Scholl reaction as a powerful tool for the synthesis of nanographenes: a systematic review

Abstract

Nanographenes, or extended polycyclic aromatic hydrocarbons, have been attracting increasing attention owing to their widespread applications in organic electronics. However, the atomically precise fabrication of nanographenes has thus far been achieved only through synthetic organic chemistry. Polycyclic aromatic hydrocarbons (PAHs) are popular research subjects due to their high stability, rigid planar structure, and characteristic optical spectra. The recent discovery of graphene, which can be regarded as giant PAH, has further stimulated research interest in this area. Chemists working with nanographene and heterocyclic analogs thereof have chosen it as their preferred tool for the assembly of large and complex architectures. The Scholl reaction has maintained significant relevance in contemporary organic synthesis with many advances in recent years and now ranks among the most useful C-C bond-forming processes for the generation of the π-conjugated frameworks of nanographene or their heterocyclic analogs. A broad range of oxidants and Lewis acids have found use in Scholl-type processes, including Cu(OTf)₂/AlCl₃, FeCl₃, MoCl₅, PIFA/BF₃-Et₂O, and DDQ, in combination with Brønsted or Lewis acids, and the surface-mediated reaction has found especially wide applications in PAH synthesis. Undoubtedly, the utility of the Scholl reaction is supreme in the construction of nanographene and their heterocyclic analogues. The detailed analysis of the progress achieved in this field reveals that many groups have contributed by pushing the boundary of structural possibilities, expanding into surface-assisted cyclodehydrogenation and developing new reagents. In this review, we highlight and discuss the recent modifications in the Scholl reaction for nanographene synthesis using numerous oxidant systems. In addition, the merits or demerits of each oxidative reagent is described herein.

Fig. 1. (a) C–C bond formation between the two aryl units (intramolecular) through the Scholl reaction. (b) C–C bond formation between the two different aryl units (intermolecular) through the Scholl reaction.

Fig. 2. Oxidative coupling of arenes through the two alternative possible pathways under Scholl reaction conditions.

Scheme 1. Synthesis of hexa-peri-hexabenzocoronene through intramolecular oxidative cyclodehydrogenation.

Scheme 2. Synthesis of tetramethoxytriphenylene using a combination of DDQ and protic acid.

Scheme 3. Synthesis of quinones via the Scholl reaction.

Scheme 4. Synthesis of perylene from naphthalene and binaphthalene.

Scheme 5. Polyphenyl synthesis via the Scholl reaction.

Scheme 6. C–C bond formation in anthraquinone-derived dyes.

Scheme 7. Naphthotetraphene synthesis.

Scheme 8. Phthaloyl carbazole synthesis.

Scheme 9. Oxidative cross coupling of 2-naphthols.

Scheme 10. Synthesis of enantiomerically pure binaphthol derivatives.

Scheme 11. Selective cross coupling induced by CuCl₂.

Scheme 12. Planar polycyclic aromatic hydrocarbon synthesis.

Scheme 13. Oxidative cyclodehydrogenation of oligophenylene to the PAH.

Scheme 14. Terylene synthesis via AlCl₃.

Scheme 15. HBC synthesis by the Scholl reaction.

Scheme 16. Arene–arene coupling by the Scholl reaction.

Scheme 17. Synthesis of quaterrylenes via the Scholl reaction.

Scheme 18. Synthesis of oligophenylphenyl–TBTQ derivatives by the Scholl reaction leading to the condensed and threefold TBTQ hydrocarbons.

Scheme 19. Tetrabenzocronene synthesis by the Scholl reaction.

Scheme 20. AlCl₃-mediated cyclodehydrogenation for C–C bond formation.

Scheme 21. AlCl₃-mediated intramolecular Scholl reaction.

Scheme 22. Scholl coupling reaction via AlCl₃.

Scheme 23. AlCl₃-mediated Scholl reaction of benzoylnaphthalene.

Scheme 24. Synthesis of perylene via the Scholl reaction.

Scheme 25. POF synthesis by the AlCl₃-mediated Scholl reaction.

Scheme 26. Triphenylene synthesis employing FeCl₃.

Scheme 27. Binaphthyl synthesis by FeCl₃ catalysis.

Scheme 28. Synthesis of pyrrole-fused azacoronenes employing FeCl₃.

Scheme 29. Synthesis of graphene nanoribbons by the Scholl reaction.

Scheme 30. Synthesis of sterically crowded o-terphenyl crown ether.

Scheme 31. Synthesis of bisindeno-annulated pentacenes.

Scheme 32. Cyclodehydrogenation of polyphenylenes by FeCl₃.

Scheme 33. Fused aromatic compound synthesis by the Scholl reaction.

Scheme 34. Twofold Scholl cyclization by FeCl₃.

Scheme 35. Hexabenzocoronene synthesis via the Scholl reaction.

Scheme 36. FeCl₃-mediated Scholl reaction for π-extension.

Scheme 37. Nanographene synthesis by the Scholl reaction using FeCl₃.

Scheme 38. Synthesis of soluble HBCs from hexaarylbenzene precursors.

Scheme 39. Synthesis of a series of HBC derivatives containing EWGs via DDQ.

Scheme 40. DDQ-induced synthesis of grossly-warped graphene molecules.

Scheme 41. Synthesis of biphenylene-based PAH by DDQ/MeSO₃H.

Scheme 42. Graphene syntheses with an embedded 7-membered ring by cyclodehydrogenation.

Scheme 43. Synthesis of triptycenes by DDQ.

Scheme 44. Synthesis of fullerene pentagons with DDQ.

Scheme 45. DDQ-promoted oxidative cycloheptatriene ring formation around the TBTQ core.

Scheme 46. DDQ-mediated Scholl-type oxidative cycloheptatriene formation; a fourfold bridging of the fenestrindanes core with the electron-rich aryl units.

Scheme 47. Skeletal rearrangement of PAH by DDQ.

Scheme 48. Synthesis of benzo-fused carbohelicene by DDQ.

Scheme 49. Synthesis of tetracene via the Scholl reaction.

Scheme 50. DDQ–TfOH-promoted Scholl macrocyclization for porous nanographene syntheses.

Scheme 51. Synthesis of nanographene unit via the Scholl reaction.

Scheme 52. Synthesis of azulene nanographene via the Scholl reaction.

Scheme 53. Synthesis of helical azulene-based nanographene via the Scholl reaction.

Scheme 54. Synthesis of eight-membered ring-embedded nanographene by the Scholl reaction.

Scheme 55. Synthesis of graphene nanobelts via the Scholl reaction.

Scheme 56. Synthesis of the π-extended nanographene from the three-fold bay-bridged TBTQ derivative by DDQ/TfOH.

Scheme 57. Synthesis of substituted triphenylenes using MoCl₅.

Scheme 58. Construction of a heptagonal ring using MoCl₅.

Scheme 59. Synthesis of thianthrene with MoCl₅.

Scheme 60. Synthesis of metasequirin via MoCl₅.

Scheme 61. Screening of the MoCl₅ oxidant for the oxidative dimerization reaction.

Scheme 62. Biomimetic synthesis of dendridine.

Scheme 63. Formation of π-extended dithia[6]helicene.

Scheme 64. Oxidative coupling of olefin.

Scheme 65. Oxidative coupling of benzyl ethers with PIFA.

Scheme 66. Oxidative coupling of bithiophenes with PIFA.

Scheme 67. Oxidative coupling of bipyrroles.

Scheme 68. Oxidative coupling of bipyrroles.

Scheme 69. Oxidative arylation of ternaphthyls.

Scheme 70. Oxidative cyclization using PIFA–BF₃·ET₂O.

Scheme 71. A controlled oxidative intramolecular arene–alkene coupling reaction.

Scheme 72. An example of benzocarbazole synthesis.

Scheme 73. Indolcarbazole synthesis.

Scheme 74. Synthesis of highly conjugated acenes.

Scheme 75. Oxidative coupling of a tertiary benzamide with toluene.

Scheme 76. Pd catalyzed APEX of PAHs.

Scheme 77. π-extension of perylene by the Scholl reaction.

Scheme 78. Oxidative coupling of secondary and tertiary aromatic amines under the Scholl reaction.

Scheme 79. Biaryl construction via Scholl reaction with TlTf₃.

Scheme 80. Biaryl coupling with vanadium oxyfloride.

Scheme 81. PAHs extension with CoF₃.

Scheme 82. Biaryl coupling via the Scholl reaction.

Scheme 83. Synthesis of triphenylenes with vanadium oxytrichloride.

Scheme 84. Synthesis of the heptagonal ring by iodoarene using the Scholl reaction.

Scheme 85. Synthesis of decinine with vanadium oxyfloride.

Scheme 86. Synthesis of phenanthrene with MnO₂/TFA.

Scheme 87. Bithiophene synthesis by HTIB-activators.

Scheme 88. C–C coupling of naphthalene units with AgSO₄.

Scheme 89. Mechanochemical cyclodehydrogenation of hexaphenylbenzene by the Scholl reaction.

Scheme 90. Oxidative cyclodehydrogenation by electrochemical synthesis.

Scheme 91. C–C coupling of porphyrin units using Au(111).

Scheme 92. Graphene nanoribbon synthesis on gold surface.

Scheme 93. Synthetic strategy for GNRs with zigzag edges by gold catalysis on the surface.

Scheme 94. On-surface cyclization of o-dihalotetracenes to four and six-membered rings.

Scheme 95. Synthetic route toward peri-pentacene on the Au(111) surface.

Scheme 96. Synthetic route to the peri-hexacene derivative on gold surface.

Scheme 97. On-surface growth dynamics of graphene nanoribbons.

Scheme 98. Trigonal porous nanographene synthesis.

Scheme 99. Non-planar porous nanographene synthesis.

Scheme 100. Synthesis of π-extended BODIPYs.

Scheme 101. Synthesis of molecular nanographenes.

Ehsan Ullah Mughal

Saleh A. Ahmed