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 Introduction
2 Organoiodine-Compound-Mediated Semipinacol Rearrangement via C–X Bond Cleavage
3 2-Iodoazolium-Salt-Catalyzed Reactions through Halogen Bonding (XB)
3.1 TMSI-Co-catalyzed Dehydroxylative Coupling of Alcohols with Organosilanes
3.2 Base-Co-catalyzed Umpolung Alkylation of Oxindoles with an Iodonium(III) Ylide
3.3 Thiourea-Co-catalyzed N-Glycofunctionalization of Amides
3.4 Thiourea-Co-catalyzed N-α-Glycosylation of Amides
4 Catalytic Reactions Using 2-Haloazolium Salts as the Brønsted Acids
4.1 N-β-Glycosylation of Amides
4.2 N-β-2-Deoxyglycosylation of Amides
5 Conclusions
Key words
halogen bonding - organocatalyst - Brønsted acid - Lewis acid - glycosylation - hypervalent iodine
