Proton transfer sequential

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... 

J.I3 Na As04 is a salt of a weak base that can accept more than one proton, (a) Write the chemical equations for the sequential proton transfer reactions of the anion with water. Identify the acid and the base in each reaction, (b) If 35.0 g of Na3As04 is dissolved in water to make 250.0 ml. of solution, how many moles of sodium cations are in the solution ... 

Cunningham AF Jr (1994) Sequential Friedel-Crafts diacetylation of ferrocene interannular proton transfers as a mechanistic probe. Organometallics 13 2480-2485... 

Examples illustrating the reactions 21-23 are given in Figures 10-12. Shown in Figure 10 is the CID mass spectrum for the desolvation of Ni2+(H2O)10. The sequence of product ions Ni2+(H20) where n = 9 to n = 4 illustrates the sequential solvent loss represented by equation 21. The CID spectra in Figure 11 demonstrate that for the n = r = 4, charge reduction via internal proton transfer (see equation 23)... 

The double proton transfer of [2,2 -Bipyridyl]-3,3 -diol is investigated by UV-visible pump-probe spectroscopy with 30 fs time resolution. We find characteristic wavepacket motions for both the concerted double proton transfer and the sequential proton transfer that occur in parallel. The coherent excitation of an optically inactive, antisymmetric bending vibration is observed demonstrating that the reactive process itself and not only the optical excitation drives the vibrational motions. We show by the absence of a deuterium isotope effect that the ESIPT dynamics is entirely determined by the skeletal modes and that it should not be described by tunneling of the proton. 

Two mechanisms were proposed for the (3-lactam-forming photoreactions, one radical involving excited-state H-atom abstraction while the other following a sequential single-electron-transfer (SET)-proton-transfer route. 

Deprotonation is essential in some cation radical reactions the corresponding examples will be described in Chapter 6. Scheme 1-43 depicts a photoreaction between phenan-threne and triethylamine. This reaction includes photoinduced sequential electron-transfer, proton-transfer, and radical-recombination processes (Lawson et al. 1999). 

O. F. Mohammed, D. Pines, J. Dreyer, E. Pines and E. T. J. Nibbering, Sequential proton transfer through water bridges in acid-base reactions, Science, 310 (2005) 83-86. 

In the second mechanism, the electron transfer from the nucleophile cluster into the aromatic ring should be facilitated by the decrease of the ionization potential (IP) of the solvent clusters as n increases. This mechanism is convincing for the ammonia or methanol clusters which show relatively low IPs when cluster size is increasing however, for water clusters, the IPs of n > 3 clusters are not known. The IPs of water and its dimer are 12.6 and 11.2 eV, respectively (Ng et al. 1977). However, these IPs are certainly higher than the one of PDFB (9.2 eV), which is not in favor of a sequential electron transfer followed by a proton transfer mechanism. This mechanism is more likely possible if one assumes, in agreement with Brutschy and coworkers, that the barrier to the reaction is lowered by a concerted electron transfer/proton transfer mechanism (Brutschy 1989, 1990 Brutschy et al. 1988, 1991, 1992, in press). 

Table 2.1), in which 5 with the longer chain was the fastest, supports our previous contention that in transamination the catalytic group also performs the protonation at the amino acid a-carbon. In the imidazole series (7-9) the shortest chain system 7 was also fastest in HC1 elimination. The striking contrast to the data for transamination fully supports the proposition that in our transamination studies the catalysis was sequential, with proton transfer by the catalytic group to the remote position of the intermediate. 

The monoanionic tungsten complex W[S2C2Ph(p-MeOPh)]3 has been reported to catalyze formation of hydrogen from water, using free radicals derived from methyl viologen as the source of electrons (112). The catalytic cycle has been proposed to involve sequential electron and proton transfer. 

Another interesting reaction type detected from the photoreaction of tertiary amine-ketone system which have found use in alkaloids chemistry [172] involves sequential electron transfer and proton transfer followed by back electron transfer to yield iminium cations (Eq. 35) leading to dealkylation products upon aqueous workup. The regiocontrol in these dealkylations is dictated by preferential deprotonation at the less branched amine cation radical a-carbon. 

We also examined the reaction rate in different mixtures of D2O and H2O, and we saw that the proton transfers were simultaneous, not sequential (19). Remarkably, the detailed data were almost identical with those observed in the enzyme process, which raised the question of whether the enzyme also used a mechanism proceeding through a five-coordinate phosphorane intermediate, rather than going to the ring-opened product directly. We examined another biomimetic system to explore this question. 

Alternatively, hydrogenation is simply explained by the sequential electron-transfer-proton-transfer reaction followed by disproportionation of the resulting radical pair (Eqs 63, 64). 

The AdN2 reaction on a carbonyl is often carried out in two separate and sequential reaction conditions if the nucleophile is a strong base (and would react with proton sources). The addition of the nucleophile occurs in basic aprotic media, followed by addition of a weak acid in the workup for the proton transfer step. The reversibility of the Ad>f2 can usually be predicted by the ApA a rule, but in protic solvents if the nucleophilic attack forms a stronger base, a following irreversible proton transfer step may make the overall reaction favorable. This very favorable p.t. step is the driving force for the cyanohydrin formation reaction below. 

More recently, it has been demonstrated that the reagent anions used for either ETD or proton transfer can be derived from the same neutral compound. The radical anions used for ETD, [M]- , are converted into even-electron proton transfer reagent anions. [M + H]-, by changing the potential on the methane Cl source.1 9 This voltage switch can be acheived in milliseconds allowing for rapid sequential ion—ion reactions and opens up the possibility of top-down sequencing of intact proteins in RF ion traps. 

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