Branched alkanes, electrophilic

Since the early 1960s, superacids have been known to react with saturated hydrocarbons to yield carbocations, even at low temperature [41]. This discovery initiated extensive studies devoted to electrophilic reactions and conversions of saturated hydrocarbons. Thus, the use of superacidic activation of alkanes to their related carbocations allowed the preparation of alkanecarboxylic acids from alkanes themselves with CO. In this respect, Yoneda et al. have found that alkanes can be directly carboxylated with CO in an HF-SbFs superacid system [42]. Tertiary carbenium ions formed by protolysis of C-H bonds of branched alkanes in HF-SbFs undergo skeletal isomerization and disproportionation prior to reacting with CO in the same acid system to form carboxylic acids after hydrolysis (eq. (9)). 

Super acid-catalyzed electrophilic hydroxylation of branched alkanes were carried out using HS03F SbF5 S02ClF with various ratios of alkane and hydrogen peroxide at different temperatures . Some of the results are summarized in Table 5. Protonated hydrogen peroxide inserts into the C—H bond of alkanes. The mechanism is illustrated in Scheme 10 with isobutane. 

The above mechanism, implying a bis-alkyl species, is the best way to explain (i) the observed selectivities and (ii) the primary and secondary products. It also takes into account important facts (i) the minor ](=SiO)2ZrH2] supported species (20-35% depending on the oxide support) is more electrophilic and activates C-H bonds of alkane faster than its homolog ](=SiO)3ZrH] (65-80%) [48] (ii) the higher selectivity in isobutene compared to that of methyl-branched pentenes is not consistent with a simple insertion of adsorbed propene into Zr-alkyl species because the concentration of Zr-propyl has to be greater than Zr-Me species (Scheme 3.12). 

CH Activation is sometimes used rather too loosely to cover a wide variety of situations in which CH bonds are broken. As Sames has most recently pointed out, the term was first adopted to make a distinction between organic reactions in which CH bonds are broken by classical mechanistic pathways, and the class of reactions involving transition metals that avoid these pathways and their consequences in terms of reaction selectivity. For example, radicals such as RO- and -OH readily abstract an H atom from alkanes, RH, to give the alkyl radical R. Also in this class, are some of the metal catalyzed oxidations, such as the Gif reaction and Fenton chemistry see Oxidation Catalysis by Transition Metal Complexes). Since this reaction tends to occur at the weakest CH bond, the most highly substituted R tends to be formed, for example, iPr-and not nPn from propane. Likewise, electrophilic reagents such as superacids see Superacid), readily abstract a H ion from an alkane. The selectivity is even more strongly in favor of the more substituted carbonium ion product such as iPr+ and not nPr+ from propane. The result is that in any subsequent fimctionalization, the branched product is obtained, for example, iPrX and not nPrX (Scheme 1). 

Normally intramolecular elimination of alkane from alkyl(hydride) complexes occurs readily and is favoured thermodynamically. There is interest, however, in the possibility of carrying out the reverse reaction, the addition of a C—H bond to an unsaturated transition metal centre. Alkanes are susceptible to electrophilic attack, for example by Lewis or Br nsted acids which convert linear alkanes into their branched isomers via carbonium ion intermediates. Linear and cyclic alkanes can be converted into aromatic hydrocarbons and hydrogen over metal surfaces such as platinum. These reactions are carried out on a large scale industrially in the reforming of petroleum. 

The term CH activatiorP emphasizes the selectivity difference between low-valent metal complexes and classical oiganic reagents. In a classical electrophilic Of radical route, radicals such as OH abstract an H atom from alkanes, PirH, but always to give the branched radical i-Pr. Sopoacids, abstract H ion from PrH, but always to give the branched ion By such classical routes, the ultimate functionalization product, t-PrX, is branched. 

---