Enforced-concerted process

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. 

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 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. 

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