Synlett 2016; 27(08): 1187-1192
DOI: 10.1055/s-0035-1561599
cluster
© Georg Thieme Verlag Stuttgart · New York

Site-Selective and Stereoselective C(sp3)–H Borylation of Alkyl Side Chains of 1,3-Azoles with a Silica-Supported Monophosphine-Iridium Catalyst

Ryo Murakami
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan   Email: sawamura@sci.hokudai.ac.jp
,
Tomohiro Iwai
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan   Email: sawamura@sci.hokudai.ac.jp
,
Masaya Sawamura*
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan   Email: sawamura@sci.hokudai.ac.jp
› Author Affiliations
Further Information

Publication History

Received: 28 February 2016

Accepted after revision: 18 March 2016

Publication Date:
30 March 2016 (online)

Abstract

Site-selective and stereoselective C(sp3)–H borylation of alkyl side chains of 1,3-azoles with bis(pinacolato)diboron was effectively catalyzed by a silica-supported monophosphine-iridium catalyst. The borylation occurred under relatively mild conditions (2 mol% Ir, 50–90 °C), affording the corresponding primary and secondary alkylboronates. This system was applicable to a variety of 1,3-(benzo)azoles such as thiazoles, oxazoles, and imidazoles.

Supporting Information

 
  • References and Notes

    • 1a Joule JA, Mills K. Heterocyclic Chemistry. 5th ed. John Wiley and Sons; Chichester: 2010
    • 1b Joule JA, Mills K. Heterocyclic Chemistry at a Glance . 2nd ed. John Wiley and Sons; Chichester: 2013
    • 1c Turchi IJ, Dewar MJ. S. Chem. Rev. 1975; 75: 389
    • 1d Bansal Y, Silakari O. Bioorg. Med. Chem. 2012; 20: 6208
    • 1e Contreras R, Flores-Parra A, Mijangos E, Téllez F, López-Sandoval H, Barba-Behrens N. Coord. Chem. Rev. 2009; 253: 1979
  • 2 Zificsak CA, Hlasta DJ. Tetrahedron 2004; 60: 8991

    • Selected recent reviews on transition-metal-catalyzed C–H borylation:
    • 3a Mkhalid IA. I, Barnard JH, Marder TB, Murphy JM, Hartwig JF. Chem. Rev. 2010; 110: 890
    • 3b Hartwig JF. Chem. Soc. Rev. 2011; 40: 1992
    • 3c Ros A, Fernández R, Lassaletta JM. Chem. Soc. Rev. 2014; 43: 3229

      Heteroatom-directed C(sp3)–H borylation:
    • 5a Cho SW, Hartwig JF. J. Am. Chem. Soc. 2013; 135: 8157
    • 5b Mita T, Ikeda Y, Michigami K, Sato Y. Chem. Commun. 2013; 49: 5601
    • 5c Zhang L.-S, Chen G, Wang X, Guo Q.-Y, Zhang X.-S, Pan F, Chen K, Shi Z.-J. Angew. Chem. Int. Ed. 2014; 53: 3899
    • 5d Cho SH, Hartwig JF. Chem. Sci. 2014; 5: 694
    • 5e Miyamura S, Araki M, Suzuki T, Yamaguchi J, Itami K. Angew. Chem. Int. Ed. 2015; 54: 846
    • 5f Murai M, Omura T, Kuninobu Y, Takai K. Chem. Commun. 2015; 51: 4583
    • 5g He J, Jiang H, Takise R, Zhu R.-Y, Chen G, Dai H.-X, Murali DharT. G, Shi J, Zhang H, Cheng PT. W, Yu J.-Q. Angew. Chem. Int. Ed. 2016; 55: 785
    • 5h Larsen MA, Cho SH, Hartwig JF. J. Am. Chem. Soc. 2016; 138: 762

    • See also ref 10. An example of C(sp3)–H borylation of aliphatic substrates without strong directing groups:
    • 5i Ohmura T, Torigoe T, Suginome M. Chem. Commun. 2014; 50: 6333

      Borylation of C(sp3)–H bonds located α or β to heteroatoms:
    • 6a Liskey CW, Hartwig JF. J. Am. Chem. Soc. 2012; 134: 12422
    • 6b Ohmura T, Torigoe T, Suginome M. J. Am. Chem. Soc. 2012; 134: 17416
    • 6c Ohmura T, Torigoe T, Suginome M. Organometallics 2013; 32: 6170
    • 6d Li Q, Liskev CW, Hartwig JF. J. Am. Chem. Soc. 2014; 136: 8755

      C(sp3)–H borylation of cyclopropanes, see:
    • 7a Liskey CW, Hartwig JF. J. Am. Chem. Soc. 2013; 135: 3375
    • 7b Murakami R, Tsunoda K, Iwai T, Sawamura M. Chem. Eur. J. 2014; 20: 13127 ; see also ref. 5e,g

      Borylation of benzylic C(sp3)–H bonds:
    • 8a Shimada S, Batsanov AS, Howard JA. K, Marder TB. Angew. Chem. Int. Ed. 2001; 40: 2168
    • 8b Ishiyama T, Ishida K, Takagi J, Miyaura N. Chem. Lett. 2001; 30: 1082
    • 8c Mertins K, Zapf A, Beller M. J. Mol. Catal. A: Chem. 2004; 207: 21
    • 8d Boebel TA, Hartwig JF. Organometallics 2008; 27: 6013
    • 8e Larsen MA, Wilson CV, Hartwig JF. J. Am. Chem. Soc. 2015; 137: 8633
    • 8f Palmer WN, Obligacion JV, Pappas I, Chirik PJ. J. Am. Chem. Soc. 2016; 138: 766

      Borylation of allylic C(sp3)–H bonds:
    • 9a Caballero A, Sabo-Etienne S. Organometallics 2007; 26: 1191
    • 9b Olsson VJ, Szabó KJ. Angew. Chem. Int. Ed. 2007; 46: 6891
    • 9c Olsson VJ, Szabó KJ. J. Org. Chem. 2009; 74: 7715
    • 9d Deng H.-P, Eriksson L, Szabó KJ. Chem. Commun. 2014; 50: 9207
    • 10a Kawamorita S, Murakami R, Iwai T, Sawamura M. J. Am. Chem. Soc. 2013; 135: 2947
    • 10b Kawamorita S, Miyazaki T, Iwai T, Ohmiya H, Sawamura M. J. Am. Chem. Soc. 2012; 134: 12924
    • 10c Iwai T, Murakami R, Harada T, Kawamorita S, Sawamura M. Adv. Synth. Catal. 2014; 356: 1563
    • 10d Iwai T, Harada T, Hara K, Sawamura M. Angew. Chem. Int. Ed. 2013; 52: 12322
  • 11 Typical Procedure for the C(sp3)–H Borylation of Alkyl Side Chains on 1,3-Azoles with a Silica-SMAP-Ir Catalyst System (Table 1, Entry 1)In a glove box, Silica-SMAP (0.07 mmol/g, 57.1 mg, 0.0040 mmol, 2 mol%), bis(pinacolato)diboron (2, 50.8 mg, 0.20 mmol), and anhydrous, degassed THF (0.3 mL) were placed in a 10 mL glass tube containing a magnetic stirring bar. A solution of [Ir(OMe)(cod)]2 (1.3 mg, 0.0020 mmol, 1 mol%) in THF (0.7 mL) and 2-ethylbenzo[d]thiazole (1a, 97.9 mg, 0.60 mmol) were added successively. The tube was sealed with a screw cap and removed from the glove box. The reaction mixture was stirred at 60 °C for 15 h, and filtered through a glass pipette equipped with a cotton filter. The solvent was removed under reduced pressure. An internal standard (1,1,2,2-tetrachloroethane) was added to the residue. The yields of the products 3a and 4a were determined by 1H NMR spectroscopy (82% and 32% yields, respectively). The crude material was then purified by Kugelrohr distillation (1 mmHg, 145 °C), to give the corresponding product 3a (43.1 mg, 0.15 mmol, 75% yield) contaminated with the diborylation product 4a (<1%) and traces of impurities, as estimated by 1H NMR spectroscopy. Total yield over 100% based on 2 indicates that HBpin formed during catalytic turnover also served as a borylating reagent (theoretical maximum yield is 200%). 1H NMR (400 MHz, CDCl3): δ = 1.24 (s, 12 H), 1.38 (t, J = 7.6 Hz, 2 H), 3.24 (t, J = 7.6 Hz, 2 H), 7.32 (td, J = 8.4, 1.2 Hz, 1 H), 7.42 (td, J = 7.6, 0.8 Hz, 1 H), 7.82 (d, J = 8.0 Hz, 1 H), 7.94 (d, J = 8.4 Hz, 1 H). 13C NMR (100 MHz, CDCl3): δ = 11.10 (br), 24.75 (4 C), 28.85, 83.36 (2 C), 121.42, 122.41, 124.42, 125.67, 135.19, 153.19, 173.81. 11B NMR (128 MHz, CDCl3): δ = 32.6. IR (ATR): 2976, 2931, 1519, 1436, 1370, 1313, 1142, 1082, 967, 845, 758 cm–1. ESI-HRMS: m/z [M + H]+ calcd for C15H21O2N10BS: 289.14169; found: 289.14170.
  • 12 The Silica-SMAP-Ir catalyst was easily separated from the reaction mixture by filtration. However, attempts to reuse the catalyst were unsuccessful.
    • 13a Ochida A, Hamasaka G, Yamauchi Y, Kawamorita S, Oshima N, Hara K, Ohmiya H, Sawamura M. Organometallics 2008; 27: 5494
    • 13b Ochida A, Hara K, Ho H, Sawamura M. Org. Lett. 2003; 5: 2671
  • 14 The C(sp2)–H borylation of heteroarenes including 1,3-benzazoles catalyzed by the Ir-Me4Phen system: Larsen MA, Hartwig JF. J. Am. Chem. Soc. 2014; 136: 4287
  • 15 In some cases, the formation of C=N reduction products of starting materials 1 was indicative by 1H NMR analyses of the crude products. Similar C=N reduction was observed in the C(sp3)–H boylation of small-ring carbocycles bearing 1,3-azoles with the Silica-SMAP-Ir catalyst system (ref. 7b). The desired products 3 or 5 could be isolated by bulb-to-bulb distillation or silica gel column chromatography.

    • Cyclooctene would act as a scavenger of H2 or HBpin. The use of alkene derivatives as a H2 or HBpin scavenger in an Ir-catalyzed aromatic C–H borylation of aldimines was also reported:
    • 16a Sasaki I, Amou T, Ito H, Ishiyama T. Org. Biomol. Chem. 2014; 12: 2041
    • 16b Sasaki I, Ikeda T, Amou T, Taguchi J, Ito H, Ishiyama T. Synlett 2016; 27 in press; DOI: 10.1055/s-0035-1561578.
  • 17 Methyl groups on the thiazole ring in 1g were necessary for the C(sp3)–H borylation. In fact, the reaction of 2-ethylthiazole with 2 in the presence of the Silica-SMAP-Ir catalyst (2 mol%, 60 °C, 15 h) gave the corresponding arylboronates exclusively via the C(sp2)–H borylation.
  • 18 Sueki S, Kuniobu Y. Org. Lett. 2013; 15: 1544
  • 19 Li L, Zhao S, Joshi-Pangu A, Diane M, Biscoe MR. J. Am. Chem. Soc. 2014; 136: 14027
  • 20 Bruno NC, Tudge MT, Buchwald SL. Chem. Sci. 2013; 4: 916
  • 21 Isolated product 7 was contaminated with 4,4′-dimethoxy-1,1′-biphenyl (2%), probably generated through homocoupling of 4-chloroanisole.
  • 22 Matteson DS. Chem. Rev. 1989; 89: 1535