SNl-like transition state

An SNl-like transition state would consist of heterolytic cleavage of the carbon—carbon bond at the transition state of the reaction, followed by ET... 

All negatively charged nucleophiles must of course have a positive counterion. If this ion is Ag+ (or some other ion that specifically helps in removing the leaving group, p. 359), rather than the more usual Na+ or K+, then the transition state is more SNl-like. Therefore... 

Some authors have referred to highly dissociative AnDn reactions as being SnI-like . If the transition state is bimolecular, with both the nucleophile and the leaving group in the reaction coordinate, then the reaction has an AnDn mechanism, regardless of the extents of bond loss or formation at the transition state. In this chapter, we will refer to highly dissociative A Dn transition states as being oxocarbenium ion-like rather than SNl-like . 

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. ... 

Two reaction mechanisms, such as SNl and SN2 mechanisms, seem to be possible for explaining formations of 158a-c (Scheme 25). The fontiCT requires a resonance-stabilized indolyl cation 165 as an intermediate, while the latter indicates the presence of a transition state like 167. The introduction of a methoxy group into the 5 position of 165 should stabilize the corresponding cation 166, in which nucleophilic substitution on indole nitrogen would become a predominant pathway. 

Substitution on 2,2-difluorovinylic carbon has been examined by using ab initio MO calculations. For a model borate, lithium 2,2-difluoro-l-mesyloxyvinyl(trimethyl) borate (1), three feasible mechanisms were investigated the SNl-like, the Sn2-type, and the addition-elimination mechanisms. Four transition states were obtained. 

Evidently, the transition state for acid-catalyzed epoxide opening has an S Z-like geometry but also has a large amount of SNl-like carbocationic character. Since the positive charge in the protonated epoxide is shared by the more highly substituted carbon atom, backside attack of Br occurs at the more highly substituted site. 

The abstraction process proceeds by homolysis and is kinetically of the first-order (SNl-like). It involves two steps formation of new radicals, via the homolytic cleavage of the nonpolar, perfectly covalent bond in H2, in absence of any electrophile or nucleophile to initiate the heterolytic pattern, and the subsequent recombination of a new radical with another radical species. The hydride exchange is an example of the kinetically second-order, the first-order in both the incoming (nucleophile) and leaving (nucleofuge) hydride groups. It proceeds via the familiar Walden-inversion Transition State (TS) in a single, concerted reaction. 

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