Archive for the ‘palladium’ Category

New Coupling Reaction???

June 1, 2007

I realize I am not the first to highlight this paper….

An interesting reaction was reported in the May 25th issue of Science.  The method is touted as a new coupling method and involves an apparent Pd-catalyzed aryl cross-coupling reaction between two different “non-activated” aromatic substrates without the formation of homo-coupled products.  This could potentially be a useful and expedient method for the synthesis of biaryl-compounds since it avoids the necessicity of using aryl-halide, -stannane, -borate, etc…  starting materials according to the traditional transition metal coupling methods.

While I think this is interesting I am not sure this method provides a general solution to the issue of transition metal catalyzed cross-coupling of “non-activated” aromatic substrates.  Setting aside the the fact that the method requires a large excess of the phenyl component, the use of indole, which is the only example inlcuded in the paper as the other component of the reaction, suggests that there is a serious limitation in scope (I must assume that other heterocyles were tried but failed to deliver the desired product) Reactions at the C3-position of indole are well known and therefore the deck seems to be stacked in the author’s favor. In addition, the authors ignore the possiblity that Pd(II) might be acting as a Lewis acid.  As I show below, Tajima and Nakatsuku have shown the same transformation is possible using AlCl3 and MnO2. Granted this is not catalytic but it shows a parallel mechanism without the

indole.gif

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