Isobutylene, formylation

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. 

Olah et al. reported the triflic acid-catalyzed isobutene-iso-butylene alkylation, modified with trifluoroacetic acid (TFA) or water. They found that the best alkylation conditions were at an acid strength of about//q = —10.7, giving a calculated research octane number (RON) of 89.1 (TfOH/TFA) and91.3 (TfOH/HaO). Triflic acid-modified zeohtes can be used for the gas phase synthesis of methyl tert-butyl ether (MTBE), and the mechanism of activity enhancement by triflic acid modification appears to be related to the formation of extra-lattice Al rather than the direct presence of triflic acid. A thermally stable solid catalyst prepared from amorphous silica gel and triflic acid has also been reported. The obtained material was found to be an active catalyst in the alkylation of isobutylene with n-butenes to yield high-octane gasoline components. A similar study has been carried out with triflic acid-functionalized mesoporous Zr-TMS catalysts. Triflic acid-catalyzed carbonylation, direct coupling reactions, and formylation of toluene have also been reported. Tritlic acid also promotes transalkylation and adaman-tylation of arenes in ionic liquids. Triflic acid-mediated reactions of methylenecyclopropanes with nitriles have also been investigated to provide [3 + 2] cycloaddition products as well as Ritter products. Tritlic acid also catalyzes cyclization of unsaturated alcohols to cyclic ethers. ... 

Irregularities occur with branched olefins. As Nienburg et aL [236] and Keulemans et aL [25] first pointed out, no quaternary carbon atoms are formed by olefin hydroformylation, at least under the standard conditions given above that is, the formyl group is not attached to a carbon atom that is branched. Accordingly, isobutylene forms almost exclusively 3-methyl-butanal [237], only about 5% of the isolated product being pivalaldehyde. 

Efforts to find conditions where the formyl group will be attached to a branched carbon atom have met with only limited success. With isobutylene at higher temperature (220 °C) and pressure (425 atm) required for catalyst stability, an 8% yield of neopentyl compounds is obtained, together with 61.6% yield of isopentyl compounds. At the same time, 25% of the starting material was hydrogenated [237]. Good yields of the neopentyl structures have been obtained only at room temperature with stoichiometric quantities of hydrocarbonyl [35] (see also p. 9). 

---