Regioselective β-Pyrrolic Electrophilic Substitution of Hydrodipyrrin–Dialkylboron Complexes Facilitates Access to Synthetic Models for Chlorophyll f

Mengran Liu, Marcin Ptaszek, Olga Mass, Daniel F. Minkler, Roger D. Sommer, Jayeeta Bhaumik, Jonathan S. Lindsey

Research output: Contribution to journalArticlepeer-review

Abstract

Substituents in ring A of chlorophylls can exert profound effects on spectral properties. A  de novo  route to synthetic chlorins employs a tetrahydrodipyrrin reactant containing pyrrole and pyrroline rings. Complexation of the tetrahydrodipyrrin with a dialkylboron motif caused electrophilic substitution (bromination, formylation) to proceed predominantly at the β 7 - rather than α-position of the pyrrole ring, whereas an analogous dihydrodipyrrin underwent substitution equally at the 7- and 8-positions. The fully unsaturated dipyrrin–difluoroboron complex is known to undergo electrophilic substitution at the 8-position. The 7-position of the hydrodipyrrin ultimately gives rise to substituents at the chlorin 2-position (ring A), which heretofore has been little accessed. The position of substitution was confirmed by four single-crystal X-ray structures. Two isomeric formylchlorins were prepared by Pd-mediated carbonylation of the corresponding bromochlorins. Access to a 2-formylchlorin relied on bromination of the tetrahydrodipyrrin–dibutylboron complex, whereas a 3-formylchlorin was prepared by installation of the bromo group in the earliest precursor, pyrrole-2-carboxaldehyde. The two formylchlorins differ in absorption spectral properties: the Q y  absorption maximum is 654 or 664 nm for the 2- or 3-formylchlorin, respectively. The synthetic formylchlorins provide initial models for understanding the strong red absorption of native 2- or 3-formylchlorophylls ( f  and  d ).
Original languageAmerican English
JournalNew Journal of Chemistry
Issue number4
StatePublished - 1 Apr 2014
Externally publishedYes

EGS Disciplines

  • Biochemistry, Biophysics, and Structural Biology

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