M. Carmen Nicasio

Regioselective Formation of 2,5-Disubstituted Oxazoles Via Copper(I)-Catalyzed Cycloaddition of Acyl Azides and 1-Alkynes

The reaction of 1-alkynes with acyl azides in the presence of [Tpm(*,Br)Cu(NCMe)]BF(4) [Tpm(*,Br) = tris(3,5-dimethyl-4-bromopyrazolyl)methane] as the catalyst provides 2,5-oxazoles in moderate to high yields. This is a novel transformation of the CuAAC type that constitutes a significant variation of the commonly observed [3 + 2] cycloaddition reaction to yield 1,2,3-triazoles.

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.

Catalytic insertion of diazo compounds into N-H bonds: the copper alternative.

The complexes TpxCu (Tpx = homoscorpionate) catalyse the insertion of diazo compounds into nitrogen-hydrogen bonds of amines and amides, under very mild conditions, with quantitative yields being obtained with equimolar ratios of reactants.

C−H Bond Activation of Benzene and Cyclic Ethers by TpIrIII Species

An unusual Fischer carbene derivative that, in addition, contains an alkyl and a hydride ligand is obtained by C−H bond activation of THF by the hydride–vinyl species [TpMe2IrH(CHCH2) (C2H4)]. This complex is also capable of activating the C−H bonds of benzene to give remarkably stable IrIII–N2 complexes (see illustration).

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.

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.

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.

Organometallic derivatives of Ni(II) with poly(pyrazolyl)borate ligands

Abstract The reaction of the alkyl or aryl derivatives Ni(R)X(PMe 3 ) 2 (R=CH 2 SiMe 3 , CH 2 CMe 3 , C 6 H 5 ; X=Cl, Br) with the potassium salt of the Bp ligand (Bp=dihydrobis(pyrazolyl)borate anion) forms the corresponding compounds BpNi(R)(PMe 3 ). In contrast, the reaction of the aryl derivatives Ni(C 6 H 4 - p -X)Br(PMe 3 ) 2 (X=H, Me, OMe, NMe 2 ) with the Bp tBu anion (Bp tBu =dihydrobis(3- t -butylpyrazolyl)borate) proceeds with formation of complexes of composition Bp tBu Ni(C 6 H 4 - p -X)(PMe 3 ) 2 , in which the polydentate ligand is bound to the metal through only one pyrazolyl group. The Tp anion leads to only aryl derivatives; the phenyl complex TpNi(C 6 H 5 )(PMe 3 ) has been obtained, and the reaction of the alkyl complex Ni(CH 2 CMe 2 Ph)Cl(PMe 3 ) 2 with KTp furnishes the aryl TpNi(C 6 H 4 - o -Bu t )(PMe 3 ), by means of a rearrangement of the neophyl ligand. The Tp ligand in these complexes is bonded in the η 2 fashion, although an X-ray analysis carried out for TpNi(Ph)(PMe 3 ) reveals the existence of an important Ni…N interaction with the third pz ring. Upon reaction with the bulky hydrotris(3- t -butylpyrazolyl)borate anion, the aryl derivatives Ni(C 6 H 4 - p -X)Br(PMe 3 ) 2 (X=H, Me, OMe, NMe 2 ) form complexes of composition Tp tBu Ni(C 6 H 4 - p -X)(PMe 3 ) 2 , in which the polydentate ligand is once more bound to the metal through only one pyrazolyl group. These complexes represent the first examples of η 1 coordination of poly(pyrazolyl)borate-type ligands. The acyl and aroyl complexes BpNi(COR)(PMe 3 ) (R=CH 2 SiMe 3 , CH 2 CMe 3 ) and TpNi(COPh)(PMe 3 ) have been obtained by carbonylation of the parent compounds. The aroyls Tp tBu Ni(COC 6 H 4 - p -X)(PMe 3 ) 2 have also been obtained from the derivatives Ni(COC 6 H 4 - p -X)Br(PMe 3 ) 2 although they evolve CO slowly in solution. An X-ray analysis carried out with Tp tBu Ni(C 6 H 4 - p -Me)(PMe 3 ) 2 confirms the η 1 -coordination mode of the Tp tBu ligand, which was deduced from NMR studies.

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.

Monohapto co-ordination of poly(tert-butylpyrazolyl)borate ligands in nickel and palladium complexes

Several four-co-ordinate nickel and palladium aryl and aroyl complexes containing the bulky bis- or tris(3-tert-butylpyrazolyl)borate ligands have been prepared, in which the poly(pyrazolyl)borate group adopts monohapto co-ordination, both in solution and in the solid state.