Rhodium complexes purine

Although this reaction represents the first time that a purine base was deliberately reacted to generate a transition metal carbene complex, no application for this rhodium complex was reported. One could have envisaged catalytic trials. However, the efficiency of the silver oxide route for these purine base carbene precursors was recognised and their potential for the synthesis of late transition metal carbene complexes reported. 

As an alternative to addition of anionic nucleophiles followed by reoxidation, rhodium(l)-catalyzed C-H activation allowed the nucleophilic addition of alkenes to the intermediate Rh(i) carbene complex <2002JA13964, 2004JOC7329>. Purine behaved anomalously compared to other heterocycles, for which selective monoalkylation was observed, and underwent sequential substitution first at C-8 and then at C-6 (Equation 8). Caffeine was monoalkylated at C-8 in low yield (15%). Selectivity for C-8-arylation was also observed in the palladium-catalyzed C-H activation of 6-phenyl-9-benzylpurine (aryl iodides, 0.05 equiv Pd(OAc)2, 3 equiv Cul, 2.5 equiv CS2CO3, DMF, 160 °C, 60 h, 48-95% yields) <2006OL5389>. 

Involvement of two nucleophilic nitrogen atoms is thus typical for the amino heterocycles. The mutual disposition of the pyridine and amine nitrogen atoms allows the formation of chelate structures for the cobalt complexes of purine, 221 and 222. Structures with the N, iV -five-membered metal cycles were proven for the tri- and tetranuclear complexes of silver ) with 8-aminoquinoline (223) (92IC4370), and polymeric copper- and rhodium-acetate clusters (224). Another coordination mode can be found in the complexes of 4-amino-3,5-bis(pyridin-2-yl)-l,2,4-triazole, (225 or... 

Prompted by this publication, Schulz and Herrmann published their findings on rhodium and iridium purine carbene complexes in the same year [98]. They did not choose the popular silver oxide route, but the less popular alcoholate route (see Chapter 1). Here, the... 

Figure 6.43 Synthesis of some purine derived rhodium(l) carbene complexes and their vCO...

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