TA-NaBr-MRNi

The latter problem was overcome by the discovery of a second modifier. Since the discovery of MRNi, whenever we got an unusual result, we explored factors such as modification and reaction conditions to determine if they might increase the EDA of MRNi. We accidentally found that anions in our water supply remarkably increased the EDA of TA-MRNi during the dry summer of 1978 when we were supplied very dirty water in Osaka. Because of this result, sodium halides were examined for their abilities as the second modifier, and NaBr was found to be the best second modifier. This led to our discovery of the practical MRNi, TA-NaBr-MRNi (37,47). 

Because catalytic activity and enantioselectivity of modified metal catalysts strongly depend on their preparation procedures, some examples of the preparation procedures of Raney Nickel catalysts modified with TA (MRNi-TA-NaBr) or AA and of supported Ni catalysts (HNi-AA-aerosil) are... 

Table 4.6. Hydrogenation of methyl (3-oxoalkanoates) on untreated and ultrasonicated MRNi-TA-NaBr catalyst.

TA-NaBr-MRNi was prepared by the reported method [3]. RNi (W-1 type) was prepared from 1.9 g of Raney nickel alloy (Kawaken Fine Chemical Co., Ni/Al = 42/58). To wash out the excess base and aluminum salts, a sufficient amount of deionized water was used with ultrasonic irradiation. The modifying solution was prepared by dissolving of (R,R)-tartaric acid (1 g) and NaBr (6 g to 10 g) in 100 ml of water and adjusting the pH to 3.2 with IN NaOH aqueous solution. RNi was heated in the modifying solution at 100 C for 1 hour, washed with water (50 ml), methanol (50 ml, twice), and THF (10 ml). The TA-NaBr-MRNi obtained by this method was immediately used for the hydrogenation. 

The durability of TA-NaBr-MRNi was greatly improved by embedding it in silicon rubber (48). 

Figure 11 shows the correlation between the concentration of NaBr and the EDA of MRNi A as well as the correlation between the amounts of TA and NaBr adsorbed on TA-NaBr-MRNiA (47). The absorbed amount of TA decreased in the presence of a trace amount of NaBr, and no further decrease of absorbed TA was observed. The EDA of the catalyst increased... 

Comparison of EDA between TA MRNi and TA NaBr-MRNi in Enantio-Differentiatiny Hydroyenations of Various Ketones... 

The rate of hydrogenation of acetone over TA MRNi was higher than over TA NaBr-MRNi (47). Thus, the NaBr inhibits partially the hydrogenation activity of TA-MRNi, and the increase in EDA of TA- MRNi with NaBr can be ascribed to the inhibition of hydrogenation at the unmodified surface of TA-MRNi. In other words, most of the hydrogenation with TA NaBr MRNi must be performed at the enantio-differentiating site. 

As shown in Table XVI, TA-NaBr-MRNi catalyzed the enantio-differentiating hydrogenation of ketones with a much higher EDA than TA-MRNi. The increase of EDA of TA-MRNi with NaBr for each substrate can be correlated with the hydrogenation rates of each substrate at the modified surface and the unmodified surface. 

Table XVII shows the comparison of results of enantio-differentiating hydrogenations of ketones which have a general structure of R—CO— CH2—X—O— over TA-NaBr-MRNi and TA-MRNi. The EDA over TA-NaBr-MRNi was twice as much as that over TA-MRNi without exception.

Fig. 15. Relation between the optical yield and amount of acetic acid in the enanliolace-dilTerenliating hydrogenation of MAA with TA-NaBr-MRNi. Catalyst standard (see Table IX). Reaction conditions M A A (11.5 ml), THF (23 ml), AcOH, 100 C, 100 kg cm2.

Diastereoface- and enantiomer-differentiating hydrogenations of 4-hy-droxy-2-pentanone (6) with TA-MRNi and TA-NaBr-MRNi (46) will be introduced as an example. 

Effect of Additives on the Optical Yield in the Enantio-Differentiating Hydrogenations of 2-Octanone and MAA with TA-NaBr-MRNi... 

Stereo-Differentiating Hydrogenation of (R)-4-Hydroxy-2-pentanone (6) to 2,4-Pentandiol (7) with TA-NaBr-MRNi"... 

Since TA-NaBr-MRNi is a nearly completed enantio-differentiating catalyst for practical use, the EDA of this catalyst will be summarized in this section. The standard method for preparation of TA-NaBr-MRNi is followed. That is, RNi prepared from 1.9 g of RNi alloy is modified with 100 ml of an aqueous solution of (R,R)-TA (1.0 g) and NaBr (10.0 g) at... 

Enantio-Differentiating Hydrogenations of fi-Diketones with (R, R )-TA-NaBr-MRNi°... 

Enantio-Differentiating Hydrogenation of RCOCH2COOCH3 with TA-NaBr-MRNi ... 

TA NaBr-MRNi has been found to be an effective catalyst for enantio-differentiating hydrogenations of ketones which have a general structure of R—CO—CH2—X—O— as shown in Table XVII (52c) and methyl ketones as shown in Table XXVI (52d). Among all, /i-diketones and /i-ketoesters are the most favorable substrate for this catalyst. Specific rotations [a] 0 of (R, R )-diols produced from /3-diketones by hydrogenation with this catalyst are summarized in Table XXVII (44). 

In the hydrogenation of /3-ketoesters, more than 80% of the enantio-differentiation was performed regardless of the type of alkyl residue in the acyl or ester moieties as shown in Tables XXVIII and XXIX, respectively (52a). When the hydrogenation of AA with (R,R)-TA-NaBr-MRNi had been stopped at the point of 70% conversion and the enantiomer-differentiation (see Section IV,B) was controlled, (R,R)-2,4-pentanediol 7 with 98% optical purity was obtained by a single distillation of the product (46). 

Although TA-NaBr-MRNi is rather more unstable than TA-MRNi in the persistency of EDA, TA-NaBr-MRNi was extremely stabilized not only with EDA but also with hydrogenation activity by embedding it in a silicone polymer as shown in Fig. 34 (48). The embedded catalyst can be stored very stably for a long period without special precautions. The embedded catalyst with high EDA is expected to be obtained in the near future. 

Fig. 34. Durability of TA -NaBr-NRNi embedded in silicone polymer (TA-NaBr MRNi/SR) with respect to EDA and HA.

The study of silk Pd began when Dr. S. Akabori led our research group. The establishment of the experimental rule with MRNi was achieved through the contributions of Mr. M. lmaida. Dr. S. Tatsumi, Dr. T. Tanabe, Dr. T. Ninomiya, and Dr. K. Okubo. The new concept of stereo-differentiation was rationalized by the efforts of Drs. A. Tai and H. Ozaki. The development of TA-NaBr-MRNi mostly depended on the efforts of Dr. T. Harada. 

The chiral hydrogenation of dialkyl ketones with high ee (80%) was performed on the TA-NaBr-MRNi catalyst in the presence of pivalic acid as co-modifier460-465. The first substrates examined were 2-alkanones (equation 48). 

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