Metal insertion Stille coupling

For those investigated, PQQ-containing dehydrogenases require a divalent metal ion for activity, Mg in one case and Ca in all other cases (see Table 1). For some enzymes, Ca removal leads also to detachment of PQQ, providing an apo enzyme that can be easily reconstituted by addition of PQQ and Ca. For other enzymes, removal of the metal ion plus PQQ is not so easy, but fortunately several bacteria produce apoenzyme gratuitously as they are still able to produce the protein part of the dehydrogenase, but not PQQ (at least not under the laboratory conditions used for cultivation). Also in these cases, reconstitution to holo enzyme is easy, in vitro as well as in vivo the latter leads to active enzyme coupled to the respiratory chain. However, indications exist that a special protein is required for inserting Ca in the case of MDH [58],... 

The reaction sequence in the vinylation of aromatic halides and vinyl halides, i.e. the Heck reaction, is oxidative addition of the alkyl halide to a zerovalent palladium complex, then insertion of an alkene and completed by /3-hydride elimination and HX elimination. Initially though, C-H activation of a C-H alkene bond had also been taken into consideration. Although the Heck reaction reduces the formation of salt by-products by half compared with cross-coupling reactions, salts are still formed in stoichiometric amounts. Further reduction of salt production by a proper choice of aryl precursors has been reported (Chapter III.2.1) [1]. In these examples aromatic carboxylic anhydrides were used instead of halides and the co-produced acid can be recycled and one molecule of carbon monoxide is sacrificed. Catalytic activation of aromatic C-H bonds and subsequent insertion of alkenes leads to new C-C bond formation without production of halide salt byproducts, as shown in Scheme 1. When the hydroarylation reaction is performed with alkynes one obtains arylalkenes, the products of the Heck reaction, which now are synthesized without the co-production of salts. No reoxidation of the metal is required, because palladium(II) is regenerated. 

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