Platinum, reaction with alkanes

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. 

This equation is in good agreement with experimental results. An analysis of this dependence results in the following ratio of constants k -.k2. k-i = 100 14 0.6 (for cyclohexane at 100 °C), i.e in effect, the uncharged complex S2PtCl2 is the most active. This may be an indication of the importance of the electrophilic properties of the platinum complex in the reaction with hydrocarbons, although an appreciable value of the rate constant of such a moiety as PtCl, in its reaction with alkanes in aqueous solution, does not fit the usual concepts of electrophiles very well. It has been shown that the particular Pt(II) complexes obtained, which should form positive ions upon solution in water, PtCI(H20)j and [Pt(H20)4] react more slowly with alkanes than PtCl2. This demonstrates a more complicated nature of interaction of the bivalent platinum and the hydrocarbon molecule than that of the interaction of an electrophile with a nucleophile. 

In many other cases, oxidative additions of alkanes occur readily to transition-metal-alkyl complexes to generate hydride dialkyl intermediates that subsequently eliminate alkane and form a new metal-alkyl complex. For example, cations related to the alkyl hydrides of iridium formed by oxidative addition undergo reaction with alkanes at or below room temperature to generate new alkyl complexes (Equation 6.34). Cationic platinum complexes undergo similar reactions with substrates containing aromatic and aliphatic C-H bonds (Equation 6.35). " The C-H activation of the platinum complexes has been studied, in part, to understand and to develop systems related to the ones reported by Shilov that lead to H/D exchange, and oxidation and halogenation of alkanes. 

The oxidative addition of disilanes occurs to palladium complexes of isonitrile ligands and platinum complexes of trialkylphosphine ligands as part of tiie catalytic silylation of alkynes and aryl halides. The addition of stannylboranes to Pd(0) complexes has also been reported,and the addition of diboron compounds to many metal systems, such as Pt(0) complexes (Equation 6.67), is now common. These reactions all occur with metal complexes that do not undergo intermolecular reactions with alkane C-H bonds, let alone C-C bonds. Thus, the Lewis acidic character of these reagents must accelerate the coordination of substrate and cleavage of the E-E bonds. 

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into lTans-l,2-diols by acid-catalyzed epoxide hydrolysis. The corresponding cis-l,2-diols can be made directly from alkenes by hydroxylation with 0s04. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. 

For transition-metal catalyzed hydroxylation of alkane C-H bonds, the reactions of alkanes with platinum(II) complexes were the most successful. In an aqueous solution of hexachloroplatinic acid and Na2PtCl4, alkanes were converted into a mixture of isomeric alkyl chlorides, alcohols, and ketones, and the platinum(IV) is reduced to platinum(II).7 The kinetics of the reaction with methane as the alkane have been described in detail.8... 

As the data in Table XIV indicate, over platinum demethylation of a ring is slow compared to C—C bond rupture within a ring. On the other hand, it is well established [e.g., Kochloefl and Bazant (161) that if one uses a supported nickel catalyst which is known to favor stepwise alkane degradation, reaction with an alkylcycloalkane is largely confined to the alkyl group (s) which are degraded in a stepwise fashion and are finally removed entirely from the ring. 

As previously mentioned, Davis (8) has shown that in model dehydrocyclization reactions with a dual function catalyst and an n-octane feedstock, isomerization of the hydrocarbon to 2-and 3-methylheptane is faster than the dehydrocyclization reaction. Although competitive isomerization of an alkane feedstock is commonly observed in model studies using monofunctional (Pt) catalysts, some of the alkanes produced can be rationalized as products of the hydrogenolysis of substituted cyclopentanes, which in turn can be formed on platinum surfaces via free radical-like mechanisms. However, the 2- and 3-methylheptane isomers (out of a total of 18 possible C8Hi8 isomers) observed with dual function catalysts are those expected from the rearrangement of n-octane via carbocation intermediates. Such acid-catalyzed isomerizations are widely acknowledged to occur via a protonated cyclopropane structure (25, 28), in this case one derived from the 2-octyl cation, which can then be the precursor... 

Palladium- and platinum compounds have been successfully used for the functionalization of alkanes [110-112], An efficient and highly selective catalytic system is a platinum complex with a bipyrimidine ligand [Pt(bpym)Cl2], which provides up to 72% yield of methanol. The major drawback of the system is the reaction mediiun. Oleum leads to a large amount of diluted sulfuric acid when the formed ester is hydrolized. 

The platinum-catalyzed reaction of alkanes with chlorine leads to alkyl chlorides and alcohols (Table 6, entry 46) with modest rates and conversions [50], Cydooctane can be easily dehydrogenated (Table 6, entry 47) in the presence of a stabilized vinylalkane by use of the neutral rhenium compound ReH7(PR3)2 [51]. By employing an iridium-based catalyst, the photochemical dehydrogenation of methylcydohexane to methylenecyclohexane is performed at room temperature... 

Bis(pinacolato)diborane(4) selectively adds to terminal aikenes and cyclic aikenes having internal strain to provide bis(boryl)alkanes in 76-86% yields 85-89 in the presence of a catalytic amount of Pt(dba)2 at 50 °C67 (Scheme 16). It is interesting to mention that Pt(dba)2 directed 1,2-addition to certain conjugated dienes, whereas 1,4-addition through a 7i-allyl-platinum(II) intermediate is an energetically more favorable process. The 1,4-addition to penta-1,3-diene at 80 °C with Pt(PPh3)4 gives 90, but the same reaction with Pt(dba)2 selectively produced the 1,2-addition product 91 at room temperature (Scheme 16). 

Alkenes can be converted to alkanes by their reaction with hydrogen over a finely divided metal catalyst such as palladium, nickel, or platinum (Following fig.). This is an addition reaction, as it involves the addition of hydrogen atoms to each end of the double bond. It is also called a catalytic hydrogenation or a reduction reaction. 

Alkynes react with hydrogen gas in the presence of a metal catalyst and the process called hydrogenation. It is an example of a reduction reaction. With a fully active catalyst like platinum metal, two molecules of hydrogen are added to produce an alkane. 

In this chapter, we will consider the reactions of C-H compounds, such as alkanes, arenes as well as some others, with platinum complexes containing mainly chloride ligands. The reactions of alkanes with platinum(II) complexes have been the first examples of true homogeneous activation of saturated hydrocarbons in solution. Complexes of Pt(II) exhibit both nucleophilic and electrophilic properties, they do not react with alkanes via a typical oxidative addition mechanism nor can they be regarded as typical oxidants. Due to this, it is reasonable to discuss their reactions in a special chapter which is a bridge between previous chapters (devoted to the low-valent complexes) and further sections of the book that consider mainly complexes in a high oxidation state. Chloride cortplexes of platinum(IV) are oxidants and electrophiles and they will constitute the first subjects in our discussion of processes of electrophilic substitution in arenes and alkanes as well as their oxidation. 

These effects are not expected to correlate with only one parameter, such as the ionization potential, even if this is considered as the potential of an electron of the orbital which is affected in the reaction with the platinum complexes. The apparent linearity of the observed log k relation with the ionization potential might be connected to a considerable extent with the fact that the effect of C-H bond differences on the rate of H-D exchange in the case of platinum complexes is relatively small. When we go from the most inert alkanes of the methane series to the most active aromatic hydrocarbons (such as phenanthrene and pyrene), the rate differs by slightly more than two orders of magnitude, whereas the ionization potential changes by over 5 eV. This would have changed the reaction rate by a factor of approximately 10 at 120 °C if the latter were determined entirely by the energy of electron transfer from the hydrocarbon molecule. 

In the absence of platinum(II), the Pt(IV) concentration deaeases auto-catalytically, the observed induction period being removed by the addition of bivalent platinum. The reaction is convenient for kinetic study (because of the absence of by-products) and obviously proceeds according to a mechanism similar to that for the oxidation of alkanes. The reaction is first order with respect to the platinum(II) and acetic acid concentrations, and is retarded on the addition of acid and Cl ions, its rate being inversely proportional to the square of chloride-ion concentration at high Cl concentrations. The order of reaction with respect to Pt(IV) changes from 0 to 1. The mechanism suggested for this reaction... 

The reaction between the PtCh ion and alkanes can be induced by irradiation [26]. When a solution of hexachloroplatinic acid and n-hexane in acetic acid is irradiated by light (A > 300 nm) [26a,c,d] or y-quanta [26b], a 7t-coinplex of hex-1-ene with platinum(II) is formed in addition to isomeric chlorohexanes. This complex has been isolated in the form of the pyridine adduct, (hex-l-cne)PtCl2Py. The yield of the it-complex in the y-induced reaction reaches 17% based on Pt. The photochemical reaction is of first order with respect to hexane. It is interesting that the photochemical reaction with hex-2-ene affords the n-complex ofhex-l-ene (Scheme VII.5). This complex can be also prepared by the reaction ofhex-l-ene with PtCL, " under irradiation. It should be noted that the photoinduced reaction of PtCl6 or PtCU with olefins is a convenient method for the synthesis of various n-o efin complexes of platinum(II) [27]. The photostimulated reaction of PtCU with stilbene does not occur, possibly due to steric restrictions. 

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