Reactivities of terminal alkene

The rate also decreases with an increase in the chain length of the alkene molecule (hex-l-ene > oct-1-ene > dodec-l-ene). Although the latter phenomenon is attributed mainly to diffusion constraints for longer molecules in the MFI pores, the former (enhanced reactivity of terminal alkenes) is interesting, especially because the reactivity in epoxidations by organometallic complexes in solution is usually determined by the electron density at the double bond, which increases with alkyl substitution. On this basis, hex-3-ene and hex-2-ene would be expected to be more reactive than the terminal alkene hex-l-ene. The reverse sequence shown in Table XIV is a consequence of the steric hindrance in the neighborhood of the double bond, which hinders adsorption on the electrophilic oxo-titanium species on the surface. This observation highlights the fact that in reactions catalyzed by solids, adsorption constraints are superimposed on the inherent reactivity features of the chemical reaction as well as the diffiisional constraints. 

Figure 10 Reactivities of terminal alkenes toward diarylcarbenium ions (-70° C, CH2CI2, reference reaction Aryl2CH+ + 2-methyl-l-pentene). (Reprinted with permission from Ref. 128. Copyright 1990 American Chemical Society.)...

The proximity of the diffusion limit also inhibits a detailed discussion of the data in Table 7, but a significant difference to the substituent effects discussed in Section III.D.4 is obvious. Whereas the reactivities of terminal alkenes, dienes, and styrenes toward AnPhCH correlate with the stabilities of the new carbenium ions and not with the ionization potentials of the 7r-nucleophiles [69], the situation is different for the reactions of enol ethers with (p-ClC6H4)2CH+ [136]. In this reaction series, methyl groups at the position of electrophilic attack activate the enol ether double bonds more than methyl groups at the new carbocationic center, i.e., the relative activation free enthalpies are not controlled any longer by the stabilities of the intermediate carbocations but by the ionization potentials of the enol ethers (Fig. 20). An interpretation of the correlation in Fig. 20 has not yet been given, but one can alternatively discuss early transition states which are controlled by frontier orbital interactions or the involvement of outer sphere electron transfer processes [220]. 

The reactivity of different alkenes toward mercuration spans a considerable range and is governed by a combination of steric and electronic factors.24 Terminal double bonds are more reactive than internal ones. Disubstituted terminal alkenes, however, are more reactive than monosubstituted cases, as would be expected for electrophilic attack. (See Part A, Table 5.6 for comparative rate data.) The differences in relative reactivities are large enough that selectivity can be achieved with certain dienes. 

Wilkinson s catalyst brings about the hydrosilylation of a range of terminal alkenes (1-octene, trimethylvinylsilane) by 2-dimethylsilylpyridine with good regioselectivity for the anti-Markovnikoff product. Both 3-dimethylsilylpyridine and dimethylphenylsilane are less reactive sources of Si-H. In contrast, these two substrates are far more reactive than 2-dimethylsilylpyridine for the hydrosilylation of alkynes by [Pt(CH2 = CHSiMe2)20]/PR3 (R = Ph, Bu ). This difference was explained to be due to the operation of the two different pathways for Si-H addition—the standard Chalk-Harrod pathway with platinum and the modified Chalk-Harrod pathway with rhodium.108... 

A modification of an earlier procedure for debromination of v/c-dibromides in the presence of catalytic amounts of diorganotellurides has allowed the synthesis of terminal alkenes and cis- and frani-l,2-disubstituted alkenes from appropriate precursors the relative substrate reactivities suggest that, as for the stoichiometric reaction, the catalytic reaction involves intermediate bromonium ion formation. The Te(IV) dibromides formed in the debrominative elimination are reduced back to the catalysts by either sodium ascorbate or the thiol glutathione. 

Pd(H) fcrf-butyl peroxidic complexes of the general formula [RCOOPdOOferf-Bu]4 are very efficient reagents for the selective stoichiometric oxidation of terminal alkenes in anhydrous nonbasic organic solvents at ambient temperature.518 They also catalyze the ketonization by tert-BuOOH. Internal olefins are not reactive at all. A peroxometallacycle intermediate (see discussion below) was suggested. 

A somewhat similar oxidation of terminal alkenes to methyl ketone and alcohol by 02 in the presence of Co(salMDPT) [salMDPT = bis(salicylideneiminopropyl)methylamine] and in ethanol solvent has recently been reported by Drago and coworkers (equation 244).560 Only terminal alkenes were found to be reactive with this catalytic system. The reaction is alcohol dependent and occurs in ethanol and methanol but not in t-butyl or isopropyl alcohols. The alcohol is concomitantly oxidized during the reaction, and may act as a coreducing agent and/or favor the formation of cobalt hydride. This oxidation might occur according to the mechanism of equation (243). 

Despite the obvious advantages the catalytic methods suffer from some limitations. A considerably lesser degree of regioselectivity was observed in the oxyamination reactions of terminal alkenes and asymmetrically disubstituted alkenes, such as 1-phenyl-1-propenes, with respect to the comparable results from stoichiometric reactions (cf. Tabic 5 and 6). Furthermore, reduced reactivity was observed, In fact, neither method (A or B) was successful with tetramethylethylene, cholesteryl acetate, diethyl ( )-2-butenedioate, 2-cyclohexenone, 1-acetoxycyclohexene or 1-phenylcyclohexene73. 

The group of Strukul [134,175] has developed a class of electron-poor Pt(II) complexes which are efficient catalysts for the epoxidation of terminal alkenes with H2O2 (Table 1.6). The complexes have general structure [(P-P)Pt(CfTt)(H2O)] [X] where (P-P) is a diphosphine and X is BF4 or OTf. Kinetic studies showed that the complexes owed their reactivity to their ability to increase the nucleophilicity of the olefin by coordination, thereby changing the traditional electrophile/ nucleophile roles of the system [176] (see Chapter 2). 

Catalyst 33a was subsequently used to epoxidize several other olefins. Again, the reactivity of the catalyst at (5 mol%) was good, but a wide range of ees was observed (Table 5.6). l-Phenyl-3,4-dihydronaphthalene was epoxidized with high enantioselectivity (95% ee and 66% yield after 35 min). 4-Phenylstyrene oxide was produced with 29% ee, one of the highest reported ees for the epoxidation of terminal alkenes using iminium salt catalysis. 

Erker and co-workers reported in 1990 that in the presence of the chiral Zr complex 82, shown in Eq. 6.16, 1-naphthol adds to ethyl pyruvate with an appreciable level of enantio-selectivity [81]. Higher optical purities were reported at lower temperatures, and interestingly, as later reported by Wipf for Zr-catalyzed carboaluminations of terminal alkenes (Scheme 6.14), addition of water leads to improvements in selectivity and reactivity [82],... 

The reactivity of the alkenes toward LiAlH -TiCl decreases in the series CH2=CHR > CH2=CR2 > RCH=CHR. Thus dienes can be hydroaluminated selectively at the terminal double bond , e.g. ... 

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