Reactions Catalyzed by 2-Halogenated Azolium Salts: From Halogen-Bond Donors to Brønsted-Acidic Salts

Abstract

Our research group has developed a variety of organocatalysts, especially bi- and multi-functional hydrogen-bond (HB)-donor catalysts. Since 2013, we have become interested in halogen-bond (XB) interactions in organic synthesis, and we have focused on the development of organocatalysts using XBs. Although it is difficult to develop otherwise inaccessible transformations using XBs as the primary interaction, we found several unique reactions that use XB interactions in combination with co-catalysts such as trimethylsilyl iodide, Proton Sponge, and Schreiner’s thiourea. During the synthesis of various 2-iodoazolium salts that can serve as XB donors, a ‘protonated’ 2-iodoazolium salt (a Brønsted-acidic salt) was unexpectedly obtained instead of the corresponding ‘alkylated’ 2-iodoazolium salt (XB donor). The obtained Brønsted-acidic salt is unprecedentedly effective for the N-glycosylation of amides. This account summarizes our findings in this area to date.1 Introduction2 Organoiodine-Compound-Mediated Semipinacol Rearrangement via C–X Bond Cleavage3 2-Iodoazolium-Salt-Catalyzed Reactions through Halogen Bonding (XB)3.1 TMSI-Co-catalyzed Dehydroxylative Coupling of Alcohols with ­Organosilanes3.2 Base-Co-catalyzed Umpolung Alkylation of Oxindoles with an ­Iodonium(III) Ylide3.3 Thiourea-Co-catalyzed N-Glycofunctionalization of Amides3.4 Thiourea-Co-catalyzed N-α-Glycosylation of Amides4 Catalytic Reactions Using 2-Haloazolium Salts as the Brønsted Acids4.1 N-β-Glycosylation of Amides4.2 N-β-2-Deoxyglycosylation of Amides5 Conclusions