Asymmetric Alkynyl Additions to Aldehydes Catalyzed by Tunable ­Oxovanadium(V) Complexes of Schiff Bases of β-Amino Alcohols

Abstract

The first example of asymmetric alkynyl additions to ­aldehydes catalyzed by oxovanadium(V) catalysts of tridentate Schiff bases of β-amino alcohols 3a-h is reported. Catalytic reactions employing the best-performing catalyst 3g furnish chiral ­propargyl alcohols in good to excellent enantioselectivities from 73% to 99% ee.

Key words

oxovanadium complex - alkynylation - Schiff bases - β-amino alcohols - aromatic aldehydes - aliphatic aldehydes - propargyl alcohols

References and Notes

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14

General Procedures for Synthesis of Oxovanadium(V) Complexes 3a-h.
To a solution of a Schiff base (1.00 mmol) in 10 mL appropriate alcohol, VO(Oi-Pr)₃ (0.24 mL, 1.0 mmol) was added at r.t. The mixture was stirred for 1 h and the solvent was removed under reduced pressure to give a quantitative yield of the product. All compounds were characterized by ¹H NMR and ¹³C NMR spectroscopy and by elemental analyses. Two sets of NMR resonances are observed for each complex due to the presence of two isomers in solution. Spectroscopic data of complexes 3c and 3g as examples are listed in the following. The two isomers are designated as major and minor based on relative intensities of resonances. ¹H NMR spectra for the two isomers in the phenyl region are overlapped and resonances of the minor are included in data of the major.
Complex 3c: dark green solid. ¹H NMR (400 MHz, CDCl₃): δ (major, 69%) = 7.56 (s, 1 H, CH=N), 7.50-6.76 (m, Ph), 6.43 (d, J = 3.2 Hz, 1 H, CHO), 5.34-5.26 (m, 2 H, OCH ₂ ), 4.20 (d, J = 6.0 Hz, 1 H, CHN), 3.61 (dd, J = 7.6, 9.2 Hz, 1 H, CH _A H _B Ph), 2.62 (dd, J = 2.0, 9.0 Hz, 1 H, CH _A H _B Ph), 1.59 (t, J = 5.2 Hz, 3 H, CH ₃ ) ppm; δ (minor, 31%) = 7.57 (s, 1 H, CH=N), 6.65 (d, J = 3.6 Hz, 1 H, CHO), 5.40-5.36 (m, 2 H, OCH ₂ ), 4.53 (d, J = 7.2 Hz, 1 H, CHN), 2.57 (dd, J = 2.4, 9.2 Hz, 1 H, CH _A H _B Ph), 2.47 (dd, J = 7.6, 9.2 Hz, 1 H, CH _A H _B Ph), 1.69 (t, J = 4.8 Hz, 3 H, CH ₃ ) ppm. ¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 163.26, 162.86, 162.71, 140.50, 137.86, 137.11, 135.82, 135.57, 132.45, 132.35, 130.19, 130.14, 128.70, 128.60, 128.55, 128.47, 127.62, 127.43, 127.01, 126.77, 126.67, 126.05, 125.95, 124.98, 119.78, 119.51, 118.84, 93.35, 90.92, 87.19, 86.39, 84.50, 79.54, 79.07, 78.62, 42.02, 36.23, 36.11, 25.58, 19.13, 17.97, 17.74. Anal. Calcd for C₂₄H₂₄NO₄V (%): C, 65.31; H, 5.48; N, 3.17. Found: C, 65.66; H, 5.08; N, 3.57.
Complex 3g: brown solid. ¹H NMR (400 MHz, CDCl₃): δ (major, 71%) = 7.45-6.77 (m, CH=N, Ph), 4.89 (d, J = 4.0 Hz, 1 H, CHN), 4.85 (ddd, J = 6.8, 11.4, 28.0 Hz, 2 H, OCH ₂), 4.05 (d, J = 12.0 Hz, 1 H, CHO), 3.83 (dd, J = 12.4, 12.4 Hz, 1 H, CH _A H _B Ph), 3.43 (d, J = 13.2 Hz, 1 H, CH _A H _B Ph), 2.16-2.06 (m, 1 H, CH₂ CH), 1.18 (s, 9 H, t-Bu), 1.03 [d, J = 6.8 Hz, 6H, CH(CH ₃ )₂] ppm; δ (minor, 29%) = 5.24 (d, J = 4.0 Hz, 1 H, CHN), 5.05 (ddd, J = 6.8, 11.0, 30.8 Hz, 2 H, OCH ₂), 4.32 (d, J = 11.6 Hz, 1 H, CHO), 3.28 (d, J = 14.0 Hz, 1 H, CH _A H _B Ph), 2.62 (dd, J = 12.4, 12.4 Hz, 1 H, CH _A H _B Ph), 2.40-2.22 (m, 1 H, CH₂CH), 1.21 (s, 9 H, t-Bu), 1.12 [d, J = 6.8 Hz, 6 H, CH(CH ₃ )₂] ppm. ¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 165.18, 163.73, 161.93, 161.78, 137.98, 137.17, 135.37, 135.08, 132.25, 132.15, 130.20, 130.03, 128.44, 128.38, 126.67, 126.60, 120.01, 119.34, 119.04, 118.97, 118.64, 118.57, 98.05, 94.60, 89.84, 88.71, 83.31, 78.13, 36.77, 36.31, 35.81, 31.91, 31.60, 27.55, 27.12, 19.47, 19.41, 18.79. Anal. Calcd for C₂₄H₃₂NO₄V (%): C, 64.13; H, 7.18; N, 3.12. Found: C, 63.72; H, 7.42; N, 3.59.