Association electrophilic path

The AdE2 is the conceptual reverse of the El elimination. The first step, Ag, (Association, Electrophilic) is a new electron flow path and has the following general form. The arrow starts from the pi bond, breaking it and forming a new sigma bond to the electrophile, creating a carbocation next to the carbon-electrophile bond. 

Path Ae, Electrophile Addition to a Multiple Bond (Association, Electrophilic)... 

The activation associated with the catalytic path also appears to cause orientational changes in a variety of the molecules. This can also be accounted for by the electrophilic nature of the reactions. In the... 

As shown in equation 90, the ionic intermediate can follow several reaction routes. The product distribution is therefore controlled not only by the nature of the intermediate, whether bridged or weakly bridged, but also by association with its nucleophilic partner and by the rate ratios derived from the different reaction paths. All these factors depend on the alkene structure, the electrophile and the reaction conditions (solvent, added salts, temperature). 

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). 

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