Proton transfer to anions

Even stronger acids than HCl can be eliminated by proton transfer from to the counteranion of highly electrophilic One of the strongest acids 

B(CgF5)3. Complex 30 reacts under H, atmosphere below room temperature to form equilibrium amounts ( 5%) of 28. On warming, methane, B(CgF5)3, nd cis-ReH(CO)4(PR3), 29, form apparently by protonation of the anion MeB(C4F5)3 by the acidic in 28. As in Eq. (9.37) 29 is not observed by NMR, but presumably quickly reacts with unreacted 27 (or 28) to form the hydride bridged dimer 27, which is a thermodynamic sink in these systems. Such protonation of a borane anion has precedence as shown in Eq. (9.51), where the H2 complex is also unstable and is 

Even stronger adds than HC1 can be eliminated by proton transfer from jj2-H2 to the counteranion of highly electrophilic [L M]+. One of the strongest acids known, triflic acid, CF3S03H, can be eliminated from an H2 complex formed from H2 gas, as seal for reaction of a dicationic complex [Ru(OTf)(CNH)( L)J [OTf] (23, L = diphosphine) containing triflate anions and a protonated cyanide ligand, CNH.68 The crystal structure of 22, which is a weak Bipnsted base, shows a 

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Step 2 Proton transfer to anionic form of tetrahedral intermediate... 

Experimental observations and test calculations pointed out a special behaviour of the nitrate anion when faced with 6arbocations. Therefore a detailed investigation with the assistance of the MINDO/3 and the Huron-Claverie method was carried out122). It appeared that in addition to the ester formation and the proton transfer to the counterion, the formation of NO+ by oxygen transfer to the cation must be considered as well (see Fig. 11). 

As is the case for cationic polymerisation, anionic polymerisation can terminate by only one mechanism, that is by proton transfer to give a terminally unsaturated polymer. However, proton transfer to initiator is rare - in the example just quoted, it would involve the formation of the unstable species NaH containing hydride ions. Instead proton transfer has to occur to some kind of impurity which is capable for forming a more stable product. This leads to the interesting situation that where that monomer has been rigorously purified, termination cannot occur. Instead reaction continues until all of the monomer has been consumed but leaves the anionic centre intact. Addition of extra monomer causes further polymerisation to take place. The potentially reactive materials that result from anionic initiation are known as living polymers. 

All carboxylic acids are weak. In an aqueous solution at equilibrium, a small fraction of the carboxylic acid molecules have undergone proton transfer to water molecules, generating hydronium ions and anions that contain the—CO2 carboyylate CH3 CO2 H((317)-FH2 0(/) CH3 CO2 (atj) + H3 0 (aq)... 

The overall rate law is, however, found to contain a term involving [ketoacid] (47) as well as the term involving [ketoacid anion]. The ready decarboxylation of the (3-ketoacid itself is probably due to incipient proton transfer to 0=0 through hydrogen-bonding in (47) ... 

Figure 6.1 A simple electrostatic adsorption mechanism illustrating the protonation-deprotonation chemistry of surface hydroxyl groups on oxide surfaces (which are neutral at the PZC) and the corresponding uptake of anionic or cationic complexes. Proton transfer to or from the surface can significantly affect the solution pH.

As Skinner has pointed out [7], there is no evidence for the existence of BFyH20 in the gas phase at ordinary temperatures, and the solid monohydrate of BF3 owes its stability to the lattice energy thus D(BF3 - OH2) must be very small. The calculation of AH2 shows that even if BFyH20 could exist in solution as isolated molecules at low temperatures, reaction (3) would not take place. We conclude therefore that proton transfer to the complex anion cannot occur in this system and that there is probably no true termination except by impurities. The only termination reactions which have been definitely established in cationic polymerisations have been described before [2, 8], and cannot at present be discussed profitably in terms of their energetics. It should be noted, however, that in systems such as styrene-S C/4 the smaller proton affinity of the dead (unsaturated or cyclised) polymer, coupled, with the greater size of the anion and smaller size of the cation may make AHX much less positive so that reaction (2) may then be possible because AG° 0. This would mean that the equilibrium between initiation and termination is in an intermediate position. 

A nucleophilic attack at an allene system of the type of 417 was described for the first time by Cainelli et al. [172], namely at 444 with the chloride ion as the nucleophile (Scheme 6.91). After the treatment of the mesylate 443 with triethylamine in the presence of lithium, sodium or tetrabutylammonium chloride, mixtures of the vinyl chlorides 445 and 447 were isolated in high yields. Since the reaction did not proceed in the absence of triethylamine, the first step should be a /3-elimination of methanesulfonic acid from 443 to generate 444, which would accept a chloride ion at the central allene carbon atom. A proton transfer to either allyl terminus of the anion thus formed (446) would lead to the products 445 and 447. 

Class (3) reactions include proton-transfer reactions of solvent holes in cyclohexane and methylcyclohexane [71,74,75]. The corresponding rate constants are 10-30% of the fastest class (1) reactions. Class (4) reactions include proton-transfer reactions in trans-decalin and cis-trans decalin mixtures [77]. Proton transfer from the decalin hole to aliphatic alcohol results in the formation of a C-centered decalyl radical. The proton affinity of this radical is comparable to that of a single alcohol molecule. However, it is less than the proton affinity of an alcohol dimer. Consequently, a complex of the radical cation and alcohol monomer is relatively stable toward proton transfer when such a complex encounters a second alcohol molecule, the radical cation rapidly deprotonates. Metastable complexes with natural lifetimes between 24 nsec (2-propanol) and 90 nsec (tert-butanol) were observed in liquid cis- and tra 5-decalins at 25°C [77]. The rate of the complexation is one-half of that for class (1) reactions the overall decay rate is limited by slow proton transfer in the 1 1 complex. The rate constant of unimolecular decay is (5-10) x 10 sec for primary alcohols, bimolecular decay via proton transfer to the alcohol dimer prevails. Only for secondary and ternary alcohols is the equilibrium reached sufficiently slowly that it can be observed at 25 °C on a time scale of > 10 nsec. There is a striking similarity between the formation of alcohol complexes with the solvent holes (in decalins) and solvent anions (in sc CO2). 

DFT calculations on the Mg(L—H)(L) complex reveal how water and acetonitrile can be lost (Scheme 9). Thus intramolecular proton transfer tautomerizes the neutral acetamide ligand in 48 into the hydroxyrmine form in 49, which can then dissociate via another intramolecular proton transfer to yield the four-coordinate adduct 50, which now contains both water and acetonitrile ligands. It is this complex that is the direct precursor to water and acetonitrile loss. Note that the reaction shown in Scheme 9 is a retro-Ritter reaction and involves fragmentation of the neutral rather than the anionic acetamide ligand, which is a bidentate spectator ligand. 

The basic catalyst in the isomerization of 1,2-butadienes to butynes acts by removing an alkenic proton from the hydrocarbon. Two different anions can be formed, each of which is stabilized by electron delocalization involving the adjacent multiple bond. Either anion can react with the solvent by proton transfer to form the starting material or an alkyne. At equilibrium the most... 

A salt with a basic anion In this case, we expect pH > 7. We follow the same procedure used for an acidic cation, except that now proton transfer to the anion results in the formation of OH- ions. We therefore use Kh, and the equilibrium table leads to a value for pOH. At the end of the calculation, we convert pOH to pH by using pH + pOH = 14.00. This procedure is illustrated in Example 10.8. 

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