Platinum complexes cyclopropane

The interaction of the complex tetracarbonyldi-fj-chlorodirhodium with cyclopropane and substituted cyclopropanes produced complexes 3 and 4a, ° respectively, with the same dimeric structure. Contrary to the reaction with the platinum complex a five-membered ring 3 or 4120 jjj which both the metal and the carbon of one of the carbonyl groups were incorporated was formed. The cleavage site was the least substituted C —C bond. Analogous products 4c and 4b (75%) were obtained with bicyclo[4.1.0]heptane and phenylcyclopropane, respective-... 

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. ... 

Nucleophilic attack on the central allyl carbon atom of (T) -allyl) palladium and platinum compounds was employed in the synthesis of cyclopropanes from allylic electrophiles and silyl enolates. Treatment with base of (r) -allyl) palladium and platinum complexes bearing a methoxymethoxy group at the 2-position afforded the corresponding oxodimethylenemethane complexes in contrast to the formation of 2-hydroxysubstitutcd-(Ti5-allyl) complexes which was observed under acidic conditions. 

An electron-releasing methoxy group enhances the regioselective insertion of platinum into a cyclopropane ring of 3 to form the complex 4, suggesting the stabilization of the incipient cation 5 with the methoxy group [6]. This result is in contrast to complexation to only the olefinic moiety of bicyclo[4.1.0] hept-3-ene under identical conditions. (Scheme 2)... 

Numerous studies aimed at the understanding of the mechanism of these processes rapidly appeared. In this context, Murai examined the behavior of acyclic linear dienyne systems in order to trap any carbenoid intermediate by a pendant olefin (Scheme 82).302 A remarkable tetracyclic assembly took place and gave the unprecedented tetracyclo[6.4.0.0]-undecane derivatives as single diastereomer, such as 321 in Scheme 82. This transformation proved to be relatively general as shown by the variation of the starting materials. The reaction can be catalyzed by different organometallic complexes of the group 8-10 elements (ruthenium, rhodium, iridium, and platinum). Formally, this reaction involves two cyclopropanations as if both carbon atoms of the alkyne moiety have acted as carbenes, which results in the formation of four carbon-carbon bonds. 

Other means of improving sulfide yields in the reaction of halides with thiolates are (1) the use of thiols and platinum(II) complex catalysts287, (2) the generation of thiolate anions by electrochemical means288 and (3) the use of phase-transfer conditions237. The first method has been used for the synthesis of thioketals from geminal diiodides and the third has been used for the conversion of gem-dichlorocyclopropanes into cyclopropane thioketals, which are effectively masked cyclopropane moieties. 

Dichloro(l, 3-propanediyl)platinum and its bis(pyridine) derivative have been studied by a number of authors. Dichloro(l,3-propanediyl)platinum, and the corresponding substituted 1,3-propanediyl platinum compounds release the parent cyclopropane on treatment with potassium cyanide, potassium iodide, a tertiary phosphine, carbon monoxide, and other ligands.2,6 Reduction by means of hydrogen or lithium aluminum hydride yields chiefly isomeric substituted propanes. Dichlorobis(pyridine)(l,3-propanediyl)platinum in refluxing benzene yields a pyridinium ylid complex, - (CH3CH2CHNC5Hs)-PtpyCla. 

It has been shown that electron-rich cyclopropanes are able to displace ethylene from dichloro(ethylene)platinum to yield four-membered metallocyclic complexes (cf. equation 37). On the other hand 1,1,2,2-tetracyanocyclopropane (171) reacts under mild conditions with zerovalent platinum and palladium complexes of the type Pt(PPh3)2 (C2H4) or ML (n = 3, 4 M = Pd or Pt L = phosphines or triphenylarsines) to give metallocyclobutane derivatives (172) (equation 118) . 

Nucleophilic attack of the central carbon of allyl ligands represents an important access to metallacyclobutanes independent from cyclopropanes as precursors. This reaction pathway competes with the more common attack at one of the terminal allylic carbons. However, several examples of metallacyclobutane formation followed by release of cyclopropane products have been reported, especially with allyl complexes of palladium, platinum, and iridium as stable precursors or reactive intermediates in catalytic cycles. Detailed experimental (see below) and theoretical studies considering chemo-, regio- and stereoselectivity of the crucial reaction steps with respect to influence of the metal, ligands, substituents and reaction conditions are available. ... 

Cyclopropane formation can be dramatically enhanced by addition of Ai,iV,Af, A -tetrame-thylethylenediamine as an additional ligand or cosolvent. This was first reported in stoichiometric conversions of 7r-allylpalladium complexes and can also be applied to palladium-catalyzed processes." Thus allyl bromides with ketene acetals in the presence of thallium(I) acetate and 7i-allylpalladium(II)(TMEDA) acetate as catalyst gives cyclopropanes 9 with trans geometry of the substituents." " Similar results are observed with platinum catalysts vide infra). 

The cyclopropane ring is known to be susceptible to insertion reactions when treated with metal complexes. Aryl-substituted cyelopropanes were shown to displace ethene from the di-chloro(ethene)platinum dimer with formation of substituted (propane-1,3-diyl)platinum dichlo-ride. The cleavage site was the bond next to the aryl substituent. 

An interesting feature of Scheme 24, devised for palladium, is that it also provides a straightforward explanation for the bond shift isomerization on platinum. An adsorbed metallocyclobutane complex is similar to the trimethylene di-tr-complex of platinum, which is readily formed from hexa-chloroplatinic acid and cyclopropane (75). Also, platinum is known to promote easily a-y exchange of some hydrocarbons with deuterium (57, 34, 76). Therefore, in the case of platinum, the direct formation of metal-locyclobutanes, without intermediacy of 7t-allylic species, would explain the remarkable ability of this metal to promote the isomerization of neopentane to isopentane (Scheme 25). 

Several platinacyclobutane complexes have been isolated from the reaction of platinum-phosphine complexes with cyclopropanes. Those structurally characterized are listed in Table 8. No other structures of metallacyclobutane-phosphine complexes have been reported, but an... 

Other Catalysts. Other catalysts with metals of rhenium and platinum have shown catalytic reactivities for cyclopropanation. Methylrhenium trioxide (MTO) was the first rhenium catalyst for catalytic cyclopropanation, with yields of 57-87% obtained for the cyclopropanation of alkyl or aryl alkenes with EDA (45). As for platinum, a number of complexes have been screened for cyclopropanation catalytic activity (46). PtCl4 was the most active, giving good yield (79%) of cyclopropane from styrene and EDA. However, all reactions had to proceed at elevated temperature. Nonmetal catalysts such as tris(4-bromophenyl)-aminium hexachloroantimonate have been utilized as catalysts for mechanistic studies of cyclopropanation of a series of raras-stilbenes with EDA (47). A cation radical mechanism for this catalysis has been proposed. 

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