Facile Formation of Stable Neutral Radicals and Cations from [22]Smaragdyrin BF₂ Complexes

Abstract

meso‐Trimesityl‐substituted [20]smaragdyrin freebase was synthesized by p‐toluenesulfonic acid catalyzed reaction of 5‐mesityldipyrromethane and 2,14‐dibromodipyrrin in an improved yield of 63 %. Unexpectedly, treatment of the [20]smaragdyrin freebase with BF3 ⋅ OEt2 and triethylamine (TEA) gave a stable radical species, in which the BF2 unit is coordinated at the tripyrrin site, probably by ready release of a hydrogen atom of a [22]smaragdyrin BF2 complex. Similar treatment of [22]smaragdyrin free base produced another [22]smaragdyrin BF2 complex, in which the BF2 unit is coordinated at the dipyrrin site. The tripyrrin site coordinated neutral radical was oxidized with AgSbF6 to give a stable antiaromatic cation; this was reduced with NaBH4 to its 22π congener, which was easily oxidized back to the neutral radical in the air and rearranged to thermodynamically stable dipyrrin site coordinated [22]smaragdyrin BF2 complex upon treatment with BF3 ⋅ OEt2 and TEA. Further, the dipyrrin site coordinated [22]smaragdyrin BF2 complex was similarly oxidized to a stable neutral radical and a stable cation in a stepwise manner. This work demonstrates a rare ability of smaragdyrin BF2 complexes to exist in multiple redox states, particularly forming a stable neutral radical by facile release of a hydrogen atom.

The unprecedented strong paratropic ring current of a bis-PdII complex of 5,10,23-trimesityl [28]heptaphyrin(1.1.0.0.1.0.0)

“… 5 Recently, we have developed a new synthetic method for expanded porphyrins on the basis of acid-catalyzed Friedel–Crafts-type cyclization. 13 In this work, we employed a similar cyclization of tetrapyrrolic BF 2 complex 1 (ref. 14 ) and α,α′-dibromotripyrrin 2 (ref.…”

Synthesis of Pyrrole‐Sharing Fused Porphyrinoid Hybrids by Post‐fabrication of Ni(II) Porphyrins

“…Synthesis of pyrrole‐sharing fused porphyrinoids is outlined in Scheme 1. A solution of meso ‐free Ni II porphyrin 1 and α , α ′‐dibromotripyrrin 2 [7] in CH 2 Cl 2 was heated at 40 °C in the presence of AlCl 3 for 24 h, giving meso ‐tripyrrin‐appended product 3 in 61 % yield via a Friedel‐Craft type aromatic substitution reaction [7g] . Reaction of 3 with 2 equivalent amounts of Pd(OAc) 2 in a 4 : 1 mixture of CH 2 Cl 2 and methanol at room temperature for 15 minutes under rigorously anhydrous conditions gave pyrrole‐sharing bicyclic porphyrinoids 4 and 5 in 36 % and 20 %, respectively (Scheme 1a).…”

Carbazole Based Smaragdyrins: Synthesis, Aromaticity Switching, and Formation of a Spiro‐Dimer

“…[5] These BF 2 complexes were converted to the corresponding [22]and [20]smaragdyrin free bases C and D, whose chemical stabilities are reversed. Namely, D is much stable than C. [5] Since this first report, interesting attributes of smaragdyrins have been continuously exploited, which include easy formation of neo-N-confused smaragdyrins, [6] extremely stable smaragdyrin radicals, [7] and facile formation of fused compounds. [8] With these in back ground, we embarked on the synthesis of carbazole-incorporated smaragdyrins.…”