Mass spectra of new heterocycles: XVI. Electron impact study of alkyl 5-aminothiophene-2-carboxylates

Electron impact mass spectra of alkyl 4-alkoxy-5-amino-3-methylthiophene-2-carboxylates were studied for the first time. These compounds, except for 4-(1-ethoxyethoxy) and 4-(ferrocenylmethoxy) derivatives, give rise to a stable molecular ion whose decomposition follows three pathways. The main fragmentation pathway of the molecular ion is elimination of alkyl radical from the 4-alkoxy group, the second pathway involves expulsion of alkoxy group from the ester moiety, and the third pathway is decomposition of the thiophene ring. The molecular ions of 4-(1-ethoxyethoxy)thiophenes decompose mainly via elimination of ethyl vinyl ether molecule with formation of [M–VinOEt]⁺ · odd-electron ion, and fragmentation of the latter follows general pathways. In the mass spectra of 4-(ferrocenylmethoxy)thiophenes the most abundant are ferrocenylmethyl ion with m/z 199 (I _rel 100%) and fragment ions derived therefrom.     

Alkenylation of thiophenes at the 2-position with magnesium alkylidene carbenoids

Treatment of magnesium alkylidene carbenoids, generated from 1-chlorovinyl p-tolyl sulfoxides with isopropylmagnesium chloride at −78 °C in toluene, with 2-lithiothiophenes gave 2-alkenylated thiophenes in good to high yields. The intermediate of this reaction was found to be an alkenylmagnesium, which could be trapped with iodoalkanes and ethyl chloroformate. This procedure offers a novel and efficient one-pot synthesis of thiophenes having a disubstituted or a trisubstituted olefin at the 2-position from thiophenes in good yields.     

A metal-free,one-pot route to substituted benzo[b]thiophenes and their hetero-fused analogs via iodine mediated intramolecular arylthiolation of in situ generated β-(het)aryl-β-cyanoenethiolates

A metal-free one-pot route to substituted 3-cyanobenzo[b]thiophenes has been developed via iodine mediated intramolecular arylthiolation of 2-(het)aryl-2-cyanoenethiolates generated in situ by base mediated condensation of arylacetonitriles and (het)aryl dithioesters. The methodology has been further extended to the synthesis of 2-aminobenzo[b]thiophenes as well as hetero-fused thiophenes such as thieno[2,3-b]thiophenes, thieno[2,3-b]indoles and thieno[3,2-c]pyrazoles. An electrophilic cyclization mechanism has been proposed based on experimental observations.     

Facile Synthesis of α-Polythienyls via 1,4-Diketones

Unsubstituted and substituted linear polythienyls including thiophenes in both even and odd numbers are conveniently prepared from the Michael addition of thiophenecarboxaldehydes to Mannich bases and the vinyl sulfone. Cyclization of the resulting 1,4-diones with Lawesson's reagent gives α-polythienyls.     

Synthesis and characterization of novel styryl-substituted oligothienylenevinylenes

A number of 3-monosubstituted bis(thienylvinyl)thiophenes, suitable for the preparation of electronically-variable poly(thienylenevinylene)s have been synthesized for the first time. These materials have been characterized by both NMR spectroscopy and mass spectrometry, and a single crystal X-ray structure analysis of (E,E)-3-(5,5-dimethyl[1,3]dioxin-2-yl)-2,5-bis(2-thien-2-ylvinyl)thiophene has shown that the planarity of the terthienylenevinylene chain is maintained on substitution at the 3 position of the central thiophene ring. UV/visible spectroscopy measurements are reported and cyclic voltammetric measurements show that, with the exception of (E,E,E)-2,5-bis(2-thien-2-ylvinyl)-3-(2-(4-dimethylaminophenyl)vinyl)-thiophene, electroactive films are produced on electrochemical oxidation of the monomers.     

New liquid crystalline compounds based on 2-arylthiophenes and 2-(biphenyl-4-yl)thiophenes

A general and versatile route to the 2-alkyl-5-(4'-cyanophenyl)thiophenes was established, involving the condensation of an aryl vinyl ketone with an aldehyde in the presence of a thiazolium catalyst (Stetter procedure) to give a 1,4-diketone which with Lawesson's reagent undergoes ring-closure to give the corresponding arylthiophene. In the final step the exchange of a bromo substituent on the phenyl ring for a cyano group is accomplished by copper(I) cyanide in N,N-dimethylforamamide at reflux. The same sequence afforded 2-alkyl-5-(4'-cyanobiphenylyl)thiophenes. Several of the 1,4-diketones were obtained by conjugate addition of nitroalkanes to aryl vinyl ketones, and treatment of the γ-nitroketones with silica gel-supported potassium permanganate. Three alkyl 5-(4-cyanophenyl)thiophene-2-carboxylates were prepared by condensing β-chlorovinylaldehydes with thioglycolates. The cyanophenylthiophenes exhibited only monotropic phases, but incorporation of an additional phenyl ring provided cyanobiphenylylthiophenes of wide nematic ranges. Transition temperatures of binary mixtures of the cyanophenylthiophenes with 4-n-pentyl-4'-cyanobiphenyl were measured, and extrapolated virtual nematic-isotropic transition temperatures were determined.     

Action of alkali metals in liquid ammonia on substituted thiophenes

The action of lithium and alcohols in liquid ammonia on α-substituted thiophenes with subsequent hydrolysis of the products leads to conversion of the thiophene fragment of the molecule to a butyryl group. A number of amino ketones of the aliphatic series were obtained from α-(ω-dialkylaminoalkyl)thiophenes.     

Selective cis-addition of C-H bonds of pyrroles and thiophenes to alkynes catalyzed by a dinuclear palladium complex

Pyrroles and thiophenes reacted with alkynes in the presence of dinuclear palladium complexes with high stereoselectivity (cis-addition) in almost all cases. While regioselectivity in the reaction with pyrroles depended on substituents on the nitrogen atom and alkynes, all reactions of thiophenes afforded 2-alkenylthiophenes.     

Site-specific preparation of 4-substituted-6-fluoro(carboalkoxyl)benzo[b]furans and benzo[b]thiophenes via base-catalyzed cyclization of enyne derivatives

A site-specific synthesis of 4-substituted-6-fluoro(carboalkoxyl)benzo[b]furans and benzo[b]thiophenes is described. The reactions of heterocyclic aromatic aldehydes with a Wittig reagent, followed by Sonogashira reaction with terminal alkynes, and subsequent base-catalyzed cyclization site-specifically provide 4-substituted-6-fluoro(carboalkoxyl)benzo[b]furans and benzo[b]thiophenes in good yields.     

A convenient approach towards 2- and 3-aminobenzo[b]thiophenes

Reaction of 1-(2-chloro-5-nitrophenyl)ethanone via Willgerodt-Kindler route with primary or secondary amines and sulfur allows a simple, efficient one-pot synthesis of 3-aminobenzo[b]thiophenes. Base-catalyzed transformation of 4-(2-chloro-5-nitrophenyl)-1,2,3-thiadiazole in the presence of primary and secondary amines offers a convenient approach towards 2-aminobenzo[b]thiophenes.     

Use of 1,3-dipolar reactions for the preparation of SF5-substituted five-membered ring heterocycles. Pyrroles and thiophenes

In situ-generated unsubstituted, “parent” azomethine and thiocarbonyl ylides are used to prepare a large variety of 3-aryl- and alkyl-substituted, 4-pentafluorosulfanylpyrroles and 3-aryl-substituted, 4-pentafluorosulfanylthiophenes, the latter of which are to our knowledge the first reported SF₅-substituted thiophenes. The 1,3-cycloadditions of these ylides with aryl and alkyl, SF₅-alkynes produce dihydro-pyrroles and thiophenes, which without isolation can then be oxidatively aromatized to the respective pentafluorosulfanylpyrroles and thiophenes in good yield.     

Synthesis of thiophenes and pyranone fused thiophenes by base induced inter and intramolecular C-S and C-C bond formation: a non-catalytic approach

An efficient and concise one pot synthesis of highly functionalized thiophenes and pyranone fused thiophenes has been delineated through base induced ring transformation of suitably functionalized 2H-pyran-2-ones by ethyl thioglycolate by inter and intramolecular C-S and C-C bond formation.     

A new synthesis of benzo[b]thiophenes utilizing an interrupted Pummerer reaction

A convenient synthesis of 3-arylbenzo[b]thiophenes utilizing an interrupted Pummerer reaction of 2-(1-arylvinyl)phenyl ethyl sulfoxides is described. Thus, treatment of these sulfoxides, which were readily prepared from 2-sulfanylphenyl ketones or 2-fluoro-5-methoxybenzaldehyde, with acetic anhydride at 100 °C afforded 3-arylbenzo[b]thiophenes in reasonable yields.     

Chiral Macrocycles Having C3 Symmetry Resulting from Orientation of Thiophene Rings

An chiral Rh^II-catalyzed cyclooligomerization reaction of thiophenes having triazolyl and vinyl substituents at the 2- and 4-positions was studied. Structurally interesting cyclic trimers, having chirality that is ascribed only to the orientation of the 2,4-disubstituted thiophene rings, are obtained. The 2,4-disubstitution of the starting thiophene monomer allows production of each of the enantiomers. The observed electronic circular-dichroism spectra are in accord with those simulated by density-functional theory calculations.     

Straightforward Synthesis of 3‐Aminothiophenes Using Activated Amides

Herein, we describe a facile approach towards the synthesis of diversely substituted 3‐aminothiophenes. A wide range of functional groups can be incorporated at the C(2), C(4), and C(5) positions of the thiophenes, and this route is also suitable for the synthesis of fused bicyclic heterocycles such as 3‐aminotetrahydrobenzothiophenes. This methodology relies on a 6π‐electrocyclization involving a vinyl sulfide linked to a keteniminium salt, the latter being formed in‐situ through activation of the corresponding amide with triflic anhydride.     

Palladium-catalyzed alkenation of thiophenes and furans by regioselective C-H bond functionalization

A palladium-catalyzed direct alkenation of thiophenes and furans has been developed in the presence of AgOAc and pyridine. A variety of olefinic substrates such as acrylates, acrylamides, and acrylonitrile can perform the direct oxidative coupling reactions with various thiophenes and furans to give the mono-alkenylated products in good yields. In most cases, the (E)-isomers were isolated as the major products.     

Dipyridyl/pyridinium thieno[2,3-b]thiophenes as new atropisomeric systems. Synthesis, conformational analysis and energy minimization

Synthesis of a new class of cofacially oriented dipyridyl(pyridinium)lthieno[2,3-b]thiophenes with or without -CO₂Et and -COMe substituents at C2, and C5 positions of thieno[2,3-b]thiophene ring was readily accomplished using a double Dieckman cyclization protocol as the key step. While C2/C5 substituted dipyridylthieno[2,3-b]thiophenes exhibited syn/anti atropisomerism at least up to 70 °C with Arrhenius energy of activation (ΔG^≠) in the range of 17-18 kcal/mol, on the other hand unsubstituted dipyridylthieno[2,3-b]thiophene and its bis-N-quaternized salt were found to show free conformational rotation with an estimated ΔG^≠ of lower than 10 kcal/mol. Conformational energy minimization using AM1 protocol revealed a slight preference for the anti over syn isomers. Compared to the unsubstituted dipyridylthieno[2,3-b]thiophenes, higher energy barriers to rotation (3.7-5.1 kcal/mol) in substituted dipyridylthieno[2,3-b]thiophenes can be attributed to steric encumbrance resulting from -CO₂Et and -COMe substituents located on the non-rotating thienothiophene platform.     

An efficient phosphine-free palladium coupling for the synthesis of new 2-arylbenzo[b]thiophenes

Straightforward and rapid access to 2-arylbenzo[b]thiophenes has been developed. It involved a catalytic coupling of 3-activated benzo[b]thiophenes with several aryl halides in the presence of a phosphine-free palladium system. In case of fragile functional groups such as aldehydes, a quaternary ammonium was used as an additive as with the other substrates, the coupling performed better and faster in the presence of a crown ether, the best one being DCH-18-C-6, with good yields and low reaction times. This method would provide a direct access to novel structures of biological interest.     

An efficient synthesis of 3-aryl-2-aryl(or methyl)sulfanylbenzo[b]thiophenes via cyclization of aryl 2-aryl(or methyl)sulfanylmethylsulfanylphenyl ketones

A convenient method for the preparation of 2-aryl(or methyl)sulfanylbenzo[b]thiophenes has been developed. Thus, aryl 2-aryl(or methyl)sulfanylmethylsulfanylphenyl ketones, easily prepared from readily available aryl 2-sulfanylphenyl ketones or 2-chloro-5-nitrophenyl phenyl ketone, are treated with LDA in 1,2-dimethoxyethane (DME) to give 3-aryl-2-aryl(or methyl)sulfanylmethylsulfanyl-2,3-dihydrobenzo[b]thiophen-3-ols, which in turn were dehydrated with thionyl chloride to afford 3-aryl-2-aryl(or methyl)sulfanylbenzo[b]thiophenes in reasonable overall yields.     

Insights into the reaction of trans-diarylethenes with thionyl chloride: a practical synthesis of chlorobenzo[b]thiophenes

The reactivity of a variety of trans-1,2-diarylethenes with thionyl chloride has been investigated. All the substrates could readily afford 3-chlorobenzo[b]thiophenes in moderate yields and a pair of threo- and erythro-1,2-dichloro-1,2-diarylethanes as the minor products under mild condition. The diastereoisomers of 1,2-dichloro-1,2-diarylethanes with a methoxy group on the benzene ring were found for the first time to be also converted into cholobenzo[b]thiophenes under treatment of thionyl chloride. The chemical transformations have been mechanistically rationalized and successfully applied to a one-pot synthesis of novel fluorescent liquid crystalline compounds containing benzo[b]thiophene and 1,3,4-oxadiazole units. Their spectroscopic and mesogenic behaviours are also described.