Dimer mechanism

In the absence of air or peroxides, only cycHc dimers are formed in the thermal dimerization of isoprene (33). Six cycHc dimers are formed in good yields four substituted cyclohexenes (3—6) and two dimethylcyclooctadienes (7—8). The latter two are, of course, not Diels-Alder dimers. There is some evidence that the isoprene dimerization mechanism differs from the usual Diels-Alder route. 

It is seen that this expression has the same form as that obtained from the singlet dimerization mechanism although the slope and intercept in the kinetic expressions for these two mechanisms have different meanings ... 

We should now examine the nature of the concentration quenching process that we proposed in our singlet dimerization mechanism. There are a number of possibilities, as follows. 

The dimerization mechanism for either the Co or the Fe catalyst system is depicted in Scheme 9 according to the postulation by Hata (65) and Miyake et al. (66). The reaction sequences involve 1,4-addition of the... 

There are two mains aspects of the role of dimerization of intermediates on the electrochemical responses that are worth investigating in some detail. One concerns the effect of dimerization on the primary intermediate on the current-potential curves that corresponds to the first electron transfer step, the one along which the first intermediate is generated. Analysis of this effect allows the determination of the dimerization mechanism (radical-radical vs. radical-substrate). It is the object of the remainder of this section. 

Calculation stability implies that At/Ay2 <0.5. The fulfillment of this condition may become a problem when fast reactions, or more precisely, large values of the kinetic parameter, are involved since most of the variation of C then occurs within a reaction layer much thinner than the diffusion layer. Making Ay sufficiently small for having enough points inside this layer thus implies diminishing At, and thus increasing the number of calculation lines, to an extent that may rapidly become prohibitive. This is, however, not much of a difficulty in a number of cases since the pure kinetic conditions are reached before the problem arises. This is, for example, the case with the calculation alluded to in Section 2.2.5, where application of double potential step chronoamperometry to various dimerizations mechanisms was depicted. In this case the current ratio becomes nil when the pure kinetic conditions are reached. 

The governing dimensionless partial derivative equations are similar to those derived for cyclic voltammetry in Section 6.2.2 for the various dimerization mechanisms and in Section 6.2.1 for the EC mechanism. They are summarized in Table 6.6. The definition of the dimensionless variables is different, however, the normalizing time now being the time tR at which the potential is reversed. Definitions of the new time and space variables and of the kinetic parameter are thus changed (see Table 6.6). The equation systems are then solved numerically according to a finite difference method after discretization of the time and space variables (see Section 2.2.8). Computation of the... 

A graphical representation of the dimensionless irreversible voltammograms obtained for the four dimerization mechanisms under pure kinetic conditions is given in Chapter 2 (Figure 2.14) together with their peak characteristics. 

Another reaction mechanism explaining the observed enhancement of A 0bs values with increasing [amine]o values in S vAr reactions is the dimer mechanism 276, which involves the self-association of the amines277-279 and which (in some cases) may be considered overlapped with mechanism of Scheme 14. A reaction pathway for dimer mechanism is shown in Scheme 17. Considering the zwitterionic intermediate 114 it is possible to have a catalysis in removing the proton and the leaving group (reaction pathway indicated by fc3). 

In terms of the dimer mechanism a term in [P]2 would also be expected in special systems according to the reacting scheme shown in Scheme 13. Actually, one molecule of pyridine would act forming the mixed associate nucleophile, and the second molecule... 

Furthermore, although the intercepts k kiK/k- ) and the slope (kikjK/k-i) are equally influenced by the dimerization constant K in equation 28, this does not imply that they should show the same effect on changing the solvent. According to the dimer mechanism , it could be expected that the base catalysed decomposition of the transition state SB2, measured by Ag, should be more depressed by small additions of protic solvents than the spontaneous decomposition measured by Ag. Indeed, the overwhelming evidence on the classical base catalysis by amines shows that usually Ag is more important in aprotic than in protic solvents1. 

If the dimer mechanism interpretation is correct, addition of a HBA co-solvent, e.g. dimethyl sulphoxide (DMSO) (/> value = 0.76)178, in catalytic amounts should increase the reaction rate by forming a mixed aggregate RNH2 OS(CH3)2 (B DMSO), equation 35, where the amine acts now as a HBD, and therefore this mixed aggregation should increase its nucleophilicity. DMSO has been shown to increase the nitrogen electron density of primary and secondary amines161. 

The interpretation of formation of homo- (or hetero-) conjugated acid BH+B by proton transfer from the intermediate and the electrophilically catalysed departure of the nucle-ofuge due to this aggregate is common to this and to the dimer mechanism and they can be formulated as essentially the same, and as reflecting different parts of a spectrum of methods for the formation of the second intermediate153. For a given nucleophile, dimer formation increases with increase of concentration, hence the relative importance that reaction via a dimer should increase with increasing nucleophile concentration. 

Several alternative mechanisms have been described here that have been reported to explain the anomalous kinetic results, such as the observed fourth-order kinetics. Further treatment of the different equations may help to understand the scope of the different proposals. In a simplified form for the dimer mechanism, only attack by the dimer nucleophile can be considered, as shown by equation 41. 

Attempts to convert 1-bromo-l-phenylacetonitrile into the dicyano derivative under liquidrliquid two-phase conditions have been unsuccessful but, on addition of aqueous sodium hydroxide, l,2-dicyano-l,2-diphenylethene is formed by an oxidative dimerization mechanism [18], Similarly, diethyl bromomalonate fails to produce the corresponding azide with lithium azide under catalytic conditions the sole product (15%) is the ethene-l,l,2,2-tetracarboxylate [19]. 

The shape of this wave and the variation with pH are both consistent with fast equ-librium reactions In the pH region lower than the value of pK, for the hydroxyl radical, the reactions of this hydroxyl radical dominate the electrochemical process. Controlled potential reduction at the potential of this first wave indicates a IF process and the principal products are dimers of the hydroxyl radical. The second wave in this acidic region is due to addition of an electron and a proton to the neutral radical. This process competes with dimerization in the mid-pH range where the two polarographic waves merge. Over the pH range 7-9, monohydric alcohol is the principal product. At pH <3 or >12, pinacols are the main products. Unsymmet-rical carbonyl compounds afford mixtures of ( )- and meso-pinacols. Data (Table 10.3) for the ( ) / meso isomer ratio for pinacols from acetophenone and propio-phenone indicate different dimerization mechanisms in acid and in alkaline solutions. 

Scheme 9 Proposed dimeric mechanism for ROP of L-lactide initiated by 54 [79]...

Other members of the homologous series containing n-hexane would be expected to be subject to the same types of ionic reactions as postulated for n-hexane. An analysis of the dimer product patterns from solid n-pentane through n-nonane33 shows a close similarity which indicates that similar mechanisms are operating throughout this series. These similarities are summarized in Table XI. It is of interest to note that the enhancement factor for the direct dimerization mechanism as shown in the last row of Table XI is a constant. 

A density functional study of the transition structures of Ti-catalyzed epoxidation of allylic alcohol was performed, which mimicked the dimeric mechanism proposed by Sharpless et al.5 Importance of the bulkiness of alkyl hydroperoxide to the stereoselectivity, the conformational features of tartrate esters in the epoxidation transition structure, and the loading of allylic alcohol in the dimeric transition structure model were pointed out. 

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