Of alkenes to alkanes

A catalyst, usually acid, is required to promote chemoselective and regioselective reduction under mild conditions. A variety of organosilanes can be used, but triethylsilane ia the presence of trifiuoroacetic acid is the most frequendy reported. Use of this reagent enables reduction of alkenes to alkanes. Branched alkenes are reduced more readily than unbranched ones. Selective hydrogenation of branched dienes is also possible. 

The substitution of trialkylphosphine for carbon monoxide also makes the metal-hydrogen bond more hydridic in character and results in increased reduction of the initially formed aldehyde to alcohol. Slaugh and Mullineaux (52) compared Co2(CO)g and [Co2(CO)8 + 2PBu3], each at reaction conditions of 150°C, 500 psi, H2/CO I.0, for the hydroformylation of 1-pentene. The products consisted of hexyl aldehydes and hexyl alcohols in the ratios of 95 5 and 30 70, respectively. In a negative aspect of the reaction, they observed 23% hydrogenation of alkene to alkane at a reaction temperature of 195°C with the phosphine-modified catalyst. Tucci (54) reported less alkane formation (4-5%) under more favorable reaction conditions (I60°C, H2/CO 1.2, 1 hour reaction time). 

Sodium triacetoxyborohydride, 283 Titanium(III) chloride, 302 Tributyltin hydride, 316 Zinc borohydride, 167 of alkenes to alkanes (R)-(-F)- and (S)-(-)-2,2 -Bis(di-phenylphosphine)-l,1 -binaphthyl, 36 [1,4-Bis(diphenylphosphine)-butanej(cycloheptatriene)-rhodium(I) tetrafluoroborate, 89 [ 1,4-Bis(diphenylphosphine)butane]-(norbornadiene)rhodium(I) tetrafluoroborate, 37... 

The porous Ni particle, or skeleton, which is left by the preparative leaching process offers such an excellent adsorbtive surface for H2 that the alloy is used as an economical means for the hydrogenation of alkenes to alkanes at low press (1 to 4 atm) and moderate temps (0 to 100°). 

The reduction of alkenes to alkanes is a reaction that is often used as a key part of a synthetic sequence. In some cases this reaction is performed in an attempt to introduce chirality into a molecule. The emphasis here is on the stereocontrolled reduction of alkenes in complex molecules. 

Metathesis reactions may be intramolecular and ring-closing diene metathesis (RCM, implicated in Scheme 1.13, see Chapter 12) allows disconnections in retro-synthetic analysis otherwise of little use. The normal disconnection of the macrocyclic amide in Scheme 1.13 would be at the amide but, because of the ready reduction of alkenes to alkanes, the alternative disconnection now becomes a viable option. And since any of the C—C linkages could be formed by RCM, such a disconnection allows far greater synthetic flexibility than the conventional disconnection at the functional group. 

With the advances in pro-catalyst design that have been witnessed over the last decade or so, the transition-metal-catalysed alkene metathesis reaction has now become a practical procedure that can be utilised by the chemist at the bench. Undeniably, this has added a new dimension to the repertoire of synthetic organic chemistry as it facilitates disconnections that, pre-metathesis, simply would not have been considered. Take, for example, a macro-cyclic amide where the normal disconnection would be at the amide. Now, with the ready reduction of alkenes to alkanes, a ring-closing diene metathesis (RCM), followed by hydrogenation, becomes an alternative disconnection. And, when one considers that any of the C—C linkages could be established in such a manner, the power of the RCM disconnection becomes obvious. 

Excellent examples of the use of nitrogen bases as catalyst poisons are seen in the selective hydrogenation of alkynes to alkenes (see Chapter 4). Quinoline is probably the base that has been most often employed for this purpose. In this selective hydrogenation, the nitrogen base effectively inhibits the hydrogenation of alkenes to alkanes... 

Cyclohexane, cyclohexene and cyclohexadiene are used as hydrogen sources in the hydrogenation of alkenes to alkanes, when they are themselves oxidized to benzene. In these reactions, the driving force is the formation of the aromatic ring. 

Although most of the reductions of carbon-carbon rr-systems have involved the reduction of triple to double bonds, Whitesides and Ehmann have described the reduction of alkenes to alkanes using sodium... 

In the case of the reduction of alkenes to alkanes, it does not appear that the stereo-selectivity or -specificity of this process has been studied. It is doubtful that the final steps in the reduction reactions differ from those proposed for the reduction of alkynes, and little stereo-selectivity or -specificity is to be anticipated. TTie mechanism for the reduction of alkenes with LiAlH4 in the presence zirconium(IV) chloride is proposed to occur via a metal exchange reaction followed by hydrolysis (equation 39). ... 

Hydrogenation of Alkenes to Alkanes by Various Transition Metal Clusters... 

More generally, a ratio of alkene to alkane is determined by the relative production rate of each. During free-radical cracking, pyrolysis of alkanes yields either alkenes by unimolecularly decomposing free radicals, alkanes by free radicals abstracting a hydrogen from another source, or both. For example, ethene can be formed by decomposition of primary radicals (including ethyl),... 

The strength of adsorption of H2 on iron is known to be weaker than that of CO. If alkali increases the coverage of the surface by growing chains and monomers, there would be fewer vacant sites available for H2 adsorption making the system less hydrogenating. This could explain why alkali promotion increases the ratio of alkenes to alkanes. 

Imines undergo addition of one mole of hydrogen in the presence of a catalyst such as palladium, platinum, or nickel to produce secondary amines (Eq. 17.10). This process is analogous to the hydrogenation of alkenes to alkanes and of carbonyl compounds to alcohols. 

R ) rearrangements is provided by the fact that alkane and alkene products show that H/D scrambling can occur within one R (but not between R and R"). In the dissociative pathway, for alkyl systems the ratio of alkenes to alkanes in the products show that all j8-hydrogen atoms have an equal tendency to be eliminated. They are, however, activated by phenyl groups. An isopropyl group is isomerized to an n-propyl ligand before decomposition occurs. In the nondissociative pathway, ease of j8-elimination from groups appears to rise PtEt < Pt Pr< Pt Bu. Rate constants for overall process (134) vary by a factor of only 4 for various R. 

Photocatalytic Hydrogenation of Alkenes to Alkanes in Alcoholic Suspensions of Palladium-Loaded Titanium(IV) Oxide Without the Use of Hydrogen Gas... 

Applicability of the photocatalytic hydrogenation of alkenes to alkanes was investigated using various aromatic or aliphatic alkenes, and Table 9.1 shows results of the photocatalytic hydrogenation in methanolic suspensions of Pd-Ti02 particles under deaerated conditions. Hydrogenation reaction proceeded in all cases, and high yield was obtained. 

Imamura K, Okubo Y, Ito T, Tanaka A, Hashimoto K, Kominami H (2014) Photocatalytic hydrogenation of alkenes to alkanes in alcoholic suspensirms of palladium-loaded titartium (IV) oxide without the use of hydrogen gas. RSC Adv 4 19883-19886... 

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