction under fairly mild conditions.

Copper(I) complexes with trispyrazolylmethane ligands: synthesis, characterization, and catalytic activity in cross-coupling reactions.

Three novel Cu(I) complexes bearing tris(pyrazolyl)methane ligands, Tpm(x), have been prepared from reactions of equimolar amounts of CuI and the ligands Tpm, (HC(pz)(3)), Tpm*, (HC(3,5-Me(2)-pz)(3)), and Tpm(Ms), (HC(3-Ms-pz)(3)). X-ray diffraction studies have shown that the Tpm and Tpm(Ms) derivatives exhibit a 2:1 Cu:ligand ratio, whereas the Tpm* complex is a mononuclear species in nature. The latter has been employed as a precatalyst in the arylation of amides and aromatic thiols with good activity. The synthesis of a Tpm*Cu(I)-phthalimidate, a feasible intermediate in this catalytic process, has also been performed. Low temperature (1)H NMR studies in CDCl(3) have indicated that this complex exists in solution as a mixture of two, neutral and ionic forms. Conductivity measurements have reinforced this proposal, the ionic form predominating in a very polar solvent such as DMSO. The reaction of Tpm*Cu(I)-phthalimidate with iodobenzene afforded the expected C-N coupling product in 76% yield accounting for its role as an intermediate in this transformation.