BINAP , structure

A transition structure model was proposed which accounts for the high selectiv-ities obtained for some of the substrates [80]. In the structure shown in Scheme 6.37 the two phosphorus atoms of the Tol-BINAP ligand and the two car-... 

A number of chiral ketones have been developed that are capable of enantiose-lective epoxidation via dioxirane intermediates.104 Scheme 12.13 shows the structures of some chiral ketones that have been used as catalysts for enantioselective epoxidation. The BINAP-derived ketone shown in Entry 1, as well as its halogenated derivatives, have shown good enantioselectivity toward di- and trisubstituted alkenes. 

The BINAP system of general structure 111 can be used in asymmetric hydrogenations the compound in which Ar = S.S-MejCgHj, R1 = R2 = 4-MeOCgH4,... 

Lautens also used this nickel-catalyzed hydroalumination methodology in the total synthesis of ionomycin 145. The starting compound was a [3.2.1]oxabicyclic alkene 143.144 Their rigid bicyclic structures can be used to introduce functional groups in a highly stereoselective manner. The synthesis of the key intermediate 144 involves the slow addition of DIBAL to the oxabicyclic alkene and the Ni(COD)2/(6T)-BINAP in toluene to afford 144 in 95% yield and 93-95% ee. (Scheme 18). 

Fig. 3.3 The structures of diphosphines with four atoms in the backbone (a) dppb (b) (-)-(R,R)-diop (c) (R)-binap.

In the hydrogenation of diketones by Ru-binap-type catalysts, the degree of anti-selectivity is different between a-diketones and / -diketones [Eqs (13) and (14)]. A variety of /1-diketones are reduced by Ru-atropisomeric diphosphine catalysts to indicate admirable anti-selectivity, and the enantiopurity of the obtained anti-diol is almost 100% (Table 21.17) [105, 106, 110-112]. In this two-step consecutive hydrogenation of diketones, the overall stereochemical outcome is determined by both the efficiency of the chirality transfer by the catalyst (catalyst-control) and the structure of the initially formed hydroxyketones having a stereogenic center (substrate-control). The hydrogenation of monohydrogenated product ((R)-hydroxy ketone) with the antipode catalyst ((S)-binap catalyst) (mis-... 

With an increase of conversion, the enantiopurity of unreacted (S)-substrate increases and the diastereoselectivity of the product decreases. Using Ru-((S)-binap)(OAc)2, unreacted (S)-substrate was obtained in more than 99% ee and a 49 1 mixture of anti-product (37% ee (2R,iR)) at 76% conversion with a higher kR ks ratio of 16 1 [46]. In the case of a racemic cyclic allyl alcohol 24, high enantiopurity of the unreacted alcohol was obtained using Ru-binap catalyst with a high kR ks ratio of more than 70 1 [Eq. (16)] [46]. In these two cases, the transition state structure is considered to be different since the sense of dia-stereoface selection with the (S)- or the (R)-catalysts is opposite if a similar OH/ C=C bond spatial relationship is assumed. 

The sense of diastereoselectivity in the dynamic kinetic resolution of 2-substi-tuted / -keto esters depends on the structure of the keto ester. The ruthenium catalyst with atropisomeric diphosphine ligands (binap, MeO-biphep, synphos, etc.) induced syn-products in high diastereomeric and enantiomeric selectivity in the dynamic kinetic resolution of / -keto esters with an a-amido or carbamate moiety (Table 21.21) [119-121, 123, 125-127]. In contrast to the above examples of a-amido-/ -keto esters, the TsOH or HC1 salt of /l-keto esters with an a-amino unit were hydrogenated with excellent cwti-selectivity using ruthenium-atropiso-... 

The high level of enantiofacial selection is made in the hydride transfer step 7C -> 7D [2], The chelating geometry in the transition state 7F decreases the activation energy. The chiral environment derived from (R)-BINAP clearly differentiates diastereomeric Si-7F and Re-7F (Fig. 32.7b). The Si structure affording the R alcohol is much more favored than the Re structure, which suffers from the Ph/R repulsion. 

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. 

The use of cyclic alkenes as substrates or the preparation of cyclic structures in the Heck reaction allows an asymmetric variation of the Heck reaction. An example of an intermolecular process is the addition of arenes to 1,2-dihydro furan using BINAP as the ligand, reported by Hayashi [23], Since the addition of palladium-aryl occurs in a syn fashion to a cyclic compound, the 13-hydride elimination cannot take place at the carbon that carries the phenyl group just added (carbon 1), and therefore it takes place at the carbon atom at the other side of palladium (carbon 3). The normal Heck products would not be chiral because an alkene is formed at the position where the aryl group is added. A side-reaction that occurs is the isomerisation of the alkene. Figure 13.20 illustrates this, omitting catalyst details and isomerisation products. 

The reaction is carried out with aryl triflates and other details such as solvent and base used are also important. Intramolecular additions of aryl halides or triflates to alkenes in a side-chain leading to cyclic compounds have been reported by Overman [24], Rather complicated ring structures can be made stereospecifically. While initially BINAP seemed the best ligand for this conversion, the number of useful ligands is increasing [25],... 

Fig. 24 Minimized structure of 47 showing the kink in the mPE backbone brought about by the binap moiety...

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