Olefin uptake

As a consequence, olefin uptake energies higher than 10 kcal/mol are required to form stable olefin complexes in the gas phase. Again, the picture is quite different in solution, since olefin coordination probably requires the displacement of a coordinated solvent molecule. The entropy loss due to the olefin coordination could be counterbalanced by the entropy gain due to the dissociation of a coordinated solvent molecule. In conclusion, it is reasonable to expect that coordination/dissociation of the olefin from the metallocene is a process with a low energy barrier and with low energy gain/loss. 

Site epimerization is estimated to be fast relative to olefin uptake and insertion. Thus, repetitive enchainment of one olefin enantiomer will occur, producing an isotactic block of poly(a-olefin) until a syndiotactic enchainment error occurs. When this happens, olefin insertion will be switched to the opposite side, and another block of isotactic poly(a-olefin) will be produced. Thus, the resulting polymer is predominantly isotactic with isolated r dyads stereoblocks of the two olefin enantiomers surround each isolated r dyad (Figure 4.23). 

Napamezole (68) is a dihydroimidazole derivative with antidepressant activity probably as a result of its combined a 2 adrenergic receptor blockuig and serotonin uptake blocking proper ties It can be synthesized by Wittig olefination of p-tetralone (65) with diethyl (cyanomethyl) phosphonate (66) and base to give nitnle 67 Imidazoline construction on the latter was smoothly... 

The free-radical reaction may be equally initiated by photoactivated sulfur dioxide (3S02)442 (equation 79). On the other hand, polysulfones are obtained by radical copolymerization of appropriate olefins with sulfur dioxide443-449, and similarly, uptake of sulfur dioxide by a radical-pair formed by nitrogen extrusion from an azo compound yields the corresponding sulfone450 (equation 80). Correspondingly, alkylbenzenes, dibenzoyl peroxide, and sulfur dioxide yield sulfones under thermal conditions451... 

The kinetics of epoxidation were measured in situ as the rate of uptake of olefin by the catalyst, since the epoxide product remains adsorbed on the catalyst surface in the absence of solvent. Addition of 2 Torr cyclohexene vapor (30 pmol) to a 14.2 mg sample of 3 (9.0 pmol Ti, 4.5 pmol V) at room temperature resulted in the loss of the o(C=C) mode in the gas phase IR spectrum over the... 

Information published from several sources about 1970 presented details on both the halide-containing RhCl(CO)(PPh3)2- and the hydride-containing HRh(CO)(PPh3)3-catalyzed reactions. Brown and Wilkinson (25) reported the relative rates of gas uptake for a number of different olefinic substrates, including both a- and internal olefins. These relative rates are listed in Table XV. 1-Alkenes and nonconjugated dienes such as 1,5-hexadiene reacted rapidly, whereas internal olefins such as 2-pentene or 2-heptene reacted more slowly by a factor of about 25. It should also be noted that substitution on the 2 carbon of 1-alkene (2-methyl-l-pentene) drastically lowered the rate of reaction. Steric considerations are very important in phosphine-modified rhodium catalysis. 

Another pertinent observation is the fact that the reaction proceeded twice as fast in -butyraldehyde (polar) as in benzene (nonpolar), even though the catalyst concentration was reduced to only one-third the comparable level. A graphic illustration of this effect is given in Fig. 9. The rate of gas uptake is plotted as a function of time for a reaction conducted in benzene and again for a second reaction conducted in butyraldehyde. The rate of reaction in the polar solvent was initially fast and decreased with time. The rate in the nonpolar benzene was initially slow, became faster as the solvent became more polar with the presence of product aldehyde, and then subsequently diminished with time. When the data were replotted as the log of unreacted olefin vs. time, the polar medium reaction showed first-order dependence on olefin concentration, whereas the nonpolar solvent reaction showed no definite order, owing to the constantly changing polarity. 

This stearolic acid has been thoroughly characterized 3 6 by the freezing-point curve, ultraviolet and infrared spectra, ozonization, and hydrogenation. It has been shown to be free both of positional isomers and of olefinic acids such as oleic and elaidic acids. Its properties include m.p. 46-46.5°, iodine number (Wijs titration, 30 minutes) 89.5, d 5 1.4510, d 5 1.4484, neutral equivalent 279.2-279.6 (theory 280.4), hydrogen uptake 95-100% of theory for a triple bond. The last trace of color is difficult to remove by recrystallization from petroleum ether. It can be removed, however, by crystallization from a 20-30% solution in acetone at —5 to —8°, or from an 8-10% solution at —20°, or by distillation (b.p. 189-190°/2mm.). 

A quantitative model requires knowledge of the diffusivity under reaction conditions and of the intrinsic activities for toluene disproportionation and xylene isomerization. While these are not easily obtained, the methodology has been worked out for the case of paraffin and olefin cracking (5). So far, we have obtained an approximate value for the diffusivity, D, of o-xylene at operation conditions from the rate of sorptive o-xylene uptake at lower temperature and extrapolation to 482°C (Table V). 

Finally, the uptake energy values only represent a contribution to the total free energy of coordination. In fact, an always unfavorable uptake entropy has to be accounted for. Although few experimental data are available, it is reasonable to assume that the -FAScontribution to the free energy of olefin coordination to group 4 metallocenes at room temperature is close to the 10 kcal/mol value observed at 300 K for Ni and Pd compounds [59], The few... 

The reduction of some polyenes is affected by the double bond migration, e.g. when a tetrasubstituted olefin is formed, since it is hydrogenated with difficulty. For example, the reduction of the second double bond was fast, but the reduction ceased after the uptake of two moles of hydrogen (equation 51)136. 

Oxidation of Elaidic Acid Ozonization Products. Aliquots of the unseparated ozonization products from elaidic acid were autoxidized at 95 °C. uncatalyzed and in acetone over reduced platinum oxide as before. Total yields of acids and esters were determined by titration and were found to be 74.6 and 19.2%, respectively, in the catalyzed reaction with uptake of 63% of the theoretical volume of oxygen. Time required for uptake of half this volume was 4 hours at 21 °C. Uncatalyzed oxidation at 95°C. of the other fraction gave 27.4% yield of esters and 74.5% yield of acids, calculated on the assumption that one original olefinic linkage can produce one ester function or two acid functions. When elaidic acid was ozonized in methyl acetate and the catalyzed oxidation performed in the same solvent, acid yield was 80.8%, and ester yield was 7.3% with a half-uptake time of 5.6 hours and 88% of the theoretical quantity of... 

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