Ruthenium allyl carbene complexes

Blechert carried out a tandem reaction of enynes in the presence of olefins instead of ethylene (Scheme 21). Treatment of cyclopentenol derivative 58a with Ic in the presence of an alkene affords 59a. The five-membered ring in estrone 58b is cleaved by Ic to give 59 and an alkene part is introduced on the six-membered C ring. However, cycloalkenyl amine derivative 60 is treated in a similar manner in the presence of an allyl alcohol derivative to give pyrrolidine derivative 61, and in this case, an alkene part is introduced on the diene moiety. Presumably, ruthenium carbene complex XVI reacts with an alkyne part to produce the pyrrolidine ring with a regioselectivity opposite to the other cases. 

Dihydropyrroles have recently become readily available by ring-closing metathesis. For this purpose, N-acylated or N-sulfonylated bis(allyl)amines are treated with catalytic amounts of a ruthenium carbene complex, whereupon cyclization to the dihydropyrrole occurs (Entries 6 and 7, Table 15.3 [30,31]). Catalysis by carbene complexes is most efficient in aprotic, non-nucleophilic solvents, and can also be conducted on hydrophobic supports such as cross-linked polystyrene. Free amines or other soft nucleophiles might, however, compete with the alkene for electrophilic attack by the catalyst, and should therefore be avoided. 

Poly(l,4-butadiene) segments prepared by the ruthenium-mediated ROMP of 1,5-cyclooctadiene can be incorporated into the ABA-type block copolymers with styrene (B-106) and MMA (B-107).397 The synthetic method is based on the copper-catalyzed radical polymerizations of styrene and MMA from the telechelic poly(butadiene) obtained by a bifunctional chain-transfer agent such as bis(allyl chloride) or bis-(2-bromopropionate) during the ROMP process. A more direct route to similar block copolymers is based on the use of a ruthenium carbene complex with a C—Br bond such as Ru-13 as described above.67 The complex induced simultaneous or tandem block copolymerizations of MMA and 1,5-cyclooctadiene to give B-108, which can be hydrogenated into B-109, in one pot, catalyzed by the ruthenium residue from Ru-13. 

Although numerous examples of successful RCM reactions have been demonstrated, a few limitations and/or side-reactions have been uncovered. Some cases where the RCM reaction proceeds with complications are depicted in Scheme 18. Sometimes RCM reactions are competitive with alkene isomerization. For example, the unexpected formation of 162 from precursor 160 was attributed to alkene isomerization (affording 161), followed by RCM to afford the ring-contracted compound 162. Later investigators went on to exploit this observation for the synthesis of cyclic enol ethers. Treatment of the allyl ether 164 with a ruthenium carbene complex catalyst affords the RCM... 

Cyclic allyl ethers generated by RCM can be isomerized into 2,3-dihydropyrans by a ruthenium complex and eventually by conditioning ruthenium carbene complex precursors [39]. This combination of sequentially Ru-catalyzed ring... 

Complex formation with flic substrate is the key stage of many catalytic processes. The formation of the following types of organometallic complexes is most typical in catalysis alkyl 7i-complexes, carbene complexes, n-complexes of substrates with the saturated bond (olefin, acetylene and allyl, complexes with carbon oxides), hydrazine complexes, and complexes with molecular oxygen and nitrogen. The structure of a ruthenium complex with CO2 obtained on the basis of an ab initio study is presented in Fig. 17.4. 

The mechanism (Scheme 60) involves coordination of the terminal alkyne to the ruthenium atom followed by the formation of the vinylidene complex 135. Coordination of the allyl alcohol followed by addition of the alcohol to the ruthenium vinylidene complex leads to the ruthenium carbene complex 136. Metalla-Claisen rearrangement produces the jr-allyl-acylruthenium complex 137, which undergoes a reductive elimination to give the product 133 and regenerates the catalytically active ruthenium species. The regioselectivity of the coupling is independent of the site of ionization and the new bond formation occurs on the more substituted terminus of the double bond of the rr-allyl-ruthenium complex. 

Isolable ruthenium vinylidene and carbene complexes are involved in the coupling of alkynes with allylic alcohols. Some of these transformations were previously known from model reactions. The system aUows the synthesis of a large range of enones (Scheme 32). While most coupling reactions, particularly those applied to organic synthesis, remain faithful to palladium, molybdenum carbonyl complexes, too, have found uses. 

First synthetic attempts with compound 11 using catalyst [Ru]-VII [17] delivered the undesired bicyclic structure 12, which results from the initial attack of the ruthenium carbene complex at the terminal olefin via intermediate 13 (Scheme 11.5). To direct the initiation of the catalytic cycle into the other double bond, through intermediate 14, Honda et al. decided to introduce steric hindrance using a disub-stituted olefin in the alkyl chain. Their results showed, however, that the resulting compound was not reactive enough to undergo metathesis under the chosen reaction conditions. With the use of a more electron-rich allyl ether moiety such as in 15, it was possible to isolate the desired product in 74% yield and successfully complete the synthesis of securinine. Ruthenium complexes [Ru]-I and [Ru]-II were... 

Summary Two catalytic reactions, i.e. silylative coupling (frans-silylation) (SC) catalyzed by complexes containing or generating Ru-H and/or Ru-Si bonds (I, O, V, VI) and cross-metathesis (CM) catalyzed by ruthenium-carbene (i.e. 1st and 2nd generation ruthenium Grubbs catalyst (ID, IV)) of vinyl and allyl-substituted hetero(N,S,B)organic compounds with commercially available vinyltrisubstituted silanes, siloxanes, and silsesquioxane have been overviewed. They provide a universal route toward the synthesis of well-defined molecular compounds widi vinylsilicon functionality. 

Recently, we found that Af-allyl-o-vinylaniline 44 gave 1,2-dihydroquinoline 45 by normal RCM and developed silyl enol ether-ene metathesis for the novel synthesis of 4-siloxy-l,2-dihydroquinoline and demonstrated a convenient entry to quinolines and 1,2,3,4-tetrahydroquinoline [13], We also have found a novel selective isomerization of terminal olefin to give the corresponding enamide 46 using ruthenium carbene catalyst [Ru] and silyl enol ether [14], which represented a new synthetic route to a series of substituted indoles 47 [12], We also succeeded an unambiguous characterization of ruthenium hydride complex [RuH] with A -heterocyclic carbene... 

Occasionally, systems with complexes of ruthenium and cobalt have been reported to catalyze intramolecular cyclopropanation of allylic diazoacetate with high enantiocontrols. For cobalt-salen 39, up to 97% ee was observed (Scheme 30) (128). More recently, methods other than diazo decomposition were applied in intramolecular cyclopropanation. Activated by irradiation, [WlCOle] catalyzed the cyclopropanation of alkynol to give good yield of cyclopropane (Scheme 31) (129). The reaction was proposed via a tungsten—carbene intermediate. 

---