Lewis Acid-Mediated Electrophilic Substitutions

Lewis adds can polarize organic residues in such a way that negatively polarized portions can react as carbanions. The most common Lewis add mediated forma- 

400 mesh, loading 1.96 mmol g based on Cl) was added to a solution of sodium ethyl acetoacetate (5.96 g, 39.2 mmol) in DMF (30 mL) and the mixture was stirred at 80 °C for 16 h. The resin was subsequently washed as 

The ketone resin (loading 1.88 mmol g based on C=0) was recovered. 

The cycHc malonic ester resin (loading 1.20 mmol g ) was thereby obtained. 

The loading of the resin was determined by back-titration with HCl after saponification with excess NaOH in EtOH. 

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Table 7. Summary of some Lewis acid mediated electrophilic substitutions. ( ) Indicates the center of attack by an electrophile. ...

Other types of heterofunctionalized cyclopropane derivatives used as starting materials in transition metal mediated conversions are acylcyclopropanes and similar systems. These systems, being electrophilic cyclopropanes, are opened by carbanion nucleophiles (e.g. cuprates, see Section l.B.2.1.3.) or nucleophilic transition-metal centers. In combination with siloxy groups, push-pull (capto-dative) substituted cyclopropanes are available as homoenolate precursors via Lewis acid mediated ring opening (see Section 1. B.2.1.4.).. This latter procedure has been used in tetrahydrofuran synthesis. 

Allyltributylstannanes are common reagents in both radical and Lewis acid-mediated reactions, although they are unknown to react with enones. However, there has been the first report of a reaction of an electrophilic allylstannane with an enone (Scheme 1). While the parent allylstannane does not react with enones, those substituted with an ester substitute smoothly at the y3-position of enones. The authors claim this to be the first non-basic and non-nucleophilic alternative to the Michael reaction. 

Study of the reactivity of aromatic C-H bonds in the presence of transition metal compounds began in the 1960s despite the quite early discovery of Friedel-Crafts alkylation and acylation reactions with Lewis acid catalysts. In 1967, we reported Pd(II)-mediated coupling of arenes with olefins in acetic acid under reflux [1], The reaction involves the electrophilic substitution of aromatic C-H bonds by a Pd(II) species, as shown in Scheme 2, and this is one of the earliest examples of aromatic C-H bond activation by transition metal compounds. Al-... 

Planar chiral orf/to-substituted (arene)Cr(CO)3-stabilized a-propargylic cations (67) may be generated from (66) in 5n1 fashion mediated by Lewis acid. They are in principle ambident electrophiles, susceptible to nucleophilic attack at the propar-gyl or allenyl positions. However, they react with 5-, N-, and jr-nucleophiles to give regioselectively the propargyl derivatives (68) in good yields and excellent diastereoselectivities, overall a process of retention by double inversion. Without the orf/to-substituent nucleophilic attack at both possible positions occurs. 

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