Carbon Carbon Bond Formation

The scope of the multiphasic system was extended to coupling reactions—like the Heck reaction—using a heterogeneous supported catalyst, such as Pd/C.61 The rationale here lay in the observation that aryl halides were activated in the multi-phasic system (as seen for hydrodehalogenation), and that therefore they should also be activated toward C-C coupling reactions.

The multiphasic system, where A336 was coupled with the use of Pd/C in a mixed aqueous-organic solvent system, promoted the Heck reaction of aryl iodides and -bromides. In particular, the reaction of aryl iodides with electron-poor olefins became up to 10 times faster in the presence of A336 than without it (Figure 6.29). Under the multiphasic conditions, A336 formed a third catalyst-philic liquid phase that allowed the catalyst, products and reagents, and base to be kept separate. This was regarded as an advantage over solventless systems (where separation and recovery of the catalyst can be complicated), and over reactions carried out in pure ionic liquids that are expensive and that require a product recovery step). In addition, the multiphasic system used a heterogeneous catalyst rather than a homogeneous one, and Pd activating phosphines could be done without.

A similar "micromembrane" (Figure 6.30) formed by TBAB on Pd/C was able to selectively synthesize biphenyls from halobenzenes using a reducing

PTC membrane

Palladium on carbon

Figure 6.30 PTC membrane hypothesis.

Palladium on carbon

Figure 6.30 PTC membrane hypothesis.

agent (formate) and a weak base (Na2CO3). In this case, the concentration of water was crucial, since an excess caused the TBAB to leach from the supported catalyst.64 Figurer 6.30 represents the same concept expressed earlier in Figure 6.17.

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