Platinacyclobutane

Plant growth regulators heterocyclic compounds as, 1, 191 Platinacyclobutane, dichloro-, 1, 666 Platinacyclobutanes reactions, 1, 667 rearrangement, 1, 667 synthesis, 1, 672 Platinacyclobutenones synthesis, 1, 666 Platinacycloheptanes synthesis, 1, 672 Platinacyclohexanes synthesis, 1, 672 Platinacyclopentanes synthesis, 1, 672... 

The platinacyclobutane 2 derived from the ds-disubstituted cyclopropane 1 is isolated and characterized as shown in Scheme 1 [5], (Scheme 1)... 

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. 

Our first observations related to the particular skeletal isomerization of Scheme I were obtained in a study of steric effects of ligands on the stability of platinacyclobutanes (9). Three products could be obtained as shown in equation 2. 

The isolated platinacyclobutane, (I), has the neopentane skeleton but the rearrangement products (II) and (III) have the isopentane skeleton. Low temperature NMR experiments, when L ... 

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

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]. 

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). 

This unexpected result promoted us to reinvestigate the decomposition of the platinacyclobutane [ PtCl2(CD2CMe2CHMe) n]. 

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. 

Schrock has proposed that the reverse reaction occurs during some catalytic alkene dimerisation reactions (20) but, in studies of decomposition of alkyl substituted platinacyclobutanes, no... 

The required platinacyclobutanes were prepared and solvolysed according to equation (8) (OMs = mesylate). 

In solution, ring-substituted platinacyclobutanes undergo an apparently general rearrangement, illustrated by the isomerizations (17) — (18) and (19) — (20). Detailed studies... 

An alternative approach to platinum(IV) metallacyclopentanes relies on the facile insertion of platinum(II) reagents into hydroxymethylcyclopropanes followed by ring expansion of the resulting platinacyclobutanes as illustrated (Scheme 30, Fig. 29) for the synthesis of the structurally characterised example 93.114... 

JCD549, 1988JCD427, 19950M2538> are intermediate between those observed for /3-allyl complexes (ca. 4000 Hz for unsubstituted allyl platinum complexes, 3500—4000 Hz for 2-alkoxyallyl platinum complexes) and platinacyclobutane complexes (<2000 Hz) <1993OM3019>. These data clearly support a contribution from the 3-allyl-like coordination mode in metallacyclobutanone complexes. 

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