Acetonitrile proton transfers

This type of charge reduction by charge transfer to the solvent molecule occurs in general when SI are polar solvent molecules of aprotic character such as dimethyl-sulfoxide, dimethyl formamide, and acetonitrile. Protic solvents such as water lead to charge reduction which involves an intracluster proton transfer reaction ... 

Figure 2.7. The rate constants for proton transfer as a function of the negative enthalpy change for the three nitrile solvents as a function of Es and otg with the remaining parameters held constant as specified in Figure 2.5. Butanenitrile = squares with Es = 8.0kcal/mol and g>q = 195 cm-1. Propanenitrile = triangles with Es — 12.0kcal/mol and mq = 179cm-1. Acetonitrile = circles with Es — 17.0kcal/mol and cdq = 164 cm-1.

Product 34 predominates in the polar aprotic solvent (acetonitrile), while in the polar protic solvent (methanol) products 35 are formed preferentially. The different products are caused by the relative rate of deprotonation against desilylation of the aminium radical, that is in turn governed by the action of enone anion radical in acetonitrile as opposed to that of nucleophilic attack by methanol. In an aprotic, less silophilic solvent (acetonitrile), where the enone anion radical should be a strong base, the proton transfer is favoured and leads to the formation of product 34. In aprotic solvents or when a lithium cation is present, the enone anion radical basicity is reduced by hydrogen bonding or coordination by lithium cation, and the major product is the desilylated 35 (Scheme 4). 

Since morpholine and piperidine are stereochemically similar but exhibit different pKa values, the difference between their rates in the reactions of the fluoro-substrates in acetonitrile could be also due to a change in mechanism, whereby proton transfer from the intermediate 1 in equation 1 becomes rate-limiting when the reagent is morpholine. The change from an uncatalysed to a base-catalysed reaction with decrease in basicity of the nucleophile is well known in ANS for both primary and secondary amines1 200. 

Eight generalizations are given arising from world-wide studies of proton transfer reactions in aqueous media carried out over the past twenty-five years. Future directions of research on proton transfer kinetics are predicted, and recent kinetic studies by the authors on proton transfer in nonaqueous media (methanol, acetonitrile, and benzonitrile) are reviewed. 

Another proton transfer studied by the E-jump technique in acetonitrile (42) is that between p-nitrophenol (AH) and tri-ethylamine (B). The extinction coefficients for each of the species in the following equilibrium have been measured by Kree-voy and Liang (3) ... 

Whereas in acetonitrile the rate limiting step is an opening of the solvent shell of a reactant, in benzonitrile the back reaction of (5) between the protonated acridine orange cation (BH ) and the 3-methyl-4-nitrophenolate ion (A ) to form the ion pair is diffusion controlled (although the overall reaction to the neutral molecules is an endothermic process). Because of its lower dielectric constant than acetonitrile, the electrostatic interactions between reactants in benzonitrile outweigh specific solvent effects. In other words, in benzonitrile a rate limiting coupling of proton transfer to the reorientation of solvent dipoles does not occur and the measured rates are very fast. The ion recombination (I) + (II) in benzonitrile has a diffusion controlled specific rate (theoretical) k = 9 -1 -1... 

All of our measurements of proton transfer rates in acetonitrile have been done by various NMR techniques. One of the simplest has been used to measure the rates of transfer from cis-Os(CO) 2 to Et N and back again (21). Whereas a mixture of... 

The kinetics and mechanism of proton transfer from diarylcyanomethanes to a proton-sponge, c/i-l,2-bis(diethylaminomethyl)cyclohexane, in acetonitrile has been studied. 

An even more extreme case is the reaction of hydroxide with acetonitrile. In the ICR spectrometer, the bare hydroxide ion yields a simple proton transfer product, by way of reaction (9). In contrast, in aqueous solution the bulk-solvated hydroxide ion reacts to hydrolyze the nitrile group to give the carboxylate ion plus... 

The fates of the radical ion pairs produced upon electron transfer depends on the nature of their production. As already mentioned, the Bp DMA" com formed from irradiation of the ground-state CT complex. Bp - DMA, is suggested by Mataga and co-workers [24] to decay only by febet, on a timescale of 85 ps. Diffusional separation to solvent separated radical ion pairs or proton transfer within Bp -DMA com are not kinetically competitive. The triplet CRIP Bp -I- DMA" ip has two decay pathways that occur on the picosecond timescale. The first process is proton transfer, fept, to generate a triplet radical pair, BpH-l- DMA ] (Scheme 2.3). In acetonitrile, this occurs with a rate constant of fept of 1.3 x 10 s [43]. The second process leading to the decay of the CRIP is diffusional separation to the SSRIP, kips, which occurs with a rate constant of 5 x 10 s (Scheme 2.3) [43]. Thus the efficiency of the... 

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 observation of decreased exciplex fluorescence intensity and increased adduct formation with increasing solvent polarity (Fig. 10) led to the proposal that adduct formation proceeds via initial one-electron transfer to yield a radical ion pair, followed by proton transfer to yield a 1,2-diphenylethyl and a-di-alkylaminoalkyl radical pair, which subsequently combines to yield 63, disproportionates or diffuses apart (114). Subsequent investigation of this reaction led to the proposal that proton transfer occurs only from the initially formed exciplex or contact radical ion pair prior to solvation to yield a solvent separated radical ion pair. The detailed mechanism for reaction of It with tertiary amines in acetonitrile solution is summarized in Fig. 11 (116c). 

Rate and equilibrium constants have been reported for the reactions of butylamine, pyrrolidine, and piperidine with trinitrobenzene, ethyl 2,4,6-trinitrophenyl ether, and phenyl 2,4,6-trinitrophenyl ether in acetonitrile, hi these reactions, leading to cr-adduct formation and/or nucleophilic substitution, proton transfer may be rate limiting. Comparisons with data obtained in DMSO show that, while equilibrium constants for adduct formation are lower in acetonitrile, rate constants for proton transfer are higher. This probably reflects the stronger hydrogen bonding between DMSO and NH+ protons in ammonium ions and in zwitterions.113 Reaction of 1,3,5-trinitrobenzene with indole-3-carboxylate ions in methanol has been shown to yield the re-complex (26), which is the likely precursor of nitrogen- and carbon-bonded cr-adducts expected from the reaction.114 There is evidence for the intermediacy of adducts similar to (27) from the reaction of methyl 3,5-dinitrobenzoate with l,8-diazabicyclo[5.4.0]undec-8-ene (DBU) cyclization eventually yields 2-aminoindole derivatives.115... 

The kinetics of proton transfer from ethyl bis(4-nitrophenyl)acetate to N-bases with guanidine-like character, in acetonitrile, are determined by basicity and steric hindrance in die vicinity of the reacting site of the N-base and also by different distributions of positive charge in protonated A-bases.147... 

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