Conversion of Aryl Azides to Aminopyridines.

A longstanding challenge in fundamental functional group interconversion has been the direct transformation of benzene into pyridine via nitrogen insertion and carbon deletion. Herein, we report a protocol for the transformation of aryl azides, easily accessible from their corresponding anilines, to 2-aminopyridines using blue light and oxygen. Mechanistic studies corroborate that the arene to pyridine conversion is achieved by nitrogen insertion into the benzene ring followed by oxidative carbon extrusion.

Expanded Helicenes: A General Synthetic Strategy and Remarkable Supramolecular and Solid-State Behavior.

A divergent synthetic strategy allowed access to several members of a new class of helicenes, the "expanded helicenes", which are composed of alternating linearly and angularly fused rings. The strategy is based on a three-fold, partially intermolecular [2+2+n] (n = 1 or 2) cycloaddition with substrates containing three diyne units. Investigation of aggregation behavior, both in solution and in the solid state, revealed that one of these compounds forms an unusual homochiral, π-stacked dimer via an equilibrium that is slow on the NMR time scale. The versatility of the method was harnessed to access a selenophene-annulated expanded helicene that, in contrast to its benzannulated analogue, exhibits long-range π-stacking in the solid state. The new helicenes possess low racemization barriers, as demonstrated by dynamic 1H NMR spectroscopy.

Enantioselective Cyclobutenylation of Olefins Using N-Sulfonyl-1,2,3-Triazoles as Vicinal Dicarbene Equivalents.

Cyclobutenes are highly useful synthetic intermediates as well as important motifs in bioactive small molecules. Herein, we report a regio-, chemo-, and enantioselective synthesis of cyclobutenes from olefins using N-sulfonyl-1,2,3-triazoles as vicinal dicarbene equivalents or alkyne [2 + 2] cycloaddition surrogates. Terminal and cis-olefins can be transformed into enantioenriched cyclopropanes via rhodium catalysis. Then, in one pot, treatment of these intermediates with tosyl hydrazide and base effects diazo formation followed by rhodium-catalyzed ring expansion to yield enantioenriched cyclobutenes. These cyclobutenes can be transformed into highly substituted, enantioenriched cyclobutanes, including structures relevant to natural product scaffolds.