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1.
J Am Chem Soc ; 141(35): 13812-13821, 2019 09 04.
Artigo em Inglês | MEDLINE | ID: mdl-31433633

RESUMO

Olefins devoid of directing or activating groups have been dicarbofunctionalized here with two electrophilic carbon sources under reductive conditions. Simultaneous formation of one C(sp3)-C(sp3) and one C(sp3)-C(sp2) bond across a variety of unbiased π-systems proceeds with exquisite selectivity by the combination of a Ni catalyst with TDAE as sacrificial reductant. Control experiments and computational studies revealed the feasibility of a radical-based mechanism involving, formally, two interconnected Ni(I)/Ni(III) processes and demonstrated the different ability of Ni(I) species (Ni(I)I vs PhNi(I)) to reduce the C(sp3)-I bond. The role of the reductant was also investigated in depth, suggesting that a one-electron reduction of Ni(II) species to Ni(I) is thermodynamically favored. Further, the preferential activation of alkyl vs aryl halides by ArNi(I) complexes as well as the high affinity of ArNi(II) for secondary over tertiary C-centered radicals explains the lack of undesired homo- and direct coupling products (Ar-Ar, Ar-Alk) in these transformations.

2.
Chemistry ; 25(64): 14512-14516, 2019 Nov 18.
Artigo em Inglês | MEDLINE | ID: mdl-31518012

RESUMO

We report a novel atom-economical Ni-catalyzed cyclization reaction of enynes with alkynylboronates. The reaction employs a non-expensive Ni salt, a phosphine-based ligand and easy-handling alkynylboronates as boron-carbon source. The reaction provides complex fused-bicyclic compounds containing borylated 1,4-cyclohexadienes in high yields in short reaction times, involving the formation of two C-C bonds in one step. A reasonable reaction mechanism is proposed based on mechanistic experimental results.

3.
Chemistry ; 24(44): 11239-11244, 2018 Aug 06.
Artigo em Inglês | MEDLINE | ID: mdl-29575256

RESUMO

Metal-catalyzed borylative cyclization reactions of polyunsaturated compounds allow concomitant formation of C-C and C-B bonds to provide cyclic boronates, which constitute synthetically useful intermediates. Recent developments using first-row transition metals as catalysts provide fully atom-economical reactions that follow mechanisms involving σ-bond metathesis.

4.
Chemistry ; 24(4): 784-788, 2018 Jan 19.
Artigo em Inglês | MEDLINE | ID: mdl-29105882

RESUMO

We report first Fe-catalyzed hydroborylative cyclization reaction. The process provides one C-C and one C-B bond in a single operation and shows a wide scope, allowing the formation of carbo- and heterocycles containing a homoallylic boryl unit that can be further functionalized. The reaction takes place in smooth conditions, with inexpensive catalytic system and full atom economy since HBpin is the borylation agent, in contrast to our previously reported Pd-catalyzed reaction. Both aryl and alkyl substituted alkynes are reactive, revealing a wide reaction scope. Mechanistic studies suggest the intermediacy of FeII -hydride active catalyst capable to react with the alkyne group prior to alkene insertion, and computational studies suggest the occurrence of barrierless σ-bond metathesis involving HBpin and Fe-C bonds along the catalytic cycle.

5.
Chemistry ; 24(11): 2653-2662, 2018 Feb 21.
Artigo em Inglês | MEDLINE | ID: mdl-29240981

RESUMO

A new family of homochiral silver complexes based on carbophilic interactions with ortho-phenylene ethynylene (o-OPE) scaffolds containing up to two silver atoms are described. These compounds represent a unique class of complexes with chirality at the metal. Chiral induction is based on the inclusion of chiral sulfoxides, which allow efficient transfer of chirality to the helically folded o-OPE, leading to circularly polarized luminescence (CPL)- and vibrational circular dichroism (VCD)-active compounds. In the presence of silver(I) cations, carbophilic interactions dominate, which promote helical structures with a defined helicity. This is one of the very scarce examples of the use of such interactions in the attractive field of abiotic foldamers. The switching event has been extensively studied by using different chiroptical techniques, including circular dichroism, CPL, and VCD, and represents one of the few CPL switches described in the literature.

6.
J Phys Chem A ; 122(8): 2250-2257, 2018 Mar 01.
Artigo em Inglês | MEDLINE | ID: mdl-29442504

RESUMO

We have performed a systematic study of the electron structure of a series of Ni(I) and Ni(II) iodo and methyl complexes with a variety of di- and tridentate nitrogen ligands to study the influence of these ligands in the structure of catalytically active complexes in cross-coupling reactions. Ni(II) compounds show the expected square-planar configuration typical of complexes of d8 metals, regardless of the kind of coordinating nitrogen atom (sp2 or sp3) found in ligands derived from either trialkylamines or pyridines. In contrast, Ni(I) complexes show different structures. Thus, the absence of orbitals capable of delocalizing the unpaired electron (such as in TMEDA and PMDTA derivatives) leads to nonplanar iodo or methyl tetracoordinate complexes. In contrast, the presence of ligands derived from pyridine allows delocalization of the unpaired electron on the ligand. This delocalization is especially effective for terpyridine species.

7.
Chemistry ; 23(7): 1584-1590, 2017 Jan 31.
Artigo em Inglês | MEDLINE | ID: mdl-27862442

RESUMO

We describe the unprecedented formation of allenes by Ni-catalyzed cross-coupling of propargyl bromides with alkylzinc halides. The reaction regioselectivity is complementary to the previously reported formation of propargyl-coupled compounds. Experiments support the formation of NiI complexes as the active species and the participation of radical intermediates. Kinetic studies showed that the reaction is first order with respect to the electrophile, zero-order with respect to the nucleophile (fast transmetalation), and one-half order with respect to the metal catalyst. Mechanistic studies support a bimetallic NiI -based pathway that involves fast homolytic cleavage of the C-Br bond by an alkyl-NiI complex, followed by radical coordination to NiI that determines the observed regioselectivity.

8.
J Am Chem Soc ; 137(43): 13818-26, 2015 Nov 04.
Artigo em Inglês | MEDLINE | ID: mdl-26452050

RESUMO

In this paper, we have systematically studied how the replacement of a benzene ring by a heterocyclic compound in oligo(phenyleneethynylene) (OPE) derivatives affects the conductance of a molecular wire using the scanning tunneling microscope-based break junction technique. We describe for the first time how OPE derivatives with a central pyrimidine ring can efficiently link to the gold electrode by two pathways presenting two different conductance G values. We have demonstrated that this effect is associated with the presence of two efficient conductive pathways of different length: the conventional end-to-end configuration, and another with one of the electrodes linked directly to the central ring. This represents one of the few examples in which two defined conductive states can be set up in a single molecule without the aid of an external stimulus. Moreover, we have observed that the conductance through the full length of the heterocycle-based OPEs is basically unaffected by the presence of the heterocycle. All these results and the simplicity of the proposed molecules push forward the development of compounds with multiple conductance pathways, which would be a breakthrough in the field of molecular electronics.

10.
J Org Chem ; 80(2): 1098-106, 2015 Jan 16.
Artigo em Inglês | MEDLINE | ID: mdl-25514331

RESUMO

Nonafluorobutanesulfonyl azide is a highly efficient reagent for the copper-catalyzed coupling of terminal alkynes to give symmetrical and unsymmetrical 1,3-diynes in good to excellent yields and with good functional group compatibility. The reaction is extremely fast (<10 min), even at low temperature (−78 °C), and requires substoichiometric amounts of a simple copper(I) or copper(II) salt (2­5 mol %) and an organic base (0.6 mol %). A possible mechanistic pathway is briefly discussed on the basis of model DFT theoretical calculations. The quantitative assessment of the safety of use and shelf stability of nonafluorobutanesulfonyl azide has confirmed that this reagent is a superior and safe alternative to other electrophilic azide reagents in use today.


Assuntos
Alcinos/química , Alcinos/síntese química , Azidas/química , Cobre/química , Di-Inos/química , Di-Inos/síntese química , Hidrocarbonetos Fluorados/química , Oxidantes/química , Catálise , Estrutura Molecular
11.
Phys Chem Chem Phys ; 17(47): 31902-10, 2015 Dec 21.
Artigo em Inglês | MEDLINE | ID: mdl-26568425

RESUMO

The design of two dimensional graphene-type materials with an anisotropic electron flow direction in the X- and Y-axes opens the door for the development of novel electronic materials with multiple functions in nanoelectronics. In the present work, we have studied the electronic transport properties of a new family of 2D graphene-graphyne hybrids presenting conformationally free phenylethylene subunits. This system ensures two different conductive pathways that are perpendicular to each other: an acene nanoribbon subunit, in the X-axis, with graphene-type conduction, and a free to rotate phenylethylene subunit, in the Y-axis, in which the magnitude of the conduction depends dynamically on the corresponding torsion angle. Our calculations have confirmed that this system presents two different conduction pathways, which are related to the presence of asymmetric Dirac-type cones. Moreover, the Dirac cones can be dynamically modified in the presence of an external gate electrode, which is unprecedented in the literature.

12.
Dalton Trans ; 53(20): 8550-8554, 2024 May 21.
Artigo em Inglês | MEDLINE | ID: mdl-38715455

RESUMO

We have synthesised and characterised the complex Ni(tpy)2 (tpy = 2,2':6',2''-terpyridine). This formally Ni(0) complex is paramagnetic both in the solid state and in solution (S = 2). The crystal structure shows an octahedral geometry, with molecules arranged in independent dimers involving π-stacking between pairs of complexes. Magnetic measurementes and DFT calculations suggest the existence of temperature-dependent intermolecular antiferromagnetic coupling in the solid state.

13.
Chemistry ; 19(48): 16187-91, 2013 Nov 25.
Artigo em Inglês | MEDLINE | ID: mdl-24249687

RESUMO

Extended reach: A calculated mechanism for long-range proton-coupled electron transfer (PCET, see picture) through an array of structured water molecules between Fe(III)-Fe(III) complexes accounts for the reaction enthalpy and kinetic isotope effect previously measured for this reaction. This mechanism may be general and occur for other hydroxo-metal complexes.

14.
Chemistry ; 19(26): 8405-10, 2013 Jun 24.
Artigo em Inglês | MEDLINE | ID: mdl-23677686

RESUMO

Suggesting novel disconnections: A powerful Ni-catalyzed cascade reaction involving cyclization followed by cross-coupling allows the formation of up to three alkyl-alkyl bonds in a single operation by using alkene-containing alkyl iodides and Grignard reagents (see scheme; acac = acetylacetonate; TMEDA = N,N',N'-tetramethyl ethylenediamine). Mechanistic experimental and computational studies suggest a Ni(I)-Ni(II)-Ni(III) catalytic cycle and the intermediacy of radicals.

15.
Chemistry ; 19(43): 14484-95, 2013 Oct 18.
Artigo em Inglês | MEDLINE | ID: mdl-24105753

RESUMO

In this article, a complete study on the selectivity of titanocene(III) cyclization of epoxypolyprenes is presented. The requirements for the formation of six- or seven-membered rings during these cyclizations are determined, taking into account the different substitution pattern in the epoxypolyprene precursor. Thus, a complete selectivity to 6-exo or 7-endo cyclization process has been achieved, yielding mono-, bi-, and even tricyclic compounds, constituting a new and efficient access to this type of derivative. Additionally, this procedure opens the possibility to prepare excellent building blocks for the synthesis of polycyclic compounds with a trisubstituted oxygenated function, which is present in several natural terpenes.


Assuntos
Compostos Organometálicos/química , Terpenos/química , Catálise , Ciclização , Compostos de Epóxi/química , Radicais Livres/química , Estereoisomerismo , Terpenos/síntese química , Termodinâmica
16.
J Am Chem Soc ; 134(17): 7219-22, 2012 May 02.
Artigo em Inglês | MEDLINE | ID: mdl-22500739

RESUMO

Good to excellent reactivity and regiocontrol have been achieved in the Cu(I)-catalyzed borylation of dialkyl internal alkynes with bis(pinacolato)diboron. The presence of a propargylic polar group (OH, OR, SAr, SO(2)Ar, or NHTs), in combination with PCy(3) as ligand, allowed maximizing the reactivity and site-selectivity (ß to the propargylic function). DFT calculations suggest a subtle orbitalic influence from the propargylic group, matched with ligand and substrate size effects, as key factors involved in the high ß-selectivity. The vinylboronates allowed the stereoselective synthesis of trisubstituted olefins, while allylic substitution of the SO(2)Py group without affecting the boronate group provided access to formal hydroboration products of unbiased dialkylalkynes.

17.
Chemistry ; 18(40): 12825-33, 2012 Oct 01.
Artigo em Inglês | MEDLINE | ID: mdl-22926987

RESUMO

A straightforward access to polyhydroxylated terpenoids based on two key titanocene(III)-mediated reactions is presented: the "head-to-tail" Barbier-type addition of prenyl chlorides to α,ß-unsaturated aldehydes, which allows the introduction of hydroxy groups at desirable positions of the acyclic precursor, and the subsequent bioinspired radical cyclisation. This methodology has been also used in the first total synthesis of pentacyclic sesterstatin 1 and a model compound of the C-D rings of aspergilloxide.


Assuntos
Compostos de Epóxi/química , Compostos de Epóxi/síntese química , Compostos Organometálicos/química , Compostos Policíclicos/química , Compostos Policíclicos/síntese química , Terpenos/química , Terpenos/síntese química , Titânio/química , Estrutura Molecular , Estereoisomerismo
18.
Chemphyschem ; 13(17): 3857-65, 2012 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-23090906

RESUMO

Molecular design of chromium arenes are theoretically studied as a model for the development of novel thermally-driven molecular fuses. This study correlates the switching event with a partial disconnection of the molecule from the metallic electrode mediated by changes in the conformational states of the molecule directed by external stimuli. Moreover, the reversibility of the process (the reconnection to the metallic electrode) is also considered for these systems when a reversal voltage pulse (reset) is applied. The energetic requirements of the on and off states are correlated with temperature through the Arrhenius equation. To carry out this study we performed density functional theory (DFT) calculations.

19.
Chemphyschem ; 13(3): 860-8, 2012 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-22271673

RESUMO

One of the central issues of molecular electronics (ME) is the study of the molecule-metal electrode contacts, and their implications for the conductivity, charge-transport mechanism, and mechanical stability. In fact, stochastic on/off switching (blinking) reported in STM experiments is a major problem of single-molecule devices, and challenges the stability and reliability of these systems. Surprisingly, the ambiguous STM results all originate from devices that bind to the metallic electrode through a one-atom connection. In the present work, DFT is employed to study and compare the properties of a set of simple acenes that bind to metallic electrodes with an increasing number of connections, in order to determine whether the increasing numbers of anchoring groups have a direct repercussion on the stability of these systems. The conductivities of the three polycyclic aromatic hydrocarbons are calculated, as well as their transmission spectra and current profiles. The thermal and mechanical stability of these systems is studied by pulling and pushing the metal-molecule connection. The results show that molecules with more than one connection per electrode exhibit greater electrical efficiency and current stability.

20.
Angew Chem Int Ed Engl ; 51(52): 13036-40, 2012 Dec 21.
Artigo em Inglês | MEDLINE | ID: mdl-23154914

RESUMO

Spring loaded: the smallest members of a family of carbon nanocoils (CNCs), adopting a fixed helical structure, have been synthesized by introduction of one or two staples in o-phenylene ethynylene oligomers. The chiroptical responses of the systems having enantiopure L-tartrate-derived staples confirmed the induced helicity. Theoretical studies suggest that these CNCs are pseudoelastic.

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