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Highly regioselective and efficient borylation of a variety of porphyrins has been achieved by reaction with bis(pinacolato)diboron through C-H bond activation under iridium catalysis on the basis of the synthetic protocol developed by Miyaura, Hartwig, and Smith. A boryl group can be selectively introduced at sterically uncongested positions in the peripheral aryl groups of porphyrin substrates whose peripheral beta-positions are sterically hindered. Curiously, beta substituents adjacent to the aryl group to be borylated have unexpectedly large effects on the regioselectivity, because the iridium catalyst can discriminate between subtle steric differences. Chemoselective borylation was also achieved for several functionalized porphyrins. This borylation protocol can be applied to various monomeric and oligomeric functional porphyrins, hence offering an efficient route to elaborate multiporphyrin-based molecular constructs.

Steric interactions between methoxy and tert-butyl groups are the reason that 2 only exists as p-phenylenebis(N-tert-butylaminoxyl) B. The p-phenylenebis(N-tert-butylaminoxyl) 1 has the p-quinonediimine N,N'-dioxide structure Q in solution as well as in the crystalline state., (© 1998 WILEY-VCH Verlag GmbH, Weinheim, Fed. Rep. of Germany.)