Thioethers: Potential ligands for asymmetric catalysis?

Journal of Molecular Catalysis, 85 (1993) 131-141 Elsevier Science Publishers B.V., Amsterdam

131

M251

Thioethers:

potential

ligands

for asymmetric

catalysis?

F. Fache, P. Gamez, F. Nour and M. Lemaire* Universith Claude Bernard Lyon I, Laboratoire de Catalyse et Synthke Organique, U.P. C.N.R.S. no. 5401, ESCIL, 43 bd du 11 novembre 1918,69622 Villeurbanne Ckdex (France); fax. ( + 33)72448209 (Received March 3,1993; accepted July 1,1993)

Abstract Different chiral dithio or azathio ether ligande have been synthesized and tested in the enantioselective reduction of acetophenone into 1-phenylethanol under atmospheric pressure of hydrogen and at room temperature, with PdC& or Pd(OAc)s. The influence of the ligand structure on the conversion and on the chemoselectivity of the reaction is studied. Low, but significant enantioselectivity (up to 16% ee) has been observed. Key words: asymmetric catalysie; chemoselectivity;

enantioeelective reduction; palladium

Introduction Sulfur-containing compounds are well known to coordinate strongly many kinds of metals or metal salts and to poison metallic catalysts. Nevertheless, several reports in the literature describe results obtained with sulfur compounds as ligands in homogeneous catalysis [ 11. Metals such as iridium [2], rhodium [ 31, copper [ 41, nickel [ 51, platinum and palladium [ 61 are concerned as well as reactions like hydrogenation [2,6] or hydroformylation [3] of olefins, conjugate additions of organometallic reagents to a&unsaturated enones [ 41 or cyclotrimerization of diphenylethyne [ 71. The sulfur can be introduced in the ligand under different forms: for example it can be a sulfoxide [ 71, a ferrocenyl amine sulfide [6] or a thioether [ 2,4,7], including thiamacrocycles [ 51. In the last decade, taking into account the growing interest for asymmetric catalysis, the number of articles concerning chiral sulfur containing ligands are increasing [ 3,8,9]. Recently [lo], 94% ee have been obtained in the addition of organocuprates to enones. Nevertheless, in most of the cases, chiral sulfur-containing liganda cannot compete with phosphines in terms of enantioselectivity. For example, Kellogg et al. [ 51 reported 46% ee for the nickel catalyzed cross coupling of a Grignard reagent with vinyl bromide using a ligand with sulfide and amine binding sites, whereas 94% ee was obtained by *Corresponding author.

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F. Fache et al. /J. Mol. Catal. 85 (1993) 131-141

Fig. 1. General procedure for the reduction of acetophenone ligand.

into 1-phenylethanol.

L*=chiial

Kumada et al. [ 111 with the best chiral phosphines. However, sulfur ligands potentially have numerous advantages over the phosphines (less expensive, less toxic and less oxidizable, . . . ) : the low efficiencies and selectivities observed with this type of ligands may be due to the lack of effort to develop them. Thus, better understanding of how sulfur ligands react and how they are complexed with the metal may lead to a wider use of them. At the same time, dithioethers [ 121 were proved to be good extractants for liquid-liquid stripping of Pd2+ from nitric aqueous solutions. Taking into account this affinity of thiapodands for palladium, such compounds could also be tested as ligands for palladium catalysis. In this paper, we describe the first part of a systematic study on chiral sulfur ligands and more specially on their use in the enantioselective catalytic reduction of acetophenone 1 into 1-phenylethanol 2 with palladium, under atmospheric pressure of hydrogen and at room temperature (Fig. 1).

Experimental General methods

The structure assignment of the reaction products was carried out by spectral analysis. ‘H NMR spectra were recorded with a Varian Anaspect EM 360 (60 MHz) or a Bruker AM 200 (200 MHz) NMR spectrometer with tetramethylsilane as internal standard. GC analyses were carried out on a Shimadxu GC-14A gas chromatograph with a J & W column (30 mx0.25 mm) packed with 86% dimethyl- 14% cyanopropylphenyl-polysiloxane. Enantiomeric excesses were measured by using both the optical rotation (Perkin-Elmer 241 polarimeter) and gas chromatography with a chiral column SGE Cydex-/3 (50 m x 0.25 mm) packed with B-cyclodextrin. Materials

All the catalysts and acetophenone and were used as received.

were commercial products (Aldrich)

F. Fache et al. /J. Mol. Catal. 85 (1993) 131-141

133

Experiments General procedure

Acetophenone (2.5 mmol) , Pd (0.125 mmol, 5%/substrate), ligand (0.125 mmol, 1 equiv./Pd) were dissolved in 2.5 ml of solvent and stirred for 18 h at room temperature under atmospheric pressure of hydrogen. After filtration, the mixture was analyzed by GC and the alcohol was purified by chromatography over SiO, before optical rotation measurement. Ligand synthesis Thiapodands

Tosylate 4 was prepared using the method of Ouchi et al. [ 131 (Fig. 2). To a stirred solution containing sodium ethoxide (56 mmol) in ethanol (90 ml) were added the dithiol (20 mmol) and 4 (50 mmol) . The suspension was refluxed for 17 h. After evaporation, the mixture was taken up in ether and washed with water. The ether layer was dried over MgSO,. After filtration and evaporation, the crude product was purified by chromatography over SiOz (heptane/CH&l,: 70/30). All the thiapodands are oily products. (3S,12S)-3,12-DimethyZ-5,IO-dithiatetradecan.e 6. Chemical yield after purification: 40%. ‘H NMR 6: 1 (12H, m, CH3), 1.6 (lOH, m, CH2-CH), 2.5 (8H, m, CH&). [c11]n20=+25” (c=8.5, EtOH). (3S,llS)-3,11-Dimethyl-5,9-dithiutridecane 6. Chemical yield after purification: 40%. ‘H NMR 6: 1 (12H, m, CH3), 1.3 (6H, m, CH,-CH), 2 (2H, m, SCH2-CH2-CH2S), 2.6 (8H, m, CH2S). [a!]n20= +21.25” (c=7.5, EtOH). (3S,IOS)-3,10-Dimethyl-5&dithiadodecane 7. Chemical yield after purification: 79%. ‘H NMR 6: 1 (12H, m, CHB), 1.3 (6H, m, CH,-CH), 2.4 (4H, m, CH,S), 2.6 (4H, s, CH2S). [a!]n20= +25” (c=5, EtOH). (7s) -2,7-Dimethyl-2-aza-5-thiunonune 9. Starting material: 2dimethyL aminoethanethiol instead of dithiol (Fig. 3). Chemical yield after purification:

Fig. 2. Synthesis of the dithia- or oxathia-podandz.

4

Fig. 3. (7S)-2,7-Dimethyl-2-aza-5-thianonane.

134

F. Fuche et al. /J. Mol. Catal. 85 (1993) 131-141

r *

HSCSHIT *\

*

NaOH. EtOH -r

sT0

HI~CSS

*\ ‘1 %Hl7

0 g-

Fig. 4. tram- ( - )-1,4-Dithiaoctane-2,3-0-isopropylidene-L-threitol.

40%.‘H NMR S: 1 (6H, m, CH3), 1.5 (3H, m, CH,-CH), 2.3 (6H, s, CH,N), 2.4 (6H, m, CHZS, CH2N). [c~]n~~= +12.7” (c=5, EtOH). trans- ( - ) -1,4-Dithiuoctane-2,3-O-i.sopropylidene-L-threitollO. Starting materials: truns- ( - )-1,4-di-O-tosyl-2,3-0-isopropylidene-L-threitol (Janssen) and octanethiol (Fig. 4). Chemical yield after purification: 58%. iH NMR 6: 0.9 (6H, t, CH3), 1.3 (6H, a, CH3), 1.5 (24H, m, CH,), 2.6 (4H, m, CH,-S), 2.9 (4H, m, CH2S), 4.1 (2H, m, CHO). [a!]n20= -4.26” (c=5, CHCL). (3S,10S)-3,10-Dimethy1-5-oxa-&thiudodecane 8. This compound was synthesized by the procedure described for the thiapodands [ 141 using potassium terbutylate in dimethylformamide instead of NaOH/EtOH and starting from 2-mercaptoethanol. Chemical yield after purification over Si02 (MeOH/ CHCl,: 10/90): 47%. ‘H NMR 6: 1 (12H, m, CHB), 1.3 (6H, m, CH2-CH), 2.4 (2H, m, CH2S), (2.6 (2H, m, CH2S), 3.5 (2H, d,J=6 Hz, CH20), 3.7 (2H, t, J=6 Hz, CH20). [a!]n20= +15” (c=8, EtOH). Amino acid derivatives

These compounds were synthesized by the procedure described for the thiapodands [ 141. For compounds 11 and 12, the starting materials were the relevant methyl- or dimethyl-amino chlorides prepared analogously to a method already described [ 11,141, starting from I-proline and l-valine (Fig. 5). All the amino acid derivatives are oily compounds. (2S)-1 -Methyl-2-pyrrolidine-(2-thiudecane) 11. Chemical yield after purification over Si02 (MeOH/CHC&: 10/90): 20%. ‘H NMR 6: 1 (3H, m, CHs), 1.3 (16H, m, CH2), 2.2 (3H, m, CH2N, CHN), 2.3 (3H, s, CH,N),2.5 (4H, m, CH,S). [cx],,~~= -75.5” (c=8, EtOH). (2S)-2-Methyl-3-kopropyl-2-aza-5-thiatridecane 12. Chemical yield after purification over SiO, (MeOH/CHC&: 10/90): 20%. ‘H NMR 6: 0.9 (9H, d, J=6Hz,CH,), 1.2 (13H,m, CH,-CH),2.2 (6H,s,CH,N),2.3 (lH,m,CHN), 2.5 (4H, m, CH2S). [a!]n2’= +12.3” (c=5, EtOH). (2R)-2-(Dimethylamino)-3-(methylthio)propylmethyl

ester 13a.

I-Cys-

teine (41 mmol) was added to a stirred solution containing sodium hydroxide (100 mmol) in C2H50H (80 ml). The suspension was refluxed for 1 h. Then, CH31 (45 mmol) in C2H50H (10 ml) was added dropwise and stirred 6 h at

135

F. Fache et al. /J. Mol. Catal. 85 (1993) 131-141

COOH

i. ii. iii, iv

‘&SC&‘,,

*

I, NW,),

NH,

L2

i :CH,O, H20.H2 HSC,H,,.

PdK; ii: LiAIH,,THF;iii

HOCC

HOW

i

SH

W

: HCl EtOH. SOCl,,CHCI,:iv : NaOH

HOOC

ii

s'

-

EtOH,

(CH,h N

ROW iii (CHh

i

: NaOH,

N

EtOH, CH,I; ii

: CH20.

H20, H2 Pd/C; iii : SOCII,

ROH.

Fig. 5. General synthesis of the amino acid derivatives.

room temperature under nitrogen. After evaporation, the crude product was used in the next step, without any purification. The dimethylamino derivative was synthesized by the Eschweiler-Clarke procedure [ 151. Esterification of the crude product was performed as follows: 2- (Dimethylamino ) -3- (methylthio)propanoic acid (50 mmol) in MeOH (40 ml) was cooled at 0’ C under nitrogen and thionylchloride (310 mmol) was added dropwise. After complete addition, the reaction mixture was refluxed for 2 h. After evaporation and extraction with E&O, the oily residue was washed with Et,N in water. Purification by chromatography on SiOz (CH&l, ). Chemical yield after purification: 11%. ‘H NMR (200 MHz) 6 2.13 (s, 3H, SCH3), 2.36 (s, 6H, NCHB), 2.74 (dd, lH, J=13 Hz, J=6.5 Hz, CHzS), 2.86 (dd, lH, 5~13 Hz, 5~6.5 Hz, CH,S), 3.40 (dd, lH, J=8.6 Hz, J=6.5 Hz, HCCOOMe), 3.75 (a, 3H, CH30). [cY]~~~=+13.8” (c=6.2, MeOH). (2R)-2-(Dimthylumino)-3-(methylthio)proggdethyl ester 13b. Product 13b was synthesized by the procedure described for product 13a,using ethanol instead of methanol in the esterification. Chemical yield after purification: 11%. ‘H NMR (200 MHz) 6 1.2 (t, 3H, J=8Hz,CH2-CH3), 2 (s,3H, SCH3), 2.2 (s,6H,NCH3), 2.6 (dd, 2H, CH,S), 3.2 (m,lH,HCOOEt),4 (q,2H, J=14Hz,J=8Hz,CHZO). [a!]n20=+17.40 (~7, EtOH).

136

F. Fache et al. /J. Mol. Catal. 85 (1993) 131-141

(2R)-2-(Dimethylamino)-3-(methylthio)propyl~opropylester 13~. Product 13c was synthesized by the procedure described for product 13a, using isopropanol instead of methanol in the esterification. Chemical yield after purification: 2%. ‘H NMR 6: 1 (t, 6H, CH3), 2 (a, 3H, SCH3), 2.4 (a, 6H, NCH3), 2.7 (dd, 2H, CH,S), 3.2 (m, lH, HCOOiPr), 4 (m, (CH3)J!HOO).

Results and discussion Compounds 6 or 6 with 3 or 4 CH2 groups as spacer, respectively, between the two thioether functions gave the best results for the liquid-liquid extraction of Pd2+ from nitric aqueous solutions [ 121. Therefore, we have used them as basic structure to begin our study on the reduction of acetophenone into lphenylethanol at atmospheric pressure of hydrogen. Thus, we were led to study the influence of different structural parameters such as the number of methylene groups between the two heteroatoms, the nature of these heteroatoms... At the same time, we have systematically considered the effect of the polarity of the solvent (heptane, methylene chloride or methanol) on the conversion ( [ 1-phenylethanol] + [ethylbenzene] / ( [acetophenone] + [ l-phenylethanol] + [ ethylbenzene] ) ) and on the chemoselectivity towards l-phenylethan01 ( [ 1-phenylethanol] / [ 1-phenylethanol] + [ ethylbenzene] ) ) and enantioselectivity (ee) of the reaction as on the nature of the catalytic system (homogeneous or heterogeneous). [ 1-phenylethanol] , [ ethylbenzene] and [acetophenone] are the concentrations of the products and the starting material determined by GC chromatography. The system is defined as homogeneous when no precipitation of palladium is noticed and heterogeneous in the reverse case. Maitlis’s tests were performed on two cases to confirm such an assumption [ 161. Different sources of Pd2+ have also been tested. Influence of the number of methykne groups between the two heteroatoms

We have first studied the influence of the bridge length between the two sulfur atoms. Without ligand, formation of Pd black is observed and ethylbenzene is obtained quantitatively. With ligands 5 or 6, whatever the solvent, the system is heterogeneous and no reaction is observed. Compound 7, which possesses an ethane bridge between the two sulfur atoms, gives 19% of conversion in heptane, 0% in methylene chloride and 4% in methanol (Table 1). The reaction medium is heterogeneous with methanol and heptane and homogeneous with methylene chloride. In all the cases, the reaction mixture is black which indicates that the palladium is reduced. The chemoselectivity (50% in heptane) is low and there is no enantioselectivity. More generally, these low conversions and selectivities could be explained by a poor stabilization of Pd(0) by the ligands in the homogeneous phase. This leads to the precipitation of Pd black in the reaction medium,

137

F. Fache et al. /J. Mol. Catal. 85 (1993) 131-141

TABLE 1 Influence of the bridge size between the two sulfur atoms (conditions: catalyst: PdClz 5% mol/ acetophenone; ligand: 1 equiv./Pd; 16 h, RT; 1 atm Hz; horn. = homogeneous, het. = heterogenous) No

Ligands Solvent

ligand

TyP

d

o^

Aspect

d

o^ %

bn

ee

%

Aspect

40

46

Z6Hl4

10

9

het.

60

0

1

het.

ZH2C12

0

0

horn.

41

0

2

het.

UeOH

3


het.

20

0

16

het.

T

Aspect

% 100

bet.

.

-

whose heterogeneous palladium is poisoned by the thiapodands. This hypothesis is confirmed by the following experiment: if we use Pd/C (10% ) without ligand, 100% ethylbenzene is obtained. With ligand 6 and Pd/C, no reaction occurs in 16 h. This tends to prove that heterogeneous palladium is poisoned by this kind of ligand. Molecular mechanics calculations published by R.D. Hancock and coworkers [ 171 on aza ligands, showed that in minimum strain chelated rings, the distance between the two nitrogen atoms, and thus between the metal and the ligand atoms, decreased from the ethane bridged compound to the butane one. Therefore, if a similar assumption is made for the thioether ligands, ligand 7 could lead to a more stable system with large Pd(0) atoms (and thus the reaction can occur) than ligands 6 and 6. The absence of induction can be explained by the very structure of the ligand: the chiral center is two atoms away from the sulfur which is unfavorable to the transmittance of the chiral information. In addition, this center is located on the side chain, which is free to rotate even when the ligand is coordinated to the metal. An asymmetric center between the two sulfur atoms should be better. We have thus tested a derivative of the classical Diop (product 10)substituted by two thioether functions close to the asymmetric carbon atoms (Table 1). Even if the two thioethers are separated by four carbon atoms, 60% of conversion are observed in heptane, 41% in methylene chloride and 20% in methanol. In this case too, heptane is the best solvent, in terms of conversion. Interestingly, a chemoselectivity of 100% in alcohol is obtained with this ligand. Heterogeneous palladium is observed in heptane and in methanol. The enantioselectivity is higher when the conversion is lower: 16% ee in methanol against 2% in methylene chloride and 1% in heptane. These results are not consistent with the above observations concerning the bridge size, but in this case, the structure is rigid and this can modify the distance between the two

138

F. Fah

et al. /J. Mol. Catal. 85 (1993) 131-141

sulfur atoms and thus the affinity of the ligand for the different palladium species. Complicated relationships between structures, reactivities and/or solubilities of the palladium complexes could be invoked to explain these results. Nevertheless, they show that thioethers are able to induce both chemo- and enantioselectivity in the reduction of ketones. Influence of the nature of the heteroatoms

We have shown (Table 1) that an ethane bridge between the two sulfur atoms gives the best results. We then tried to modify this basic structure by changing the nature of one of the heteroatoms, always keeping one sulfur atom. When the heteroatom is replaced by an oxygen (compound 8), 100% ethylbenzene are obtained in heptane with PdC12. In contrast to the sulfur atoms, the oxygen atom is a weak ligand for palladium. Thus, Pd(0) is allowed to precipitate and the heterogeneous processus may occur. This oxathia ligand can also poison the heterogeneous Pd (0) and therefore, the reaction proceeds slowly (16 h for 100% conversion into ethylbenzene with this ligand instead of 3 h for PdCl, alone). Table 2 shows the results obtained when the other heteroatom is a nitrogen (ligand 9). In this case, with PdC12, we observe 14% of conversion in heptane, 89% in methylene chloride and 100% in methanol, probably because of the higher solubility of the complex ligand-palladium in these last two solvents. In all cases, the solution appears homogeneous during and at the end of the reaction. The chemoselectivity is higher in heptane (55% ) than in methylene chloride (30% ) and methanol (46% ). The results obtained with compound 9 are the best observed so far for the conversion (compared to 7 and 8) and the chemoselectivity. We can thus assume that azathia ligands stabilize in a better way Pd (0) in the homogeneous phase and allow an increase of the reactivity and of the chemoselectivity of the reduction. With PdC12, the production of hydrochloric acid could induce hydrogenolysis. It is also known that chlorine anions poison catalysts [ 181. Therefore, TABLE 2 Influence of the nature of the heteroatom (conditions: catalyst: 5% mol/acetophenone; equiv./Pd, 16h; RF, 1 atm H,; horn. = homogeneous; het. = heterogeneous)

ligand 1

Ligand No Solvent

d

PdCl2 PdClz

Aspect

& 46

CaH14 CHzC12

ligand

;tFq-

Catalyst

8 27

d

0” 90

46 6 62

horn. horn.

MeOH

PdCl2

46

54

horn.

MeOH MeOH

Pd(OAc)z Pd(acetytacetonatc,

91 2

8 0

horn. horn.

Aspect 96

0

100

het.

65 0

35 100

het. het.

F. Fache et al. /J. Mol. Catal. 85 (1993) 131-141

139

taking into account the good results observed with ligands 9 and PdClz in methanol, we have studied the influence of the Pd source in this solvent. Without ligand, Pd( acetylacetonate) gives a black palladium precipitate and 100% ethylbenzene. With compound 9,2% conversion in 17 h and no precipitation of palladium black are observed. This may be due to the competition between acetylacetonate and acetophenone. Without ligand, using Pd(OAc)2, 100% conversion and 65% chemoselectivity are obtained. With compound 9, 92% chemoselectivity at 99% conversion are performed which is the best result obtained so far with our sulfur containing ligands if chemoselectivity is concerned. This led us to choose Pd (OAc) 2 as the source of palladium for the rest of our study. No induction is observed with compound 9, probably because of its structure and for the same reason that no induction is obtained with the dithioethers described above. Therefore, we synthesized other axathia ligands from amino acids. These ones have been widely used as starting materials for the synthesis of enantioselective auxiliaries, phosphines [ 111 as well as thioethers [5] with a N-functionality close to the asymmetric carbon atom. Among them, we chose to test a proline 11, a valine 12 and cysteine derivatives 13a-c (Table 3). With ligand 11, we obtained the same conversion as with ligand 9 (99% ). With compound 12, the conversion is slightly lower, probably because of the steric hindrance of the isopropyl group. Ligands having an ester group (cysteine derivatives 13a-c),giverise to low conversions. The carbonyl group of the ester can be considered as a third anchoring point of the ligands 13a-c which competes with the substrate coordination. In all the cases, the chemoselectivity remains high (less than 1% ethylbenzene). Low but significant enantiomeric excesses are obtained and measured by two different methods: by gas chromatography on a chiral capillary column and by polarimetry. With all TABLE 3 Comparison of different azathia derivatives (conditions: catalyst: Pd (OAc )s: 5% mol/acetophenone; ligand 1 equiv./Pd; 16 h, RT; 1 atm H,)

“With a ratio Pdlacetophenone of 10% mol. bin EtOH instead of MeOH. ‘In iPrOH instead of MeOH; in 5 h 30 min.

140

F. Fade et al. /J. Mol. Catal. 85 (1993) 131-141

the ligands described in Table 3, the reaction medium appears homogeneous, confirming the hypothesis made with ligand 9. Taking into account the large possibilities of modification of both structures of azathia ligands and reaction conditions (temperature, pressure... ) optimizationa are now in progress in our laboratory and may lead to useful results. Homogeneous versus heterogeneous

It is a crucial point to know whether homogeneous or heterogeneous species are responsible for the reaction. In the previous discussion, we have qualified the reaction of homogeneous or heterogeneous on the basis of the reaction mixture. To confirm this assumption, we have used Maitlis’s test [ 161 in two cases: one with a ligand which leads to a heterogeneous-like mixture and gives conversion (Table 1, entry 1, ligand 7) and the other one with a homogeneouslike system (Table 2, entry 2, ligand 9). Reduction of PdCIZ in the presence of the ligand was performed and the reaction mixture was then filtered off on active charcoal. The filtrate in one side and the charcoal in the other side were used for the catalytic reduction of acetophenone by the previously described procedure: (i) with ligand 7, the system was defined as heterogeneous and indeed the filtrate is inactive; (ii) in contrast, with ligand 9, the system was defined as homogeneous and when using the filtrate, the reaction occurs at the same rate as that without filtration. The charcoal is inactive. We can reasonably assume that these tests performed on the other ligands would lead to the same conclusions and that the aspect of our reaction mixture is well correlated to the nature of the catalytic species (homogeneous or heterogeneous).

Conclusion This study reveals that dithioethers and above all azathia compounds are suitable ligands for homogeneous palladium catalysis. That is not the case for the oxathia ligands. It is thus possible to control the chemoselectivity of the reduction of acetophenone into 1-phenylethanol by an appropriate choice of ligand, source of palladium and solvent. The observed stereoselectivity was low but significant. More generally, carbonyl compounds are not easy to reduce by catalytic hydrogenation using palladium as a catalyst and other metals, such as rhodium or ruthenium are known to be more efficient for ketone reduction. Thus, even if thio ethers turn out to induce very low enantioselectivity in catalytic reduction of ketones with palladium, we are still confident of their potentiality in general asymmetric catalysis.

F. Fache et al. /J. Mol. Catal. 85 (1993) 131-141

141

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