Vinylidene, coordinated ligands, nucleophilic addition

Nucleophilic Addition to Coordinated lsocyanide and Vinylidene Ligands... 

II = Nucleophilic addition to coordinated isocyanide and vinylidene ligands. 

These reactions illustrate the importance of ruthenium vinylidene species, as activated forms of terminal alkynes, in catalysis, because they favor the addition of O-nudeophiles (carbamic and carboxylic acids, alcohols, water) to terminal alkynes and completely reverse the expected regioselectivity of the addition. These examples also show that the activation processes are very sensitive to the nature of the nucleophiles, and the success of the awtt-Markovnikov addition to terminal alkynes is highly dependent on both the electron richness and steric hindrance of the ancillary ligands coordinated to the active site. 

When coordinated to a metal, alkyne and vinylidene ligands show a marked difference in reactivity (Scheme 3.1). While coordination of either tautomer to a cationic metal center increases their electrophilicity, the site of nucleophilic attack varies between alkynes and vinylidenes. In the case of alkynes, nucleophilic attack follows Markovnikov s rule, whereas for vinylidenes, an examination of the frontier molecular orbitals demonstrates that the lowest unoccupied molecular orbital (LUMO) is located on the metal-bound a-carbon (Scheme 3.2). The net result of nucleophilic attack is formal anti-Markovnikov addition. A typical example of this difference in reactivity is that the coordination of alkyne and vinylidene ligands to suitable metal complexes may catalyze the nucleophilic attack of water to give ketones and aldehydes respectively [16]. 

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