Ruthenium catalysts enantioselective allylic substitutions

Enantioselective allylic substitutions catalyzed by transition-metal complexes are a powerful method for constructing complex organic molecules [4f,55]. Palladium-based catalysts have often given excellent results. To expand the scope of the reaction, a new enantioselective allylic alkylation catalyzed by planar-chiral ruthenium complexes was developed [56]. For example, the reaction of l,3-diphenyl-2-propenyl ethyl carbonate with sodium dimethyl malonate in the presence of 5 mol% of a planar chiral (S)-ruthenium complex (Figure 5.3) at 20 °C for 6 h in THE resulted in the formation of the corresponding chiral allylic alkylated product of dimethyl 2-((2 )(lS)-l,3-diphenylprop-2-enyl)propane-l,3-dioate in 99% yield vsdth 96% e.e. (Eq. 5.33). 

An 5 n2 allylic substitution of l,3-diphenyl-2-propenyl acetate by the enamine nucleophile produced from ketones and aldehydes gives C -substituted aldehydes and ketones in moderate to high yields (60-95%) and high enantioselectivities (79-98% ee). The reaction requires the presence of a palladium catalyst, pyrrolidine, and water. The palladium catalyst was prepared from a chiral ferrocene P,N ligand when a ketone was used, but with a ruthenium-based P,P ligand when an aldehyde was used in the reaction. 

Much effort has been devoted to developing catalysts that control the enantioselectiv-ity of these substitution reactions, as well as the regioselectivity of reactions that proceed through unsymmetrical allylic intermediates. A majority of this effort has been spent on developing palladium complexes as catalysts. Increasingly, however, complexes of molybdenum, tungsten, ruthenium, rhodium, and iridium have been studied as catalysts for enantioselective and regioselective processes. In parallel with these studies of allylic substitution catalyzed by complexes of transition metals, studies on allylic substitution catalyzed by complexes of copper have been conducted. These reactions often occur to form products of Sj 2 substitution. As catalylic allylic substitution has been developed, this process has been applied in many different ways to the synthesis of natural products. ... 

In the asymmetric intramolecular cyclopropanation of 40 (Scheme 32), comparative studies of chiral copper, rhodium, and ruthenium catalysts showed none of the catalysts is omnipotent in providing high enantiocontrol for all substrates (117). Complementary factors between these catalysts were observed. For instance, for the copper/7 catalyzed reaction, whereas high enantioselection was achieved with substituted allylic diazoacetates, any substitution at and R resulted in low percentage of ee. Interestingly, the opposite observation was observed with the rhodium catalyst 37. In the case of ruthenium catalyst 25, while high enantioselectivities were obtained for substrates with substitutions... 

The catalytic activation of allylic carbonates for the alkylation of soft car-bonucleophiles was first carried out with ruthenium hydride catalysts such as RuH2(PPh3)4 [108] and Ru(COD)(COT) [109]. The efficiency of the cyclopen-tadienyl ruthenium complexes CpRu(COD)Cl [110] and Cp Ru(amidinate) [111] was recently shown. An important catalyst, [Ru(MeCN)3Cp ]PF6, was revealed to favor the nucleophilic substitution of optically active allycarbonates at the most substituted allyl carbon atom and the reaction took place with retention of configuration [112] (Eq. 85). The introduction of an optically pure chelating cyclopentadienylphosphine ligand with planar chirality leads to the creation of the new C-C bond with very high enantioselectivity from symmetrical carbonates and sodiomalonates [113]. 

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