Cupric tetrafluoroborate

Another, potentially extremely useful reaction, is the Diels-Alder reaction. The reaction is illustrated here with N-nitroso-dehydropiperi dine and cyclopentadiene. This reaction is sluggish without catalysis that can be accomplished by addition of various metal salts, such as cupric tetrafluoroborate. 

As discussed in Section 10.3, the system consisting of a diazonium ion and cuprous ions can be used for hydroxy-de-diazoniation at room temperature in the presence of large concentrations of hydrated cupric ions (Cohen et al., 1977 see Schemes 10-7 to 10-9). With (Z)-stilbene-2-diazonium tetrafluoroborate under these conditions, however, the major product of ring closure of the initially formed radical was phenanthrene (64%). When the cupric nitrate was supplemented by silver nitrate the yield increased to 86% phenanthrene. Apparently, the radical undergoes such rapid ring closure that no electron transfer to the cupric ion takes place. 

The nitrite ion behaves as a nitrogen nucleophile with vinyliodonium ions. Thus, admixture of the vinyliodonium tetrafluoroborates shown in equations 188 and 189 with sodium nitrite results in the production of nitroalkenes119,125,126. Cupric sulfate is apparently necessary for the success of the latter reaction, although its role has not been clarified. The (cyclohexenyl)iodonium salt also reacts with sodium thiophenoxide to give the corresponding vinyl phenyl sulfide125,126. 

Nitration Acetone cyanohydrin nitrate. Acetyl nitrate. /-Amyl nitrate. Cupric nitrate-Acetic anhydride. Dinitrogen pentoxide. Dinitrogen tetroxide. Dinitrogen tetroxide-Boron trifluoride. Nitronium tetrafluoroborate. Nitryl chloride. Polyphosphoric acid-Nitric acid. Potassium f-butoxide-Amyl nitrate. Tetranitromethane. 

Methyl o-iodobenzoate is converted under these conditions (equation 11) into methyl salicylate (main product) and dimethyl diphenate (minor product). If the cuprous salt is replaced by the cupric salt, hydrolysis is essentially the only reaction (equation III). If ammonium tetrafluoroborate is added, methyl benzoate is the only product. 

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