Platinum complexes metallacyclobutanes

R.R. Schrock, M.I.T. Have you or anyone else prepared platinum(IV) metallacyclobutane complexes with alkoxide ligands in place of chlorides One might expect the alkoxide complexes to behave considerably differently than the chloro complexes, perhaps like early transition metal complexes. 

R. J. Puddephatt No. Nobody has prepared such complexes and the synthesis is not trivial. Substitution of halide ligands in octahedral platinum(IV) derivatives is typically very slow, and a better route (suggested by J. K. Kochi) might involve oxidation of platinum(II) metallacyclobutanes with peroxides. It would certainly be worthwhile to attempt this synthesis in view of the promise of enhanced reactivity. 

Another example of this type starts from exo,< xo-tetracyclo[3.3.1.0. 0 ]nonane, which reacted with Zeise s dimer, [(C2H4)PtCl2]2, to form an insoluble platinum complex. This product is solubilized with pyridine and was shown to be a metallacyclobutane complex 4. Synthesis of cxo-tricyclo[3.3.1.0 ]non-6-ene was achieved in 45-55% yield if 4 was treated with dimethyl sulfoxide. ... 

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

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. ... 

A detailed recent study has been made to try and elucidate whether metallacyclobutanes rearrange to alkenes by p elimination or by a elimination to give an intermediate ylide complex, which can rearrange to an alkene complex. Using deuterium-labeled platinum(IV) platinacyclobutanes it is concluded that the pathway involves a [1,3] H shift (a elimination) rather than a [1,2] H shift ()3 elimination). Platinacyclopentanes have also been formed by an alkene coupling between Pt(cod)2 and butadiene. Addition of PMcs gives complex (66 equation 2(K)). ... 

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... 

The use of platinum(II) alkene complexes in the synthesis of metallacyclobutane complexes of platinum has been reviewed. ... 

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