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Orthopalladation of GFP-Like Fluorophores Through C-H Bond Activation: Scope and Photophysical Properties

Abstract : The orthopalladation of 4‐aryliden‐5(4H)‐oxazolones and ‐imidazolones through C–H bond activation is regioselective and gives dimers where only the ortho position of the 4‐arylidene ring is activated. The carboxylate ligands can be replaced by acetylacetonate, affording rigid neutral mononuclear complexes. The photophysical properties of these rigid mononuclear complexes have been examined and confirm the importance of rigidification of imidazolones. The luminescence of oxazolones R1‐C6H4CH=CC(O)O‐CN(R2) (1a–1j) and imidazolones R1‐C6H4CH=CC(O)NR3CN(R2) (1k–1q) has been examined. The new GFP‐like imidazolones (GFP = Green Fluorescent Protein) (1k–1q) have been prepared by reaction of the oxazolones with amines H2NR3 and bis(trimethylsilyl)acetamide. The most intense fluorescence was found in push‐pull systems containing simultaneously strong electron‐donating and electron‐withdrawing substituents. The incorporation of the Pd atom into the molecular skeleton of oxazolones and imidazolones notably changes their luminescence. The reaction of oxazolones (1a–1j) and imidazolones (1k–1q) with Pd(OAc)2 (1:1 molar ratio) in carboxylic acids gives the dinuclear [Pd{R1‐C6H3CH=CC(O)OCN(R2)}(µ‐carboxylate)]2 (2a–2j) and [Pd{R1‐C6H3CH=CC(O)NR3CN(R2)}(µ‐carboxylate)]2 (2k–2q) through regioselective C–H bond activation of the ortho position of the respective 4‐arylidene rings. Complexes 2a–2q react with LiCl in MeOH to give the chloride‐bridge derivatives [Pd{R1‐C6H3CH=CC(O)OCN(R2)}(µ‐Cl)]2 (3a–3j) and [Pd{R1‐C6H3CH=CC(O)NR3CN(R2)}(µ‐Cl)]2 (3k–3q), which further react with Tl(acac) (acac = acetylacetonate) to give the mononuclear species [Pd(R1‐C6H3CH=CC(O)OCN(R2)(acac)] (4a–4j) and [Pd{R1‐C6H3CH=CC(O)NR3CN(R2)(acac)}] (4k–4q). Complexes 4o and 4q, having an orthopalladated push‐pull imidazolone, are strongly fluorescent, showing a notable increase of the quantum yield with respect to the free ligands 1o and 1q up of one order of magnitude.
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Submitted on : Tuesday, February 19, 2019 - 11:09:26 AM
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Sandra Collado Ruiz, Alejandro Pueyo, Christine Baudequin, Laurent Bischoff, Ana Isabel Jiménez, et al.. Orthopalladation of GFP-Like Fluorophores Through C-H Bond Activation: Scope and Photophysical Properties. European Journal of Organic Chemistry, Wiley-VCH Verlag, 2018, Special Issue: C‐H Activation in Organic Synthesis, 2018 (44), pp.6158-6166. ⟨10.1002/ejoc.201800966⟩. ⟨hal-02024490⟩



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