Cesium Alkyltrifluoroborates from Asymmetric Boronic Esters

Abstract

Cesium alkyltrifluoroborates are precipitated in high yields by the reaction of pinanediol (alkyl)boronates in diethyl ether with equivalent amounts of a mixture of aqueous hydrofluoric acid and cesium fluoride.

Key words

boron - asymmetric synthesis - fluorine - cesium - trifluoroborate

References and Notes

• 1a Vedejs E. Chapman RW. Fields SC. Lin S. Schrimpf MR. J. Org. Chem.  1995,  60:  3020 
• 1b Thadani AN. Batey RA. Tetrahedron Lett.  2003,  44:  8051 
• 1c Quach TD. Batey RA. Org. Lett.  2003,  5:  1381 
• 1d Molander GA. Ito T. Org. Lett.  2001,  3:  393 
• 1e Molander GA. Fumagalli T. J. Org. Chem.  2006,  71:  5743 
• 1f Molander GA. Ham J. Org. Lett.  2006,  8:  2767 
• Reviews:
• 1g Darses S. Genet J.-P. Eur. J. Org. Chem.  2003,  4313 
• 1h Molander GA. Figueroa R. Aldrichimica Acta  2005,  38:  49 
• 2 Matteson DS. Kim GY. Org. Lett.  2002,  4:  2153 
• 3 Kim BJ. Matteson DS. Angew. Chem. Int. Ed.  2004,  43:  3056 ; Angew. Chem. 2004, 116, 3118
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4

¹H NMR and ¹³C NMR data: data are for CD₃CN solutions; ¹H NMR, 300 MHz; ¹³C NMR, 75.47 MHz, broad BC resonance was usually not detected.
Compound 2a: ¹H NMR: δ = 0.07 (m, 1 H), 0.95 (m, 2 H), 1.15 (m, 3 H), 1.61 (m, 5 H). ¹³C NMR: δ = 27.3, 28.2, 27.4.
Compound 2b: ¹H NMR δ = 0.05 (m, 2 H), 0.86 (t, J = 7.1 Hz, 3 H), 1.20 (m, 4 H). ¹³C NMR: δ = 14.8, 27.4, 29.2 (d or unresolved q, J _CF = ca. 2.3 Hz).
Compound 2c: ¹H NMR: δ = 0.02 (m, 2 H), 0.89 (t, 3 H, J = 6.9 Hz), 1.27 (m, 12 H). ¹³C NMR: δ = 14.5, 23.5, 26.9, 30.4, 30.9, 32.9, 34.7.
Compound 2d: ¹H NMR: δ = 7.16 (m, 3 H), 7.45 (m, 2 H). ¹³C NMR: δ = 126.5, 127.7, 132.4.
Compound 2e: ¹H NMR: δ = 1.60 (m, 2 H), 6.97 (m, 1 H), 7.12 (m, 4 H). ¹³C NMR: δ = 51 (br), 122.2, 127.1, 128.5.
Compound 2f: ¹H NMR: δ = 0.39 (m, 2 H), 2.51 (m, 2 H), 7.10 (m, 1 H), 7.20 (m, 4 H). ¹³C NMR: δ = 33.2, 125.5, 128.9.
Compound 2g: ¹H NMR: δ = 2.67 (dd, J = 15.0, 12.3 Hz, 1 H), 3.13 (dd, J = 15.0, 2.5 Hz, 1 H), 3.21 (m, 1 H), 7.15 (m, 1 H), 7.25 (m, 4 H).
Compound 2h: ¹H NMR: δ = 0.51 (m, 1 H), 0.58 (d, 3 H, J = 6.0 Hz), 2.06 (dd, J = 13.5, 11.1 Hz, 1 H), 2.84 (dd, J = 13.5, 3.3 Hz, 1 H), 7.10 (m, 1 H), 7.20 (m, 4 H). ¹³C NMR: δ = 15.4, 40.6, 125.5, 128.7, 130.0, 147.2.
Compound 2i: ¹H NMR: δ = 1.71 (s, 3 H), 2.46 (dd, J = 13.9, 10.1 Hz, 1 H), 2.78 (dd, J = 13.9, 4.4 Hz, 1 H), 3.10 (m, 1 H), 5.89 (m, 1 H), 7.10 (m, 1 H) 7.20 (m, 4 H). ¹³C NMR: δ = 23.6, 38.9, 125.9, 128.6, 130.27, 144.5, 169.8.
Compound 2j: ¹H NMR: δ = 2.75 (m, 2 H), 2.94 (m, 1 H), 4.26 (d, J = 11.9 Hz, 1 H), 4.42 (d, J = 11.9 Hz, 1 H), 7.20 (m, 10 H). ¹³C NMR: δ = 39.3, 72.7, 125.9, 127.7, 128.5, 128.7, 129.0, 130.4, 142.1, 145.4.
Compound 2k: ¹H NMR: δ = 0.93 (t, 3 H, J = 7.3 Hz), 1.51 (m, 2 H), 2.60 (m, 1 H), 4.46 (d, J = 11.9 Hz, 1 H), 4.52 (d, J = 11.9 Hz, 1 H), 7.3 (m, 5 H). ¹³C NMR: δ = 11.5, 23.9, 71.6, 127.3, 128.2, 128.6, 141.6.

5

Analysis Data (Desert Analytics, Tucson, AZ)
Compound 2a: Anal. Calcd for C₆H₁₁BCsF₃: C, 25.39; H, 3.91; B, 3.81; Cs, 46.82; F, 20.08. Found: C, 25.40, 25.17; H 3.71, 3.93; B, 3.79; F, 18.95.
Compound 2d: Anal. Calcd for C₆H₅BCsF₃: C, 25.94; H, 1.81; B, 3.89; Cs, 47.84; F, 20.52. Found: C 25.82, H 1.90, B 3.80, Cs 47.86, F 20.69.
Compound 2i: Anal. Calcd for C₁₀H₁₂BCsF₃NO: C, 33.09; H, 3.33; N, 3.86. Found: C, 33.22; H, 3.15; N, 3.82.
Compound 2j: Anal. Calcd for C₁₅H₁₅BCsF₃O: C, 43.73; H, 3.67. Found: C, 43.73; H, 3.36.
Compound 2k: Anal. Calcd for C₁₀H₁₃BCsF₃O: C, 34.32; H, 3.74; B, 3.09. Found: C, 34.40; H, 3.59; B, 3.16. Cs and F were inexplicably low: Cs calcd, 37.98; found 36.94; F calcd, 16.29; found 14.64.
Tetrabutylammonium cyclohexyltrifluoroborate: Anal. Calcd for C₂₂H₄₇BF₃N: C, 67.16; H, 12.04; B, 2.75; F, 14.49; N, 3.56. Found: C, 67.49; H, 12.01; B, 2.79; F, 14.00.

9

It is convenient to prepare a stock solution. CsF (1 mol) was added in small portions to stirred 48% aq HF (2 mols) in a polyethylene bottle cooled with an ice-water bath. [CAUTION: exothermic. Also, HF is highly toxic and contact with skin or breathing of vapors must be strictly avoided]. The density of the resulting solution of CsHF₂ and HF is ca. 2.23, and the weight of 10 mmol is 2.35 g.

10

The (1-benzyloxypropyl)trifluoroborate (2k) was prepared by a slightly modified procedure. The reaction was allowed to proceed for 24 h, though the effect of reaction time was not investigated. In addition to the 70% yield indicated in Table [1] , 17% more of slightly impure 2k was obtained by concentration of the H₂O/saturated ether phase from the original reaction mixture followed by treatment of the residue with pentane to remove the pinanediol. However, 2k was not appreciably soluble in anhyd Et₂O.