Hydrogenation reactions, of alkenes

Until recently, iron-catalyzed hydrogenation reactions of alkenes and alkynes required high pressure of hydrogen (250-300 atm) and high temperature (around 200°C) [21-23], which were unacceptable for industrial processes [24, 25]. In addition, these reactions showed low or no chemoselectivity presumably due to the harsh reaction conditions. Therefore, modifications of the iron catalysts were desired. 

Hydrogenation of n-AIkenes. - The activity of oxides with alkali metals in the hydrogenation of alkenes is similar to the activity of EDA complexes of alkali metals with organic electron acceptors described by Tamam. Hydrogenation of alkenes occurs at 423-473 K under normal pressure. In Table 5 are given the initial rates of hydrogenation reactions of alkenes in the presence of oxides doped with sodium and potassium vapours. 

An alternate method for the preparation of such dusters [213] is based on the impregnation of a dehydrated zeolite with alcoholic solutions of sodium azide, followed by the controlled decomposition of the azide. The ionic sodium clusters were shown by ESR spectroscopy to be Na4 and are formed in the zeolite pores. The clusters exhibit catal)dic properties for both isomerization and hydrogenation reactions of alkenes and alkynes. 

The types of reactions that can be catalyzed by transition metal complexes are now very numerous and are very widely used in synthesis. We have already met a number of them—osmium in catalysis of dihydroxylation reactions, titanium in Sharpless epoxidation, various metals in hydrogenation reactions of alkenes, and the Ziegler-Natta process for polymerization. In this section, we will just highlight a few types that have been popular—an oxidation, some hydrogenations, and some coupling reactions. Although outline reaction mechanisms will be given, this is for interest only—they are beyond the scope of this text, and many are more complicated than is shown here. 

A breakthrough in the study of iridium-catalyzed reactions was reported by Crabtree in 1977 regarding the hydrogenation reactions of alkenes [38, 39]. Only recently, iridium complexes have been utilized on alkynes for other reactions than the hydrogenation. 

Palladation products formed from arylmercurials, carboalkoxymercurials, and alkylmercurials, which have no /3-hydrogen, are used in situ for the reaction of alkenes[367]. Particularly, the arylation of alkenes is synthetically useful. Styrene derivatives 402 and 403 are formed by the reaction of a... 

We have already discussed one important chemical property of alkynes the acidity of acetylene and terminal alkynes In the remaining sections of this chapter several other reactions of alkynes will be explored Most of them will be similar to reactions of alkenes Like alkenes alkynes undergo addition reactions We 11 begin with a reaction familiar to us from our study of alkenes namely catalytic hydrogenation... 

Acid-Gatalyzed Synthesis. The acid-catalysed reaction of alkenes with hydrogen sulfide to prepare thiols can be accompHshed using a strong acid (sulfuric or phosphoric acid) catalyst. Thiols can also be prepared continuously over a variety of soHd acid catalysts, such as seoHtes, sulfonic acid-containing resin catalysts, or aluminas (22). The continuous process is utilised commercially to manufacture the more important thiols (23,24). The acid-catalysed reaction is commonly classed as a Markownikoff addition. Examples of two important industrial processes are 2-methyl-2-propanethiol and 2-propanethiol, given in equations 1 and 2, respectively. 

Oxo or Hydroformylation and Hydroesterification. Reactions of alkenes with hydrogen and formyl groups are cataly2ed by HCo(CO)4... 

Rhodium and cobalt participate in several reactions that are of value in organic syntheses. Rhodium and cobalt are active catalysts for the reaction of alkenes with hydrogen and carbon monoxide to give aldehydes, known as hydroformylation,281... 

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. 

The hydroformylation reaction ( oxo reaction ) of alkenes with hydrogen and carbon monoxide is established as an important industrial tool for the production of aldehydes ( oxo aldehydes ) and products derived there from [1-6]. This method also leads to synthetically useful aldehydes and more recently is widely applied in the synthesis of more complex target molecules [7-15,17], including stereoselective and asymmetric syntheses [18-22]. 

The kinetics of the catalytic oxidation of cyclopentene to glutaraldehyde by aqueous hydrogen peroxide and tungstic acid have been studied and a compatible mechanism was proposed, which proceeds via cyclopentene oxide and /3-hydroxycyclopentenyl hydroperoxide. " Monosubstituted heteropolytungstate-catalysed oxidation of alkenes by t-butyl hydroperoxide, iodosobenzene, and dioxygen have been studied a radical mechanism was proved for the reaction of alkenes with t-BuOOH and O2, but alkene epoxidation by iodosobenzene proceeds via oxidant coordination to the catalyst and has a heterolytic mechanism. ... 

Figure 1.36 Olefinic region of the H NMR spectrum for the product of the hydrogenation reaction of 3-hexyne-l-ol in presence of [RuCp (alkene)] (alkene = 3-hexenoic acid) and para-enriched hydrogen, under mild reaction conditions (300 K, 1 bar of H2) and low conversion rate.

A number of ruthenium-based catalysts for syn-gas reactions have been probed by HP IR spectroscopy. For example, Braca and co-workers observed the presence of [Ru(CO)3l3]", [HRu3(CO)ii]" and [HRu(CO)4] in various relative amounts during the reactions of alkenes and alcohols with CO/H2 [90]. The hydrido ruthenium species were found to be active in alkene hydroformylation and hydrogenation of the resulting aldehydes, but were inactive for alcohol carbonylation. By contrast, [Ru(CO)3l3]" was active in the carbonylation of alcohols, glycols, ethers and esters and in the hydrogenation of alkenes and oxygenates. 

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