Synlett 2013; 24(10): 1229-1232
DOI: 10.1055/s-0033-1338842
cluster
© Georg Thieme Verlag Stuttgart · New York

SNAr-Derived Decomposition By-products Involving Pentafluorophenyl Triazolium Carbenes

Xiaodan Zhao
Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA   Fax: +1(970)4911801   Email: rovis@lamar.colostate.edu
,
Garrett S. Glover
Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA   Fax: +1(970)4911801   Email: rovis@lamar.colostate.edu
,
Kevin M. Oberg
Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA   Fax: +1(970)4911801   Email: rovis@lamar.colostate.edu
,
Derek M. Dalton
Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA   Fax: +1(970)4911801   Email: rovis@lamar.colostate.edu
,
Tomislav Rovis*
Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA   Fax: +1(970)4911801   Email: rovis@lamar.colostate.edu
› Author Affiliations
Further Information

Publication History

Received: 19 April 2013

Accepted after revision: 30 April 2013

Publication Date:
17 May 2013 (online)

Abstract

Pentafluorophenyl triazolium carbenes, widely used in NHC catalysis, can decompose by several mechanisms. Under high concentration conditions, the azolium may undergo a pentafluorophenyl exchange by a proposed SNAr mechanism to give an inactive salt. In the presence of appropriate substrates, cyclization on the ­ortho-position of the arene can occur, also by SNAr. These adducts provide a potential pathway for catalyst decomposition and serve as a caveat to the development of new reactions and catalysts.

Supporting Information

 
  • References and Notes

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  • 9 Typical Experimental Procedure for Decomposition of 1: In a 5-mL vial were subsequently added triazolium salt 1 (0.2 mmol), KOAc (0.2 mmol) and MeOH (3 mL). The solution was stirred under argon at 23 °C for 16 h, and then concentrated. The resulting residue was purified by column chromatography on silica gel [eluent: hexanes–EtOAc (1:1) then 100% EtOAc then 100% MeOH] to give 2 and 3. The salt 3 can be further purified by trituration with EtOAc. Compound 2: yellow solid. 1H NMR (300 MHz, CDCl3): δ = 8.26 (d, J = 12.5 Hz, 1 H), 7.30 (br s, 4 H), 7.00 (br s, 0.5 H), 6.31 (br s, 0.5 H), 4.71–4.74 (m, 1 H), 4.63 (t, J = 4.7 Hz, 1 H), 4.21–4.38 (m, 2 H), 3.26 (dd, J = 16.9, 5.1 Hz, 1 H), 3.12 (d, J = 16.9 Hz, 1 H). 13C{1H} NMR (75 MHz, CDCl3): δ = 163.8, 158.5, 139.3, 128.7, 128.4, 127.4, 125.3, 123.3, 64.3, 57.2, 37.9, 29.3. 19F NMR (282 MHz, CDCl3): δ = –142.8 (s, 0.5 F), –143.2 (s, 0.5 F), –145.8 (d, J = 19.8 Hz, 1 F), –152.6 (t, J = 21.4 Hz, 0.5 F), –153.4 (t, J = 21.4 Hz, 0.5 F), –160.8 (t, J = 19.5 Hz, 1 F), –161.5 (s, 1 F). LRMS (ES+): m/z [M]+ calcd for C18H12F5N3O2: 397.09; found: 398.10. Compound 3: slightly yellow solid. 1H NMR (400 MHz, CDCl3): δ = 13.23 (s, 1 H), 7.71 (d, J = 7.5 Hz, 1 H), 7.23–7.33 (m, 3 H), 6.50 (br s, 1 H), 4.98–5.05 (m, 3 H), 3.32 (dd, J = 4.2, 17.1 Hz, 1 H), 3.19 (d, J = 17.1 Hz, 1 H). 13C{1H} NMR (101 MHz, CDCl3): δ = 150.6, 148.1 (br), 139.8, 134.7, 129.7, 128.0, 125.4, 124.2, 77.5, 62.8, 60.3, 37.5. 19F NMR (282 MHz, CDCl3): δ = –143.7 (d, J = 18.4 Hz, 2 F), –145.7 (t, J = 21.8 Hz, 1 F), –157.9 to –158.1 (m, 2 F). HRMS (ESI): m/z [M – OH]+ calcd for C18H11F5N3O: 380.0817; found: 380.0816.
  • 10 Salt 3 proved difficult to purify to analytically pure material, resulting in significant loss of material. Thus, we do not report an isolated yield here.
  • 11 Compound 4: orange solid. 1H NMR (300 MHz, CDCl3): δ = 7.35 (d, J = 4.1 Hz, 2 H), 7.13–7.17 (m, 1 H), 6.71 (d, J = 7.8 Hz, 1 H), 6.01 (t, J = 3.5 Hz, 1 H), 5.05–5.17 (m, 3 H), 3.43 (dd, J = 17.0, 4.5 Hz, 1 H), 3.34 (d, J = 17.0 Hz, 1 H), 1.86–1.62 (br s, 2 H from H2O). 13C{1H} NMR (100 MHz, CDCl3): δ = 150.7, 139.7, 134.4, 130.3, 128.6, 126.0, 123.8, 98.7, 77.8, 62.9, 62.8, 60.1, 36.9. 19F NMR (376 MHz, CDCl3): δ = –142.3 to –142.4 (m, 1 F), –144.1 (tt, J = 21.6, 4.4 Hz, 1 F), –144.2 to –144.3 (m, 1 F), –144.6 to –144.7 (m, 1 F), –147.3 to –147.4 (m, 1 F), –156.5 (td, J = 21.8, 6.8 Hz, 1 F), –156.8 (td, J = 21.8, 6.8 Hz, 1 F), –163.2 (t, J = 23.0 Hz, 1 F), –163.5 (t, J = 23.0 Hz, 1 F). HRMS (ESI): m/z [M + H]+ calcd for C24H11F9N3O2: 544.0708; found: 544.0712.
  • 12 This structure is somewhat disordered with what appears to be partial protonation of the phenolic oxygen. There is a corresponding counterion, also disordered, whose identity could not be determined (hydroxide vs. fluoride). Both structures (4 and 9) have been submitted to the Cambridge Crystallographic Data Centre as CCDC 935024 and CCDC 935025.
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  • 15 Experimental Procedure for the Formation of 9: In a 5-mL vial were subsequently added triazolium salt 8 (0.05 mmol), 7 (0.05 mmol), K2CO3 (0.05 mmol) and THF (4 mL). The vial was capped. The mixture was stirred at 23 °C for 48 h, and then concentrated. The resulting residue was purified by preparative TLC [eluent: hexanes–EtOAc (1:1)] to give the adduct 9 as a solid. Compound 9: slightly yellow solid. 1H NMR (400 MHz, CDCl3): δ = 7.81 (d, J = 8.2 Hz, 1 H), 7.25 (td, J = 2.4, 8.3 Hz, 1 H), 7.01–7.07 (m, 2 H), 4.22–4.33 (m, 2 H), 3.34 (s, 3 H), 2.88–2.92 (m, 2 H), 2.54 (p, J = 7.5 Hz, 2 H), 1.64 (s, 9 H). 13C{1H} NMR (101 MHz, CDCl3): δ = 177.2, 164.6, 158.8, 158.7, 149.5, 145.5, 138.7, 134.5, 128.7, 124.7, 123.8, 114.5, 109.9, 84.0, 50.2, 49.3, 28.1, 26.2, 21.2. 19F NMR (282 MHz, CDCl3): δ = –134.9 (ddd, J = 5.3, 8.8, 22.5 Hz, 1 F), –150.7 (dd, J = 9.0, 20.5 Hz, 1 F), –153.87 (dt, J = 5.1, 21.0 Hz, 1 F), –160.43 (t, J = 22.0 Hz, 1 F). HRMS (ESI): m/z [M + H]+ calcd for C27H23F4N4O5: 559.1599; found: 559.1603.
  • 16 DiRocco DA, Oberg KM, Rovis T. J. Am. Chem. Soc. 2012; 134: 6143
  • 17 A catalyst deactivation pathway has been observed for N-phenyl triazolium precursors, see: Hao L, Du Y, Lv H, Chen X, Jiang H, Shao Y, Chi YR. Org. Lett. 2012; 14: 2154