A novel method for the reduction of alkenes using the system silane/oxo-rhenium complexes Rita G. de Noronha a , Carlos C. Romão a , Ana C. Fernandes b, * a Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Av. da República - EAN, 2781-157 Oeiras, Portugal b Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal article info Article history: Received 26 October 2009 Revised 10 December 2009 Accepted 14 December 2009 Available online 16 December 2009 Keywords: Alkenes Reduction Oxo-rhenium complexes Silanes abstract In this work, we report the first practical application of high-valent oxo-complexes for the reduction of alkenes to the corresponding alkanes. The catalytic system Ph(Me) 2 SiH/ReIO 2 (PPh 3 ) 2 (5 mol %) proved to be very efficient for the reduction of mono- and disubstituted alkenes under solvent-free conditions. Ó 2010 Elsevier Ltd. All rights reserved. The reduction of alkenes is a fundamental reaction in organic synthesis. The most commonly used method for reducing C–C double bonds involves homogeneous or heterogeneous catalytic hydrogenation. 1 However, it requires the handling of the highly flammable hydrogen gas and pressurized conditions. As an alterna- tive, catalytic hydrogen transfer methods that make use of solvents such as amines, alcohols and water as the hydrogen source have been widely employed for the reduction of alkenes. 2 The reagent systems NaBH 4 /Pd/C, 3 NaBH 4 /RuCl 3, 4 NaBH 4 / Ru(PPh 3 ) 4 H 2 5 and NaBH 4 /InCl 3 6 can also be used as reducing agents, but functional group compatibility of metal hydride re- agents often becomes a problem. Organosilanes are mild and environmentally benign reagents, which have applications in many kinds of reactions. However, they are known to be poor reducing agents, due to their low capacity to donate hydrogen atoms or hydrides. To overcome this limitation, a variety of modified silanes with weaker Si–H bonds and composite reducing systems based on a combination of a silane/transition- metal catalyst have been developed. 7 Recently, Toste reported the first example of Si–H bond activa- tion by a high-valent oxo-complex. 8 The novel catalytic system silane/ReIO 2 (PPh 3 ) 2 proved to be very efficient for the hydrosilyla- tion of aldehydes and ketones. We extended this result to the activation of Si–H bond and B–H bond by other oxo-rhenium complexes and oxo-molybdenum complexes. We also described the efficient hydrosilylation of aldehydes and ketones 9 and the reduction of several functional groups such as aromatic nitro com- pounds, 10 imines, 11 amides, 12 esters, 13 sulfoxides 14 and pyridine N- oxides 14a with the catalytic system silane/oxo-complexes or bor- ane/oxo-complexes. Royo and co-workers 15 reported the use of high-valent oxo-molybdenum and -rhenium complexes as cata- lysts for the hydrogenation of alkynes to the corresponding al- kenes. However, these catalytic systems were inefficient for the reduction of alkenes. It is known that the catalytic addition of silanes to alkenes usu- ally gives alkylsilicon derivatives. Various complexes of transition metals have been used as effective catalysts for this reaction, including Rh, Ru and Ir. 16 Alkenes can also be reduced by silanes to the corresponding alkanes in the presence of Pd/C, 17 PdCl 2 17b or Pd(OAc) 2 . 18 The system silane/Pd/C proved to be efficient for the reduction of mono- and disubstituted alkenes, but the system silane/PdCl 2 or silane/Pd(OAc) 2 only reduces terminal double bonds. During our previous work about the deoxygenation of aromatic nitro compounds with the catalytic system PhMe 2 SiH/ReIO 2 (PPh 3 ) 2 (5 mol %), we observed the reduction of the double bond in the sub- strates m-nitrostyrene and ethyl p-nitrocinnamate (see Eqs. 1 and 2). 10 These results suggested that the system silane/oxo-rhenium complexes could be appropriate for the reduction of alkenes. To the best of our knowledge, this is the first example of the reduction of a double bond catalyzed by high-valent oxo-complexes. 0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2009.12.061 * Corresponding author. Tel.: +351 218419388; fax: +351 218464457. E-mail address: anacristinafernandes@ist.utl.pt (A.C. Fernandes). Tetrahedron Letters 51 (2010) 1048–1051 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet