Ruthenacyclopentene

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... 

Interestingly, experimental results indicate that pathway II might be operative in some ruthenium(n)-catalyzed [5 + 2]-reactions. Cyclized products implicating /3-hydride elimination and subsequent reductive elimination from ruthenacyclopentenes have been reported (Scheme 17).26 A direct comparison with rhodium catalysts using these specific substrates has not been reported. 

Epimerization of vinylcyclopropanes by Grubbs I-type ruthenium catalysts (28) has been explored.33 The reaction can also be effected by the Grubbs-Hoveyda catalyst (29) provided that an additional phosphine is added. Mechanistic studies (experimental and theoretical) suggest that the epimerization goes through a ruthenacyclopentene intermediate (30). 

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],... 

As another example of the reaction via the ruthenacyclopentene, 1,4-diketones are formed by the Ru-catalysed reaction of terminal alkynes with vinyl ketones in aqueous DMF in the presence of NH4PF6 and InCl3. The reaction is explained by the generation of the ruthenacyclopentene 422, followed by addition of H2O to the double bond. Elimination of /5-hydrogen and reductiuve elimination afford 1,5-diketones [165a],... 

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. 

A similar mechanism,based on a ruthenacyclopentene, can be proposed for the coupling of alkynes and allylic alcohols to lead to y,<5-unsaturated aldehydes and ketones. When (C5H5)RuC1(COD) was used as a catalyst, the ruthenium-catalyzed coupling between alkynes and substituted allylic alcohols afforded y,<5-unsaturated ketones. The linear isomer was the major product [39] (Eq. 28). Similarly, the linear derivative was also obtained when an allylsi-lylether or an allylic amide was used in place of the allyl alcohol, leading to 1,4-dienes [40]. 

Finally, benzenepolycarboxylates were obtained by ruthenium-catalyzed cross-benzannulation of acetylenedicarboxylates with allylic compounds [52] (Eq. 39). A ruthenacyclopentene is postulated to occur via oxidative coupling of one molecule of alkyne with allylic alcohol. Subsequent insertion of another molecule of alkyne gives the corresponding polysubstituted benzene derivatives. 

Ruthenacyclopentenes have also been proposed as intermediates in the intramolecular coupling reaction of the C=C bond and the C=C bond. Thus, the complex CpRu(CH3CN)3PF6 catalyzed the cycloisomerization of a variety of... 

The ruthenacyclopentene intermediate can also undergo insertion of ethylene to give a ruthenacycloheptene. Subsequent unexpectedly observed /1-hydride elimination occurred and led then to cyclization products with a propenylidene substituent [79] (Eq. 58). Various enynes.with substituents on triple or double bonds, have been cyclized to form carbocyclic and heterocyclic compounds in good yields. 

Quite recently, the alkenylative cyclization of enynes with ethylene was achieved using Cp RuCl(cod) as a precatalyst at room temperature [92]. This mild and selective transformation was applied to a sulfonamide 95 to produce the corresponding pyrrolidine derivative 96 (Eq. 37). A ruthenacyclopentene intermediate was proposed for this novel cyclization. 

The [2 -I- 2] cycloaddition of an alkene and an alkyne is a valuable route leading to cyclobutene derivatives. The ruthenium(0)-catalyzed [2 -1- 2] cycloaddition of a strained cycloalkene, norbornene 40, vith highly electron-deficient DMAD afforded the cyclobutene 74 (Scheme 4.28) [62]. As expected, the reaction took place at the exo face of 40 via the ruthenacyclopentene intermediate 75, that ivas formed by the oxidative cyclization of DMAD and norbornene. In addition to the parent 40, various norbornene derivatives can also be used as alkene components. When the Ru" precatalyst 17 ivas employed, electronically neutral alkynes participated in the [2 -1- 2] cycloaddition with norbornene and its derivatives [63]. A similar [2 -1- 2] cycloaddi-... 

A similar ruthenium complex (C5H5)RuCl(cod) catalyzes a totally different reaction pathway for alkynes and allylic alcohols to produce y,d-unsaturated ketones, which involves a ruthenacyclopentene intermediate, rather than a jt-allylmthenium intermediate [39]. 

The mechanism which involves an intermediary ruthenacyclopentene K is proposed (Scheme 12.6). Coordination of the enyne to the coordinatively unsaturated cationic cyclopentadienylmthenium species I, tautomerization of the resulting ruthenium-enyne complex J to the ruthenacyclopentene K, /3-hydrogen elimination to form a vinylmthenium L, followed by reductive elimination yields the 2-alkenyl-1-alkylidenecydopentane 58 and regenerates the catalyst I. 

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]. 

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