Simultaneous protonation

All three elimination reactions--E2, El, and ElcB—occur in biological pathways, but the ElcB mechanism is particularly common. The substrate is usually an alcohol, and the H atom removed is usually adjacent to a carbonyl group, just as in laboratory reactions. Thus, 3-hydroxy carbonyl compounds are frequently converted to unsaturated carbonyl compounds by elimination reactions. A typical example occurs during the biosynthesis of fats when a 3-hydroxybutyryl thioester is dehydrated to the corresponding unsaturated (crotonyl) thioester. The base in this reaction is a histidine amino acid in the enzyme, and loss of the OH group is assisted by simultaneous protonation. 

Some cyclic thioacetals have an A-SE2 hydrolysis mechanism,206 as do some 2-aryl-2-methyl-l,3-dithianes, except for the 4-NO2 derivative, which looks more A2-like.207 In 10 vol% dioxane/aqueous HC104 mixtures, reactive 2-aryl-2-phenyl-l,3-dithianes are believed to have an A-SE2 hydrolysis mechanism, whereas the least reactive ones have an A2 mechanism.130 Isothiocyanates are believed to hydrolyze by a mechanism that involves simultaneous proton transfer to nitrogen and attack of water at carbon in a cyclic transition... 

The addition of HC1 to 1,3-butadiene in the gas phase at total pressures lower than 1 atmosphere and at temperatures in the range of 294-334 K yielded mixtures of 3-chloro-l-butene and ( )- and (Z)-l-chloro-2-butenes, in a ratio close to unity44,45. Surface catalysis has been shown to be involved in the product formation (Figure 1). The reaction has been found to occur at the walls of the reaction vessel with a high order in HC1 and an order less than unity in diene. The wall-catalyzed process has been described by a multilayer adsorption of HC1, followed by addition of butadiene in this HC1 layer. This highly structured process is likely to involve near simultaneous proton and chloride transfers. 

For this reaction the transition state drawn in Equation 11.82 involving simultaneous proton and hydride transfers has been proposed to explain the observed isotope effects. 

The parent compound and a set of monosubstituted bis(acylamino)diarylspiro-X4-sulfanes (360 X = H, Me, MeO, Cl, NO2) undergo hydrolysis to the corresponding sulfoxides (361). The probable mechanism involves rate-determining cleavage of one of the S—N hypervalent bonds in the spiro ring with simultaneous proton transfer to the nitrogen atom. The hydroxide ion which is formed thereby then attacks the sulfur atom in a fast step to form a diaryl(acylamino)hydroxy-k4-sulfane (362), which is converted into the sulfoxide (361) (Scheme 47). ... 

This definition for aliphatic amines can be used to direct protonation of these species although it should be noted that our ionization model uses more complex definitions to prevent simultaneous protonation of both nitrogen atoms in a piperazine ring. 

Wong, A. S. Robertson, J. D. 1993. Multi-elemental analysis of coal and its by-products by simultaneous proton-induced gamma-ray/X-ray emission analysis. Journal of Coal Quality, 12, 146-150. 

Figure 2. Pattern of simultaneous protonation of acetalic O-atoms (compare with 1)...

Mixtures of gaseous HC1 and 1,3-butadiene at 294-334 K and <1 atm of total pressure give mixtures of 3-chloro-l-butene and (E)- and (Z)-l-chloro-2-butene with the ratio of 1,2- to 1,4-addition products being approximately unity. Kinetic measurements in pyrex cells, using FT IR spectroscopy, revealed that surface catalysis is required and that the reaction most probably occurs between a multilayer of adsorbed HC1 and gaseous or weakly adsorbed butadiene. This highly structured process is believed to proceed with nearly simultaneous proton and chloride transfer77,78. 

Parker and Williams recently reported NAND logic action in the terbium complex 16.[S8] The delayed emission of the lanthanide ion is switched off when H+ and 02 are present simultaneously. Protonation of the phenanthridine side chain causes its triplet excited state to approach theTb(m) 5D4 excited state energetically. This leads to equilibration of these two excited states and sharing of their properties. Thus, the metal-centered state displays the 02 sensitivity usually only found in organic triplets. 

Thus, the reaction mechanism of monooxygenation (hydroxylation) consists of oxygen atom transfer from hydrogen peroxide molecule to CH4 molecule by adding OH group to it and simultaneous proton transfer from methane molecule to the matrix. The well-known oxynoid mechanism of monooxygenation is not fulfilled. 

The primary radical first attacks the vinyl group of the monomer and then the free electron from this group is transferred to the central metal ion. This is stimulated by the presence of a conjugation chain (long coplanar re-electronic system) and chain propagation does not start until a complete reduction of Cu(II) to Cu(I). Simultaneously proton -elimination from the vinyl group, which is attacked by the primary radical, and its addition to pyrrol nitrogen occur. 

The authors thanks are due to Prof. K. U. Ingold, who called their attention (Ingold, 1967) to a mistake in their discussion (Simonyi et al., 1967b) of the non-simultaneous proton and electron transfer. 

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