Detection of propagating metallacyclobutane complexes

In Section 3.6 we shall consider cases of initiation by metallacyclobutane complexes. In the present section we are concerned with the detection of metallacyclobutane complexes in systems initiated by metal carbene complexes. 

We have seen from Fig. 3.4 that some initiator remains after all the monomer has reacted with W(=CHCMe3)(Br)2(OCH2CMc3)2/GaBr3. This is still the case when the initiator and monomer are mixed at —78°C in CD2CI2 confirming that this is not due to inefficient mixing. If the temperature is raised slowly, reaction begins at about -53°C but the initial spectra are different from those observed at room temperature. The species formed at low temperature have been identified as intermediate transoid metallacyclobutane complexes 7. 

These complexes disappear when the temperature is raised to 20°C and are 

The complex 7b, produced from c t/o,e /o-5,6-dimethylnorbomene, is an order of magnitude more stable than 7a. It can be obtained in 70% yield from the initiator at -38°C which allows the kinetics of its first-order rearrangement to be studied at -28 to -18°C Fig. 3.7. The Arrhenius parameters are /I = 7 x lO s and 

When the initiator has a doubly substituted carbene ligand the initially formed metallacyclobutane complex Xi is not detected. This is because it has both cisoid and transoid substituents at C-8, and cisoid structures are never detected. In this case, for the reaction of, say, norbomene, the first product seen in the H NMR spectrum is Pi, followed by the two transoid diastereomers of X2. When the initiator has a monosubstituted carbene ligand, and the norbomene carries a syn-1-methyl substituent, again the Xi species is not observed. One can expect from the structure of 7 that a syn-methyl group at C-7 will have the same sort of destabilizing 

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