Raney Nickel: An Efficient Reagent to Achieve the Chemoselective Hydrogenation of α,β-Unsaturated Carbonyl Compounds

Abstract

Raney Nickel is an effective reagent to achieve the chemoselective reduction of conjugated olefins in a,b-unsaturated carbonyl compounds that also contain isolated double bonds. Its use is also compatible with a variety of other functional groups.The selective reduction of conjugated olefins in a,b-unsaturated carbonyl compounds, that also contain isolated carbon-carbon double bonds, constitutes an important challenge in organic synthesis. This can be achieved by treating the carbonyl derivative with triethylsilane, in the presence of catalytic amounts of (tris(triphenylphosphine)rhodium chloride; 2a was converted in 2b following this procedure. 1 Some organic hydrides also allow this transformation to be carried out. Thus, L-, K-selectride and lithium triethylborohydride have been used to reduce a,b-unsaturated esters and ketones; however, this reactivity is not general and depends upon the steric hindrance of the alkene and of the ring size in cyclic ketones, b-alkylation usually completely supresses the 1,4-reduction. For example, treatment of 1a with an equivalent of K-selectride afforded 1b in a quantitative yield; 2 2a gave the dienic alcohol as the only product under similar conditions. 2 Enantioselective reduction of a,b-unsaturated carboxylates with sodium borohydride in the presence of cobalt complexes has been reported. 3 Semmelhack et al have demonstrated that copper salts modify the reactivity of lithium and sodium hydridoaluminate derivatives so that the reagents deliver hydride in a 1,4 fashion to alkenones. 4 Other copper hydrides have also been successfully used for this purpose. 5-8 Keinan et al described that a threecomponent system comprised of a soluble palladium catalyst, hydridosilane and zinc chloride is capable of efficient conjugate reduction of a,b-unsaturated carbonyl compounds. 9 A ruthenium complex combined with cyanoborohydride has also been reported for carrying out this transformation. 10 Very recently the selective 1,4-reduction of a,b-unsaturated carbonyl compounds by combined use of bis(1,3-diketonate)cobalt(II) complex and diisobutylaluminium hydride has been described. 11 On the other hand, conventional catalytic hydrogenation, using platinum or palladium, is not suitable to achieve the conjugate reduction because of its low selectivity, procuring in most cases the complete reduction of carbon-carbon double bonds. 12,13 Following the authors' research into the synthesis of valuable compounds from available natural terpenes we have found that Raney Nickel exhibited a good selectivity to carry out the conjugate reduction of a,b-unsaturated carbonyl compounds bearing other isolated carbon-carbon double bonds. In order to establish the scope and synthetic applications of this reaction, the behaviour of Raney Nickel against different a,b-unsaturated carbonyl compounds has been studied and compared with that of palladium over carbon (Table). In most cases, treatment with Raney Nickel in tetrahydrofuran for 20-30 minutes at room temperature afforded the 1,4-reduction compound in...

Hydrogenation of allyl alcohols catalyzed by aqueous palladium and platinum nanoparticles

“…[3][4][5] At the contrary, a useful building block for chemical and pharmaceutical processes, 1-propanol, 6 could be obtained by hydrogenation of 2-propen-1-ol, which, in turn, can be produced by deoxydehydration of glycerol, a by-product of biodiesel production, available in large amounts and at relatively low costs. [17][18][19][20] The selectivity in the reduction of the C=C group in the presence of other functional substituents (e.g., C=O or OH), can be controlled by a variety of parameters, such as the nature of the metal catalyst, the presence of a second metal (bimetallic catalysts), the size of metal particles and the nature of the supporting materials (electron-donating or -withdrawing ligand effects, metal-support interactions and solvent effect). 16 Hydrogenations are normally performed using hydrogen gas and heterogeneous metal catalysts, such as Raney Nickel or Palladium, Rhodium or Platinum metals.…”

Reactivity of Alcohols in Chemoselective Transfer Hydrogenation of Acrolein over Magnesium Oxide as the Catalyst

“…The main route for their synthesis is the selective hydrogenation of a,b-unsaturated carbonyl compounds. Conventional reduction of these compounds by gaseous dihydrogen over many metal catalysts results in preferable hydrogenation of the C=C bond [1][2][3][4][5]. Recently, metallic gold dispersed on FeOOH or c-Fe 2 O 3 has shown a remarkable chemoselectivity in the reduction of the conjugated C=O bond in the a,b-unsaturated carbonyl compounds [6,7].…”

Synthesis of platencin core structures via twist-brendane

“…This was achieved by refluxing 8 in NaOH/MeOH for 12 h followed by diazomethane workup which provided a separable mixture of diastereomers 9a/9b in 1:1 exo/endo ratio of methyl ester (Scheme 2). Reduction enone 9a was carried out using Raney nickel 33,37 in THF to obtain the keto ester 10 as a single diastereomer in 83% yield. Several two-step protocols are available in the literature for the conversion of ketone to enone moiety.…”