Cymene ruthenium complexes

The (5 )-selective DKR of alcohols with subtilisin was also possible in ionic liquid at room temperature (Table 14). " In this case, the cymene-ruthenium complex 3 was used as the racemization catalyst. In general, the optical purities of (5 )-esters were lower than those of (R)-esters described in Table 5. 

Kinetic resolution.Allylic alcohols are resolved with a combination of lipase and a (p-cymene)ruthenium complex. 

The group also found that cymene ruthenium complexes, depicted in Scheme 4.6, were also active for the racemisation of alcohols in the presence of TEA. As shown in Scheme 4.6, a noticeable feature of these catalysts was their high activity towards allylic alcohols, since their DKR was possible even at room temperature by using PS-C and p-chlorophenyl acetate as the acyl donor. 

In addition, the activated hydride form of a cymene ruthenium complex was shown to be elfective as a racemising catalyst in ionic liquids such as... 

Ruthenium complexes have also been reported as active species for enan-tioselective Diels-Alder reactions. Faller et al. prepared a catalyst by treatment of (-)-[( ] -cymene)RuCl(L)]SbF6 with AgSbFe resulting in the formation of a dication by chloride abstraction [95]. The ligand was (-l-)-IndaBOx 69 (Scheme 36) and the corresponding complex allowed the condensation of methacrolein with cyclopentadiene in 95% conversion and 91% ee. As another example, Davies [96] prepared the complex [Ru(Fl20)L ( i -mes)] [SbFe]2 (with 70 as L in Scheme 36), and tested its activity in the same reaction leading to the expected product with similar activity and lower enan-tioselectivity (70%). 

NMR spectra of (p-cymene)ruthenium (II) Schiff base complex, derivative of (S)-(a-methylbenzyl) and 3,5-di-ferf-butylsalicylaldimine, at room temperature in CDCI3 solution evidenced the presence of diastereomers at the ratio of 88 12.93 On the basis of a detailed analysis of 2D NMR spectra (ROESY) measured at 293 and 233 K, the (RRu,Sc) configuration of the major diastereomer in solution was suggested. 

Scheme 3 shows the details of the synthetic strategy adopted for the preparation of heteroleptic cis- and trans-complexes. Reaction of dichloro(p-cymene)ruthenium(II) dimer in ethanol solution at reflux temperature with 4,4,-dicarboxy-2.2 -bipyridine (L) resulted the pure mononuclear complex [Ru(cymene)ClL]Cl. In this step, the coordination of substituted bipyridine ligand to the ruthenium center takes place with cleavage of the doubly chloride-bridged structure of the dimeric starting material. The presence of three pyridine proton environments in the NMR spectrum is consistent with the symmetry seen in the solid-state crystal structure (Figure 24). 

Sinou and coworkers evaluated a range of enantiopure amino alcohols derived from tartaric acid for the ATH reduction of prochiral ketones. Various (2R,iR)-i-amino- and (alkylamino)-l,4-bis(benzyloxy)butan-2-ol were obtained from readily available (-I-)-diethyl tartrate. These enantiopure amino alcohols have been used with Ru(p-cymene)Cl2 or Ir(l) precursors as ligands in the hydrogen transfer reduction of various aryl alkyl ketones ee-values of up to 80% have been obtained using the ruthenium complex [93]. Using (2R,3R)-3-amino-l,4-bis(benzyloxy)butan-2-ol and (2R,3R)-3-(benzylamino)-l,4-bis(benzyloxy)butan-2-ol with [lr(cod)Cl]2 as precursor, the ATH of acetophenone resulted in a maximum yield of 72%, 30% ee, 3h, 25 °C in PrOH/KOH with the former, and 88% yield, 28% ee, 120 h with the latter. 

Ruthenium complexes of a novel silsesquioxane-based tridentate phosphine ligand have been prepared and characterized by Mitsudo et al The synthesis of the ligand 178 is depicted in Scheme 60. Reactions of 178 with several late transition metal complexes were examined. A typical example is the reaction with three equivalents of [RuCl2(cymene)]2, which produced the red triruthenium complex (c-C5H9)7Si709[0SiMe2CH2CH2PPh2RuCl2(cymene)]3 (179) in almost quantitative yield. 

Quite recently, some mononuclear ruthenium complexes such as [(p-cymene)RuX-(CO)(PR3)]OTf (X = Cl, OTf, R = Ph, Cy) have been found to work as catalysts for the propargylation of aromatic compounds such as furans, where some ruthenium complexes were isolated as catalytically active species from the stoichiometric reactions of propargylic alcohols (Scheme 7.27) [31]. The produced active species promoted the propargylation of furans vdth propargylic alcohols bearing not only a terminal alkyne moiety but also an internal alkyne moiety, indicating that this propargylation does not proceed via allenylidene complexes as key intermediates. 

Allenylidene-ruthenium complex Ib readily promotes the ROMP of norbornene, much faster than the precursor RuCl2(PCy3)(p-cymene) [39] (Table 8.1, entry 1). The ROMP of cyclooctene requires heating at 80 °C (5 min), however a pre-activation of the catalyst allows the polymerization to take place at room temperature. The activation consists, for example, in a preliminary heating at 80 °C or UV irradiation of the catalyst before addition of the cyclic aikene, conditions under which rearrangement into indenylidene and arene displacement take place [39] (Table 8.1, entries 2,3). The arene-free allenylidene complexes, the neutral RuCl2(=C=C=CPh2)... 

From 1995 to 2000, catalyst profiles of several ruthenium catalysts bearing pyridine-diimide 1 [13], diiminocarbene 2 [14], diamine-arene 3 [15],phos-phino-arene 4 [16], and substituted cyclopentadienyl 5 and 6 [17, 18] were shown to have good activity for the cydopropanation (Fig. 1). At the relatively high reaction temperature of 60-100 °C,they also gave moderate-to-high yields over 90%. It is interesting in that the dipyridine-diimide complex 1 and the p-cymene-carbene complex 2 show high trans selectivity, 86 14 and 82 18, respectively. 

The bisacetato ruthenium complex 28, on heating in 2-propanol, leads to the bridged hydrido dinuclear complexes 73 and 74. The bistrifluoroacetato complex 28 also leads to complex 73. The Tj2-acetato complex 39 was transformed in hot 2-propanol to another bridged hydrido derivative (75, arene = durene, mesitylene, p-cymene, hexamethylbenzene 60-70%). The introduction of alkyl substituents on the benzene ring is reflected by a shift of the p FI resonance toward high field (14,53). 

An attempt to achieve the complexation of the cyclophane iron complex 276 by treatment with the solvated (p-cymene)ruthenium derivative 7 results in disruption of the arene-iron bond and formation of the (p-cymene)[[22](l,4)cyclophane]ruthenium(II) salt 277 as the only-product (164) (Scheme 29, p. 224). 

RuCl2(PMe3)(C6Me6) and RuCl2(PMe3)(p-cymene) appear to be much more efficient catalysts than other mononuclear ruthenium complexes, or Ru3(CO)12, for a variety of secondary amines and terminal alkynes, such as diethylamine, morpholine, piperidine, and pyrrolidine (65,201), except for acetylene itself (202,203). The regioselective addition to the terminal carbon suggests that that the reaction proceeds via an arene ruthenium vinyl-idene intermediate that has been characterized (66) (Section II,A,3,d). 

Recently, new types of ruthenium catalyst precursors that perform the Markovnikov addition of carboxylic acids to terminal alkynes have been developed. The most representative examples are [RuCl2(p-cymene)]2/P(furyl)3/base [50], Ru-vinylidene complexes such as RuCl2(PCy3)2(=C=CHt-Bu), RuCl2(PCy3)(bis(mesityl)imidazolyli-dene)(=C=CHf-Bu), [RuCl(L)2(=C=CHt-Bu)]BF4 [51], and the ruthenium complexes shown in Figure 8.1 [52-54]. 

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