Novel porphyrinoids, 15. Syntheses of novel expanded [26]porphyrins with conformational control by the “helical effect”

Abstract

Novel [26]porphyrinogen~ [~I 24-28 were conveniently prepared in few steps from the pyrrole building blocks 19-23. The selectivity of the cyclization is explained by a conformational helical effect due to steric congestion of the pyrrole psubstituents. The aromaticity of the [26]porphyrins[*I 30-33, obtained e y dehydrogenation of the porphyrinogens, is evi-dent from the 'H-NMR spectra displaying 25-ppm shift differences for the inner and outer protons of the porphyrins. These pigments belong to the most intensive chromophores known so far, showing VIS absorptions at 544-550 nm with lg E values up to 5.9. Present addresses: I+] Bayer AG, D-47829 Krefeld; I++] Boehringer Ingelheim KG, D-55216 Ingelheim; I+++] Solvay Deutschland GmbH, D-30173 Hannover. pared to the formation of [ 18lporphyrinogens. A computeraided inspection[l0] of three-dimensional conformations of P-substituted vinylogous tetrapyrroles revealed an increasing support of the helical conformation B (Scheme 2) with the size of the P-substituents. Scheme 1. Porphobilinogen (1) and the biomimetic transformation of its derivative 2 to [26]porphyrinogen 4 and [26]porphyrin 3 Et Et t CH2OH Et 2 Et E t a E t NCH, H , C N~ Br, Et a E t NCH, HSCN Et \@I / / / Et Et / \ \ Et E t Et Et E t 3 4 2 Bre t / / NCH, H, CN \ NCH, H, CN \ The pyrroles 10-13 (Scheme 3) were chosen as suitable precursors to investigate steric effects on the biomimetic synthesis of [26]porphyrinogens. Three of them were syn-Novel Porphyrinoids, 15 505 Scheme 5. Preparation of the pyrrole building blocks 20-23 and the transformation of 19-23 to the [26]porphyrinogens 24-28 12 10 13 11 KOH/DMSO, CH, I 1 7 5 z 187 % 176 % 180 Z CH, 19 20 21 22 23

Expanded Porphyrins and Their Heterologs

“…Dehydrogenation of macrocycles 215 − 217 furnished the corresponding aromatic congeners 219 − 221 , which exhibit similar spectroscopic properties to 218 . Moreover, all these [26]porphyrins(3.3.3.3) exhibit an equivalent diamagnetic ring current effect in the NMR (cf., Δδ = 25.0 to 25.3 ppm) compared to the isoelectronic [26]porphyrin(5.1.5.1) 204 (Δδ = 24.1 ppm) …”

“…Progressing from the above series, the next obvious expanded porphyrins encompass compounds in which all four of the single atom meso bridges are expanded. In 1986, Franck and Gosmann synthesized the first member in this series of tetravinylogous porphyrins via a biomimetic sequence analogous to porphyrin biosynthesis. , Here, an acid-catalyzed cyclotetramerization of the N -methylpyrrylpropenol ( 208 ) followed by dehydrogenation of the resultant macrocycle 213 gave the bisquaternary octaethyl[26]porphyrin ( 218 ) in just two steps (Scheme ). This expanded porphyrin exhibits a remarkably strong diatropic ring-current effect in the 1 H NMR, such that the internal protons of the methine bridges and the N -methyl groups appear as a triplet at δ = −11.64 ppm and a singlet at −9.09 ppm.…”

“…The most striking feature of this synthetic strategy, is the selectivity observed for the formation of macrocycle 213 (e.g., 27%). Evidence that this results from a conformational helical effect came from the synthesis of the β-unsubstituted and β-substituted derivatives 214 − 217 (Scheme ); where, an increase in yield (from 0.1% to 52%) of the latter macrocycles is observed as the bulkiness of the side chains progressively increases . Clearly, the steric congestion of the pyrrolic β side chains force the tetrapyrrolic precursors to adopt the more favorable helical conformation 222 immediately prior to the final ring closure.…”

Octaethyltetraselenaporphyrin Dication

“…The helical effect [13] of the ethyl groups, to which the relatively high yields of porphyrinogens from 2-(hydroxymethyl)-3,4-diethylfuran and -thiophene is ascribed, evidently plays a negligible role in the case of 7.…”

Porphyrinoid Macrocycles Based on Thiophene—The Octaethyltetrathiaporphyrin Dication

“…[14] a) Cambridge Structural Database, Update October 1995, Mean value of 386 structures; searching conditions: 1. no disorder, 2. R factor I lo%, 3. temperature below -5 3 T ; b) the measured C-CI bond length for 1 in the gas phase is 177.2 pm, R. J. Myers, W. D. Gwinn,J Chem.…”