Summary
A series of xanthobilirubinic acid (XBR) esters (i.e.,1c to1p) and amides (2a–2c) have been prepared by a procedure involving detachment of the chromophore previously ester-bound to a functionalized, insoluble polystyrene (1b). Detachment is achieved by treatment with the respective alcohol in aqueous alkali or with the amines, yielding directly the corresponding esters or amides. With primary, short-chained alcohols the ester yields are high (60% forn-C4H9-OH to 99% for C2H5-OH), but decrease rapidly with chain length (40% forn-C10H21-OH and 0% forn-C16H33-OH). The same trends are observed with the amines. These results are interpreted in terms of an (increasingly) unfavourable entropic interaction between the polymer matrix to which the chromophore is bound and the (growing) alcohol chain. The impairment by the long chained nucleophile to reach the transesterification center could also contribute, and, for the most lipophylic alcohols, their low solubility in aqueous alkali is an additional drawback. With secondary and tertiary alcohols, no ester is obtained, in agreement with a BAC2 mechanism involving a tetrahedral intermediate.
Zusammenfassung
Unlösliche, durch eine Estergruppe an Polystyrol gebundene Xanthobilirubinsäurederivate liefern mit Alkoholen die monomeren Ester (1c–1p), mit Aminen die monomeren Amide (2a–2c). Die Ausbeuten für primäre kurzkettige Alkohole sind hoch (n-C4H9-OH: 60%, C2H5-OH: 99%), mit steigender Kettenlänge sinken sie rasch (n-C10H21-OH: 40%,n-C16H33-OH: 0%). Denselben Trend beobachtet man bei Aminen. Die Ergebnisse werden durch eine zunehmend ungünstige entropische Wechselwirkung zwischen Chromophor an der Matrix und Alkohol interpretiert. Durch die steigende Kettenlänge des Alkohols könnte auch aus räumlichen Gründen die Annäherung des Nucleophils an das Reaktionszentrum erschwert werden. Die geringe Löslichkeit der höheren lipophilen Alkohole in wäßrigem Alkali wirkt sich ebenfalls ungünstig auf den Reaktionsverlauf aus. Für sek. und tert. Alkohole wird keine Reaktion erhalten, wie es für einen BAC2-Mechanismus mit tetraedrischem Zwischenprodukt zu erwarten ist.
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Serra, S., Sesé, M.L. & Trull, F.R. Polymer bound pyrrole compounds, VII: Xanthobilirubinic acid esters and amides from an insoluble, polystyrene-supported precursor. Monatsh Chem 122, 673–682 (1991). https://doi.org/10.1007/BF00811466
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DOI: https://doi.org/10.1007/BF00811466