Synlett 2012; 23(11): 1678-1682
DOI: 10.1055/s-0031-1291164
letter
© Georg Thieme Verlag Stuttgart · New York

Palladium-Catalyzed C–H Cyclization in Water: A Milder Route to 2-Arylbenzothiazoles

Kiyofumi Inamoto*
Graduate School of Pharmaceutical Sciences, Tohoku University, 6-3, Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan, Fax: +81(22)7955917   eMail: inamoto@m.tohoku.ac.jp   eMail: ykondo@m.tohoku.ac.jp
,
Kanako Nozawa
Graduate School of Pharmaceutical Sciences, Tohoku University, 6-3, Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan, Fax: +81(22)7955917   eMail: inamoto@m.tohoku.ac.jp   eMail: ykondo@m.tohoku.ac.jp
,
Yoshinori Kondo*
Graduate School of Pharmaceutical Sciences, Tohoku University, 6-3, Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan, Fax: +81(22)7955917   eMail: inamoto@m.tohoku.ac.jp   eMail: ykondo@m.tohoku.ac.jp
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Publikationsverlauf

Received: 29. Februar 2012

Accepted after revision: 19. April 2012

Publikationsdatum:
13. Juni 2012 (online)


Abstract

Water was successfully employed as a reaction medium in palladium-catalyzed C–H cyclization of thiobenzanilides. Reactions efficiently proceeded under considerably mild conditions such as 40 °C in water, providing a more practical, greener method for the synthesis of 2-arylbenzothiazoles. For some substrates, the addition of an amphiphilic surfactant greatly enhanced the process. The method represents a rare example of palladium-catalyzed C–H functionalization processes performed in water.

Supporting Information

 
  • References and Notes

  • 9 Pd-catalyzed C–H functionalization processes performed under mild conditions such as room temperature still remain an important challenge, see: Haffemayer B, Gulias M, Gaunt MJ. Chem. Sci. 2011; 2: 312 ; and references cited therein
  • 10 Use of other surfactants such as SDS, TPGS, PTS, Brij 30, and Brij S-100 led to somewhat decreased yields
    • 11a As preliminary mechanistic studies, reactions using the isotopically labeled substrates were carried out, which resulted in the observation of KIEs (kinetic isotope effects) of 2.3–2.7. For details, see Supporting Information
    • 11b For C–S bond-forming reductive elimination from Pd(IV) complexes, see: Zhao X, Dong VM. Angew. Chem. Int. Ed. 2011; 50: 932
  • 12 Representative Procedure for the Synthesis of 2-Arylbenzothiazoles (Table 2, Entry 5, Conditions B) Under an O2 atmosphere, a mixture of N-(4-nitrophenyl)-thiobenzamide (1e, 37.2 mg, 0.14 mmol), Pd2(dba)3 (7.4 mg, 0.0081 mmol), tris(2-methylphenyl)phosphine (9.6 mg, 0.32 mmol), Rb2CO3 (36.2 mg, 0.16 mmol), and Triton X-100 (29.7 mg, 0.048 mmol) in H2O (3 mL) was stirred for 24 h at 40 °C. The reaction mixture was diluted with sat. aq NH4Cl (5 mL) and extracted with CHCl3 (3 × 30 mL), and then the combined organic layer was dried over Na2SO4. The solvent was removed under a reduced pressure, and the residue was purified by SiO2 column chromatography (eluent; 1% EtOAc in hexane) to give 6-nitro-2-phenylbenzothiazole (2e, 28.3 mg, 77%). Recrystallization from EtOAc–hexane gave pale orange prisms, mp 191–192 °C. 1H NMR (400 MHz, CDCl3/TMS): δ = 7.50–7.56 (m, 3 H), 8.09–8.12 (m, 3 H), 8.34 (dd, 1 H, J = 9.0, 2.3 Hz), 8.81 (d, 1 H, J = 2.3 Hz) ppm. 13C{1H} NMR (100 MHz, CDCl3/TMS): δ = 118.1, 121.8, 123.2, 127.8, 129.2, 132.1, 132.6, 135.2, 144.8, 157.7, 173.6 ppm. LRMS (EI): m/z = 256 [M+]. HRMS: m/z calcd for C13H8N2O2S: 256.0307; found: 256.0287. IR (neat): 1518, 1333 cm–1