Alkenyl carbenium ions

Upon contacting 1.3 Torr propene enzene (1/6) mixture with HP at 453 K, fast formation of cumene and the disappearance of the OH bands (the active centres of reaction) were observed. After the first 20 minutes the development of cumene bands stopped, while the parallel formation of alkenyl carbenium ions increased slightly and continuously with reaction time. 

Fig. 9 shows the UV-VIS spectra of those IR wafers which were exposed to propene, benzene, cumene and 1/6 propene/benzene mixture at 453 K. From the comparison of these spectra, it is clearly seen that benzene gives no surface species detectable by UV-VIS spectroscopy (as observed in IR experiments). The spectrum of the sample with cumene shows a large shoulder at around 320 nm, characteristic of the alkenyl carbenium ions formed by decomposition of cumene. Two sharp bands below 300 nm and that at 410, 430 and 520 nm... 

Under adsorption of propene/benzene mixtures, slow and fast formation at 295 and 453 K of cumene occurs respectively, together with the generation of alkenyl carbenium ions. 

Spectroscopic results suggest that alkenyl carbenium ions, regarded as precursors of carbonaceous deposites are formed from the oligomeric surface species. 

The first step in double-bond isomerization (DBI) is chemisorption of I-butene on the catalytic surface. Transition state intermediates can be generated at the active sites allowing the H-addition and elimination reactions of DBI to proceed. Alkyl carbenium ions (I) form on Bronsted acid sites and alkenyl carbenium ions (II) form on Lewis acid sites. 

Table 22.1 lists three examples of cyclic alkenyl carbenium ions that live long enough in zeolites to be detected by NMR [6]. Obviously, alkoxide formation is not favored and the proton affinities of their parent hydrocarbon compounds are so large that they win the competition with the zeolite framework for the proton. 

The formation of alkenyl carbenium ions, characterized by an IR band at ca. 1510 cm", was successfully observed by adsorption of 1-methylcyclopentene, methylenecyclopentane and 1-methylcyclopentanol on zeolite Y. At temperatures as low as 150 K, the carbenium ions were formed soon after introducing the olefins. The characteristic band at ca. 1510 cm increases in intensity with the elevation of temperature and is stable up to 373 K. For MCPOH, the characteristic band appears at temperatures above 245 K after its dehydration. The formed species were assigned to be dimerized alkenyl carbenium ions by calibrating the acid sites of HY using 1-butene at 235 K. UV-vis spectroscopic studies confirm the formation of momoenylic and dienylic carbenium ions, with bands at 323 and 400 nm, respectively. 

This triad of bands is attributed to the ti-ti transitions of mono-, di- and trialkenyl carbenium ions. Basically, alkenyl carbenium ions can form by (i) protonation of dienes, (ii) hydride ion abstraction from olefins on Lewis acid... 

The capability for alkenyl ion formation from different hydrocarbons and their derivatives follows the same sequence as observed in superacid solutions dienes>olefins>alcohols>paraffms. Decreasing the Brpnsted and the overall acidity has the consequence of slowing down the formation of alkenyl carbenium ions. Thus, protonation and hydride ion abstraction are the triggering steps. 

The wavelengths of the band maximum characteristic for alkenyl carbenium ions formed in zeolites agree approximately with those calculated from the Sorensen equation... 

Within the scope of this review we shall only consider those compounds possessing one or more alkenyl functions susceptible to activation by electrc hilic attack. Included in this family is a vast array of monomers varying in basicity from ethylene, which is so resistant to protonation that the ethyl carbenium ion has hitherto eluded observations even under the most drastic conditions (see below), and which in fact is equally resistant to cationic polymerisation, to N-vinylcarbazole, whose susceptibility to this type of activation is so pronounced that it can be polymerised by almost any acidic initiator, however weak. We shall also deal with olefins which, because of steric hindrance, can only dimerise (e.g., 1,1-diphenylethylene) or cannot go beyond the stage of protonated or esterified monomeric species (e.g., 1,1-diphenylpropene). The interest of such model compounds is obvious they allow clean and detailed studies to be conducted on the kinetics and mechanism of the initiation steps and on the properties of the resulting products which simulate the active species in cationic polymerisation. The achievements and shortcomings of the latter studies will be discussed below. 

Since our present interest is the reaction of r-bonded systems, the carbon-carbon double bond of alkenyl monomers, with electrophiles to give classical carbenium ions (or esters) as end products, we will concentrate on these entities. Whether nonclassical carbonium ions ( TT-complexes ) are involved in these reactions as hi -energy transition states, is still uncertain, and this interesting aspect will be discussed specifically in later sections (see Chaps. Ill and IV). 

The mode of attack of stable carbenium ions on to alkenyl monomers has been proved in some cases to proceed via direct electrophilic addition. However, for certain monomers, e.g., styrene and a-methylstyrene, the mechanism of this reaction has not yet been established unambiguously. We believe that studies on model compounds such as substituted styrenes incapable of polymerising because of steric impediment could provide an answer to this important question. 

Acid sites catalyse reactions by forming carbocations. Alkenyl cations have been seen by NMR and allylic ones by IR. Simple secondary or tertiary carbenium ions have yet to be observed. There are many questions here. For example, why is isobutane formed aiid why is there an induction period in n-hexane cracking ... 

The "right half of the sesquiterpene (+)-p-selinene (as drawn below) includes (R)-(+)-limonene as a substructure. Retrosynthetic disconnection to (if)-(+)-limonene leads to the intermediate carbenium ions la and lb via 15-nor-ll-eudesmen-4-one (carbonyl alkenylation) and 15-nor-13-chloro-2-eudesmen-4-one (dehydrogenation, protective masking of the double bond in the side chain). These carbenium ions arise from (if)-9-chloro-p-menth-l-ene and the acylium ion Ic (synthone) originating from 3-butenoic acid as reagent (synthetic equivalent). (i )-/7-Menth-l-en-9-ol, on its part obtained by hydroboration and oxidation of (if)-(-l-)-limonene, turns out to be the precursor of the chloromenthene. 

Bamford-Stevens and Shapiro Reactions. The Bamford-Stevens reaction is used to obtain unsaturated compounds from tosylhydrazones. A base is required to generate its monoanion, which thermally decomposes to yield the corresponding di-azo derivatives. These reactive species evolve to give an aUcene through carbenium ions in protic solvents or carbenes in aprotic solvents. The thermal decomposition of the monoanions of trisyl-hydrazones is commonly used to obtain diazoalkanes for different applications such as functionalization of solid supports, epox-idation and alkenylation of aldehydes, or the study of radicals and carbenes The functionalization of a Merrifield resin with... 

The parameters E and N can be used to compare the reactivity of compounds and even to predict the possibility that a reaction occurs. The parameter E represents the electrophiUcity parameter, which is defined by the use of a reference electrophilic compound. The values of E for many carbenium ions have been determined [6]. Contrary to common sense, which considers the carbenium ion to be intractable, it is possible to classify several carbenium ions that differ in elec-trophihdty by more than 16 orders of magnitude. Their stability is strictly connected to the presence of aryl, alkenyl, or alkynyl groups directly connected to the carbenium ionic center. For these reasons, sometimes the phrase n activated alcohol [7] is encountered for this type of compounds. The tables compiled by Mayr (Figure 26.2) offers a more readable and precise definition for the stability and reactivity of carbenium ions. Carbenium compounds with E < 0 (benzhy-dryhum ion) are almost impossible to isolate and to store without their rapid decomposition [8]. 

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