Shilov systems

Synthetic organic chemistry applications employing alkane C-H functionalizations are now well established. For example, alkanes can be oxidized to alkyl halides and alcohols by the Shilov system employing electrophilic platinum salts. Much of the Pt(ll)/Pt(rv) alkane activation chemistry discussed earlier has been based on Shilov chemistry. The mechanism has been investigated and is thought to involve the formation of a platinum(ll) alkyl complex, possibly via a (T-complex. The Pt(ll) complex is oxidized to Pt(iv) by electron transfer, and nucleophilic attack on the Pt(iv) intermediate yields the alkyl chloride or alcohol as well as regenerates the Pt(n) catalyst. This process is catalytic in Pt(ll), although a stoichiometric Pt(rv) oxidant is often required (Scheme 6).27,27l 2711... 

The oxidation of />-toluenesulfonic acid to the corresponding alcohol and aldehyde was achieved using the Shilov system and when employing oxidants other than Pt(iv), including peroxydisulfate or phosphomolybdic acid, only moderate turnovers were observed (Equation (18)).28... 

Figure 19.14. Mechanism of methanol formation in the "Shilov" system...

During the 1970s Shilov published extensively on the reactions of alkanes in aqueous solutions of platinum(II) complexes [3]. The reactions are typically carried out in aqueous hydrochloric acid as solvent at <100°C with chloride salts of Pt(II) as catalyst and the chloride salts of Pt(IV) as the stoichiometric oxidant. Typical reaction yields, based on added methane, are less than 3% with >75% selectivity to methanol and methyl chloride. It was proposed the reaction proceeded via C-H activation to generate alkyl platinum intermediates in reactions with alkanes and later results are consistent with this proposal [4]. This system is one of the first systems proposed to operate via the C-H activation reaction and to generate potentially useful functionalized products. The key disadvantages of the Shilov system were the low rates (catalyst tum-over-frequency, TOF, <10 s ), short catalyst life (turnover-number, TON, <20), and the use of Pt (IV) as a stoichiometric oxidant. 

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]. 

For the Shilov system a mechanism was proposed which consists of three basic steps (1) activation of the alkane by a Pt" species, followed by (2) a two-electron oxidation forming a Pt intermediate and (3) reductive elimination of the oxidized alkane as shown in Scheme 4. 

While it has been generally assumed that heterolytic C-H bond cleavage is involved in the Shilov system (Eq. 4), the possibility that C-H activation proceeds through an oxidative addition step (Eq. 3) resulting in the intermediacy of a Pt(IV)(alkyl)(hydride) has been raised based on studies of model systems... 

SCHEME 11.22 Two proposed mechanisms for C—H activation by the Pt(II) Shilov system. 

The above exchange results require an intermediate with significant lifetime such as a a complex, and logical extension to the Shilov system suggests that PtIV(R)(H) and alkane adducts are transients in alkane activation. It is important, however, that formation of the PtH(R) intermediate (A) in Scheme 2 does not seem to arise from direct deprotonation of a complex E, but rather from the Pt RXH) tautomer C in equilibrium with it after it is converted to or D. Such a Pt"(RH) Pt,v(R)(H) equilibrium is analogous to well-known M(H2) M(H)2 equilibria. H/D isotopic exchange with benzene-46 very similar to that in Scheme 3 is seen in closely related complexes such as 33.142 Although the C,H-j 2-benzene... 

Among catalytic alkane conversions, the most important is the Shilov system and its descendents [108]. Discovered around 1970, these involve Pt(II) salts in aqueous solvents. Initially, the reaction studied was H/D exchange with D2O, where polydeuteration of alkanes was seen. The selectivity for attack at the terminal methyl groups of long chain alkanes made it clear that one was not dealing with classical electrophilic chemistry. The intervention of colloidal Pt was also excluded. 

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. 

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). ... 

Similarly, Ziegler et al. [80, 81] carried out DFT calculations to investigate the pure [PtCU] (Shilov system) and its 1 2 mixtures with [PtX2(H20)2] forX = F , Cl , Br , I , etc. They also indicated that ligand substitution indeed takes place and the real active species is dependent not only on their equilibrium concentration but also on their reactivity toward methane. Their calculations demonstrated that in the former case, the role that [PtCl2(H20)2] plays is negligible since its concentration is... 

In this regard, the Catalytica catalyst behaves like a self-assembled system as it can be readily formed by mixing Pt black or indissolvable Pt salt with bypm in fumed H2SO4. Noteworthily, forming Pt black or Pt salt is just the formidable problem in the Wacker system [82] as well as in the Shilov system [76]. 

The first alkane reactions involving organometallic intermediates were investigated by Shilov. The best current understanding of the pathway involved is shown in equation la. The metal M appears to bind the C—H bond to form a side-on a complex of a type discussed in more detail below. In the Shilov systems, this species loses a proton (equation la) to give a metal alkyl which is usually unstable and goes on to give a variety of catalytic reactions. Occasionally, however, the metal alkyl has been observed directly. 

In the second type of process the metal acts as a carbenoid and inserts into the C—H bond, a process generally termed oxidative addition in organometallic chemistry (equation 1 b). This reaction is believed to go via the same sort of alkane complex as in the Shilov system, but, instead of losing a proton, it goes instead to an alkylmetal hydride. This may be stable, in which case it is observed as the final product, or it may react further. 

Abstract The Shilov system, a mixture of di- and tetravalent chloroplatinate salts in aqueous solution, provided the first indication of the potential of organotransition metal complexes for activating and functionalizing alkanes under mild conditions the participation of higher-valent species plays a crucial role. In this chapter, we discuss the experimental and computational studies that have led to detailed mechanistic understanding of C-H activation and functionalization by both the original Shilov system and the many subsequent modifications that have been developed, and assess the prospects for practical, selective catalytic oxidation of alkanes using this chemistry. 

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