Substituent Effects in NHC–Boranes: Reactivity Switch in the Nucleophilic Fluorination of NHC–Boranes

Abstract

Substituents on the boron atom of NHC–boranes direct the reactivity of the ligated boreniums obtained through hydride abstraction. Depending on the electronics of the substituent, the reaction is selectively steered toward either B-substitution or Lewis base exchange.

Key words

boron - carbene complexes - fluorine - cations - Lewis ­acids

• References and Notes


For reviews, see:
• 1a De Vries TS, Prokofjevs A, Vedejs E. Chem. Rev. 2012; 112: 4642
• 1b Piers WE, Bourke SC, Conroy KD. Angew. Chem. Int. Ed. 2005; 44: 5016

For recent references, see:
• 1c Tsurumaki E, Hayashi S.-Y, Tham FS, Reed CA, Osuka A. J. Am. Chem. Soc. 2011; 133: 11956
• 1d Someya CI, Inoue S, Praesang C, Irran E, Driess M. Chem. Commun. 2011; 47: 6599
• 1e Prokofjevs A, Vedejs E. J. Am. Chem. Soc. 2011; 133: 20056
• 1f Mansaray HB, Rowe AD. L, Phillips N, Niemeyer J, Kelly M, Addy DA, Bates JI, Aldridge S. Chem. Commun. 2011; 47: 12295
• 1g Ines B, Patil M, Carreras J, Goddard R, Thiel W, Alcarazo M. Angew. Chem. Int. Ed. 2011; 50: 8400
• 1h Del GA, Singleton PJ, Muryn CA, Ingleson MJ. Angew. Chem. Int. Ed. 2011; 50: 2102
• 1i Del GA, Helm MD, Solomon SA, Caras-Quintero D, Ingleson MJ. Chem. Commun. 2011; 47: 12459
• 1j De Vries TS, Prokofjevs A, Harvey JN, Vedejs E. J. Am. Chem. Soc. 2009; 131: 14679
• 2a Matsumoto T, Gabbaï FP. Organometallics 2009; 28: 4252
• 2b Tsai J.-H, Lin S.-T, Yang RB.-G, Yap GP. A, Ong T.-G. Organometallics 2010; 29: 4004
• 2c McArthur D, Butts CP, Lindsay DM. Chem. Commun. 2011; 47: 6650
• 2d Solovyev A, Geib SJ, Lacôte E, Curran DP. Organometallics 2012; 31: 54

For a review on NHC-borane chemistry, see:
• 2e Curran DP, Solovyev A, Makhlouf Brahmi M, Fensterbank L, Malacria M, Lacôte E. Angew. Chem. Int. Ed. 2011; 50: 10294
• 3 Solovyev A, Chu Q, Geib SJ, Fensterbank L, Malacria M, Lacôte E, Curran DP. J. Am. Chem. Soc. 2010; 132: 15072
• 4a Eisenberger P, Bailey AM, Crudden CM. J. Am. Chem. Soc. 2012; 134: 17384
• 4b Farrell JM, Hatnean JA, Stephan DW. J. Am. Chem. Soc. 2012; 134: 15728
• 4c Chen J, Lalancette RA, Jäkle F. Chem. Commun. 2013; 49: 4893
• 5 Walton JC, Makhlouf Brahmi M, Monot J, Fensterbank L, Malacria M, Curran DP, Lacôte E. J. Am. Chem. Soc. 2011; 133: 10312
• 6a Hudnall TW, Gabbaï FP. J. Am. Chem. Soc. 2007; 129: 11978
• 6b Chiu C.-W, Gabbaï FP. Organometallics 2008; 27: 1657
• 7a De Vries TS, Vedejs E. Organometallics 2007; 26: 3079
• 7b Funke M.-A, Mayr H. Chem. Eur. J. 1997; 3: 1214
• 8 General Procedure To a solution of NHC–borane complex (1 equiv) in CH₂Cl₂ (0.07 M) was added the triphenylcarbenium derivative (1 equiv), then phenol (1 equiv). The reaction mixture was stirred at r.t. for 5 min. The solvent was then evaporated in vacuo, and the residue was purified by flash chromatography. Typical Characterization Data for Compound 2a Mp 222–227 °C. IR (diamond): ν = 2960, 2930, 2870, 2360 (B–F), 1460, 1260, 1060, 1015, 930, 800, 760, 735 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.50 (t, J = 7.8 Hz, 2 H, p-H of IPr), 7.28 (d, J = 7.8 Hz, 4 H, m-H of IPr), 7.04 (s, 2 H, NCH), 6.94 (t, J = 6.9 Hz, 1 H, p-H of Ph), 6.88 (t, J = 6.8 Hz, 2 H, m-H of Ph), 6.77 (d, J = 6.6 Hz, 2 H, o-H of Ph), 2.61–2.55 [m, 4 H, CH(CH₃)₂], 1.20 [d, J = 6.8 Hz, 12 H, CH(CH ₃)₂], 1.13 [d, J = 6.8 Hz, 12 H, CH(CH ₃)₂]. ¹³C NMR (100 MHz, CDCl₃): δ = 145.5 (C arom.), 134.2 (C arom.), 131.7 (CH arom.), 130.4 (CH arom.), 126.6 (CH arom.), 125.9 (CH arom.), 123.8 (NCH), 123.8 (CH arom.), 29.0 [CH(CH₃)₂], 25.9 (CHCH ₃CH₃), 22.3 (CHCH₃CH ₃). ¹¹B NMR (133 MHz, BF₃·OEt₂): δ = 4.8 (br s). ¹⁹F NMR (376 MHz, CFCl₃): δ = –153.2 (br s). HRMS: m/z calcd. for C₃₃H₄₁N₂ ¹¹BF₂Na [M + Na]⁺: 537.3223; found: 537.3224.