Carbonium reactions with ethylene

Reaction of the ierf-butyl carbonium ion with octene (ethylene tetramer), for example, will produce isobutane and an unsaturated carbonium ion which may form a diene by loss of a proton or which may cyclize to yield an ethylcyclohexyl carbonium ion ... 

Catalytic activity measurements and correlations with surface acidity have been obtained by numerous investigators. The reactions studied most frequently are cracking of cumene or normal paraffins and isomerization reactions both types of reactions proceed by carbonium ion mechanisms. Venuto et al. (219) investigated alkylation reactions over rare earth ion-exchanged X zeolite catalysts (REX). On the basis of product distributions, patterns of substrate reactivity, and deuterium tracer experiments, they concluded that zeolite-catalyzed alkylation proceeded via carbonium ion mechanisms. The reactions that occurred over REX catalysts such as alkylation of benzene/phenol with ethylene, isomerization of o-xylene, and isomerization of paraffins, resulted in product distribu-... 

Mechanistically, two pathways are logical (Scheme 3). The ethyl cation can directly alkylate methane via a pentacoordinated carbonium ion (Olah) (path a), or alternatively, although a less favorable pothway (b), the ethyl cation could abstract a hydride ion from methane. The methyl cation thus formed, which is less stable by 39 kcal/mole (26), could then react directly with ethylene. In the latter case, propylene and/or polymeric material would probably be formed since the hydrogen required for a catalytic reaction has been consumed by the formation of ethane. 

Isomerization from the secondary to the more stable tertiary carbonium ion precedes reaction with the next molecule of monomer. A 1,3 polymerization has therefore occurred. Similarly, 4-methyl-l-pentene gives a 1,4 polymer (probably involving two successive 1,2 hydride shifts) that has a structure corresponding to an ethylene-isobutylene copolymer (Reaction 25). 

Let us look at some of the evidence for this mechanism. If a carbonium ion is the intermediate, we might expect it to react with almost any negative ion or basic molecule that we care to provide. For example, the carbonium ion formed in the reaction between ethylene and bromine should be able to react not only with bromide ion but also— if these are present—with chloride ion, iodide ion, nitrate ion, or water. 

From a mechanistic point of view, two different ionic mechanisms have to be considered (due to the presence of oxygen the radical chain mechanism plays no role in the technical process) first, the uncatalyzed reaction of ethylene and chlorine and second, the metal halide catalyzed reaction. Both routes compete in this process. The uncatalyzed halogenation was studied extensively for the bromina-tion of olefins [14, 15] (Scheme 4). It is commonly accepted that the halogenation of olefins starts with formation of a 1 1 -complex of halogen and alkene followed by formation of a bromonium ion. Subsequent nucleophilic attack of a bromine anion leads to the dibromoalkane. However, when highly hindered olefins (such as tetraneopentylethylene) are used, formation of a 2 1 r-complex, as an intermediate between 1 1 ir-complex and a bromonium ion, is detectable by UV spectroscopy. In the catalyzed reaction the metal halide polarizes the chlorine bond, thus leading to formation of a chloronium or carbonium ion. Subsequent nucleophilic attack of a chloride anion gives the dichloroalkane [12] (Scheme 5). 

Taylor attempted the reaction of MA with ethylene at 25 C. An excess of aluminum chloride was needed. Interestingly, the reaction mixture absorbed two moles of ethylene before slowing down. A 15% yield of the unsaturated keto acid 165 was obtained. Isobutylene residue in the latter may reflect isomerization of a carbonium intermediate. No by-products were explored but if dimerization precedes acylation, contributions by ene reaction and polymerizations cannot be ruled out. 

Muscarine derivatives were the target of another synthesis, which starts off with a simple aldol condensation between (65) and (66). A novel method of dihydrofuran formation was achieved by reaction of the dienone derivative (67) with ethylene glycol and p-toluenesulphonic acid to give (68). ° The success of the reaction, which would appear to contravene Baldwin s Rules, was attributed to the formation of the delocalized carbonium ion (69). 

According to this mechanism, the reaction rate is proportional to the concentration of hydronium ion and is independent of the associated anion, ie, rate = / [CH3Hg][H3 0 ]. However, the acid anion may play a marked role in hydration rate, eg, phosphomolybdate and phosphotungstate anions exhibit hydration rates two or three times that of sulfate or phosphate (78). Association of the polyacid anion with the propyl carbonium ion is suggested. Protonation of propylene occurs more readily than that of ethylene as a result of the formation of a more stable secondary carbonium ion. Thus higher conversions are achieved in propylene hydration. 

In normal carbonium-ion chemistry, reaction proceeds from a precursor with a tetrahedral carbon capable of asymmetry hence, the stereochemistry of displacement in an aliphatic system can be ascertained by observation of the fate of the chiral center from reactant to product. An ethylenic system, of course, has no such chiral center, and hence there can be no change in optical configuration as the reaction proceeds. However, the stereochemistry of vinylic displacement and hence the symmetry and geometry of the intermediate can be... 

Detailed discussion of the energetics of the cationic polymerisation of ethylene shows that it cannot yield linear polymethylenes, in agreement with experiment. Since the cationic polymerisation of diazomethane does yield linear polymethylenes it cannot proceed through carbonium ions, as suggested by other workers. The following equations summarise the mechanism proposed here for this reaction ... 

If this mechanism is correct, the aconitase reaction is an excellent illustration of the influence of the stereochemistry of the metal, as well as its charge, upon the course of a biochemical reaction. The charge on the iron is, of course, responsible for the formation of the resonating carbonium ions A and B from C, D, or E. In C and D the flow of electrons toward iron severs the bond between carbon and the hydroxyl group, whereas in E the proton is released from coordinated water and attached to one of the two ethylenic carbon atoms. The stereochemistry of the iron atom can be credited with holding the organic molecule and the hydroxide in their proper spatial relationship in A and B. It has been recently demonstrated that the complexes of the aconitase substrates with nickel have the structures postulated by Speyer and Dickman and shown in Figure 3 (19). 

Tphe excellent catalytic activity of lanthanum exchanged faujasite zeo-A lites in reactions involving carbonium ions has been reported previously (1—10). Studies deal with isomerization (o-xylene (1), 1-methy 1-2-ethylbenzene (2)), alkylation (ethylene-benzene (3) propylene-benzene (4), propylene-toluene (5)), and cracking reactions (n-butane (5), n-hexane, n-heptane, ethylbenzene (6), cumene (7, 8, 10)). The catalytic activity of LaY zeolites is equivalent to that of HY zeolites (5 7). The stability of activity for LaY was studied after thermal treatment up to 750° C. However, discrepancies arise in the determination of the optimal temperatures of pretreatment. For the same kind of reaction (alkylation), the activity increases (4), remains constant (5), or decreases (3) with increasing temperatures. These results may be attributed to experimental conditions (5) and to differences in the nature of the active sites involved. Other factors, such as the introduction of cations (11) and rehydration treatments (6), may influence the catalytic activity. Water vapor effects are easily... 

Propane as a degradation product of polyethylene (a byproduct in the reaction) was ruled out because ethylene alone under the same conditions does not give any propane. Under similar conditions but under hydrogen pressure, polyethylene reacts quantitatively to form C3 to C6 alkanes, 85% of which are isobutane and isopentane. These results further substantiate the direct alkane alkylation reaction and the intermediacy of the pentacoordinate carbonium ion. Siskin also found that when ethylene was allowed to react with ethane in a flow system, n-butane was obtained as the sole product, indicating that the ethyl cation is alkylating the primary C—H bond through a five-coordinate carbonium ion [Eq. (5.66)]. 

The problem of distinguishing between carbenoid and carbonium ion mechanisms of decomposition of diazoalkanes in protic media arises also in interpreting the base-induced decomposition of tosylhydrazones. In the original procedure for this widely-used reaction (W. R. Bamford and Stevens, 1952), the tosylhydrazone of a carbonyl compound is treated with the sodium salt of ethylene glycol in refluxing glycol. A mixture of olefins and alkoxyethanol is produced (equation 6). Many... 

In on attempt to generate primary (trivalent) cations and to simulate the ethylene-methane alkylation, ethyl chloride was reacted with methane (eq. 3) under alkylation reaction conditions (28). When no propane or propylene product was observed, the energetically more favorable reaction of methyl chloride with ethane was carried out (eq. 3a). These two reactions proceeded without any involvement of the alkane and provide evidence that the ethylene-methane alkylation proceeds through a more stabilized species such as a pentacoordinoted carbonium ion. The behavior of these alkyl chlorides will be discussed separately after the alkylation chemistry. 

Oxidmion of ketone acetals and ethers. Ketones can be regenerated from the ethylene acetal derivatives by treatment with trityl fluoroborate in dry dichloro-methanc (Nj) at room temperature. Thus the reaction of trityl fluoroborate with cyclohexanone ethylene acetal results in cyclohexanone (80%) and triphenylmethane. The reaction thus involves hydride transfer to the triphenyl carbonium ions. Triethyl-oxonium fluoroborate can also be used but is somewhat less effective than trityl fluoroborate. 

Similarly, Siskin " found that when ethylene was allowed to react with ethane in a flow system, only n-butane was obtained. This was explained by the direct alkylation of ethane by ethyl cation through a pentacoordinated carbonium ion (equation 127). The absence of a reaction between ethyl cation and ethylene was explained by the fact that no rearranged alkylated product (isobutane) was observed. 

Pinacol Rearrangement.12 Certainly one of the best-known examples of a carbonium ion rearrangement is the pinacol transformation in which polysubstituted ethylene glycols are converted into substituted ketones by the action of acidic reagents such as mineral acid, acetyl chloride, or acetic acid and iodine. In accordance with Whitmore s theory of carbonium ion rearrangements,11 the mechanism of the reaction can be outlined as follows, with pinacol itself as an example ... 

Reactions of alkenes with hypervalent iodine compounds lead mostly to vicinally functionalised alkanes. This is the case with PhI(OAc)2, PhIO, PhI(OH)OTs, PhI(OTf)0(TfO)IPh and other related reagents.230,231,233,239-247 poj. example, treatment of alkenes with PhI(OH)OTs, (HTIB), affords vie bis(tosyloxy)alkanes with a syn stereospecificity.239,241 n generally admitted that this reaction proceeds by the electrophilic attack of the hypervalent iodine species on the ethylenic double bond to afford a carbonium ion intermediate (140). This intermediate undergoes two consecutive Sn2 substitution reactions to eventually give the final products. (Scheme 5.17)... 

The carbonium ions are known to be the important intermediates in the reactions involving formation and breaking of the carbon-carbon bonds. In the case of a solid-acid catalyst, the Bronsted acid sites are considered to be the active sites for initiation of carbonium ion formation, which in turn lead to various reactions such as polymerization, alkylation and aromatization[5]. However, a comparision of the data in Fig.2 and Table 1 shows that the conversion of ethylene over ZSM-5 and HZSM-5 sanq)les is either unaffected or shows an increase while the concentration of the hydroxyl groups reduced to 25-40% with the increase in pretreatment tenq)erature fi om 300 to 700°C. Similarly, while the concentration of the... 

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