Vinylcarbene ruthenium complex

The highly substituted, chelating, vinylcarbene ruthenium complex 22 has been prepared by Fiirstner [40] via the reaction of 6 with ortho-isopropoxyethynylbenzene in dichloromethane at room temperature. Silver chloride (AgCl) was added to this reaction as a phosphine scavenger. Complex 22, which results from a formal insertion of the triple bond into the Ru=C via a [2-1-2] cycloaddition, was not evaluated as a metathesis catalyst in this report. 

Recently, cyclopropane derivatives were produced by a ruthenium-catalyzed cyclopropanation of alkenes using propargylic carboxylates as precursors of vinylcarbenoids [51] (Eq. 38). The key intermediate of this reaction is a vinylcarbene complex generated by nucleophilic attack of the carboxylate to an internal carbon of alkyne activated by the ruthenium complex. Then, a [2+1] cycloaddition between alkenes and carbenoid species affords vinylcyclo-propanes. 

From the mechanistic point of view, the observed competitive reactions can be explained by considering two different pathways (Scheme 114). The intermediacy of ruthenacyclopentadiene 453 or biscarbenoid 452, formed from the reaction of a diyne and a ruthenium(ll) complex, is postulated in the proposed mechanism. Cyclopropanation of the alkene starts with the formation of ruthenacyclobutane 456, which leads to the generation of the vinylcarbene 457. Then, the second cyclopropanation occurs to afford the biscyclopropyl product 458. Insertion of the alkene 459 into the ruthenacyclopentadiene 453 affords the ruthenacycloheptadiene 454. The subsequent reductive elimination gives the cyclotrimerization product 455. The selectivity toward the bis-cyclopropyl product 458 is improved with an increasing order of haptotropic flexibility of the cyclopentadienyl-type ligand. 

Since the vinylcarbenes la-c and the aryl substituted carbene (pre)catalyst Id, in the first turn of the catalytic cycle, both afford methylidene complex 3 as the propagating species in solution, their application profiles are essentially identical. Differences in the rate of initiation are relevant in polymerization reactions, but are of minor importance for RCM to which this chapter is confined. Moreover, the close relationship between 1 and the ruthenium allenylidene complexes 2 mentioned above suggests that the scope and limitations of these latter catalysts will also be quite similar. Although this aspect merits further investigations, the data compiled in Table 1 clearly support this view. 

The relative ease of ring formation from dienes decreases in the order six->seven->five-membered rings. Hoshi et al. [138] have recently reported the synthesis of a number of disilacycloalkenes via RCM of bis(allyldimethyl-silyl)substituted compounds, with the ruthenium vinylcarbene complex Cl2(PCy3)2Ru(=CHCH=CPh2) used as a catalyst. Successful formation of seven-and eight-membered rings has been achieved under mild reaction conditions. 

Scheme 14.13 Ruthenium indenylidene complexes as precursors of first-generation ben-zylidene and vinylcarbene complexes.

A series of ruthenium vinylcarbene catalysts have been prepared by Furstner by insertion of alkynes into the ruthenium-alkylidene bond of various complexes [37], Complexes 25, 26, and 27 were obtained in 72%, 59%, and 75% yields, respectively (Scheme 15). 

Addition of diazo compounds to metallic complexes allows the formation of metal carbenoid species which can react with unsaturated molecules to form new carbon-carbon bonds. The Cp RuCl(cod)-catalyzed addition of diazo compoimds to alkynes led to the selective synthesis of functional 1,3-dienes by the combination of two molecules of diazoaUcane and one molecule of alkyne [115,116] [Eqs. (53) and (54)]. The ruthenium carbene, generated from diazo compound, reacts with the C=C bond to produce vinylcarbene intermediate able to add a second molecule of diazo compotmd to generate dienes. The stereoselective formation of these conjugated dienes results from the selective creation of two C=C bonds, probably due to the possibility for (C5Me5)RuCl moiety to accomodate two cis carbene ligands. This reaction occurred with terminal or internal alkynes as well as 1,3-diynes [115] and was applied successfully to alkynylboronates [116]. 

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