Search

The synthesis of spiro compounds featuring seven- and six-membered rings in the spirobicyclic motif is successfully achieved through a cascade process encompassing a rhodium(I)-catalyzed cycloisomerization followed by a highly selective Diels-Alder homodimerization. The scope of the reaction is analyzed based on a series of synthetic substrates, and control experiments and DFT calculations led us to justify the exquisite degree of selectivity observed We are grateful for financial support from the Ministerio de Ciencia e Innovación (PID2020-113711GB-I00 MCIN/AEI/ 10.13039/501100011033), the Generalitat de Catalunya (Project 2017-SGR-39), and the UdG for an IF predoctoral grant to J.V. Open Access funding provided thanks to the CRUE-CSIC agreement with ACS

The development of catalytic methodologies involving the formation of C−C bonds to enable the generation of cyclic systems constitutes a field of great relevance in synthetic organic chemistry. One paradigmatic process to accomplish this goal efficiently is the transition-metal-catalyzed [2 + 2 + 2] cycloaddition reaction, since it permits the formation of a wide range of highly functionalized 6-membered carbo- and heterocyclic molecules in a single step with high efficiency and perfect atom economy. A key feature of these transformations is the mechanistic pathway that they follow, since a deep knowledge of this mechanism may enable us to understand and improve the efficiency of the reaction. This review covers the mechanistic aspects, studied both from theoretical and experimental points of view, of the transition-metal-catalyzed [2 + 2 + 2] cycloaddition reaction involving all kinds of unsaturated substrates with metals such as Co, Ni, Ru, Rh, Ir, Pd, Zr, Ti, Ta, and Nb. A thorough overview is undertaken, from the seminal studies until the present day, of the key mechanistic aspects that influence the reactivity and selectivity of the reaction, comparing the involvement of different unsaturated substrates as well as the different transition metals used We gratefully acknowledge the financial support of our own research in this area by the Spanish Ministry of Economy and Competitivity (MINECO) (project CTQ2017-85341-P) and the Generalitat de Catalunya (project 2017-SGR-39, Xarxa de referencia en Química Teórica i Computacional, and ICREA Academia 2014 Award to M.S.)

Density functional theory calculations were used to systematically explore the effects of axial ligation by solvent molecules on the reactivity and selectivity of dirhodium tetracarboxylates with diazo compounds in the context of C–H insertion into propane. Insertions on three types of diazo compounds—acceptor/acceptor, donor/acceptor, and donor/donor—promoted by dirhodium tetraformate were tested with and without axial solvent ligation for no surrounding solvent, dichloromethane, isopropanol, and acetonitrile. Magnitudes, origins, and consequences of structural and electronic changes arising from axial ligation were characterized. The results suggest that axial ligation affects barriers for N2 extrusion and C–H insertion, the former to a larger extent.

Density functional theory calculations were used to systematically explore the effects of axial ligation by solvent molecules on the reactivity and selectivity of dirhodium tetracarboxylates with diazo compounds in the context of C−H insertion into propane. Insertions on three types of diazo compounds - acceptor/acceptor, donor/acceptor, and donor/donor - promoted by dirhodium tetraformate were tested with and without axial solvent ligation for no surrounding solvent, dichloromethane, isopropanol, and acetonitrile. Magnitudes, origins, and consequences of structural and electronic changes arising from axial ligation were characterized. The results suggest that axial ligation affects barriers for N2 extrusion and C−H insertion, the former to a larger extent

A novel methodology to transform bisallenes into a variety of polycyclic derivatives employing rhodium(I) catalysis has been developed. This transformation encompasses an intramolecular Rh-catalyzed cycloisomerization of bisallenes 1 to deliver a reac-tive cycloheptadiene, which concomitantly undergoes a regioselective [4+2] cycloaddition with alkenes. A complete mechanistic study of this transformation has been undertaken including DFT calculations. Overall, the methodology presented here constitutes a new and straightforward entry to polycyclic dihydroazepine and dihydrooxepine derivatives employing catalytic methods We are grateful for the financial support by the Spanish Ministry of Economy and Competitivity (MINECO) (Projects CTQ2017- 85341-P and CTQ2017-83587-P, FI predoctoral grant to A.A., RyC contract RYC2012-11112 to A.L.), the Generalitat de Catalunya (Project 2017-SGR-39, Xarxa de Referència en Química Teòrica i Computacional, ICREA Academia 2014 prize for M.S.) and UdG for IF predoctoral grant to J.V

The electronic and steric factors that favour the formation of 1,2,4- and 1,3,5-regioisomers in the intermolecular [2+2+2] cyclotrimerisation of terminal alkynes are not well understood. In this work, this problem was analysed from a theoretical and experimental point of view. Density functional theory (DFT) calculations of the [2+2+2] cyclotrimerisation of p-X-substituted phenylacetylenes (X = H, NO2, and NH2) catalysed by [Rh(BIPHEP)]+ were carried out to determine the reaction mechanism in each case and analyse the effect that the electronic character of the substituents has on the regioselectivity. For the rate-determining step corresponding to the oxidative coupling leading to the rhodacyclopentadiene intermediate, we have taken into account two reaction pathways: the reaction pathway with the lowest energy barrier and the reaction pathway through the most stable transition state (Curtin-Hammett pathway). Our results show that the theoretical results conform experimental outcomes for different p-X-substituted phenylacetylenes (X = NO2, F, H, Me, tBu, OMe, NMe2) only when the Curtin-Hammett reaction pathway is considered. A fairly good correlation has been obtained between the electronic nature of the substituents (as expressed by the Hammett σpara constant values) and the regioisomeric ratios experimentally obtained and computationally predicted Financial support from the Spanish Ministry of Economy and Competitivity (MINECO) (Project No. CTQ2017-85341-P) and the DIUE of the Generalitat de Catalunya (Project No. 2017SGR39 and ICREA Academia 2014 to M. S.) are acknowledged. Thanks are due to the Generalitat de Catalunya for FI predoctoral grants to Ò.T., M.F. and À.D.-J. The EU under the FEDER grant UNGI10-4E-801 has also funded this research

Two palladium complexes containing triene-coordinated ligands, 3 and 4, open-chain analogues of the macrocyclic palladium(0) complexes of type 1, have been synthesized. A comparative study of both open-chain and macrocyclic complexes has been carried out, based on a NMR and X-ray structural analysis, a DSC thermal behavior assessment, and a catalytical activity evaluation in the arenediazonium and organotrifluoroborate cross-coupling Suzuki−Miyaura reaction. The 1,6,11-triene sequence has been revealed to be crucial in the great stability of this family of palladium(0) complexes.

Two synthetic routes to "Janus"-type dendrimers possessing ammonium groups on one side and fluorescent dansyl derivatives on the other side are described. These surface-block dendrimers are obtained by the coupling of two different dendrons, built from the hexafunctional cyclotriphosphazene core. Their characterization and their photophysical behavior are reported. The largest compound possesses 10 ammonium groups and 5 dansyl groups; it is potentially useful as a fluorescent label in materials science and biology.

A novel type of chiral and stable palladium(0) complexes of polyunsaturated aza-macrocyclic ligands were prepared and fully characterized by means of NMR spectroscopy and X-ray diffraction. Fifteen-membered alkene−alkyne type ligands as well as 20- and 25-membered polyolefinic ligands showed a preference for tricoordination with the metal. The stereochemical complexity of these complexes is related to the different isomers that can be formed by complexation of the metal to either of the two faces of each of the olefins involved. The palladacyclopropane formulation of the palladium−olefin interaction offers a clear picture of the stereogenicity of the olefin carbon atoms when they are coordinated to the metal. The preorientation of the macrocyclic ligand and the prepositioning of the olefinic bonds for complex formation facilitated by six-membered chelate rings explain the stability of the structures. The conformation of the palladacyclohexanic rings is found to be crucial in the stereoisomers formed. Thes...

Allylic substitution of PhCH(OAc)CHCHPh by CH2(COMe)2 in a basic MeOH/H2O mixture could be achieved in the absence of a palladium catalyst and lead to a mixture of PhCH(CH(COMe)2)CHCHPh and PhCH(OMe)CHCHPh in 40% and 55% yields, respectively. The process is induced by water, and nucleophilic attack addition occurred on a stabilized carbocation as the intermediate. Addition of a catalytic amount of PdCl2(CH3CN)2 did not accelerate the reaction but improved the selectivity, and PhCH(CH(COMe)2)CHCHPh was then obtained in 92% yield while PhCH(OMe)CHCHPh was observed in trace amounts. An ESI-MS analysis of the reaction mixture led us to assume that a palladium acetylacetonate complex is involved in the formation of PhCH(CH(COMe)2)CHCHPh.

Selective and efficient preparation of a new type of nitrogen-containing 15-membered triacetylenic macrocycles named 1,6,11-tris(arylsulfonyl)-1,6,11-triazacyclopentadeca-3,8,13-triynes (1) (Ar1 = Ar2 = Ar3 = 4-methylphenyl, 1aaa; Ar1 = Ar2 = 4-methylphenyl, Ar3 = ferrocenyl, 1aab) has been achieved. Macrocycle 1aaa forms a very stable complex with Pd0, being the first palladium(0) complex with a macrocyclic triyne. The three triple bonds in 1aaa are responsible for the complexation. The X-ray structures of both the ligand 1aaa and its Pd0 complex are presented. Furthermore, compound 1aaa undergoes cycloisomerization into the corresponding 2,5,8-tris[(4-methlphenyl)sulfonyl]-2,5,8-triazatrindane, 11aaa, when treated with Pd(PPh3)4 in refluxing toluene.

The air- and moisture-stable phosphine-free palladium(0) complex 1 is a highly active and recoverable catalyst for Heck olefination of aryl diazonium tetrafluoroborates. The reactions were performed under aerobic conditions at room temperature to give the coupling products in excellent yields. [reaction: see text]

A series of nickel(0) complexes, 3, 4, and 5, containing unsaturated 15-membered azamacrocyclic ligands have been synthesized. Structural characterization of the complexes is based on NMR spectroscopy, X-ray diffraction analysis, and differential scanning calorimetry. In addition, the catalytic activity of the complexes in the Suzuki-Miyaura cross-coupling of arylboronic acids and aryl halides has been tested The project was funded by the program Acciones Integradas Hispano-Alemanas between Germany (DAAD, BMBF) and Spain (MICINN, Spanish Ministry of Science and Innovation, Project AIB2010DE-00262). Financial support from the MICINN (Projects CTQ2008-05409, CTQ2011-23121) and the Generalitat de Catalunya (Project 2009SGR637) is also gratefully acknowledged. S.B. thanks the University of Girona for a predoctoral grant

A rhodium-catalyzed [2 + 2 + 2] cycloaddition of cyano-yne-allene scaffolds followed by a dehydrogenative process enabling the direct synthesis of unsaturated pyridine-containing compounds that can be conveniently converted to 2,6-naphthyridine derivatives is reported Financial support from the Spanish Ministry of Education and Science (MINECO) (Project No. CTQ2011-23121, CTQ2014-54306-P, and a RyC contract to A.L.) and the DIUE of the Generalitat de Catalunya (Project No. 2014SGR931) are acknowledged