Iridium complexes carbene transfer reactions

The ligand was then used to form a variety of transition metal carbene complexes [207] (see Figure 3.72). Interestingly, more than one method for the formation of transition metal carbene complexes was successfully employed presence of an inorganic base (IC COj) to deprotonate the imidazolium salt and the silver(I) oxide method with subsequent carbene transfer to rhodium(I), iridium(I) and copperfi), respectively. The silver(I) and copper(I) carbene complexes were used for the cyclopropanation of styrene and indene with 1,1-ethanediol diacetate (EDA) giving very poor conversion with silver (< 5%) and qnantitative yields with copper. The diastereomeric ratio (endolexo) was more favonrable with silver than with copper giving almost a pnre diastereomer for the silver catalysed reaction of indene. 

A series of benzylidene and low-valent alkylidene complexes have also been prepared from sulfur ylides. Deprotonation of benzyl or alkyl diphenylsulfonium salts with a strong base, such as KN(SiMe3)2, KNPr j or LiNPr generates the corresponding sulfur ylide. Reaction of this ylide with low-valent ruthenium, osmium, rhodium, or iridium complexes containing a labile ligand leads to transfer of the carbene unit from sulfur to... 

Not surprisingly, these rhodium and iridium carbene complexes were tested for their catalytic behaviour in the transfer hydrogenation of benzophenone and acetophenone (M +3), the hydrosilylation of alkynes (M +1) and also the catalytic cyclisation of acetylenic carboxylic acids (M +1). Hydrogenation works better for iridium than rhodium and for aromatic than for aliphatic ketones [40,43,44]. The iridium(I) complex is the first iridium catalyst showing activity for the cyclisation of acetylenic carboxylic acids [40]. The results for the hydrosilylation reactions were very moderate. 

A similar complex bearing a normal and an abnormal coordinated carbene moiety was reported by Zuo and Braunstein its catalytic activity in the transfer dehydrogenation of cyclooctene was however markedly lower than that of the complexes by Chianese/ Remarkably, CCC pincer complexes of iridium(III) developed by Chianese and by the gyoup of Herrmann and Kuhn " turned out to be active catalysts also for another C—H functionalization reaction, namely theborylation of arenes. Activities were however low compared to iridium(III) complexes with other ligands which were extensively optimized over the years. 

---