Platinum complexes oxidative reductive elimination

The carbene complexes can also be formed by direct oxidative addition of ze-rovalent metal to an ionic liquid. The oxidative addition of a C-H bond has been demonstrated by heating [MMIM]BF4 with Pt(PPh3)4 in THF, resulting in the formation of a stable cationic platinum carbene complex (Scheme 15) (189). An effective method to protect this carbene-metal-alkyl complex from reductive elimination is to perform the reaction with an imidazolium salt as a solvent. 

A mechanism for catalysis by platinum compounds was proposed in 1965 by Chalk58) and has since been supported by increasing knowledge about silyl-metal systems and by the direct detection of Pt-Si211) and Rh-Si61,18s) complexes in the reaction mixtures. The suggested mechanism requires olefin coordination to the Pt(II) species (in the case of H2PtCl6 formed by reduction by the silicon hydride), oxidative addition of the silane, formation of an intermediate in which silicon and alkyl are both bonded to the platinum center, and reductive elimination of alkylsilane, probably assisted by coordination of more olefin ... 

Monomeric platinum(III) complexes have been observed frequently as transient species in electrochemical or pulse radiolysis studies and they are proposed as an intermediates in reductive elimination and oxidative addition reactions of platinum(IV) and platinum(II) respectively.382-388... 

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). 

Organothorium complexes such as [Th(r 3-allyI )4] supported on dehydroxylated y-alumina have been shown to exhibit activities rivaling those of the most active platinum metal catalysts.123 Thorium maintains its original +4 oxidation states at all times that is, the mechanism does not follow the usual oxidative addition-reductive elimination pathway. Partially hydrogenated products cannot be detected... 

All reactions belonging to this class involve an inner-sphere atom-transfer oxidation-reduction path. These reactions are generalized in Scheme 23. Path ii results in a net reduction to platinum(H) complexes, whereas pathways iii, iv and v represent net formal substitutions at platinum(IV). Each of these pathways can be regarded as an oxidative addition to the platinum(II) complex formed in step i. The combination of step i with iii, iv or v is therefore known as a reductive elimination oxidative addition (REOA) reaction. 

When the bis(isopropylamino)iodocyclopropenylium iodide is reacted with platinum black in acetonitrile, the reaction takes a different course, affording mainly the trans-bis[bis(diisopropylamino)cyclopropenylidene] diiodoplatinum complex (equation 277)351. A plausible pathway for this reaction involves two consecutive oxidative additions to platinum leading to the hexacoordinated intermediate Ptlv-complex [ -Pr2N)2C3]Ptf4, followed by reductive elimination of I2 to form the product (cf Section VI. A. 1. a). 

Attempts have been made to mimic proposed steps in catalysis at a platinum metal surface using well-characterized binuclear platinum complexes. A series of such complexes, stabilized by bridging bis(diphenyl-phosphino)methane ligands, has been prepared and structurally characterized. Included are diplati-num(I) complexes with Pt-Pt bonds, complexes with bridging hydride, carbonyl or methylene groups, and binuclear methylplatinum complexes. Reactions of these complexes have been studied and new binuclear oxidative addition and reductive elimination reactions, and a new catalyst for the water gas shift reaction have been discovered. 

Oxidative addition of the Si-aryl carbon bond in the silacyclobutene ring to Pt gives the optically active intermediate Pt-complex. Further coordination of (+)-l-methyl-l-(l-naphthyl)-2,3-benzosilacyclobut-2-ene to the complex and cr-bond metathesis will provide the cyclic dimer Pt-complex. Reductive elimination from the intermediate platinum complex gives cyclic polymers and oligomers. Preference of cr-bond metathesis over reductive elimination gives polymers of higher molecular weight. The presence of EtsSiH in the system results in the formation of linear products via cr-bond metathesis. 

Platinum-lead complexes undergo a cleavage of Pt—Pb bond with halogens and halogen acids506,507 510. These reactions are believed to occur through an electrophilic attack on Pt(II) leading to oxidative addition with the formation of a hexa-coordinated Pt(IV) complex. Reductive elimination of a plumbane results in the observed products (equation 193). 

In the majority of catalytic reactions discussed in this chapter it has been possible to rationalize the reaction mechanism on the basis of the spectroscopic or structural identification of reaction intermediates, kinetic studies, and model reactions. Most of the reactions involve steps already discussed in Chapter 21, such as oxidative addition, reductive elimination, and insertion reactions. One may note, however, that it is sometimes difficult to be sure that a reaction is indeed homogeneous and not catalyzed heterogeneously by a decomposition product, such as a metal colloid, or by the surface of the reaction vessel. Some tests have been devised, for example the addition of mercury would poison any catalysis by metallic platinum particles but would not affect platinum complexes in solution, and unsaturated polymers are hydrogenated only by homogeneous catalysts. 

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