Alkynes partial hydrogenation

The species [Ir(cod)(PPh3)(pz)] or [Ir(cod)(pz)]2 in the presence of PPhj (pz = pyrazolato) dissolves in acetone to yield catalytic solutions which partially hydroformylate and hydrogenate alkynes and alkenes. Results were reported for the hydroformylation of hept-2-yne. The hydrosilylation of styrene by [Si(Cl)2(H)Me] is catalyzed by complexes of the type [ML,L , ] (M = Ir, Co, Ni, Ru, Rh, Os, Pd, Pt L = halide, NO2 , SCN L = DMSO, Et2SO, Et2S, NHj). The catalytic activity and regioselectivity of these catalysts have been examined after immobilization on AVI 7X8 or KU 2X8 anion exchangers. ... 

The reaction occurs with stepwise addition of hydrogen to first give an alkene. The second step occurs even faster than the first. Therefore, catalytic hydrogenation with transition metal catalysts cannot be used to partially hydrogenate alkynes and stop at an alkene. The hydrogenation goes all the way to the alkane. 

Noting that cis alkenes are intermediates in the hydrogenation of alkynes leads us to con sider the possibility of halting hydrogenation at the cis alkene stage If partial hydrogena tion of an alkyne could be achieved it would provide us with methods for preparing... 

A useful alternative to catalytic partial hydrogenation for converting alkynes to alkenes IS reduction by a Group I metal (lithium sodium or potassium) m liquid ammonia The unique feature of metal-ammonia reduction is that it converts alkynes to trans alkenes whereas catalytic hydrogenation yields cis alkenes Thus from the same alkyne one can prepare either a cis or a trans alkene by choosing the appropriate reaction conditions... 

CoF is used for the replacement of hydrogen with fluorine in halocarbons (5) for fluorination of xylylalkanes, used in vapor-phase soldering fluxes (6) formation of dibutyl decalins (7) fluorination of alkynes (8) synthesis of unsaturated or partially fluorinated compounds (9—11) and conversion of aromatic compounds to perfluorocycHc compounds (see Fluorine compounds, organic). CoF rarely causes polymerization of hydrocarbons. CoF is also used for the conversion of metal oxides to higher valency metal fluorides, eg, in the assay of uranium ore (12). It is also used in the manufacture of nitrogen fluoride, NF, from ammonia (13). 

Dia ene deductions. Olefins, acetylenes, and azo-compounds are reduced by hydrazine in the presence of an oxidizing agent. Stereochemical studies of alkene and alkyne reductions suggest that hydrazine is partially oxidized to the transient diazene [3618-05-1] (diimide, diimine) (9) and that the cis-isomer of diazene is the actual hydrogenating agent, acting by a concerted attack on the unsaturated bond ... 

Alkynes can be reduced to yield alkenes and alkanes. Complete reduction of the triple bond over a palladium hydrogenation catalyst yields an alkane partial reduction by catalytic hydrogenation over a Lindlar catalyst yields a cis alkene. Reduction of (he alkyne with lithium in ammonia yields a trans alkene. 

Among the tasks remaining is the replacement of the C-16 hydroxyl group in 16 with a saturated butyl side chain. A partial hydrogenation of the alkyne in 16 with 5% Pd-BaS04 in the presence of quinoline, in methanol, followed sequentially by selective tosylation of the primary hydroxyl group and protection of the secondary hydroxyl group as an ethoxyethyl ether, affords intermediate 17 in 79% overall yield from 16. Key intermediate 6 is formed in 67 % yield upon treatment of 17 with lithium di-n-butylcuprate. 

A synthetically useful virtue of enol triflates is that they are amenable to palladium-catalyzed carbon-carbon bond-forming reactions under mild conditions. When a solution of enol triflate 21 and tetrakis(triphenylphosphine)palladium(o) in benzene is treated with a mixture of terminal alkyne 17, n-propylamine, and cuprous iodide,17 intermediate 22 is formed in 76-84% yield. Although a partial hydrogenation of the alkyne in 22 could conceivably secure the formation of the cis C1-C2 olefin, a chemoselective hydrobora-tion/protonation sequence was found to be a much more reliable and suitable alternative. Thus, sequential hydroboration of the alkyne 22 with dicyclohexylborane, protonolysis, oxidative workup, and hydrolysis of the oxabicyclo[2.2.2]octyl ester protecting group gives dienic carboxylic acid 15 in a yield of 86% from 22. 

Thus far, chemists have been able to influence the stereoselectivity of macro-cyclic RCM through steric and electronic substrate features or by the choice of a catalyst with appropriate activity, but there still exists a lack of prediction over the stereochemistry of macrocyclic RCM. One of the most important extensions of the original metathesis reaction for the synthesis of stereochemi-cally defined (cyclo)alkenes is alkyne metathesis, followed by selective partial hydrogenation. 

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 hydrogenation of alkynes is a very interesting reaction, since the selectivity toward the partially or the fully reduced product allows the in-situ comparison of the ability of a catalyst to reduce C=C versus C=C bonds. This is perhaps the area in which duster catalysis has been most extensively developed, as recently reviewed by Cabeza [27], Adams and Captain [4], and Dyson [28]. A good number of metal clusters have been employed as catalyst precursors in alkyne hydrogenation, the majority of them containing ruthenium. 

The use of dispersed or immobilized transition metals as catalysts for partial hydrogenation reactions of alkynes has been widely studied. Traditionally, alkyne hydrogenations for the preparation of fine chemicals and biologically active compounds were only performed with heterogeneous catalysts [80-82]. Palladium is the most selective metal catalyst for the semihydrogenation of mono-substituted acetylenes and for the transformation of alkynes to ds-alkenes. Commonly, such selectivity is due to stronger chemisorption of the triple bond on the active center. 

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. 

Woelk and Bargon have studied the catalytic hydrogenation of alkynes in scCOj [34], using catalysts based on transition metal colloids, carried out in the TCA setup. Under mild experimental conditions (15 bar hydrogen partial pressure, at 323 K) they have found extremely high TOFs (as high as 4 X 10 h" ... 

Preparation of cis-alkenes Lindlar s catalyst, which is also known as poisoned catalyst, consists of barium sulphate, palladium and quinoline, and is used in selective and partial hydrogenation of alkynes to produce c/s-alkenes. Hydrogen atoms are delivered simultaneously to the same side of the alkyne, resulting in syn addition (cw-alkenes). Thus, the syn addition of alkyne follows same procedure as the catalytic hydrogenation of alkyne. 

A more challenging task is the selective partial hydrogenation (semihydrogenation) of alkynes to yield alkenes. This is a selectivity problem similar to the hydrogenation of dienes in that that the alkyne is hydrogenated preferentially in the presence of an alkene. The possibility of the formation of geometric isomers from nonterminal acetylenes raises the problem of stereoselective semihydrogenation. 

Quite remarkably partial hydrogenation of alkynes to trans alkenes is also possible with homogeneous rhodium complexes 168169... 

As was discussed in Sections 11.1.5 and 11.2.3, the stereoselective partial hydrogenation of alkynes to either cis or trans alkenes is of key importance. Chemical reductions can also be applied to achieve both selective transformations. 

Since that time several other metal-solvent systems have been found useful, effective, and selective in the partial hydrogenation of alkynes. The mechanism of the transformation depends on the nature of the solvent and the substrate. Detailed studies were carried out with the Na-HMPA-tert-BuOH system,197 resulting in the suggestion of the mechanism depicted in Scheme 11.10. 

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