Push-pull structures with a pyrazine core and hexatriene chain: synthesis and light-emitting properties

In this paper, we describe the synthesis of various push-pull molecules with a central pyrazine unit connected to a hexatriene chain terminated by various p-substituted phenyl groups. The key steps involve metallation and subsequent transmetallation of 2-chloro and 2,6-dichloropyrazine followed by a Negishi cross-coupling reaction of the intermediate organozinc derivative with (2E,4E)-5-bromopentadienal. The aldehydes are then submitted to a Wittig reaction with the appropriate phosphonium salts readily obtained from various substituted benzyl alcohols. The light-emitting properties of the so obtained molecules are then investigated in terms of absorption and emission spectra and non-linear optics experiments have been carried out in two cases.     

Synthesis of a novel ditopic ligand incorporating directly bonded 1,10-phenanthroline and 2,2′:6′,2″-terpyridine units

The synthesis of a rigid ditopic ligand incorporating a 1,10-phenanthroline directly connected through its 3-position to the 5-position of a 2,2′:6′,2″-terpyridine is described. The synthesis is based on a series of palladium(0)-catalyzed cross-coupling reactions (Stille and Suzuki couplings) starting from 1,10-phenanthroline and bromo-substituted pyridines.     

Toward poly(aminophthalimide), structures of dimers and trimers

Poly(aminophthalimide) (PAP) dimers and trimers have been synthesized by palladium-catalyzed cross-coupling reactions of 3-aminophthalimides with 3-chloro- and 3,6-dichlorophthalimide, respectively. When Pd(OAc)₂, XPhos (2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl), and K₃PO₄ are used, the C-N bond-forming reactions proceed quantitatively. The structures of those oligomers are examined by experimental and theoretical techniques including NMR, IR, single-crystal X-ray diffraction, and DFT calculations. The strong preference for cisoid structure of the diphthalimidylamine unit bearing a bifurcate hydrogen bonding is disclosed. Therefore, the aminophthalimide backbone is a highly promising candidate for the construction of a dynamically ordered helical structure.     

2,5,7-Trisubstituted benzo[b]furans through a copper- and/or palladium-catalyzed assembly and functionalization process

A straightforward approach to the synthesis of 2,5,7-trisubstituted benzo[b]furans from 2-bromo- and 2-chloro-6-iodo-4-substituted phenols through a consecutive copper- and/or palladium-catalyzed assembly and functionalization process is described. Functionalization at the C(7) position is carried out by Suzuki-Miyaura cross-coupling and C-N bond forming reactions.     

One-pot synthesis of 5,5′-dibromo-2,2′-dipyridylacetylene and its boronic acid derivative

5,5′-Dibromo-2,2′-dipyridylacetylene was prepared from 2,5-dibromopyridine and (trimethylsilyl)acetylene via the new one-pot synthesis approach using a regioselective palladium-catalyzed coupling reaction with a 60% yield. Several protocols of lithium-halogen exchange were then attempted to synthesize 6,6′-(1,2-ethynediyl)bis[3-pyridylboronic acid] from 5,5′-dibromo-2,2′-dipyridylacetylene. The former was successfully obtained with a 54% yield by a reverse addition method using toluene and THF and it showed potential as a useful building block for cross-coupling reactions in the formation of carbon-carbon bonds.     

Palladium-catalysed reactions of 6-halogeno-1,1′-binaphthyl derivatives. A detailed investigation of structure/reactivity and structure/selectivity relationships

Five 6-halogeno-binaphthyl derivatives of different structure were synthesised starting from 2,2′-dihydroxy-1,1′-binaphthyl 1. Several new 6-substituted binaphthyl compounds were obtained via the palladium-catalysed reactions of these derivatives. The reactivity of 6-iodo derivatives was much greater in most cases. In cross-coupling reactions the 6-bromo compounds were converted into the products using longer reaction times and/or higher temperatures. The reactivity difference between the two types of substrates was especially marked in aminocarbonylation and Heck reactions.     

An efficient synthesis of 2-substituted 6-methylpurine bases and nucleosides by Fe- or Pd-catalyzed cross-coupling reactions of 2,6-dichloropurines

Fe-catalyzed cross-coupling reactions of 9-substituted or protected 2,6-dichloropurines with 1 equiv of methylmagnesium chloride gave regioselectively 2-chloro-6-methylpurines in good yields. The same reactions with 3 equiv of methylmagnesium chloride or Pd-catalyzed reactions with trimethylaluminum afforded 2,6-dimethylpurines. The 2-chloro-6-methylpurines underwent another coupling with phenylboronic acid to give 6-methyl-2-phenylpurines. All reactions were perfomed for Bn- and THP-protected purine bases as well as for acyl-protected ribosides and 2-deoxyribosides. After deprotection, free purine bases and nucleosides were obtained.     

Synthesis of planar-chiral bridged bipyridines and terpyridines by metal-mediated coupling reactions of pyridinophanes

We have accomplished efficient synthesis of planar-chiral bridged 2,2′-bipyridine (S)-6, C₂-symmetric bipyridinophane (S,S)-7, bridged 2,2′:6′,2″-terpyridines (S)-11, and C₂-symmetric terpyridine (S,S)-12 by metal-mediated biaryl cross-coupling or homo-coupling reactions of the corresponding 6-halo[10](2,5)pyridinophanes. Stille-type and Negishi cross-coupling reactions are particularly useful for the syntheses of 6, 11, and 12. On the other hand, nickel-mediated homo-coupling reaction worked best for achieving the synthesis of structurally unique bipyridinophane 7.     

Synthesis, characterization and primary evaluation of the synthetic efficiency of supported vinyltins and allyltins

Supported vinyltins and allyltins grafted to an insoluble cross-linked polystyrene matrix were prepared using methods usually employed in solution, like hydrostannylation of alkynes, transmetallation of a tin halide with organomagnesium or organozinc reagents, and substitution of an allyl halide by a supported stannylanion or S_N2′ substitution of a supported β-stannylacrolein acetal by cyanocopper reagents in the presence of boron trifluoride etherate. The insoluble grafted organotin reagents were analysed by HRMAS NMR, allowing an unambiguous assignment of their isomeric distribution or the identification of side products. When involved in Stille cross-coupling reactions (vinyltins) or in addition on aldehydes (allyltins), these supported reagents exhibit similar reactivity and similar stereoselectivity when compared to the tributyltin analogues, with the advantage to prevent problems due to the contamination by tin residues.     

Synthesis of aminopyrimidylindoles structurally related to meridianins

The synthesis of new meridianin derivatives substituted at the C-5′ position of the 2-aminopyrimidine ring by various aryl groups and substituted or not by a methyl group on the indole nitrogen is described. The 2-aminopyrimidine ring was obtained via a Bredereck synthesis. Aryl groups were introduced by Suzuki cross-coupling after bromination of the 2-aminopyrimidine ring at the C-5′ position.     

Six- and seven-membered ring carbenes: Rational synthesis of amidinium salts, generation of carbenes, synthesis of Ag(I) and Cu(I) complexes

Reactions of neat 1,3- and 1,4-dibromides with N,N′-diarylformamidines in the presence of diisopropylethylamine (DIPEA) afford corresponding amidinium salts in high yields (>80%). Six- and seven-membered ring amidinium salts bearing bulky Mes (2,4,6-Me₃C₆H₂) and Dipp (2,6-ⁱPr₂C₆H₃) aryl groups were prepared using this method. Free six-membered ring carbene 6-Dipp was generated from amidinium salt using LiHMDS as a base. NHC-Ag(I) complexes were obtained by the reactions of amidinium salts with Ag₂O. NHC complexes of Pd and Rh are not accessible by deprotonation of amidinium salts, nor by transmetallation of Ag(I) complexes. However NHC-Cu(I) complexes were obtained by transmetallation of NHC-Ag(I). Thus, transmetallation of six- and seven-membered NHC-Ag(I) complexes was documented for the first time.     

Synthesis of chiral binaphthalenes using the asymmetric Suzuki reaction

The synthesis of atropisomeric 1,1′-binaphthalenes can be achieved using an asymmetric Suzuki cross-coupling reaction. The Suzuki reaction leading to such hindered compounds is challenging and competing hydrolytic deboronation frequently dominates unless carefully chosen conditions are employed. The simple, standard mechanism is inadequate when describing the Suzuki coupling of hindered partners. Evidence suggests that the key step leading to asymmetry is transmetallation (delivery of the organometallic by the asymmetric ligand) and the reactions operate under kinetic control. Reductive elimination (itself likely to be triggered by oxidative addition of another molecule of halide) is fast compared with equilibration (epimerisation and/or cis-trans isomerisation).     

Palladium-catalyzed aryl-aryl cross-coupling reaction using ortho-substituted arylindium reagents

A range of biaryl compounds (aryl-aryl, aryl-heteroaryl, and heteroaryl-heteroaryl) can be efficiently prepared by a palladium-catalyzed cross-coupling reaction between ortho-substituted triarylindium reagents and aryl halides. The triarylindium reagents are prepared by directed ortho-lithiation and transmetallation to indium from the corresponding benzene derivatives using various directed metallation groups (DMGs). The reaction proceeds smoothly in high yields and short reaction times with high atom economy (the three aryl groups attached to indium are efficiently transferred).     

The synthesis, structures, and electrochemistry of 1′-heteroaryl-2,5-dimethylazaferrocenes

We report the synthesis of 1′-heteroaryl-2,5-dimethylazaferrocenes (heteroaryl = 2-bromothiophen-5-yl and 2-bromopyrid-6-yl) via palladium catalysed cross-coupling reactions along with their crystallographically determined structures. The newly synthesized compounds as well as the parent 2,5-dimethylazaferrocene have been investigated by cyclic voltammetry (CV) and were found to exhibit reversible first oxidations followed by an irreversible oxidation at considerably higher potential.     

Competitive ionization of tetraphenylporphyrin in a laser-generated metal ion plasma

The ionization of tetraphenylporphyrin (TPP) in a laser-desorbed metal ion plasma is examined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Competitive reaction pathways observed to generate abundant molecular ion species include electron detachment, cation attachment, charge exchange, metallation, and transmetallation in the positive ion mode and electron capture, metallation, and transmetallation in the negative ion mode. In general, cation attachment reactions dominate positive ion spectra below the laser irradiance threshold for plasma ignition, although the metallation product from [TPP]⁺ reaction with the metal atom, M, is observed. Negative ion products are not observed in the FT-ICR spectrum when a plasma is not formed. Under plasma ignition conditions, positive ion spectra include [TPP]⁺ formed by charge exchange with M⁺, which is also present in the spectrum. Negative ion spectra are dominated by [TPP]^?; which is formed by attachment to thermal electrons generated in the plasma. Metallation reactions involving TPP and the metal substrate are examined. Positive ion metallation products are observed both in the absence of a plasma through reaction of [TPP]⁺ with M and by a second pathway under plasma ignition conditions through reaction of TPP with M⁺. In negative ion mode, metallation is only observed under plasma ignition conditions through reaction of [TPP]^? with M. Observation of metallated products is found to be consistent with formation of stable metal oxidation states in the metallated porphyrin.     

Stepwise construction of novel zig-zag shaped thiophene-based back-to-back terpyridine ligands with acetylenic tethers

A series of structurally well-defined molecular-scale wires bearing one to five 2,5-diethynyl-3,4-dibutylthiophene spacers and 2,2′:6′,2′′-terpyridine capping units were synthesized by a combination of cross-coupling reactions and deprotection steps. Palladium-catalyzed cross-coupling of halogenated thiophene units with acetylene grafted terpyridine effectively yields ethynylated thiophenes. Deprotection of the propargylic fragment provides additional building blocks, which could be used in an iterative sequence of reactions. Reliable and practical synthetic routes are now available for the construction of very large polytopic frameworks.     

A selective and direct synthesis of 2-bromo-4-alkylthiophenes: Convenient and straightforward approaches for the synthesis of head-to-tail (HT) and tail-to-tail (TT) dihexyl-2,2′-bithiophenes

A straightforward method for the synthesis of 2-bromo-4-alkylthiophenes was developed, and the desired products were obtained in the highest chemical yields (>90%) reported to date. 2-Bromo-4-alkylthiophenes were synthesized by regioselective lithiating of 3-alkylthiophenes with n-BuLi and quenching with bromine at −78 °C. Moreover, a simple and efficient protocol for the synthesis of dihexyl-2,2′-bithiophenes was developed by employing 2-bromo-4-hexylthiophene instead of the commonly used monomer, 2-bromo-3-hexylthiophene. Kumada and Suzuki cross-coupling reactions were conducted to synthesize the desired products as head-to-tail (HT) and tail-to-tail (TT) regioisomers in high yields and excellent selectivity.     

Synthesis of 6,8,9-tri- and 2,6,8,9-tetrasubstituted purines by a combination of the Suzuki cross-coupling, N-arylation, and direct C-H arylation reactions

Novel practical methodology of synthesis of a several types of di-, tri-, and tetraarylpurine derivatives by a combination of regioselective Suzuki cross-coupling reactions and/or Cu-catalyzed N-arylation with direct C-H arylations was developed. 6,8-Diaryl- and 2,6,8-triaryl-9-isopropylpurines were prepared by one or two cross-couplings of 6-chloro- or 2,6-dichloro-9-isopropylpurine with arylboronic acids followed by Pd-catalyzed C-H arylation by aryl halides to position 8. 6-Chloropurine and adenine underwent Cu-catalyzed N-arylation to position 9 with boronic acids, followed by cross-coupling with AlMe3 and/or C-H arylation to obtain 8,9-diaryl-6-methylpurines or 8,9-diaryladenines (accompanied by products of partial N-arylation of adenine in position 6). The methodology is suitable for construction of small libraries of modified purines.     

Intramolecular arylation reactions: first efficient synthesis of novel fused pyridoimidazoquinolinones or pyridoimidazoazepinones libraries

An efficient method for the synthesis of new polycyclic skeletons: pyrido[2′,1′:2,3]imidazo[5,4-c]quinolin-6(5H)-ones and pyrido[2′,1′:2,3]imidazo[5,4-c]azepin-7(6H)-ones libraries is described via Pd-catalyzed intramolecular arylation involving C(sp2)-H activation. This method permits the synthesis of polycyclic derivatives in good yield. The process tolerates a variety of aryl substituents as well as alkyl imidazo[1,2-a]pyridine-2-carboxamide structures. The resultant compounds, 10(11)-chloro-pyrido[2′,1′:2,3]imidazo[5,4-c]quinolinones or azepinones are functionalized under Suzuki cross-coupling conditions to give polyfunctional compounds.     

Synthesis of novel planar-chiral [2.2]paracyclophane derivatives as potential ligands for asymmetric catalysis

The synthesis of a variety of new 4,5-disubstituted [2.2]paracyclophane derivatives has been achieved employing different cross-coupling reactions. By this methodology, a heteroatom-variation of successful catalyst ligands was achieved, giving rise to a modular ligand system. The X-ray structure of 4-hydroxy-5-(1′-hydroxy-1′-phenylethyl)-[2.2]paracyclophane was determined to elucidate the configuration. Additionally, a diastereoselective synthesis of planar- and central-chiral 4-([2.2]paracyclophanyl)ethylamine was achieved, thus resulting in a planar- and central-chiral phenyl ethylamine analogue.