Acceptor reaction mechanisms

The transfer of energy must proceed with net conservation of spin. In the usual case, the acceptor molecule is a ground-state singlet, and its reaction with the triplet state of the sensitizer will produce the triplet state of the acceptor. The mechanism for triplet photosensitization is outlined below ... 

Michael acceptors and 1,4-addiiion of alkyl group is a normal process. The reaction mechanism is not clear, but the process via addition of alkyl radicals and subsequent elimination of NO radical is one of the possible routes. Recently, several related reactions have been reported, as shown in Eq. 4.76, Eq. 4.77, and Eq. 4.78, in which alkyl radicals are involved. The reaction of trialky Igalliiim compounds with nitrostyrene gives also a similar snbsdtiidon product fEq. 4.791. ° ... 

We postulated a reaction mechanism with participation of an aromatic radical cation which was formed by one electron transfer from an aromatic hydrocarbon to copper(II) chloride. Activated alumina has electron-acceptor properties, and formation of a radical cation of an aromatic hydrocarbon adsorbed on alumina has been observed by ESR (ref. 13). Therefore, it seemed to us that alumina as a support facilitates the generation of the radical cation of the aromatic hydrocarbon. 

With these results, we named the new enzyme as phenylacetaldoxime dehydratase (EC 4.99.1.7). It was also suggested that the enzyme utilizes FMN as an electron acceptor, because the value was increased about five times under anaerobic condition and the sulfite ion could replace FMN, although the enzyme requires oxidized form of FMN. It was revealed that the enzyme is a quite unique enzyme whose apparent function is to catalyze a dehydration reaction. The reaction mechanism is of much interest. 

The dependence of g0 on the position of the Fermi level at the surface of the catalyst is presented in Fig. 11 by a heavy curve, in accordance with (80) and (7). We see that within the framework of the mechanism considered above the oxidation of CO falls into the category of acceptor reactions (i.e., it is retarded as the Fermi level is lowered). 

J. F. Robyt and T. F. Walseth, The mechanism of acceptor reactions of Leuco-nostoc mesenteroides B-512F dextransucrase, Carbohydr. Res., 61 (1978) 433 145. 

Studies aimed at the elucidation of reaction mechanisms have been performed by many groups, notably by those of Backvall [28]. In test reactions, typically enantiopure 1-phenylethanol labeled with deuterium at the 1-position (8) is used. The compound is racemized with acetophenone (9) under the influence of the catalyst and after complete racemization of the alcohol, the deuterium content of the racemic alcohol is determined. If deuterium transfer proceeds from the a-carbon atom of the donor to the carbonyl carbon atom of the acceptor the deuterium is retained, but if it is transferred to the oxygen atom of the acceptor it is lost due to subsequent exchange with alcohols in the reaction mixture (Scheme 20.4). 

Halogen Transfer Reactions from bis-Amino Halonium Ions to Acceptor Olefins Mechanism and Strategies for Chiral Halogenation... 

Nucleophilic substitution is the widely accepted reaction route for the photosubstitution of aromatic nitro compounds. There are three possible mechanisms11,12, namely (i) direct displacement (S/v2Ar ) (equation 9), (ii) electron transfer from the nucleophile to the excited aromatic substrate (SR wlAr ) (equation 10) and (iii) electron transfer from the excited aromatic compound to an appropriate electron acceptor, followed by attack of the nucleophile on the resultant aromatic radical cation (SRi w 1 Ar ) (equation 11). Substituent effects are important criteria for probing the reaction mechanisms. While the SR wlAr mechanism, which requires no substituent activation, is insensitive to substituent effects, both the S/v2Ar and the Sr+n lAr mechanisms show strong and opposite substituent effects. 

The mechanism for the acceptor reactions involve the binding of the acceptor at the active-site in such a manner that one of their hydroxyl groups is specifically oriented to make a nucleophilic attack onto C-1 of the enz3rme linked glucosyl unit or dextranosyl... 

Mechanisms for the synthesis of branch linkages and for acceptor reactions of B-512F dextransucrase. 

In this paper, it has been shown that glucansucrases with different synthetic specificities can synthesize glucans with different structures and can synthesize oligosaccharides with different structures by acceptor reactions. The mechanisms by which these enzymes carry out the polymerization and by which the acceptor reactions occur has been discussed. The products resulting from the... 

Biosynthesis of Digalloylglucose. Besides the above mentioned acyltransferase, oak leaves also contained a completely different type of acyltransferase that catalyzed the formation of digalloylglucose (41). It became evident that this ester was synthesized by a new reaction mechanism in which / -glucogallin was utilized as both acyl donor and acceptor this conclusion was supported by the isolation of analogous acyltransferases related to other metabolic pathways (cf. Table III). Recent studies (54) have shown, in accordance with previous proposals (5,7,8), that 1,6-O-digalloylglucose was produced by the enzyme, and that the stoichiometry of the reaction... 

The reaction mechanism proposed for the LiBr/NEta induced azomethine ylide cycloadditions to a,p-unsaturated carbonyl acceptors is illustrated in Scheme 11.10. The ( , )-ylides, reversibly generated from the imine esters, interact with acceptors under frontier orbital control, and the lithium atom of ylides coordinates with the carbonyl oxygen of the acceptors. Either through a direct cycloaddition (path a) or a sequence of Michael addition-intramolecular cyclization (path b), the cycloadducts are produced with endo- and regioselectivity. Path b is more likely, since in some cases Michael adducts are isolated. 

Dealing with type II processes, the question arises whether the substrate or the oxygen is activated when it enters the termination reaction which leads to the final produces) AOa. Therefore, we may distinguish an acceptor-activation mechanism from an oxygen-activation mechanism. Furthermore, in each of these mechanisms the activated species may be either the electronically excited A or 02, respectively, or the electronically excited complexes of the primarily excited species with A or 02, respectively (Chart I). 

This particular reaction mechanism belongs to the family of hydrogen-transfer reactions. This pathway implies the direct transfer mechanism between the donor and the acceptor groups. The MPV mechanism is common for main group elements acting as Lewis acids catalysts such as aluminum isopropoxydes or... 

ATP -I- 2 -deoxynucleoside = ADP + 2 -deoxynucleoside 5 -phosphate (<1>, compulsory ordered steady-state reaction mechanism with formation of a ternary complex with the phosphate donor and acceptor [2] the enzyme from embryonic cells of Drosophila melanogaster differs from other deoxynucleoside kinases [EC 2.7.1.76 (deoxyadenosine kinase) and EC 2.7.1.113 (deoxyguanosine kinase)] in its broad specificity for all four common deoxynucleosides)... 

The second important feature of the reaction mechanism of the epoxy compound curing under the action of amines (primary, secondary and tertiary) and their mixtures consists in formation of various hetero-, auto-, inter- and intramolecular donor-acceptor complexes between the components of the reaction system — the starting substances and reaction products. Consideration of this complex formation can adequately explain the reaction kinetics. 

---