Bis-acyl dications

Monocationic acyl ions are readily prepared as persistent species in solutions of low nucleophile strength.68 These acyl ions have been thoroughly characterized by IR and NMR spectroscopy, and several acyl ion salts have been characterized by X-ray crystallography. The monocationic acyl ions are often prepared in situ from carboxylic acids, esters, or anhydrides, by the action of a strong Brpnsted acid, or the ions can be prepared from ionization of an appropriate acid halide with a strong Lewis acid. Both methods have been used to prepare acyl-centered dications, some of which can be considered distonic superelectrophiles. As described previously, dicarboxylic acids cleave to the bis-acyl ions in superacid (FSChH-SbFs) provided that the acyl cations are separated by at least three methylene units (eq 54).55 The first bis-acyl dications were reported by Olah and Comisarow, being prepared by the reactions of dicarboxylic acid fluorides with superacidic SbFs (eq 72).69... 

Bis-acyl dications have also been generated from terephthalic acid derivatives. Ionization of terephthaloyl fluorides (211a-c) with SbFs has been used to prepare bis-acyl cations (212a-C, eq 73),69... 

A number of cyclic systems have produced bis-carboxonium dications (Table 1, entries 6-9). In the case of succinic anhydride (82), however, the product formed at — 80°C in FS03H-SbF5-S02 solution was the acyl-carboxonium dication (96, eq 26).37... 

When the solution is warmed to —40°C, NMR suggests that the acyl-carboxonium dication 96 rearranges through the bis-carboxonium dication 83 leading to the acyl-carboxonium dication 96. 

Asymmetric Diels—Alder catalysis was more successful with dication-like versions of the Zr-EBTHI system, and using conformationally better defined acyl-oxazoline dieno-philes. The bis(triflate) [Zr(EBTHI)(OTf)2] (Scheme 8.47) induced high levels of ee (>90%) in the cycloaddition to cyclopentadiene at low temperatures, especially in the polar solvent 2-nitropropane [87]. 

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