Proton transfer irreversible

A direct irreversible proton transfer in limiting stage of 1-ethoxybut- l-en-3-yne hydration is confirmed by the value of kinetic isotopic effect k ilk = 2.9. For fast reversible proton transitions this value is less than 1. 

Excited-state intramolecular proton transfer (ESIPT) exhibits different regularities [49, 50]. Commonly, this is a very fast and practically irreversible reaction proceeding along the H-bonds preexisting in the ground state. Therefore, only the reaction product band is seen in fluorescence spectra. Such cases are not interesting for designing the fluorescence reporters. The more attractive dual emission is... 

The redox and proton transfer reactions undergone by the flavin prosthetic group are summarized in Scheme 5.2. The vertical reactions are oxidations by Q regenerating P. From the standard potential values (V vs. SCE) of the four flavin redox couples that are involved in Scheme 5.2 and those of the mediators (Table 5.1), all four oxidation steps may be regarded as irreversible. The horizontal reactions are deprotonations by the bases present in the buffer. From the pA values of the various flavin acid-base couples indicated in Scheme 5.2 (over or below the horizontal arrows), reactions H2 and H4 may be regarded as irreversible and reactions HI and... 

For an EGB to work efficiently on a synthetic scale, the proton transfer reaction (Eq. 3) has to appear essentially irreversible. 

When the EGB is an anion or a dianion, irreversible follow-up reaction of the deprotonated substrate, S , is the only way by which an unfavorable proton transfer equilibrium can be driven toward products. A disadvantage is that when the follow-up reaction of S is with an added electrophile, competing reaction between the EGB and the electrophile is often observed. Or - if the EGB is a dianion - the monoprotonated form may react with the electrophile. 

To achieve LCP, one needs to start by choosing the initiator, coinitiator, and other components of a reaction so that there is no nucleophile present that can irreversibly terminate the propagating cationic species. Basic components also need to be avoided to minimize P-proton transfer. However, even with the most judicious choice of reaction system, P-proton transfer is still present because monomer itself is a base. One needs to minimize P-proton transfer to monomer to achieve LCP. 

We turn to the chemical behavior of cycloalkane holes. Several classes of reactions were observed for these holes (1) fast irreversible electron-transfer reactions with solutes that have low adiabatic IPs (ionization potentials) and vertical IPs (such as polycyclic aromatic molecules) (2) slow reversible electron-transfer reactions with solutes that have low adiabatic and high vertical IPs (3) fast proton-transfer reactions (4) slow proton-transfer reactions that occur through the formation of metastable complexes and (5) very slow reactions with high-IP, low-PA (proton affinity) solutes. 

The GLE may be generalized to include space and time dependent fiiction and then this coordinate dependence is naturally included. Such a generaUzation has been considered by a number of author T62 8 gjjj recently by Antoniou and Swhwartz who foimd in a munerical simulation of proton transfer that the space dependence of the friction can lead to considerable changes in the magnitude of the rate of reaction. The GLE can also be generalized to include irreversible effects in the form of an additional irreversible time dependence of the random force. ... 

Buncel et a/.215 found that at a higher temperature, or in the presence of an excess of nucleophile, subsequent transformation of 173 irreversibly leads to new products (Scheme 12). A proton transfer occurs first from thes/ 3 carbon atom to the N-oxide group. The resulting intermediate 175 changes by two routes, leading to 176, presumably by an intramolecular shift of the oxide substituent similar to a Smiles rearrangement, and to anion 167 by loss of OH and subsequent MeO attachment. 

It should be noted that there is a kinetic isotope effect on the normal reaction (9.11) when the a-deuterated compound is used as the substrate. A similar effect is found when the deuterated suicide inhibitor is used. Thus, both reactions involve a proton transfer in the rate-determining step of the reaction. It has also been shown that a sample of the allenic intermediate that is prepared chemically does in fact irreversibly inhibit the enzyme.18... 

C kinetic isotope effects (KIEs) of four cinnamyl alcohol oxidations have been determined by 13C NMR spectroscopy using competition reactions with reactants at natural 13C abundance. Primary 13C KIEs of the Pd(II)-catalysed oxidation and of the MnC>2 oxidation are similar ( 1.02) and indicate the C—H bond cleavages to be the irreversible and rate-limiting steps in the respective reactions. Low primary 13C KIEs in Swern and Dess-Martin oxidations, however, indicate that the initial C—H bond breakings and proton transfers are not the irreversible steps in these mechanisms, which control the rate.284... 

Under conditions of kinetic control, the mixed Aldol Addition can be used to prepare adducts that are otherwise difficult to obtain selectively. This process begins with the irreversible generation of the kinetic enolate, e.g. by employing a sterically hindered lithium amide base such as LDA (lithium diisopropylamide). With an unsymmetrically substituted ketone, such a non-nucleophilic, sterically-demanding, strong base will abstract a proton from the least hindered side. Proton transfer is avoided with lithium enolates at low temperatures in ethereal solvents, so that addition of a second carbonyl partner (ketone or aldehyde) will produce the desired aldol... 

The dynamism of these systems is further illustrated in the chemistry of the adduct of vinyl acetate (Entry 12), where reversible coordination of the C=0 function dominates, but competes with electrophilic attack of the vinyl group of the ester moiety (Scheme 18).132 Irreversible proton transfer from the OH tautomer of this minor olefin adduct results in the expulsion of CeFsH and formation of the internally coordinated adduct shown as the eventual thermodynamic product in the reaction. A related chelating RC(=0)0R species results from the hydroboration of H2C=C(H)CH2CH2C(=0)0Et using HB(C6F5)2.30... 

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