A Homage to Siegfried Hünig and His Research

One-pot nucleophilic substitution-double click reactions of biazides leading to functionalized bis(1,2,3-triazole) derivatives

The nucleophilic substitution of benzylic bromides with sodium azide was combined with a subsequent copper-catalyzed (3 + 2) cycloaddition with terminal alkynes. This one-pot process was developed with a simple model alkyne, but then applied to more complex alkynes bearing enantiopure 1,2-oxazinyl substituents. Hence, the precursor compounds 1,2-, 1,3- or 1,4-bis(bromomethyl)benzene furnished geometrically differing bis(1,2,3-triazole) derivatives. The use of tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine (TBTA) as ligand for the click step turned out to be very advantageous. The compounds with 1,2-oxazinyl end groups can potentially serve as precursors of divalent carbohydrate mimetics, but the reductive cleavage of the 1,2-oxazine rings to aminopyran moieties did not proceed cleanly with these compounds.

The Story of the Little Blue Box: A Tribute to Siegfried Hünig

The tetracationic cyclophane, cyclobis(paraquat-p-phenylene), also known as the little blue box, constitutes a modular receptor that has facilitated the discovery of many host-guest complexes and mechanically interlocked molecules during the past 35 years. Its versatility in binding small π-donors in its tetracationic state, as well as forming trisradical tricationic complexes with viologen radical cations in its doubly reduced bisradical dicationic state, renders it valuable for the construction of various stimuli-responsive materials. Since the first reports in 1988, the little blue box has been featured in over 500 publications in the literature. All this research activity would not have been possible without the seminal contributions carried out by Siegfried Hünig, who not only pioneered the syntheses of viologen-containing cyclophanes, but also revealed their rich redox chemistry in addition to their ability to undergo intramolecular π-dimerization. This Review describes how his pioneering research led to the design and synthesis of the little blue box, and how this redox-active host evolved into the key component of molecular shuttles, switches, and machines.

Tf2O/TTBP (2,4,6-Tri-tert-butylpyrimidine): An Alternative Amide Activation System for the Direct Transformations of Both Tertiary and Secondary Amides

Ten types of Tf₂O/TTBP-mediated amide transformation reactions were investigated. The results showed that compared with pyridine derivatives 2,6-di-tert-butyl-4-methylpyridine (DTBMP) and 2-fluoropyridine (2-F-Pyr.), TTBP can serve as an alternative amide activation system for the direct transformation of both secondary and tertiary amides. For most surveyed examples, higher or comparable yields were generally obtained. In addition, Tf₂O/TTBP combination was used to promote the condensation reactions of 2-(tert-butyldimethylsilyloxy)furan (TBSOF) with both tertiary and secondary amides, the one-pot reductive Bischler-Napieralski-type reaction of tertiary lactams, and Movassaghi and Hill's modern version of the Bischler-Napieralski reaction. The value of the Tf₂O/TTBP-based methodology was further demonstrated by the concise and high-yielding syntheses of several natural products.