Tetraoxa[22]porphyrin(2.2.2.2) Dication and Tetraoxa[24]porphyrinogens(2.2.2.2)

Abstract

In addition to the porphyrin homologues ([22] porphyrin(l.l , I .l)dication 1 (R = H), which is formally analogous to the N.N',N",N'"-tetramethylporphyrin dications 2,[61 as an aromatic system. Recently, with the synthesis of a n octaethyl derivative of 1 (R = H)"] and tetraalkyl derivatives of 1 (R = alkyl) ['] substituted in the meso positions, access to these porphyrinoids has been decisively improved. The porphyrinoid analogue, tetraoxaporphyrin(2.0.2.0) dication 3 (tetraoxaporphycene dication). which is an isomer of 1, was also prepared by E. Vogel et a1.18] We describe here for the first time the next highest homologue to I . the tetraoxa[22]porphyrin(2.2.2.2)dication 4. Tetraoxaporphyrinogen(2.2.2.2) 5 ([24]annulene tetroxide), the required precursor to 4. was prepared in 1969 by J. A. Elixrgl by self-condenis obtained from the isomeric mixture (E/Z)-7 by recrystalhzation. McMurry dimerization of (E)-7 was carried out with TIC!,/ Zn/Cu in anhydrous tetrahydrofuran. After chromatography on silica gel/CH,CI, and silica gel/cyclohexane/THF, 5 was obtained in 7 YO yield (stable black-violet crystals, m.p. > 300 "C). The melting point and spectroscopic data of our compound are not consistent with either of the isomers (5a, b) described by J. A. Elix; thus we will refer to the isomer we obtained as 5c.

Synthetic Expanded Porphyrin Chemistry

“…Efforts to prepare it gave rise to a compound reported earlier by Märkl et al, [37] namely the cis,trans,cis,trans isomer 18 a. Compound 18 was calculated to be 90 kcal mol À1 higher in energy than its bis-trans isomer 18 a.…”

Aromaticity Switching in Porphyrinoids

“…The impact of expansion through a stepwise increase in the length of the meso linker within the tetraoxatetraphyrin( m.n.m.n ) class has been inventively explored by Märkl and co‐workers, as exemplified by tetraoxatetraphyrin(2.2.2.2) 23 (Scheme ) . Antiaromatic character has also been observed for this derivative, as documented by 1 H NMR spectroscopy, for which the outer protons are relocated upfield ( trans bridge: δ =5.30 ppm, cis bridge: δ =4.72 and 4.66 ppm β‐furans: δ =5.50/5.45 and 5.73/5.56 ppm), whereas the only hydrogen pointed towards the center of the macrocycle resonates at δ =12.38 ppm.…”

The McMurry Coupling and Related Reactions

“…The tandem McMurry reaction has been successfully used for the synthesis of cyclophanes (Figure 6.22; Table 6.8) such as 133 and 134 [114,115,173], and is a classical route to ''expanded'' porphyrin systems. After the preparation of porphycene and its substituted derivatives such as 135 [174], a variety of porphyrin analogues were isolated, in which pyrrole units were replaced with furan or thiophene rings and/or extended by additional ring systems [135,[175][176][177][178][179][180][181][182][183][184][185] 6.24). Some of these molecules are of current interest as effective photosensitizers for biomedical applications and are employed in the photodynamic therapy of tumors and viral inhibition.…”