Synthesis of Substituted Phenanthrenes via Intramolecular Condensation Based on Temperature-Dependent Deprotonation Using a Weak Carbonate Base

Katrien Monsieurs; Geert Rombouts; Pál Tapolcsányi; Péter Mátyus; Bert U. W. Maes

doi:10.1055/s-2006-956451

LETTER

Synthesis of Substituted Phenanthrenes via Intramolecular Condensation Based on Temperature-Dependent Deprotonation Using a Weak Carbonate Base

Katrien Monsieurs^a, Geert Rombouts^a, Pál Tapolcsányi^b, Péter Mátyus*^b, Bert U. W. Maes*^a
^a Department of Chemistry, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium
Fax: +32(3)2653233; e-Mail: bert.maes@ua.ac.be;
^b Department of Organic Chemistry, Semmelweis University, Högyes E. u. 7., 1092 Budapest, Hungary
Fax: +36(1)217085; e-Mail: matypet@szerves.sote.hu;

Abstract

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Abstract

Construction of substituted phenanthrenes via intramolecular condensation of 2′-methylbiphenyl-2-carbaldehydes using a mild base at 200 °C is described. The required high temperature can be quickly reached and easily maintained using microwave flash heating.

Key words

phenanthrene - Suzuki reaction - condensation - carbonate base - microwave irradiation - 11H-benzo[a]carbazole

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References

References and Notes

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General Procedure for the Synthesis of Phenanthrenes 3a-g and 11-Methyl-11 H -benzo[ a ]carbazole (6): An
80-mL Greenchem vessel was charged with biphenyl compound 2a-g (0.75 mmol), Cs₂CO₃ (3 mmol) and anhyd DMF (5 mL). The vessel was flushed with argon under magnetic stirring for a few minutes. Subsequently, the vessel was sealed and heated to 200 °C in a Mars multi-mode microwave oven (CEM). The set power was 300 W. The total irradiation time (including the ramp time to the set temperature) was 90 min, unless indicated otherwise. After cooling the reaction mixture was poured into H₂O (100 mL) and was extracted with EtOAc (3 × 100 mL). The combined organic fractions were dried over MgSO₄, evaporated to dryness and purified via flash column chromatography on silica gel.

It was not possible to reach 200 °C when DME was used as the solvent. At a temperature of 193 °C the autogenic pressure rose to the maximum allowed value of 200 psi. The safety settings of the microwave apparatus stopped microwave irradiation of the vessel. The reaction mixture was subsequently held at 193 °C not to exceed 200 psi. Figure [2] shows the heating profile.

Figure 2

The isolation of unsubstituted phenanthrene and 3-methyl-phenanthrene was hampered by their volatility. It is possible that this also contributes to the lower yields obtained for these compounds.

General Procedure for the Synthesis of Biphenyls 2a-g: A two-necked flask was charged with phenylbromide 1a-g, arylboronic acid (1.5 equiv), Pd(PPh₃)₄ (5 mol%), and DME (6 mL/mmol 1). The flask was connected to a reflux condenser and flushed with nitrogen (via the second neck) for 2 min under magnetic stirring. Subsequently, an aq solution of 10% Na₂CO₃ (1 mL/mmol 1) was added and the reaction mixture was stirred and refluxed overnight in an oil bath under a N₂ atmosphere. After cooling, the reaction mixture was poured into H₂O and extracted with CH₂Cl₂. The combined organic layers were dried over MgSO₄, evaporated to dryness and purified by flash column chromatography on silica gel.

1-Hydroxy-2-nitrophenanthrene: mp 160 °C (Lit.³⁵ 161-162 °C). ¹H NMR: (acetone-d ₆): δ = 8.82-8.88 (m, 1 H), 8.44 (d, J = 9.5 Hz, 1 H), 8.38 (d, J = 9.2 Hz, 1 H), 8.27 (d, J = 9.5 Hz, 1 H), 8.07-8.12 (m, 1 H), 8.03 (d, J = 9.1 Hz, 1 H), 7.77-7.83 (m, 2 H). A derivatization of this compound was carried out to provide an extra confirmation of its structure.
1-Methoxy-2-nitrophenanthrene was synthesized by methylation of 1-hydroxy-2-nitrophenanthrene; mp 120-121 °C (Lit.³⁵ 122-123 °C).

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