Counterions proton transfer

In the El mechanism, the leaving group has completely ionized before C—H bond breaking occurs. The direction of the elimination therefore depends on the structure of the carbocation and the identity of the base involved in the proton transfer that follows C—X heterolysis. Because of the relatively high energy of the carbocation intermediate, quite weak bases can effect proton removal. The solvent m often serve this function. The counterion formed in the ionization step may also act as the proton acceptor ... 

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). 

Kovacs, G., Ujaque, G. and Lledos, A. (2008) The Reaction Mechanism of the Hydroamination of Alkenes Catalyzed by Gold(I)-Phosphine The Role of the Counterion and the N-Nucleophile Substituents in the Proton-Transfer Step. Journal of the American Chemical Society, 130, 853-864. 

The reactions of the vinylcarbenes 7 and 15 with methanol clearly involve delocalized intermediates. However, the product distributions deviate from those of free (solvated) allyl cations. Competition of the various reaction paths outlined in Scheme 5 could be invoked to explain the results. On the other hand, the effect of charge delocalization in allylic systems may be partially offset by ion pairing. Proton transfer from alcohols to carbenes will give rise to carbocation-alkoxide ion pairs that is, the counterion will be closer to the carbene-derived carbon than to any other site. Unless the paired ions are rapidly separated by solvent molecules, collapse of the ion pair will mimic a concerted O-H insertion reaction. 

A hypothesis which may explain the experimental observations can be developed as follows Transfer has been assumed to occur by proton transfer to monomer. Previous studies (18,19) indicate that propagation and transfer have similar transition states in cationic polymerizations. For this reason it is possible that these two processes may both occur within the ion-counterion-monomer complex. Termination has been assumed to occur by ion-counterion collapse (20), for example, for EtAlCl2 ... 

During the y-radiolysis of vitreous solutions containing only biphenyl (0.1 M) or only pyrene (0.02 M), the yield of Ph2 and Py- at 77K is high enough for them to be recorded at an irradiation dose of 1019 eV cm-3. At 77 K these particles have been observed to decay spontaneously (Fig. 5), evidently, due to proton transfer from alcohol molecules (the most probable process in the case of Ph2 anion radicals [14]) or to recombination with counterions formed during radiolysis. Naphthalene and pyrene additives to solutions of Ph2 essentially accelerate the decay of the Ph2 anion radical at 77 K which is naturally accounted for by electron transfer from Ph2 to Nh and Py. In agreement with this conclusion the decay of Py in the presence of Ph2 is slower than its spontaneous decay in the absence of Ph2. ... 

Irradiation of aqueous solutions of 1-isobutenyl- and l-propenyl-2-pyridinones containing perchloric acid leads to formation of 2,3-dihydrooxazolo[3,2-a]pyridinium salts. It is suggested that the mechanism involves an initial excited state electrocyclization to generate a pyridinium ylide (255). The latter is rapidly trapped by proton transfer from water. In the absence of perchloric acid the hydroxide counterion acts as a nucleophile and opens the ring to the monocyclic alcohol (256) (79JA3607). 

Both in the case of sensory rhodopsin in humans and of bacteriorhodopsin (a heptahelical membrane protein in halobacteria which is not coupled to a G protein) translocation of a Schiff-base proton is the essential step in making the protein functional (reviewed in ref 58). In rhodopsin the conversion of the inactive AH state to the AHI state that binds to the G protein is coupled to proton transfer from the Schiff base to the counterion, Glul 13, and proton uptake from the medium to the highly conserved Glul34, which serves as proton acceptor. Based on that similarity, one could consider sensory rhodopsin as an incomplete proton pump. Furthermore, a property shared by all G-protein-coupled receptors is a triplet, formed by residues 134-136 in rhodopsin, consisting of Glu-Arg-Tyr. The consequences of mutational replacement of Glul34 supports the notion that the state of protonation of this amino add is crudal for activity, and that its protonation triggers the conformational transition of the receptor from the inactive to the active state. 

At the L M transition, two protons neutralize the counterions [233] One proton transfers from the Schiff base to the terminal group of Asp 85 [271,272], and another transfers from the terminal group of Tyr 185 to that of Asp 212 [273]. Note that this process follows the deprotonation of Asp 96, including an additional negative charge on the terminal group —COOH of Asp 85 and Asp 212. This is the mechanism of additional decrease of the Schiff s base... 

The stereochemistry observed in proton exchange reactions of carbanions is dependent on the conditions under which the anion is formed and trapped by proton transfer. The dependence on solvent, counterion, and base is the result of the importance of ion pairing effects. The base-catalyzed cleavage of 1 is illustrative. The anion of 1 is cleaved at elevated temperatures to 2-butanone and 2-phenyl-2-butyl anion, which under the conditions of the reaction is protonated by the solvent. Use of resolved... 

Cw-platin diacetate binds to ethidium bromide and yields a thermochromic 1 1 complex. The transition at 490 nm (orange) shifts to 640 nm (blue) as the temperature is increased and a proton transfer occurs between the coordinated amino group of the dye and the acetate counterion (Fig. 8.6.19). Ternary ethid-ium-cisplatin-DNA complexes are also known (Ren et al., 1993). 

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