Periana system

The Periana system is currently the most active catalytic system for the C-H activation of methane. The proposed reaction mechanism (Scheme 6) is also based on three steps, C-H activation, oxidation, and functionalization. An important feature of the overall process is that the methyl ester is less reactive with the catalyst than methane. This is attributed to greater inhibition of the presumed electrophilic reaction of the C-H bonds of methylbisulfate in comparison with methane as a result of the electron-withdrawing ability of the bisulfate group... 

Later, Periana and coworkers proposed (2,2 -bipyrimidyl)platinum(ll)dichloride as a catalyst ( Periana system see a recent review [4d]). Fuming sulfuric acid is the oxidant in this case. A simplified scheme of the catalytic cycle is shown in Fig. 1.2. It can be seen that some intermediates contain a-methyl-platinum bonds. 

Figure 1.2 The simplified catalytic cycle for the methane oxidation by the Periana system. Adapted from Reference 4d.

Oxidation of Methane. A variety of new catalyst systems have been disclosed, and new reagents were developed with the aim to perform selective transformation of methane to methanol, methyl esters, and formaldehyde. Much work was carried out in strongly acidic solutions, which enhances the electrofilicity of the metal ion catalyst, and the ester formed is prevented from further oxidation. An important advance in the selective oxidation of methane to methanol is Periana s 70% one-pass yield with high selectivity in sulfuric acid solution under moderate conditions.1073 The most effective catalyst is a Pt-bipyrimidine complex. Pt(II) was shown to be the most active oxidation state generating a Pt-methyl intermediate that is oxidized to yield the product methyl ester. A density functional study... 

Shilov chemistry, developed from 1970, employs [Pt(II)CLt] salts to oxidize alkanes RH to ROH or RCl with modest efficiency. Pt(IV) is an efficient (but economically impractical) primary oxidant that makes the process catalytic. This discovery strongly contributed to the continuing activity in CH activation. Periana developed a related and much more efficient system for methane oxidation to methanol using 2,2 -bipyrimidine ligands and sulfuric acid as solvent. In this case, the sulfuric acid is the primary oxidant and the methanol formed is protected by being converted in situ to MeOSOsH, an ester that strongly resists further oxidation. This area is more fully described under the entry Alkane Carbon-Hydrogen Bond Activation. 

In an important development, Periana made Shilov-hke chemistry more practically useful with a series of methane conversion catalysts. The first such system involved Hg(n) salts in H2SO4 at 180°, the latter being both a solvent and a mild reoxidant (equation 3). Methane was converted to the methanol ester, methyl bisulfate, MeOSOsH, in which the -OSO3H provides a powerful deactivating group to... 

The activation of C-H bonds is one of the elementary steps in chemistry. Intensive research has lead to homogeneous as well as heterogeneous systems which can activate the strong C-H bonds (cf. Section 3.3.6). There are numerous experimental studies which have more recently often been accompanied by theoretical calculations. The two best known examples for the activation of methane are the so-called Shilov system K2PtCl4 [1], which was one of the first systems reported, and the [Pt(bpym)Cl2] system of Periana, which is currently the most active system reported for the direct, low-temperature, oxidative conversion of methane to methanol by platinum salts such as dichloro( /-2-[2,2 -bipyrimidyl])platinum(II) [Pt(bpym)Cl2] with yields of more than 70% and a selectivity of 80% [2]. 

Periana et al. have reported a mercury system that catalyzes the partial oxidation of methane to methanol.81 Hg(II) is typically considered to be a soft electrophile and is known to initiate electrophilic substitution of protons from aromatic substrates. The catalytic reaction employs mercuric triflate in sulfuric acid, and a key step in the catalytic cycle is Hg(II)-mediated methane C—H activation. For methane C—H activation by Hg(II), an oxidative addition reaction pathway via the formation of Hg(IV) is unlikely. Thus, an electrophilic substitution pathway has been proposed, although differentiation between proton transfer to an uncoordinated anion versus intramolecular proton transfer to a coordinated anion (i.e., o-bond metathesis) has not been established. Hg(II)-based methane C H activation was confirmed by the observation of H/D exchange between CH4 and D2S04 (Equation 11.9). 

A later variant involved incorporation of an oxidant, Pt(IV), which led to formation of functionalized species, RX, from alkane, RH. In the typical chloride-rich Shilov systems, X is commonly Cl and OH. The Pt(IV) oxidant is reduced to Pt(II) during the reaction, but it has proved hard to replace this expensive oxidant by a cheaper one while retaining activity. A remarkable system of this type discovered by Periana [109] uses cone. H2SO4 as both oxidant and solvent and a Pt(II) 2,2 -bipyrimidine complex as catalyst with the result that CH3OSO3H, a methanol derivative, is formed from methane. 

The process is quantitative at 213 K, but the hydridocyclopropyl complex rearranges into the rhodiumcyclobutane compound when warmed to room temperature, rather than eliminating ty-CsH. Periana and Bergman remarked that this observation suggests that C-H activation in the rhodium system is kinetically preferred but the C-C activation product is the most stable from a thermodynamic point of view. 

Catalytic Hg Pt and Pd systems discovered by Periana represent a major advance for the... 

Periana discovered that use of bispyrimidine (bpym) as a ligand leads to stabilization of Pt(ll) in the Shilov system. It was shown that Pt(0) would actually be taken back into solution using bpym in strongly acidic media. At 220 °C in sulfuric acid solution, he showed that Pt(bpym)(0S03H)2 could catalyze the oxidation of methane to methylbisulfate (Scheme 3). The proposed mechanism involved methane activation by a Pt(ll) cation and oxidation to Pt(iv) by SO3 (formed in the hot H2SO4). ... 

Several systems for selective catalytic reactions based on Shilov s system have been developed with oxidants more practical than platinum(IV). Periana reported two different systems for the oxidation of methane in sulfuric acid containing SO,. One of the catalysts is a simple mercuric halide, and reactions catalyzed by this mercury compound generated methyl sulfate with turnover frequencies of 10" s" . The second system is more reactive and is based on a platinum complex containing a bipyrimidine ligand (Equation 18.7). In this case, methane is converted to methyl bisulfate with 81% selectivity, greater than 500 turnovers, and a turnover frequency of 10 s" . These reactions are selective for the functionalization of methane to this methanol derivative because the electron-withdrawing... 

Progress on the addition of aromatic C-H bonds to olefins has been made by Periana with iridium catalysts - - and Gunnoe with ruthenium catalysts. - Both systems illustrate that the anti-Markovnikov addition products can be generated in larger quantities than the Markovnikov products, although mixtures of regioisomers are still observed. Intramolecular additions of the C-H bonds of electron-rich heterocycles to electron-deficient alkenes have also been reported (Equation 18.65). Most recently, Tilley has reported the addition of the C-H bond of methane across an olefin catalyzed by scandocene complexes. This reaction occurs, albeit slowly, with Markovnikov regiochemistry. 

Following up on their earlier report of surprisingly selective mercury-catalyzed oxidation of methane to methyl bisulfate by sulfuric acid (which was also the reaction medium) [52], Periana and coworkers discovered that a bipyrimidine complex of Pt(II) worked even better, generating the same product in over 70% yield - a remarkable achievement, given that selective oxygenation of methane to methanol or derivatives thereof rarely surpasses yields of a few percent. A mechanism closely akin to that of the Shilov system was proposed (Scheme 15), with SO3 replacing Pt(IV) as the oxidant to convert RPt(II) to RPt(IV) whether the initial C-H activation involved an RPt(IV)H intermediate or not was left an open question [52]. 

In their pioneering work, Periana et al. [88] reported that Pd(II)/H2S04 catalytic system catalyzed direct conversion of methane to methanol and acetic acid with a combined selectivity of >90% at 455 K in liquid sulfuric acid. It was concluded that carbon atoms in acetic acid originate from methane and methanol, with the latter being primarily formed from methane. The reaction is initiated by the electrophilic CH activation with Pd(II) to yield Pd-CHs species. The activation is accelerated by sulfuric acid. The primarily formed Pd-CHs species is further transformed to methanol with simultaneous reduction of Pd(II) to Pd(0). The reduced Pd species are also formed upon methanol oxidation to some CO species. The latter participate in the carbonylation of the Pd-CHs species. To close the catalytic cycle Pd(0) is oxidized to Pd(II) by sulfuric acid. Since gas-phase O2 did not influence the reaction rate and the selectivity, free radicals were excluded as reaction intermediates. 

Fig. 5 Catalytic systems based on Ir (Bergman [21] and Periana [91, 92]), Re (Jones) [93], (Green) [94], and Os° (Hood) [95] that undergo CH activation with hydrocarbons but do not give oxy-functionalized products...

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