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

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.

Key words

palladium - catalysis - heterocycles - water - C–H functionalization

• References and Notes

• 1a Organic Reactions in Water . Lindstorm UM. Wiley-Blackwell; New York: 2007
• 1b Simon M.-O, Li C.-J. Chem. Soc. Rev. 2012; 41: 1415

For reviews on the use of amphiphilic surfactants for organic synthesis, see:
• 2a Dwars T, Paetzold E, Oehme G. Angew. Chem. Int. Ed. 2005; 44: 7174
• 2b Lipshutz BH, Ghorai S. Aldirichimica Acta 2008; 41: 59

For selected recent reviews on transition-metal-catalyzed C–H functionalization, see:
• 3a Cho SH, Kim JY, Kwak J, Chang S. Chem. Soc. Rev. 2011; 40: 5068
• 3b Baudoin O. Chem. Soc. Rev. 2011; 40: 4902
• 3c Wencel-Delord J, Dröge T, Liu F, Glorius F. Chem. Soc. Rev. 2011; 40: 4740
• 3d Wendlandt AE, Suess AM, Stahl SS. Angew. Chem. Int. Ed. 2011; 50: 11062
• 3e Liu Q, Zhang H, Lei A. Angew. Chem. Int. Ed. 2011; 50: 10788
• 3f Beccalli EM, Broggini G, Fasana A, Rigamonti M. J. Organomet. Chem. 2011; 696: 277
• 3g Lyons TW, Sanford MS. Chem. Rev. 2010; 110: 1147
• 3h Ashenhurst JA. Chem. Soc. Rev. 2010; 39: 540
• 3i Ackermann L. Chem. Commun. 2010; 46: 4866
• 3j Sun C.-L, Li B.-J, Shi Z.-J. Chem. Commun. 2010; 46: 677
• 4a Inamoto K, Hasegawa C, Hiroya K, Doi T. Org. Lett. 2008; 10: 5147
• 4b Inamoto K, Hasegawa C, Kawasaki J, Hiroya K, Doi T. Adv. Synth. Catal. 2010; 352: 2643

For other examples of C–S formation via transition-metal-catalyzed or -mediated C–H functionalization, see:
• 5a Inamoto K, Arai Y, Hiroya K, Doi T. Chem. Commun. 2008; 5529
• 5b Zhao X, Dimitrijević E, Dong VM. J. Am. Chem. Soc. 2009; 131: 3466
• 5c Fukuzawa S.-i, Shimizu E, Atsumi Y, Haga M, Ogata K. Tetrahedron Lett. 2009; 50: 2374
• 5d Joyce LL, Batey RA. Org. Lett. 2009; 11: 2792
• 5e Chu L, Yue X, Qing F.-L. Org. Lett. 2010; 12: 1644
• 5f Zhang S, Qian P, Zhang M, Hu M, Cheng J. J. Org. Chem. 2010; 75: 6732
• 5g Saidi O, Marafie J, Ledger AE. W, Liu PM, Mahon MF, Kociok-Köhn G, Whittlesey MK, Frost CG. J. Am. Chem. Soc. 2011; 133: 19298
• 5h For selected recent examples for the synthesis of 2-arylbenzothiazoles from thiobenzanilides, see: Bose DS, Idrees M. J. Org. Chem. 2006; 71: 8261
• 5i See also: Bose DS, Idrees M, Srikanth B. Synthesis 2007; 819
• 5j See also: Moghaddam FM, Zargarani D. J. Sulfur Chem. 2009; 30: 507
• 5k See further: Wang H, Wang L, Shang J, Li X, Wang H, Gui J, Lei A. Chem. Commun. 2012; 48: 76

For selected recent reports on the use of amphiphilic surfactants in transition-metal-catalyzed processes, see:
• 6a Nishikata T, Abela AR, Lipshutz BH. Angew. Chem. Int. Ed. 2010; 49: 781
• 6b Krasovskiy A, Duplais C, Lipshutz BH. Org. Lett. 2010; 12: 4742
• 6c Lessi M, Masini T, Nucara L, Bellina F, Rossi R. Adv. Synth. Catal. 2011; 353: 501
• 6d Gottardo M, Scarso A, Paganelli S, Strukul G. Adv. Synth. Catal. 2010; 352: 2251
• 6e Cavarzan A, Scarso A, Strukul G. Green Chem. 2010; 12: 790
• 6f Cicco SR, Farnola GM, Martinelli C, Naso F, Tiecco M. Eur. J. Org. Chem. 2010; 2275
• 6g Lipshutz BH, Ghorai S, Abela AR, Moser R, Nishikata T, Duplais C, Krasovskiy A. J. Org. Chem. 2011; 76: 4379
• 6h Lipshutz BH, Ghorai S, Leong WW. Y, Taft BR. J. Org. Chem. 2011; 76: 5061
• 6i Samant BS, Kabalka GW. Chem. Commun. 2011; 47: 7236
• 6j Inamoto K, Nozawa K, Yonemoto M, Kondo Y. Chem. Commun. 2011; 47: 11775

Only a few examples of Pd-catalyzed C–H functionalization processes using water as a reaction medium have so far been reported, see:
• 7a Turner GL, Morris JA, Greaney MF. Angew. Chem. Int. Ed. 2007; 46: 7996
• 7b Nishitaka T, Lipshutz BH. Org. Lett. 2010; 12: 1972
• 7c Ref. 6a

Several processes involving Ru-catalyzed C–H functionalization in water have recently been reported, see:
• 8a Arockiam PB, Fischmeister C, Bruneau C, Dixneuf PH. Angew. Chem. Int. Ed. 2010; 49: 6629
• 8b Ackermann L, Fenner S. Org. Lett. 2011; 13: 6548
• 8c Ackermann L, Wang L, Wolfram R, Lygin AV. Org. Lett. 2012; 14: 728
• 8d Ackermann L, Lygin AV. Org. Lett. 2012; 14: 764
• 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