Reductions of alkynes to alkenes

A few remarkable, but rather uncommon, transfer hydrogenations also deserve mention within the context of this chapter namely, the reduction of alkynes to alkenes using a chromium catalyst, and the reduction of double bonds using diimines. 

Alkynes may also be hydrogenated, initially to alkenes, and then further to alkanes. By suitable modification of the catalyst, it has proved possible to stop the reaction at the intermediate alkene. Typically, platinum or palladium catalysts partially deactivated (poisoned) with lead salts are fonnd to be suitable for reduction of alkynes to alkenes. Again, syn addition is observed. 

Reduction of alkynes to alkenes. The zinc-copper couple of Smith and Simmons (1, 1292) reduces internal alkynes to (Z>alkcncs exclusively in yields generally >95%.2 Terminal alkynes are reduced to 1-alkenes.3 Example ... 

Although it does not act as selectively as the Pd catalysts, finely divided Ni is sometimes useful for reduction of isolated triple bonds, especially in the presence of nitrogenous bases such as pyridine, ammonia, or piperidine. The heterogeneous catalyst NaH/RONa/Ni(OAc)2 presents practical advantages it is cheap, easily and reproducibly prepared, and can be stored for long periods. By monitoring the uptake of Hj, selective reduction of alkyne to alkene and then to alkane is achieved, particularly in the presence of quinoline. Only mild conditions are required to reduce disubstituted or terminal alkynes to cis-alkenes in high yields ... 

Lindlar s catalyst is often used for the reduction of alkynes to alkenes. The catalyst consists of palladium which is deposited in a finely divided state of barium sulfate, which is subsequently treated with a heterocyclic amine called quinoline. Which of the following is the most likely function of quinoline ... 

The hydrosilylation of terminal alkynes disclosed by Trost can be applied to internal alkynes as well. i Remarkably, the (Z)-isomer is generated in this process, resulting from trans addition during hydrosilylation. The protodesilylation of these sily-lated products in the presence of copper(I) iodide and tetrabuty-lammonium fluoride (TBAF) or silver(I) fluoride (eq 15) leads to internal fraws-olefins. This two-step method is a useful synthetic transformation to access ( j-alkenes from internal alkynes. In contrast, the chemoselective reduction of alkynes to the corresponding ( -alkenes is conventionally accomplished readily with Lindlar s catalyst. The complementary process to afford ( )-olefins has proven much more difficult. Methods involving metal hydrides, dissolving metal reductions, low-valent chromium salts provide the desired chemical conversion, albeit with certain limitations. For example, functional substitution at the propargylic position (alcohols, amines, and carbonyl units) is often necessary to achieve selectivity in these transformations. Conversely, the hydrosilylation/protodesilyla-tion protocol is a mild method for the reduction of alkynes to ( )-alkenes. 

Stereoselective and chemoselective semihydrogenation of the internal alkyne 208 to the ew-alkene 210 is achieved by the Pd-catalyzed reaction of some hydride sources. Tetramethyldihydrosiloxane (TMDHS) (209) i.s used in the presence of AcOH[116]. (EtO)3SiH in aqueous THF is also effective for the reduction of alkynes to di-alkenes[l 17], Semihydrogenation to the d.v-alkene 211 is possible also with triethylammonium formate with Pd on carbon[118]. Good yields and high cis selectivity are obtained by catalysis with Pd2fdba)3-Bu3P[119],... 

Partial reduction of alkynes to Z-alkenes is an important synthetic application of selective hydrogenation catalysts. The transformation can be carried out under heterogeneous or homogeneous conditions. Among heterogeneous catalysts, the one that... 

The first experiments on chemoautotrophic theory were carried out by Stetter at the University of Regensburg. It was found that synergy in the FeS/H2S system determined the reductive effect, for example, in the conversion of nitrate to ammonia or of alkynes to alkenes. The conditions used corresponded to those present in hydrothermal systems aqueous phase, 373 K, almost neutral pH and anaerobic conditions (Blochl et al 1992). Two years later, the formation of an amide bond without the use of a condensation agent was successfully demonstrated in the same laboratory (Keller et al 1994). 

Scheme 20.29 Reduction of alkynes to trans-alkenes by chromous sulfate.

Chromium complexes,163 cationic homogeneous complexes of rhodium164 and ruthenium,165 166 and supported palladium complexes141167 have been found to be extremely selective in reduction of alkynes to cis alkenes ... 

Selective reduction of alkynes to cis alkenes using hydrosilane functions immobilized on silica gel in acetic acid in the presence of a Pd(0) catalyst system has been reported.239... 

Catalytic reduction of alkynes to ds-alkenes. This reduction is not possible with 10% Pd/C alone because this metal is too reactive and the alkane is formed readily. The selective reaction is possible if the Pd/C is deactivated by either Hg(0) or Pb(0), obtained by reduction of metal acetate with NaBH4. Sodium phosphinate, H2P02Na, is the preferred hydride donor. Since this donor is not soluble in the Organic solvents used, a phase-transfer catalyst, benzyltriethylammonium chloride, is added.3... 

A review which covered the different known methods for the preparation of chiral amines and analysis of the different chiral catalysts used, correlating them according to their efficiency, selectivity, and flexibility, has been presented.279 Reduction reactions of alkenes, arenes, alkynes and allenes resulting in the formation of two or more C-H bonds280 and reduction and addition reactions of alkynes to alkenes to form one or more C=C bonds281 have been reviewed. 

Reduction of alkynes to cis-alkenes.1 This relatively stable copper hydride (1) reduces terminal alkynes at 25° internal alkynes are reduced at 80° to cis-alkenes exclusively. These reductions require 2 hydride equivalents of the complex, and 5-10 equiv. of H20 is necessary for satisfactory yields. Under these conditions, a tertiary propargyl acetate was converted to an allene (equation I). 

Water is essential for this reduction. This system reduces alkynes to rij-alkenes with high stereoselectivity. Addition of methyl propynoate (10 mole %) increases the reactivity and can effect reduction of alkynes to alkanes. 

Birch reduction of aromatic compounds involves reaction with an electron-rich solution of alkali metal lithium or sodium in liquid ammonia (sometimes called metal ammonia reduction). Usually a proton donor such as tert-butanol or ethanol is used to avoid the formation of excess amount of LiNH2 or NaNH2. The major product is normally a 1,4-diene. This reaction is related to the reduction of alkynes to frans-alkenes ° (section 6.2.2). 

For the electrochemical oxidation and reduction of alkynes and alkenes an analogy may be drawn with their relative reactivities towards electrophilic and nucleophilic attack. Alkynes are the more easily attacked by nucleophiles and are slightly easier to reduce. Alkynes are, however, much less prone to electrophilic attack than alkenes and are correspondingly more difficult to oxidize electro-chemically. 

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