Rhodium complexes caffeine

Figure 6.41 Synthesis of the rhodium (iridium) caffeine carbene complex using the ethanolate...

Concerning ring size expansion, since the first anionic sbc-membered example derived from malonic acid described by Cesar et al. [8], other sophisticated five-[5a,9], sbc-[10], and seven-membered [11] NHCs have been described and coordinated to rhodium and iridium, including sbc-membered amino/amido carbenes [10b] and caffeine-based diamino carbenes [12]. Also related to this type of ligands, Conejero and coworkers described a general method for the preparation of 2-pyridylidene-based rhodium complexes by decarboxylation of pyridinium car-boxylates [13], a method previously described by Crabtree and coworkers [14]. 

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>. 

Methylated caffeine [75,95,96] was reacted with silver oxide to generate the silver(I) bis-carbene complex [97] in near quantitative yield (see Figure 6.40). Carbene transfer to rhodium(I) using [Rh(cod)Cl]2 afforded the respective rhodium(I) carbene complex quantitatively. 

Figure 6.40 Synthesis of a rhodium (I) carbene complex from methylated caffeine as starting...

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