N-Heterocyclic Carbenes in Late Transition Metal Catalysis

In the investigation of efficient chemical transformations, the carbon-carbon bond-forming reactions play an outstanding role. In this context, organocatalytic processes have achieved considerable attention. 1 Beside their facile reaction course, selectivity, and environmental friendliness, new synthetic strategies are made possible. Particularly, the inversion of the classical reactivity (Umpolung) opens up new synthetic pathways. 2 In nature, the coenzyme thiamine (vitamin B1), a natural thiazolium salt, utilizes a catalytic variant of this concept in biochemical processes as nucleophilic acylations. 3 The catalytically active species is a nucleophilic carbene. 4

Organocatalysis by N-Heterocyclic Carbenes

Organocatalytic Reactions Enabled by N-Heterocyclic Carbenes.

Modern synthetic methods have revolutionized polymer chemistry through the development of new and powerful strategies for the controlled synthesis of complex polymer architectures. 1-5 Many of these developments were spawned by new classes of transition metal catalysts for the synthesis of new polyolefin microstructures, 5 the design of highly efficient families of “living” polymerization strategies for the synthesis of block, graft, and star polymers, 6-12 controlled methods for the synthesis of dendritic macromolecules, 3,13,14

Organocatalytic ring-opening polymerization.

Organocatalyzed reactions represent an attractive alternative to metal-catalyzed processes notably because of their lower cost and benign environmental impact in comparison to organometallic catalysis. In this context, N-heterocyclic carbenes (NHCs) have been studied for their ability to promote primarily the benzoin condensation. Lately, dramatic progress in understanding their intrinsic properties and in their synthesis have made them available to organic chemists. This has resulted in a tremendous increase of their scope and in a true explosion of the number of papers reporting NHC-catalyzed reactions. Here, we highlight the ever-increasing number of reactions that can be promoted by N-heterocyclic carbenes.

N‐Heterocyclic Carbenes as Organocatalysts

Chiral N-heterocyclic carbenes as asymmetric acylation catalysts

Refluxing of a mixture of benzaldehyde (1a), 1, 3-dimethylbenzimidazolium iodide (7), p-nitroaniline (9b) as an oxidizing agent, and 1, 8-diazabicyclo[5, 4, 0]-7-undecene (DBU) in MeOH afforded methyl benzoate (2a) in good yield. Other methyl arenecarboxylates 2 were similarly obtained from arenecarbaldehydes 1. We showed that this aroylation proceeds via the 2-aroyl-1, 3-dimethylbenzimidazolium salt (8). The 1, 2, 4-triazolium salt (18) and the naphtho[1, 2-d]imidazolium salt (19) were also effective catalysts for this oxidative aroylation. However, the aroylation did not proceed with the imidazolium salt (20). In the presence of flavins (25a-c), arenecarbaldehydes 1 reacted in MeOH under aerobic conditions catalyzed by the benzimidazolium salt 7 to give the corresponding methyl arenecarboxylates 2. 1-Methyl-3-[3-(10-phenylisoalloxazin-3-yl)propyl]benzimidazolium bromide (27) is an effective complex catalyst for this oxidative aroylation.

Catalytic Action of Azolium Salts. VIII. Oxidative Aroylation with Arenecarbaldehydes Catalyzed by 1, 3-Dimethylbenzimidazolium Iodide

Benzoin reaction in water as an aqueous medium catalyzed by benzimidazolium salt

Nucleophilic acylation of arylfluorides catalyzed by imidazolidenyl carbene.

In the presence of 1, 3-dimethylimidazolium iodide (1), 6-chloro-9-phenyl-9H-purine (7) and 4-chloro-5, 6-dimethylpyrrolo[2, 3-d]pyrimidines 40-42 undrwent uncleophilic aroylation with arenecarbaldehydes (5) to give the corresponding fused aroylpyrimidines 8 and 43-45. 1, 3-Dimethylbenzimidazolium iodide (2) was an effective catalyst for the similar synthesis of 7-aroyl-3-phenyl-3H-1, 2, 3-triazolo[4, 5-d]pyrimidines 16-21. In the synthesis of 4-aroyl-1H-pyrazolo[3, 4-d]pyrimidines 26-32, both azolium salts 1 and 2 were effective as catalysts. Moreover, 4-aroyl-7H-pyrrolo[2, 3-d]pyrimidines 43-45 were obtained in good yields via the 4-tosyl derivatives, in the presence of catalytic amounts of sodium p-toluenesulfinate (46) and the imidazolium salt 1. This catalytic aroylation was found to be a facile and useful method for the synthesis of 6-aroyl-9H-purines and their analogues.

Catalytic Action of Azolium Salts. IX. Synthesis of 6-Aroyl-9H-purines and Their Analogues by Nucleophilic Aroylation Catalyzed by Imidazolium or Benzimidazolium Salt

Yumiko Suzuki

Papers

Chiral N-heterocyclic carbenes as asymmetric acylation catalysts

Catalytic Action of Azolium Salts. VIII. Oxidative Aroylation with Arenecarbaldehydes Catalyzed by 1, 3-Dimethylbenzimidazolium Iodide

Benzoin reaction in water as an aqueous medium catalyzed by benzimidazolium salt

Nucleophilic acylation of arylfluorides catalyzed by imidazolidenyl carbene.

Catalytic Action of Azolium Salts. IX. Synthesis of 6-Aroyl-9H-purines and Their Analogues by Nucleophilic Aroylation Catalyzed by Imidazolium or Benzimidazolium Salt