Aerobic C(sp²)–H Hydroxylations of 2-Aryloxazolines: Fast Access to Excited-State Intramolecular Proton Transfer (ESIPT)-Based Luminophores

Abstract

The direct hydroxylation of 2-aryloxazolines via a deprotonative magnesiation using TMPMgCl•LiCl and subsequent oxidation with molecular oxygen or air as a green oxidant is reported. This method proceeds under mild conditions at room temperature with high regioselectivity and chemoselectivity. The obtained phenols exhibit tunable luminescence properties, induced by excited-state intramolecular proton transfer. This method opens a new opportunity for the sustainable synthesis of luminescent organic molecules.

Emission Color-Tunable Oxazol(in)yl-Substituted Excited-State Intramolecular Proton Transfer (ESIPT)-Based Luminophores

“…Oxazoline-substituted hydroxyfluorenes Oxa-OH, (Oxa) 2 -OH and (Oxa-OH) 2 were synthesized according to our previously reported procedures in very good yields. 13,14 Hydroxylated benzoxazole (BO-OH), naphthoxazole (NO-OH), benzothiazole (BS-OH) and benzoselenazole (BSe-OH) were achieved using either a deprotonative metalation/aerobic hydroxylation method 14 or a Pd-catalyzed hydroxylation with 1,4-dioxane as oxygen source. 15 The optical properties of the respective precursor oxazolinyland arylchalcogenazolyl-substituted fluorenes were examined in different solvents (see the supporting information) as well as the final hydroxylated compounds which were additionally analyzed in their crystalline form.…”

“…Rf = 0.26 (SiO2, CH:EE 20: 192.2, 157.0, 154.1, 144.6, 136.8, 135.9, 130.8, 130.7, 126.6, 123.2, 122.7, 120.5, 47.4, 26.9 (2x) 11,14 2.3.4 2-(7-Bromo-9,9-dimethyl-9H-fluoren-2-yl)-4,4-dimethyl-4,5-dihydrooxazole (Oxa)…”

Recent Advancements on Transition‐Metal‐Catalyzed, Chelation‐Induced ortho‐Hydroxylation of Arenes

“…[12] Recently, a descent protocol on ortho-hydroxylation using Grignard reagent has been reported by Nachtsheim group of workers. [13] However, most of these protocols have limitations in terms of numbers of steps, use of toxic solvents, harsh condition, limited substrates scope, average yields along with lack of regioselectivity. Therefore, attention was focused on the development of efficient transition metal catalyzed chelation assisted chemo-and regioselective ortho-hydroxylation reactions (Scheme 1).…”

“…reported the regioselective ortho ‐hydroxylation of benzamide using molecular oxygen in presence of s ‐BuLi, TMEDA where ortho ‐lithiation followed by hydroxylation proceeds via the coordination of carbonyl oxygen [12] . Recently, a descent protocol on ortho ‐hydroxylation using Grignard reagent has been reported by Nachtsheim group of workers [13] . However, most of these protocols have limitations in terms of numbers of steps, use of toxic solvents, harsh condition, limited substrates scope, average yields along with lack of regioselectivity.…”

Clickable azide-functionalized bromoarylaldehydes – synthesis and photophysical characterization

“…Our group is highly interested in the de novo synthesis of small organic luminophores and in this regard, we recently developed efficient methods for the synthesis of ESIPT-based luminophores [70][71][72]. Herein, we present the efficient functionalization of derivatives of the potent luminophores 1 and 2 with "clickable" azide functionalities to target the structures 3-5 and further investigated the influence of this functionalization, both in the unlinked azide state and the linked triazole state, on the emission properties of these compounds (Figure 1).…”