Expanding the repertoire of cyclopropenium ion phase transfer catalysis: Benzylic fluorination

Conspectus Cyclopropenium ions are the smallest class of aromatic compounds, satisfying Hückel’s rules of aromaticity with two π electrons within a three-membered ring. First prepared by Breslow in 1957, cyclopropenium ions have been found to possess extraordinary stability despite being both cationic and highly strained. In the 65 years since their first preparation, cyclopropenium ions have been the subject of innumerable studies concerning their synthesis, physical properties, and reactivity. However, prior to our work, the reactivity of these unique carbocations had not been exploited for reaction promotion or catalysis. Over the past 13 years, we have been exploring aromatic ions as unique and versatile building blocks for the development of catalysts for organic chemistry. A major portion of this work has been focused on leveraging the remarkable properties of the smallest of the aromatic ionscyclopropeniumsas a design element in the invention of highly reactive catalysts. Indeed, because of its unique profile of hydrolytic stability, compact geometry, and relatively easy oxidizability, the cyclopropenium ring has proven to be a highly advantageous construction module for catalyst invention. In this Account, we describe some of our work using cyclopropenium ions as a key element in the design of novel catalysts. First, we discuss our early work aimed at promoting dehydrative reactions, starting with Appel-type chlorodehydrations of alcohols and carboxylic acids, cyclic ether formations, and Beckmann rearrangements and culminating in the realization of catalytic chlorodehydrations of alcohols and a catalytic Mitsunobu-type reaction. Next, we describe the development of cyclopropenimines as strong, neutral organic Brønsted bases and, in particular, the use of chiral cyclopropenimines for enantioselective Brønsted catalysis. We also describe the development of higher-order cyclopropenimine superbases. The use of tris(amino)cyclopropenium (TAC) ions as a novel class of phase-transfer catalysts is discussed for the reaction of epoxides with carbon dioxide. Next, we describe the formation of a cyclopropenone radical cation that has a portion of its spin density on the oxygen atom, leading to some peculiar metal ligand behavior. Finally, we discuss recent work that employs TAC electrophotocatalysts for oxidation reactions. The key intermediate for this chemistry is a TAC radical dication, which as an open-shell photocatalyst has remarkably strong excited-state oxidizing power. We describe the application of this strategy to transformations ranging from the oxidative functionalization of unactivated arenes to the regioselective derivatization of ethers, C–H aminations, vicinal C–H diaminations, and finally aryl olefin dioxygenations. Collectively, these catalytic platforms demonstrate the utility of charged aromatic rings, and cyclopropenium ions in particular, to enable unique advances in catalysis.

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“…Notably, Dudding has developed a range of phase-transfer catalysts that also incorporate the cyclopropenium ring. − …”

Section: Cyclopropeniums As Phase-transfer Catalystsmentioning

confidence: 99%

Cyclopropenium Ions in Catalysis

Wilson

Lambert

2022

Acc. Chem. Res.

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“…[2][3][4] Among the differently substituted cyclopropenium cations,t he aminocyclopropenium ions first described by Yoshida in 1971, [5] have received special attention. [6] Most recent work has focused on their use as phase transfer, Lewis acid or organocatalysts, [7][8][9][10][11][12][13][14][15][16] electrophotocatalysts, [17] ligands for catalytic metal complexes, [18][19][20] ionic liquids, [21][22][23][24][25] persistent radical cations, [26] redox active polymers for redox flow batteries, [27][28][29][30][31][32] fluorescent materials, [33][34][35] aromatic cations in hybrid halide perovskites, [36] biologically active compounds such as transfection agents, [37][38][39] and nanoparticles. [40,41] Surprisingly, the self-assembly of cyclopropenium compounds into liquid crystalline phases has not been reported.…”

Section: Introductionmentioning

confidence: 99%

Self‐Assembly of Aminocyclopropenium Salts: En Route to Deltic Ionic Liquid Crystals

et al. 2020

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Aminocyclopropenium ions have raised much attention as organocatalysts and redox active polymers. However, the self‐assembly of amphiphilic aminocyclopropenium ions remains challenging. The first deltic ionic liquid crystals based on aminocyclopropenium ions have been developed. Differential scanning calorimetry, polarizing optical microscopy and X‐ray diffraction provided insight into the unique self‐assembly and nanosegregation of these liquid crystals. While the combination of small headgroups with linear p‐alkoxyphenyl units led to bilayer‐type smectic mesophases, wedge‐shaped units resulted in columnar mesophases. Upon increasing the size and polyphilicity of the aminocyclopropenium headgroup, a lamellar phase was formed.

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Carbocations

Moreira

2021

Organic Reaction Mechanisms Series

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Expanding the repertoire of cyclopropenium ion phase transfer catalysis: Benzylic fluorination

Cited by 14 publications

References 74 publications

Cyclopropenium Ions in Catalysis

Cyclopropenium Ions in Catalysis

Self‐Assembly of Aminocyclopropenium Salts: En Route to Deltic Ionic Liquid Crystals

Carbocations

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