Ruthenacyclopentene Intermediates

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... 

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). 

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]. 

A similar ruthenacyclopentene intermediate can be proposed for the coupling of alkynes and allylic alcohols to lead to Y,8-unsaturated aldehydes and ketones. When [Cp Ru(CH3CN)3]PFg was used as a catalyst, the catalytic coupling between propargylic amides and allylic alcohols selectively provided piperidine derivatives by cyclization of the highly reactive aminoaldehyde intermediate [65] [Eq. (27)]. Only the branched isomer was formed by oxidative coupling, allowing the intramolecular condensation. 

The synthesis of 1,3-dienes via a ruthenacyclopentene intermediate is also a possible pathway. It results from intracyclic p-hydride ehmination, and this process takes place only when an exocychc p-elimination is not possible or not favored. Thus, the Cp RuCl(cod)-catalyzed hydrovinylation of ynamides proceeded in the presence of ethylene to selectively afford 2-amino-l,3-dienes [66] [Eq. (28)]. 

The coupling between alkynes and alkenes can also afford cycUzation reactions and leads to strained carbocycles. Most of these reactions are performed via a constrained ruthenacyclopentene intermediate which cannot undergo p-hydride elimination and leads to cyclobutenes via a reductive elimination as in the following example [68] [Eq. (30)]. 

The expected cyclobutenes are not always formed and other pathways than [2+2] cycloadditions can occur from ruthenacyclopentene intermediate. For example, a cyclopropanation reaction of various bicyclic alkenes could be performed when secondary propargyhc alcohols were used as alkynes in the presence of the Cp RuCl(cod) catalyst in THF [84] or when tertiary propargylic acetates were reacted in the presence of the CpRuCl(PPh3)2 catalyst in dioxane [85] [Eq. (34)]. 

A ruthenium(II)-catalysed intramolecular [2 + 2 -i- 2] cyclization of allene-yne-enes has been reported to proceed via a ruthenacyclopentene intermediate formed from the allene-yne or ene-yne moiety to give tricyclic compounds stereospecifically (Scheme 158).2°4... 

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