De Novo Synthesis of Stable Bacteriochlorins

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Title: De Novo Synthesis of Stable Bacteriochlorins
Author: Kim, Han-Je
Advisors: Jonathan S. Lindsey, Committee Member
David A. Shultz, Committee Member
Daniel L. Comins, Committee Member
Christopher B. Gorman, Committee Member
Abstract: Hydroporphyrins perform a wide variety of essential functions in living systems. Hydroporphyrins differ from porphyrins in having fewer pi bonds along the perimeter of the macrocycle. Efficient routes for the preparation of stable, dehydrogenation-resistant analogues of naturally occurring hydroporphyrins (e.g., chlorins, bacteriochlorins, isobacteriochlorins, corrins) are essential for fundamental studies and diverse applications. To develop such routes, a collection of 24 hydrodipyrrins has been prepared wherein each hydrodipyrrin contains a pyrrole ring and a pyrroline ring. The pyrroline ring bears a geminal-dimethyl group to lock-in the hydrogenation level. The alpha-substituents on the pyrrole and pyrroline rings provide different reactivity combinations (Nu/E⁺, E⁺/E⁺, or E⁺/Nu). Selected hydrodipyrrins have been employed in six exploratory routes to stable bacteriochlorins. The availability of straightforward routes to various hydrodipyrrins should facilitate development of syntheses of diverse hydroporphyrins (Chapter III). Bacteriochlorins are attractive for diverse photochemical applications owing to their strong absorption in the near-infrared spectral region, as exemplified by the bacterial photosynthetic pigment bacteriochlorophyll a, yet often are labile toward dehydrogenation to give the chlorin. An eight-step synthesis for preparing stable bacteriochlorins begins with p-tolualdehyde and proceeds to a dihydrodipyrrin-acetal (IV-1) bearing a geminal-dimethyl group and a p-tolyl substituent. Self-condensation of IV-1 in CH₃CN containing BF₃.OEt₂ at room temperature afforded a readily separable mixture of two free base bacteriochlorins and a ring-contracted, B,D-tetradehydrocorrin. Each bacteriochlorin contains two geminal-dimethyl groups to lock-in the bacteriochlorin (tetrahydroporphyrin) hydrogenation level, p-tolyl substituents at opposite (2,12) beta positions, and the absence (H-BC) or presence (MeO-BC) of a methoxy group at the 5- (meso) position. The B,D-tetradehydrocorrin (TDC) lies equidistant between the hydrogenation levels of corrin and corrole, is enantiomeric, and contains two geminal-dimethyl groups, 2,12-di-p-tolyl substituents, and an acetal group at the pyrroline-pyrrole junction. Examination of the effect of the concentrations of IV-1 (2.5 - 50 mM) and BF₃.OEt₂ (10 - 500 mM) revealed a different response surface for each of H-BC, MeO-BC, and TDC. The highest isolated yield of each was 49%, 30%, and 67%, respectively. The hydroporphyrins are stable to routine handling in light and air. The spectral features of H-BC are exemplary, including strong near-IR absorption (lambda[subscript Qy] = 737 nm, epsilon[subscript Qy] = 130,000 M⁻¹cm⁻¹) and emission (lambda[subscript em] = 744 nm, phi[subscript f] = 0.14). A crystal structure was obtained for MeO-BC. In summary, the ease of preparation of stable bacteriochlorins having characteristic spectral features should facilitate a wide variety of applications (Chapter IV).
Date: 2005-03-29
Degree: PhD
Discipline: Chemistry

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