The presence of five membered ring systems in many target molecules has produced varied methods of their preparation. One of the most successful of these is the trimethylenemethane [3 +2] cycloaddition. This process is catalysed by palladium (0) complexes produced in situ by the reduction of Pd(OAc)2 by Pd(i-PrO)3. In these animations a Pd(PH3) complex has been used for modelling purposes.
‘Click’ the different stages to view the 3D models of each reaction:
This stage of the reaction forms the trimethylenemethane unit necessary for the cycloaddition from a [(trimethylsilyl)methyl]-2-propen-1yl acetate.
The palladium Pi-allyl complex is formed followed by the removal of the trimethylsilyl group under nucleophilic attack from the resulting acetate ion.
Overall the mechanism for this reaction is proposed to be a stepwise ‘cycloaddition’ reaction, whereby the carbanion attacks the cyclopentenone first to form an enonlate. The resulting enolate can then cyclise to form exo five membered cycloaddition product.
B. M. Trost, J. P. Stambuli, S. M. Silverman and U. Schwörer, J. Am. Chem. Soc., 2006, 128, 13328–13329.
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