Mechanism synchronous

The different possibilities for the creation of the pyrazole ring according to the bonds formed are shown in Scheme 46. It should be noted that this customary classification lacks mechanistic significance actually, only two procedures have mechanistic implications the formation of one bond, and the simultaneous formation of two bonds in cycloaddition reactions (disregarding the problem of the synchronous vs. non-synchronous mechanism). 

Since the quantum chemical calculations used to parameterize equations 6 and 7 are relatively crude semiempirical methods, these equations should not be used to prove or disprove differences in mechanisms of decomposition within a family of initiators. The assumption made in the present study has been that the mechanism of decomposition of initiators does not change within a particular family of initiators (reactions 1-4). It is generally accepted that trow5-symmetric bisalkyl diazenes (1) decompose entirely by a concerted, synchronous mechanism and that trans-phenyl, alkyl diazenes (2) decompose by a stepwise mechanism, with an intermediate phenyldiazenyl radical (37). For R groups with equal or larger pi-... 

The observed stereoselectivity with the alkenes utilized in this particular study led the authors to postulate that the reaction proceeded via a synchronous mechanism. However, there was also the possibility of a stepwise electrophilic mechanism involving carbocationic intermediates, as represented in Scheme 15. 

Fig. 3.6 Similar stabilization of isopolar TS and CS (concerted synchronous mechanism).

It is apparent there is a definite advantage to operating under solvent-free conditions. The specific microwave effect is here of low magnitude, but evident, because after 3 min the yield increases from 64 to 98%. Prolongation of the reaction time with classical heating led to an equivalent result. The microwave effect is rather limited here, because of a near-synchronous mechanism. 

Two alternative mechanisms were considered for the reaction (a) synchronous mechanism involving a cyclic transition state 91, and (b) stepwise electrophilic addition proceeding through formation of a carbocation 92 (Scheme 35). 

Reactions of unsymmetrical dienes and/or dienophiles have also been studied101 103 104. For these reactions ab initio calculations predict concerted non-synchronous mechanisms. The values of the potential energy barriers are very sensitive to the level of calculation and reasonable values are only obtained when electron correlation is included up to the MP3 level103. 

Power Supply Electrical precipitators are generally energized by rectified alternating current of commercial frequency. The voltage is stepped up to the required value by means of a transformer and then rectified. The rectifying equipment has undergone an evolution which began with the synchronous mechanical rectifier in 1904 and was... 

Using a valence bond scheme parametrized with an effective Hamiltonian technique, it was shown that the mechanistic preference for a synchronous pathway with an aromatic transition state versus a non-synchronous mechanism via biradicaloid intermediate can be controlled by two factors (1) the stability of the long bond in the Dewar valence bond structure, and (2) the softness of the Coulomb interaction between the end methylene groups in the 1,5-diene chain. This means that the mechanism of rearrangement (equation 153) can strongly depend on substituents218. 

Nitrene intermediates are formed in the thermal decomposition reactions of most alkyl azides, aryl azides, sulfonyl azides, and azidoformates. Some decompositions, however, must be regarded as proceeding by a synchronous mechanism. They are discussed in this section. 

These reaction products would result if an intermediate nitrene were formed (see Table I). The values of the entropy of activation (Table III) are also in good agreement with a synchronous mechanism. Some values, however, are rather high for such a reaction. If, nevertheless, in these cases the loss of nitrogen would precede the rearrangement, the lifetime of the intermediate nitrene should be so short that it would not have time to react with the solvent. 

When imines derived from a,(3-unsaturated aldehydes react with ketenes, formal [2+2] and [4+2] cycloadditions are possible [11, 41] (Scheme 5). Although a [ji2s+ji4s] concerted (but not necessarily synchronous) mechanism is conceivable,... 

A Sizable primary KIE s have been observed in the gas-phase thermal decomposition of 2,2-dimethyl-4-phenyl but-3-enoic acid528,529, consistent with a synchronous mechanism via a six-membered cyclic ideal TS, 462. Recently531 the mechanism of thermal decarboxylation of 461 and its derivatives HR1C=CR2CH2COOH (R1 and R2 = H, F, Me, Et and Cl) has been studied again530,531 from the theoretical point of view by ctb initio MO calculations and a twisted chair six-membered cyclic TS (463) has been constructed. 

The nomenclature used in describing bimolecular electrophilic substitutions involving cyclic transition states reflects, in part, the above-mentioned difficulty. Ingold3 has adopted the nomenclature of Winstein et al.1 and refers to such substitutions as SEi, but to the present author this is not a particularly appropriate choice since it does not indicate the bimolecular nature of the substitution. Dessy et al.8 have used the term SF2 to describe a mechanism, such as that in reaction (5), in which a four-centred transition state is formed, but not only is such a term too restricted, it also provides no indication that the mechanism is one of electrophilic substitution. The view of Reutov4 is that the cyclic, synchronous mechanism is very close to the open mechanism and that both can be described as SE2 mechanisms. Dessy and Paulik9 used the term nucleophilic assisted mechanisms to describe these cyclic, synchronous mechanisms and Reutov4,10 has recently referred to them in terms of internal nucleophilic catalysis , internal nucleophilic assistance , and nucleophilic promotion . Abraham, et al,6 have attempted to reconcile these various descriptions and have denoted such mechanisms as SE2(cyclic). 

The observed first-order kinetics and independence of the nature of the attacking base make the bimolecular E2 or E lcB mechanisms unlikely. The E1 mechanism or fraws-elimination would involve the formation of the amide anion NR2. Even when this cannot be ruled out, this possibility seems less probable in aqueous solutions. A synchronous mechanism, possibly with ring formation, seems to be more probable. 

A full paper on the conversion of alcohols into f-butyl ethers by treatment with t-butyl dicarbonate (B0C2O) and Mg(C10g)2 [or, less effectively, A1(C104)3] in CH2CI2 has elaborated upon the proposal made in a 2005 communication (see ORM 2005, p. 58). The process is now seen to involve initial formation of a mixed anhydride (18), which, catalysed by the metal, progresses to the product via a synchronous mechanism in a six-membered ring transition state (Scheme 6).42... 

Challenge-response schemes—These look like one-time password generators and use a similar synchronization mechanism however, additional user actions are required for authentication. It involves a challenge/response exchange with a new key being used at each login. 

The kinetics of the gas-phase elimination of ethyl and /-butyl carbazates have been studied in a static reactor system over the temperature range 220.3-341.7 °C and pressure range 21.1-70.0 torr.13 Theoretical calculations on the thermal decomposition of ethyl carbazate (4) suggest that the reaction proceeds by a concerted non-synchronous mechanism, through a quasi-three-membered ring transition state (Scheme 4). In contrast, the transition state structure for the thermal decomposition of /-butyl carbazate is an almost planar six-membered ring. 

Here curve I corresponds to water formation at a maximum rate in the respiration process. Since H+ and e formation is the general process for conjugated processes, their synchronization mechanism may be presented by the following scheme ... 

Similar to simulated enzymes, their biomimetic analogs frequently implement synchronous mechanism of the substrate transformation rather than the stage-by-stage one. Many metal ions enter to the composition of enzymes. As a rule, they form a coordination bond, preserve neutrality of charges and participate in catalytic processes. 

The above results indicate that the counterion does not participate in the transition state, which contradicts the synchronous mechanism. This con-... 

In all cases of hindered substituent-Cs bond rotations discussed above, an uncorrelated, stepwise or one-ring rotation rather than a correlated, synchronous mechanism has been proposed, which appears to be the norm with alkyl groups attached to planar frameworks (216). 

Because of the weakness of the Bronsted acidity of the surface hydroxyl groups, one can suppose that the synchronous mechanism is relatively efficient for these substitution reactions. The simplest reaction of this type is H/D isotopic heteroexchange with D-containing molecules. The heteroexchange with water is known to proceed rapidly at a temperature as low as 20°C. With molecular deuterium, such exchange occurs at 350-400°C with an activation energy of 15-20 kcal/mol (55). 

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