Synthesis of Bench-Stable N-Quaternized Ketene N,O-Acetals and Preliminary Evaluation as Reagents in Organic Synthesis

<div>N-Quaternized ketene N,O-acetals are typically an unstable, transient class of compounds most commonly observed as reactive intermediates. In this report, we describe a general approach to a variety of benchstable N-quaternized ketene N,O-acetals via treatment of pyridine or aniline bases with ethoxyacetylene and an appropriate Brønsted or Lewis acid (triflic acid, triflimide, or scandium(III) triflate). The resulting pyridinium and anilinium salts may be used as reagents or synthetic intermediates in multiple reaction types. For example, N-(1-ethoxyvinyl) pyridinium or anilinium salts can thermally release highly reactive O-ethyl ketenium ions that undergo electrophilic aromatic substitution with electron rich arenes. N-(1-ethoxyvinyl)-2-halopyridinium salts can also be utilized in peptide couplings as a derivative of Mukaiyama reagents, or react with alcohol and amines in nucleophilic aromatic substitutions.</div>

Synthesis of Bench-Stable N-Quaternized Ketene N,O-Acetals and Preliminary Evaluation as Reagents in Organic Synthesis

<div>N-Quaternized ketene N,O-acetals are typically an unstable, transient class of compounds most commonly observed as reactive intermediates. In this report, we describe a general approach to a variety of benchstable N-quaternized ketene N,O-acetals via treatment of pyridine or aniline bases with ethoxyacetylene and an appropriate Brønsted or Lewis acid (triflic acid, triflimide, or scandium(III) triflate). The resulting pyridinium and anilinium salts may be used as reagents or synthetic intermediates in multiple reaction types. For example, N-(1-ethoxyvinyl) pyridinium or anilinium salts can thermally release highly reactive O-ethyl ketenium ions that undergo electrophilic aromatic substitution with electron rich arenes. N-(1-ethoxyvinyl)-2-halopyridinium salts can also be utilized in peptide couplings as a derivative of Mukaiyama reagents, or react with alcohol and amines in nucleophilic aromatic substitutions.</div>

Strategies for the Synthesis of N-Arylammonium Salts

The N-arylation of tertiary amines to provide sp3 quaternary ammonium salts is a challenge in organic chemistry. To date, no general method for such arylations has been established. Here, we summarize a variety of strategies that have been tested, starting with harsh nucleophilic aromatic substitutions, through to the use of copper catalysis and the application of strong electrophiles, such as phenyl cations and benzynes. The achievements and limitations of each method are summarized, and the challenges yet to be met in the synthesis of charged ammonium compounds are described.1 Introduction2 Alkylation of Anilines: The Menshutkin Reaction3 Arylations3.1 Nucleophilic Aromatic Substitutions by Tertiary Amines3.2 Preparation of N-Arylpyridinum Salts from Zincke and Pyrylium Salts3.3 Arylations Using Phenyl Cations3.4 Copper-Catalyzed Arylation of N-Heteroarenes3.5 Benzynes as Aryl Electrophiles4 Conclusions and Perspective