Electrophilic promoters

As already shown in Figure 6.3b the system exhibits remarkable electrophilic promotional behaviour with p values up to 20.64 This is also shown in Fig. 8.60 which depicts a galvanostatic transient. Application of a negative current between the Pt catalyst-working electrode and the Au counter electrode causes a sharp increase in all reaction rates. In the new steady state of the catalyst (achieved within lhr of current application) the catalytic rate increase of C02 and N2 production is about 700%, while lesser enhancement (250-400%) is observed in the rates of CO and N20 production. The appearance of rate maxima immediately after current application can be attributed to the reaction of NO with previously deposited carbon.64... 

The electrophile-induced cyclization of heteroatom nucleophiles onto an adjacent alkene function is a common strategy in heterocycle synthesis (319,320) and has been extended to electrophile-assisted nitrone generation (Scheme 1.62). The formation of a cyclic cationic species 296 from the reaction of an electrophile (E ), such as a halogen, with an alkene is well known and can be used to N-alkylate an oxime and so generate a nitrone (297). Thus, electrophile-promoted oxime-alkene reactions can occur at room temperature rather than under thermolysis as is common with 1,3-APT reactions. The induction of the addition of oximes to alkenes has been performed in an intramolecular sense with A-bromosuccinimide (NBS) (321-323), A-iodosuccinimide (NIS) (321), h (321,322), and ICl (321) for subsequent cycloaddition reactions of the cyclic nitrones with alkenes and alkynes. 

The electrophile-promoted cyclization in cyclic systems is a highly regio- and stereoselective reaction that can be carried out under various conditions and with a range of electrophiles. The bicyclic lactone products have the cfs-ring fusion, indicating that the electrophile approaches from the face of the molecule anti to the carboxyalkyl side chain. Such halolactonizations constitute a useful approach to bicyclic and polycyclic compounds. 

Stereoselective conversion of 2-amino-3-hydroxy-4-pentenoic acid35 into the highly functionalized 2-furanones with the m-(4-hydroxy, 4-haloalkyl) relationship has been obtained through electrophile-promoted cyclization, due to the propensity of the hydroxy group to direct a cis substitution. Thus, treatment of (2.S, 3S)-2-tcrt-butoxycarbonylamino-3-hydroxy-4-pentenoic acid (32) with /V-bromosuccinimide in tetrahydrofuran gives the corresponding (35,4/ ,55 )-bromolactone 33 in 70% yield as the sole product35. 

Oxazolidinones have proven to be useful auxiliaries in reactions of radicals substituted a to carboxiimides bearing this group. The use of this auxiliary requires a Lewis acid to control the conformation of the oxazolidinone relative to the reactive radical center, and the complexation of the Lewis acid to the radical makes the radical very electrophilic, promoting reactions to unactivated olefins. The equilibrium between radical and Lewis acid is shown for structures 29A and B. The con-... 

Reitz AB, Nortey SO, Maryanoff BE, liotta D, Monahan R III (1987) Stereoselectivity of electrophile-promoted cyclizations of y-hydroxyalkenes. An investigation of carbohydrate-derived and model substrates. J Org Chem 52 4191-4202... 

Scheme 1. Electrophile-promoted addition of an alcohol to protected D-glucal 1.

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