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Synlett 2015; 26(11): 1520-1524
DOI: 10.1055/s-0034-1381006
DOI: 10.1055/s-0034-1381006
letter
Rhodium(III)-Catalyzed C–H Activation: An Oxidative Intramolecular Heck-Type Reaction Directed by a Carboxylate
Weitere Informationen
Publikationsverlauf
Received: 15. April 2015
Accepted after revision: 13. Mai 2015
Publikationsdatum:
15. Juni 2015 (online)
![](https://www.thieme-connect.de/media/synlett/201511/lookinside/thumbnails/st-2015-b0274-l_10-1055_s-0034-1381006-1.jpg)
Abstract
Carboxylates effectively direct C–H activation for rhodium(III)-catalyzed intramolecular Heck-type reactions. A catalytic amount of Cu(OAc)2 is used as the external oxidant with oxygen likely acting as the terminal oxidant. Additionally, a novel electron-deficient rhodium(III) catalyst was found to be more effective than [RhCp*Cl2]2 with some substrates. A wide variety of complex dihydrobenzofurans, dihydrobenzopyrans, and other bicycles that can be easily further functionalized are now accessible through relatively mild reaction conditions.
Key words
C–H activation - Heck reaction - rhodium(III) catalysis - cyclization - dihydrobenzofuransSupporting Information
- Supporting information for this article is available online at http://dx.doi.org/10.1055/s-0034-1381006.
- Supporting Information
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References and Notes
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- 4 For a recent review on carboxylate-directed C–H activation, see: Shi G, Zhang Y. Adv. Synth. Catal. 2014; 356: 1419
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- 13 Use of Z-disubstituted alkene substrates led to low conversions and poor Z/E ratios of the products.
- 14 A screen of different Cp catalysts revealed [RhCp(CF3)2ArCl2]2 to be the most selective in terms of di- vs. trisubstituted alkene selectivity and E/Z selectivity.
- 15 General Procedure for the Synthesis of 2a A 1.5 dram vial was charged with a stir bar, the acid substrate (0.1 mmol, 1 equiv), [RhCp*Cl2]2 (1.5 mg, 2.5 μmol, 0.025 equiv) or [RhCp(CF3)2ArCl2]2 (2.5 mg, 2.5 μmol, 0.025 equiv), Cu(OAc)2 (8.0 mg, 40 μmol, 0.4 equiv), and K2CO3 (38.4 mg, 200 μmol, 2 equiv), followed by addition of DCE–H2O (1:1, 500 μL, 0.2 M). The vial was flushed with oxygen, sealed, and stirred at 60 °C for the indicated time. The reaction mixture was allowed to cool and was quenched with 1 M aq HCl until fully precipitated (pH ca. 1). The suspension was then extracted twice with EtOAc. The combined organic layers were washed once with H2O and once with sat. aq NaCl before drying over MgSO4. The solution was filtered and concentrated by rotary evaporation. The crude residue was purified by column chromatography (EtOAc–hexanes). Compound 2a: Rf = 0.29 (25% EtOAc–hexanes). IR (film): 3082, 2962, 2925, 2879, 1696 cm–1. 1H NMR (400 MHz, CDCl3): δ = 7.56 (d, J = 7.6 Hz, 1 H), 7.25 (dd, J = 7.6 Hz, 1 H), 7.05 (d, J = 8.0 Hz, 1 H), 6.17 (dd, J = 17.6, 10.8 Hz, 1 H), 5.14 (d, J = 10.4 Hz, 1 H), 5.09 (d, J = 17.6 Hz, 1 H), 4.40 (d, J = 8.4 Hz, 1 H), 4.27 (d, J = 8.4 Hz, 1 H), 1.64 (s, 3 H); acid proton signal too broadened to assign. 13C NMR (100 MHz, CDCl3): δ = 171.4, 160.9, 142.0, 135.6, 128.6, 127.0, 123.6, 115.0, 113.6, 84.1, 49.4, 23.3. MS (ESI + APCI): m/z calcd for C12H11O3 [M – H]–: 203.1; found: 203.1.
For recent reviews on Rh(III) C–H activation, see:
Examples of Rh(III)-catalyzed amidoarylations with alkynes:
Amidoarylations with alkenes:
Olefinations:
Halogenations:
Allylations:
For examples of enantioselective Rh(III)-catalyzed reactions, see:
Examples of new or improved reactivity from the variation of Rh(III) Cp ligands: