Elimination of proton

The formation of a double bond during anodic oxidations can result from eliminations of protons, carbon dioxide or acylium cations. The electrooxi dative aromatization of dihydropyridine derivatives and heterocycles containing nitrogen atom (di-hydroquinoxalines, tetrahydrocinnolines) involves an ECE mechanism as previously... 

In the case of protonated pyrroles, the p Ta value lies in the range between 4 and —4, whereas the pATa value of acetonitrile is about —10. Therefore, the oligomerization of pyrrole in pure acetonitrile may already stop at the level of a-intermediates of hi- or more likely of tetrapyrrole. Acetonitrile is a weaker base than the a-intermediates. Consequently, a stronger base must be used to initiate the elimination of protons. Water fulfills this condition. Pyrrole can be polymerized in acetonitrile in the presence of 1% water [6, 37]. A similar effect results from the application of a sterically hindered base such as 2,6-di-tert-butylpyridine [38]. However, the concentration should be kept low because, at high concentrations proton, abstraction from the monomeric radical cation may occur, thus forming a neutral radical [28d]. The base effect can be also observed in the case of thiophenes. 

The mechanism for replacement of a methoxyl group by cyanide in these reactions follows Scheme 6.7. The radical-cation reacts with cyanide ion at the point of highest positive charge density. Oxidation of the radical so formed to the carbon-ium ion is followed by elimination of proton and formaldehyde [79]. The elimination step is analogous to the conversion of cyanhydrins to the carbonyl compound and cyanide ion in basic solution. 

Similar process of decomposition of betaines has been known in case of aromatic amines, but proceeding with elimination of protonated amine [17j. 

Studies reported on the hydrocarbon monomers show that there are three main areas of ionicities which produce different initiation, termination and termination reactions. The strong cationic systems involve the transfer or elimination of protons or carbonium ions. This has been well reviewed by Kennedy and Langer (1). At the other extreme, strong anionic systems react by hydride transfer. For the olefinic monomers, this region extends to include alkyl aluminum which undergo easy exchange to produce dimers (72). 

Fig. 7-26. Elimination of proton and formaldehyde from the quinone methide intermediate during alkali pulping (Gierer, 1970).

Simulations that have been carried out for a vast number of different rate parameters and mechanistic variants show that only the slow second-order coupling steps between dimers and the subsequent slow elimination of protons can explain the trace-crossing (Fig. 2). In any case, the so-called nucleation loop in cyclic voltammetry is not the result of a nucleation step, but of slow, homogeneous follow-up reactions of soluble oligomers in the diffusion layer. 

At very low pH (< 1), Fe exists as the purple, hexa-aquo ion, [Fe(H20)6] . Hydrolysis involves the stepwise elimination of protons from the six water moleeules that surround the central Fe eation to form mono- and binuclear speeies. These species then interact further to produce species of higher nuclearity (Sylva, 1972 Johnston and Lewis, 1986 Bottero et al. 1994 Rose et al. 1997 Schwertmann et al. 1999). The latter finally precipitate as a more or less crystalline product, the nature of which depends on the rate and conditions of the reaction. Examples of hydrolysis and polymerization reaetions are ... 

Generation of ammonia from glutamine and its excretion as NH4 is an important mechanism for elimination of protons, particularly during severe metabolic acidosis, when it becomes a significant mode of nitrogen excretion. Most renal glutamine is derived from muscle (Chapters 17 and 22). Glutamine provides two molecules of NH3 ... 

The elimination of protons bound to carbon-12 in proton-detected correlations becomes trivial (suppression ratio of 1 100) and even the selection of natural abundance fragments becomes feasible... 

The process is known to be stereoselective the S enantiomer of squalene epoxide is formed and only the C-20-R isomer of the steroids is formed. All steroids are formed through the protosteroid cation by elimination of protons at C-7, C-9, C-11, or C-19 to give lanostadienol, lanosterol, parkeol and cycloartenol, respectively. All have the same absolute configuration at C-5-C-10, the enzyme being responsible for enantiomeric selection. 

Acyl halides bearing a-hydrogen atoms, and acetyl chloride in particular, have limited stability in the presence of strong Lewis acids. Elimination of proton from the acylium ion leads to the ketene. This intermediate undergoes ready Friedel-Crafts acylation, acetyl chloride eventually forming the diacetylace-tylium ion, ° which is a poor acylating agent. For this reason, excessive reaction temperatures and times should be avoided in Friedel-Crafts acetylations. 

Scheme 61 Oxidation - nucleophilic addition - elimination of proton...

The most straightforward and widely applied routine for quantification of reactive surface groups possessing either basic or acidic properties and thus capable of binding or elimination of protons or hydroxyl ions is probably pH potentiometric titration. If it is known that there are no ions other than protons (hydroxyl ions) that can react with the surface to any appreciable extent—a rather exceptional situation, as will be shown later—the number, of reacted surface groups per... 

In Eq. 1, a monomer, M is oxidized at the electrode surface and releases an electron to form a radical cation. In Eq. 2, two of the radical cations couple to produce a dimer. The dimer then immediately releases two protons. Street et al. [17] found a decrease in pH during electrochemical polymerization, which provides supporting evidence for the elimination of protons. Equations 1 and 2 continuously repeat to increase the length of the growing oligomer chain. Equation 3 represents the net reaction in which jc monomeric units link to form a polymer HMJH. Overall, (2x - 2) protons and an equal number of electrons are released. In Eq, 4, the polymer HMJH is oxidized to form the polycarbonium ion, and y electrons. The... 

When the cationic i7 -allylpalladiuin(II) complex is formed in the absence of nucleophiles, elimination of proton and conjugate diene formation ensues (see Sect V.2.5.1 for a specific coverage of eliminations). Formally this is a dehydration of allylic alcohols in neutral medium by prior conversion into allylic carbonate however, in some cases a tertiary base is added to accelerate proton elimination. Tsuji s group has reported the elimination in steroidal and related compounds. Different configurations at C-3 (A ring) afford different diene systems (Scheme 50). The combination of Pd(0Ac)2 and tributylphosphine is the precatalytic combination preferred by this group, which uses NMR to control the quality of the catalytic mixture. ... 

The elimination of proton from C-10 of agroclavine can be achieved by butyl lithium, giving a carbanion. Because the C-10 carbanion can tautomerise to... 

Decomposition via Elimination of Protonated Carbene (Imidazolium Salt)... 

---