Pentacoordinate palladium pathway

The normal neutral pathway (22 24 25 27) was ruled out by conducting the reaction with monodentate phosphine BINAP ligand mimics (Scheme 12.5). The products obtained were of low enantiomeric excess relative to reactions employing BINAP. The direct cationic pathway (24-> 26) was also eliminated due to the fact that the opposite stereochemistry was obtained under cationic conditions with the addition of silver salts. The switch in stereoselectivity in the presence of silver salts, moreover, indicates that oxidative insertion is not the enantioselective step. j8-Hydride elimination was also discounted as the enantioselective step due to the influence of the double-bond geometry of the starting material on the enantioselectivity of the cyclization. The proposed enantioselective step is the formation of the cationic intermediate 26 by an associative displacement (24-> 28-> 26). In the case of square planar pafladium(n) complexes, substitution chemistry can occur through associative processes. Axial coordination of the alkene would form the pentacoordinate pafladium(II) complex 28. Reports of isolated and characterized pentacoordinate palladium(II) species provide support for this proposed intermediate. 

Much of the recent hteiature on the mechanism of the enantioselective intramolecular Mizoroki-Heck reaction has focused on the anionic mechanism, o-iodoanilide substrates, pathways involving neutral pentacoordinate palladium intermediates and the influence of additives. The new examples and mechanistic findings indicate that the potential may exist to control the stereoselectivity of the intramolecular Mizoroki-Heck reaction through pathways other than the cationic mechanism. However, further research is needed to obtain the level of effectiveness of the traditional cationic pathway. 

The 1,2-migration could also be explained by a jS-hydride elimination from the tetracoordinated alkenylpalladium(II) chloride complex generating a transient pentacoordinated palladium(II) intermediate, but the non-isomerized products obtained using DPPF as the auxiliary ligand rule this pathway out [2]. 

The isomerization process was analyzed in detail by the group of Espinet [81] in the case of complex 7, formed by the oxidative addition of CgCl2F3l to [Pd(PPhj)]4 (Scheme 1.5). The isomerization of cis-7 to trans-8 is a rather complex process that can take place by four major competitive pathways. Two of these pathways involve associative replacements of PPh3 by an iodide ligand of a second palladium complex. Two additional routes involve two consecutive Berry pseudorotations on pentacoordinated species formed by coordination of the solvent tetrahydroluran (THF) [81]. 

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