Trigonal bipyramidal transition state intermediate

Nucleophilic substitution at RSO2X is similar to attack at RCOX. Many of the reactions are essentially the same, though sulfonyl halides are less reactive than halides of carboxylic acids. The mechanisms are not identical, because a tetrahedral intermediate in this case (148) would have five groups on the central atom. Though this is possible (since sulfur can accommodate up to 12 electrons in its valence shell) it seems more likely that these mechanisms more closely resemble the Sn2 mechanism, with a trigonal bipyramidal transition state (148). There are two major experimental results leading to this conclusion. 

The mechanism of phosphate ester hydrolysis by hydroxide is shown in Figure 1 for a phosphodiester substrate. A SN2 mechanism with a trigonal-bipyramidal transition state is generally accepted for the uncatalyzed cleavage of phosphodiesters and phosphotriesters by nucleophilic attack at phosphorus. In uncatalyzed phosphate monoester hydrolysis, a SN1 mechanism with formation of a (POj) intermediate competes with the SN2 mechanism. For alkyl phosphates, nucleophilic attack at the carbon atom is also relevant. In contrast, all enzymatic cleavage reactions of mono-, di-, and triesters seem to follow an SN2... 

Fig, 5. Possible general reaction profile for gold(III) ligand replacements, dominated by a trigonal-bipyramidal transition state but allowing for square-pyramidal intermediates. 

The regiochemistry of cleavage of the cyclic sulfite upon attack by small and large nucleophiles has been rationalized by invoking trigonal bipyramidal transition states or intermediates, as shown in transition states (38) and (39) (Scheme 2.27). 

It is generally claimed that phosphoryl transfer may follow basically two pathways (see O Fig. 5.1). In the dissociative mechanism a trigonal metaphosphate intermediate is formed (Xu and Guo 2008), while the associative mechanism involves a relatively stable, trigonal bipyra-midal intermediate (Lahiri et al. 2003). Note that an intermediate refers to a local energy minimum on the reaction path. However, a third option has to be mentioned, too, this is the classical Sn2 mechanism with a trigonal bipyramidal transition state, referring to a maximum on the reaction path (Bernardi et al. 2002). The preferred pathway is determined by the nature of the phosphorus electrophile, the nucleophile, and the reaction medium (solvent or enzyme active site). Earlier computer simulations indicate that associative and dissociative mechanisms are similarly favored in the aqueous phase (Floridn and Warshel 1998), and also calculations for different enzymes support either dissociative or associative pathways depending on a variety of factors (Klahn et al. 2006) (O Fig. 30-9). 

Water exchange proceeds through a trigonal bipyramidal reactive intermediate [Li(H20)5]+, reached via a late transition state. In accordance with the above-mentioned experimental observation of a very fast exchange process, the activation barrier... 

Mechanistic details are very similar for DMSO and water exchange. The reaction proceeds through a distorted trigonal bipyramidal reactive intermediate [Li(DMSO)5]+ that is reached via a late transition state. The enthalpy profile (see Fig. 13) is in line with the experimentally observed very fast exchange process. The five-coordinate intermediate is computed to be 7.9kcalmol 1 less stable than [Li(DMS0)4]+ and free DMSO, while an overall activation barrier of only 8.4kcalmol 1 is computed. Obviously,... 

The stereochemistry of substitutions at sulfinyl sulfur that proceed with inversion has usually been explained in terms of the reaction proceeding through a trigonal bipyramidal intermediate (or transition state) [46] in which... 

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