Hydridoruthenium species

The mechanism of the homocoupling of dienes is one of the representative reactions proceeding through a n-allylruthenium intermediate. Indeed, a bis 7r-allylruthenium complex was produced by oxidative cyclization of two dienes and the coupling of the terminal carbon atoms led to a cationic (diene) (allyl)hydridoruthenium species. 

The intermediate hydridoruthenium species is most probably RuHzCPhjP), as was observed in RuCl2(PhjP)rcatalyzed hydrogen transfer reactions. The observation that RuH2(PhjP)4 exhibits the same activity as RuCl2(PhjP)3 in the Ru/TEMPO catalyzed aerobic oxidation of 2-octanol is consistent with this notion. 

Functionalized dienes can be obtained by C-C bond formation between 1,3-dienes and alkenes via oxidative coupling with electron-rich ruthenium catalysts but also via insertion into Ru-H and then Ru-C bonds. For example, Ru(COD)(COT) catalyzed the selective codimerization of 1,3-dienes with acrylic compounds to give 3,5-dienoic acid derivatives [18] (Eq. 13). -coordination of 1,3-diene to a hydridoruthenium leads to a 7r-allylruthenium species to selectively give, after coupling with the C=C bond and isomerization, the functionalized conjugated 1,3-dienes. 

Sixteen-electron ruthenium(O) species of type (rj6-arene)(L)Ru(0) and containing two-electron ligands are probable intermediates for C—H bond activation and formation of metallacyclic complexes (Section II,A,3,c). A variety of 18-electron complexes of general formula (arene)(L1)(L2)Ru(0) have been prepared by H. Werner and co-workers either by deprotonation of hydride ruthenium(II) complexes or by reduction of cations RuX(L)2-(arene)+. Some of these Ru(0) complexes have already been discussed with the formation of alkyl or hydridoruthenium complexes (Sections... 

The alcohol oxidations discussed above involve as a key step the oxidative dehydrogenation of the alcohol to form low-valent hydridoruthenium intermediates. On the other hand, high-valent oxoruthenium species are also able to dehydrogenate alcohols via an oxometal mechanism (see Figure 5.6). It has long been known that ruthenium tetroxide, generated by reaction of ruthenium dioxide with periodate, smoothly oxidizes a variety of alcohols to the corresponding carbonyl compounds [47]. 

---