Transformation of 6′‐aminosubstituted spironaphthoxazines induced by Pb(II) and Eu(III) cations

Abstract

The transformation of spiroindolinonaphth[2,1‐b][1,4]oxazines (SNOs) conjugated with aza‐15(18)‐crown‐5(6) ether moieties or morpholine in the presence of Pb2+ or lanthanoid cations in MeCN solution was studied by UV–Vis, NMR and surface‐enhanced resonance Raman scattering spectroscopy methods. The Pb2+ or Eu3+ cations induce the formation of the betaine merocyanine form of SNO. When the merocyanine complexes are kept in the dark, the corresponding oxazole derivatives are formed irreversibly and regioselectively. Copyright © 2005 John Wiley & Sons, Ltd.

Cation-induced ring-opening and oxidation reaction of photoreluctant spirooxazine–quinolizinium conjugates

“…Moreover, the 13 C NMR signal of C2' also exhibited a downfield shift from δ C = 100.7 to 170.8. Thereby, both the 1 H and the 13 C NMR shifts of the indolinium-naphthoxazole unit were in good agreement with the reported ones of similar derivatives [17,[73][74][75][76]. At the same time, the addition of 2 equiv of Cu 2+ to 3b also resulted in the formation of the corresponding oxazole derivative 4b (Scheme 3), as shown by 1 H NMR spectroscopic and mass spectrometric analysis.…”

“…As the substrates 3a and 3b turned out to be resistant towards the photoinduced merocyanine formation, we tested whether the ring-opening reaction of 3a and 3b may be achieved by the treatment with metal ions as this has been shown for other derivates already [17][18][19][20][21][22][23][24][25][26]. Since the color change upon conversion of the substrate can usually be seen by the naked eye, a colorimetric screening was performed with a series of different metal cations to check their propensity to induce a reaction of 3a and 3b.…”

“…It has already been shown that spirooxazines may react in certain cases to oxazole derivatives. However, depending on the employed conditions, different mechanisms for the formation of these oxidation products have been proposed [17,[73][74][75][76][77][78][79][80]. For example, Fedorova et al have shown that after the cation-induced ring opening of spirooxazine derivatives, the respective merocyanine forms undergo a slow oxidation to the corresponding oxazoles in the dark only under aerobic conditions [17,80].…”

“…These classes of compounds possess advantageous properties, such as good synthe-tic accessibility, which allows a broad range of structural modifications and high quantum yields of the photoreaction [12,15,16]. The photochromism is based on a reversible electrocyclic reaction that proceeds through the UV light-induced cleavage of the C2'-O bond of the closed, colorless form to give the colored, metastable merocyanine form, whereas the Scheme 1: Photo-or cation-induced ring-opening reaction of spirooxazine 1a SO ; M n+ = Pb 2+ , La 3+ , Eu 3+ , Tb 3+ [17]. back reaction can be initiated thermally or by the irradiation with visible light, as is exemplarily shown for the spiroindolinonaphthoxazine 1a (Scheme 1) [15].…”

“…The merocyanine form 1a MC essentially possesses a planar structure, and because of the extended π system, the absorption maximum is significantly red-shifted in comparison to the closed form (e.g., 1a SO : λ abs = 317 nm, 1a MC : λ abs = 602 nm, in MeCN) [16]. In some instances, the ring-opening reaction may also be induced in the dark through the addition of particular metal cations that are able to coordinate to the phenolate oxygen atom and/or other additional coordinating atoms and substituents that have been introduced into the structure of the spirooxazine and spiropyran derivatives [17][18][19][20][21][22][23][24][25][26]. Regardless of whether the ring-opening reaction is induced by cations or irradiation, the coordination of metal ions to the substrate usually results in a negative photochromism, i.e., a blue shift of the absorption maximum of the merocyanine form [5,19,[27][28][29], and increased stability of the latter towards the thermal back reaction [21,28,[30][31][32][33][34].…”