Article Cite This: J. Org. Chem. XXXX, XXX, XXX−XXX
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Porphyrin−Hexaphenylbenzene Conjugates via Mixed Cyclotrimerization Reactions Max M. Martin,† Maximilian Dill,† Jens Langer,‡ and Norbert Jux*,† †
Department Chemie und Pharmazie & Interdisciplinary Center for Molecular Materials (ICMM), Friedrich-Alexander-Universität Erlangen-Nürnberg, Nikolaus-Fiebiger-Strasse 10, 91058 Erlangen, Germany ‡ Inorganic and Organometallic Chemistry, Egerlandstrasse 1, 91058 Erlangen, Germany
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ABSTRACT: Mixed cyclotrimerization reactions of diarylacetylenes (tolans) were applied to generate a library of multiple porphyrin− hexaphenylbenzene (HPB) architectures. Successful reactions, which could be influenced by the ratio of tolan starting materials, were conducted using dicobaltoctacarbonyl as a catalyst. Separation of the reaction products was performed by chromatographic and crystallization techniques. The physical properties were investigated with respect to the number of porphyrins per HPB and their substitution pattern.
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INTRODUCTION Multiple porphyrin architectures with different structural features were synthesized for mimicking the processes of photosynthesis.1−10 One approach, which is based on hexaphenylbenzene (HPB) scaffolds meso-connected to porphyrins, was first reported in 2002 and showed similar properties to the light-harvesting antenna LH2 in purple bacteria.11,12 This was followed by further HPB lightharvesting arrays.13−16 The concept was expanded by the introduction of different functional groups, e.g. antenna molecules or pH sensitive dyes, close to the porphyrins, resulting in systems which mimic crucial photophysical processes such as nonphotochemical quenching.17,18 Utilizing the 6-fold geometry of HPBs as a template succeeded in constructing cyclic, wheel shaped light-harvesting architectures.19−24 As HPBs and related compounds emerged as a versatile building block in organic synthesis, much effort is being invested into the development of novel synthetic strategies.25−29 Yet, their synthesis is dominated by two major routes: the [2 + 2 + 2] metal-catalyzed cyclotrimerization30 of tolans and the [4 + 2] Diels−Alder reaction31 of tetraarylcyclopentadienones with tolans. Attractive features such as a low number of synthetic steps as well as high yields still qualify these as reactions of choice for HPB derivatives. The remarkable variety of possible HPB substitution pattern allows for, if controllable, the specific placement of “active” components for light harvesting and energy/electron transfer such as fullerenes or porphyrins.13−18 The precise control over the molecular structure of functionalized HPBs is intended to study the correlation of physical properties with the respective substitution pattern. Hence, a library of porphyrin−HPBs was needed, in which the relative configuration as well as the number of chromophores is © XXXX American Chemical Society
altered. We have recently introduced a method for controlling the substitution patterns of HPBs which may be suitable for synthesis of porphyrin−HPBs.27 However, a statistical approach of mixed cyclotrimerization reactions seems to be a promising choice for producing a series of light-harvesting porphyrin−HPB systems faster with less effort in synthesis but with possible separation and purification issues. Therefore, the following paper reports the synthesis of porphyrin−HPBs via mixed cyclotrimerization reactions and their isolation and characterization.
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RESULTS AND DISCUSSION The literature provides one example for a mixed cyclotrimerization reaction with porphyrins (Scheme 1),17 in which the doubly substituted porphyrin tolan 1 was reacted with a monosubstituted tolan 2. The desired product 3 was obtained in 18% yield as well as hexa-substituted HPB 4 as a byproduct. However, additional five porphyrin−HPB byproducts are expected in this statistical reaction, which were not mentioned in the publication. With the goal of producing a library of multiporphyrin− HPBs for studying their structure−physical property relation, we investigated the concept of mixed cyclotrimerizations in more detail. Therefore, we started with para-substituted tolans 5 and 6 (Scheme 2) having both sides occupied either with porphyrins or tert-butyl groups, respectively. Cyclotrimerization in this case is intended to yield three porphyrin−HPB products and additionally hexa-tert-butyl substituted HPB 7. Porphyrin−tolan 5 as well as tert-butyl-tolan 632 were prepared under standard Sonogashira conditions (see Experimental Received: November 14, 2018 Published: December 31, 2018 A
DOI: 10.1021/acs.joc.8b02907 J. Org. Chem. XXXX, XXX, XXX−XXX
Article
The Journal of Organic Chemistry Scheme 1. Example of a Mixed Cyclotrimerization in the Literature17
Scheme 2. Mixed Cyclotrimerization Aa
a
Two equiv of 5 and one equiv of 6 yielding three porphyrin−HPB products. Hexa-tert-butyl-HPB 7 was formed only in minor quantities (< 1 mg), and therefore, no yield was determined.
Scheme 3. Mixed Cyclotrimerization Ba
a
Two equivalents of 11 and one equivalent of 6 yielding six porphyrin−HPB products. Hexa-tert-butyl-HPB 7 was formed only in minor quantities (
