Cyclo[4]pyrrole with α-β direct linkages

A cyclo[4]pyrrole bearing pyrrole C(α)-C(β) direct linkages, a contracted porphyrin analogue with no meso-carbon bridge, was synthesized from an oligoketone-related precursor. X-ray crystallography and StrainViz analysis revealed a non-planar structure with a total strain of 20.8 kcal mol-1. The cyclo[4]pyrrole emits fluorescence in the visible region with a quantum yield of 0.026. The NICS calculations indicated a local 6π-aromatic character for each pyrrole unit, and the global π-electronic communication among the four pyrrole units was shown by the frontier orbitals in the neutral form. The cyclo[4]pyrrole underwent reversible stepwise electrochemical two-electron oxidation and the spin density of the radical cation intermediate localized on the 3,2':5',3''-terpyrrole moiety. However, spectroelectrochemical measurements and theoretical calculations indicated the contribution of a triplet diradical dication form with the spin density delocalized across the four pyrrole units and smaller dihedral angles between neighboring aromatic rings. The structural and electrochemical behavior of cyclo[4]pyrrole demonstrated the effects of ring contraction and recombination of the pyrrole-pyrrole linkages on the cyclo[n]pyrrole system.

Transition-Metal-Catalyzed Silylation and Borylation of C-H Bonds for the Synthesis and Functionalization of Complex Molecules

The functionalization of C-H bonds in organic molecules containing functional groups has been one of the holy grails of catalysis. One synthetically important approach to the diverse functionalization of C-H bonds is the catalytic silylation or borylation of C-H bonds, which enables a broad array of downstream transformations to afford diverse structures. Advances in both undirected and directed methods for the transition-metal-catalyzed silylation and borylation of C-H bonds have led to their rapid adoption in early-, mid-, and late-stage of the synthesis of complex molecules. In this Review, we review the application of the transition-metal-catalyzed silylation and borylation of C-H bonds to the synthesis of bioactive molecules, organic materials, and ligands. Overall, we aim to provide a picture of the state of art of the silylation and borylation of C-H bonds as applied to the synthesis and modification of diverse architectures that will spur further application and development of these reactions.