Ruthenium complexes bonding studies

Another focus of this chapter is the alkynol cycloisomerization mediated by Group 6 metal complexes. Experimental and theoretical studies showed that both exo- and endo- cycloisomerization are feasible. The cycloisomerization involves not only alkyne-to-vinylidene tautomerization but alo proton transfer steps. Therefore, the theoretical studies demonstrated that the solvent effect played a crucial role in determining the regioselectivity of cycloisomerization products. [2 + 2] cycloaddition of the metal vinylidene C=C bond in a ruthenium complex with the C=C bond of a vinyl group, together with the implication in metathesis reactions, was discussed. In addition, [2 + 2] cycloaddition of titanocene vinylidene with different unsaturated molecules was also briefly discussed. 

Photochemical methods offer a convenient tool to study intra- and interprotein ET because of their time resolution and selectivity. Various mechanistic and design approaches based on photochemistry of metal complexes have been undertaken. Most of the studies on protein electron transfer processes have been done for hae-moproteins using among others ruthenium complex as a photosensitizer, modified haemoproteins in which haem iron is substituted by another metal (mainly Zn), and CO-bonded haem proteins [6,7],... 

Besides Cu and Rh, various other metals are known to catalyze the decomposition of diazo compounds [6,7,8,9,10]. Palladium complexes, e.g., are efficient catalysts for the cyclopropanation of electron-deficient C-C double bonds with diazoalkanes [19,20, 21], in contrast to Cu and Rh catalysts which are better suited for reactions with electron-rich olefins. Unfortunately, attempts to develop chiral Pd catalysts for enantioselective cyclopropanation have not been successful so far [22]. More promising results have been obtained with cobalt and ruthenium complexes. These and other chiral metal catalysts, that have been studied besides Cu and Rh complexes, are discussed in chap. 16.3. The same chapter also covers a new direction of research that has recently been taken with the development of catalytic enantioselective Simmons-Smith reactions. 

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