Loose Sn2 transition state

The opening of the bromonium ion is often regioselective. The nucleophile usually attacks the more substituted carbon atom of the ring, because this carbon atom is more positively polarised. The reaction proceeds via a loose SN2 transition state . 

Fig. 10 The relationship between the looseness (the nucleophile-leaving group distance) of the Sn2 transition state and the magnitude of the secondary a-deuterium KIE as determined by the Ca-H(D) out-of-plane bending vibrations in the transition state. Reprinted with permission from Ref. [55]. Copyright (1997) American Chemical Society.

The last two examples add the same group (OPr) to the same compound (an epoxide) to give different products. We can tell that the first is SnI as PrOH adds to the more substituted (tertiary and benzylic) position. Inversion occurs because the nucleophile prefers to add to the less hindered face opposite the OH group. If you said that it is an Sn2 reaction at a benzylic centre with a loose cationic transition state, you may well be right. 

Figure 7 Qualitative depiction of the energy profile along the reaction co-ordinate for the Sn2 reaction Cl ICII3CI >CICn3 I Cl, which involves nucleophilic substitution of the chloride of methylchloride by a chloride ion. The potential energy curves drop as the two reactants approach until a loose complex is formed. Then the energy rises rapidly to the transition state, which has two equal C-Cl interatomic distances (zero on the abscissa). The energy profile looks quite different in the gas and solution phases. Compared to the reactants (or products), the loose complex and the TS are poorly solvated, so the energies for these are much higher in solution than in a vacuum.

When 1-methylcyclohexene is used as the starting material, there is additionally a question of regioselectivity. The alcohol attacks the more hindered end of the bromonium ion—the end where there can be greatest stabilization of the partial positive charge in the loose 5 2 transition state. This reaction really does illustrate the way in which a mechanism can lie in between Sj l and Sn2. We see a configurational inversion, indicative of an Sn2 reaction, happening at a tertiary centre where you would usually expect S l-... 

The Hammett p value of -4.1 suggests a carbocation intermediate as does the regioselectivity of the reaction (MeOH attacks the benzyhc position) but the stereochemistry (the reaction occurs with inversion) and a modest negative entropy of activation (AS = -48 JmoH K ) suggest rather an Sn2 reaction with a loose transition state having substantial positive charge at the benzylic carbon. Neither piece of evidence alone would be enough to define the mechanism. 

A model of the potential surface is needed to understand why an SN2 reaction is slow. The double minimum surface shown in Figure 1 can accommodate the experimental results for this system (1-7). Although the central barrier is at a lower energy than that of the reactants, the reaction proceeds slowly because the transition state associated with the central barrier is tight and the sum of states associated with it is smaller than that associated with the loose transition state for decomposition back to reactants. The rate constant for the reaction is given by the rate constant for formation of the complex multiplied by the fraction of complexes that go on to products. This branching fraction is the ratio of the forward step over the sum of the forward and back steps and can be related to the efficiency, which is the reaction rate divided by the collision rate. 

Before leaving the discussion of possible mechanisms, it should be pointed out that they ntav not be as distinctly separated in concept as it may have appeared in their individual descriptions. The distinction between SNI and SN-2 in solvolysis reactions is blurred by the probability of varying degrees of nucleophilic solvent participation in the S, I transition slate [72]. Within SN2 (eq. 2.8) there can be different extents of bond breaking and bond making in the transition state at one extreme, a loose transition state with a nearly broken bond to I. but little bond making to Nu could be described as SNl-like (72. Second-order kinetics may also be expected if Nu reacts with an ion pair formed by rapid, reversible ionization of RL [73. ... 

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