Ruthenacyclopentene, formation

As the steric bulk of the propargylic substituents increased, the preference for the formation of the seven-membered ring increased as well. Formation of a ruthenacyclopentene intermediate with sterically hindered substrates involves a large amount of A(1,3) strain, leading to preferential formation of a 7r-allyl species. This novel cycloisomerization process is very sensitive to alkene substitution the requirement for a m-methyl group was evidenced by the failure of 70 to give... 

Coupling of 1-alkyne 410 with 1-alkene 411, catalysed by CpRu(cod)Cl in aqueous DMF, affords the diene 414 as an ene-type product in good yield. One explanation of the reaction is the formation of the (71-al ly I )(> 2-al kync)i n termediate from the 1 -alkene and insertion of the alkyne [161]. However, formation of the ruthenacyclopentene 412, subsequent /1-elimination to form 413, and reductive elimination offer a more easily understandable mechanism. A formal synthesis of altemaric acid (415) was achieved by this reaction [162],... 

One of the most reported pathways for C=C and C=C bonds coupling involves the oxidative coupling and the ruthenacyde intermediate formation. The first ruthenium-catalyzed Unear codimerization of disubstituted alkynes and alkenes involved acrylates or acrylamides and selectively produced 1,3-dienes [33] (Eq. 23). The proposed mechanism involves a ruthenacyclopentene via oxidative coupUng on the Ru(0) catalyst Ru(COD)(COT). The formation of 1,3-di-ene results from intracyclic /1-hydride eUmination, this process taking place only when a favored exocyclic /1-elimination is not possible. 

Given the preferential complexation of an alkyne compared with an alkene to ruthenium, the notion that alkene-alkyne coupling (Scheme 1.5) would occur seemed remote. However, to the extent that formation of the ruthenacyclopentene occurs, it can become irreversible because there exists a low-energy pathway by which it can further react, namely (5-hydrogen elimination. A final reductive elimination then completes a catalytic cycle wherein an alkene and an alkyne couple to form a 1,4-diene. 

What happens when p-hydrogcn elimination in the ruthenacyclic intermediate 45 is preduded, as in the case when vinyl ketones are the alkene partners (Equation 1.55) Given the extraordinary ability of Ru to interconvert easily among numerous oxidation states, one can imagine that the Ru can activate the double bond towards additions. For example, in the presence of water, protonation at the carbon (5 to Ru in the ruthenacyclopentene followed by nucleophilic addition of hydroxide can lead to 1,5-diketone formation. Indeed, terminal alkynes undergo smooth three-component coupling to form 1,5-diketones as shown in Equation 1.56 [52]. 

In a complementary manner, the regioselective coupling of ynamides and ethylene has been reported to be mediated by a low valent ruthenium catalyst (Cp RuCl(cod)). The formation of a ruthenacyclopentene 209 where the ruthenium... 

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