Facile approach to N,O,S-heteropentacycles via condensation of sterically crowded 3 H-phenoxazin-3-one with ortho- substituted anilines

Abstract

A convenient method for the synthesis of a series of 2-(arylamino)-3H-phenoxazin-3-ones based on the nucleophilic substitution reaction between sterically crowded 3H-phenoxazin-3-one and arylamines performed by short-term heating of the melted reactants at 220-250 °C is described, and the compounds were characterized by means of single-crystal X-ray crystallography, NMR, UV-vis, and IR spectroscopy, as well as cyclic voltammetry. The reaction with o-amino-, o-hydroxy-, and o-mercapto-substituted arylamines widened the scope and provided an access to derivatives of N,O- and N,S-heteropentacyclic quinoxalinophenoxazine, triphenodioxazine and oxazinophenothiazine systems.

Keywords: 3H-phenoxazin-3-one; fluorescence; molecular structure; pentacyclic heterocycles; synthesis.

Scheme 1

Synthesis of 6,8-di-tert-butyl-N-aryl-3H-phenoxazin-3-imines 3 [6] and 6,8-di-tert-butyl-2-(arylamino)-3H-phenoxazin-3-ones 4.

Figure 1

DFT-calculated molecular geometry (B3LYP/6-311++G(d,p) level) and distribution of electronic density in 6,8-di-tert-butyl-3H-phenoxazin-3-one (1): Mulliken charges and molecular electrostatic potential (MEP, isovalue = 0.004).

Scheme 2

6,8-Di-tert-butyl-2-(arylamino)-3H-phenoxazin-3-ones 4 prepared by the one-pot reaction between 6,8-di-tert-butyl-3H-phenoxazin-3-one (1) and aromatic amines 2b (the yield is given in parentheses).

Figure 2

Molecular structure of 6,8-di-tert-butyl-2-(o-nitrophenylamino)-3H-phenoxazin-3-one (4f). a) Selected bond distances (Å) and angles: N(1)–C(11) 1.3061(19), N(1)–C(14) 1.3836(18), O(23)–C(4) 1.2314(19), N(15)–C(3) 1.3765(19), N(15)–C(16) 1.383(2), C(11)–N(1)–C(14) 117.55(12), C(3)–N(15)–C(16) 131.12(14). b) Crystal packing of 4f. Important crystallographic parameters and bond distances are given in Tables S2 and S5, Supporting Information File 1. Thermal ellipsoids are drawn at the 50% probability level.

Figure 3

UV–vis spectra of 6,8-di-tert-butyl-2-(arylamino)-3H-phenoxazin-3-ones 4a–h (toluene, c = 2⋅10⁻⁵ M, l = 1 cm, T = 293 K).

Scheme 3

Synthesis of 14H-quinoxaline[2,3-b]phenoxazines 5 and 6.

Scheme 4

Relative stability of the tautomers 7 and 7a,b of quinoxaline[2,3-b]phenoxazine calculated at the DFT B3LYP/6-311++G(d,p) level.

Scheme 5

Preparation of quinoxaline[2,3-b]phenoxazine (7) from 2-amino-3H-phenoxazin-3-one (8) [10] and 2-ethoxy-3H-phenoxazin-3-one (9) [11], respectively.

Figure 4

Molecular structure of ethyl 2,4-di-tert-butyl-14H-quinoxalino[2,3-b]phenoxazine-10-carboxylate (5c), with atom numbering scheme. Selected bond distances (Å) and angles: N(1)–C(5) 1.390(2), N(1)–C(6) 1.365(3), N(2)–C(14) 1.363(3), N(2)–C(15) 1.336(3), N(3)–C(8) 1.338(3), N(3)–C(9) 1.356(3), C(6)–N(1)–C(5) 122.13(17), C(15)–N(2)–C(14) 116.88(17), C(8)–N(3)–C(9) 116.66(17). All bond lengths, angles, and important crystallographic parameters are given in Tables S6 and S7, Supporting Information File 1. Hydrogen atoms are omitted for clarity.

Scheme 6

Triphenodioxazine and oxazinophenothiazine derivatives 10 via condensation of 3H-phenoxazin-3-one 1 with o-aminophenol and o-mercaptoaniline derivatives 2d.

Figure 5

a) UV–vis (solid lines) and fluorescence emission (λ_ex = 365 nm, dashed) spectra of compounds 5a–c (toluene, c = 2 10⁻⁵ M, l = 1 cm). b) Solutions of compounds 5a–c in toluene before irradiation (no emission) and c) during irradiation (photoluminescence, λ_ex = 365 nm) at room temperature.

Figure 6

UV–vis (solid lines) and fluorescence emission (dashed, λ_ex = 365 nm) spectra of compounds 6a,b in toluene (c = 2 10⁻⁵ M, l = 1 cm) at room temperature.

Figure 7

UV–vis, fluorescence emission (λ_ex = 500 nm), and fluorescence excitation (λ_obs = 590 nm) spectra of benzo[5,6][1,4]oxazino[2,3-b]phenothiazine 10c in toluene (c = 2 10⁻⁵ M (UV–vis) or c = 2 10⁻⁶ M (fluorescence), l = 1 cm) at room temperature.