Alkenes conversion

High alkylation activity that allows quantitative alkene conversions at adequate... 

Alkene conversion to epoxide (%) Epoxide selectivity (%) Epoxide conversion (%) Cyclic carbonate selectivity (%)... 

The treatment leads to a significant improvement in alkene conversion in cyclohexene epoxidation in the case of Ti-MCM-41 and Ti-MCM-48 (273). Although epoxide selectivity improved in the former case, there was a decrease in the latter. In the case of hexane oxidation, silylation did not improve the conversion. An enhancement in the number of turnovers and selectivity for the epoxide on silylation was also observed in the cyclohexene epoxidation with TBHP catalyzed by Ti-SBA-15 (Table LII) (274). Ti-SBA-15 was claimed to be thermally more stable than Ti-MCM-41. Ti leaching was absent. 

In the case of manganese porphyrin catalyzed epoxidations, the axial ligands have been used alone or together with other additives like carboxylic acids (Banfi and coworkers) and soluble bases (Johnstone and coworkers). For example, Mansny and coworkers showed that in the presence of imidazole, 2-methylimidazole or 4-imidazole chloromanganese(tetra-2,6-dichlorophenylporphyrin) catalyzes the epoxidation of varions aUtenes including 1-alkenes by Under these conditions alkene conversion... 

Natural bite angle, isomeric ratio of LRh(CO)2H, normal ove iso ratio, normaZ-aldehyde product, isomerized product 2-octene, and turnover frequency were determined at 20% alkene conversion. 

Turnover frequency in molmol h was determined at 40% alkene conversion. 

In the two-stage process the alkene is reacted with the catalyst system (100-110°C, 10 atm). The reduced catalyst solution containing Cu2Cl2 is reoxidized with air in a second reactor under the same conditions. In both cases about 95% oxo yield is achieved at 95-99% alkene conversion. Because of the explosion hazard of mixing ethylene or propylene with pure oxygen, most commercial operations favor the less hazardous two-stage process. 

When the enthalpies of reaction between branched ketones and the corresponding 1,1-disubstituted alkenes are calculated using the multiple enthalpies of formation available for the latter, the following ranges are obtained Me/i-Pr, 196.6 to 200.5 Et/i-Pr, 201.2 to 206.6 and Me/t-Bu, 200.5 to 205.1 kJmol-1. Perhaps it is reasonable to conclude that the reaction enthalpies for the branched compounds either will be approximately constant, as for the unbranched ketone/alkene conversions, or will be more endothermic with branching, as in the branched aldehyde/alkene conversions. In either case, the least endothermic reaction enthalpy for the Me/i-Pr conversion above seems inconsistent and therefore the enthalpies of formation for 2,3-dimethyl-l-butene from References 16 or 26, which are essentially identical, should be selected. These enthalpies were also selected in a previous section. However, there is too much inconstancy, as well as too much uncertainty, in the replacement reactions of carbonyls and olefins to be more definitive in our conclusions. 

Temperature (°C) [C6H6]/[Alkene] Contact Time (s) Alkene Conversion (%)... 

Entry Alkene Conversion (%) Contribution of cleavage route (%) Product (composition in mol.%)... 

While not extensively explored, vinyl acetates have also been reduced to the corresponding alkenes with iron pentacarbonyl at elevated temperatures (Scheme 34). In addition, vinyl acetates have been subjected to hydroboration, but elimination of the boronate ester in the intermediate and subsequent hy-droboration of the resulting alkene are so rapid that this method is not synthetically useful for ketone to alkene conversion. 

The fust important test of this methodology came in Hanessian s investigation of the spiroketal portion of avermectin Bu- This highly convergent approach incorporates all the oxidation levels and functionality required for carbons C(15)-C(28), except for the necessity of alkyne to alkene conversion. The lithium alkynide was prepared at -78 C and then mixed with boron trifluoride etherate under the conditions of Yamaguchi (Scheme 19). (Direct condensation of the lithium salt and lactone lead to substantial amounts of a, -unsaturated lactone.) Addition of the lactone in stoichiometric amounts to the solution of the modified alkynide led to the formation of the desired hemiketal in acceptable yield. Further improvements could be obtained by the recycling of starting material. ... 

When 8% loss of alkene conversion was used as the criterion for comparison, the regenerated swing reactors combined for 23 fold increases in productive time on stream, for the 2h/2h tests, versus the non-regenerated control case. The cumulative C5+ and TMP yields were increased by 24 fold and 22 fold, respectively. The C5+ and TMP productivities were essentially unchanged versus the control, with slight increases for Cs+ and slight decreases for TMPs. 

Finally, we address the question of the reasons for the superiority of Co(II) compounds as the catalysts for the alkene epoxidation by 02/aldehyde system. We have studied the catalytic activity of Co(II) compounds using imKS-stilbene as a model substrate. Kinetic curves for the Iran -stilbene epoxide accumulation are given in Fig. 3. The kinetic curves show autocatalytic character. The time of the complete alkene conversion depends on both the induction time and the rate of the reaction after the completion of the induction period. It should be noted that the induction time increases considerably with decreasing aldehyde concentration and goes through a maximum with increasing Co(II) concentration (in the range 10 - 10 M). One can see from Fig. 3 that the rate of the epoxidation lowers in the order PW1 iCo, CoW 12 > CoNaY, Co(N03)2 >CoPc. 

The synthesis of such eight-membered rings by the nickel-catalysed dimerisation of butadienes, is used here with 2-methylbutadiene. The regio-selectivity and stereo-specificity of the hydro-boration of symmetrical 124 are as expected for a trisubstituted alkene. Conversion of the alcohol 125 into a good leaving group 126 made it reactive enough to carry out electrophilic addition on the other alkene. In aqueous solution, the final nucleophile was water and the product a mixture of diastereoisomers of the tertiary alcohol 127. 

Reaction conditions alkene 0.30 mmol, isobutyraldehyde 2.28 mmol, CoNaY 35 mg, solvent 3 ml, air 1 atm, 24°C. Determined by H NMR. For another sample of 3 the time of 100% alkene conversion was 9 h. Only monoepoxide is formed, 4p,5a-/4a,5P-epoxide=85 15. Mono-/diepoxide=2 l. l,2-/8,9-epoxide=24 l, tra s-/c -epoxide=47 26 (trans-lcis- indicate the orientation of the epoxide oxygen relative to isopropenyl group in the 6-membered ring), Mono-/diepoxide=l 3 1. 

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