Northern–Southern Route to Synthetic Bacteriochlorins

Abstract

A new route to bacteriochlorins via Northern-Southern (N-S) self-condensation of a dihydrodipyrrin-acetal complements a prior Eastern-Western (E-W) route. Each bacteriochlorin was prepared in five steps from an α-halopyrrole and a 2,2-dimethylpent-4-ynoic acid. The first three steps follow Jacobi's synthesis of dihydrodipyrrins: Pd-mediated coupling to form a lactone-pyrrole, Petasis reagent treatment for methenylation, and Paal-Knorr type ring closure to form the 1,2,2-trimethyl-substituted dihydrodipyrrin. Subsequent steps entail conversion of the 1-methyl group to the 1-(dimethoxymethyl) unit and acid-catalyzed self-condensation of the resulting dihydrodipyrrin-acetal. The essential differences between the N-S and E-W routes lie in (1) the location of the gem-dimethyl group (with respect to the 1-acetal unit) at the 2- versus 3-position in the dihydrodipyrrin-acetals, respectively, (2) the method of synthesis of the dihydrodipyrrins, and consequently (3) access to diverse substituted bacteriochlorins including those with substituents at the meso-positions. Ten new bacteriochlorins bearing 0-6 total aryl, alkyl, and carboethoxy substituents at the β-pyrrole and/or meso-positions have been prepared, with yields of macrocycle formation of up to 39%. Four single-crystal X-ray structures (two intermediates, two bacteriochlorins) were determined. The bacteriochlorins exhibit characteristic bacteriochlorophyll-like absorption spectra, including a Q band in the region 713-760 nm.

Synthesis and Spectral Properties of meso-Arylbacteriochlorins, Including Insights into Essential Motifs of their Hydrodipyrrin Precursors

“…Such an approach requires use of the N-S route. Thus, the Pd-mediated coupling of acid precursor 9 [ 40 ] and iodopyrrole 10 [ 19 ] gave the lactone–pyrrole 11 in 68% yield. Treatment of the latter with the Petasis reagent afforded 12 in 67% yield.…”

“…As a compromise between the simplicity of porphyrin hydrogenation and the complexity of natural bacteriochlorin total synthesis, we have been developing de novo synthetic routes to stable, tailorable bacteriochlorins [ 18 , 19 , 20 ]. Such bacteriochlorins contain a gem-dimethyl group in each pyrroline ring to protect the macrocycle from adventitious oxidation.…”

“…Such bacteriochlorins contain a gem-dimethyl group in each pyrroline ring to protect the macrocycle from adventitious oxidation. Two routes rely on the self-condensation of a dihydrodipyrrin–acetal, either in an “Eastern-Western” (E-W) [ 18 ] or “Northern-Southern” (N-S) fashion, defined by the respective 3- versus 2-position of the gem-dimethyl group in the pyrroline ring of the dihydrodipyrrin–acetal (compare III and IV , Scheme 1 ) [ 19 ]. The routes along with creative extensions by others [ 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 ] have enabled preparation of >100 bacteriochlorins with specific substituent patterns for diverse applications [ 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 ].…”

Synthesis, properties and photovoltaic performance in dye-sensitized solar cells of three meso-diphenylbacteriochlorins bearing a dual-function electron-donor

“…29 To the best of our knowledge, there have been very few reports of airstable, meso-aryl-substituted bacteriochlorins. 28,29 In our previous work, PCE of the DSC reached 4.67% with the use of the LS-01 bacteriochlorin. This value outperforms that of a reference porphyrin dye with a similar structure (H2PE1, 2.06%, structure not shown).…”

“…Owing to this unique and important property, bacteriochlorins play key roles in the bacterial photosynthetic reaction centers, including light harvesting, energy transduction, charge separation, and electron transfer, etc. [20][21][22][23][24] Inspired by the pioneer work of Lindsey and co-workers, [25][26][27][28] we recently reported the synthesis, fundamental properties, and photovoltaic performance of two air-stable, meso-substituted bacteriochlorins (denoted as LS-01 and LS-11, Fig. 1).…”