Pyrrole

5 Key excerpts on "Pyrrole"

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  eBook - PDF

  Pyrroles, Volume 48, Part 1

  • Richard A. Jones(Author)
  • 2009(Publication Date)
  • Wiley-Interscience

    (Publisher)

  The very low basicity compared with typical secondary amincs is also evidence for the delocalization of the lone pair of electrons on nitrogen. Several different types of theoretical and physical criteria have also been cited in support of the aromaticity of Pyrroles (see Chapter I ) . The resonance energy of Pyrrole has been estimated by several different authors from thermochemical data and also by using equilibrium data’-5 as, for example, the comparison of the free-energy change resulting from protonation at positions 2 and 3 of the Pyrrole ring with that for the corresponding 295 296 Reactivity of the 1 H-Pyrrole Ring System protonation of model dihydropyridines and dihydroPyrroles. The average of the values obtained2 is about 100 kJ mol- ', which agrees with calculations6 of the resonance energy obtained by the valence-bond method (103 kJ mol- '). This is somewhat less than the value for benzene (130-140 kJ mol- ') and accords with the accepted view that Pyrrole is less aromatic than benzene. Resonance energy determinations of the related 5-membered aromatic heterocycles, thiophen and furan, give the order of aromaticity: benzene > thiophen > Pyrrole > f ~ r a n . ~ Further physical evidence suppporting delocalization of the Ir-electrons of Pyrrole is provided by bond-length measurements and dipole moments. Elec- tron diffraction, and microwave, spectroscopic measurements, show that the C2-C3 bond (133.2 pm) is slightly longer, the C 3 X 4 bond (141.7 pm) slightly shorter, and the N-C2 bond (137.0 pm) much shorter than the corresponding bonds in cyclopentadiene (1 34, 147, and 15 1 pm, respectively). Comparisons with the bond lengths in benzene, thiophen, and furan again lead to the relative order of reactivity benzene > thiophen > Pyrrole > furan, on the basis of the criterion that the more aromatic the ring, the closer are the C .

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  Progress in Heterocyclic Chemistry

  • Gordon Gribble, John A. Joule, Gordon W. Gribble(Authors)
  • 2016(Publication Date)
  • Elsevier

    (Publisher)

  H functionalization. Although the total synthesis of natural products is not a focus of this monograph, certain key steps may be covered. Review articles published in 2015 will be noted in the appropriate sections.

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  5.2.2. Synthesis of Pyrroles

  Advances in the synthesis and application of natural and artificial bioactive Pyrrole derivatives were reviewed (15EJMC176 ). The role of Pyrrole as a small molecule in medicinal compounds was covered by Bhardwaj (15RSCA15233 ). Titanium-catalyzed multicomponent couplings as efficient one-pot syntheses of Pyrroles and nitrogen heterocycles were reviewed (15ACR2822 ).

  5.2.2.1. Intramolecular Approaches to Pyrroles

  5.2.2.1.1. Intramolecular Type a

  Gevorgyan and coworkers demonstrated that imine 1 , upon treatment with a gold catalyst, underwent a migratory cycloisomerization process to furnish Pyrrole 2 . If silylated imines were instead used as substrates for the reaction, a double migratory process occurred leading to 2,3,4-substituted Pyrroles (such as 3 ). Notably, if aldehydes were used in place of the imines, substituted furans were obtained in excellent yields (15TL3251 ).

  Tetrasubstituted Pyrrole 5 was prepared by the treatment of 1,3-enyne 4 with aniline under copper catalysis. The sequence is tolerant of a variety of electron-donating or electron-withdrawing groups on the aniline; if phenylhydrazine was instead used as the nitrogen source, pyrazole 6 was obtained in 93% yield (15OBC2786 ).

  Jiang and colleagues reported a novel, palladium-catalyzed approach to N -aryl Pyrrole 9 . The reaction is proposed to proceed first by an intermolecular Heck reaction with boronic acid 8 and the pendant alkene on 7 . The intermediate then undergoes an intramolecular aza-Wacker cyclization to give the final product. An advantage of this approach is the ready accessibility of both of the aniline and boronic acid starting materials as well as the good yields of the polysubstituted Pyrrole products (15JOC1235 ).

  5.2.2.1.2. Intramolecular Type c

  Wan reported the base-selective synthesis of Pyrrole 11 and bicycle 12 from a common scaffold, enyne 10 . If R=OPh, the reaction is suggested to proceed through an intermediate allene, which cyclizes onto the enamine to provide vinyl Pyrrole 11 in moderate to good yields. If, however, R is an aryl or alkyl group, the intermediate allene is proposed to undergo a [2 + 2]-cycloaddition with the alkene, followed by a formal 1,3-hydrogen shift and 1,3-sulfonyl migration leading to heterocycle 12 (which was supported by a series of deuterium labeling studies) (15OL3944

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  Progress in Heterocyclic Chemistry

  • Gordon Gribble, J. Joule, Gordon W. Gribble(Authors)
  • 2011(Publication Date)
  • Elsevier Science

    (Publisher)

  Chapter 5.2

  Five-membered ring systems: Pyrroles and benzo analogs

  Erin T. Pelkey [email protected]     Hobart and William Smith Colleges, Geneva, NY 14456

  5.2.1 INTRODUCTION

  Pyrroles and indoles are likely the most studied and reported of all heterocyclic ring systems and there was a noticeable increase in citations this year compared to previous years.

  Pyrrole chemistry investigated by the Banwell (Pyrrole alkaloids) <05COC1589 > and Jacobi (phytochrome) <05SL2861 > research groups has been reviewed. A number of indole review articles have been published that are based on the work of a number of leading research groups including: Borschberg (Aristotelia alkaloids) <05COC1465 >, Gribble (electron-deficient and [b ]-fused indoles) <05COC1493 >, Knölker (carbazole alkaloids) <05COC1601 >, Takayama (Corynanthe alkaloids) <05COC1445 >, and Umani-Ronchi (stereoselective alkylation reactions) <05SL1199 >. A monograph detailing the preparation and reactions of 5,6-dihydroxyindoles and indole-5,6-diones was published <05AHC1 >. A comprehensive review of the synthesis and elaboration of indoles utilizing palladium catalysts has appeared <05CR2873 >. Additional review articles will be mentioned later in the text.

  5.2.2 SYNTHESIS OF PyrroleS

  In order improve the organization of this section, Pyrrole syntheses have been categorized utilizing the systematic approach utilized by Sundberg <96CHEC-II119 >. Intramolecular approaches (type I) and intermolecular approaches (type II) are classified by the number and location of the new bonds that describe the Pyrrole ring forming step as shown opposite.

  5.2.2.1 Intramolecular Approaches

  A few metal-mediated type Ia cyclizations have been reported. A gold(I)-catalyzed addition of an azide to a proximal alkyne was utilized to prepare 2,5-disubstituted Pyrroles and higher functionalized Pyrroles <05JA11260 >. For example, treatment of azido alkyne 1 with a gold(I) catalyst led to the formation of Pyrrole 2 presumably via gold(I) activation of the alkyne. In a related sequence utilized to prepare N -bridgehead Pyrroles 3 , silver nitrate was utilized to induce the addition of vinylogous carbamates onto pendant alkynes <05EJO505

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  Isocyanide Chemistry

  Applications in Synthesis and Material Science

  • V. Nenajdenko(Author)
  • 2012(Publication Date)
  • Wiley-VCH

    (Publisher)

  Figure 11.1  – is soluble in organic solvents, and is converted into insoluble tetrabenzoporphyrin (TBP) by heating at 150 °C. This process is employed in the fabrication of a p-type semiconductor that is based on TBP and used in solar cells [16] or field-effect transistors (FETs) [17], via a solution process.

  Figure 11.1  Typical Pyrrole derivatives found in nature or used in the materials sciences.

  The boron–dipyrromethene (BODIPY) dye, when fused with a phenanthrene ring, demonstrates a bright, long-wavelength fluorescence that is useful for detection purposes in microscopy [18]. In addition, cyclo[8]Pyrroles with long alkyl side chains have potential utility as chemosensors for nitroaromatic explosives [19]. In order to prepare such materials, Pyrroles fused with either a bicyclic ring or an aromatic ring, or substituted with long alkyl groups are required. In this situation, the Barton–Zard reaction is more suitable for the synthesis of such Pyrroles, and is far superior to the classical Knorr method.

  11.2 Synthesis of Pyrroles Using TosMIC

  In 1972, van Leusen and coworkers reported a useful method for the synthesis of Pyrroles, by using TosMIC and electron-deficient alkenes. The process is shown in Scheme 11.1 , where the isocyano function undergoes typical α-addition reactions while the tosyl group serves two functions, serving as a leaving group and also enhancing the acidity of the α-carbon. Today, TosMIC is better known as van Leusen’s reagent, and has been used extensively in the synthesis of a variety of heterocyclic compounds [3]. Thus, Pyrroles with carbonyl, cyano, and nitro groups at the β-positions may be readily prepared, as shown in Scheme 11.1 . Quinone can be also used for the van Leusen reaction, with fused Pyrrole 2 being obtained via reaction with naphthoquinone [20] (Scheme 11.2

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  eBook - PDF

  Progress in Heterocyclic Chemistry

  • Gordon Gribble, J. Joule, Gordon W. Gribble(Authors)
  • 2009(Publication Date)
  • Elsevier Science

    (Publisher)

  The total synthesis of 77 is still an unsolved problem. N N HN N HN N O H 2 N H NH 2 Cl NH 2 HO H 12 17 20 77 N H H N O O N H N H Br Br NH N N NH NH 2 NH 2 78 N N B F F 79 Five-membered ring systems: Pyrroles and benzo analogs 133 Reports of total syntheses of complex Pyrrole natural products continue unabated. An enantioselective total synthesis of dimeric Pyrrole natural product sceptrin 78 was achieved; this compound served as the key precursor to structurally related natural products including ageliferin <07JACS4762>. Several total syntheses of dibromophakellstatin 41 were reported <07S2756; 07JOC8076; 07JACS7768>. Other notable total syntheses reported include: ageladine A <07JOC4892; 07T9112>, manzacidin B <07OL1765>, tambjamines <07OL5127>, lukianol A <07S608>, and longamide B 45 <07OL2357; 07TL3459>. 5.2.4.2 Pyrrole Materials Due to space constraints, only a few review articles that discuss the synthesis and chemistry of novel classes of Pyrrole materials will be discussed here. A comprehensive review of the important class of dyes, known collectively as BODIPY dyes (difluoro-4-bora-3a,4a-diaza-s -indacene 79 ), appeared <07CRV4891>. Reviews of carboporphyrins <07EJO5461> and acyclic oligoPyrroles <07EJO5313> were also published. 5.2.5 SYNTHESIS OF INDOLES Whether in the pursuit of specific targets for pharmaceutical application or general methods for directing bond forming processes, enthusiasm for the synthesis of indoles continues to be manifest in an outpouring of new literature on that topic. And as the repertoire of methodology for the construction or functionalzation of the indole nucleus grows, so too do the challenges set forth by mother nature as new discoveries continue to reveal the modest indole core nestled within a fascinating range of ornamental frameworks.

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