Ruthenium-mediated Reactions

The formation of cyclic Fischer carbene complexes by ruthenium-mediated activation of 3-butyn-l-ol and 4-pent5m-l-ol has been reported before (147,155,158-164). Correspondingly, a reaction of [Ru(bdmpza)Cl(PPh3)2] (24) with these terminal alkynols results... 

CEJ1358> and the ruthenium mediated isomerization of double bonds (cf. Scheme 89, Section 8.11.7) <2007TL137> are recent examples of transition metal catalyzed manipulations at the side chain carbon atoms of 1,3-heterocycles. A novel side-chain addition reaction of aldehydes to 6-alkylidene-l,3-dioxin-4-ones was used for the construction of intermediates of lophotoxin <2006CJC1226>. An acid-catalyzed intramolecular cycloaddition of a hydroxy group to an alkene has been effected by the presence of an adjacent 1,3-dithiane moiety <2006TL4549>. 

Chloromethyl polystyrene can be converted to a free-radical initiator by reaction with 2,2,6,6-tetramethylpipcridinc-/V-oxyl (TEMPO). Radical polymerization of various substituted alkenes on this resin has been used to prepare new types of polystyrene-based supports [123]. Alternatively, cross-linked vinyl polystyrene can be copolymerized with functionalized norbornene derivatives by ruthenium-mediated ringopening metathesis polymerization [124],... 

Figure 9-18. The sequence of reactions thought to be involved in the ruthenium-mediated oxidation of 9.8. In this figure the structure of 9.8 is reduced to the minimal structural features, and [Ru] = [Ru(bpy)2].

Internal acetylenes, Ni-mediated reactions, 10, 546 Internal alkenes, ethylene co-polymers, 4, 1145 Internal alkynes in alder-ene reaction, 10, 567 intermolecular hydrosilylation with ruthenium, 10, 802 with yttrium, 10, 801 silylboration, 9, 163 silylformylation, 11, 483... 

Grignard additions, 9, 59, 9, 64 indium-mediated allylation, 9, 687 in nickel complexes, 8, 150 ruthenium carbonyl reactions, 7, 142 ruthenium half-sandwiches, 6, 478 and selenium electrophiles, 9, W11 4( > 2 in vanadocene reactions, 5, 39 Nitrites, with trinuclear Os clusters, 6, 733 Nitroalkenes, Grignard additions, 9, 59-60 Nitroarenes, and Grignard reactivity, 9, 70 Nitrobenzenes, reductive aminocarbonylation, 11, 543... 

Strategies to pyridines include a ruthenium-mediated [2+2+2] cycloaddition to produce annulated products <20010L2117>. Reaction of 1,6-heptadiynes with electron-deficient nitriles yields the pyridine (Equation 175), whereas the same strategy using isocyanates leads to the 2-pyridone (Equation 176). 

The copper and palladium transition metal catalysts noted in Table 18 proved to be superior to nickel, ruthenium and rhodium catalysts. The nature of the reacting species has not been unequivocally defined, but the following experimental observations may provide some insight (i) tetrahydrofuran solvent is essential for the palladium-mediated reactions, since complex reaction mixtures (presumably containing carbinols) were observed when the reactions were performed in either benzene or methylene chloride (ii) the reaction is truly catalytic with respect to palladium (2 mmol alkylaluminum, 0.05 mmol of Pd(PPh3)4), whereas the copper catdyst is stoichiometric and (iii) in the case where a direct comparison may be made (entries 1-8, Table 18), the copper-based system is superior to palladium catalysis with regard to overall yield. 

P. Hu and A. Alavi (2001) Insight into electron-mediated reaction mechanisms Catalytic CO oxidation on a ruthenium surface. J. Chem. Phys. 114, p. 8113... 

Other metal-mediated reactions of azide reagents to terminal alkynes have also been reported. Indium(ll) triflate catalyzed tandem azidation/l,3-dipolar cycloaddition of various (o,(o-dialkoxyalkynes 134 with trimethylsilyl azide yielded fused 1,2,3-triazoles 135 <05TL8639>. A new ruthenium-catalyzed process for the regioselective synthesis of 1,5-disubstituted-1,2,3-triazoles has been developed <05JA15998>. 

Ruthenium chloride complexes, such as dichlorotris(triphenylphosphane)ruthenium(II), effectively catalyze the addition of polyhalocarbons to double bonds5 13 18. In a mechanistic and stereochemical study, carbohalogenation of cyclohexene with carbon tetrachloride in the presence of dichlorotris(triphenyIphosphane)rulhenium(II) gave l-chloro-2-(trichloromethyl)cyclohexane (2) in 77% yield and a diastereomeric ratio (transjeis) of 96 419. In comparison, the same conversion promoted by dibenzoyl peroxide is considerably less selective and gives the same product in only 10% yield with a 53 47 ratio of the trans, cis-isomets. This striking difference led to the conclusion that the ruthenium-catalyzed version does not proceed via a free-radical mechanism, as assumed in the peroxide-mediated reaction. 

Non-carbenoid, ruthenium-catalyzed intramolecular enyne metathesis shares a mechanistic resemblance to the Pd-mediated reactions above. 

DFT calculations on the ruthenium-mediated hydroxylation show that the low-spin reaction trajectory is preferred throughout, in accord with general expectations for the behavior of second-row transition metals The ruthenium analog was found to be more electrophilic than its iron complex, having lower hydrogen abstraction barriers. Thus, the data for the iron and ruthenium porphyrin systems is in accord with the predictions of theory that a radical rebound process is viable for iron which has an accessible high-spin state but not for ruthenium which is always low-spin. 

During the past few decades, a wide variety of molecules with transition metal-carhon mulhple bonds have been studied. The chemistry of doubly bonded species - carbenes - is particularly interesting because it leads to several synthetically important transformations, and for this reason, metal carbenes are the main subject of this chapter. Our discussion begins with a classification of metal-carbene complexes based on electronic structure, which provides a way to understand their reactivity patterns. Next, we summarize the mechanistic highlights of three metal-carbene-mediated reactions carbonyl olefinafion, olefin cyclopropanafion, and olefin metathesis. Throughout the second half of the chapter, we focus mainly on ruthenium-carbene olefin metathesis catalysts, in part because of widespread interest in the applications of these catalysts, and in part because of our expertise in this area. We conclude with some perspectives on the chemistry of metal carbenes and on future developments in catalysis. 

The importance of relativistic phenomena both in coordination complexes and in chemisorption has been reviewed. For complexes containing coordinated ethene or other unsaturated hydrocarbons, comparable quantitative information on all the Group 10 metals is extremely hard to come by, but calculations on various ethene and ethyne complexes (Table 4.13) performed by the non-local quasi-relativistic DF method are instructive. For each complex the bond energy is in the sequence Ni > Pt > Pd marked differences in the stabilities and reactivities of complexes of the type M"P2(CH3) (M = Pd, Pt P = PPhs) were also noted. In this context, it is never remarked that nearly all reactions homogeneously catalysed by metal salts or complexes, and metal-mediated reactions, involve elements from the first and second rows, and very rarely a third row element. Ruthenium, rhodium and palladium feature often osmium, iridium and platinum hardly at all. This is because very generally the complexes of the third row elements are too stable to be reactive. 

Ruthenium-mediated regioselective intermolecular homo- and heterodimerization of substituted propiolates leads to 2H-pyran-2-one-5-carboxylates and 2H-pyran-2-one-6-carboxylates (140L652). 6-Aryl-2J-f-pyran-2-ones arise from a palladium-catalyzed oxidative aimulation of internal alkynes with acrylic acid with excellent regioselectivity and in high yields (140L2146). A one-pot isothiourea-promoted cascade reaction of (phenylthio) acetic acids with a,P-unsaturated trifluoromethyl arylke-tones provides 4-aryl-6-trifluoromethyl-2f/-pyran-2-ones (Scheme 42) (140L964). 

Supercritical carbon dioxide represents an inexpensive, environmentally benign alternative to conventional solvents for chemical synthesis. In this chapter, we delineate the range of reactions for which supercritical CO2 represents a potentially viable replacement solvent based on solubility considerations and describe the reactors and associated equipment used to explore catalytic and other synthetic reactions in this medium. Three examples of homogeneous catalytic reactions in supercritical CC are presented the copolymerization of CO2 with epoxides, ruthenium>mediated phase transfer oxidation of olefins in a supercritical COa/aqueous system, and the catalyic asymmetric hydrogenation of enamides. The first two classes of reactions proceed in supercritical CO2, but no improvement in reactivity over conventional solvents was observed. Hythogenation reactions, however, exhibit enantioselectivities superior to conventional solvents for several substrates. 

A mechanism has been proposed by Blechert for this metathesis cascade, which involved the formation of a number of carbon-carbon bonds (in principle, a ruthenium-mediated [2-I-2+2] cycloaddition is also plausible for this transformation [49]). This postulated mechanism, as shown for the conversion of triyne 141 into the substituted aromatic system 142, is depicted in Scheme 17.27 [50]. Initially, complex 1-Ru adds to the less hindered acetylene of 141 to afford the vinyl carbene complex 143, which then undergoes an intramolecular metathesis reaction to afford 144 via 145. The conjugated complex 144 can then undergo a further RCM reaction to yield the product 142. 

---