Ruthenium allylic substitution reactions

Although few investigations have been made to determine the stereochemical course of the reaction of Jt-allylmthenium complexes with nucleophiles, Harman and coworkers recently reported that the reaction with soft nucleophiles exclusively proceeded via an anti mechanism [58]. The observations described here, together with information in the literature, suggest that the ruthenium-catalyzed allylic substitution reaction proceeds via a double inversion (i.e., a net retention) mechanism. 

A vast majority of the allylic substitution reactions have been reported with palladium catalysts. However, complexes of other metals also catalyze allylic substitution reactions. In particular, complexes of molybdenum,tungsten, ruthenium, rhodium, and iridium " have been shown to catalyze the reactions of a variety of carbon nucleo-pliiles. In addition, complexes of ruthenium, rhodium, and iridium catalyze the reactions of phenoxides, alkoxides, amines, and amine derivatives. " The regioselectivity of the allylic substitution process witli these metals can often complement the regioselectivity of the reactions catalyzed by palladium complexes. The regioselectivity... 

In order to complement earlier studies showing that IV-alkyl-substituted ben-zimidazolylidene-ruthenium complexes were efficient catalysts for the regiose-lective alkylation of cinnamyl carbonate by dimethyl malonate or 1,3-diketones, as well as etherification of allylic halides by phenols, Bruneau and co-workers prepared a wide range of imidazolinium 38, tetrahydropyrimidinium 39, and benzimidazolium salts 40 that were screened as NHC ligand precursors in various allylic substitution reactions (Equation (7.9)). Unfortunately, linear versus branched selectivities were only modest, and no characterisation of the [(NHC)Ru(Cp )] complexes assumed to take part in the reaction could be achieved, thereby preventing any further rational ligand modification that would have helped refine the catalytic system. 

Table VI lists the known trifluoroacetato complexes of iron, ruthenium, and osmium. Photochemical substitution reactions of various tricovalent phosphorus-donor ligands with (7r-allyl)Fe(C0)2(02CCF3) and (iT-Cp)Fe(C0)3(02CCF3), analogous to those described in the preceding for Mn(C0)5(02CCF3), have afforded (ir-allyl)Fe(CO)(cis-Ph2PCH=CHPPh2)(02CCF3), (7r-Cp)Fe(C0)(PR3)(02CCF3) (where...

The regioselectivity of the attack of the Jt-allyl moiety of ruthenium complexes by nucleophiles is also notable, since the reaction exclusively occurs at the more-substituted allylic terminus in jt-allylmthenium complexes [29]. These reactions can be carried out catalytically by choosing appropriate ruthenium catalysts and reaction conditions (vide infra). 

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). 

The reactions of vinyltrisubstituted silanes with allyl substituted heteroorganic compounds proceed in the presence of ruthenium alkylidene complexes (catalyst IV) as well as catalysts including an Ru-H bond (I) according to the following equation, giving two isomeric products (E + Z) and ethylene (Scheme 4). 

Mechanistic implications of a general cross-metathesis of vinylsilicon with allyl-substituted heteroorganic compounds have been studied in detail for the reaction with allyl alkyl ethers [13]. The detailed NMR study of the stoichiometric reaction of Grubbs catalyst with allyl-n-butyl ether has provided information on individual steps of the catalytic cycle. A general mechanism of the cross-metathesis of vinyltri(alkoxy, siloxy)silanes (as well as octavinylsilsesquioxane) with 3-heteroatom-containing 1-alkenes in the presence of ruthenium carbene is shown in Scheme 5. 

The two reactions catalyzed by ruthenium complexes, i.e. silylative coupling (SC) (trans-silylation) catalyzed by I, II, V, and VI and cross-metathesis (CM) (catalysts III and IV) of vinyl- and allyl-substituted hetero(N,S,B)organic compounds with commercially available vinyltrisubstituted silanes, siloxanes, and silsesquioxanes provide a universal route toward the synthesis of well-defmed molecular compounds with vinylsilicon functionality. 

Star-shaped molecules containing cationic arene complexes of iron and ruthenium have been reported by Astruc and co-workers.298 Utilizing the activating nature of the cyclopentadienyliron moiety on the complexed arene, 260 was converted into 262 via bromobenzylation. The photolysis of 262 gave 263, which was subsequently reacted with 264 to give the hexametallic complex 265. Further nucleophilic aromatic substitution reactions with phenol 266 gave the allyl-substituted complex 267 (Scheme 2.70). 

As a matter of convenience, the results of oxidation of longifolene with lead tetraacetate, and ruthenium tetraoxide will also be summarized here steric diversion may have little role in forming the products of these reactions. One of the important reactions of lead tetraacetate with olefins is allylic substitution/rearrangement (J02). Since, this pathway is blocked for longifolene, the major product of this reaction is the ring-expanded enol acetate (154) (Chart 17), exactly parallel to what happens with camphene (8J, 99). 

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. 

The 5 2 allylic substitution of cinnamyl chloride by water in the presence of an (5)-Cp Ru catalyst and NaHC03 in aqueous THF at room temperature occurs with complete regiospeciflcity, giving a 93-99% yield of the branched allylic alcohol, with between 89 and 94% eeP A r-allyl-ruthenium complex is believed to form during the reaction. A tracer study using H2 0 showed that water is the nucleophile in the hydrolysis 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. ... 

Atom-transfer radical cyclization (ATRC) is an atom-economical method for the formation of cyclic compounds, which proceeds under mild conditions and exhibits broad functional group tolerance. Okamura and Onitsuka described a planar-chiral Cp-Ru complex 124-catalyzed asymmetric auto-tandem allylic amidation/ATRC reaction in 2013. This protocol proceeds highly regio, diastereo, and enantioselec-tively to construct optically active y-lactams from readily available substrates in a one-pot manner (Scheme 2.32). In this process, a characteristic redox property of ruthenium complexes would work expediently in different types of catalyzes involving mechanistically distinct allylic substitutions (Ru /Ru ) and atom-transfer radical cyclizations (Ru /Ru ), thus leading to the present asymmetric auto-tandem reaction [48]. 

The ruthenium complex 78 is a good catalyst for allylic substitution of various allyl derivatives (acetates, carbonates, etc.) to give the allylated products 209 (Scheme 96). These reactions were classified as an electrophilic and a nucleophilic allylation. It has been concluded that there exists the possibility that the intermediate (rj -allyl)ruthenium complexes can alternately function as a nucleophile and an electrophile, i.e., as an ambiphile, depending on the reactivity of the substrates [134]. Table 43 gives some typical examples of the allylation of AT- and C-nucleophiles with allylic carbonates [134d]. 

The most important ruthenium-catalyzed domino process is based on a metathesis reaction. Nonetheless, a few other ruthenium-catalyzed processes have been employed for the synthesis of substituted 3,y-unsaturated ketones, as well as unsaturated y-lactams and allylic amines. 

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