Proton transfer, alternative mechanisms

Cyclopropanations. A variety of electron-deficient alkenes react with this reagent In aprotic solvents to produce good 3delds of ethoxycarbonyTsubstItuted cyclopropanes (eq 1). EtOH and presumably other protic solvents allow alternative reaction pathways which seem to be dependent on proton transfer. The mechanism can be thought of as 1,4-addition followed by in tramolecular ring closure with loss of dimethyl sulfide. In general, the reaction succeeds for double bonds activated by one or two... 

An alternative view of these addition reactions is that the rate-determining step is halide-assisted proton transfer, followed by capture of the carbocation, with or without rearrangement Bromide ion accelerates addition of HBr to 1-, 2-, and 4-octene in 20% trifluoroacetic acid in CH2CI2. In the same system, 3,3-dimethyl-1-butene shows substantial rearrangement Even 1- and 2-octene show some evidence of rearrangement, as detected by hydride shifts. These results can all be accoimted for by a halide-assisted protonation. The key intermediate in this mechanism is an ion sandwich. An estimation of the fate of the 2-octyl cation under these conditions has been made ... 

Photosynthetic electron transport, which pumps into the thylakoid lumen, can occur in two modes, both of which lead to the establishment of a transmembrane proton-motive force. Thus, both modes are coupled to ATP synthesis and are considered alternative mechanisms of photophosphorylation even though they are distinguished by differences in their electron transfer pathways. The two modes are cyclic and noncyclic photophosphorylation. 

FIGURE 7.2. Two alternative mechanisms for the catalytic reaction of serine proteases. Route a corresponds to the electrostatic catalysis mechanism while route b corresponds to the double proton transfer (or the charge relay mechanism), gs ts and ti denote ground state, transition state and tetrahedral intermediate, respectively. 

The small solvent isotope effect shows clearly that a proton transfer is not part of the rate-determining step of the hydrolysis. Christensen (1966, 1967) favors an SN2-type mechanism (164) for the hydrolysis, with nucleophilic attack of water on a sulfonyl group synchronous with the departure of ArSO. The alternative formulation (165), however, where a pentacovalent inter-... 

These similarities indicate that the mechanism for (188) is apparently exactly the same as that for (135) except that the attack of water occurs on a sulfonyl group in (188), instead of on a sulfinyl group as in (135), and that a proton transfer is also part of the rate-determining step of the spontaneous hydrolysis of cr-disulfones. It may be recalled that in the case of the spontaneous hydrolysis of sulfinyl sulfones we determined that the purpose of the proton transfer was either to assist the attack of a water molecule on the substrate (136) or to assist the departure of the ArSOz group (137), but could not make a definite decision between the two alternatives from the information available. Thus the mechanism for the spontaneous hydrolysis of cr-disulfones is either as in (189) (where attack of water on a sulfonyl group is aided by the removal of a... 

An Alternative Mechanism. Considering the facility of the electron transfer reactions to which a great deal of this symposium has been devoted, we have to worry whether our "proton transfer" reactions may not really be the result of electron transfer in the reverse direction followed by hydrogen transfer. As Bergman (26) has recently reported that another hydride anion may act as a one-electron reducing agent, and as we have evidence implicating 0s(C0) H as an intermediate in a number of... 

Scheme 12. The proposed mechanism for the reaction of water with 60. Alternative pathways, involving either initial protonation at nitrogen followed by proton transfer to oxygen, or direct protonation at oxygen, are illustrated in the upper right section of the scheme.

An alternative route for the synthesis of TV-methyl amino acids without racemization is shown in Scheme 8.[98 This method includes the use of TBPB in the presence of copper(I) octanoate. The proposed mechanism of this free radical reaction is given in Scheme 8. Electron transfer from copper(I) to TBPB affords the copper(II), benzoate, and tBuO radical 4, which undergoes (3-scission to acetone and methyl radical 5. In turn, electron transfer from the urethane to the copper(II) ion, followed by proton transfer, affords the corresponding urethane radical 6, which reacts with the methyl radical 5 to give the desired product in overall yields of 54% (Z derivative) or 57% (Boc derivative), respectively. 

The mechanism shown in Scheme 4.9 has been proposed for the hydrogen atom transfer from phenols (ArOH) to radicals (Y ) in non-aqueous solvents, a kinetic effect ofthe solvent (S) being expected when ArOH is a hydrogen bond donor and the solvent a hydrogen bond acceptor. Steps with mechanistic rate constants k, k-1 and k>, involve proton transfer (the latter two near to the diffusion-controlled limit), and kj involves electron transfer. The step with rate constant fco involves a direct hydrogen atom transfer, and the other path around the cycle involves a stepwise alternative. 

When a proposed intermediate is so unstable that it cannot exist, i.e. it would have a lifetime less than that of a bond vibration ( 10 13 s), the reaction must proceed by a concerted mechanism (see reference [11], p. 5). The proton-transfer steps and other covalent bondforming and bond-breaking processes are concerted but with varying degrees of coupling between their motions. However, it is still not clear whether a concerted mechanism can occur when the intermediates, which would be formed in an alternative stepwise mechanism, have significant lifetimes. This is an important question for reactions catalysed by enzymes because the nature of the intermediate itself will control whether the enzyme- and non-enzyme-catalysed mechanisms are forced to be similar if the sole criterion for a concerted mechanism is the stability of the intermediate. 

To conclude this section on proton transfer, we have examined an alternative mechanism to that of von Grotthuss for proton conduction [105-110], We carried out B3LYP/6-31+G and PW91 DFT calculations on model compounds (1,2,3,4-tetrasubstituted benzenes, e.g., 99) showing that these compounds could play the role of proton conductors [104],... 

The chiral discrimination in the self-association of chiral l,3a,4,6a-tetrahydroi-midazo[4,5-d]imidazoles 3 has been studied using density functional theory methods [37], (Scheme 3.20). Clusters from dimers to heptamers have been considered. The heterochiral dimers (RR SS or SS RR) are more stable than the homochiral ones (RR RR or SS SS) with energy differences up to 17.5 kJ mol-1. Besides, in larger clusters, the presence of two adjacent homochiral molecules imposes an energetic penalty when compared to alternated chiral systems (RR SS RR SS...). The differences in interaction energy within the dimers of the different derivatives have been analyzed based on the atomic energy partition carried out within the AIM framework. The mechanism of proton transfer in the homo- and heterochiral dimers shows large transition-state barriers, except in those cases where a third additional molecule is involved in the transfer. The optical rotatory power of several clusters of the parent compound has been calculated and rationalized based on the number of homochiral interactions and the number of monomers of each enantiomer within the complexes. 

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