title: π-Electron manipulation of the 5,6-dihydroxyindole/quinone system by 3-alkynylation: mild acid-mediated entry to (cross)-conjugated scaffolds and paradigms for medium-tunable chromophores.
abstract: 5,6-Dihydroxyindole-based systems engender increasing interest for the design and implementation of new functional aromatic scaffolds and eumelanin-like materials with tailored absorption and electronic properties. However, studies aimed at elucidating the influence of external π-conjugating groups on the redox properties and acid-induced reactivity of these highly oxidizable indolic platforms are lacking. We report herein the synthesis (as acetyl derivatives) and chemical/quantum chemical characterization of the first π-extended 5,6-dihydroxyindole derivatives, 3-ethynyl-5,6-dihydroxyindole (1) and 3,3'-(1,2-ethynediyl)bis-5,6-dihydroxyindole (2), in order to understand whether and how β extension of the enamine-like pyrrole sector affects the absorption properties, redox behavior, and protonation equilibria at both the o-diphenol and quinone levels. Oxidation of 1 and 2 proceeded smoothly to generate dark insoluble materials with eumelanin-like UV properties. On exposure to phosphate buffer at pH 3, 1 was rapidly converted to 3-acetyl-5,6-dihydroxyindole (5) and, in the presence of 5,6-dihydroxyindole, to the cross-conjugated 3,3'-ethenylidenebis-5,6-dihydroxyindole (6). DFT calculations on 1 and 2 and their quinones in their pristine states and after protonation provided a mechanistic frame to rationalize the unusual acid-mediated chemistry of 1 and disclosed 2-quinone as the prototype of a novel class of medium-dependent chromophores. The ethynyl(ene) structural motif is thus proposed as the key to new tunable π-electron extended 5,6-dihydroxyindole/5,6-indolequinone paradigms for the rational design of alkyne-containing hybrid eumelanin-type polymers.
authors.affiliation: Department of Organic Chemistry and Biochemistry, University of Naples Federico II, Naples, Italy.
authors.forename: Luigia
authors.initials: L
authors.lastname: Capelli
authors.forename: Orlando
authors.initials: O
authors.lastname: Crescenzi
authors.forename: Paola
authors.initials: P
authors.lastname: Manini
authors.forename: Alessandro
authors.initials: A
authors.lastname: Pezzella
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Barone
authors.forename: Marco
authors.initials: M
authors.lastname: d'Ischia
mesh_terms: D000042:Absorption; D000480:Alkynes / Q000737:chemistry*; D016227:Benzoquinones / Q000737:chemistry*; D001704:Biopolymers / Q000737:chemistry; D003116:Color; D004583:Electrons; D006863:Hydrogen-Ion Concentration; D007211:Indoles / Q000737:chemistry*; D008543:Melanins / Q000737:chemistry; D010084:Oxidation-Reduction; D019800:Phenol / Q000737:chemistry; D011789:Quantum Theory
chemical_list: D000480:Alkynes; D016227:Benzoquinones; D001704:Biopolymers; D007211:Indoles; D008543:Melanins; C041877:eumelanin; D019800:Phenol; C004532:quinone; C033871:5,6-dihydroxyindole