Elimination compared with alkene isomerization

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

The full paper on the conversion of 1,2-diols into alkenes by reduction-elimination of appropriate 1,3,2-dioxaphospholans has appeared (see Organophosphorus Chemistry , Vol. 8, p. 123). 2 method was examined with particular regard to the conversion of cyclodecane- and cyclododecane-1,2-diols into the cycloalkenes, as mixtures of cis- and tmns-isomers, and consists of the treatment of the cyclic ethyl esters or AW-dimethylamides with lithium in liquid ammonia or with Ti-THF. The kinetics of the methanolysis of thiophosphoryl chlorides have been studied. The rather unusual isomerization of 2-hydroxyethyl phosphorothioates (70) yields the 2-mercaptoethyl isomers after several days at room temperature instead of the expected thiono-esters. This observation should be compared with those reported in ref. 83. 

Photocatalysis of alkene isomerizations and related processes has also been noted for a number of other common carbonyls including Fe(CO)s, Fe(CO)4PPh3, Mo(CO)6, W(C0)6, Ru(CO)4PPh3 and Ru3(CO)i2 [61,74,79-82], In some cases quantum yields substantially in excess of unity have been reported consistent with the photolytic formation of a catalytically active species capable of numerous turnovers. The photocatalysis of 1-pentene hydrogenation by Fe(CO)s is accompanied by isomerization of the substrate to 2-pentene [57], Such a pathway could occur via a -hydride elimination pathway comparable to the reverse of step 2 in Scheme I, but a more likely pathway would be via a C-H insertion reaction of the intermediate Fe(CO)3(alkene) to give an alkyl hydride intermediate as in eq. 20. 

It has long been known that thiyl radicals add reversibly to double bonds (cf. Scheme 1) [17]. The (Z)-( ) interconversion of olefins by the addition-elimination sequence of thiyl radicals [18] is now an established methodology in chemical synthesis [19] and has been applied successfully as the key step in the synthesis of elaborate molecules such as the antifungal macrocyclic lactone (-)-gloeosporone [20a] and the antibiotic-antitumor agent (+)-hitachimycin [20b]. The E/Z ratio after equilibration generally reflects the thermodynamic stability of (Z)- and ( )-alkenes. It has recently been shown that equilibrium Z/E-18/82) for ( )- and (Z)-hexen-l-ol is reached with PhS radical in 1 h at 80°C [21]. Comparatively, the same isomeric composition is reached in 4h and lOh with Bu3Sn and (TMS)3Si respectively under similar conditions. 

Analysis Weigh the distillate and determine the yield of products. Test the distillate for unsaturation using the bromine and Baeyer tests (Secs. 25.8A and B. respectively). Analyze your distillate by GLC or submit a sample of it for analysis. After obtaining the results, calculate the relative percentages of the isomeric alkenes formed assume that the response factors are identical for the isomers. A typical GLC trace of the alkenes from this elimination is shown in Figure 10.10. Obtain IR and NMR spectra of your starting material and compare your spectra with those of an authentic sample (Figs. 10.11 and 10.12). 

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