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1.
Artigo em Inglês | MEDLINE | ID: mdl-34590773

RESUMO

In recent years, metal-free organic synthesis using triarylboranes as catalysts has become a prevalent research area. Herein we report a comprehensive computational and experimental study for the highly selective synthesis of N-substituted pyrazoles through the generation of carbenium species from the reaction between aryl esters and vinyl diazoacetates in the presence of catalytic tris(pentafluorophenyl)borane [B(C6 F5 )3 ]. DFT studies were undertaken to illuminate the reaction mechanism revealing that the in situ generation of a carbenium species acts as an autocatalyst to prompt the regiospecific formation of N-substituted pyrazoles in good to excellent yields (up to 81 %).

2.
Chemistry ; 2021 Sep 02.
Artigo em Inglês | MEDLINE | ID: mdl-34473849

RESUMO

Density functional theory computation indicates that bridge splitting of [PtII R2 (µ-SEt2 )]2 proceeds by partial dissociation to form R2 Pta (µ-SEt2 )Ptb R2 (SEt2 ), followed by coordination of N-donor bromoarenes (L-Br) at Pta leading to release of Ptb R2 (SEt2 ), which reacts with a second molecule of L-Br, providing two molecules of PtR2 (SEt2 )(L-Br-N). For R=4-tolyl (Tol), L-Br=2,6-(pzCH2 )2 C6 H3 Br (pz=pyrazol-1-yl) and 2,6-(Me2 NCH2 )2 C6 H3 Br, subsequent oxidative addition assisted by intramolecular N-donor coordination via PtII Tol2 (L-N,Br) and reductive elimination from PtIV intermediates gives mer-PtII (L-N,C,N)Br and Tol2 . The strong σ-donor influence of Tol groups results in subtle differences in oxidative addition mechanisms when compared with related aryl halide oxidative addition to palladium(II) centres. For R=Me and L-Br=2,6-(pzCH2 )2 C6 H3 Br, a stable PtIV product, fac-PtIV Me2 {2,6-(pzCH2 )2 C6 H3 -N,C,N)Br is predicted, as reported experimentally, acting as a model for undetected and unstable PtIV Tol2 {L-N,C,N}Br undergoing facile Tol2 reductive elimination. The mechanisms reported herein enable the synthesis of PtII pincer reagents with applications in materials and bio-organometallic chemistry.

3.
J Org Chem ; 86(17): 12237-12246, 2021 Sep 03.
Artigo em Inglês | MEDLINE | ID: mdl-34410728

RESUMO

Hypervalent iodine (HVI) compounds are efficient reagents for the double oxidative dearomatization of electron-rich phenols to o-quinones. We recently reported that an underexplored class of iodine(V) reagents possessing bidentate bipyridine ligands, termed Bi(N)-HVIs, could dearomatize electron-poor phenols for the first time. To understand the fundamental mechanistic basis of this unique reactivity, density functional theory (DFT) was utilized. In this way, different pathways were explored to determine why Bi(N)-HVIs are capable of facilitating these challenging transformations while more traditional hypervalent species, such as 2-iodoxybenzoic acid (IBX), cannot. Our calculations reveal that the first redox process is the rate-determining step, the barrier of which hinges on the identity of the ligands bound to the iodine(V) center. This crucial process is composed of three steps: (a) ligand exchange, (b) hypervalent twist, and (c) reductive elimination. We found that strong coordinating ligands disfavor these elementary steps, and, for this reason, HVIs bearing such ligands cannot oxidize the electron-poor phenols. In contrast, the weakly coordinating triflate ligands in Bi(N)-HVIs allow for the kinetically favorable oxidation. It was identified that trapping in situ-generated triflic acid is a key role played by the bidentate bipyridine ligands in Bi(N)-HVIs as this serves to minimize the decomposition of the ortho-quinone product.

4.
Nat Commun ; 12(1): 4065, 2021 07 01.
Artigo em Inglês | MEDLINE | ID: mdl-34210971

RESUMO

Strategies that enable intermolecular site-selective C-H bond functionalisation of organic molecules provide one of the cornerstones of modern chemical synthesis. In chloroalkane synthesis, such methods for intermolecular site-selective aliphatic C-H bond chlorination have, however, remained conspicuously rare. Here, we present a copper(I)-catalysed synthetic method for the efficient site-selective C(sp3)-H bond chlorination of ketones, (E)-enones and alkylbenzenes by dichloramine-T at room temperature. A key feature of the broad substrate scope is tolerance to unsaturation, which would normally pose an immense challenge in chemoselective aliphatic C-H bond functionalisation. By unlocking dichloramine-T's potential as a chlorine radical atom source, the product site-selectivities achieved are among the most selective in alkane functionalisation and should find widespread utility in chemical synthesis. This is exemplified by the late-stage site-selective modification of a number of natural products and bioactive compounds, and gram-scale preparation and formal synthesis of two drug molecules.


Assuntos
Domínio Catalítico , Cobre/química , Cetonas/química , Sulfonamidas/química , Produtos Biológicos/química , Carbono/química , Catálise , Halogenação , Hidrogênio/química , Temperatura
5.
J Org Chem ; 86(9): 6566-6576, 2021 05 07.
Artigo em Inglês | MEDLINE | ID: mdl-33872505

RESUMO

Hypervalent iodine(V) reagents are a powerful class of organic oxidants. While the use of I(V) compounds Dess-Martin periodinane and IBX is widespread, this reagent class has long been plagued by issues of solubility and stability. Extensive effort has been made for derivatizing these scaffolds to modulate reactivity and physical properties but considerable room for innovation still exists. Herein, we describe the preparation, thermal stability, optimized geometries, and synthetic utility of an emerging class of I(V) reagents, Bi(N)-HVIs, possessing datively bound bidentate nitrogen ligands on the iodine center. Bi(N)-HVIs display favorable safety profiles, improved solubility, and comparable to superior oxidative reactivity relative to common I(V) reagents. The highly modular synthesis and in situ generation of Bi(N)-HVIs provides a novel and convenient screening platform for I(V) reagent and reaction development.


Assuntos
Iodo , Nitrogênio , Indicadores e Reagentes , Estrutura Molecular , Oxirredução
6.
J Am Chem Soc ; 2021 Mar 15.
Artigo em Inglês | MEDLINE | ID: mdl-33719443

RESUMO

The donor-acceptor ability of frustrated Lewis pairs (FLPs) has led to widespread applications in organic synthesis. Single electron transfer from a donor Lewis base to an acceptor Lewis acid can generate a frustrated radical pair (FRP) depending on the substrate and energy required (thermal or photochemical) to promote an FLP into an FRP system. Herein, we report the Csp3-Csp cross-coupling reaction of aryl esters with terminal alkynes using the B(C6F5)3/Mes3P FLP. Significantly, when the 1-ethynyl-4-vinylbenzene substrate was employed, the exclusive formation of Csp3-Csp cross-coupled products was observed. However, when 1-ethynyl-2-vinylbenzene was employed, solvent-dependent site-selective Csp3-Csp or Csp3-Csp2 cross-coupling resulted. The nature of these reaction pathways and their selectivity has been investigated by extensive electron paramagnetic resonance (EPR) studies, kinetic studies, and density functional theory (DFT) calculations both to elucidate the mechanism of these coupling reactions and to explain the solvent-dependent site selectivity.

7.
J Org Chem ; 86(3): 2998-3007, 2021 02 05.
Artigo em Inglês | MEDLINE | ID: mdl-33502190

RESUMO

Iodosobenzene (PhIO) is known to be a potent oxidant for alcohols in the presence of catalytic bromide in water. In order to understand this important and practical oxidation process, we have conducted density functional theory studies to shed light on the reaction mechanism. The key finding of this study is that PhIO is not the reactive oxidant itself. Instead, the active oxidant is hypobromite (BrO-), which is generated by the reaction of PhIO with bromide through an SN2-type reaction. Critically, water acts as a cocatalyst in the generation of BrO- through lowering the activation energy of this process. This investigation also demonstrates why BrO- is a more powerful oxidant than PhIO in the oxidation of alcohols. Other halide additives have been reported experimentally to be less effective catalysts than bromide-our calculations provide a clear rationale for these observations. We also examined the effect of replacing water with methanol on the ease of the SN2 reaction, finding that the replacement resulted in a higher activation barrier for the generation of BrO-. Overall, this work demonstrates that the hypervalent iodine(III) reagent PhIO can act as a convenient and controlled precursor of the oxidant hypobromite if the right conditions are present.


Assuntos
Brometos , Água , Álcoois , Catálise , Oxirredução
8.
Chemistry ; 27(10): 3552-3559, 2021 Feb 15.
Artigo em Inglês | MEDLINE | ID: mdl-33210327

RESUMO

A simple gold-catalyzed annulation of 1,8-dialkynylnaphthalenes utilizing a cationic gold catalyst was developed. Such a peri-position of two alkynyl substituents has not been studied in gold catalysis before. Dependent on the substrate, the reactions either follow a mechanism involving vinyl cation intermediates or involve a dual gold catalysis mechanism which in an initial 6-endo-dig-cyclization generates gold(I) vinylidene intermediates that are able to insert into C-H bonds. Indenophenalene derivatives were obtained in moderate to high yields. In addition, the bidirectional gold-catalyzed annulation of tetraynes provided even larger conjugated π-systems. The optoelectronic properties of the products were also investigated.

9.
J Org Chem ; 86(2): 1758-1768, 2021 01 15.
Artigo em Inglês | MEDLINE | ID: mdl-33377772

RESUMO

We have identified and extensively investigated the photochemical activation and reaction of a hydroxyquinone-derived phenyliodonium ylide in the presence of visible light using experiment and theory. These studies revealed that in its photoexcited state this iodonium is capable of facilitating a range of single-electron transfer (SET) processes, including hydrogen atom transfer (HAT), a Povarov-type reaction, and atom-transfer radical addition chemistry. Where possible, we have employed density functional theory (DFT) to develop a more complete understanding of these photoinduced synthetic transformations.

10.
Org Biomol Chem ; 18(40): 8103-8108, 2020 Oct 21.
Artigo em Inglês | MEDLINE | ID: mdl-33006357

RESUMO

A combination of iodosobenzene (PhIO) and molecular iodine (I2) is well-documented to produce a key species capable of conducting various organic reactions through radical mechanisms. This key species is identified here by density functional theory (DFT) calculations to be the hypoiodite radical (IO˙). The calculations show that two equivalents of IO˙ are generated when I2 reacts with two equivalents of PhIO. One of the ensuing IO˙ species acts as a hydrogen abstractor and thus forms an organic radical and the other one is involved in oxidation of the resultant organic radical to afford the final product.

11.
Dalton Trans ; 49(39): 13566-13572, 2020 Oct 12.
Artigo em Inglês | MEDLINE | ID: mdl-32969451

RESUMO

The oxidant diphenyldiselenide reacts with MIIMe2(bipy) (bipy = 2,2'-bipyridine) to form a pre-equilibrium involving weak adducts, from which [MMe2(bipy)]2·Ph2Se2 undergoes rate-limiting dissociation of phenylselenide preceded by the oxidative addition step to obtain [Me2(bipy)M-MMe2(bipy)(SePh)]+. Coordination of PhSe- gives the neutral MIII-MIII bonded dimers [MMe2(bipy)(SePh)]2. The dimers fragment in the presence of light to give radicals [MIIIMe2(bipy)(SePh)]˙. After reorientation in the solvent cage, the radicals interact to form triplet adducts [MIIIMe2(bipy)(SePh)·(bipy)MIIIMe2(SePh)]˙˙ with π-stacked 'SePh·bipy', followed by transformation via a Minimum Energy Crossing Point allowing [SePh]˙ transfer to give MIIMe2(bipy) and MIVMe2(bipy)(SePh)2. The regenerated MII reagent reacts with Ph2Se2 through the above sequence, allowing completion of reaction to give the MIV product only. The reaction of PtMe2(bipy) with diphenyldisulfide has been studied in an analogous manner to assist with interpretation of DFT results for reactions of diphenyldiselenide. In short, this study shows that photochemical cleavage of metal-metal bonds (Pd, Pt) via excitation to an M-M antibonding orbital facilates disproportionation of the MIII-MIII complex to MII and MIV complexes.

12.
Chemistry ; 26(67): 15629-15635, 2020 Dec 01.
Artigo em Inglês | MEDLINE | ID: mdl-32696509

RESUMO

Density Functional Theory studies of square-planar PtII pincer structures, (4-Z-NCN)PtCl ([4-Z-NCN]- =[4-Z-2,6-(Me2 NCH2 )2 C6 H2 -N,C,N]- , Z=H, NO2 , CF3 , CO2 H, CHO, Cl, Br, I, F, SMe, SiMe3 , tBu, OH, NH2 , NMe2 ), enable characterisation of mesomerism for the pincer-Pt interaction. Relationships between Hammett σp substituent parameters of Z and DFT data obtained from NBO6 and AOMix computation are used to probe the interaction of the 5dyz orbital of platinum with π-orbitals of the arene ring. Analogous computation for 2,6-(Me2 CH2 )2 C6 H3 Z (Z=H, CF3 , CHO, Cl, Br, I, F, SMe, SiMe3 , tBu, OH, NH2 ) and (4-H-NCN)PtZ allows an estimation of the relative substituent effects of "(CH2 NMe2 )2 PtZ" on π-delocalisation in the pincer system.

13.
Angew Chem Int Ed Engl ; 59(36): 15492-15496, 2020 Sep 01.
Artigo em Inglês | MEDLINE | ID: mdl-32485034

RESUMO

Herein we report a facile, mild reaction protocol to form carbon-carbon bonds in the absence of transition metal catalysts. We demonstrate the metal-free alkenylation reactions of aryl esters with α-diazoesters to give highly functionalized enyne products. Catalytic amounts of tris(pentafluorophenyl)borane (10-20 mol %) are employed to afford the C=C coupled products (31 examples) in good to excellent yields (36-87 %). DFT studies were used to elucidate the mechanism for this alkenylation reaction.

14.
Org Biomol Chem ; 18(6): 1117-1129, 2020 02 14.
Artigo em Inglês | MEDLINE | ID: mdl-31994575

RESUMO

Density functional theory (DFT) at the SMD/M06-2X/def2-TZVP//SMD/M06-2X/LANL2DZ(d),6-31G(d) level was used to explore the regioselective double oxidation of phenols by a hypervalent iodine(v) reagent (IBX) to give o-quinones. The oxidative dearomatization commences with the ligand exchange between IBX and phenol, yielding a phenolate complex, followed by the first redox process, which reduces iodine(v) to iodine(iii). Both the processes (the ligand exchange and the first redox reaction) were found to be mediated by a less stable isomer of iodine(v) species. We found that although the first redox process preferentially proceeds via an associative pathway, an electron withdrawing substituent on the phenol ring decreases its accessibility. The inspection of the electronic structure of the redox transition state indicates that the phenolate involved in the iodine(v) reduction has some phenoxenium character. The intrinsic stability of a phenoxenium ion is calculated to be highly sensitive to the substituent on the phenol ring. Since the electron withdrawing substituents considerably decrease the stability of the phenoxenium, they render the iodine(v) to iodine(iii) reduction energy consuming. Once the first redox step has completed, a catechol-iodine(iii) complex is formed, from which the second redox process produces the final o-quinone product via a carboxylate-assisted transition structure. This transition structure gains stability by hydrogen bond interaction between the catechol OH and carboxylate group. Such an interaction results in the phenolate not having any phenoxenium character in the transition structure, thus making the activation barrier to the second redox step independent from the substituent on the phenol ring.

15.
Angew Chem Int Ed Engl ; 59(1): 471-478, 2020 Jan 02.
Artigo em Inglês | MEDLINE | ID: mdl-31622542

RESUMO

We report a switchable synthesis of acylindoles and quinoline derivatives via gold-catalyzed annulations of anthranils and ynamides. α-Imino gold carbenes, generated in situ from anthranils and an N,O-coordinated gold(III) catalyst, undergo electrophilic attack to the aryl π-bond, followed by unexpected and highly selective 1,4- or 1,3-acyl migrations to form 6-acylindoles or 5-acylindoles. With the (2-biphenyl)di-tert-butylphosphine (JohnPhos) ligand, gold(I) carbenes experienced carbene/carbonyl additions to deliver quinoline oxides. Some of these epoxides are valuable substrates for the preparation of 3-hydroxylquinolines, quinolin-3(4H)-ones, and polycyclic compounds via facile in situ rearrangements. The reaction can be efficiently conducted on a gram scale and the obtained products are valuable substrates for preparing other potentially useful compounds. A computational study explained the unexpected selectivities and the dependency of the reaction pathway on the oxidation state and ligands of gold. With gold(III) the barrier for the formation of the strained oxirane ring is too high; whereas with gold(I) this transition state becomes accessible. Furthermore, energetic barriers to migration of the substituents on the intermediate sigma-complexes support the observed substitution pattern in the final product.

16.
J Org Chem ; 85(2): 515-525, 2020 Jan 17.
Artigo em Inglês | MEDLINE | ID: mdl-31876155

RESUMO

Density functional theory was utilized to investigate plausible mechanisms for amine and alcohol oxidation by an iodine(V) hypervalent reagent (IBX). In this contribution, we found that amine and alcohol oxidation both proceed by similar mechanisms. The reactions initiate from ligand exchange to give four coordinate intermediates followed by a redox process giving an iodine(III) species and oxidized substrates. Interestingly, for both the ligand-exchange and the redox steps a hypervalent twist is required for the reaction to proceed via an energetically more accessible route. The ligand-exchange process was found to be mediated by a proton-shuttling agent such as water, a second IBX, or a second substrate. While the ligand-exchange step for both amine and alcohol occurs with almost identical activation energy (particularly when water is considered as the shuttling agent), the redox step for the amine takes place with much lower activation energy than that for the alcohol. Finally, we ascertained that five coordinate amide iodine(V) complexes are unreactive toward redox reactions due to the fact that in such cases two electrons from the coordinated amide are required to occupy a 3c-4e σ* orbital which is too high in energy to be reachable.

17.
J Am Chem Soc ; 141(49): 19513-19520, 2019 12 11.
Artigo em Inglês | MEDLINE | ID: mdl-31769667

RESUMO

This Article demonstrates a mild oxidatively induced C(sp3)-H activation at a high-valent Ni center. In contrast with most C(sp3)-H activation reactions at NiII, the transformation proceeds at room temperature and generates an isolable NiIV σ-alkyl complex. Density functional theory studies show two plausible mechanisms for this C-H activation process involving triflate-assisted C-H cleavage at either a NiIV or a NiIII intermediate. The former pathway is modestly favored over the latter (by ∼3 kcal/mol). The NiIV σ-alkyl product of C-H cleavage reacts with a variety of nucleophiles to form C(sp3)-X bonds (X = halide, oxygen, nitrogen, sulfur, or carbon). These stoichiometric transformations can be coupled using N-fluoro-2,4,6-trimethylpyridinium triflate as a terminal oxidant in conjunction with chloride as a nucleophile to achieve a proof-of-principle NiII/IV-catalyzed C(sp3)-H functionalization reaction.

18.
J Am Chem Soc ; 141(32): 12872-12879, 2019 08 14.
Artigo em Inglês | MEDLINE | ID: mdl-31379153

RESUMO

This Article describes the development of a stable NiIV complex that mediates C(sp2)-H trifluoromethylation reactions. This reactivity is first demonstrated stoichiometrically and then successfully translated to a NiIV-catalyzed C-H trifluoromethylation of electron-rich arene and heteroarene substrates. Both experimental and computational mechanistic studies support a radical chain pathway involving NiIV, NiIII, and NiII intermediates.

19.
J Am Chem Soc ; 141(33): 13261-13267, 2019 08 21.
Artigo em Inglês | MEDLINE | ID: mdl-31408327

RESUMO

The treatment of pyridine- and pyrazole-ligated NiII σ-aryl complexes with Selectfluor results in C(sp2)-F bond formation under mild conditions. With appropriate design of supporting ligands, diamagnetic NiIV σ-aryl fluoride intermediates can be detected spectroscopically and/or isolated during these transformations. These studies demonstrate for the first time that NiIV σ-aryl fluoride complexes participate in challenging C(sp2)-F bond-forming reductive elimination to yield aryl fluoride products.

20.
Chemistry ; 25(52): 12180-12186, 2019 Sep 18.
Artigo em Inglês | MEDLINE | ID: mdl-31310400

RESUMO

Precise control of the selectivity in organic synthesis is important to access the desired molecules. We demonstrate a regiospecific annulation of unsymmetrically substituted 1,2-di(arylethynyl)benzene derivatives for a geometry-controlled synthesis of linear bispentalenes, which is one of the promising structures for material science. A gold-catalyzed annulation of unsymmetrically substituted 1,2-di(arylethynyl)benzene could produce two isomeric pentalenes, but both electronic and steric effects on the aromatics at the terminal position of the alkyne prove to be crucial for the selectivity; especially a regiospecific annulation was achieved with sterically blocked substituents; namely, 2,4,6-trimetyl benzene or 2,4-dimethyl benzene. This approach enables the geometrically controlled synthesis of linear bispentalenes from 1,2,4,5-tetraethynylbenzene or 2,3,6,7-tetraethynylnaphthalene. Moreover, the annulation of a series of tetraynes with a different substitution pattern regioselectively provided the bispentalene scaffolds. A computational study revealed that this is the result of a kinetic control induced by the bulky NHC ligands.

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