Proton abstraction transfer

The proton transfer equilibrium that interconverts a carbonyl compound and its enol can be catalyzed by bases as well as by acids Figure 18 3 illustrates the roles of hydroxide ion and water m a base catalyzed enolization As m acid catalyzed enolization protons are transferred sequentially rather than m a single step First (step 1) the base abstracts a proton from the a carbon atom to yield an anion This anion is a resonance stabilized species Its negative charge is shared by the a carbon atom and the carbonyl oxygen... 

Only the a hydrogens are replaced by deuterium m this reaction The key intermediate IS the enolate ion formed by proton abstraction from the a carbon atom of cyclopen tanone Transfer of deuterium from the solvent D2O to the enolate gives cyclopentanone containing a deuterium atom m place of one of the hydrogens at the a carbon... 

Alkylation of isobutylene and isobutane in the presence of an acidic catalyst yields isooctane. This reaction proceeds through the same mechanism as dimerization except that during the last step, a proton is transferred from a surrounding alkane instead of one being abstracted by a base. The cation thus formed bonds with the base. Alkylation of aromatics with butylenes is another addition reaction and follows the same general rules with regard to relative rates and product stmcture. Thus 1- and 2-butenes yield j -butyl derivatives and isobutylene yields tert-huty derivatives. 

From a study of the decompositions of several rhodium(II) carboxylates, Kitchen and Bear [1111] conclude that in alkanoates (e.g. acetates) the a-carbon—H bond is weakest and that, on reaction, this proton is transferred to an oxygen atom of another carboxylate group. Reduction of the metal ion is followed by decomposition of the a-lactone to CO and an aldehyde which, in turn, can further reduce metal ions and also protonate two carboxyl groups. Thus reaction yields the metal and an acid as products. In aromatic carboxylates (e.g. benzoates), the bond between the carboxyl group and the aromatic ring is the weakest. The phenyl radical formed on rupture of this linkage is capable of proton abstraction from water so that no acid product is given and the solid product is an oxide. 

For polymerizations of butadiene in toluene at 50°C with the Ba-Li catalyst, we have observed a reduction in molecular weight and the incorporation of benzyl groups in chains of polybutadiene. We conclude from this result that proton abstraction from toluene occurs to give benzyl carbanions which are capable of forming new polymer molecules in a chain transfer reaction. 

In the mechanism study of /V-benzyl-/V -alkyl hydroxylamines, regarding oxidation with HgO and p-benzoquinone, it has been proposed on the basis of intra- and intermolecular kinetic isotope effects that, initially, there takes place a one-electron transfer from a nitrogen atom to the oxidant, with a subsequent proton abstraction (106—108). 

FIGURE 7.34 Decomposition of the symmetrical anhydride of A-methoxycarbonyl-valine (R1 = CH3) in basic media.2 (A) The anhydride is in equilibrium with the acid anion and the 2-alkoxy-5(4//)-oxazolone. (B) The anhydride undergoes intramolecular acyl transfer to the urethane nitrogen, producing thelV.AT-fcwmethoxycarbonyldipeptide. (A) and (B) are initiated by proton abstraction. Double insertion of glycine can be explained by aminolysis of the AA -diprotected peptide that is activated by conversion to anhydride Moc-Gly-(Moc)Gly-0-Gly-Moc by reaction with the oxazolone. (C) The A,A -diacylated peptide eventually cyclizes to the IV.AT-disubstituted hydantoin as it ejects methoxy anion or (D) releases methoxycarbonyl from the peptide bond leading to formation of the -substituted dipeptide ester. 

One aspect of asymmetric catalysis has become clear. Every part of the molecule seems to fulfill a role in the process, just as in enzymic catalysis. Whereas many of us have been used to simple acid or base catalysis, in which protonation or proton abstraction is the key step, bifunctional or even multifunctional catalysis is the rule in the processes discussed in this chapter.Thus it is not only the increase in nucleophilicity of the nucleophile by the quinine base (see Figures 6 and 19), nor only the increase in the electrophilicity of the electrophile caused by hydrogen bonding to the secondary alcohol function of the quinine, but also the many steric (i.e., van der Waals) interactions between the quinoline and quinuclidine portions of the molecule that exert the overall powerful guidance needed to effect high stereoselection. Important charge-transfer interactions between the quinoline portion of the molecule and aromatic substrates cannot be excluded. 

The Brpnsted coefficient /3b = 0.52 for deprotonation of 3-phenylcoumaran-2-one (108) by a series of bases in 50% (v/v) water-dioxane, and q bh = 0.48 for reprotonation by the conjugate acid of the buffer, are indicative of a fairly symmetrical transition state for proton transfer, although the primary KE, ku/ku = 3.81, found for proton abstraction by HO is lower than expected. " The moderate intrinsic rate constant for deprotonation of (108) suggests that generation of the charge in the transition state is accompanied by only a small amount of molecular and solvent reorganization. In acidic solution, below pH 5, O-protonation of (110) occurs initially to form (109)... 

Alonso et al. (2005) described anion-radical proton abstraction from prochiral organic acids. If the anion radicals were formed from homochiral predecessors, asymmetric deprotonation can be reached. However, low reactivity of the anion radical is required Slow proton transfer, that is, high activation energy of the reaction discriminates well between diastereoselective transition states. 

The O2 ion in a solution promotes proton abstraction from a substrate or a solvent. This results in the formation of organic bases, which are conjugated with the appropriate H acids. All H acids with a pX value lower than 23 can take part in such proton transfers (Sawyer and Gibian 1979). For this reason, even organic acids (HB), which are weaker than water, enter the exothermic reaction, namely, HB -I- 2O2 —>02 + HOO + B. ... 

Recently, detailed kinetic studies of the hybrid[type II , 02 - type RH] photo-oxidations of cyclohexane and cyclohexane-dn in both NaY and BaY have been reported. A kinetic isotope effect kulko of 5.7 was determined for X > 400 nm in BaY. This substantial isotope effect, which is nearly identical to the isotope effect on the kinetic acidity of cyclohexane, requires that the proton abstraction step, k, in the alkane radical cation superoxide ion pair be smaller than the back-electron transfer, k, to regenerate the charge-transfer complex (Fig. 18). If kpT were larger than k, the rate expression, Eq. (A) in Fig. 18, would be reduced to Eq. (B) and only a small isotope effect on et would be anticipated. 

In general acid catalysis, the reaction rate increases because the transition state for the reaction is lowered by proton transfer from a Bronsted acid in general base catalysis, the reaction rate increases by virtue of proton abstraction by a Bronsted base. 

The presence of organolithium compounds in etheric solvents at temperatures above 0°C may lead to extensive decomposition of the solvent and solute a slow electron transfer side reaction of lithium naphthalene or sodium naphthalene with the THE solvent (equation 5) has been reported . The three isomeric forms of BuLi were shown to induce extensive decomposition of THE. The main path for this process is metallation at position 2 of THE, leading to ring opening and elimination of ethylene. An alternative path is proton abstraction at position 3, followed by ring opening. The presence of additives such as (—)-sparteine (24), DMPU (25), TMEDA and especially HMPA does not prevent decomposition but strongly affects the reaction path. ... 

Transfer of a P-proton from the propagating carbocation is the most important chain-breaking reaction. It occurs readily because much of the positive charge of the cationic propagating center resides not on carbon, but on the P-hydrogens because of hyperconjugation. Monomer, counterion or any other basic species in the reaction mixture can abstract a P-proton. Chain transfer to monomer involves transfer of a P-proton to monomer with the formation of terminal unsaturation in the polymer. 

Excluding polymerizations with anionic coordination initiators, the polymer molecular weights are low for anionic polymerizations of propylene oxide (<6000) [Clinton and Matlock, 1986 Boileau, 1989 Gagnon, 1986 Ishii and Sakai, 1969 Sepulchre et al., 1979]. Polymerization is severely limited by chain transfer to monomer. This involves proton abstraction from the methyl group attached to the epoxide ring followed by rapid ring cleavage to form the allyl alkoxide anion VII, which isomerizes partially to the enolate anion VIII. Species VII and VIII reinitiate polymerization of propylene oxide as evidenced... 

The two-electron process is either a hydride transfer or a proton abstraction followed by two-electron transfer. 

Enzymology of Proton Abstraction and Transfer Reactions Irwin A. Rose... 

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