Carbocations proton elimination

The involvement of carbocations accounts for the side reactions that accompany isomerization. Carbocations are known to undergo p scission to yield low-molecular-weight cracking products. They can also undergo proton elimination to form alkenes that, in turn, participate in condensation (oligomerization), cycli-zation, and disproportionation reactions. 

This reaction is an dehydration acid-catalyzed.12 The hexaaquocop-per cation behaves as a weak cationic acid in copper-salt solution.13 Protonation of the hydroxy group produces an oxonium ion that decomposes unimolecularly into carbocation 21 and water. Water is removed from the reaction equilibrium by means of a water-separating device. Carbocation 21 eliminates an -proton with formation of the energetically favorable conjugated diene 9. 

Although indan formation is significant in styrene polymerizations, j3-proton elimination is much faster than intramolecular alkylation [292]. Unsaturated styrene and a-methylstyrene dimers are prepared quantitatively under high dilution at elevated temperatures without cyclization to indan derivatives [293]. In this case, the carbocations must be quenched before intramolecular cyclization becomes significant at high conversion. However, indan formation competes with depropagation at temperatures above 50° C, which is much too high for unsaturated styrene dimers (D =) to homopolymerize. As outlined in Eq. (95), the unsaturated dimers form indans (Din) in the presence of acid. 

Relative to living cationic polymerization, the structure of a-methylsty-rene is both advantageous and disadvantageous. Because of the additional methyl group on the a-carbon, the growing carbocation is tertiary and should be thermodynamically more stable, but it would also be prone to undergo j8-proton elimination (chain transfer) due to the increase in the number of abstractable protons. Another important aspect of this monomer is its low ceiling temperature that requires low temperatures for polymerization. 

All components of controlled systems must be selected with respect to their low basicity in order to decrease the possibility of /3-proton elimination from growing carbocations. 

A The silver ion reacts rapidly with the chloride whilst the acetate anion is a relatively weak delocalized nucleophile. Consequently, the primary carbocation may rearrange to the more stable tertiary carbocation or eliminate a proton prior to attack by the nucleophile. 

On the other hand, the bending of the acetylene unit aided by its association with the proton would allow for the approach of the amino group to the other acetylene carbon. The transition state corresponding to this idea (XI) would evolve into either carbocation VI (see Scheme 12.1) or cation XII depending on which C-0 bond is concomitantly broken. Proton elimination would finish this sequence. 

A biosynthetic pathway involving intramolecular cyclization to form a non-classical carbocation (corner-protonated cyclopropane) or its equivalent between C(l) and the terminal 14,15-double bond of geranylgeranyl pyrophosphate (66), followed by proton elimination from the original C(l) to yield casbene (65) with its fused cyclopropane and macrocyclic ring system, has been proposed (equation 4) This process is mechanisti-... 

Clear evidence for a C-C protonated C4H1C ion (55) (which would resemble 52) has been obtained by Siskin/ while studying the I II TaFs catalyzed ethylation of excess ethane with ethylene in a flow system [Eq. (5.12)]. n-Butane was obtained as the only product no isobutane was detected. This remarkable result can be explained by C-H bond ethylation of ethane by the ethyl cation, thus producing the hypercoordinate 55 carbocation intermediate, which, subsequently, by proton elimination, yields n-butane (56). The use of a flow system that limits the contact of the product n-butane (56) with the acid catalyst is essential. Prolonged contact causes isomerization of n-butane to isobutane to occur (see Chapter 6). 

The nucleophilic olefin is electron deficient, e.g., treating the protected ( )-2,3,6,7-un-saturated carboxylic acid 1 with acetic acid and sulphuric acid protonates the tetrasubstituted olefin in the first step generating a tertiary carbocation 2. Attack of the electron-deficient nucleophilic double bond is slow enough to allow the initial cation to undergo configurational relaxation. Termination of cyclization then takes place via a concerted alkylation, proton-elimination step and the trans-decalin 3 is formed stereoselectively21. 

The reaction of 4-cyclooctene-l-tosylate (and brosylate) has been studied6. After breaking the tosylate carbon bond a carbocation remains which is transannularly attacked by the nucleophilic double bond. The resulting secondary cation can be stabilized by nonstereoconlrolled attack of water or acetic acid, by proton elimination or by Wagner-Meerwein rearrangement. In the quoted example, the main product is the bicyclo[3.3.0]octane skeleton and only a small amount of rearranged bicyclo[3.2.1]octane is found as a byproduct. 

The radical cation of a reactant formed via electron-hole oxidation may also be subject to nucleophilic attack. For example, the radical cation of p-dimeth-oxybenzene is attacked by cyanide and leading the formation of cyanoanisole [Eq. (13)] [118]. Similarly, selective fluorination of triphenylmethane on irradiated TiOj in the presence of AgF has been reported [Eq (14)] [119]. A stable carbocation, which is formed after a sequential electron transfer and proton elimination from the reactant, is key for successful fluorination (Fig. 4). Phogocatalytic fluorination employs safe and easy-to-handle reagents and eliminates the need for toxic fluorine gas or other problematic fluorination reagents. 

The carbocation may eliminate a proton or another electrophile from the adjacent atom (known as El) to yield a stable compound (Scheme 2.14). 

Spontaneous proton elimination is unlikely, but /8-protons can be abstracted by basic compounds (solvent, counteranion, additives) in the system, including the monomer in competition with propagation (94). Mayr and co-workers (91) found that in the case of 1,2-dialkyl-substituted ethylenes, electrophilic attack on the double bond and hydride abstraction have comparable activation energies, so these systems are riddled with chain transfer, while in the case of 1,1-dialkyl and higher alkylated ethylenes the attack of carbocations at the C=C double bond is preferred. 

The field of cationic polymerization of vinyl monomers is apparently well established, since it has a long history, in which considerable smdies had been performed from different aspects. However, almost no progress was made in controlling the polymerization reaction until the 1960s. This difficulty even with polar monomers made polymer chemists believe that suppressing side reactions, such as p-proton elimination, in cationic process would be impossible, since carbocation is inherently unstable and highly active. 

Step 3 IS new to us It is an acid-base reachon m which the carbocation acts as a Br0n sted acid transferrmg a proton to a Brpnsted base (water) This is the property of carbo cations that is of the most significance to elimination reactions Carbocations are strong acids they are the conjugate acids of alkenes and readily lose a proton to form alkenes Even weak bases such as water are sufficiently basic to abstract a proton from a carbocation... 

Elimination unimolecular (El) mechanism (Section 5 17) Mechanism for elimination characterized by the slow for mation of a carbocation intermediate followed by rapid loss of a proton from the carbocation to form the alkene Enamine (Section 17 11) Product of the reaction of a second ary amine and an aldehyde or a ketone Enamines are char actenzed by the general structure... 

In the El mechanism, the leaving group has completely ionized before C—H bond breaking occurs. The direction of the elimination therefore depends on the structure of the carbocation and the identity of the base involved in the proton transfer that follows C—X heterolysis. Because of the relatively high energy of the carbocation intermediate, quite weak bases can effect proton removal. The solvent m often serve this function. The counterion formed in the ionization step may also act as the proton acceptor ... 

When the radicals have p hydrogens, alkenes are formed by a process in which carbocations are probably bypassed. Instead, the oxidation and the elimination of a proton probably occur in a single step through an alkylcopper species. The oxidation state of copper in such an intermediate is Cu(III). 

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