Ruthenium catalyzed trans addition

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. 

The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was first reported by Nishiyama et al., who used catalyst 31 in combination with either iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)2], or TBHP for the oxidation of trans-stilbene [130]. In the best result, using PhI(OAc)2 as oxidant, they obtained trans-stilbene oxide in 80% yield and 63% ee. More recently, Beller and coworkers have reexamined this catalytic system and found that asymmetric epoxidations could be performed using ruthenium catalysts 30 and 31 and 30% aqueous hydrogen peroxide [131-133]. A development of the pybox ligand led to ruthenium complex 32, which turned out to be the most efficient catalyst for asymmetric alkene epoxidation. Thus, using 5 mol% of 32 and slow addition of hydrogen peroxide, a number of aryl substituted alkenes were epoxidized in yields up >99% and enantioselectivity up to 84% (Scheme 2.25). 

Denmark pursued intramolecular alkyne hydrosilylation in the context of generating stereodefined vinylsilanes for cross-coupling chemistry (Scheme 21). Cyclic siloxanes from platinum-catalyzed hydrosilylation were used in a coupling reaction, affording good yields with a variety of aryl iodides.84 The three steps are mutually compatible and can be carried out as a one-pot hydro-arylation of propargylic alcohols. The isomeric trans-exo-dig addition was also achieved. Despite the fact that many catalysts for terminal alkyne hydrosilylation react poorly with internal alkynes, the group found that ruthenium(n) chloride arene complexes—which provide complete selectivity for trans-... 

The Michael addition of malonates to cyclic enones, catalyzed by chiral Ru( 6-arcnc)(p-lolucncsulfonyl-1,2-diaminc), has been performed to afford the adduct with excellent enantiomeric excess [91,92]. A related catalyst was designed to perform sequentially the Michael addition to cyclic enone and the enantioselective hydrogenation of the ketone. Thus, the chiral ruthenium catalyst B containing trans hydride and borohydride ligands was able to enan-tioselectively (96% ee) promote the Michael addition of malonate to cyclo-hexenone. Further in situ catalytic hydrogenation (400 psi H2) was performed and led to excellent diastereoselectivity trans/cis 30/1 [93] (Scheme 43). 

Several water-soluble ruthenium complexes, with P = TPPMS, TPPTS, or PTA ligands (cf. Section 2.2.3.2), catalyze the selective reduction of crotonaldehyde, 3-methyl-2-butenal (prenal), and trans-cinnamaldehyde to the corresponding unsaturated alcohols (Scheme 2) [33—36]. Chemical yields are often close to quantitative in reasonable times and the selectivity toward the aUyhc alcohol is very high (> 95%). The selectivity of the reactions is critically influenced by the pH of the aqueous phase [11] as well as by the H2 pressure [37]. The hydrogenation of propionaldehyde, catalyzed by Ru(II)/TPPTS complexes, was dramatically accelerated by the addition of inorganic salts [38], too. In sharp contrast to the Ru(II)-based catalysts, in hydrogenation of unsaturated aldehydes rhodium(I) complexes preferentially promote the reaction of the C=C double bond, although with incomplete selectivity [33, 39]. 

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