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Procedure: 50 mL
1-neck flask, stirbar, septum, N2 inlet Dissolved
0.512 g of alkyne I in 13 mL of
absolute MeOH. Added 0.138 g of
Lindlar's catalyst (II). Stirred suspension at rt under 1 atm of
H2. After 2 h TLC
(10:90 EtOAc-hexanes, PMA) still showed unreacted I at Rf 0.48.
The reaction mixture was filtered through Celite and 0.143 g of fresh
Lindlar's catalyst was added.
Stirring was continued under 1 atm of H2. After 45 min, the catalyst was removed
by filtration and the solvent was removed from the reaction mixture by rotary
evaporation. The product was
isolated by flash chromatography on silica gel using 10:90 EtOAc-hexanes as
eluant. The product was a clear,
colorless oil. 1H NMR (CDCl3,
400 MHz) d 5.65 (dddd, J = 11.4, 6.6, 5.5, 1.2 Hz, H2), 5.61 (ddq, J
= 15.4, 6.6, 0.9 Hz, H7), 5.36 (ddt,
J = 11.2, 8.2, 1.8 Hz, H3), 5.34 (ddq,
J = 15.3, 7.4, 1.6 Hz, H6), 4.28 (ddd,
J = 13.7, 6.4, 1.8 Hz, H1), 4.16 (ddd,
J = 13.7, 5.4, 1.9 Hz, H1), 4.09 (m, H4), 3.86
(dd(apparent t), J = 7.1 Hz, H5),
2.70 (d, J = 3.1 Hz, OH), 1.66
(dd, J = 6.4, 1.4 Hz, H8), 0.88
(s, SiC(CH3)), 0.87 (s, SiC(CH3)), 0.05 (s, Si(CH3)),
0.04 (s, Si(CH3)2), 0.02 (s, Si(CH3)). notes Lindlar's catalyst (see a) Lindlar, H. Helv. Chim. Acta 1952, 35, 446. b) Lindlar, H.; Dubois, R. Org. Synth. Col. Vol. 5, 1973, 880) consists of palladium on CaCO3 which has been treated or ÒpoisonedÓ with lead, usually Pb(OAc)2. Catalytic hydrogenation employing Lindlar's catalyst is a popular method for preparing cis-alkenes from alkynes. The reduction generally halts at the alkene stage and over-reduction is not a problem, although that is not exclusively the case. While Lindlar reductions are highly selective, cis/trans ratios of products may vary. The explanation for the catalyst's preference for the hydrogenation of alkynes over the hydrogenation of alkenes is thought to involve a reduction in the surface area of available Pd(0) after treatment with the "poison" (Pb(OAc)2), thus attenuating the strength of the catalyst (see McEwen, A.B.; Guttieri, M.J.; Maier, W.F. J. Org. Chem. 1983, 48, 4436). | ||||||||||||||||||||||||||||||||||||||||||||||
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