Enforced concerted

Table 1 includes partitioning data only for carbocations that are sufficiently stable to form in the nucleophilic aqueous/organic solvents used in these experiments. For example, it is not possible to obtain values of ks for reaction of secondary aliphatic carbocations in water and other nucleophilic solvents, because the chemical barrier to ks is smaller than that for a bond vibration.83 The vanishingly small barriers for reaction of secondary carbocations with nucleophilic solvents results in enforced concerted mechanisms2-3 for the nucleophilic substitution and elimination reactions of secondary derivatives in largely aqueous solvents.83-84... 

The description of the chemisorption in terms of cycloaddition reactions is useful if it leads to reliable predictions of the reaction products for most reactions, a variety of products are possible, yet only one will result from a particular cycloaddition mechanism. Central to the applicability of such schemes is the notion that the silicon dimers contain a weak 7r bond responsible for the enforced concerted motion of the two electrons involved. However, in reality there is little evidence to support the presence of even a weak 7r bond within the dimers. While DFT calculations that enforce spin pairing depict the bond as a singlet biradical [32], spin-polarized calculations predict a triplet ground state for the unbuckled dimer [33] with no 7T character whatsoever. The decoupling of the two silicon electrons means that their motion is not likely to be concerted so that a [2+2] cycloaddition reaction becomes better represented as an independent [1 + 2+1] process, a notation that recognizes the independence of the silicon free radicals. This mechanism is also illustrated in Fig. 3. In practice such a reaction is unlikely to proceed in a concerted fashion, and a key signature for it would be the... 

The theoretical study [22] lends no support to the existence of zwitterionic species resulting from water addition to phosphonic and phosphoric acids in the gas phase. Instead, these reactions are calculated to proceed through enforcedly concerted transition... 

Mech. 3 proceeds by Zn 5 ( OCH3) acting as a general base to promote delivery of methanol to P in an enforced-concerted process having instantaneous decomposition of a short-lived phosphorane intermediate via metal-assisted LG departure. 

One of the most widely applied cycloaddition techniques for the preparation of thietanes is the reaction of sulfenes with enamines. The stereochemistry of these reactions has been extensively investigated by Truce and Rach. Whether the mechanism is a two-step or a concerted process, both in accordance with the stereoselective formation of the cis form in Scheme 1, is still unresolved. The special orientation of the 1,4-dipolar intermediate 64, in which the charged phenyl and dimethylamino moieties are in proximity, enforces the cis geometry of the resulting thietane dioxide. In the concerted mode of reaction, formation of the orthogonal oriented unsaturated system, 65 should also yield the cis cycloadduct. 

These data could also be rationalized if there were a change in the ratedetermining step for a stepwise reaction, from k s for acid-catalyzed addition of solvent to o-l to k for the reaction of 81. However, it is unlikely that k will be rate determining for the stepwise addition to 81. This would require k s > k for Scheme 47. A value of k i > 1010 s 1 is expected for the strongly favorable deprotonation of H-81+ by water,162 and there is evidence for a significant barrier to k s for addition of water to -SR-substituted benzylic carbocations. For example, a value of k s = 5 x I07s 1 has been determined for addition of an aqueous solvent to the l-4-(thiomethylphenyl)ethyl carbocation. The concerted mechanism is favored because it avoids formation of the unstable acidic intermediate H-81+. It is possible, but not proven, that the concerted mechanism is enforced by the absence of a vibrational barrier to the step k i for deprotonation of this very strongly acid reaction intermediate by water.163,164... 

The reaction of hydroxide ion with carbocation 82 is expected to occur at the diffusion-controlled limit because proton transfer from the /i-OI I is thermodynamically favorable, and the activation barrier for collapse of the resulting zwitterion (108, Scheme 32) to reform the epoxide ring should be absent or very small. A concerted mechanism for this reaction is enforced if zwitterion 108 is too unstable to exist as an intermediate.104 The attack of hydroxide ion as a base on 82 as shown in Scheme 38 must be energetically more favorable than attack as a nucleophile to yield tetrol product. The exact yield of tetrol from direct combination of hydroxide ion with carbocation 82 could not be estimated, but is negligible or very small. 

The equilibrium constant for zwitterion formation (logATgq = -15.95) may be obtained from the equilibrium constant for formation of the neutral adduct (logA q = -13.39)" and the values of hydroxyl and imidazolyl of the adduct (8.12 and 5.56 respectively), which are determined by calculation. The decomposition rate constant Z aic can be determined for the putative zwitterion intermediate by combining the equilibrium constant and the observed rate constant (0.583 M" sec" ) to give 4.96 x 10 sec". This rate constant exceeds the value expected for a vibration (10 sec" ) and indicates that the intermediate cannot have a discrete existence and that the mechanism must be a concerted process enforced by the short lifetime of the intermediate. In other words the putative intermediate cannot pass enough reaction flux to support the observed reactivity. 

However, general acid catalysis is observed for the (1375 times slower) cyclization of the C-methyl compound 4.3 (R = CH3, Scheme 2.32) evidently methyla-tion destabilizes the vinyl carbanion to the point where it no longer has a significant lifetime in water, and the preassociation-concerted mechanism is enforced once more [67]. 

If there is no, or very weak, coupling between the motions of bond making and breaking because of unfavourable geometry and orbital overlap a concerted mechanism can only occur when it is enforced i.e. when there is no activation barrier for one of the steps decomposing the hypothetical intermediate [1]. 

Transition states for bond formation and breakage mirror each other. Such a condition requires that diffusion apart of the complex and either group is slower than bond formation with either group. If one group could depart then so could the other, which is not the case. This is an enforced preassociation mechanism. The only alternative to this mechanism that is consistent with the data would arise if the intermediate had too short a lifetime to exist and the reaction was concerted, proceeding via a penta-covalent intermediate or transition state. 

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