Formylation, superelectrophilic

The reaction of trivalent carbocations with carbon monoxide giving acyl cations is the key step in the well-known and industrially used Koch-Haaf reaction of preparing branched carboxylic acids from al-kenes or alcohols. For example, in this way, isobutylene or tert-hutyi alcohol is converted into pivalic acid. In contrast, based on the superacidic activation of electrophiles leading the superelectrophiles (see Chapter 12), we found it possible to formylate isoalkanes to aldehydes, which subsequently rearrange to their corresponding branched ketones. 

Formylation of isobutane with carbon monoxide in the presence of an excess of A1C13 was first reported by Nenitzescu to yield, among others, methyl isopropyl ketone (31%).168 A new highly efficient superelectrophilic formylation-rearrange-ment of isoalkanes by Olah and coworkers has been described.282 Selective formation of branched ketone in high yield with no detectable branched acids, that is, the Koch products, was achieved. A particularly suitable acid is HF—BF3, which transforms, for example, isobutane to methyl isopropyl ketone in 91% yield. The... 

Olah et al.450 have recently described a new, highly efficient superelectrophilic formylation-rearrangementofisoalkanes. Branched ketones are formed in high yields and with high selectivity with no detectable branched acids (Koch products) in the presence of moderately strong superacids such as HF-BF3 or triflic acid-BF3. Carbonylation of isobutane under such conditions gives isopropyl methyl ketone in high yield [Eq. (5.164)] The transformation was interpreted with the involvement of... 

The reaction yield increases with acid strength, further suggesting the intermediacy of the protosolvated, superelectrophilic formylating species (9). 

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