Stille reactions donor-acceptor

Distance The affects of electron donor-acceptor distance on reaction rate arises because electron transfer, like any reaction, requires the wavefunctions of the reactants to mix (i.e. orbital overlap must occur). Unlike atom transfer, the relatively weak overlap which can occur at long distances (> 10 A) may still be sufficient to allow reaction at significant rates. On the basis of work with both proteins and models, it is now generally accepted that donor-acceptor electronic coupling, and thus electron transfer rates, decrease exponentially with distance kji Ve, exp . FCF where v i is the frequency of the mode which promotes reaction (previously estimated between 10 -10 s )FCF is a Franck Condon Factor explained below, and p is empirically estimated to range from 0.8-1.2 with a value of p 0.9 A most common for proteins. 

Starting from the findings of the racemic cross-benzoin condensation [66], and assuming that aldehydes not accepted as donor substrates might still be suitable acceptor substrates, and vice versa, a mixed enzyme-substrate screening was performed in order to identify a biocatalytic system for the asymmetric cross-carboligation of aromatic aldehydes. For this purpose the reactions of 2-chloro-(40a), 2-methoxy- (40b) and 2-methylbenzaldehyde (40c), respectively, were studied with different enzymes in combination with benzaldehyde (Scheme 2.2.7.23) [67]. The three ortho-substituted benzaldehyde derivatives 40a-40c were... 

The trimolecular rate constants were determined in earlier works with no account of any donor-acceptor interactions in the system 26,50), but later on an attempt has been made to consider such reactions I4,16,17,29 30). The effective kinetic and thermodynamic parameters that are independent of conversion are useful in describing the kinetic curve (such as that in Fig. 5), although the interpretation of the physical meaning of these constants is still very tentative. Such a situation seems to be typical... 

Very little is known about the nature of the weak interactions of CAs in solutions where a vast majority of their chemical reactions has been studied. Particularly, the study of donor-acceptor complexes of CAs by modern physical-chemical methods is still of great interest. Besides, complexation of CAs with donors or acceptors of electron density is a useful tool for modifying the stability, reactivity and spectral properties of CAs. Systematic investigations of the redox properties of CAs are needed in order to elucidate the role of electron transfer in the transformations of CAs. 

Figure 27 contrasts the original reactive trajectory to the perturbed one, shown in panel (a). Panel (b) shows that after a delay, the hydride donor-acceptor distance begins to deviate from the original reactive trajectory, unable to reach its minimum without the full compression of the donor side residues, i.e. 65 shown in panel (c) and the perturbed 31 (not shown). Additionally, the absence of the compression prevents the relaxation of the acceptor side residues, for example, of 106 shown in panel (d). The donor-acceptor distance comes closer, since at that time residue 106 is still compressing and the perturbed residue 31 has a weaker, delayed compression. Due to this, the hydride starts to transfer. However, since the compression-relaxation transition does not occur, the reaction is not completed. 

In summary, the body of work described above indicates that the polyproline n secondary protein structure provides an extremely effective ET pathway fliat allows electrons to be transferr over very long distances (>40 A). It is interesting to note that the small distance attenuation factors observed for these systems are of the same order of magnitude as that originally claimed by Barton and co-workers [62] in DNA-based donor-acceptor compounds. However, the specific mechanism for the efficiency of the proline-bridged ET reactions is still not understood. 

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