Reductive Elimination from Pt IV

2 Proposed Five-Coordinate Platinum(IV) Intermediates 2.1 Reductive Elimination from Pt(IV) 

Reductive elimination from a six-coordinate octahedral Pt(IV) center results in the coupling of two formally anionic ligands and the formation of a four-coordinate square-planar Pt(II) species. It has been typically observed that coordinatively saturated six-coordinate Pt(IV) organometallic complexes are remarkably stable toward reductive elimination. It is generally only after loss of an ancillary ligand from the octahedral complex that reductive elimination takes place from Pt(IV). 

The first suggestion that ligand loss was important in reductive eliminatimi reactions from Pt(lV) complexes dates back more than 40 years. In 1969, Ettore observed that added iodide inhibited the reductive elimination of Phi from L2PtPh2l2 in methanol and suggested iodide loss to form a six-coordinate solvento intermediate [13]. Several years later, a five-coordinate intermediate was proposed by Puddephatt in studies of C-C reductive eliminations from Pt(IV) complexes (see Sect. 2.1.1) [15]. Since then, the involvement of five-coordinate intermediates has been supported so consistently in both experimental (e.g., [10,13-42]) and computational studies (e.g., [29, 30, 43 8]) of alkyl C-C, C-H, and C-X reductive elimination that it is now accepted as the norm in mechanistic schemes for reductive elimination from Pt(IV). 

Nucleophihc attack on a Pt(IV) alkyl has been proposed as the product forming step in Shilov catalysis wherein alkanes are converted to alcohols and alkyl chlorides using Pt salts in aqueous solution [40, 41, 53, 54]. Kinetic studies of the proposed product-forming step using the model platinum complex [MePtCls] found that the loss of a chloride ligand occurs prior to nucleophilic attack [40,41]. As these investigations were carried out in aqueous solution, it was not possible to 

Five-coordinate Pt(IV) intermediates are also proposed in C-H reductive elimination from Pt(IV) alkyl hydrides. Protonation of Pt(II) alkyls at low temperature (—78 °C) with HX resulted in the first spectroscopic observations of Pt(IV) aUcyl hydride 

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C. Five-coordinate Pt(IV) Intermediates and Pt(II) a-Complexes in Reductive Elimination from Pt(IV) and Oxidative Addition to Pt(II)... 

There are now a number of quite stable Pt(IV) alkyl hydride complexes known and the synthesis and characterization of many of these complexes were covered in a 2001 review on platinum(IV) hydride chemistry (69). These six-coordinate Pt(IV) complexes have one feature in common a ligand set wherein none of the ligands can easily dissociate from the metal. Thus it would appear that prevention of access to a five-coordinate Pt(IV) species contributes to the stability of Pt(IV) alkyl hydrides. The availability of Pt(IV) alkyl hydrides has recently allowed detailed studies of C-H reductive elimination from Pt(IV) to be carried out. These studies, as described below, also provide important insight into the mechanism of oxidative addition of C-H bonds to Pt(II). 

Further insight into the carbon-oxygen reductive elimination from Pt(IV) and the involvement of five-coordinate Pt(IV) intermediates has been provided recently. The first direct observation of high-yield C-0 reductive elimination from Pt(IV) was described and studied in detail (50,51). Carbon-oxygen coupling to form methyl carboxylates and methyl aryl ethers was observed upon thermolysis of the Pt(IV) complexes ( P2 )PtMe3(OR) ( P2 =bis(diphenylphosphino)ethane or o-bis(diphenyl-phosphino)benzene OR=carboxylate, aryl oxide). As shown in Scheme 47, competitive C-C reductive elimination to form ethane was also observed. 

Scheme 25 Proposed mechanism of the kinetically controlled methyl iodide reductive elimination from Pt(IV). Due to the reversibility of the reductive elimination step, the thermodynamic products of CH3-CH3 coupling are eventually formed as the major species...

Fig. 3 Proposed transition state for the aryl-idiode reductive elimination from Pt(IV) complexes...

Williams BS, Holland AW, Goldberg KI (1999) Direct observation of C-O reductive elimination from Pt(IV). J Am Chem Soc 121 252-253... 

Reductive eliminations to form C-C bonds from platinum complexes also include those containing both Pt(II) and Pt(IV) centers. Reductive elimination of biaryls and cyclopropanes from Pt(II) complexes were shown in Equations 8.39-8.41. Reductive eliminations from Pt(IV) were shown in Equations 8.35 and 8.37. ... 

These opposite trends for reductive elimination to form carbon-heteroatom bonds from Pt(IV) and Pd(II) result from the differences in the mechanisms for reductive elimination from the two types of complexes. Reductive elimination from Pt(IV) relies on dissociation of X" and, therefore, is favored by complexes containing X ligands that are stable as the free aruon. In contrast, reductive elimination to form carbon-heteroatom bonds from Pd(II) occurs by a concerted three-centered mechanism. As a result, the stability of the anion is less important, and reactions from Pd(ll) are faster from complexes containing more covalently bound heteroatom ligands (Equation 8.52). ... 

In most studies of C-H reductive elimination from Pt(IV) alkyl hydride complexes, a mechanism involving dissociation of a ligand and formation of a five-coordinate Pt(IV) intermediate prior to concerted C-H coupling has been proposed... 

Five-coordinate Pt(TV) species with silyl ligands are poised to perform Si-C or Si-H reductive elimination from Pt(IV). Note that the microscopic reverse of the latter reaction, Si-H oxidative addition, was used to synthesize the first five-coordinate Pt(TV) complexes with silyl ligands (2a-c) [84]. Complex 6, which has Pt-Me groups and a Pt-SiMe3 group, was observed to react over time at room temperamre to form tetramethylsilane, the product of Si-C reductive elimination, and intractable Pt products [91]. The five-coordinate complex ( pypyr)Pt(H>2SiEt3, 7a, was found to react with HSiMeaEt to form product 7b. Study of this reaction showed that Si-H reductive eliminaticm from 7a was rate-determining and it occurred directly from the five-coordinate complex [97]. Reaction of 7a with phosphines at room temperature led to the formation of a Pt(II)H(PR3) complex and free silane, the product of Si-H reductive elimination. Complex 7a was observed to be an active catalyst for the hydrosilylation of ethylene, tert-butylethylene, and alkynes [97]. 

A substantial body of C-H activation chemistry can be initiated by reductive elimination from Pt(IV) species either of ethane from stable, five-coordinate (p-diketiminate)Pt Me3 or of alkane from six-coordinate TpPt HR2. In some cases, usually involving arene activation, new stable Pt(IV) products are obtained the course of benzene activation by the TpPt system has been examined theoretically [80]. For alkane activation, the final product is often a Pt(II)-olefin hydride... 

Although the C-X reductive elimination from Pd(IV) complexes has been established, it was recently suggested that Pd(III) dimers can also participate in the reductive elimination (Pig. 2) [57, 58]. Such, and relevant Pt(III) dimers, have been isolated and structurally characterized [59-63], however, it is not yet clear whether the mechanistic interpretation of the C-X elimination from these species should be treated differently from the established elimination fi om M(IV). As one... 

Stoichiometrically, the liberation of RX from R-Pt(IV)-X amounts to reductive elimination however, mechanistically, there are issues. Does the reaction proceed directly from six-coordinate Pt(lV), or is prior dissociation of a ligand required There have been extensive studies on the mechanism of reductive elimination from Pt(lV) in general, especially with regard to C-C bond formation, where preformation of a five-coordinate intermediate is required, although some (kinet-ically more favored) C-H eliminations from Pt(lV) appear to proceed directly... 

Relatively soon after the discovery that aqueous solutions containing PtCl - and PtClg- can functionalize methane to form chloromethane and methanol, a mechanistic scheme for this conversion was proposed (16,17). As shown in Scheme 4, a methylplatinum(II) intermediate is formed (step I), and this intermediate is oxidized to a methylplatinum(IV) complex (step II). Either reductive elimination involving the Pt(IV) methyl group and coordinated water or chloride or, alternatively, nucleophilic attack at the carbon by an external nucleophile (H20 or Cl-) was proposed to generate the functionalized product and reduce the Pt center back to Pt(II) (step III) (17). This general mechanism has received convincing support over the last two decades (comprehensive reviews can be found in Refs. (2,14,15)). Carbon-heteroatom bond formation from Pt(IV) (step III) has been shown to occur via nucleophilic attack at a Pt-bonded methyl, as discussed in detail below (Section V. A). 

For the oxidative addition pathway, however, it is not obvious why the C-H bond cleavage reaction should be more facile if the hydrocarbon first binds in the coordination sphere of the metal (Scheme 5, c). One argument could be that the equilibrium between the Pt(II) alkane complex and the five-coordinate Pt(IV) alkyl hydride has an intrinsically low activation barrier. Insight into this question together with detailed information about the mechanisms of these Pt(II) a-complex/Pt(IV) alkyl hydride interconversions has been gained via detailed studies of reductive elimination reactions from Pt(IV), as discussed below. 

Thus to date, virtually all studies of C-C reductive elimination to form alkanes from Pt(IV) have found that these reactions proceed via five-coordinate intermediates. Only very recently have stable examples of Pt(IV) alkyl hydrides been synthesized (53-69). Detailed studies of C-H reductive elimination to form alkanes from these related complexes have identified similar five-coordinate intermediates on the reaction pathway (see following section). 

The observation of stable Pt(IV) alkyl hydrides upon protonation of Pt(II) alkyls has provided support for the idea that the methane which had been observed in earlier studies (89-92) of protonation of Pt(II) methyls could be produced via a reductive elimination reaction from Pt(IV). An extensive study of protonation of Pt(II) methyl complexes was carried out in 1996 (56) and an excellent summary of these results appeared in a recent review article (14). Strong evidence was presented to support the involvement of both Pt(IV) methyl hydrides and Pt(II) cr-methane complexes as intermediates in the rapid protonolysis reactions of Pt(II) methyls to generate methane. The principle of microscopic... 

The experimental data available to date consistently indicate that ligand dissociation precedes reductive elimination from six-coordinate platinum(IV). In the reverse direction (oxidative addition), it seems necessary that the hydrocarbon molecule coordinates in the square plane of platinum(II). C-H bond cleavage then forms a five-coordinate Pt(IV) species consistent with the principle of microscopic reversibility. 

Probing C—H addition/elimination in Pt(ll)/Pt(IV) systems The importance of oxidative addition of aromatic and aliphatic C—H bonds to Pt(II) centers and its microscopic reverse, reductive elimination of C—H from Pt(IV) species, is ubiquitous in the context of both catalysis and synthesis. It is thus inevitable that the chemical, mechanistic, and kinetic facets of such reactions have become a prominent focus of group 10 poly(pyrazolyl)borate research, although this remains a relatively nascent area. 

More recently, the reductive elimination of alkanes from Pt(IV) complexes has become an intense area of investigation as it represents the reverse of the C-H oxidative addition reaction that has been shown to be catalytic in the presence of the Pt(II)/Pt(fV) couple. ... 

SCHEME 11.23 Proposed mechanism for C—H reductive elimination from observable Pt(IV) intermediate. 

Recently, Milstein and coworkers reported an interesting CH3-I reductive elimination chemistry from Rh(III) complexes (Scheme 31) [74]. The reactions, driven by steric bulk of the pincer ligands, represent the first example of the directly observed reductive elimination from metal complexes other than group 10. It was proposed that the reactions proceed via a concerted three-centered transition state rather than the SN2-type back attack of the halide at the methyl group, as was proposed for the isoelectronic Pt(IV) complexes. To the best of our knowledge, no... 

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