Arene ruthenium carbene derivatives

Fig. 9 a ruthenium(Il)-arene complexes b rhodium and ruthenium carbene complexes c rhodium, iridium and ruthenium tripodal phosphine complexes d gold [1,3-(dimesitylmeth)dimidazolinium)] chloride complexes e metal carboxamide derivatives... 

Enantioselective cyclopropanation is currently being explored. The ruthenium complex shown previously in Figure 7 also reacts with EDA and styrene to afford a transxis ratio of 4 1, with 46% ee of the trans isomer. The cis isomer is nearly racemic (<10% ee). The use of four-substituted stjrrene derivatives dramatically increases the diastereoisomeric excess of the trans isomer, with 4-fluorostyrene giving an 11 1 ratio, with 50% ee (74). Conversely, as shown in Figure 21, the porphyrin-like [RuCl(PNNP)]+ precatalyst reacts with EDA/ styrene to afford the cis isomer at a ratio of 10 1, with an enantiomeric excess of 87% (76). These types of ruthenium complexes have also been described as epoxi-dation catalysts above clearly there are mechanistic similarities between the oxo-and carbene- intermediates, which could help elucidate the reasons behind such variable enantioselectivity. Other ruthenium complexes that catalyze cyclopropanation include CpRu(II) catalysts, arene ruthenium complexes, and ruthenium-salen complexes. Cp Ru(cod)Cl is also known to catalyze the related reaction of diazo compounds with alkynes, affording the corresponding 1,3-diene (Figure 22) (77). 

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