Synlett 2020; 31(08): 805-808
DOI: 10.1055/s-0039-1690812
letter
© Georg Thieme Verlag Stuttgart · New York

Palladium-Catalyzed Synthesis of Fluorenes by Intramolecular C(sp2)–H Activation at Room Temperature

Yutaka Tanji
,
,
This work was supported by JSPS KAKENHI Grant Number 18H04257: Precisely Designed Catalysts with Customized Scaffolding (T. Fujihara), and Grant Number 17H03096 in Grant-in-Aid for Scientific Research B (Y. Tsuji) from MEXT, Japan. Y. Tanji is grateful for a research fellowship from JSPS for Young Scientists.
Further Information

Publication History

Received: 27 December 2019

Accepted after revision: 15 January 2020

Publication Date:
04 February 2020 (online)


Abstract

The synthesis of fluorenes by intramolecular Pd-catalyzed C(sp2)–H activation of 2-arylbenzyl chlorides was conducted at room temperature by using commercially available triphenylphosphine and pivalic acid as ligands. The desired reactions proceeded efficiently at room temperature, and various substrates were converted into the corresponding fluorene derivatives in excellent yields.

Supporting Information

 
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  • 11 9H-Fluorene (3a); Typical Procedure In advance, Cs2CO3 was dried by heating under vacuum for 3 h then stored in a glovebox. A 10-mL Schlenk flask was charged with the dried Cs2CO3 (0.16 g, 0.50 mmol) in a glovebox, then taken out of glovebox and dried by heating under a vacuum for at least 5 min. The flask was backfilled with argon and then PdCl2(PPh3)2 (1.4 mg, 0.0020 mmol, 1.0 mol%) and pivalic acid (1c; 20 mg, 0.20 mmol, 1.0 equiv) were added. The flask was then evacuated and backfilled with argon three times. THF (0.50 mL) and 2a (37 μL, 0.20 mmol) were added, and the mixture was stirred at 25 °C for 18 h. The mixture was then analyzed by GC with tetradecane (50 μL) as an internal standard. H2O (5 mL) and EtOAc (5 mL) were added, and the mixture was extracted with EtOAc (3 × 5 mL). The collected organic layers were combined, washed with brine (5 mL), dried (MgSO4), and concentrated. The residue was purified by chromatography (silica gel, hexane–acetone) to give a colorless solid; yield: 32.1 mg (96%). 1H NMR (400 MHz, CDCl3): δ = 7.80 (d, J = 7.8 Hz, 2 H), 7.56 (d, J = 7.3 Hz, 2 H), 7.39 (t, J = 7.3 Hz, 2 H), 7.31 (td, J = 7.4, 1.1 Hz, 2 H), 3.92 (s, 2 H). 13C NMR (100 MHz, CDCl3): δ = 143.2, 141.7, 126.7, 126.7, 125.0, 119.8, 36.9. EI-HRMS: m/z: [M – H]+ calcd for C13H9: 165.0704; found: 165.0702.