Stereoselective metal-carbene transformations

When the cis/trans stereoselectivity of cyclopropanation with ethyl diazoacetate in the presence of CuCl P(0-z-Pr)3, Rh6(CO)16 or PdCl2 2 PhCN was plotted against that obtained with Rh2(OAc)4, a linear correlation was observed in every case, with slopes of 1.74,1.04 and 0.59, respectively (based on 22 olefins, T = 298 K) S9). These relationships as well as the results of regioselectivity studies carried out with 1,3-dienes point to the similar nature of the intermediates involved in Cu-, Rh-and Pd-catalyzed cyclopropanation. Furthermore, obvious parallels in reactivity in the transformations of Scheme 45 for a variety of catalysts based on Cu, Rh, Fe, Ru, Re and Mo suggest the conclusion that electrophilic metal carbenes are not only involved in cyclopropanation but also in ylide-forming reactions66. ... 

Trimethylsilyloxyfuran reacted stereoselectively with chiral tungsten carbene complexes in a Mukaiyama-Michael addition fashion to provide -products, as shown in Equation (18) <2005AGE6583>. The metal carbene in the butenolide product serves as a useful functional group for further transformations. 

Diazo compounds are commonly used as a carbene source in organic chemistry. A few systems based on metals such as Ru and Rh have been reported for the transfer of carbenes from diazo compounds. P6rez and coworkers reported NHC-copper systems for carbene and nitrene transfer. In the first report, [Cu (Cl)(IPr)] was used for the transfer to olefins, amines, and alcohob [64]. The main transformation was the cyclopropanation of styrene with ethyl diazoacetate (Scheme 8.24). Monitoring of the reaction showed a fast formation of the product (90% conversion in 6 h). The absence of styrene does not lead to the decomposition of EDA even after a long period of time (13 h). Decent stereoselectivity was obtained with styrene (cisitrans 32/68) and cyclooctene exolendo 73/27). 

The fact that carbenes are characterized by their stereospecific transformations has effectively paved the way for numerous innovative developments. The discovery that rhodium acetate can function as a catalyst for carbene generation from diazoacetates [15, 16] with much higher efficiency than copper salts, which had been traditionally used, provided an opening into the vast possibilities offered by transition metal catalysis. The increasing number of novel, stereoselective reactions of carbenoid species have become an indispensable toolbox for the modern practitioner of chemical synthesis [17-22],... 

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