2-Hetaryl-1,3-tropolones based on five-membered nitrogen heterocycles: synthesis, structure and properties

Abstract

A series of derivatives of 2-hetaryl-1,3-tropolone (β-tropolone) was prepared by the acid-catalyzed reaction of 2-methylbenzoxazoles, 2-methylbenzothiazoles and 2,3,3-trimethylindoline with 3,4,5,6-tetrachloro-1,2-benzoquinone. The molecular structures of the three representative compounds were determined by X-ray crystallography. In crystal and (as shown by the DFT PBE0/6-311+G** calculations) in solution, 2-hetaryl-4,5,6,7-tetrachloro- and 2-hetaryl-5,6,7-trichloro-1,3-tropolones exist in the NH-tautomeric form with a strong resonance-assisted intramolecular N-H···O hydrogen bond. The mechanism of the formation of 1,3-tropolones in the reaction of methylene-active five-membered heterocycles with o-chloranil in acetic acid solution has been studied using density functional theory (DFT) methods. The reaction of 2-(2-benzoxa(thia)zolyl)-5,6,7-trichloro(4,5,6,7-tetrachloro)-1,3-tropolones with alcohols leads to the contraction of the seven-membered tropone ring with the formation of 2-(2-benzoxa(thia)zolyl)-6-alkoxycarbonylphenols. The molecular structure of 2-(2-ethoxycarbonyl-6-hydroxy-3,4,5-trichlorophenyl)benzoxazole has been determined by X-ray diffraction. 2-(2-Benzoxa(thia)zolyl)-6-alkoxycarbonylphenols display intense green fluorescence with anomalous Stokes shifts caused by the excited state intramolecular proton transfer (ESIPT) effects.

Keywords: X-ray analysis; fluorescence; intramolecular hydrogen bond; tautomerism; β-tropolones.

Scheme 1

1,3-Tropolones 2–4 prepared by the reaction of o-chloranil with methylene active compounds.

Scheme 2

General scheme of the synthesis of 2-(2-hetaryl)-5,6,7-trichloro-1,3-tropolones 5 and 2-(2-hetaryl)-4,5,6,7-tetrachloro-1,3-tropolones 6. Method A – heating under reflux of dioxane solution of the reactants. Method B – running reaction in acetic acid solution.

Scheme 3

The mechanism for the formation of 5,6,7-trichloro-1,3-tropolones 5 and 4,5,6,7-tetrachloro-1,3-tropolones 6 in the reaction of methylene-active five-membered heterocycles with o-chloranil in acetic acid solution.

Figure 1

Molecular structure of 2-(3,3-dimethylindolyl)-5,6,7-trichloro-1,3-tropolone 5g. Thermal ellipsoids are drawn on the 30% probability level. Selected bond lengths (Å): O(1)–C(1) 1.210(3), O(2)–C(3) 1.248(3), N(1)–C(8) 1.333(3), C(1)–C(2) 1.468(4), C(1)–C(7) 1.723(3), C(2)–C(3) 1.447(4), C(2)–C(8) 1.415(4), C(3)–C(4) 1.472(4), C(4)–C(5) 1.326(4), C(5)–C(6) 1.440(5), C(6)–C(7) 1.328(4), C(8)–C(9) 1.536(4); selected bond angles (^o): O(1)–C(1)–C(2) 125.7(3), C(3)–C(2)–C(1) 119.7(2), C(3)–C(2)–C(8) 118.4(2), O(2)–C(3)–C(2) 123.5(2), N(1)–C(8)–C(2) 120.2(2).

Figure 2

Molecular structure of 2-(5-chlorobenzothiazolyl)-4,5,6,7-tetrachloro-1,3-tropolone 6e. Thermal ellipsoids are drawn on the 30% probability level. Selected bond lengths (Å): O(1)–C(1) 1.229(3), O(2)–C(3) 1.230(3), N(1)–C(8) 1.346(3), C(1)–C(2) 1.444(4), C(1)–C(7) 1.503(4), C(2)–C(3) 1.433(4), C(2)–C(8) 1.419(4), C(3)–C(4) 1.499(4), C(4)–C(5) 1.339(4), C(5)–C(6) 1.455(4), C(6)–C(7) 1.345(4), S(1)–C(8) 1.734(3); selected bond angles (^o): O(1)–C(1)–C(2) 122.5(3), C(3)–C(2)–C(1) 126.2(2), C(3)–C(2)–C(8) 116.8(2), O(2)–C(3)–C(2) 123.4(3), N(1)–C(8)–C(2) 124.1(3). In the elementary unit cell of compound 6e containing four molecules of the compound is also present a molecule of benzene located in the center of symmetry (0,0,1/2).

Scheme 4

The fast prototropic N–H···O N···H–O equilibrium in solutions of 2-hetaryl-5,6,7-trichloro- and 4,5,6,7-tetrachloro-1,3-tropolones.

Scheme 5

Two reaction paths for coupling 2-hetaryl-1,3-tropolones 5 and 6 with alcohols.

Figure 3

Molecular structure of 2-(3,3-dimethylindolyl)-5,7-dichloro-6-ethoxy-1,3-tropolone 13. Selected bond lengths (Å): O(1)–C(1) 1.217(2), O(2)–C(3) 1.2563(19), N(1)–C(8) 1.334(2), C(1)–C(2) 1.471(2), C(1)–C(7) 1.508(2), C(2)–C(3) 1.444(2), C(2)–C(8) 1.422(2), C(3)–C(4) 1.475(2), C(4)–C(5) 1.335(2), C(5)–C(6) 1.461(2), C(6)–C(7) 1.343(2), C(8)–C(9) 1.546(2); selected bond angles (^o) O(1)–C(1)–C(2) 123.71(16), C(3)–C(2)–C(1) 120.98(15), C(3)–C(2)–C(8) 118.71(14), O(2)–C(3)–C(2) 123.16(15), N(1)–C(8)–C(2) 120.27(14), N(1)–H(2)–O(2)= 136.5(9).

Figure 4

Molecular structure of 2-(2-ethoxycarbonyl-6-hydroxy-3,4,5-trichlorophenyl)benzoxazole 11b. Selected bond lengths (Å): C(1)–C(2) 1.4071(13), C(2)–C(3) 1.4130(13), C(2)–C(8) 1.4511(14), N(1)–C(8) 1.3084(12), O(1)–C(8) 1.3621(12), O(2)–C(3) 1.3397(12), O(2)–H(2) 0.845(17); selected bond angles (^o) N(1)–C(8)–O(1) 114.99(9), O(1)–C(8)–C(2) 119.76(8), N(1)–C(8)–C(2) 125.24(9), C(3)–C(2)–C(8) 117.85(8), C(1)–C(2)–C(8) 122.45(9).

Figure 5

Electronic absorption (1, 2), fluorescence emission (λ_exc = 350 nm) (3, 4) and fluorescence excitation [λ_obs = 495 nm (5), λ_obs = 510 nm (6)] spectra of 11c (1, 3, 5) and 11d (2, 4, 6) in heptane solution (c = 2∙10⁻⁵ mol × L⁻¹, l = 1 cm) at 293 K.