Activation Of Thiophene Research Articles

Iron-Catalyzed C-H Activation for Heterocoupling and Copolymerization of Thiophenes with Enamines.

C-H/C-H coupling via C-H activation provides straightforward synthetic access to the construction of complex π-conjugated organic molecules. The palladium-catalyzed Fujiwara-Moritani (FM) coupling between an arene and an electron-deficient olefin presents an early example but is not applicable to enamines such as N-vinylcarbazoles and N-vinylindoles. We report herein iron-catalyzed C-H/C-H heterocoupling between enamines and thiophenes and its application to copolymerization of bisenamine and bisthiophene using diethyl oxalate as an oxidant and AlMe3 as a base, as a result of our realization that synthetic limitations in oxidative C-H/C-H couplings imposed by the high redox potential of the Pd(II)/Pd(0) catalytic cycle can be circumvented by the use of iron, which has a lower Fe(III)/Fe(I) redox potential. The trisphosphine ligand provides a coordination environment for iron to achieve the reaction's regio-, stereo-, and chemoselectivity. The reaction includes C-H activation of thiophene via σ-bond metathesis and subsequent enamine C-H cleavage triggered by nucleophilic enamine addition to the Fe(III) center, thereby differing from the FM reaction in mechanism and synthetic scope. The copolymers synthesized by the new reaction possess a new type of enamine-incorporated polymer backbone.

An Unconventional trans - exo -Selective Cyclization of Alkyne-Tethered Cyclohexadienones Initiated by Rhodium(III)-Catalyzed C–H Activation via Insertion Relay

Different from the established trans-endo-selective cyclization of alkyne-tethered electrophiles that involve an E/Z isomerization process, herein, the authors present a novel strategy to allow tra...

Mechanistic Study of Pd/Ag Dual-Catalyzed Cross-Dehydrogenative Coupling of Perfluoroarenes with Thiophenes

The mechanism of the Pd/Ag dual-catalyzed cross-dehydrogenative coupling (CDC) between perfluoroarenes and thiophenes was clarified. The exact role of the catalytic resting state of the homo-biaryl-Pd(II) complex, which was previously overlooked in Pd/Ag dual-catalyzed CDC, was carefully investigated. Detailed experimental studies indicated that a homo-biaryl-Pd(II) complex occurred in the catalytic off-cycle as a resting state and prevented the homocoupling of perfluoroarenes. In addition, these studies indicated that the C–H bond activation steps of perfluoroarenes and thiophenes proceeded by different mechanisms. While the C–H bond of perfluoroarenes was immediately activated by Ag(I) salts, the C–H bond of thiophenes was activated by the Pd(II) catalyst. Kinetic studies revealed that the Pd(II)–C–H activation of thiophenes is most likely the rate-determining step.

Visible-light-mediated C2-amination of thiophenes by using DDQ as an organophotocatalyst.

In this work, a direct C-H activation of thiophenes was presented via an oxidation pathway under visible-light irradiation, in which the thiophene radical cation serves as the key intermediate. Various thiophenes and azoles could be transformed into the corresponding amination products well, and H2O was the only byproduct which is environmentally benign. Our results showed that tert-butyl nitrite (TBN) served as the electron transfer mediator and O2 as the terminal oxidant to regenerate the photocatalyst DDQ and revive the photocatalytic cycle.

Synthesis of Fused Metallaaromatics via Intramolecular C–H Activation of Thiophenes

A convenient method to synthesize novel fused ruthena-/osmacycles via intramolecular C–H activation of thiophenes has been developed. Treatment of HC≡CCH(OH)R (R = 2-thienyl) with RuCl2(PPh3)3 or OsCl2(PPh3)3 afforded hydroxyl-coordinated metal vinyl compounds 1 and 4. Reaction of 1/4 with acid produced metal alkenylcarbene complexes 3/5, which can further convert to the corresponding fused metallaaromatics 2/7 via the C(sp2)–H activation of the thienyl groups. 7 is the first example of a metallabenzyne with a fused five-membered ring (thiophene ring). These fused metallaaromatics are thermally stable both in solution and in the solid state in air. The X-ray crystallographic analysis, NMR spectra, and DFT calculations all suggest that these fused metallaaromatics (2 and 7) show aromatic character.

Synthesis of aromatic ruthenabenzothiophenes via C-H activation of thiophenes.

The first ruthenabenzothiophenes have been achieved via the C-H activation of thiophene. These species feature high thermal stability and resistance of a moderate oxidant, which constitute valuable addition to the rare metallaaromatic containing second-row transition metals.

Oxidation Desulfurization of Thiophene Using Phase Transfer Catalyst/H2O2 Systems

In our study, an effective phosphomolybdic acid/hexadecyltrimethyl-ammonium bromide catalyst for oxidative desulfurization of thiophene in a model compound was formed. The oxidation activities of thiophene for a series of heteropoly acids were estimated. The results show that the oxidation activity of thiophene increased with increasing oxidation time, oxidation temperature, and the volume of 30% H2 O2 oxidant. The optimal values are 150 min, 40°C, and 3 mL, and sulfur removal attained 96.3% when phosphotungstic acid/hexadecyltrimethylammonium bromide was used as a catalyst.

C–S Bond Activation and Partial Hydrogenation of Thiophene by a Dinuclear Trihydride Platinum Complex

Abstract[(dppp)2Pt2H3]ClO4 [I, dppp = Ph2P(CH2)3PPh2] was found to activate the C–S bond and to cause partial hydrogenation of thiophene. Thus, the reaction of I with neat thiophene (T) at reflux temperature yields [(dppp)Pt(SC4H4‐C,S)] (II) and [(dppp)2Pt2(μ‐SC4H5‐C,S)]ClO4 (III) at a 2:3 molar ratio. The same reaction in toluene or benzene solvent affords the same complexes II and III but in a 1:9 molar ratio. The concomitant formation of II and III was interpreted on the basis of theoretical calculations (DFT), which have provided a detailed insight into the reaction mechanism. Thus, the presence of T causes I to dissociate into [(dppp)PtH2] and [(dppp)PtH(C4H4S‐κS)], this process being a common step for the two ensuing reaction pathways. After dissociation, the activation of the thiophene C–S bond to yield II involves participation of [(dppp)PtH2] exclusively. However, the reaction leading to III requires the internal migration of the coordinated hydride to the thiophene ligand in [(dppp)PtH(C4H4S‐κS)] and the subsequent assistance of the {(dppp)Pt} fragment formed from [(dppp)PtH2] by hydrogen elimination. As a result of the involvement of [(dppp)PtH2] in the formation of both II and III, the two reaction pathways are competitive. The reactivity of II and III with various sources of hydride ligands and different protonic acids has also been examined. Thus, the reaction of II with HBF4 leads to the thiolate‐bridged dinuclear complex [(dppp)2Pt2(μ‐SC4H5)2](BF4)2 (IV). In the case of III the addition of HBF4 leads to [(dppp)2Pt2(μ‐SC4H6)](BF4)2 (V), which transforms into III by the addition of a base. The X‐ray structural characterization of the unprecedented complex V is reported here.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Antifungal Activity of Tagetes patula Extracts

In the present paper the methanol extracts obtained from 10 cultivars of Tagetes patula were assayed on two phytopathogenic fungi: Botrytis cinerea and Fusarium moniliforme. B. cinerea showed a high dose-dependent inhibition, with a marked difference between light and dark treatment. F. moniliforme seems to be a more resistant test that does not appear to be affected by the different treatment conditions (light-dark) even at the highest dose. However, it can be asserted that Tagetes patula is a possible source of antifungal substances and that thiophene activity is, in general, strongly increased by UV-A irradiation. During the experiment the method of chromatographic plates was used to evaluate plant extracts bioactivity. The obtained data indicate that it is a rapid method than can be used as an alternative to Petri dish tests.

Reaction coordinate analyses of transition metal catalyzed C [sbnd]H and C [sbnd]S activation steps

The application of first-principle quantum chemical calculations toward analyzing and understanding heterogeneously catalyzed C X bond activation is reviewed. More specifically, two industrially relevant systems, methane activation and thiophene desulfurization are discussed. Density functional theoretical (DFT) calculations are used to examine C H activation of acetate a precursor to acetate decar☐ylation and C S activation of thiophene. Both systems require a sufficient cluster size model for the catalyst in order to predict energetic information, and hence model periodic trends. C H activation of acetate closely mimics the activation of methane. The reaction path is characterized by a late transition state with respect to the C H bond stretch. There is considerable M H and M C bond formation. The predicted barrier is +115 kJ/mol. Thiophene hydrodesulfurization which occurs via η1 adsorption of thiophene, is initially activated by hydrogenating thiophene to 2,5 dihydrothiophene (DHT). DHT activation proceeds via a metal atom insertion into the C S bond. The formation of an additional M S bond (two-fold sulfur) stabilizes the metallocycle intermediate and lowers the barrier of DHT C S bond breaking from +250 to +150 kJ/mol.

Activation of Thiophenes by Superacids: Protonation and Polymerization

TiCl4 forms an adduct with tetramethylthiophene which reacts with HCl to give the Ti2Cl9- salt of C4Me4HS+. When C4H4S is used in place of C4Me4S, the TiCl4/H+ mixtures gives polymers C4nH4nSn(TiCl4)n/3. This work directly relates thiophene protonation to polymerization.

C−H and C−S Activation of Thiophene by Rhodium Complexes: Influence of the Ancillary Ligands on the Thermodynamic Stability of the Products

The reaction of thiophene with the rhodium compound [Tp*Rh(C2H4)(PMe3)] (1a; Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate) leads to a mixture of the C−H and C−S activation products [Tp*Rh(H)(2-C4H3S)(PMe3)] (2a) and [Tp*Rh(CHCHCHCHS)(PMe3)] (3a), respectively. In contrast to previous observations, the former is the thermodynamically preferred isomer. For the PEt3 derivative [Tp*Rh(C2H4)(PEt3)] (1b), an even higher selectivity toward C−H activation is observed.

Theoretical study of the Diels–Alder reaction between the S-methylthiophenium ion and ethene

The capability and activation of thiophene as a diene for the Diels–Alder reactions has been investigated by ab initio methods. The reactants and transition structures are optimized with RHF/3-21 + G*, RHF/6-31 + G* and MP2/6-31 + G*. Energies are evaluated with the same level of theory and by single point calculations employing MP2/6-31 + G*//RHF/6-31 + G* and MP3/6-31 + G*// MP2/6-31 + G* methods. The ab initio calculated geometries are compared with experimental data, where available. The relative reactivity of buta-1,3-diene, thiophene and the S-methylthiophenium ion were estimated by comparing their FMO energy gap with ethene as the dienophile. The predicted activation energies are in full agreement with the qualitative determination of reactivity and suggest that thiophene is not a suitable diene for the Diels–Alder reaction. For the S-methylthiophenium ion, the predicted activation energy may be reached under normal reaction conditions. Thus S-methylthiophenium should be a suitable diene for the Diels–Alder reactions.