Reaction path concerted - -

In the pure concerted reaction there is no need to invoke the cationic or anionic intermediates in describing the transition state, but it now becomes evident that some deviation from this idealized route may be possible, and then we need a way to comment upon and to measure the extent to which the cationic or anionic character is mixed in in the transition state. This is now widely accomplished with the aid of energy surfaces of the type shown schematically in Fig. 5-19. Depending on the nature of the surface, the reaction path may follow a route far from the diagonal representing the pure concerted reaction, and the primary goal is to identify the location of the transition state on this surface. 

Figure 5-19. Two-dimensional energy surface for generalized reaction R —> P showing the cationic fl i and anionic fl ) possible intermediates. The dashed line is the reaction path for the pure concerted reaction.

Thus, the coordinate x measures the progress of the main reaction from R to P, whereas (y — j) is a measure of the perpendicular displacement from the pure concerted reaction path. 

The results are critically dependent on the level of theory. However, a stepwise mechanism with closed shell structures along the reaction path was found to be lower in energy than a concerted reaction. An all-cw conformer of 172 is reported to be a transition state rather than an intermediate. Similarities of the conformational isomers of the intermediate 2-butenedithial 172 with the dinitrosoethylenes discussed in Section IV,c are evident. 3,6-Diamino-substituted dithiins are predicted to be more stable in the open-chain bisthioamide structure [95JST51]. The... 

The activation energies were computed to 3.0 (toward 183), 0.3 (toward 182), and 21.8 kcal/mol (toward 184) at the B3-LYP/6-31G level, and thus the mechanism leading to 182 is the preferred one. The transition states of all three reactions belong to concerted but asynchronous reaction paths. The transacetalization of 177 with acylium cations results in the formation of the thermodynamically stabilized 187 (Scheme 121) [97JCS(P2)2105]. 186 is less stable than 187, and 185 is destabilized by 32.5 kcal/mol. Moreover, transacetalization of 177 with sulfinyl cations is not a general reaction. Further computational studies on dioxanes cover electrophilic additions to methylenedioxanes [98JCS(P2)1129] and the influence... 

The reaction of methyl vinyl ketone with furan catalyzed by BF3 was studied by Babiano et al. [22]. The transition states predicted were also relatively concerted and highly asynchronous for all the reaction paths studied. 

Influence of ionic strength on the reaction rate constant. The influence of the ionic strength on the reaction rate constant was studied using KCl as electrolyte. The results obtained in this study are listed in Table 4, where we can see that the reaction rate constant for N-Br-alanine decomposition undergoes an increment of 40 % upon changing the ionic strength from 0.27M to IM, while in the case of N-Bromoaminoisobutyric acid the increment of the reaction rate constant is of about 12 %. This is an evidence of a non ionic mechanism in the case of the decomposition of N-Br-aminoisobutyric acid, as it is expected for a concerted decarboxylation mechanism. For the decomposition of N-Br-proline the increase on the reaction rate constant is about 23 % approximately, an intermediate value. This is due to the fact both paths (concerted decarboxylation and elimination) have an important contribution to the total decomposition process. 

Sulfate monoesters can react by dissociative paths, and this is the favored path. Whether such reactions are concerted or involve a very short-lived sulfur trioxide intermediate has been the subject of debate. ° Benkovic and Benkovic reported evidence suggesting that the nucleophile is present (though there is little bond formation) in the transition state for the reaction of amines with p-nitrophenyl sulfate. Alkyl esters of sulfuric or sulfonic acids normally react with C-0 cleavage only when this is disfavored, as in aryl esters, does one see S-0 cleavage. Sulfate diester... 

Cycloaddition reactions result in the formation of a new ring from two reactants. A concerted mechanism requires that a single transition state, and therefore no intermediate, lie on the reaction path between reactants and adduct. The most important example of cycloaddition is the Diels-Alder (D-A) reaction. The cycloaddition of alkenes and dienes is a very useful method for forming substituted cyclohexenes.1... 

The reactions of the vinylcarbenes 7 and 15 with methanol clearly involve delocalized intermediates. However, the product distributions deviate from those of free (solvated) allyl cations. Competition of the various reaction paths outlined in Scheme 5 could be invoked to explain the results. On the other hand, the effect of charge delocalization in allylic systems may be partially offset by ion pairing. Proton transfer from alcohols to carbenes will give rise to carbocation-alkoxide ion pairs that is, the counterion will be closer to the carbene-derived carbon than to any other site. Unless the paired ions are rapidly separated by solvent molecules, collapse of the ion pair will mimic a concerted O-H insertion reaction. 

Preliminary calculations of reaction paths have proved encouraging. Thus singlet carbene is predicted to insert into CH bonds, and to add to double bonds, by concerted processes involving no activation the critical geometries are as indicated in 32 and 33. The latter is of course that predicted by Skell56) and supported experimentally by ClossS7) it is also in accord with predictions based on considerations of orbital symmetry or Evans principle 31). The total lack of discrimination shown by carbene in reactions of this type also indicates that the activation energies must be zero or close to zero. 

The Diels-Alder reaction is the best known and most widely used pericyclic reaction. Two limiting mechanisms are possible (see Fig. 10.11) and have been vigorously debated. In the first, the addition takes place in concerted fashion with two equivalent new bonds forming in the transition state (bottom center, Fig. 10.11), while for the second reaction path the addition occurs stepwise (top row, Fig. 10.11). The stepwise path involves the formation of a single bond between the diene (butadiene in our example) and the dienophile (ethylene) and (most likely) a diradical intermediate, although zwitterion structures have also been proposed. In the last step, ring closure results with the formation of a second new carbon carbon bond. Either step may be rate determining. 

A regiospecific concerted syn addition (path (a)) competing with a radical chain reaction (path (b)) has been proposed to rationahze the addition of TeCl4 to olefins. 

Some years ago, Dr. Cross and I put forward a description of concerted reductive elimination (and, by implication, concerted oxidative addition) processes at transition metal centres, assuming the conservation of orbital symmetry, within a single dominant configuration, for the most obvious reaction path This picture had unexpected implications which some recent work has rendered quite explicit, and which are discussed in Part II of this article. 

Every discussion of the Diels-Alder reaction for 1,3-butadiene includes the observation that cycloaddition should occur only from the s-cis conformer to produce czs-cyclohexene. This conformational selectivity, however, further implies that cycloaddition of s-trans- 1,3-butadiene should lead to trans-cyclohexene, but the s-trans region of this potential surface has remained unexplored. A study of this problem shows that the concerted and stepwise reaction paths exist for both diene conformers, connecting them to the respective cyclohexene isomers.661 It is also demonstrated that the usual paradigm for the Diels-Alder reactions is incomplete a thorough understanding of this archetypal reaction requires consideration of the full range of processes shown in Scheme 6.10, not just those involving the s-cis conformer. 

There is a question whether Equation 6.56 shows all the intermediates on the reaction path. If, instead of rearrangement being concerted with loss of halide ion as shown in Equation 6.56, the halide ion departed first, then a nitrene162 would be formed as shown in Equation 6.57. To date no nitrene inter-... 

Dynamics effects, which were described in previous sections, on reaction pathways, concerted-stepwise mechanistic switching, and path bifurcation have in most cases been examined for isolate systems without medium effects. Since energy distribution among vibrational and rotational modes and moment of inertia of reacting subfragment are likely to be modified by environment, it is intriguing to carry out simulations in solution. The difference or similarity in the effect of dynamics in the gas phase and in solution may be clarified in the near future by using QM/MM-MD method. Such study would provide information that is comparable with solution experiment and help us to understand reaction mechanisms in solution. 

In reactions of mechanistic borderline, the reaction pathway may not follow the minimum energy path, but the reaction proceeds via unstable species on the PES. In other cases, the reacting system remains on the IRC but does not become trapped in the potential energy minimum. In some cases, intermediates are formed in reactions that should be concerted, whereas in other reactions a concerted TS gives an intermediate. Thus, the question of concerted versus stepwise appears too simple and the definition of concerted and stepwise reactions becomes unclear. In some reactions, the post-TS dynamics do not follow IRCs, and path bifurcation gives two types of products through a common TS. 

A fully concerted mechanism for reaction 299 has been eliminated as inconsistent with 14C and 15N KIEs and also with the observed inverse solvent D2O effect. The reaction path for the deamination of AMP has been formulated613 as a stepwise conversion involving the formation of tetrahedral intermediate 515 characterized by full-bonded hydroxyl and amino groups (equation 300). The TS for slow formation of 515, resulting from the attack of the hydroxyl from enzyme zinc-activated water at the C(6), is characterized by the C(6) OH bond order of 0.8 0.1 (late TS) and fully bonded NH2, that is by the nearly complete conversion to sp3 at C(6), and by nearly complete protonation of Nq), 516, The protonation of NH2 (in 515) and departure of NH3 (with TS 517) take place in the subsequent rapid steps as shown in equation 300, Zinc hydroxide is formed prior to attack514 at C(6). Enzymatic degradation of [6-14C]AMP has been carried out to prove the position of the radiolabel in 513 (equation 301). No radioactivity in the allantoin... 

DFT calculations were performed for the double proton transfer in bicyclic 2,2 -bis(4,5,6,7-tetrahydro-l,3-diazepine) (Figure 8) <2001CPL591>. Both a concerted and a stepwise mechanism for proton transfer are considered. Though the concerted transition state has two imaginary eigenfrequencies, dynamical calculations have demonstrated that it has to be taken into account in the mechanism of the proton transfer even if it is not a true reaction path. 

Scheme 7.15] or S -type mechanism [Equation (7.9)]. Depending on the nature of the nucleophile and catalyst employed, the subsequent nucleophilic substitution of the metal can follow either via a-elimination [path A, Equations (7.8) and (7.9), Scheme 7.15], via an SN2 reaction (path B) or via an SN2 -type reaction (path C). For reasons of clarity, only strictly concerted and stereospecific SN2- or SN2 -anti-type mechanistic scenarios are shown in Scheme 7.15. The situation might, however, be complicated if, e.g., the initial S l -anti ionization event is competing with an Sn2 -syn reaction. Erosion in stereo- and regioselectivity can be the result of these competing reactions. Furthermore, fluxional intermediates such as 7t-allyl Fe complexes are not shown in Scheme 7.15 for reasons of clarity. These intermediates are known for a variety of late transition metal allyl complexes and will be referred to later. Moreover, apart from these ionic mechanisms, radicals might also be involved in the reaction. So far no distinct mechanistic study on allylic substitutions has been published.

Schmidt81 has commented that symmetry-forbidden, concerted modes of reaction may be quite accessible if the reaction path is short, the force constants in reactant and product are both small, and the anharmonicities large. He cites a case where a forbidden reaction occurs readily with an activation energy of only 25 kJ mol-1. 

The potential (6.37) corresponds with the previously discussed projection of the three-dimensional PES V(p,p2,p3) onto the proton coordinate plane (pi,p3), shown in Figure 6.20b. As pointed out by Miller [1983], the bifurcation of reaction path and resulting existence of more than one transition state is a rather common event. This implies that at least one transverse vibration, q in the case at hand, turns into a double-well potential. The instanton analysis of the PES (6.37) was carried out by Benderskii et al. [1991b], The existence of the onedimensional optimum trajectory with q = 0, corresponding to the concerted transfer, is evident. On the other hand, it is clear that in the classical regime, T > Tcl (Tc] is the crossover temperature for stepwise transfer), the transition should be stepwise and occur through one of the saddle points. Therefore, there may exist another characteristic temperature, Tc2, above which there exists two other two-dimensional tunneling paths with smaller action than that of the one-dimensional instanton. It is these trajectories that collapse to the saddle points at T = Tcl. The existence of the second crossover temperature Tc2 for two-proton transfer was noted by Dakhnovskii and Semenov [1989]. 

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