Cation-olefin reaction

Cation-olefin reaction 2,2 -Dihydroxy-1,1 -binaphthyl, 113 2,4,4,6-Tetrabromo-2,5 -cyclohexadi-enone, 285... 

Closely related to the polyepoxide cascade procedure for the synthesis of polycyclic systems is Corey s biomimetic-type, nonenzymatic, oxirane-initiated (Lewis acid-promoted) cation-olefin polyannulation. By this strategy, compound 96, containing the tetracyclic core of scalarenedial, was constructed by exposure of the acyclic epoxy triene precursor 95 to MeAlCl2-promoted cyclization reaction conditions (Scheme 8.25) [45]. 

Recently, Kennedy and Rengachary11 studied cationic olefin model and polymerization reactions. Important conclusions of the model study were ... 

Cationic olefin polycyclizations represent important transformations in the field of domino-type reactions. 

For a long time, it was considered that the formation of a bromonium ion from olefin and bromine is irreversible, i.e. the product-forming step, a cation-anion reaction, is very fast compared with the preceding ionization step. There was no means of checking this assumption since the usual methods—kinetic effects of salts with common and non-common ions—used in reversible carbocation-forming heterolysis (Raber et al., 1974) could not be applied in bromination, where the presence of bromide ions leads to a reacting species, the electrophilic tribromide ion. Unusual bromide ion effects in the bromination of tri-t-butylethylene (Dubois and Loizos, 1972) and a-acetoxycholestene (Calvet et al, 1983) have been interpreted in terms of return, but cannot be considered as conclusive. 

The Shapiro reaction provides a convenient, easy and straightforward method to convert ketones into a plethora of olefinic substances in high yields. Many of these vinyllithium derivatives are useful for further synthetic manipulations. No attempt is made in this chapter to cover all the applications of the Shapiro reaction and only few representative examples will be described. A variety of polyolefins such as 119, used for cation olefin cyclization, can be stereospecifically formed in a concise and modular approach in a single step from the components shown in equation 42 via the Shapiro reaction . 

Ethylene is the template for olefin reactions, but ethylene itself is rather unreactive, undergoing electrophilic attack by moderately strong Lewis acids. Nucleophilic attack on the bond even by the strongest Lewis bases has not been reported. The following sequence involves intramolecular addition of a carbanion to an unactivated olefin [111, 112]. The reaction is undoubtedly facilitated by active participation of the lithium cation as a Lewis acid [113]. 

The pulse radiolysis studies of liquid alkanes have relevance to the radiolysis of polyethylene and related polymers. In liquid alkanes at ambient temperature, the reaction intermediates such as alkane radical-cations, olefin radical-cations, olefine dimer-cations, excited states, and alkyl radicals have been observed after the electron-pulse irradiation [90-93]. According to the nanosecond and subnanosecond studies by Tagawa et al., the observed species were alkane radical cations, excited states, and alkyl radicals in n-dodecane excited states and cyclohexyl radical were observed in cyclohexane, and only radicals in neopentane [91, 93]. Olefin radical-cations were also detected in cyclohexane containing carbon tetrachloride [92],... 

Figure 3.42 shows an addition of a carboxylic acid to isobutene, which takes place via the ferf-butyl cation. This reaction is a method for forming fert-butyl esters. Because the acid shown in Figure 3.42 is a /3-hydroxycarboxylic add whose alcohol group adds to an additional isobutene molecule, this also shows an addition of a primary alcohol to isobutene, which takes place via the ferf-butyl cation. Because neither an ordinary carboxylic acid nor, of course, an alcohol is sufficiently addic to protonate the olefin to give a carbenium ion, catalytic amounts of a mineral or sulfonic acid are also required here. 

It was suggested that the olefm radical cations, formed upon irradiation of the CT complex of the olefins with SbCl5, react with the corresponding neutral olefins to give dimeric radical cations, whose reactions with molecular oxygen and subsequent reductions would lead to the dioxanes. Similar results wee also obtained in the TCA-sensitized photooxygenation of 9 [71] ... 

Discussion Point DP2 Olefin polymerization is an exothermic process. Estimate the heat of reaction released per day by a polymerization reactor with a typical production capacity of about 1000 t of PE or PP per day. Identify the means which can be used to remove this amount of heat from the reactor. What are their relative merits (and limits) in terms of energy use or recycling Some (e.g. cationic) polymerization reactions proceed rapidly and give products with excellent properties when conducted at temperatures below 0°C. What makes such processes uneconomical ... 

Figure 11 Formation of a cationic olefin-containing reaction complex (right) by reaction of a zirconocene dialkyl precursor complex with a cationization reagent (left), via a highly reactive contact ion pair containing an alkyl zirconocenium cation (middle).

The most extensively studied diketone has been biacetyl,24 12g) a detailed mechanistic study of its reactions with a number of olefins has appeared 128). It was suggested that a triplet exciplex is the precursor for formation of 161. The occurrence of electron transfer, presumably subsequent to exciplex formation, has been demonstrated 157,l58) in the reaction of biacetyl with tetramethyl-l,3-dioxole (167). Esr-spectroscopy of irradiated mixtures indicated the presence of biacetyl radical anion and dioxole radical cation. This reaction, which produced a complex mixture of products, was suggested to involve the excited singlet state of biacetyl. 

Although Reactions C have been widely accepted as the major method of Initiating the formation of t-C Hg whenever n-butenes are employed as feed olefins. Reaction B Involving polymer cations (R ) dissolved In the acid phase Is considered to be more important especially when sulfuric acid Is employed as the catalyst. Albright and LI (11) have previously discussed reasons for this choice, and recent experimental information further substantiates this choice. Two reasons are as follows ... 

Main Olefin Reactions. Although some olefins react to form t-butyl cations, i.e. to initiate the alkylation steps as shown in Table I, most olefins react by Reactions G through K, as summarized in Table II. Reaction G-1 is the reaction in which either 2-butene or Isobutylene reacts with a t-C Hn to form a TMP . This reaction is of major importance, and it is widely accepted as being a key step in alkylation (9,10). It should be emphasized, however, that the reaction as postulated here can be but is not necessarily part of a chain sequence of reactions. 

The purpose of this study was to investigate the mechanism of cationic olefin polymerizations by model experiments using alkyl-aluminum/alkyl halide initiator systems and to correlate the results of model experiments with corresponding polymerization reactions. 

The first part of this paper is a critical review of model studies in cationic polymerization. In the second part we describe and discuss our investigations exploring the effect of a variety of alkylaluminum/alkyl halide initiating systems under a variety of conditions on the competitive reactions of the Keimedy-Gillham scheme (9). This scheme represents a comprehensive set of model reactions developed for the study of competitive reactions in cationic olefin polymerizatioa It involves the cationation of a nonpolymerizable (steric-hindrance) olefin under simulated polymerization conditions and the complete analysis of reaction products which in turn reflect initiation, propagation, termination and transfer. 

The relative rates of chain transfer to monomer (briefly transfer), termination and propagation, determine molecular weights and ultimate conversions in most cationic olefin polymerizations. The corresponding model reactions are proton elimination, alkylation by the counteranion and formation of Cfg and higher fractions. Thus, a quantitative analysis of [Cfa]. [C13 or C14] and [Cj ] could give clues as to the relative rates of these competing reactions. The following equations further illustrate the concept ... 

Carborane-metal complexes, 8, 87-113 Catalysis by organometallics carbonylation, 6, 158-163 hydroformylation, 6, 128-136 olefin reactions, 7, 199-202 organic syntheses by nickel compounds, 8, 48-83 reviews, 10, 331-336 Cationic metal carbonyls, 8, 117-159 Chromium carbonyls, 8, 133-159, see also Benchrotrenes Cobaloximes, 7, 161, 203 Cobalt carbonyl, 6, 119-163 8, 152-155 Cotton-Kraihanzel method, 10, 213-214 Coupling reactions, on nickel, 8, 30-39, 82-83... 

The now-accepted mechanism of epimerization, first proposed by Busico and Cipullo,238 and recently confirmed by Yoder and Bercaw by means of an elegant double-labeling study,237 involves the reaction product of /3-H transfer to the metal, a metal cation-polymeryl olefin complex, and is shown in Scheme 12. Allylic activation on the same metal cation-olefin complex has been proposed to generate internal double bonds in PE2 9 and vinylidene unsaturations in PP (Scheme 13).249... 

Oligomerization After the primary reaction forms a Cg+ carbocation, a second olefin reacts to form a higher molecular weight hydrocarbon (e.g., C12) and another f-butyl cation. Further reactions can result in even larger products (e.g., CifiS, etc.). 

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