Brønsted Acid Catalyzed Carbocyclizations Involving Electrophilic Activation of Alkynes

Abstract This review focuses on the carbocyclization reactions of alkyne-containing substrates catalyzed by Brønsted acids. The electrophilic activation occurs either by direct protonation of the alkyne or by formation of an intermediate cation that further reacts with the alkyne to give a key vinyl cation intermediate. Specific focus is placed on the selectivity of the various methodologies described herein and on mechanistic rationales. 1 Introduction 2 Brønsted Acid Catalyzed Alkyne Carbocyclization Involving C(sp) Reaction Partners 2.1 Brønsted Acid Catalyzed Cyclization of Diynes 2.2 Brønsted Acid Catalyzed Cyclization of Allenynes 3 Brønsted Acid Catalyzed Alkyne Carbocyclization Involving C(sp2) Reaction Partners 3.1 Brønsted Acid Catalyzed Cycloisomerization of Enynes 3.1.1 Initial Protonation of the Alkene Function 3.1.2 Initial Protonation of the Alkyne Function 3.1.3 A Conia-Ene Reaction 3.2 Electrophile-Induced Carbocyclization Involving Other C(sp2)-Based Reaction Partners 3.2.1 Iminium Electrophiles 3.2.2 Oxonium Electrophiles 3.2.3 Isocyanate and Thioisocyanate Electrophiles 3.3 Brønsted Acid Catalyzed Cycloisomerization of Aryl-alkynes 3.3.1 Reactions Using Excess or Stoichiometric Amounts of Brønsted Acid 3.3.2 Reactions Using Substoichiometric Amounts of Brønsted Acid 3.3.3 Heteroatom-Substituted Alkynes 3.3.4 Enantioselective Brønsted Acid Catalyzed Cycloisomerization of Aryl-alkynes 4 Brønsted Acid Catalyzed Alkyne Carbocyclization Involving C(sp3) Reaction Partners: Alkyne-alkane Cycloisomerization 5 Conclusion and Outlook