Other Transition Metals as Catalysts

Catalytic reactions aimed at the formation of Csp -S and Csp -Se bonds are known to take place with rhodium, ruthenium and cobalt complexes. A particularly promising area with tremendous development in recent years concerns the investigation and possible application of iron catalysts. 

In a subsequent study this reaction was further extended to include Arl and Ar (Het)SH with high selectivity and good yields (with the exception of PhCH2SH) in the FeCl3 H20/cationic 2,2 -bipyridyl (L) system in water [97]. The catalyst with the cationic ligand L in water solutions preserved the activity over several cycles  

Although much interest has been shown recently in the possible application of various iron catalysts, care should be taken to understand the nature of the active species in the catalytic cyde. A recent study by Buchwald and Bolm raised a question on the role of trace quantities of other metals, particularly Cu 98]. 

It is important to understand the mechanistic difference between these two approaches for C—Z bond formation. Chemical selectivity is a main concern in the design of cross-coupling reactions in order to improve the ratio of hetero-/homo-coupling products and to avoid reduction of the C-halogen bond. However, examination of the addition reactions raised other important problems - stereoselectivity in the case of the addition of dichalcogenides and regioselectivity for the addition of thiols and selenols [99]. 

In this section we describe the available literature on the addition reaction utilizing dichalcogenides RZZR as reagents. We do not discuss non atalytic addition reactions carried out without transition metal catalysts, this topic has already been addressed in several publications (see [100-103] and references therein). It was shown that the non-catalytic reactions led to different products - a mixture of -/Z-isomers is very often formed in the addition of RZZR to alkynes. Our goal is to concentrate on the selective formation of Z-isomers in the catalytic addition of RZZ R to the triple bond of alkynes. 

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