jo000913b_si_002.pdf (191.47 kB)
Rational Synthesis of β-Substituted Chlorin Building Blocks
journal contribution
posted on 2000-10-21, 00:00 authored by Thiagarajan Balasubramanian, Jon-Paul Strachan, Paul D. Boyle, Jonathan S. LindseyChlorins bearing synthetic handles at specific sites about the perimeter of the macrocycle constitute
valuable building blocks. We previously developed methodology for preparing meso-substituted
chlorin building blocks and now present methodology for preparing several complementary
β-substituted chlorin building blocks. The chlorins bear one or two β substituents, one meso
substituent, a geminal dimethyl group to lock in the chlorin hydrogenation level, and no flanking
meso and β substituents. The synthesis involves convergent joining of an Eastern half and a Western
half. New routes have been developed to two β-substituted bromo-dipyrromethane monocarbinols
(Eastern halves). A new β-substituted Western half was prepared following the method for preparing
an unsubstituted Western half (3,3-dimethyl-2,3-dihydrodipyrrin). Chlorin formation is achieved
by a two-flask process of acid-catalyzed condensation followed by metal-mediated oxidative
cyclization. β-Substituted chlorins have been prepared in 18−24% yield bearing a 4-iodophenyl
group at the 8-position, a 4-iodophenyl group or a 4-[2-(trimethylsilyl)ethynyl]phenyl group at the
12-position, and a 4-iodophenyl group and a 4-[2-(trimethylsilyl)ethynyl]phenyl group at diametrically opposed β-positions (2, 12). The latter building block makes possible the stepwise
construction of linear multi-chlorin architectures. The chlorins exhibit typical absorption and
fluorescence spectra. A systematic shift in the absorption maximum (637−655 nm for the free base
chlorins, 606−628 nm for the zinc chlorins) and intensity of the chlorin Qy band (ε up to 79 000
M-1 cm-1) is observed depending on the location of the substituents. The characteristic spectral
features and location of substituents in defined positions make these chlorins well suited for a
variety of applications in biomimetic and materials chemistry.