Cationic complexes platinum

Platinum ammine complexes have been a fertile area for studying transinfluence. Table 3.21 lists data for a range of ammines showing how /(195Pt-15N) depends upon the trans-atom [153]. (A further selection of data can be found in R.V. Parish, NMR, NQR, EPR and Mossbauer Spectroscopy in Inorganic Chemistry, Ellis-Horwood, Chichester, 1991, pp. 76, 87.) Possibly the most detailed study (of complexes of tribenzylphosphine) examined over a hundred neutral and cationic complexes [154] (Table 3.22). 

The compound 70 has also been reported showing the ambident character (both C- and N-coordination) of the cyano-stabilized ylide as ligand. The authors have also transposed their work concerning the keto-bis-ylide and palladium, with the synthesis of the C-bonded complex 71 or the new cycloplatinated or-thometallated compound 72. The latter by various treatments allows one to obtain other ylidic cationic complexes of platinum such as 73. A C,C,C-terdentate coordination of the keto bis-ylide, already observed with the palladium is also obtained from the reaction of 73 with gold derivatives. 

The first structurally characterized example of a platinum(II) derivative containing a saturated tetraamine macrocycle, 6,13-dimethyl-l,4,8,ll-tetraazacyclotetradecane-6,13-diamine has been reported (80).251 The species crystallizes as the colorless tetra-cationic complex from dilute HC104 solution by slow evaporation, where the two pendant primary amines are protonated. Other macrocyclic tetraamine complexes including [Pt([14]aneN4)]Cl2 have also been described.252... 

Until recently, the hydroformylation using palladium had been scarcely explored as the activity of palladium stayed behind that of more active platinum complexes. The initiating reagents are often very similar to those of platinum, i.e., divalent palladium salts, which under the reaction conditions presumably form monohydrido complexes of palladium(II). A common precursor is (39). The mechanism for palladium catalysts is, therefore, thought to be the same as that for platinum. New cationic complexes of palladium that are highly active as hydroformylation catalysts were discovered by Drent and co-workers at Shell and commercial applications may be expected, involving replacement of cobalt catalysts. 

The proposed mechanism starts with a methyl group abstraction on platinum complex 416 with the borane reagent in the presence of diyne 414 (Scheme 105). The square-planar cationic diyne-platinum(n) complex 417 is converted to the octahedral platinum(rv) hydride intermediate 418 through oxidative addition of the hydrosilane. This complex decomposes rapidly with methane release to form another tetracoordinated platinum(n) species 419, followed by platinasilylation of the triple bond. The resulting vinylplatinum 420 undergoes an intramolecular carboplatination to... 

The rhodium-catalyzed cyclization/hydrosilylation of internal diyne proceeds efficiently with high stereoselectivity (Scheme 106). However, terminal diynes show low reactivity to rhodium cationic complexes. Tolerance of functionalities seems to be equivalent between the rhodium and platinum catalysts. The bulkiness of the hydrosilane used is very important for the regioselectivity of the rhodium-catalyzed cyclization/hydrosilylation. For example, less-hindered dimethylethylsilane gives disilylated diene without cyclization (resulting in the double hydrosilylation of the two alkynes), and /-butyldimethylsilane leads to the formation of cyclotrimerization compound. 

Fig. 11.24. Positive-ion ESI spectrum of a cationic dinuclear platinum hydride complex from dichloromethane solution. The insets compare experimental and theoretical isotopic patterns. By courtesy of P. Hofmann, University of Heidelberg.

Platinum-diphosphine complexes in [C4CiIm][PPg] have been studied via ip NMR [67,68]. Here, the insertion reaction of SnClj into the Pt-Cl bond of PtCljCbdpp) was observed, as well as the formation of an additional cationic complex. The structure of the catalyst could be elucidated via characteristic Jpp and Jpt p coupling constants. 

Activation of the triple bond of enynes with electrophilic metal derivatives, especially cationic gold complexes, platinum salts such as PtCl2, and ruthenium derivatives, has been reviewed.117 These catalysts make possible nucleophilic addition of the double... 

Such an example has been demonstrated by Johnson and Sames, who chose a platinum-mediated dehydrogenation as a key step in the synthesis of the antimitotic rhazinilam 33 (Scheme 6) [20], The key intermediate 27 was converted into the imine 28, which was allowed to react with Me Pt(//-SMe2)]2 to afford the platinum complex 29. Subsequent treatment with triflic acid resulted in elimination of methane and furnished the cationic complex 30. Upon thermolysis in trifluoroethanol, the complex lost a second methane molecule, which resulted in the activation of the ethyl group. A subsequent /1-hydride elimination gave the hydrido-Pt(n) complex 31. Treatment with aqueous KCN followed by hydrox-ylamine removed the platinum and yielded the liberated amine 32. Johnson and Sames added a homologization and a macrolactamization and completed the total synthesis of rhazinilam (33) by removal of the carboxyl group. 

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