BINAP complexes

Asymmetric hydrogenation has been achieved with dissolved Wilkinson type catalysts (A. J. Birch, 1976 D. Valentine, Jr., 1978 H.B. Kagan, 1978). The (R)- and (S)-[l,l -binaph-thalene]-2,2 -diylblsCdiphenylphosphine] (= binap ) complexes of ruthenium (A. Miyashita, 1980) and rhodium (A. Miyashita, 1984 R. Noyori, 1987) have been prepared as pure atrop-isomers and used for the stereoselective Noyori hydrogenation of a-(acylamino) acrylic acids and, more significantly, -keto carboxylic esters. In the latter reaction enantiomeric excesses of more than 99% are often achieved (see also M. Nakatsuka, 1990, p. 5586). 

The Ru(II)-BINAP complex,2 [Et2NH2]+[Ru2Cl5(BINAP)2]-,3 is prepared as a toluene solvate in nearly pure form by this procedure. Typical crystallized product shows no other signals in the phosphorus NMR and gives a good combustion analysis. The material is quite stable and can be routinely handled in air. Storage under nitrogen will extend its shelf life, however. 

Fig. 1. P MAS NMR spectrum of (a)Ru-BrNAP/PTA/y-Al203, and (b)Ru-BINAP crt rlex In order to find the characteristics of the immobilized catalyst, asymmetric hydrogenation of the prochiral C=C bond was performed as a model reaction. Firstly, three different homogeneous Ru-BINAP complexes including [RuCl2((R)-BINAP)], [RuCl((R)-BINAP)(p-cymene)]Cl and [RuCl((R)-BINAP)(Benzene)]Cl were immobilized on the PTA-modified alumina. Reaction test of immobilized catalysts showed that [RuCl2((R)-BINAP)] was the most active and selective so all the experiment were done using this catalyst afterwards.

Further investigation of complexes using open rathenocen (2, Salzer) species (Scheme 23.5), which had been noticed as possible precursors for synthesis of BINAP complexes (2) were considered. Several complexes were synthesized through procedures inspired from Salzer s work ... 

TaniaPhos active catalyst discussion As shown by Salzer (2) such complexes with half sandwich stracture result in the catalyst cycle into a hydride species where the pentadienyl moiety can be hydrogenolyticaUy liberated (2, 6). This was verified in the case of BINAP complexes (2, diss. Podewils, Geyser). In accordance to this fact and other mechanistic aspects from Noyori s work (3, 5) it is likely that the pre-catalyst species undergoes the same reaction pathway and that the reactive part of the pre-catalyst, the pentadienyl moiety, will be liberated under hydrogenolytic conditions as shown below in Scheme 23.9 ... 

Many enantioselective catalysts have been developed for reduction of functional groups, particularly ketones. BINAP complexes of Ru(II)C12 or Ru(II)Br2 give good enantioselectivity in reduction of (3-ketoesters.49 This catalyst system has been shown to be subject to acid catalysis.50 Thus in the presence of 0.1 mol % HC1, reduction proceeds smoothly at 40 psi of H2 at 40° C. 

BINAP complexes (7 in Fig. 7.7) are among the most efficient chiral catalysts for enantioselective hydrogenations, hydrosilylations, etc. Heterogeniza-tion of this complex is highly desired because of the high price of the complex. 

In asymmetric reactions, chiral phosphine ligands such as BINAP derivatives are used as effective chiral ligands in silver complexes. In particular, an Agr-BINAP complex activates aldehydes and imines effectively and asymmetric allylations,220-222 aldol reactions 223 and Mannich-type reactions224 proceed in high yield with high selectivity (Scheme 51). 

Although the asymmetric isomerization of allylamines has been successfully accomplished by the use of a cationic rhodium(l)/BINAP complex, the corresponding reaction starting from allylic alcohols has had a limited success. In principle, the enantioselective isomerization of allylic alcohols to optically active aldehydes is more advantageous because of its high atom economy, which can eliminate the hydrolysis step of the corresponding enamines obtained by the isomerization of allylamines (Scheme 26). 

The enantioselective isomerization of allylic alcohols using cationic rhodium(l)/BINAP complex was reported.9,11 Although the enantioselectivities were lower than those achieved by the isomerization of the corresponding enamines, 3,3 -disubstituted allylic alcohols were isomerized to the corresponding aldehydes in moderate yield and enantioselectivity (Scheme 27). 

Tandem hydroacylation-isomerization of 5-alkynals catalyzed by a cationic rhodium(l)/BINAP complex was applied to the short synthesis of dihydrojasmone (Scheme 49).88... 

Very recently, Wiedenhoefer272 has devised the first asymmetric 1,6-enyne hydrosilylation/cyclization tandem process using a rhodium(l) catalyst with (R)-276 as chiral ligand where rhodium-BINAP complexes were not effective (Scheme 70). More developments on this reaction are covered in Chapter 11.13. 

The catalytic hydrogenation of enamide 34 in the presence of 0.5-1 mol% of (/ )-BINAP complex in a 5 1 mixture of ethanol and dichloromethane under 1-4 atm of hydrogen affords 35 with quantitative yield and higher than 99.5% ee. The (fi)-isomer is not reduced under similar reduction conditions. This approach provides a route to a number of alkaloid compounds (Scheme 6-19).47... 

Isomerization of allylic amines is another example of the application of the BINAP complex. Rh BINAP complex catalyzes the isomerization of N,N-diethylnerylamine 40 generated from myrcene 39 with 76-96% optical yield. Compound (R)-citronellal (R)-42. prepared through hydrolysis of (R)-41, is then cyclized by zinc bromide treatment.49 Catalytic hydrogenation then completes the synthesis of (—)-menthol. This enantioselective catalysis allows the annual production of about 1500 tons of menthol and other terpenic substances by Takasago International Corporation.50... 

From a practical point of view, the catalytic asymmetric hydrogenation of the corresponding diones will be the preferred method if high yields and high enantioselectivity can be ensured. Recently, over 98% yield with more than 99% ee has been achieved by optimizing the reaction conditions.64 For example, asymmetric hydrogenation of 2,4-pentanedione catalyzed by Ru-BINAP complex in the presence of hydrochloric acid gave 2,4-pentanediol in more than 95% yield and over 99% ee (Scheme 6-29).64... 

It is well accepted that the asymmetric reduction of simple dialkyl ketones generally proceeds with low enantioselectivity.68 Ohkuma et al.69 reported that hydrogenation of simple ketones can be achieved using Ru(II) catalysts in the presence of diamine and alcoholic KOH in 2-propanol. Promising results have been achieved in the asymmetric hydrogenation of alkyl aryl ketones with a mixture of an Ru-BINAP complex, chiral diamine, and KOH (Scheme 6-33). 

BINAP Ru catalyst and (lR,25 )-ephedrine (Scheme 8-53). This result is similar to that obtained when catalyzed by pure (R)-BINAP. In pure (R)-BINAP complex-catalyzed hydrogenation, (S )-2-cyclohexenol can also be obtained with over 95% ee. This means that in the presence of (R)-BINAP-Ru catalyst, (R)-cyclohexenol is hydrogenated much faster than its (S )-enantiomer. When ephedrine is present, (R)-BINAP-Ru will be selectively deactivated, and the action of (S -BINAP-Ru leads to the selective hydrogenation of (S)-2-cyclohexenol, leaving the intact (R)-2-cyclohexenol in high ee. 

Table 9.1 Asymmetric hydrogenation of (3-keto esters using [Ru(BiNAP)] complexes (results according to the relevant publications).

Ru(II)-BINAP complexes (1) can effect enantioselective hydrogenation of pro-chiral ally lie and homoallylic alcohols, without hydrogenation of other double bonds in the same substrate.1 The alcohols geraniol (2) and nerol (3) can be reduced to either (R)- or (S)-citronellol (4) by choice of either (R)- or (S)-l. Thus the stereochemical outcome depends on the geometry of the double bond and the chirality... 

Asymmetric hydrogenation of fi-keto esters.7 The Ru(OAc)2(BINAP) complexes are ineffective catalysts for asymmetric hydrogenation of (i-keto esters, but on treatment with HX (2 equiv.) are converted into complexes with the empirical formula RuX2(BINAP), which are effective catalysts for this enantioselective hydrogenation. Complexes of (R)-BINAP catalyze hydrogenation to (R)-(S-hydroxy esters in >99% ee, whereas the enantiomeric (S)-P-hydroxy esters are obtained... 

Such an isomerization of 4-hydroxy-2-cyclopentenone (2) results in 1,3-cyclopen-tanedione (3) via the keto enol. On exposure of racemic 2 to the optically active Rh-BINAP complex (R)-l, the (S)-enantiomer isomerizes more rapidly than (R)-2 to give, after 14 days at 0°, a mixture of 3 and (R)-2 in 91% ee. 

BINAP was introduced by Noyori [18], It has been particularly explored for reduction with ruthenium catalysts. While the first generation rhodium catalysts exhibited excellent performance with dehydroamino acids (or esters), the second generation of hydrogenation catalysts, those based on ruthenium /BINAP complexes, are also highly enantioselective for other prochiral alkenes. An impressive list of rather complex organic molecules has been hydrogenated with high e.e. s. 

Asymmetric reduction of carbonyl via hydrogenation catalyzed by ruthenium(II) BINAP complex. 

A further example of ion-exchange of an organometallic complex onto a layered support has been provided by the anion exchange of a sulfonated ruthenium BINAP complex onto the external surface of layered double hydroxides [119]. Although achvihes and enantioselechvities for the hydrogenation of dimethyl itaconate were comparable to the homogeneous catalyst, and catalyst deactivation was not detected, with geraniol as substrate, catalyst deactivation was unavoidable. 

Aldol reactions of silyl enolates are promoted by a catalytic amount of transition metals through transmetallation generating transition metal enolates. In 1995, Shibasaki and Sodeoka reported an enantioselective aldol reaction of enol silyl ethers to aldehydes using a Pd-BINAP complex in wet DMF. Later, this finding was extended to a catalytic enantioselective Mannich-type reaction to a-imino esters by Sodeoka s group [Eq. (13.21)]. Detailed mechanistic studies revealed that the binuclear p-hydroxo complex 34 is the active catalyst, and the reaction proceeds through a palladium enolate. The transmetallation step would be facilitated by the hydroxo ligand transfer onto the silicon atom of enol silyl ethers ... 

H. Yamamoto reported an enantioselective allylation of aldehydes catalyzed by AgF-p-tol-BINAP complex, [Eq. (13.24)]. High enantioselectivity was obtained... 

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