Reduction of Olefins Using Ruthenium Carbene Catalysts and Silanes

Abstract

Ruthenium carbene complexes are able to mediate reduction of olefins in the presence of trialkylsilanes. Under these ­reduction conditions, when kinetically favorable ring-closing ­metathesis is possible, a one-pot cyclization-reduction sequence can be performed.

reduction - hydrosilylation - stereoselective synthesis

References

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Hydrosilylation of alkynes:

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10 For the use of silanes in selective reduction of olefins see: Jurkauskas V. Sadighi JP. Buchwald SL. Org. Lett.  2003,  5:  2417  and references cited therein

11

Product ratio was determined by GC-MS.

12

2,6-Dimethyl-8-triethylsilyloxyundec-2,6-diene (20).
To a solution of olefin 15 (200 mg, 1.03 mmol, 1 equiv) in degassed CH₂Cl₂ (2 mL) were added the silane reagent (0.65 mL, 4.12 mmol, 4 equiv) and catalyst 1 (21 mg, 25.7 µmol, 2.5 mol%) at r.t. The reaction was stirred at reflux until total conversion of the starting material. The solution was concentrated under reduced pressure, and the crude product was purified by flash chromatography on silica gel using a gradient of eluent (pentane-EtOAc). Colorless oil (200 mg, 0.65 mmol, 63%); R _f = 0 .57 (pentane-EtOAc, 95:1). IR (neat): 1475, 1400, 1260, 1080, 760 cm^-1. ¹H NMR (300 MHz, CDCl₃, E- and Z-isomers): δ = 5.10-5.00 (m, 2 H), 4.30-4.20 (m, 1 H), 2.05-1.89 (m, 4 H), 1.61 (dd, J = 4.9, 1.1 Hz, 3 H), 1.54 (dd, J = 4.5, 1.5 Hz, 6 H), 1.35-1.15 (m, 4 H), 0.87 (t, J = 7.9 Hz, 9 H), 0.84 (t, J = 6.4 Hz, 3 H), 0.49 (q, J = 7.5 Hz, 6 H). ¹³C NMR (75 MHz, CDCl₃): E-isomer δ = 134.4, 131.4, 129.5, 124.1, 69.3, 40.9, 39.6, 26.3, 25.6, 18.6, 17.5, 16.4, 14.1, 6.8, 5.0. Z-Isomer: δ = 134.6, 131.7, 130.3, 124.1, 68.9, 41.2, 32.4, 26.5, 23.4, 18.8, 17.6, 16.4, 14.1, 6.8, 4.9. MS (EI, 70 eV): E-isomer m/z (%) = 310 (8) [M⁺], 267 (100), 173 (24), 75 (39), 69 (40). MS (EI, 70 eV): Z-isomer m/z (%) = 310 (20) [M⁺], 267 (25), 173 (97), 135 (59), 115 (38), 107 (32), 103 (100), 75 (54), 69 (45).

Selected conjugate reductions using metal catalysts:

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14

4-(4-Methoxyphenyl)butan-2-one (25).
To a solution of compound 21 (42 mg, 0.24 mmol, 1 equiv) in degassed CH₂Cl₂ (0.5 mL) at r.t. were added triethylsilane (0.10 mL, 0.70 mmol, 2.5 equiv) and catalyst 1 (5 mg, 7 mol, 2.5 mol%). The resulting solution was stirred until total conversion of the starting material. The solution was concentrated under reduced pressure, and the crude product was purified by flash chromatography on silica gel using a gradient of eluent (pentane-EtOAc). Colorless oil (30 mg, 0.17 mmol, 70%); R _f = 0.63 (pentane-EtOAc, 4:1). IR (neat): 1720, 1610, 1510, 1250, 1035 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 7.05 (d, J = 8.7 Hz, 2 H), 6.75 (d, J = 8.7 Hz, 2 H), 3.70 (s, 3 H), 2.70 (m, 4 H), 2.05 (s, 3 H). ¹³C NMR (75 MHz, CDCl₃): δ = 208.0, 157.8, 132.9, 129.1, 113.8, 55.1, 45.3, 29.9, 28.7. MS (EI, 70 eV): m/z (%) = 178 (39) [M⁺], 121 (100).

15 The relative stereochemistry of the major isomers was attributed on the basis of the J values observed in the ¹H NMR spectra. Moreover, 28 was transformed to the known β-hydroxy ester (TBAF, overnight), and the spectral data of the desilylated compound was compared with the literature data: Bouzide A. Org. Lett.  2002,  4:  1347 

16

As one of our referees pointed out, it is likely that hydrogenation occurred by the metathesis-inactive catalyst after the RCM reaction was completed, because the RCM was much faster than the modification of carbene catalyst by silanes or hydrogen, generated from dimerization of silanes.