Replacing Stoichiometric Reactions with Catalytic Cycles

Replacing Stoichiometric Reactions with Catalytic Cycles 

Another interesting alternative combines catalysis and electrochemistry [35]. In the reductive coupling cycle of two bromobenzene molecules to biphenyl, the Pd catalyst provides just two electrons. This reaction can be performed in an electrochemical cell, where the electrons are supplied from an outside source [36]. We thus see that catalysis gives us a variety of green alternatives to the classic stoichiometric process. There are no hard and fast rules as to which route to choose - it all depends on the reaction conditions, and on the chemicals available. 

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Replacing Stoichiometric Reactions with Catalytic Cycles... 

Such a reaction could be quite useful on an industrial scale, but unfortunately, it is stoichiometric in highly expensive Pt complex. Since its discovery in 1972, Shilov chemistry has been the focus of numerous investigations, many of which have sought to replace the Pt(IV) stoichiometric oxidant with a cheaper alternative. In the course of this research, the mechanism of the Shilov C-H bond activation has been elucidated. The following scheme shows the catalytic cycle involved. 

An interesting selectivity was uncovered in the direct cross-dehydrogenative coupUng between N-protected indoles and arenes (Scheme 11.40) [151]. Thus, whereas 2-arylated indoles 67a were preferentially obtained from N-acetyhndole in the presence of Cu(OAc)2, the reaction of N-pivalolyUndole with AgOAc led to 67b, with excellent selectivities. The reason for this C-2/C-3 selectivity is most likely due to the formation of higher-order palladium clusters or paUadium/copper clusters under the different reaction conditions. A related reaction between aryl-boronic acids and arenes or heteroarenes also proceeds under oxidative conditions with Pd(OAc)2 as catalyst [76]. A catalytic cycle initiated by an electrophihc attack of Pd(II) on the arene, followed by transmetallation with the aryl boronic acid and reductive elimination, was suggested. In this transformation, Cu(OAc)2 as stoichiometric oxidant could be replaced by O2, and for indoles, arylation at C-2 was observed. 

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