Alkyl halides platinum complexes

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 cis/trans isomerization of platinum(II) complexes is a subject which will be discussed in some detail when the halide (Group VII) complexes are covered. Nevertheless the importance of reductive elimination reactions of platinum(II) alkyl and aryl complexes makes it imperative that this reaction be discussed here for alkyl and aryl platinum(II) compounds. 

Hydrosilylations of fluorine-containing alkenes are free radical reactions initiated by UV light or organic peroxides The direction of addition is the same as with fluonnated alkyl halides However, the reaction between hydrosilanes and fluorine-containing olefins catalyzed by platinum group metal complexes may result in bidirectional addition and/or formation of a vinylic silane, the latter by de hydrogenative silylation[/] The natures of both the silane and the catalyst affect the outcome of the reaction[7] A random selection of some typical new reactions of silanes are shown in Table 1 [1, 2, 3 4]... 

The most common route to alkyl or aryl complexes of the type [AuRL] is by the treatment of a halide complex with an alkyl- or aryllithium reagent. The first reactions of this type were performed (15) in 1959 [Eq. (5)], and the methyl and phenyl compounds were found to have chemical and thermal stabilities intermediate between those of the previously known organopalladium and -platinum complexes. 

Vanhoye and coworkers [402] synthesized aldehydes by using the electrogenerated radical anion of iron pentacarbonyl to reduce iodoethane and benzyl bromide in the presence of carbon monoxide. Esters can be prepared catalytically from alkyl halides and alcohols in the presence of iron pentacarbonyl [403]. Yoshida and coworkers reduced mixtures of organic halides and iron pentacarbonyl and then introduced an electrophile to obtain carbonyl compounds [404] and converted alkyl halides into aldehydes by using iron pentacarbonyl as a catalyst [405,406]. Finally, a review by Torii [407] provides references to additional papers that deal with catalytic processes involving complexes of nickel, cobalt, iron, palladium, rhodium, platinum, chromium, molybdenum, tungsten, manganese, rhenium, tin, lead, zinc, mercury, and titanium. 

Relative Rates for 8 2 Reactions of Chloride, Azide, and Thiocyanate Ions with Alkyl Halides, Methyl Tosylate, Aryl Halides and a Platinum Complex in Methanol (M) and in DMF (D), at 25°C... 

Oxidative addition of a Sn-H bond, which produces a species containing a M-Sn bond, was considered earlier (10.3.8.3). Platinum complexes, in particular, also react by oxidative addition of bonds between Sn and halide, aryl, alkyl, or Sn to produce species containing Pt-Sn bonds. 

Much of the mechanistic understanding of oxidative addition reactions comes from work oi the reaction between alkyl halides and zerovalent platinum complexes. The work is full] summarized in texts covering organometallic chemistry no details will be given here bu... 

This reaction is part of the mechanism for reductive eliminations of alkyl halides from Pt(IV). As noted in Chapter 8 (reductive elimination), this reaction occurs by initial dissociation of iodide to generate a cationic Pt(IV) methyl complex and subsequent attack of iodide on the platinum(IV) methyl group to generate methyl iodide (Equation 11.13). The reductive elimination of methyl aryl ethers, methyl acetate, and methyl trifluoroacetate... 

Shilov reported some of the earliest evidence that transition metal complexes could selectively cleave the C-H bonds of alkanes in a catalytic fashion. Shilov showed that H/D exchange would occur between alkanes and deuterated acid in the presence of platinum complexes (Equation 18.5 and Table 18.1). In addition, Shilov showed that the oxidation of alkanes occurred in the presence of a platinum(II) catalyst, although a platinum(IV) complex was needed as the oxidant. These reactions led to a mixture of alkyl halides formed from the halide of the Pt(IV) oxidant (Equation 18.6) and trifluoroacetate from the trifluoroacetic acid solvent. The cost of platinum(IV) as an oxidant makes this reaction impractical. However, these results provided hope that selective alkane functionalization could be developed because H/D exchange occurred faster at primary C-H bonds than at secondary C-H bonds (Table 18.1), and some selectivity for oxidations of primary C-H bonds over secondary C-H bonds was observed. As noted in Chapter 6, these results motivated a large number of groups to seek transition metal complexes that would insert into, or by other means selectively cleave, the C-H bond of alkanes and create products from this bond cleavage that could be observed directly. 

Some reactions of Pd(PPh3)4 are given in Fig. 5.18. The platinum derivative behaves similarly. Sulphur dioxide acts as a Lewis acid. The reactions with alkyl halides usually follow the Sj 2 mechanism, although free radical pathways can also participate. The reactions of aryl halides, which are normally rather unreactive towards nucleophiles are of interest. The palladium complexes catalyse the arylation of alkenes (the Heck reaction) by the following cycle ... 

One final type of coordinatively-unsaturated complex where stereochemical studies have demonstrated the nucleophilic character of the metal towards alkyl halides is [Pd(PRj)3]. These reactions contrast with those of the analogous platinum complexes where the participation of free radicals has been demonstrated (Stille and Lau, 1977 and references therein). 

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