Platinum intramolecular processes

Numerous other aromatic nitrogen compounds are known, and a significant number have been used to prepare complexes of platinum(II). Phthalazine (101) is one such compound, and the complex ds-PtCl(phthalazine)(PEt3)2 has been prepared in order to compare its fluxional properties with the phenanthroline and naphthyridine analogues. The monodentate complex is fluxional, but in this phthalazine case the orientation of the lone pairs is unsuitable for the five-coordinate intermediate required by an intramolecular process and the mechanism of exchange becomes dissociative.11 6... 

Kinetics of intramolecular processes of tungsten(O) complexes of thio-ligands are mentioned in Section 8.11.2 in connection with inversion at sulfur coordinated to platinum(IV). 

Intramolecular Processes. What is claimed to be the first direct and unambiguous determination of barriers to restricted rotation about sulphur—sulphur bonds relates to compounds Ph CHa S S R, in various solvents. Determined barriers to rotation about S—S are 7.0 < AG < 9.4kcalmol S 6.6 < A/f <9.1 kcalmol" and — 7 < LS < — 1 cal deg mol . Far-i.r. spectra of sulphur tetrafluoride suggest that the barrier to intramolecular pseudorotation for this molecule in the gas phase is 10.0 kcal mol . This figure may be compared with the activation energy of about 4.5 kcal mol for intermolecular fluoride exchange in the liquid phase. Two references to inversion at sulphur bonded to platinum or palladium are discussed in the chapter on complexes of those metals (Part II, Chapter 2). 

Carbon Disulphide.—The kinetics of reaction of /rartJ-[Pt(X)(H)(PRs)a], where R = Et or Ph, with carbon disulphide in methanol indicate at least two stages in the reaction. The first step has a dependence of rate on the concentration of the platinum complex and of the carbon disulphide the rate of the second step is independent of carbon disulphide concentration, suggesting an intramolecular process. The whole sequence proposed is shown in Scheme 2. Whereas the product (18) of this reaction is thought... 

The conversion of a Jt-bonded olefinic ligand to o-alkyl ligand requires the participation of an internal or external nucleophile. Scheme 11.16. The rearrangement of a platinum(ll) ethylene hydride complex is an example of an intramolecular process, 3 Scheme 11.17 (L = PMePh2). 

Intramolecular c/ s-addition processes with platinum catalysts 805... 

A mild one-pot procedure based on a platinum-catalyzed diborylation of 1,3-butadienes (see Eq. 30) gives doubly allylic boronate 144, which adds to an aldehyde to form a quaternary carbon center in the intermediate 145 (Eq. 105). The use of a tartrate auxiliary in this process leads to good levels of enantiose-lectivity in the final diol product, which is obtained after oxidation of the primary alkylboronate intermediate. Although examples of aliphatic, aromatic, and unsaturated aldehydes have been described, enantioselectivities vary widely (33 to 74% ee), and are good only for aliphatic aldehydes. An intramolecular variant of this interesting tandem reaction is also known. ... 

Sulfine complexes of platinum(II) can be formed by oxidative addition to Pt(PPh3)3. The initial step involves the formation of an rj2-CS complex which undergoes intramolecular oxidative addition of a C—S bond (equation 542).1869 Use of Pt(cod)2 and PCy3 gives the tricyclohexylphosphine analogue.1870 The reaction gives two stereoisomers.1871 The coordination stabilization of sulfines allows their synthesis in the coordination sphere of platinum, but the cyclic process is not very efficient.1872... 

The platinum-catalysed intramolecular domino annulation reaction of o-alkynylben-zaldehydes has been described as a versatile approach to naphthalenes with annulated carbocycles or heterocycles of various sizes (Scheme 32).94 A plausible mechanism for the platinum(II)-catalysed annulation reaction shows that the double annulation process most probably proceeds through the benzopyrylium cation (117), which results from the nucleophilic attack of the carbonyl oxygen at the alkyne, activated by the Lewis-acidic platinum salt. A subsequent intramolecular Huisgen-type 3 + 2-cycloaddition of the second alkyne is assumed to generate intermediate (118). Rearrangement to (119) and the formal 4 + 2-cycloaddition product (118) leads to the aromatized final (116), liberating the active catalyst. In the case of FeCl3 as the Lewis acid, we assume that intermediate (118) is oxidatively transformed to (121). 

Any detailed description of the mechanism of an octahedral substitution must also account for the stereochemical changes that accompany reaction. Werner recognized this and made use of it in his discussions of the stereochemistry of reactions of cobalt(III) complexes. The available experimental results can be explained on the basis of possible molecular rearrangements and some cautious predictions can even be made. The base hydrolysis of cobalt III)ammines appears to be unique in that it often occurs with rearrangement it also affords the few known examples of optical inversion. These results can be explained by formation of a 5-coordinated species with a trigonal bipyramidal structure. Optically active metal complexes racemize by either an intramolecular or an in-termolecular process. Substitution reactions of platinum metal complexes often occur with retention of configuration. 

For the reactions with tertiary phosphines, intermediates of the form [HPt(/r-H)( -dppm)2PtH(PR3)]+ have been detected, and equilibrium constants for their formation at low temperature have been determined (110). Exchange of PR3 takes place between the two platinum centers. The elimination of H2 has been shown to be intramolecular, and the rate-determining step is the elimination itself. Rearrangement to a second intermediate without a bridging hydride is believed to occur prior to reductive elimination, but it is not possible to determine with certainty whether the elimination occurs at one platinum center or by means of a concerted dinuclear process. Mechanisms involving each type of elimination process have been proposed (Scheme 8) (110). 

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