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Transcript of Hydroporphyrins. Reactivity, spectroscopy and ... · PDF file Volume II Structure and...

  • T H E PORPHYRINS Volume I I

    Structure and Synthesis, Part Β

    Edited by

    DAVID DOLPHIN Department of Chemistry

    University of British Columbia Vancouver, British Columbia, Canada

    ACADEMIC PRESS New York San Francisco London 1978

    A Subsidiary of Harcourt Brace Jovanovich, Publishers

  • Contents

    List of Contributors ix

    General Preface xi

    Preface xii i

    Contents of Other Volumes xv

    1 Synthesis and Stereochemistry of Hydroporphyrins

    H U G O S C H E E R

    I . Introduction 1 I I . Synthesis of Hydroporphyrins 4

    I I I . Stereochemistry 23 I V . Conclusions 34 V . Appendix: Recent Studies in Chlorophyll Chemistry 35

    References 37

    2 Hydroporphyrins: Reactivity, Spectroscopy, and Hydroporphyrin Analogues

    H U G O S C H E E R A N D H A N S H E R L O F F I N H O F F E N

    I . Reactivity of Hydroporphyrins 45 I I . Spectroscopy of Hydroporphyrins 62

    I I I . Oxy and Oxo Analogues of the Hydroporphyrins 71 References 85

    3 The Porphyrinogens

    D . M A U Z E R A L L

    I . Introduction 91 I I . Synthesis 91

    ν

  • vi C O N T E N T S

    I I I . Properties 96 I V . Thermodynamic Stability and Evolution 99

    References 101

    4 Oxophlorins (Oxyporphyrins)

    P. S. C L E Z Y

    I . Introduction 103 I I . Preparation of the Oxophlorins 104

    I I I . Properties of the Oxophlorins 111 IV. Oxidation of Hemes 121

    References 128

    5 Irreversible Reactions on the Porphyrin Periphery (Excluding Oxidations, Reductions, and Photochemical Reactions)

    J . - H . F U H R H O P

    I . Reactivity Patterns of the Porphyrin Periphery 131 I I . Introduction of Carbon Substituents into the Porphyrin Periphery 137

    I I I . Introduction of Nitrogen Substituents into the Porphyrin Periphery 147 I V . Introduction of Sulfur Substituents into the Porphyrin Periphery 152 V. Halogenation of the Porphyrin Periphery 153

    References 156

    6 Chemical Transformations Involving Photoexcited Porphyrins and Metalloporphyrins

    F R E D E R I C K R . H O P F A N D D A V I D G . W H I T T E N

    I . Introduction 161 I I . Photoprocesses Not Involving Permanent Transformation of the

    Porphyrin 162 I I I . Photoprocesses Involving Permanent Transformations of the

    Porphyrin 171 References 190

    7 Linear Polypyrrolic Compounds

    A L B E R T G O S S A U E R A N D J Ü R G E N E N G E L

    I . Introduction 197 I I . Nomenclature 198

    I I I . Bipyrrolic Compounds 200 I V . Tripyrrolic Compounds 224 V. Tetrapyrrolic Compounds 231

    References 246

  • C O N T E N T S v i i

    8 Metal Complexes of Open-Chain Tetrapyrrole Pigments

    J. S U B R A M A N I A N A N D J . - H . F U H R H O P

    I . Introduction 255 I I . Composition and Electronic and Vibrational Spectra of Open-Chain

    Tetrapyrrolic Metal Complexes 256 I I I . EPR Studies 263 I V . X-Ray Structures 268 V . Chemical Reactivity 272

    V I . A Comparison of Open-Chain Tetrapyrrole Chromophores with Related Macrocyclic Systems 279 References 283

    9 Stereochemistry and Absolute Configuration of Chlorophylls and Linear Tetrapyrroles

    H A N S B R O C K M A N N , JR.

    I . Introduction 288 I I . Chlorophylls of Higher Plants 288

    I I I . Bacterial Chlorophylls 306 IV . Stereochemistry of the Esterifying Alcohols 312 V. Linear Tetrapyrroles 317

    References 321

    10 Pyrrolic Macrocycles Other than Porphyrins

    R . G R I G G

    I . Corroles 328 I I . 227r-Electron Macrocycles: Sapphyrins and Related Compounds 351

    I I I . 1-Substituted and 1,19-Disubstituted Metallotetradehydrocorrins 356 I V . 1-Substituted and 1,19-Disubstituted Corrins 372

    References 388

    Author Index 393

    Subject Index 419

  • 2 Hydroporphyrins: Reactivity,

    Spectroscopy, and Hydroporphyrin Analogues

    HUGO SCHEER and HANS HERLOFF INHOFFEN

    I . Reactivity of Hydroporphyrins . . . . . . . 45 A. Chlorins and Bacteriochlorins . . . . . . 45 B. Phlorins 52 C. Porphodimethenes and Others . . . . . . 60

    I I . Spectroscopy of Hydroporphyrins . . . . . . 62 A. UV-VIS Spectra 62 B. Fluorescence Spectra . . . . . . . . 68 C. Triplet Spectra of Chlorophylls 68 D. Vibrational Spectra 69 E. Polarimetrie Spectra 69 F. Nuclear Magnetic Resonance Spectra . . . . . 70 G. Mass Spectra and X-Ray Photoelectron Spectra . . . 71

    I I I . Oxy and Oxo Analogues of the Hydroporphyrins . . . 71 A. Meso-oxygenated Hydroporphyrins . . . . . 72 B. Hydroporphyrins Oxygenated at jS-Pyrrolic Positions . . 77 C. Analogues of Hydroporphyrins . . . 8 1 D. Miscellaneous . . . . . . . . . 84 References . . . . . . . . . . 85

    I . R E A C T I V I T Y O F HYDROPORPHYRINS

    A. Chlorins and Bacteriochlorins

    The chlorin macrocyclic system, as compared to that of porphyrins, shows two distinct differences in reactivity. The methine positions next to the pyrroline ring have a higher electron density and are, thus, susceptible to

    45

  • 46 H U G O SCHEER A N D H A N S HERLOFF I N H O F F E N

    electrophilic attack, and chlorins are more easily oxidized both by one- and two-electron oxidants. Both features have been linked to the reactivity of chlorophylls, which are, with the exception of the chlorophylls c, either chlorins or bacteriochlorins. The easy isotope exchange of the δ-Η in chloro­ phyll a (1) is a direct result of the increased electron density at this position, and especially the photooxidation to the cation radical is of eminent biological importance as the genuine light conversion step in photosystem I (see Volume I I , Chapter 6; Volume IV, Chapters 3-5; and Volume V, Chapters 2 and 9).

    However, chlorophyll chemistry is determined not only by the central Mg ion, but by the functional groups of the peripheral substituents. The most prominent one is probably the enolizable ß-ketoester grouping at the iso- cyclic ring E , which is (together with the central Mg) responsible for the strong and specific self-aggregation of chlorophylls 1 , 2 and for which the involvement in photosystem I I oxygen evolution has been discussed.3,4

    Chlorophyll chemistry and chlorophyll aggregation have been reviewed tho rough ly 2 , 5 - 8 (see also Volume V, Chapter 9). Besides the basic features of the chlorin nucleus, only one special aspect of chlorophyll chemistry shall be discussed here, namely, the metalation or the free phorbin ligand with Mg. A brief summary of the recent research in chlorophyll chemistry is provided in the appendix of Chapter 1.

    1. E L E C T R O P H I L I C SUBSTITUTION

    The increased electron density at the methine positions next to the pyrroline ring in chlorins was first concluded by Woodward and Skaric 9 from reactivity studies. I t could be shown that the product obtained on reaction of chlorins with H C 1 / H 2 0 2 was not a dioxychlorin, as originally proposed, 1 0 but rather a 3-chloro-7,8-chlorin. The formation of this mesosubstituted chlorin by a selective electrophilic attack was rationalized in a new formulation of the

    Fig. 1. Schematic representation of the porphyrin 7r-electron charge densities, ac- cording to Woodward. 1 1

    6*/·'

  • 2. H Y D R O P O R P H Y R I N S 47

    Fig. 2. Schematic representation of the chlorin ττ-electron charge densities, according to Woodward. 1 1

    porphyrin macrocycle,1 1 in which the four pyrrolic subunits retain, to a certain degree, their aromaticity ( = 6π systems) by borrowing electron density from the neighboring methine bridges (Fig. 1). In the chlorins, the reduced pyrroline ring can no longer be such an aromatic subunit and, thus, the electron density at the neighboring methine positions is increased (Fig. 2). This difference between the "chlorin-type" methine protons next to the pyrroline ring (y, δ in formula 1), and the "porphyrin-type" methine protons

    (α, β in 1), respectively, was supported by M O calculations, 1 2" 1 6 and is now an experimentally well established fact.

    The chlorin-type methine protons next to the reduced ring are easily exchangeable against 2 H with ac id , 9 , 1 7 while, under the same conditions, the remote porphyrin-type methine protons are nonexchangeable. This exchange

  • 48 H U G O SCHEER A N D H A N S HERLOFF INHOFFEN

    is even more pronounced in some metal complexes and proceeds in the chlorophylls already in neutral solutions. 1 7 The differential X H / 2 H exchange of a series of pheophorbides and bacteriopheophorbides has been studied 1 8

    in an attempt to establish the methyl substitution site in the bacteriochloro- phylls d (chlorobium chlorophylls 660), from which substitution at a or β was suggested. The results were not equivocal, however, and there is now positive proof for a δ-substituted structure instead. 1 9 - 2 2 The selective electro­ philic attack at the chlorin-type methine positions has been demonstrated, for example, during nitration, 2 3 chlorination, 9 , 2 4 bromination, 2 0 a and formyl- ation (of the hemins 2 5). Further support for the increased electron density comes from the oxidative cleavage of H 2 (OEC) or Zn(OEC) with thallic trifluoroacetate yielding selectively the A-dihydrobiliverdin 2 via hydrolysis

    Η Η Η

    of a meso-trifluoroacetoxy intermediate 2 6 and cleavage of the oxochlorin phlorin. 2 7 The selective cleavage at l-δ in δ-m