Chapter IV.2 New Chiral Rh(I) and Ru(II) Complexes: Highly Efficient Catalysts for Homogeneous Asymmetric Hydrogenation

322

Chapter IV.2

NEW CHIRAL Rh(1) and Ru(I1) COMPLEXES: HIGHLY EFFICIENT CATALYSTS FOR HOMOGENEOUS ASYMMETRIC HYDROGENATION Hidemasa Takaya,*l Tetsuo Ohta,' and Ryoji Noyori*2

Kazushi Mashima,'

Masato Kitamura

1 Department of Industrial Chemistry, Faculty of Engineering, Kyoto University, Yoshida, Kyoto 606 (Japan) 2 Department of Chemistry, Faculty of Science, Nagoya University, Chikusa, Nagoya 464 (Japan)

ABSTRACT Practical methods for the synthesis of ( R ) - or (S)-2,2'-bis(diaryl- or dialky1phosphino)-1,l'-binaphthyls (BINAPs) have been developed. These new axially dissymmetric di-tert -phosphines serve as excellent ligands for Rh(1)-catalyzed asymmetric hydrogenation of a-acylaminoacrylic acids and asymmetric isomerization of allylamines to enamines. New mononuclear BINAP-Ru(I1) dicarboxylate complexes and cationic BINAP-Ru(I1)-arene complexes have also been prepared. These complexes and their derivatives are highly efficient catalysts for asymmetric hydrogenation of enamides, alkyl- and aryl-substituted acrylic acids, P,y-unsaturated carboxylic acids, allylic and homoallylic alcohols, and a variety of functionalized ketones such as P-keto esters, a-amino ketones, etc. INTRODUCTION An established way to accomplish an asymmetric catalytic reaction is based on a creation of chiral centers under the influence of transition metal complexes bearing chiral organic ligands. The molecular designing and synthesis of new effective chiral ligands are, therefore, the most important requirements for developing synthetically useful asymmetric catalytic reactions with stereoselectivity as high as those of enzymes. Several years ago, we reported first synthesis of optically pure ( R ) - or ( S ) - 2 , 2 ' bis(dipheny1phosphino)-1,l'-binaphthyl [abbreviated to ( R ) - or (S)-BINAP] (1) which possesses numerous salient structural features (refs. 1, 2). We have found that Rh(1) and Ru(I1) complexes of BINAP and its derivatives are highly efficient catalysts for asymmetric hydrogenation of various kinds

323

of olefinic and ketonic substrates, and asymmetric conversion of allylamines to enamines.

SYNTHESIS OF OPTICALLY PURE BINAPS The easily accessible optically pure diamine, 2,2'-diamino-l ,l'-binaphthyl, seemed to be an attractive starting material for the synthesis of 1, but attempted conversion of the diamine to 1 resulted in considerable racemization (ref. 3). The failure led us to optical resolution of racemic 1 using the chiral Pd(I1) complex 2 (refs. 1, 2). Recently, we provided the more practical entry by resolution of the racemic diphosphine dioxide 3 (abbreviated to BINAPO) with carnphorsulfonic acid (4) or 2,3-0 dibenzoyltartaric acid (5) (ref. 4). Fig. 1 shows O R T E P drawing of one of the diastereomeric adduct of 3, 4 , and acetic acid in 1:l:l ratio. One of the oxygen atoms of the two P=O groups interacts with camphorsulfonic acid through hydrogen bonding and the other oxygen atom has a hydrogen bond interaction with acetic acid, minimizing the quantity of the chiral resolving agent. Reduction of the resolved 3 with trichlorosilane in the presence of triethylamine affords optically pure BINAP.

0

Fig. 1. O R T E P drawing of the complex of (S)-BINAPO, (1R)camphorsulfonic acid, and acetic acid. Selected bond distances: O(1)-0(3) = 2.414(5) A, O(2)-0(7) = 2.609(6) A.

324

2

4

3

5

In a similar manner, several BINAP analogues 6-10 prepared in optically pure form.

pTolBlNAP

6

LPrBINAP 9

ptBUPhBlNAP 7

have been

CyBlNAP 8

p-MeOPhBINAP 10

ASYMMETRIC REACTIONS BY USE OF BINAP-Rh(1)

COMPLEXES

Asvmmetric hvdrogenation of a-acvlaminoacrvlic acids Reaction of (Sj-BINAP [(S)-1] and [Rh(nbd)2]C104 (nbd = norbornadiene) in dichloromethane gave (S)-11, whose molecular structure and absolute configuration have been established by X-ray analysis (Fig. 2) (ref. 5).

(S)-11

The rhodium(1) atom has nearly square planar coordination. The seven-membered chelate ring is fixed to the A conformation, and four

325

phenyl groups are placed in the alternating edge-face arrangement. angle between the planes through the two naphthyl rings is 74.3'.

The

Fig. 2. O R T E P drawing 01 (I?)-11. Selected interatomic bond distances (A) and angles (O): Rh-P(l) 2.305(1), Rh-P(2) 2.321(1); P(l)-Rh-P(2) 91.82(5). In methanol, the (5)-11 absorbed exactly two mole equivalent of hydrogen to give (5)-12 and norbornane accompanied by 10% of the methanol insoluble dinuclear complex. Methanol coordinated to rhodium atom was lost under reduced pressure to give air-sensitive [Rh(binap)]C104, which reverted to 1 2 in methanol. The complex 12 serves as excellent catalyst for the asymmetric hydrogenation of prochiral a-acylaminoacrylic acids and esters at room temperature under initial hydrogen pressure of 34 atm (equation 1) (refs. 1, 2). On the other hand, the .vethano1 insoluble dimeric complex was a poor catalyst for the above reaction. These facts advise us to carefully control the reaction conditions s that only a selected catalyst with high chiral recognition ability is created.

(S)-12

(R)-12

+ H2

+ RCH,-C-COOR' NHCOR~

S 70-100%

ee

(11

326

Asymmetric conversion of allvlamines to ena m i n a Otsuka (Osaka Univ.) and his coworkers found that chiral cobalt complexes catalyze the conversion of allylamines to the corresponding It enamines by 1,3-hydrogen transfer in up to 33% optical yield (ref. 6). was also revealed that the cationic rhodium complexes 11, 12, and even the complex of the type [ R h ( b i n a ~ )C~l]o 4 cause highly enantioselective isomerization of allylamines (refs. 7, 8). For instance, diethylgeranylamine (equation 2) or diethylnerylamine (equation 3) is converted in THF to Combination of the citronella1 (E)-diethylenamine in 95- 99% e e . geranylamine and (R)-BINAP-Rh(1) affords the S product, and the geranyl/S or neryllR combination gives R enantiomer.

R

E R = (CH3)&=CHCH&H2

95-99%

ee

[Rh((S)-binap)]*

Z

N(C2H32

(3)

s R = (CH3)2C=CHCH&H2

95-99%

ee

Citronella1 and related compounds serve as key substances in synthesis of optically active terpenic compounds. Thus, this new asymmetric catalysis has realized a practical commercial synthesis of (-)-menthol (Scheme I). Scheme I

We have investigated the stereochemistry of this important catalytic 1,3-migration using optically active deuterated substrate 13 (ref. 9). The overall 1,3-hydrogen shift of one of the enantiotopic C ( l ) hydrogens of

327

allylamines occurs in a suprafacial manner from the s-trans conformers of the flexible substrates (Scheme 11).

type

Scheme II

13 s-trans

N

It

-

F

13 s-cis

canonical conformations

A similar asymmetric 1.3-hydrogen shift was observed for allylic alcohols. When the racemic allylic alcohol 14 was exposed to (R)-12, the S enantiomer was consumed more rapidly to leave (R)-14 (72% conversion, 91% ee) which is an important chiral building block in prostaglandin synthesis. The extremely high crystallinity of the diketone 16 allowed easy separation of (R)-14 from the reaction mixture. The optically pure ( R ) - 1 4 was obtained by conversion to the crystalline 0-silyl derivative 17 (ref. 10).

0 -b

0 0.5 mol% (R)-12

HO 14

THF, 0 "C, 14 days kf,rt~krlow = 5

72% conv.

=

HO

15

0 16

b

Rd

(R)-14 R = H, 91% ee 17 R = t-C4Hg(CH3)2Si

ASYMMETRIC REACTION BY USE OF BINAP-Ru(I1) COMPLEXES Svnthesis of new mononuclear BINAP-Ru(I1) dicarboxvlate comulexes For the purpose of expanding the utility of BINAP ligands, new types of ruthenium complexes, Ru(OCOR)2(binap) (18), have been prepared. The

328

complexes have been synthesized in 71- 87% yields by the treatment of [RuC12(cod)], with ( R ) - or (S)-BINAP (or its derivatives) and triethylamine and then with sodium carboxylate. R

The molecular structure of A - ( S ) - l 8 b has been determined by X - r a y crystallography (Fig. 3), which revealed interesting structural features of these complexes (ref. 11). The whole structure approximates C 2 chirality. The dissymmetry of the (S)-BINAP ligand fixes the &conformation of the seven-membered diphosphine- and Ru-containing chelate ring. The bidentate ligation of the pivalate moieties to Ru occurs stereoselectively, bringing about exclusive formation of the A diastereomer.

Fig. 3. Stereoview of A - ( S ) - l l b (data collected at -60 "C). Hydrogen atoms of the tert-butyl groups were omitted for simplicity. Selected bond distances (A) and angles (O): Ru-P( 1 ) 2.241(3), Ru-P(2) 2.239(3); P( l)-Ru-P(2) 90.6( 1). Asvmmetric hvdroeenation of enamides catalvzed bv 1 8 The hydrogenation of the 2-enamide substrates 19 in the presence of 0.5-1.0 mol% of (R)-18 in a mixture of ethanol and dichloromethane leads 20 having 1R configuration in 96-100% ee (ref. 12). The E- isomer of 19 is inert to such catalytic conditions.

329

CH,O c H 3 0 ~ N C O R CHZO

H z ( 4 (R)-lB am) cat:

c

H

3

0 19

~

'

0

c

OCH,

H

3EtOH-CHzClp room temp.

L

s i c 100-200 20

R = H, CH3, CeH5, etc.

This enantioselective reaction of the N-acyl group leads easily (22), ( R ) - and (S)-trimethoquinol purities. A simple 1-methylene safsolidine (25).

followed by removal and/or to tetrahydropapaveline (21). (23), norreticurine (24) in analogue of 1 9 gives, after

modification laudanosine high optical deacylation,

HO CH3 OCH, 21, R = H 22, R = CH3

OCH, OCH,

cH30mNH 23, R and S

24

CH,O

25

This method has been successfully extended to the enantioselective synthesis of morphine, benzomorphans, and morphinans such as 27-3 1 starting from the enamide 2 6 using Ru(OCOCF3)2[(R)-tolbinap] or its enantiomer as the hydrogenation catalyst (ref. 13).

26

>97% ee

330

27 morphine

metazocine

28 R = CH3

29 R = CH2CH=C(CH& pentazocine

30 R = R' = CH3 dextromethorphan 31 R = H; R'= C H ~dextrorphan

Asvmmetric hvdrogenation of unsaturated carboxvlic acids catalvzed bv 18 We have found that the complexes of the type 18 are excellent catalysts for asymmetric hydrogenation of acrylic acids 3 2 having only carboxylic acid functionality whose hydrogenation in high enantioselectivity has been rarely attained using any of the catalyst systems designed so far. The representative results are summarized in Table 1 (ref. 14).

7=roH

R2

(:,3)4Y

*

BINAP-RU(II) CHaOH, rt

R'

R2

R'

32 33 TABLE 1. BINAP-Ru(I1) Catalyzed Asymmetric Hydrogenation of a$-Unsaturated Carboxylic Acids.a

R'

Substrate 32 R2

R3 H CH3 C6H5 H

Condition H2 Time S/Cb atm h

Product 33 ee % Config.

160 279 590 397 145 106 129 110 106 145

91 87 85 92 93 95 93 88 83 95

4 101 104 112 86 98 100 100 4

4

12 12 70 24 16 12 12 12 12 12

2R 3s 3s 2R 3R 3R

3s

3s 2R

c

Complex (R)-18a was used as catalyst. The reaction was carried out in methanol at 15-30 "C. The conversion was 100%. b S/C: substrate/catalyst mole ratio. c Not determined.

The optimum reaction conditions are highly dependent on the structures of the olefinic substrates. The utility of the reaction has become more general by extension of the substrates to include various oxygenfunctionalized unsaturated carboxylic acids. The synthetic significance of

331

this asymmetric catalysis is obvious. For instance, (S)-naproxen (35), a useful anti-inflammatory agent, was prepared by hydrogenation of 3 4 with (S)-18a as catalyst in 92% yield and in 97% ee. Certain P,y-unsaturated carboxylic acids were also hydrogenated in 81-88% ee.

35 (9-Naproxen

34

Recently, asymmetric hydrogenation of trisubstituted acrylic acids catalyzed by a cationic chiral (aminoalky1)ferrocenyldiphosphine-rhodium complex has been reported. Very high enantioselectivity (up to 98.4% ee) has been accomplished (ref. 15). Asvmmetric hvdrogenation of allvlic and homoallvlic alcohols catalyzed by

Ls

The BINAP- Ru(I1) dicarboxylate complexes 18 catalyze efficiently enantioselective hydrogenation of prochiral allylic and homoallylic alcohols (ref. 16). Geraniol and nerol are hydrogenated in methanol under the initial hydrogen pressure of 30-100 atm at room temperature to give citronellol in nearly quantitative yield and with 9 6 9 9 % ee (Scheme 111). Scheme 111 (S)-BINAP--RU(II)

\

-OH Geranwl

*

/

(R)-Ciimnellol

(R)-BINAP--RU(II)

Nerol

\OH

(9-Citronellol

Hydrogenation of geraniol under variable reaction conditions are shown in Table 2. The substrate/catalyst mole ratio approaches 50,000. The allylic and nonallylic double bonds in the starting olefinic alcohols can be clearly differentiated. Hydrogenation of homogeraniol also proceeds smoothly to give 4,8-dimethylnon-7-enol in 92% e e .

332

TABLE 2. BlNAP-Ru(I1)

Catalyzed Asymmetric Hydrogenation of Geraniol.= SIC

Catalyst

Ru(OCOCH3)2[(S)-binap] Ru(OCOCH3)2[(R)-binap] Ru[ OCOC(CH3)3]2[(S)-binapl Ru(OCOCF3)2[(S)-binap] Ru(OCOCH3)2[(S)-p-tolbinapl Ru( OCOCH3)2[(R)-p-tolbinap] Ru(OCOCH3)2[(S)-p-tolbinap] Ru(OCOCF3)2[(S)-p-tolbinap] a

b C

%

530 570 500 500OOc 550 520 10000~ 500OOc

H 2 100 a m . 18-20 OC, 24 h, 98-100% Based on optical rotation. H2, 30 a m , 8-14 h.

Citronellol o p . purityb (% ee) Config.

Yield

99 99

98

99 99 97

96

100

99 97

100

100

-

(96) (97) (98) (-->

(98) (97) (96)

(-1

conversion.

The present work marks the first example of the highly enantioselective hydrogenation of simple prochiral olefinic alcohols. This homogeneous catalysis has been successfully applied to the synthesis of (3R , 7 R ) - 3 , 7 , 1 1 -trimethyldodecanol (36), a versatile intermediate for synthesis of a-tocopherol (vitamin E) (Scheme IV). Scheme IV 1. (CHdzCO,

1. Rh[(S)-blnap]* 2. H20

UHO aq NaOH 2. H2, Pt/C

b

98% ee H

1. Pa,, Py 2. NaOCOCH3 ______z

3. aq NaOH 4. fractional dlstlllatlon

-OH

H2 (S)-18a

* 36

3R.7R 98% 3S,7R 1% 3RV7S 1%

333

Chiral allylic secondary alcohols can be resolved efficiently by ( R ) - or ( S ) - 1 8 (ref. 17). As shown in Scheme V, a combination of intramolecular and intermolecular asymmetric inductions allows preparation of unsaturated and saturated alcohols of either chirality in high enantiomeric purity.

Scheme V (S)-I 8a COOCH,

HZ, 4 atm

L O O C H 3

~

25 "C, 11 h 76% conv ktlk. = 16

>99% ee

I

37% ee threo:erythro = 49:l OH

(R)- or (S)-18a HZ

threo:erythro = 23:l

This method has been applied to the resolution of racemic 4-hydroxy2-cyclopentenone (14) with (S)-18, affording slow-reacting (R)-14, a versatile starting material of prostaglandin synthesis, in high optical purity. Conversion of ( R ) - 1 4 to 1 7 followed by recrystallization gave the homochiral enone. (S)-18

Hz, 4 atm HO 14

30 "C, 21 h kt/k, 3 11

68% conv.

>

b +

Rd

HO

(4-14 R = H, 98% ee 17 R = f-C4He(CH&Si

Asvmmetric hydrogenation of functionalized ketones catalvzed bv BINAPRu(I1) comDlexeS 0-Functionalized secondary alcohols are an extremely important class of compounds for the synthesis of physiologically active compounds. In the presence of the BINAP-Ru(I1) complexes derived from 18 and two equiv of HX (X = C1, Br, or I) (refs. 18,19),as well as R ~ ~ C l ~ ( b i n a p ) ~ ( C(ref. ~ H 20), ~)~N a wide range of functionalized ketones are hydrogenated in high enantioselectivities. The hydrogenation proceeds smoothly in alcohols at room temperature with initial hydrogen pressure of 70-100 atm. Hydrogenation of P-keto carboxylic esters 37 using a substrate to catalyst mole ratio of >1,000 proceeds smoothly in methanol (H2 70- 100

334

atm) to give the corresponding P-hydroxy esters 38 in nearly quantitative yields and with exceptionally high (up to 100%) enantioselectivities (Table 3). Esters of methyl, primary, secondary, and tertiary alcohols were equally employable. The efficiency of the present purely chemical manipulation rivals or is even superior to that of biological conversion.

38

37

TABLE 3. B INAP-Ru

Substrate 37 R R'

Catalyzed Asymmetric Hydrogenation of P-Keto Esters.a Condition H2 S/C atm

Catalyst

RuC12[(R)-binap] Ru C12 [ ( S ) -bin ap] RuBr2[(R)-binap] RuI2[(S)-binap] RuC12[(R)-binap] RuBq[(R)-binap] RuCI2[(R)-binap] RuBr2[(R)-binap] RuC12[(S)-binap] RuCI2 [( R )-bin ap] RuBr~[(R)-binap]

2000 1400 2100 1400 1000 1100 1000 1200 850 1100 760

100 83

100 100 103 73 70 98 94 100 91

Product 3 8 Time ee h % Config. 36 40 43 40 58 34 34 52

58 61 106

S9 S9 >99 S9

99 98 98 100 98

R S R

S

R R R R

S >99s 85 S

a Reaction was carried out at 23-30

chemical yield was 95-99%. equiv of HX were used.

O C . The conversion was 100% and The catalysts derived from 18 and two

Various functionalities including dialkylamino, hydroxyl, alkoxyl, siloxyl, keto, alkoxycarbonyl, alkylthiocarbonyl, dialkylaminocarbonyl, carboxyl, etc., can act as efficient directing functional groups (Scheme VI). The general sense of asymmetric induction suggested that the key intermediate in the stereoselection may be the simultaneous coordination of the heteroatoms, X or Y, to the ruthenium atom making five- or sixmembered chelate ring, respectively. When prochiral symmetrical Q - or P-diketones were hydrogenated, a mixture of the diols possessing meso and dl structures were obtained. The enantiomeric excesses of the dl isomers were uniformly high (99-100% ee) (refs. 18, 21).

335

Scheme VI H2 (S)-BINAP-RU

H2 (R)-BINAP-RU

OH

* H2 (R)-BINAP-RU

R/'

H2 (S)-BINAP-RU R

X ,Y = 0, N, Br, etc. c = sp2 or nonstereogenic sp3 carbon

SYNTHESIS OF NEW CATIONIC BINAP-Ru(I1) IN ASYMMETRIC HYDROGENATIONS

COMPLEXES AND USE

Catalyst precursors for asymmetric reactions are desired to have the following properties; (1) easy preparation in high purity and in high yield, (2) sufficient stability and easy handling, (3) easy generation of catalytically active species under mild conditions, (4) high catalytic activity and high enantioselectivity. The cationic ruthenium complexes are selected to meet the above requirements (ref. 22). Treatment of 39a or 39b with one equivalent of (S)-BINAP afforded cationic complexes (S)-4 Oa and (S)-40b, respectively. Chloride ion of ( S ) 40a was easily replaced by BF4 and B(CgH5)4 to give (RuCl(CgH6)[(S)binapIJBF4 [(S)-40d] and (RuC1(C6H6)[(S)-binap] ]B(CgHg)4 [(S)-4Oe]. A similar reaction of the iodide complex 39c with (S)-BINAP afforded rather unstable (S)-40c, which is prone to lose benzene ligand in solution.

39 a:X=CI b:X=Br

ax=I

(S)-40 a: X = CI 90% yield b:X=Br 94% ax= I -

The p-cymene complexes, (RuCl(p-cymene)[(S)-binapIJC1 [(S)-41a] and (RuBr(p-cymene)[(S)-binap]]Br [(S)-41b], which have been prepared by the reaction of [RuX2(p-cymene)12 (X = C1 and Br) with (S)-BINAP, are more stable than the corresponding benzene complexes 4 0 , and even iodide complex (S)-41c could be isolated in pure form in 94% yield by the reaction of [RuI2(p-cymene)I2 (Scheme VII).

puj2

336

Scheme VII

RuCI3*nHzO

1. 50 "C, 5 h, 90% EtOH

+

2. KI, 50X EtOH

(S)-B I N A P EtOH-CHpCIz,

50 "C, 1 h

'

*

I-

'I

(S)-41C

An O R T E P drawing of the complex (S)-40d determined by X - r a y crystallography is shown in Fig. 4. Ruthenium atom has a pseudooctahedral geometry defined by chloride, two phosphorus atoms of BINAP, and a tridentate benzene ligand. Another characteristic feature is the dihedral angle between the two naphthyl planes. The value of 75.7(2)' is comparable to that of 74.4' for ( R ) - 1 1 and much larger than the value of 65.6" for (S)-18b (Fig. 5). These facts demonstrate that the BINAP ligands flexibly change their dihedral angles to form stable chelate complexes depending on the nature of the central metal atoms and auxiliary ligands.

Fig. 4. ORTEP view of (S)-40d. Selected interatomic distances (A) and angles ('): Ru- P(l) 2.379(3), Ru- P(2) 2.334(3), Ru- C 1 2.393(4); P(l+Ru-P(2) 91.4(1), P(l+Ru-CI 89.1(1), P(~+Ru-CI 84.9( 1).

337

a P 2.305 A 91.8" 74.4" 2.321 Ru[OCOC(CH3)3]2((S)-binap) 2.241 90.6 65.6 2.239 [RuCl((s)-binap)(C6H6)]BF4 2.379 91.4 75.7 2.334 b4-P

[Rh((R)-binap)(nbd)]ClO4

Fig. 5. Definition of angles a and p. Thus, the complexes 4 0 and 4 1 can be prepared in high yields and in high purity. Moreover we found that the arene ligands are easily liberated under the catalytic conditions to afford coordinatively unsaturated species which exhibit sufficient catalytic activity and selectivity in the hydrogenation of a number of unsaturated substrates. Some representative results are given in Table 4. Hydrogenation of 37 (R = R' = CH3) in the presence of (S)-40 or (S)-41 gives methyl (S)-3-hydroxybutyrate in 9799% ee. Geraniol was hydrogenated in the presence of (S)-41c to ( R ) citronellol in 96% ee. (E)-2-Methyl-2-butenoic acid and 3 4 were converted to (S)-2-methylbutanoic acid and (S)-naproxen (35) in up to 89% and 96% e e , respectively. TABLE 4. Cationic BINAP-Ru(I1)-arene

Substrate Methyl 3-0x0butanoate

Catalyzed Asymmetric Hydrogenations.

Catalyst S/C

Solvent

H2 Temp. Time e e atm OC h % Config.

98 97 99 98

S S S

30

44 35 35 35

105

30

40

99

S

1000 1300

4 4

20 65

92 17

89 86

S S

(S)-4 1 c

200

116

-20

17

96

S

(S)-4 1 c (S)-4 1 c

1900 5000

100

20 60

8 10

96 R 95 R

(S)-4 (S)-4 (S)-4 (S)-4

0a 0c 1c

1c

2000 2100 2500 2200

95 100 100 100

N,N-Dimethylaminoacetone

(S)-4 1 c

1100

(E)-Z-Methyl-Zbutenoic acid

(S)-40d (S)-4 1c

34 G e raniol

112

17 50

30

S

338

FUTURE TREND OF ASYMMETRIC CATALYSIS We have synthesized unique BINAP ligands for the first time and shown that BINAP-Rh(1) and BINAP-Ru(I1) complexes are highly efficient catalysts for asymmetric hydrogenation of a number of unsaturated substrates. The high synthetic applicability of these catalyses is obvious. ee by the asymmetric For example, citronellol has been prepared in 96-99% hydrogenation of geraniol or nerol by use of the catalyst 18. (S)-citronellol of optical purity up to 92% can be obtained in a limited quantity from rose oil, while (R )-citronello1 has been obtained by hydrogenation of natural citronella1 of 4 5 % ee. Thus, our catalytic system can provide citronellol with optical purity much higher than those of natural origin. Moreover, both R and S enantiomers are accessible by either variation of the substrate Asymmetric geometry or choice of handedness of the catalysts. hydrogenation of P-functionalized ketones has brought us a powerful means to prepare a variety of optically active secondary alcohols of great synthetic utility. The present method is clean, operationally simple, economical, and suitable for a large-scale production. Thus, we believe that asymmetric catalysis becomes more and more important methodology in both synthetic and industrial chemistry and will be used cooperatively with biological or biochemical processes. ACKNOWLEDGMENT The authors wish to express their thanks to all of the coworkers described in the references, especially to Professors S. Otsuka and K. Tani (Osaka Univ.), and Dr. S. Akutagawa (Takasago Int. Co.). REFERENCES

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