BINAP mechanism

The mechanism of such reactions using unsaturated carboxylic acids and Ru(BINAP)(02CCH3)2 is consistent with the idea that coordination of the carboxy group establishes the geometry at the metal ion.26 The configuration of the new stereocenter is then established by the hydride transfer. In this particular mechanism, the second hydrogen is introduced by protonolysis, but in other cases a second hydride transfer step occurs. 

Some of the details of the mechanism may differ for various catalytic systems. There have been kinetic studies on two of the amination systems discussed here. The results of a study of the kinetics of amination of bromobenzene using Pd2(dba)3, BINAP, and sodium r-amyloxide in toluene were consistent with the oxidative addition occurring after addition of the amine at Pd. The reductive elimination is associated with deprotonation of the animated palladium complex.166... 

Another setup used for the hydrogenation of DMI with Ru-BINAP was equipped with dense PDMS elastomer membranes (Jacobs et al. [48]). The catalyst solution was present in a submerged membrane system, prepared as a sealed PDMS capsule . The catalytically active complex was retained by the membrane while substrate and products, dissolved in the bulk phase, could cross the membrane under the influence of the concentration difference without the need for mechanical pressure. 

By contrast, a recent, detailed mechanism of the enantiomeric hydrogenation of a-(acylamino)acrylic esters catalyzed by Ru((S)-binap)(OAc)2 follows that of Scheme 3.3, where both H atoms from the dihydrogen add to the C=C double bond [85]. The high enantioselectivity of the process is produced, in part, by the chelation of the alkene substrate via the C=C double bond and by a carbonyl oxygen of the substrate [86]. 

Scheme 3.9 A possible mechanism of the hydrogenation of tiglic acid catalyzed by Ru((S)-binap)(OAc)2 (as adapted from [84]). The stereochemistry of the metal center and coordination geometries are speculative at this stage.

Scheme 3.11 Partial mechanistic scheme for the hydrogenation of aryl ketones to give the (S)-alcohol catalyzed by RuCI2((R)-binap)((R,R)-dpen)/KO Bu/H2 as based on the observed mechanism for RuH2((R)-binap)(NH2CMe2CMe2NH2).

Fig. 31.15 Mechanism of the enantioselective hydrogenation of enamides by Ru BINAP, giving the opposite stereochemical course to the corresponding Rh catalyst. Note the heterolytic nature of the addition process with one of the two hydrogens arising from solvent.

In the case of hydrogenation using [Ru(BINAP)Cl2]n as the catalyst precursor, the reaction seems to occur by a monohydride mechanism as shown in Scheme 6-31. On exposure to hydrogen, RuC12 loses chloride to form RuHCl species A, which in turn reversibly forms the keto ester complex B. Hydride transfer occurs in B from the Ru center to the coordinated ketone to form C. The reaction of D with hydrogen completes the catalytic cycle.67... 

Figure 8-5. Proposed mechanism for the catalytic arylation of 2,3-dihydrofuran with phenyl triflate in the presence of Pd(OAc)2-(i )-BINAP catalyst.

The proposed mechanism is illustrated in Figure 8-5.60a Oxidative addition of the phenyl triflate to the palladium(0)-BINAP species A gives phenylpalla-dium triflate B. Cleavage of the triflate and coordination of 2,3-dihydrofuran on B yields cationic phenyl palladium olefin species C. This species C bears a 16-electron square-planar structure that is ready for the subsequent enantio-selective olefin insertion to complete the catalytic cycle (via D, E, F, and G). The base and catalyst precursor have profound effects on the regioselectivity and enantioselectivity. 

Figure 1.3. Catalytic hydrogenation of A-acylated dehydroamino esters via an unsatu-rated/dihydride mechanism the p substituents in the substrates are omitted for clarity [P-P = (i ,R)-DIPAMP, (i ,i )-CHIRAPHOS, or (R)-BINAP S = solvent or a weak ligand].

SCHEME 40. Possible mechanisms of BINAP-Ru-catalyzed hydrogenation. 

Structural Characteristics. BINAP-Ru(II) complexes catalyze a variety of synthetically useful stereoselective hydrogenations. Although the exact mechanism of the enantioface differentiation is yet to be eluci-... 

The results of deuterium labeling experiments shown in Scheme 37 clearly show the operation of a monohydride mechanism in the BINAP-RuOI) catalyzed hydrogenation of unsaturated carboxylic acids. However, with many olefinic substrates with a neutral, rather than anionic, secondary binding site, the products exhibit a similar degree of isotope incorporation at the two hydrogenated centers (Scheme 39). The out-... 

The mechanism involving simple nitrogen-coordinated complexes also accounts for reactivities of certain sterically constrained systems. For instance, 3-(diethyamino)cyclohexene undergoes facile isomerization by the action of the BINAP-Rh catalyst (Scheme 18). The atomic arrangement of the substrate is ideal for the mechanism to involve a three-centered transition state for the C—H oxidative addition to produce the cyclometalated intermediate. The high reactivity of this cyclic substrate does not permit any other mechanisms that start from Rh-allylamine chelate complexes in which both the nitrogen and olefinic bond interact with the metallic center. On the other hand, fro/tt-3-(diethylamino)-4-isopropyl-l-methylcyclohexene is inert to the catalysis, because substantial I strain develops during the transition state of the C—H oxidative addition to Rh. 

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