Relative reactivities of olefins

Relative Reactivities of Olefins towards Carbenoids Generated in Different Ways0... 

Table VII. Relative Reactivities of Olefins Towards Oxidizing Species, X.

The composition of ethylene-propylene copolymers allows the relative reactivities of olefins to be estimated. In the copolymerization of ethylene and propylene on titanium trichloride the reactivity ratio is 16 (at 75 °C) should be hi er in the homopolymerization of each component. This corresponds to the ratio ( 100 for... 

The reaction of a-olefins with (CO)5W[C(p-CgH4Me)2] supports the preferential formation of the ot,a -metallacycle (see Table 3). Only trace amounts of the olefins coming from the a,jS-substituted metallacycle are formed. Competition studies demonstrate that the relative reactivity of olefins toward metathesis is 1-pentene > 2-methylpropene > cis-2-butene > > 2-methyl-2-hexene. This stability pattern parallels the stability of 7c-olefin-metal complexes. 

Table 4. Reaction of Various Olefins with the Nonisolated (COIjWCfHPh), Organic Products, Stereochemistry and Relative Reactivity of Olefins...

The present author, over several years, has published a number of papers on the relative reactivity of olefins towards addition of HBr, using competition methods, and has derived some mechanisms without obtaining absolute rates [65, 66]. The reaction is a complex one. The initial radical is in an excited state, and is deactivated most effectively by... 

Table V. Relative Reactivities of Olefins by Product Analysis"...

Relative reactivities of olefins were determined by Adams (132) by feeding a mixture of the olefin to be tested with 1-butene. The rate of oxidation was found to be a strong function of the olefin structure, being inversely related to the strength of the allylic C—H bond. These results will be discussed in a later section. 

The relative reactivities of olefins with the peroxo Mo(VI) complexes are in the order 1-octene < styrene < 2-octene < cyclohexene 2-methylbutene-l < 2-methylbutene-2 < tetramethylethylene. Thus, the relative reactivities of olefins in reaction (253) are similar to the catalytic epoxidation of these substrates using hydroperoxides. It has been proposed [95] that the epoxidation of an olefin by [MoO(02)2(HMPA)j occurs in two steps, reversible coordination of olefin, equation (254), and irreversible decomposition of the olefin complex to give epoxide, equation (255). 

The relative reactivities of olefins with 9-BBN have been summarized in decreasing order (Table 5.2) [7]. 

Relative reactivity of olefins with bromophenylcarbene generated either (1) by irradiation of 3-bromo-3-phenyldiazirine or (2) from base-induced a-elimination of a,a-dibromotoluene in the presence and in the absence of 18-crown-6 showed that addition of 18-crown-6 (via the complex of carbene with KCl/ KO-t-butanolate) as well as photolysis of diazirine afforded the free carbene. a-Ehmination normally proceeds via carbenoids (Scheme 3). 

Purely parallel reactions are e.g. competitive reactions which are frequently carried out purposefully, with the aim of estimating relative reactivities of reactants these will be discussed elsewhere (Section IV.E). Several kinetic studies have been made of noncompetitive parallel reactions. The examples may be parallel formation of benzene and methylcyclo-pentane by simultaneous dehydrogenation and isomerization of cyclohexane on rhenium-paladium or on platinum catalysts on suitable supports (88, 89), parallel formation of mesityl oxide, acetone, and phorone from diacetone alcohol on an acidic ion exchanger (41), disproportionation of amines on alumina, accompanied by olefin-forming elimination (20), dehydrogenation of butane coupled with hydrogenation of ethylene or propylene on a chromia-alumina catalyst (24), or parallel formation of ethyl-, methylethyl-, and vinylethylbenzene from diethylbenzene on faujasite (89a). 

Radical additions lo double bonds are, in general, highly exothermic processes and rates increase with increasing temperature. The rcgiospccificity of addition to double bonds and the relative reactivity of various olefins towards radicals are also temperature dependent. Typically, specificity decreases with increasing temperature (the Reactivity-Selectivity Principle applies). However, a number of exceptions to this general rule have been reported. 8 63... 

The relative reactivity of cyclopentadiene and ds-dichloroethylene toward triplet cyclopentadiene was found to be greater than 20 1 while that for cyclopentadiene and trans-dichloroethylene is less than 5 1. Thus the trans isomer is about four times more reactive toward the triplet cyclopentadiene than the cis isomer. An interesting temperature dependence of the product distribution of this reaction has been reported (Table 10.8). The data in Table 10.8 indicate that the relative amount of 1,4 addition [products (39) and (40)] is much more sensitive to temperature than 1,2 addition [products (35)—(38)], especially for the trans-olefin. The data also indicate that some rotation about the CHC1-CHC1 bond occurs in intermediate radicals derived from both cis- and trans-dichloroethylene. However, rotational equilibrium is not established at ring closure since the ratios of ds-dichlorocyclobutanes... 

Excess MeCl2SiH added alowly to A at reflux gave 82% of compound I and 18% of II. One equivalent of MeCl2SiH was added to solutions of one equivalent of both A and B or A and C. In either case the recovered olefin was largely C. The relative yields of I and II from the competitive reactions would indicate relative reactivities of B/A/C of 1/0.07/0.02. B was apparently more reactive than A or C. This suggests that C formed from B and A after most of the II or I was made. If A and B formed C before hydrosilation, the three olefins should exhibit relative reactivities close to 1/1/1, and they should each have made the same mixture of adducts. 

In scrutinizing the various proposed reaction sequences in Eq. (26), one may classify the behavior of carbene complexes toward olefins according to four intimately related considerations (a) relative reactivities of various types of olefins (b) the polar nature of the metal-carbene bond (c) the option of prior coordination of olefin to the transition metal, or direct interaction with the carbene carbon and (d) steric factors, including effects arising from ligands on the transition metal as well as substituents on the olefinic and carbene carbons. Information related to these various influences is by no means exhaustive at this point. Consequently, some apparent contradictions exist which seem to cast doubt on the relevance of various model compound studies to conventional catalysis of the metathesis reaction, a process which unfortunately involves species which elude direct structural determination. 

Pampus and co-workers (65) established the relative reactivity of a series of olefins to be 1-butene > 2-butene > isobutylene. This order of reactivity has been confirmed by others, and exactly parallels the reported order of stability of transition metal (Rh) complexes with these olefins (66), thus clearly implicating precomplexation of the olefin with the transition metal prior to metathesis. On a limited scale, Schrock observed a similar order of reactivity for olefins in reactions with (175-C5H5 )TaCl2[=CH(CH3 )3 ], which is known to possess a nucleophilic car-bene carbon (64). This complex also provides the requisite empty coordination site needed for precomplexation. In that study, cyclopropanes or metathesis olefins were not observed as products. 

Once again, use of these donors as solvent may shift the reaction equilibrium towards the desired product. Since the reactivity of olefins is lower than that of carbonyl compounds, higher reaction temperatures are usually required to achieve acceptable TOFs, and then the relatively higher boiling hydrogen donating solvents mentioned above may be the best choice. 

The gas-phase reactivity of various terpenes has been measured. Stephens and Scott were the first to include two terpenes (pinene and a-phel-landrene) with their study of the relative reactivity of various hydrocar ns. Both monoterpenes showed the high reactivity predicted by their olefinic structure. Conversion of nitric oxide to nitrogen dioxide in e presence of isoprene is at a rate intermediate between those for ethylene and trans-2-butene, and Japar et al, reported rate constants for the a-pinene and terpinolene-ozone reactions. Grimsrud et a/. measured the rate con-... 

The reason for the low yield of hexene and high yield of nonene apparently lies in the relative reactivity of the 2-methyl-2-pentene and the original propene. In other words, the tertiary hexyl carbonium ion (XVII) adds to the olefinic double bond more rapidly than does the secondary propyl ion (XVI). 

Because the addition steps are generally fast and consequently exothermic chain steps, their transition states should occur early on the reaction coordinate and therefore resemble the starting alkene. This was recently confirmed by ab initio calculations for the attack at ethylene by methyl radicals and fluorene atoms. The relative stability of the adduct radicals therefore should have little influence on reacti-vity 2 ). The analysis of reactivity and regioselectivity for radical addition reactions, however, is even more complex, because polar effects seem to have an important influence. It has been known for some time that electronegative radicals X-prefer to react with ordinary alkenes while nucleophilic alkyl or acyl radicals rather attack electron deficient olefins e.g., cyano or carbonyl substituted olefins The best known example for this behavior is copolymerization This view was supported by different MO-calculation procedures and in particular by the successful FMO-treatment of the regioselectivity and relative reactivity of additions of radicals to a series of alkenes An excellent review of most of the more recent experimental data and their interpretation was published recently by Tedder and... 

TABLE 15.1 Relative reactivity of some olefins toward bromine in acetic acid at 24°Ceo... 

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