Putative anionic mechanism

Two proposed mechanisms for the polymerization of a,a -bis(dialkyl-sulfonio)-p-xylene dihalides (463) are shown in Fig. 70 [302]. Both mechanisms begin with the abstraction of an a-proton to produce ylid 464. The 1,6-elimination of dialkyl sulfide produces the p-xylylene pseudo-diradical 465. One mechanism involves the formation of polymer from this species via the dimerization of 465 to give the dication diradical 466. This species was proposed to grow rapidly to high molecular weight polymer (467) by head-to-tail additions to both ends [301]. An alternative mechanism involving polymer formation from 465 via a putative anionic mechanism has been proposed [302]. There were two main factors in the proposal of... 

Fig. 15. A proposed mechanism coupling the formation of the His-Tyr bond to the oxidation of ring III of the heme in HPII. The mechanism begins with the formation of compound I shown in A. A concerted series of reactions, possibly triggered by either Aspl97/His395 or by a putative anionic species bound to compound I, results in the transfer of a hydroxyl to the heme from the H2O2 shown in C, which would facilitate spirolactone cyclization to form the final product containing the His-Tyr bond and the modified heme shown in D. Reprinted with permission of Cambridge University Press from Bravo et al. (.93).

Fig. 2 Putative catalytic mechanism of ActVA-Orf6 as proposed by Sciara et al. [109] Proton transfer from the substrate to Tyr72 results in an anionic form, which reacts with molecular oxygen. The peroxy intermediate is stabilized by hydrogen bonds to Asn62 and TyrSl. The reaction is completed by protonation and deavage of the peroxy intermediate. It is not quite clear, however, why Arg86 would be deprotonated at the beginning of the catalytic cycle...

Various mechanisms for electret effect formation in anodic oxides have been proposed. Lobushkin and co-workers241,242 assumed that it is caused by electrons captured at deep trap levels in oxides. This point of view was supported by Zudov and Zudova.244,250 Mikho and Koleboshin272 postulated that the surface charge of anodic oxides is caused by dissociation of water molecules at the oxide-electrolyte interface and absorption of OH groups. This mechanism was put forward to explain the restoration of the electret effect by UV irradiation of depolarized samples. Parkhutik and Shershulskii62 assumed that the electret effect is caused by the accumulation of incorporated anions into the growing oxide. They based their conclusions on measurements of the kinetics of Us accumulation in anodic oxides and comparative analyses of the kinetics of chemical composition variation of growing oxides. 

This suggests that the attack of the thiolate anion, at least with this product, occurs principally on the 3-position of the furoxan ring. An alternative mechanism to that discussed above was proposed to explain NO-donation by this product. It implies the preliminary cleavage of the 1-2 bond of the furoxan ring, rather than of the 2-3 bond as suggested by Feelisch, to give a tertiary nitroso intermediate. Reasonable mechanisms may be put forward to explain the production of different NO-redox forms from this intermediate [20] (Scheme 6.9). Interestingly, some furoxans, such as 31 and related compounds, produce NO, detected as nitrite, spontaneously without the assistance of thiols [21]. 

Furthermore, the authors were the first to gamer evidence that the back electron-transfer (BET) from the CO2 anion-radical to the cation-radical of the ACT, leading to the formation of the activator s excited singlet state. The AG bet values were calculated on the basis of the CIEEL sequence (Scheme 44), so this finding contributes further to confirm this mechanism. However, data obtained on two less commonly used activators 9,10-dimethoxyanthracene and, particularly, 9,10-dicyanoanthracene do not fit into the correlations obtained for the other activators, implying that details of this mechanism still require clarification. A putative explanation for the fact that 9,10-dimethoxyanthracene and 9,10-dicyanoanthracene do not correlate as predicted by CIEEL is the involvement of an alternative pathway, in which CO2 cation radical and the anion radical of the activator are formed by initial electron transfer from the peroxide to the activator (Scheme 45). ... 

An examination of Table 2 reveals that although mercuric acetate and mercuric nitrate have often been used as electrophilic reagents, there are but few instances in which independent evidence as to their mechanism of reaction has been put forward. Positive kinetic salt effects have been observed in the substitution of sec.-butylmercuric acetate by mercuric acetate (with lithium nitrate in solvent ethanol)2, the substitution of di-sec.-butyl mercury by sec.-butylmercuric nitrate (with lithium nitrate in solvent ethanol)11, and the substitution of tetraethyltin by mercuric acetate (with tetra-n-butylammonium perchlorate in methanol)7. In the latter case, it was suggested7 that the observed very large positive kinetic salt effect was possibly due to anion exchange between mercuric acetate and the perchlorate ion. 

The fact that known anionic initiators for MMA can act as catalysts for GTP and the need for low amounts of catalysts in itself nearly puts to rest the associative mechanism. Seven of the other factors support the dissociative process. Except for the low temperature exchange studies, none supports the associative mechanism. Based on the lack of exchange of added silyl fluoride with silyl ketene acetal ends it looks like fluoride and bifluoride catalysts operate by irreversible generation of ester enolate chain ends [1] (Scheme 19b). On the other hand carboxylate catalysts appear to operate by reversible generation of ester enolate ends as evidenced by rapid exchange of silyl acetate with silyl ketene acetal ends [36] (Scheme 19c). 

Finally, the cyclopropyl anion appears to be exclusively dehydrogenated by reaction with NzO, for which the mechanism in (72) has been put forward. 

---