P-Proton elimination

Although the efficiency of the reaction was poor due to the Wurtz coupling and the p-proton elimination reactions, the block copolymer was synthesized by this transformation mechanism with polydispersity around 1.1 (Scheme 28). Fortunately, the problem related to coupling reactions can be avoided by using Grignard intermediates. Additionally, the P-proton elimination can be achieved by the use of xylene dibromide in the halogenation process. Obviously, such difficulties led to poor transformation efficiency. 

If the carbenium ion undergoes termination or chain transfer, the polymerization system deviates from living behavior. In Scheme 2.10, P-proton elimination is a common type of chain transfer that generates another carbenium ion while producing vinylidene (1,1-disubstituted) alkenes as... 

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. 

If the Friedel-Crafts alkylation in Eq. (10) is inhibited and instead deprotonation of the carbocationic intermediate takes place exclusively, then the resulting polymer will be linear and have unsaturated instead of cyclic units in the main chain. Mechanistically, this case is an extension of the selective linear dimerization of styrene (Eq. (11)), which can be regarded as a polyaddition reaction involving protonation of the carbon-carbon double bond of the monomer and subsequent P-proton elimination from the dimer cation (of. Sect. 3.2) ... 

In the El cb mechanism, the direction of elimination is governed by the kinetic acidity of the individual p protons, which, in turn, is determined by the polar and resonance effects of nearby substituents and by the degree of steric hindrance to approach of base to the proton. Alkyl substituents will tend to retard proton abstraction both electronically and sterically. Preferential proton abstraction from less substituted positions leads to the formation of the less substituted alkene. This regiochemistry is opposite to that of the El reaction. 

In fact, a mechanism for this reaction can be drawn that does not involve Pd at all, but let s assume that Pd is required for it to proceed. Cl- must be nucleophilic. It can add to Cl of the alkyne if the alkyne is activated by coordination to Pd(II). (Compare Hg-catalyzed addition of water to alkynes.) Addition of Cl- to an alkyne-Pd(II) complex gives a o-bound Pd(II) complex. Coordination and insertion of acrolein into the C2-Pd bond gives a new a-bound Pd(II) complex. In the Heck reaction, this complex would undergo P-hydride elimination, but in this case the Pd enolate simply is protonated to give the enol of the saturated aldehyde. 

Gao et al. (2006) considered the data on an electron double resonance spectra of the cation-radical in conjunction with the results of calculation within the DFT. The authors established that the methyl group at the double bond of the cyclohexene ring is responsible for deprotonation of the P-carotene cation-radical. This route of proton elimination produces the most stable radical leaving the Jt-conjugation system to be intact. Deprotonation at the polyene methyl groups would... 

By means of in situ NMR spectroscopy combined with deuterium incorporation experiments, van Leeuwen has elucidated the mechanism of termination by protonolysis, showing that the fl-chelates are in equilibrium with their enolate form by a p-H elimination/hydride migration process (Scheme 7.19). The enolate intermediates are regioselectively protonated at the C2 carbon atom by either MeOH or H2O to give Pd-OMe or Pd-OH and keto terminated copolymer. The enolate formation has been reported to be rate determining in the chain transfer [19]. 

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. 

Palladium-catalyzed allylic oxidations, in contrast, are synthetically useful reactions. Palladium compounds are known to give rise to carbonyl compounds or products of vinylic oxidation via nucleophilic attack on a palladium alkene complex followed by p-hydride elimination (Scheme 9.16, path a see also Section 9.2.4). Allylic oxidation, however, can be expected if C—H bond cleavage precedes nucleophilic attack 694 A poorly coordinating weak base, for instance, may remove a proton, allowing the formation of a palladium rr-allyl complex intermediate (89, path by694-696 Under such conditions, oxidative allylic substitution can compete... 

Dehydrogenation of alkylbenzene over the platinum component yields phenylalkenes. Protonation of the phenylalkene over the acid component forms a carbonium ion. (A phenylallyl cation may be produced by proton addition to phenylbutadiene or by hydride ion removal from a phenylalkene.) Attack of this carbonium ion on the aromatic ring closes either a five- or six-membered ring. Stabilization of the product occurs by proton elimination or hydride abstraction. This step may be followed by dehydrogenation to the thermodynamically most-stable species (e.g., to an alkylnaphthalene in the case of six-membered ring closure). (See p. 308.)... 

However, there is always the possibility of some E2 elimination taking place as well. Neverthless, substitution is usually favoured overelimination, even when using strong bases like HO" or EtO". If E2 elimination of a primary halide is desired, it is best to use a strong bulky base like ferf-butoxide [(CH3)3C-0"]. With a bulky base, the elimination product is favoured over the substitution product since the bulky base experiences more steric hindrance in its approach to the electrophilic carbon than it does to the acidic p-proton. 

Many nucleophiles, such as water, alcohols, and carboxylates, are compatible with the Pd(II) complex and can attack the complcxed alkene from the side opposite the palladium. The attack of the nucleophile is regioselective for the more substituted position. This parallels attack on bromoni-um ions but is probably governed by the need for the bulky palladium to be in the less hindered position. The resulting Pd(II) <7-alkyl species decomposes by p-hydride elimination to reveal the substituted alkene. Reductive elimination of a proton and the leaving group, usually chloride, leads to palladium (0). The weakness of this reaction is that the catalytic cycle is not complete Pd(II) not Pd(0) is needed to complex the next alkene. 

The migration reaction diminishes the total electron count of the complex by two, and creates a vacant site at the metal P-elimination does the opposite. P-Elimination requires a vacant site at the metal centre, and the electron count of the complex increases by two electrons during the process. The reaction resembles the P-elimination reaction occurring in many organic processes, but the difference lies in the intramolecular nature of the present process, as the eliminated alkene may be retained in the complex. In organic chemistry the reaction may well be a two-step process, e.g. proton elimination with a base followed by the leaving of the anion. In transition metal chemistry, however, it is the availability of d orbitals that greatly facilitates a concerted cis P-elimination. 

The kinetically stabilized alkyls shown in Figure 4.5 do not have the eliminations 3-M-H elimination route available to them owing to the absence of P-protons. An alternative decomposition route, however, arises when these are bound to highly unsaturated metal centres, involving loss of an cx-C-H which may be either transferred (Figure 4.27) to (a) the metal centre itself (a-elimination), (b) a co-ligand or (c) an external basic or electrophilic reagent (a-abstraction). 

Activation of the acetylene by coordination of the triple bond to the silver cation enables a 5-endo-dig cydization via nucleophilic attack of the amine [14]. Protonation of the resulting vinyl silver complex leads to an iminium ion. Subsequent p-hydride elimination affords metallic silver and a pyrrylium ion which aromatizes by proton loss to the pyrrole. For trimethylsilyl-substituted homopropargylamines (R = SiMes), the resulting pyrrole (R = SiMe3) undergoes protodesilylation to the 1,2-disubstituted pyrrole. 

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