Alkenes intermediate

Alkynes are reduced to alkanes by addition of TI2 over a metal catalyst. The reaction occurs in steps through an alkene intermediate, and measurements indicate that the first step in the reaction is more exothermic than the second step. 

The reagent titanocene dichloride reduces carboxylic esters in a different manner from that of 10-86, 19-36, or 19-38. The products are the alkane RCH3 and the alcohol R OH. The mechanism probably involves an alkene intermediate. Aromatic acids can be reduced to methylbenzenes by a procedure involving refluxing first with trichlorosilane in MeCN, then with tripropylamine added, and finally with KOH and MeOH (after removal of the MeCN). The following sequence has been suggested ... 

Many of these catalysts are derived from metal complexes which, initially, do not contain metal hydride bonds, but can give rise to intermediate MH2 (al-kene) species. These species, after migratory insertion of the hydride to the coordinated alkene and subsequent hydrogenolysis of the metal alkyl species, yield the saturated alkane. At first glance there are two possibilities to reach MH2 (alkene) intermediates which are related to the order of entry of the two reaction partners in the coordination sphere of the metal (Scheme 1.2). 

In general, the rate of alkene hydrogenation is typically ordered as follows terminal>di-substituted>tri-substituted >tetra-substituted. In fact, this allows terminal or di-substituted olefins to be hydrogenated selectively in the presence of tri- or tetra-substituted ones. Additionally, the rate of hydrogenation of al-kynes is much slower than that of alkenes, although the cis-alkene intermediate... 

Based on this model, the regiochemical outcome of both oxabicycHc alkenes and vinyl epoxides can be explained by the relative stability of the two cr-intermediates. With oxabicyclic alkenes intermediate 41 is generated directly and then goes on to give product, whereas with the vinyl epoxides a more highly strained intermediate 40 is first produced which must first isomerize to 41 prior to product formation. The regiochemical outcome with each substrate may therefore arise from the commonahty of a cr-in-termediate 41 to both reaction pathways (Scheme 9.13). 

Table 2. Average cisdrans ratio of alkene intermediate in IPP hydrogenation.

Rhodium-catalyzed cyclization/silylation/carbonylation of 1,6-diynes was proposed to occur via a-migratory insertion of CO into the Rh-C bond of dialkylidene cyclopentane intermediate Ik to form acylrhodium alkene intermediate Ilk (Scheme S-endo-[5-M gX2itory insertion of the silylated alkylidene moiety into the Rh-G bond of Ilk would form... 

The reduction of alkynes occurs in two steps addition of one mole of hydrogen atoms to form alkenes, and then addition of the second mole of hydrogen to alkenes to form alkanes. This reaction proceeds through a cis-alkene intermediate, hut cannot he stopped at this stage except with the use of a special catalyst. 

Aromatization of n-octane yielded ethylbenzene and o-xylene, the products of direct 1,6 closure, but m- and p-xylenes were also formed.205 Isomeric dimethyl-hexanes, expected to give only one aromatic compound each according to 1,6 carbon-carbon closure, produced all isomeric alkylbenzenes.206 Alkanes that cannot undergo dehydrogenation did not yield aromatic compounds, which pointed to the importance of alkene intermediates in aromatization. 

Oxidation of Other Arenes. Aromatic compounds with longer alkyl side chains can be converted to ketones or carboxylic acids. All the previously discussed reagents except Cr02Cl2 usually afford the selective formation of ketones from alkyl-substituted arenes. Oxidation with Cr02Cl2 usually gives a mixture of products. These include compounds oxidized in the P position presumably formed via an alkene intermediate or as a result of the rearrangement of an intermediate epoxide.110,705... 

Homoallyl chloroformates cyclize catalytically to a-methylene- y-butyrolactones in moderate yields at 130 C with tetrakis(triphenylphosphine)palladium(0) (equation 44).l0s This reaction only gives 1-2% of product when it is carried out intermolecularly. The presumed intermediate in the homoallyl chlorofor-mate cyclization has been isolated and kinetic measurements show that that cyclization is inhibited by an excess of triphenylphosphine. A chelated ir-alkene intermediate is proposed.1 6... 

Cracking and disproportionation in the reaction of hexane could be suppressed by the addition of cycloalkanes (cyclohexane, methylcyclopentane, cyclopentane).101 Furthermore, 3-methylpentane and methylcyclopentane also reduced the induction period. These data indicate that reactions are initiated by an oxidative formation of alkene intermediates. These maybe transformed into alkenyl cations, which undergo cracking and disproportionation. When there is intensive contact between the phases ensuring effective hydride transfer, protonated alkenes give isomerization products. 

The resulting secondary alcohol can then be dehydrated to the same alkene intermediate prepared in the preceding synthetic scheme. 

A recent investigation of the hydrosilation of alkynes by Et3SiH in the presence of the bridged derivative (PhCH2)Me2SiPt[P(cyclo-hexyl)3](p.-H) 2, analogous to (LVIII), has shown that the rate increases as the 7r-acceptor power of the alkyne increases, consistent with the formation of Si-Pt-alkyne complexes as intermediates 416). This complements earlier kinetic studies on the hydrosilation of alkenes catalyzed by Co, Rh, and Pt compounds, in which similar Si-metal-alkene intermediates were postulated 85, 134). 

The reaction involves the addition of one molecule of hydrogen to form na alkene intermediate which then reacts with a second... 

Catalytic hydrogenation takes place in two stages, with an alkene intermediate. With efficient catalysts such as platinum, palladium, or nickel, it is usually impossible to stop the reaction at the alkene stage. 

Higher alcohols can also be carbonylated, although the rate decreases sharply, MeOH >> EtOH > PrOH. Alkene intermediates are involved, and propanol gives... 

There are two possible ways to approach the M(H)n(alkene) intermediate in a hydrogenation reaction the hydride route <2), implies an initial reaction with hydrogen followed by coordination of the substrate, while the unsaturated... 

The consecutive alkenyl mechanism (Figure 3) was put forward as the route for oxidation of unsaturated reactants. The weakly adsorbed intermediates are presumed to be in equilibrium with the gas phase, which enables furan to be seen as a product in butene oxidation (22,24,27). In contrast to the previous work, this study included an examination of the fact that none of the alkene intermediates desorb from the catalyst. It was proposed that the reaction proceeded via more strongly adsorbed alkoxide intermediates that would remain on the surface for the whole oxidation sequence (Figure 5). 

Alkyne hydrogenation remains a largely forgotten area of homogeneous catalytic hydrogenation. There are clear commercial benefits to be obtained in the conversion of alkynes to alkenes however, homogeneous catalysts are not usually sufficiently selective towards the alkene intermediate. 

Unlike the previous work, this study examined the fact that none of the alkene intermediates desorb from the surface. Zhang-Iin and coworkers proposed that the reaction proceeded via more strongly adsorbed alkoxide intermediates that would remain on the surface for the whole oxidation scheme (Scheme 12.5). 

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