All naturally occurring tetrapyrrolic macrocyles, the ‘pigments of life’, carry alkyl substituents at their pyrrolic b-positions;6 yet, the majority of synthetic tetrapyrrole model systems that were studied were derived from b-unsubstituted meso-tetraarylporphyrins. The syntheses and functionalization of the latter compound class is frequently simple,37-40 including their conversion to hydroporphyrins and hydroporphyrin analogues.13, 41-43 While octaalkylporphyrins and -chlorins, such as protoporphyrin IX or the chlorophylls, are readily available in large quantities from slaughterhouse wastes44 or plant sources,45 respectively, their derivatization is hampered by problems associated with the lack of regioselectivity.46-47 The total syntheses and/or functionalization of most b-alkylporphyrins and -hydroporphyrins is also non-trivial,43, 47-48 even though the synthetic methodologies toward b-alkyl-substituted hydroporphyrins recently
N
7
developed by the Lindsey group allow now the preparation of an impressive selection of finely tunable hydroporphyrins.43, 48-50
Among the functionalities at the b-position of a porphyrinic chromophore (on meso-aryl as well as b-alkylporphyrins) that influence its electronics in profound ways are the oxo-groups.51-57 For example, Lindsey and co-workers synthesized a series of mono- and dioxochlorins, such as 7,17-dioxobacteriochlorin 9, and studied the (auxochromic) effects to the oxo-functionality.54, 57-58 The electronic structure of the 7,17-dioxobacteriochlorin framework was determined to be essentially a red-shifted chlorin, and not a typical bacteriochlorin, as the substitution pattern could have suggested.58 The strong electronic influences of the b-oxo-functionality are also seen in a special case of b-oxoporphyrinoids, the porpholactones. For instance, in dilactone 10 an astonishingly large electronic difference is noted between the two regioisomers 107,17 and 107,18, and none of them exhibit a typical bacteriochlorin UV-vis spectrum.59
Select dioxochlorin chromophores are also found in nature. Heme d1 11, incorporating a 2,7-dioxoisobacteriochlorin, is the prosthetic group in cytochrome cd1, the nitrite to nitrous oxide
O N
8
reducing enzyme of some chemo-autotrophic bacteria.60 The 7,17-dioxobacteriochlorin chromophore is the basis of tolyporphin A 12, one member of a family of green tetrapyrrolic pigments isolated from a cyanobacterium-microbial ecological unit of unknown function but endowed with intriguing medicinal properties.61-65 Model system for both chromophores (such as 9 as a model for 12) were studied.58, 66
b-Octaethylporphyrin (OEP) is the most readily accessible synthetic b-alkylporphyrin.67-6869 Its symmetry simplifies its derivatizations, and a number of methods were reported toward its functionalization,70 including its conversion to porphyrinoids containing non-pyrrolic building blocks.32-33One utmost simple modification method a b-alkylporphyrins is their treatment with H2O2
in conc. H2SO4; this reaction goes back to experiments by the group of Hans Fischer in the mid-1920’s,71 albeit at that time the reaction products were not correctly identified. The true connectivity of the major product oxochlorin 13 as the result of the treatment of OEP with H2O2/H2SO4 was identified in 1964 by the group of Johnson (Scheme 1-2).72-73 The syntheses of the oxochlorins 13 through 18 from OEP proceeded as principally described by Inhoffen and Nolte74 and modified in 1980 by Chang,75 in 2 g batches of OEP in 200 mL 96% H2SO4 and using 3% H2O2, at ice temperatures over the course of about 15 min (at which the starting material OEP was converted quantitatively).
9
Scheme 1-2. Oxidation of OEP to the corresponding oxochlorin and dioxoisobacteriochlorin- and dioxobacteriochlorin-type isomers.72, 74, 76
Column chromatography allowed the isolation of, in order of increasing polarity, oxochlorin 13 (pink, 20% isolated yield), a mixture of dioxochlorins 17, 16, and 18, then 14 (dark green, 4%
isolated yield), and finally 15 (light green, 6% isolated yield); subsequent preparative plate chromatography separated dioxochlorins 17 (purple, 5% isolated yield), 16 (blue, 0.5% isolated yield), and 18 (brown, 4% isolated yield) from other oxidation products (triketones, ring-opened, and meso-oxygenated molecules).74 Thus, this reaction allowed the isolation and identification of
N
+ triketones, meso-ketone, and meso-ring-opened products
Porphyrin
Chlorin-
Isobacteriochlorin-
type substitution patterns N
10
all isomers of the diketones: the two possible isomers of the bacteriochlorin series 14 to 16, and the three isomers of the isobacteriochlorin series 17 and 18 (as well as triketones and ring-opened products).73-76 A dramatic color change when the ketone is positioned at different place is observed (Figure 1-1).
Figure 1-1. The color of corresponding diketone isomers: Purple 17, blue 16, light purple 18, light green 14 and green 15
The Johnson group reasoned that the oxochlorin products were formed along single and double epoxidation ® epoxide opening by water ® pinacol-pinacolone rearrangement of the resulting trans-diol pathway. None of the intermediates were observed but chlorin cis-diol 19 can be prepared independently, and it was shown by Chang and co-workers to be susceptible to a pinacol-pinacolone rearrangement, forming oxochlorin 13.77 The applicability of the H2O2/H2SO4
reaction to other porphyrins than OEP was also demonstrated.78
Assuming a stepwise process of the formation of the dioxochlorins via oxochlorin 13, the modification step of 13 is not subject to any (major) regioselectivity since the combined yields of the isobacteriochlorin-type (14, 15, and 16) and bacteriochlorin-type products (17 and 18) are essentially the same. This distinguishes this reaction from many other reactions that convert chlorins to (iso)bacteriochlorins.79 This is also different compared to the dynamics of the formation
11
of the two dilactone regioisomers 107,17 and 107,18 from a common precursor.59 Four of the five dioxo-regioisomers are also formed in comparable yields (their small differences are likely more a reflective of the relative difficulty of their isolation), with one exception: 2,8-dioxoisobacteriochlorin 16 is formed in about an order of magnitude lower yields that all others.
Since then, the chemical properties of the OEP-derived oxochlorins with respect to reduction,80-83 carbonyl C-methylation,75 N-methylation,84 meso-deuteration,85 osmylation,86 and thionation87 reactions were studied. We also found oxochlorin 13 to be a versatile starting material for the preparation of a number of meso-derivatives and a pyrrolinone ring-expanded product.35,
88-90 The group of Stolzenberg prepared and studied a range of metal complexes.80, 91 We recently reported the single crystal X-ray structures of the free base and Ni(II) complex of oxochlorin 13, their oximes,88 the free base N-oxide,89 a mono- and a bis-meso-chloride;90 the structure of dioxobacteriochlorin 17 was reported previously.87 The Pt(II) complex of 13 was used as an optical oxygen sensor.92 The crystal structures and/or metal-based reactivity of the Ni(II),80 Cu(II),93 Co(II),94 and Fe(III)91, 95 complexes of 13 and the Ni(II),96-97 Co(II),94 and Fe(III),66 Cu(II)66, 98 complexes of dioxoisobacteriochlorin isomer 14 were conveyed, also.
Thus, the mono- and select di-oxochlorins are not entirely unexplored. It is surprising, however, that no comprehensive comparative study of the electronic properties of all octaethyl-mono- and di-oxochlorins was reported, particularly given that they have been known now for nearly 50 years.
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