Platinacyclobutanes elimination mechanism

At this point it should be noted that this mechanism is unexpected. Simple platinum alkyls decompose by 3-elimination whenever possible and there are no well-established examples of a-elimination [10]. All previous studies have indicated that metallacyclobutanes decompose by 3-elimination, even for tantalum and titanium derivatives for which a-elimination is a frequent mechanism for decomposition of the simple alkyls [11, 12]. There is even a labelling study which appears to prove the 3-elimination mechanism for decomposition of platinacyclobutanes (equation 3) [13]. 

Thus it seems that all platinacyclobutanes decompose by the a-elimination mechanism of Scheme II, and the reactions can be understood in terms of the following empirical rules. 

Platinacyclobutanes eliminate cyclopropane in the presence of iodide or thiocyanate (or Y ) in methyl cyanide solution. Kinetic studies suggest a mechanism consisting of reactions (149)-(152), the last being rate limiting. 

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. 

III) at -10°C. The mechanism shown in Scheme II was therefore suggested, involving skeletal isomerization and a-elimination from the platinacyclobutane. 

It is, of course, still possible that alkene complexes are formed by 8-elimination. This is most easily investigated using the platinacyclobutane [ PtCl2(CD2CHMeCHMe) n]. The predicted products according to the two opposing mechanisms are shown in equation (4). 

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