Catalytic Hydrosilylation of Carbonyl Compounds with Zinc(II) Acetate: Asymmetric Induction Collaborated with N₂S₂ Ligands

Abstract

Zinc acetate proved to be an efficient catalyst for hydro­silylation of ketones and aldehydes in the combination with (EtO)₂MeSiH, and a good to excellent asymmetric induction was observed in the presence of chiral N₂S₂ ligands.

Key words

reduction - hydrosilylation - zinc - asymmetric catalysis - chiral ligand

References and Notes

• 1a Andersson PG. Munslow IJ. Modern Reduction Methods   Wiley-VCH; New York: 2008. 
• 1b Burke SD. Danheiser RL. Handbook of Reagents for Organic Synthesis, Oxidizing and Reducing Agents   John Wiley and Sons; Chichester: 1999. 
• For reviews, see:
• 2a Bullock RM. Angew. Chem. Int. Ed.  2007,  46:  7360 
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• 2d For papers: Shaikh NS. Enthaler S. Junge K. Beller M. Angew. Chem. Int. Ed.  2008,  47:  2497 
• 3a Mimoun H. J. Org. Chem.  1999,  64:  2582 
• 3b Mimoun H. de Saint Laumer JY. Giannini K. Scopelliti R. Floriani C. J. Am. Chem. Soc.  1999,  121:  6158 
• 4a Bette V. Mortrex A. Lehmann CW. Carpentier J.-F. Chem. Commun.  2003,  332 
• 4b Bette V. Mortorex A. Savoia D. Carpentier J.-F. Tetrahedron  2004,  60:  2837 
• 5 Mastranzo VM. Quintero K. de Parrodi CA. Juaristi E. Walsh PJ. Tetrahedron  2004,  60:  1781 
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• 7 Gérard S. Pressel Y. Riant O. Tetrahedron: Asymmetry  2005,  16:  1889 
• 8 Bandini M. Melucci M. Piccinelli F. Sinisi R. Tommasi S. Umani-Ronchi A. Chem. Commun.  2007,  4519 
• Hydrosilylation of imines with chiral zinc catalysts:
• 9a Ireland T. Fontanet F. Tchao G.-G. Tetrahedron Lett.  2004,  45:  4383 
• 9b Park B.-M. Mun S. Yun J. Adv. Synth. Catal.  2006,  348:  1029 
• 10a Nishiyama H. Furuta A. Chem. Commun.  2007,  760 
• 10b Furuta A. Nishiyama H. Tetrahedron Lett.  2008,  49:  110 
• For compound 6, see:
• 11a Albano VG. Bandini M. Melucci M. Monari M. Piccinelli F. Tommasi S. Umani-Ronchi A. Adv. Synth. Catal.  2005,  347:  1507 
• 11b Albano VG. Bandini M. Barbarella G. Melucci M. Monari M. Piccinelli F. Tommasi S. Umani-Ronchi A. Chem. Eur. J.  2006,  12:  667 

12

Typical Procedure for Hydrosilylation of Methyl Biphenyl-4-yl Ketone (1) Zinc acetate (9.2 mg, 0.05 mmol; Wako 260-01881, lot LTM1219) and the ketone (196 mg, 1.0 mmol) were placed in a flask. Under an argon atmosphere, absolute THF (3.0 mL) was added at r.t. The mixture was stirred for 10 min at 65 ˚C, and (EtO)₂MeSiH (320 µL, 2.0 mmol) was then added by a syringe. The mixture was stirred for 24 h at 65 ˚C. The reaction was monitored by TLC examination; the ketone was consumed, and the silyl ether product was observed. At 0 ˚C, aq HCl (2 N, 2 mL) was added to quench the reaction. After stirring for 1 h, the mixture was extracted with EtOAc (3 × 10 mL), and the extract was washed with brine and aq NaHCO₃ and dried over Na₂SO₄. After concentration, the residue was purified by silica gel column chromatography (hexane-EtOAc as eluent) to give the corresponding desired alcohol 2 (196 mg, 0.99 mmol) in 99%.
Asymmetric Hydrosilylation of Methyl α-Naphthyl Ketone
Under the same reaction conditions above described in the typical procedure, the ligand 7a (27.4 mg, 0.06 mmol) and methyl α-naphthyl ketone (170 mg, 1.0 mmol) were used to obtain the alcohol 13 (163 mg, 0.95 mmol) in 95% and 92% ee (S); analysis, CHIRALCEL OJ-H [hexane-2-PrOH (95:5), 0.8 mL min^-¹]; t _R (S) = 34.2 min, t _R (R) = 43.5 min.

13

Preparation of Ligands 7a and 7b
A mixture of (1R,2R)-cyclohexane-1,2-diamine (116 mg, 1.0 mmol), 4-phenylthiophene-2-carbaldehyde (392 g, 2.1 mmol, commercially available), MgSO₄ (2.4 g) in THF (10 mL) was stirred at r.t. for 40 h. After diluted with EtOAc (10 mL), the mixture was filtered through Celite and was concentrated to give white solids (ca. 470 mg). A MeOH solution (15 mL) of the solids was treated with NaBH₄ (392 mg) at r.t. for 18 h. Then, H₂O (15 mL) was added, and the mixture was extracted with EtOAc. The extract was washed with brine and dried over Na₂SO₄. After concentration, the residue was purified by silica gel column chromatography with hexane-EtOAc to give white solids (265 mg, 0.58 mmol) in 58% yield.
Compound 7a: mp 113-115 ˚C. IR (KBr): ν = 3100, 3056, 2927, 2853, 1451, 737, 688 cm^-¹. ^¹H NMR (300 MHz, CDCl₃): δ = 0.91-2.37 (m, 14 H), 3.90-3.94 (m, 2 H), 4.13-4.18 (m, 2 H), 7.24-7.39 (m, 8 H), 7.54-7.57 (m, 4 H). ^¹³C (75 MHz, CDCl₃): δ = 25.1, 31.6, 45.7, 60.4, 118.8, 123.4, 126.0, 126.7, 128.5, 135.8, 141.3, 145.7. Anal. Calcd (%) for C₂₈H₃₀N₂S₂: C, 73.32; H, 6.59; N, 6.11. Found: C, 72.91; H, 6.69; N, 6.01; [α]_D ^²9 -17.0 (c 1.00, CHCl₃).
Synthesis of Compound 7b
Starting from 2,6-diisopropylaniline via 2,6-diisopropyl-phenyliodide, 2,6-diisopropylphenyl boronic acid was prepared. The mixture of the boronic acid (463 mg, 2.25 mmol), 4-bromothiophene-2-carbaldehyde (318 mg, 1.5 mmol, commercially available), Pd(OAc)₂ (3.4 mg), S-Phos (12.7 mg), K₃PO₄ (650 mg, 3.0 mmol) in toluene (3.0 mL) at 100 ˚C for 24 h. The mixture was diluted with EtOAc and filtered through Celite. After concentration, the residue was purified by silica gel column chromatography to give
4-(2′,6′-diisopropylphenyl)thiophene-2-carbaldehyde (354 mg, 1.3 mmol) in 87%. A mixture of (1R,2R)-cyclohexane-1,2-diamine (46 mg, 0.4 mmol), thiophene-2-carbaldehyde (218 mg, 0.8 mmol, commercially available), and MgSO₄ (960 mg) in THF (5.0 mL) was stirred at r.t. for 24 h. After diluted with EtOAc, the mixture was filtered through Celite and was concentrated to give white solids. A MeOH solution (10 mL) of the solids was treated with NaBH₄ (151 mg) at r.t. for 24 h. Then, H₂O (10 mL) was added, and the mixture was extracted with EtOAc. The extract was washed with brine and dried over Na₂SO₄. After concentration, the residue was purified by silica gel column chromatography with hexane-EtOAc to give the desired amine 7b (178 mg, 0.284 mmol) in 71% yield.
Compound 7b: oil. IR (film): ν = 3055, 2959, 2927, 2861, 1459, 751, 673 cm^-¹. ^¹H NMR (300 MHz, CDCl₃): δ = 1.09-1.10 (m, 24 H), 1.15-1.40 (m, 6 H), 1.85 (m, 2 H), 2.23 (m, 2 H), 2.42 (m, 2 H), 2.83 (m, 4 H), 4.00 (d, J = 14.9 Hz, 2 H), 4.19 (d, J = 14.9 Hz, 2 H), 6.82 (s, 2 H), 6.93 (s, 2 H), 7.22-7.27 (m, 4 H), 7.38 (m, 2 H). ^¹³C (75 MHz, CDCl₃): δ = 24.2, 24.3, 24.5, 24.6, 25.1, 30.3, 30.4, 31.6, 45.5, 60.2, 121.0, 122.1, 126.9, 127.6, 134.4, 139.2, 144.0, 147.4. HRMS-FAB: m/z calcd for C₄₀H₅₅Cl₂N₂S₂ ⁺ [M + H]: 627.3807; found: 627.3805. [α]_D ^²9 -23.4 (c 1.00, CHCl₃).