Olefins reactivity

Fig. 2. Dependence of olefin reactivity on its carbon atom number when linear a-olefins are copolymerized with ethylene.

Some notable reversals in the order of olefin reactivities were observed in contrast to the tantalum carbenes. Further, in comparing the reactivities of (CO)5W=CPh2 and (CO)5W=CHPh, Casey (70) noted additional striking contrasts The monophenyl carbene complex reacted rapidly at... 

If the nucleophilic site (HOMO) involves a nonbonded pair of electrons (path a), a stable covalently bonded complex will form. If the HOMO is a a bond, direct reaction is unlikely unless the bond is high in energy and sterically exposed, as in a three-membered ring, but if the bond is to H, hydride abstraction may occur (path b, steps 1 and 2) or a hydride bridge may form (path 6, step 1). The last two possibilities are discussed further in Chapter 10. If the HOMO is a n bond, a n complex may result (path c, step 1), or, more commonly, donation of the n electrons results in the formation of a a bond at the end where the n electron density was higher, the other end becoming Lewis acidic in the process (path c, steps 1 and 2). The effects of substituents on olefin reactivity were discussed in Chapter 6. 

If a pathway which involved metal ion activation followed by oxygen transfer in the coordination spere (Reaction 10) were operative, olefin reactivity would be expected to parallel the coordinative ability of the... 

It is clear that the mechanism in Scheme 25 parallels (at least from the qualitative point of view) the mechanism of the addition of bromine to olefins shown in Scheme 11. Kinetic investigations indicate that the oxymercuration reaction involves a series of fast equilibria until the mercuronium ion (53) is formed. The subsequent nucleophilic attack of the solvent is probably the rate-limiting step, as indicated by steric requirements in bulky alkenes111. In the bromine addition, the formation of the bromonium ion is the rate-limiting step (or the rate-limiting equilibrium). However, the olefin reactivities in both reactions (bromination and oxymercuration) are identical when steric effects in the TS of the two addition reactions are taken into account110. 

In general, the olefin reactivity increases with electronic density. Dichlorocarbene, for example, possesses electrophilic properties very similar to Br. but steiic effects play a very important role ... 

Readsorption steps (J3r,n) decrease the total termination probability (/3t, ) by reversing the olefin termination step. Readsorption can be enhanced by increasing the olefin reactivity (i.e., /3r, the ratio of readsorption to propagation rate constants), or by increasing olefin concentrations within pellets and reactors. 

In general it has been postulated that the substitution of an electron-donating group enhances the olefin reactivity toward carbenes and that substitution by electron-withdrawing groups reduces this reactivity (equation 30), for instance enol ethers (124)... 

The results discussed above for Cp2Zr(R)(THF)+ complexes are significant in that they represent the first cases in which simple group 4 metallocene alkyl complexes react directly with olefins. Much higher olefin reactiv-... 

The rings show aromatic rather than olefinic reactivity the existence of only three acetyl ethyl ferrocenes proves that the rings can rotate freely. Oxidation produces the blue cation (C5H5)2Fe. ... 

Properties of the metal alkyl usually determine whether olefin insertion occurs, but trends are observed with various olefin substituents. For alkyl-substituted olefins, the rate of insertion parallels the coordinating ability of the olefin (C2H4 > CH2CHR > CH2=CR2 > RCH=CHR) smaller olefins react most rapidly Electron-withdrawing substituents enhance olefin reactivity toward insertion for transition metal systems. Since the activated olefins also exhibit better coordinating ability than unactivated ones, it is not clear whether the effect arises from enhanced precoordination of olefin or accelerated migratory insertion. For aluminum alkyls. 

A comparison of olefin reactivities toward CeHjHgCBrCt in benzene at 80° with the reactivities of the same olefins toward sodium trichloroacetate in 1,2-dimethoxy-ethane at 80° established near identity of the relative reactivities toward both reagents, a result which favors the interpretation that both reactions involve free dichlorocarbene as an intermediate. Of practical significance is the fact that yields are consistently higher by the mercurial route. Thus the latter route proved effective as applied to olefins of low reactivity toward dihalocarbenes generated by other procedures. Examples are formulated ... 

Initially, there were two limiting factors in the utility of metathesis selective formation of a desired olefinic product from the mixture of possible olefins and formation of that product as a single cis or trans stereoisomer. A model for olefin reactivity has allowed for selective formation of a single product in a number of cases... 

Olefins. Olefins are the most reactive class of hydrocarbons in photochemical smog and have been studied extensively (I, 17, 18, 19). In general, as was perhaps first noted by Schuck and Doyle (20), the mechanism for olefin decomposition apparently involves electrophilic attack (by atomic oxygen, ozone, and other species) on the double bond. Thus, for most of the chemical reactions related to smog formation, olefin reactivity generally increases with additional alkyl (or other electron-donating) groups attached to the two carbon atoms joined by the double bond. 

Thus, the differences between polymer and model olefin reactivities are small. This is reasonable because the small size of the singlet oxygen molecule should allow it to interact with the olefin groups in a polymer without appreciable steric interference from the rest of the polymer chain. 

A comparison of olefin reactivities in oxidation over bismuth molybdate at 460° to those observed in aUyl hydrogen abstraction in solution at 65° by methyl radicals (140) is shown in Fig. 4. Considering the differences in phase and temperature, the correlation is quite good. The triangular points that are significantly off the line are for cis-and trons-2-butenes. An equally good correlation line was also obtained for comparison of oxidation with abstraction by fert-butoxy (141), again with the exception of three internal olefins, which seem to have abnormally low reactivity in the catalytic oxidation. 

A more quantitative treatment of the olefin reactivity data can be... 

A few remarks about the selectivity data of Table X are in order here. These are understandable in terms of the mechanism and the relative olefin reactivities. With the possible exception of 2-methyl-1-propene, initial reaction of the olefins appears to be very selective. 

Fig. S. Olefin reactivities for oxidation over bismuth molybdate at 460°.

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