Pergamon Tetrahedron Letters 40 (1999) 2857-2858 TETRAHEDRON LETTERS Efficient Semihydrogenation of the C-C Triple Bond Using Palladium on Pumice as Catalyst Michelangelo Gruttadauria a*, Renato Noto, a'b Giulio Deganello c'd and Leonarda F. Liotta c aDipartimento di Chimica Orgmfica "E. Pammb",Vialedelle Scienze, 90128 Palermo. blstitutodi Chimica e Tecnologia dei ProdottiNaturali(ICTPN-CNR) (associatedwith NationalInstitute for the Chemistryof Biological Systems - CNR), via Ugo La Malfa 153,90146 Palermo. Clstitutodi Chimica e Tecnologia dei ProdottiNaturali(ICTPN-CNR),via Ugo La Malta 153,90146 Palermo. dDipartimento di Chimica Inorganica, Vialedeile Scicnze,90128Palermo. Received 25 November 1998; revised 5 February 1999; accepted 9 February 1999 AbsWact: An efficientsemihydrogenation of C-C triple bondswas achievedusingpalladium on pumice as the catalyst with a metal loadingof 3% wt. The results obtainedshowed better selectivitywhen compared with Pd/C and better activity when computed with Lindlar's catalyst. Hydrogenationof stericallyhindo'edalkynesis slow on Pd/pumice.The selectivity with respect to alkenesis probably due to the presence of sodium ions in the pumice structure that increases the electron density on the supportedmetal. © 1999ElsevierScienceLtd. All rightsreserved. The semihydrogenation of carbon-carbon triple bonds is a particularly valuable and frequently used application of heterogeneous catalysis in synthetic chemistry. 1 The hydrogenation of an alkyne can be virtually stopped at the semihydrogenation stage because the alkyne is more strongly bound than the alkene, and competes effectively for the catalytic sites, blocking re-adsorption of the alkene, or displacing it. Moreover, high selectivity can be achieved by adding to the reaction mixture a nitrogen base, such as quinoline, pyridine or ethylenediamine, that is reversibly adsorbed onto the catalyst in competition with the alkyne and the product alkene. 1,2 Because the semihydrogenation reaction is a powerful tool in synthetic organic chemistry, we considered it worth developing a new catalyst for such reactions. As a part of an ongoing investigation into the preparation of cis-homoallylic alcohols as starting materials for the stereoselective synthesis of tetrahydrofurans,3 we studied the selective synthesis of cis-alkenes by hydrogenation of an alkyne using palladium on pumice with a metal loading of 3% wt as catalyst,4 avoiding the use of a nitrogen base. OH ..R OH cat. OP la-e OP 2a-e la : P = TBDMS, lb : P = TBDMS, lc : P = 4-Me-C6H4-SO 2, ld : P = 4-Me-C6H4-SO 2, le : P = 4-Me-C6H4-SO 2, R = CH2CH2OBn R = CH2OBn R = C5H 11 R = C(CH3) 3 a = Si(CH3) 3 The substrates la-e were prepared by reaction of the protected glycidol with the appropriate alkyne in the presence of BuLi/BF3.Et20. Stining a suspension in absolute ethanol of the hydroxyalkyne la-e (typically 0.023 M) and palladium on pumice (molar ratio alkyne/Pd = 55) at 25 °C with a flow of hydrogen of 1.0 or 0.2 nL/min (pressure 1 atm) provided the cis-homoaUylic alcohols 2a-e after filtration through Celite, concentration and flash column chromatography. 5 All the reactions were monitored by HPLC via recording the disappearance of the alkyne and the formation of the cis-homoallylic alcohols6 and the saturated alcohols. The results were compared with hydrogenation reactions performed using Pd/C (10%) or the Lindlar's catalyst (5%) as catalysL The data obtained are shown in table together with the reaction times and selectivities at 99% conversion of the alkyne. These data show that the Pd/C and Pd/pumice catalysts effect similm" rates except for compounds ld,e where R is a bulky group. However the Pd/pumice provides better selectivities as compared to Pd/C in all cases examined. Although Lindlar's catalyst shows good selectivities, the reactions proceed at much slower rates. 0040-4039/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved. PII: S0040-4039(99)00311- I