Concerted synchronous

The conversion of the intermediate, II, to product, III, involves the removal of a proton from nitrogen and the breaking of the bond to the leaving group. These two events may occur in a single, concerted, synchronous process, and a variety of transition states, consistent with such a process, are possible. These warrant discussion. 

Equation 1.7). A concerted synchronous transition state [15] (the formation of new bonds occurs simultaneously) and a concerted asynchronous transition state [16] (the formation of one reaction depends on the nature of the reagents and the experimental conditions [17]. 

Most Diels-Alder reactions, particularly the thermal ones and those involving apolar dienes and dienophiles, are described by a concerted mechanism [17]. The reaction between 1,3-butadiene and ethene is a prototype of concerted synchronous reactions that have been investigated both experimentally and theoretically [18]. A concerted unsymmetrical transition state has been invoked to justify the stereochemistry of AICI3-catalyzed cycloadditions of alkylcyclohexenones with methyl-butadienes [12]. The high syn stereospecificity of the reaction, the low solvent effect on the reaction rate, and the large negative values of both activation entropy and activation volume comprise the chemical evidence usually given in favor of a pericyclic Diels-Alder reaction. 

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-... 

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

These experimental secondary deuterium KIEs observed in Diels-Alder reactions have been compared with the respective theoretical KIEs for the stepwise mechanism involving a diradical intermediate (equation 88a) and for concerted synchronous and asynchronous mechanisms (equation 88b) for the Diels-Alder reaction of butadiene with ethylene207. 

The barrier to reactions in which two processes occur in a concerted (synchronous) fashion has been proposed to be approximately equal to the sum of the barriers to these individual steps in a fuUy stepwise reaction [(A -f B), Fig. 2.4(1)]... 

This analysis does not require a specification of the timing of the bondforming events. This could correspond (1) to a fully concerted, synchronous pathway, (2) to the cyclization of benzene to a biradical referred to as pre-fulvene followed by addition of the olefin, or (3) to the bonding of the olefin to meta positions and subsequent cyclopropane formation, as indicated for the case of meta cycloaddition in Scheme 18. 

The average AV of -16 + 1 cm mol and the solvent independence of the process suggested that the reaction follows a nonpolar concerted, synchronous one-step mechanism. The observed pressure acceleration is very similar to that found for the insertion of dipropylcyanamide and l-(diethylamino)propene into the metal-carbene bond of pentacarbonyl(methoxyphenyl-carbene)chromium and -tungsten (shown in Scheme 1.2) for which varies between —17 and —25 cm mol ... 

This kind of catalysis is usually refered to as acid-base concerted (synchronous, or push-pull) bifunctional catalysis, which for brevity is expres.sed as acid-base bifunctional catalysis in some articles. 

In summary, the MC-SCF results suggest that for the 1,3 dipolar cycloaddition of fulminic acid to acetylene the concerted synchronous pathway is preferred, in agreement with the mechanism proposed by Huisgen. However, contrary to Huisgen s prediction, the related transition state is found to be planar. 

For this allowed reaction, at the 4-31G level, there are three different reaction paths,i.e. a concerted synchronous path which leads to cyclohexene,a concerted asynchronous path which leads again to cyclohexene via a syn-gauche attack and which is disfavoured by 2.2 Kcal/mol with respect to the synchronous mechanism and a two-step path which leads to vinylcyclobutane via an anti-gauche attack and which is disfavoured by 11 Kcal/mol with respect to the synchronous mechanism. The potential... 

Again for this allowed reaction two different reaction paths,a concerted synchronous path and a two-step path which is disfavoured by 7 Kcal/mol with respect... 

The position of the critical point on the concerted synchronous pathway for the 2s+2s reaction occurs at a position which is also a critical point in -T and in Q (the values of 3Q/3R and -3T/3R in Table 1 are both almost zero). Since Q is very flat this is the only possibility. Thus the maximum in -T coincides approximately with the local maximum on the energy surface shown in Fig. 2. The position of the critical point with respect to r for the 1,3 dpolar cycloaddition reaction occurs just before the minimum in Q (ie on the attractive part of Q) while -T is still decreasing in magnitude. Thus for the 1,3 dipolar cycloaddition reaction the transition state occurs only just before the critical... 

From the preceding discussion we see that, while the general shapes of the Q and T surfaces for the 3 reactions are similar, the condition for a critical point (slopes of Q and -T opposite or equal to zero) and the nature of the critical point (transition state or local maximum) are delicately intertwined and yield a very different topology on the surface that corresponds to the total energy. The critical points on the concerted synchronous pathway for the 2s+2s reaction and the 1,3 dipolar cycloaddition reaction... 

It was first assumed that the olefin metathesis proceeds by concerted (synchronous) breaking of the original C = C bonds and the formation of new C = C bonds. This reaction is forbidden by symmetry rules however, it is allowed in the presence of catalysts [equation (13.164)]. Another, pairwise (involving two alkene molecules), nonconcerted... 

In the reaction with 1-hexene, borane reacts as a Lewis acid and the alk-ene is a Lewis base. Donation of two electrons from the x-bond to the boron is accompanied by cleavage of the x-bond and formation of a new C-B a-covalent bond. In addition, a hydrogen atom from boron must be transferred to carbon, forming a new C-H o-covalent bond as the H-B bond is broken. So far, this is similar to the reaction of an alkene with HBr. However, experiments have shown that there is no intermediate, so there is no carbocation. If there is no intermediate, the C-H o-bond must be formed almost simultaneously with the C-B o-bond. Based on the experimental evidence that there is no intermediate, the reaction of borane with an alkene is said to be a concerted reaction. Formally, it classified as concerted asynchronous rather than concerted synchronous. A reaction that has no intermediate is called a concerted reaction. If the bond-making and bond-breaking events do not occur simultaneously, the reaction is said to be asynchronous. 

This is generally a concerted synchronous process with both new bonds forming simultaneously. Combinations of some highly polar substrates undergo concerted asynchronous reactions in which the two new o bonds are formed at different rates. In some rare cases, the reaction proceeds in a stepwise fashion with the absence of expected stereoselectivity. Orfanopoulos reported an example of a stepwise hetero Diels-Alder reaction in 2009. ... 

Fignie 7.21. A representation of an idealized E2 reaction in which the bond-making and bond-breaking steps occnr in concert, synchronously and to the same extent. Thus, in this ideahzed transition state, each bond is formed (broken) to the same extent at the same time, that is, exactly halfway A two-dimensional representation of this process is shown in Figure 7.22. 

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