View metadata, citation and similar papers at core.ac.uk brought to you by CORE ARTICLE IN PRESS TET19911_proofprovided by Repositorio 26 October Institucional 2009 de la Universidad 1/7 de Alicante Tetrahedron xxx (2009) 1–7 Contents lists available at ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet 55 1 56 2 Transfer hydrogenation of olefins catalysed by nickel nanoparticles 57 3 58 4 59 ** * 5 Francisco Alonso , Paola Riente, Miguel Yus 60 6 Departamento de Quı´mica Orga´nica, Facultad de Ciencias and Instituto de Sı´ntesis Orga´nica (ISO), Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain 61 7 62 8 63 9 article info abstract 64 10 65 11 Article history: Nickel nanoparticles have been found to effectively catalyse the hydrogen-transfer reduction of a variety 66 12 Received 22 September 2009 of non-functionalised and functionalised olefins using 2-propanol as the hydrogen donor. The hetero- 67 13 Received in revised form geneous process has been shown to be highly chemoselective for certain substrates, with all the cor- 68 14 October 2009 responding alkanes being obtained in high yields. A synthesis of the natural dihydrostilbene brittonin 69 14 Accepted 15 October 2009 A is also reported based on the use of nickel nanoparticles. 15 Available online xxx 70 16 Ó 2009 Published by Elsevier Ltd. 71 17 Keywords: PROOF 72 18 Hydrogen transfer 73 19 Reduction 74 Olefins 75 20 Nickel nanoparticles 21 76 22 77 23 78 24 79 25 1. Introduction of chiral ligands. In contrast with the reduction of carbonyl com- 80 26 pounds, the hydrogen-transfer reduction of olefins has been little 81 27 The reduction of carbon–carbon double bonds is one of the studied, mainly involving noble-metal catalysts. Phosphane– 82 28 fundamental reactions in organic chemistry. For this trans- ruthenium complexes can transfer hydrogen from alcohols, formic 83 1 5 29 formation, catalytic hydrogenation, either under homogeneous or acid, and hydroaromatic compounds to olefins. Formic acid is the 84 2 6a–c 30 heterogeneous conditions, is generally preferred to other non- hydrogen donor of choice under palladium catalysis (1,4-cyclo- 85 3 6d 31 catalytic chemical methods. Homogeneous catalysts have exhibi- hexadiene has been recently used ), whereas rhodium and iridium 86 7 32 ted high activity and selectivity with special applications in complexes have been rarely applied. In the above studies, a narrow 87 33 asymmetric catalysis, although they are often expensive and their substrate scope has been tested, mainly covering activated olefins. 88 34 separation and reuse troublesome. In recent years, however, het- In the search for cheaper catalytic systems, nickel appears as an 89 35 erogeneous catalysis has experienced an enormous progress, with alternative to the noble metals since it is about 100-fold cheaper 90 36 the catalysts being, in some cases, even more selective than their than palladium and ruthenium, and much cheaper than rhodium 91 37 homogeneous counterparts. Moreover, heterogeneous catalysts are and iridium (referred to their chlorides). As a recent example, clay- 92 38 easy to separate and reuse, minimising the presence of metal traces entrapped nickel nanoparticles have been found to efficiently 93 39 in the product, improving the handling and process control and, catalyse the reduction of styrenes using hydrazine as the hydrogen 94 8 40 therefore, reducing the overall costs. At any rate, catalytic hydro- source. On the other hand, 2-propanol is a very popular hydrogen 95 41 genation requires special care in the handling of hydrogen (a highly donor since it is cheap, non-toxic, volatile, possesses good solvent 96 42 flammable and explosive gas) and, in some cases, rather expensive properties and it is transformed into acetone, which is environ- 97 43 catalysts and high pressures are mandatory for the reaction to occur. mentally friendly and easy to remove from the reaction system. 98 44 In this sense, the hydrogen-transfer reduction of organic com- Despite the attractiveness of the combination Ni/i-PrOH, only two 99 4 45 pounds is an advantageous methodology since: (a) the hydrogen reports describe its application to the transfer hydrogenation of 100 46 source is easy to handle (no gas containment or pressure vessels are olefins. In the first one, Raney nickel (10–50 wt % of total substrate) 101 47 necessary), (b) possible hazards areUNCORRECTED minimised, (c) the mild reaction was used under reflux, showing high conversions for cinnamates 102 9 48 conditions used can afford enhanced selectivity and, (d) catalytic and cyclic olefins and low conversions for acyclic olefins. In the 103 49 asymmetric transfer hydrogenation can be applied in the presence second report, activated metallic nickel, prepared by thermal de- 104 50 composition of in situ generated nickel diisopropoxide in boiling 105 51 2-propanol, was more effective in the reduction of non-function- 106 10 52 * Corresponding author. Fax: þ34 965903549. alised and non-activated olefins (10–30 mol % Ni, 95–100 C). 107 11 53 ** Corresponding author. Due to our continued interest on active metals, some years ago, 108 54 E-mail addresses: [email protected] (F. Alonso), [email protected] (M. Yus). we reported that active nickel, prepared from stoichiometric 109 110 0040-4020/$ – see front matter Ó 2009 Published by Elsevier Ltd. 111 doi:10.1016/j.tet.2009.10.057 Please cite this article in press as: Alonso, F. et al., Tetrahedron (2009), doi:10.1016/j.tet.2009.10.057 ARTICLE IN PRESS TET19911_proof 26 October 2009 2/7 2 F. Alonso et al. / Tetrahedron xxx (2009) 1–7 112 Table 1 nanoparticles15 and that it was particularly active in hydrogen- 178 113 Hydrogen-transfer reduction of 1-octene in the presence of different nickel catalysts transfer reactions,16 namely: the a-alkylation of methyl ketones 179 Ni catalyst 114 with primary alcohols,16a,b the transfer hydrogenation of carbonyl 180 115 i-PrOH, 76 °C compounds,16c,d and reductive amination of aldehydes.16e We wish 181 116 to present herein an improved methodology for the reduction of 182 a 117 Entry Ni catalyst/substrate (mmol) T ( C) t (h) Yield (%) olefins based on the catalytic transfer hydrogenation with nickel 183 118 1 NiNPs 1:10 76 24 0 nanoparticles and 2-propanol as hydrogen donor. 184 2 NiNPs 1:5 76 3 100 119 185 3 NiNPs 1:5 20 3 70 120 4 Raney Nib 1:5 76 8 100 2. Results and discussion 186 121 5 Raney Nib 1:5 20 24 0 187 b 122 6 Ni–Al 1:5 76 24 0 The nickel nanoparticles (NiNPs) were initially generated from 188 7 Ni/SiO –Al O b 1:5 76 24 0 123 2 2 3 anhydrous nickel(II) chloride, lithium powder and a catalytic 189 8 NiOb 1:5 76 24 0 0 124 9 None 76 24 0 amount of DTBB (4,4 -di-tert-butylbiphenyl, 5 mol %) in THF at 190 125 room temperature. A blank experiment, consisting in a standard 191 a GLC yield. 126 192 b Commercially available catalyst. reaction in the absence of the substrate but in the presence of the 127 hydrogen source (i.e., NiCl2, Li, DTBB, THF, i-PrOH, 76 C, 1 h), 193 128 confirmed the formation of NiNPs.17 Transmission electron mi- 194 129 croscopy (TEM) analysis revealed the presence of spherical and 195 20 mol% NiNPs 130 CC CC highly uniform nanoparticles within the range 0.75–2.88 nm (ca. 196 131 i-PrOH, 76 °C 1.75Æ1.00 nm).16c A preliminary study was carried out using 197 132 1-octene as model substrate in order to optimise the amount of 198 Scheme 1. 133 catalyst and compare with other nickel catalysts (Table 1). A 1:10 199 134 NiNPs/substrate molar ratio at 76 C was shown to be inactive 200 135 NiCl2$2H2O, lithium, and a catalytic amount of an arene, can ef- (entry 1), whereas a quantitative conversion into the product 201 136 fectively reduce alkenes.12 Later on, we introduced the catalytic n-octanePROOF was observed with a 1:5 NiNPs/substrate molar ratio 202 137 hydrogenation of alkenes using the system NiCl2(40 mol %)-Li- (20 mol % Ni) (entry 2). Interestingly, the reaction occurred with 203 138 [naphthalene(cat.) or polymer-supported naphthalene(cat.)] and the latter even at room temperature, albeit with incomplete con- 204 139 external molecular hydrogen.13 Alternatively, molecular hydrogen version (entry 3). A series of experiments were performed with 205 140 could be generated in situ from ethanol.14 More recently, we dis- commercially available nickel catalysts. Raney nickel behaved 206 141 covered that the above generated active nickel was in the form of similarly to the NiNPs but longer reaction time was needed in 207 142 208 143 209 144 Table 2 210 Hydrogen-transfer reduction of non-functionalised alkenes catalysed by NiNPsa 145 211 146 Entry Alkene t (h) Productb Yieldc (%) 212 147 1 3 >99 213 148 214 149 2 Ph 2 Ph >99d 215 150 216 151 3 2 >99 217 152 218 4 1 60 153 219 154 220 Ph Ph d e 155 5 Ph 1 Ph >99 (40) 221 156 222 157 223 6 1 >99 158 Ph Ph 224 159 225 160 7 4 >99d 226 161 Ph Ph Ph Ph 227 162 228 163 229 8 2 >99 164 230 165 231 166 232 167 UNCORRECTED 233 168 9 72 >99 234 169 235 170 236 171 237 10 5 >99 172 238 173 239 a 174 Alkene (5 mmol), NiNPs (1 mmol), 2-propanol (5 mL), 76 C. 240 b 1 175 All isolated products were 95% pure (GLC and/or H NMR).