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Increase in the Enantioselectivity of Asymmetric Hydrogenation in Water Influenced by Surfactants or Polymerized Micelles.

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H) = 1 l . Y Hr, 1 H. 2'-H,): 7 : 6 = 3.26 (dd. 'J(2-H. 3-H,,,,) = 3.2. 'J(2-H, 3H ,,,,,,,,) = 7 . 9 H ~ .I H: 2-H). 4.23 (dd, 'J(2'-H,*. 2 - H J =11.6. 'J(2-Hd.I'H) = 1.4 HL. 1 H : 2'-H,,). 4.92 (dd. 'J(Z'-H,, 2-H,) =11.6. 3J(2'-H,. 1'H) = 2.3 HL. I H: 2'-H,).
[I71 X-ruy diffraction analysis of 7.crystal dimensions 0.15 x 0.22 x 0.2 mm, space
group I'hcu. u =13.818(3). b =10.098(2), c =14.086(4) A, V=1965.6.&',
Z = 8. pLd,id=1.46
Siemens-P3 diffractometer. Mo,, radiation, 2Hrange 4 54 . measuring method w/2H-scan, scan rate 3.5-29.3- min-'.
7 = 293 K. 2161 independent reflections measured, 1100 of which observed
( / > L Y 6 u ( / ) ) . 131 variables refined. direct methods (SHELXTL-plus). leastsquare tit. R = 0.0682. R , = 0.0536. Further details of the crystal structure
investigation arc available on request from the Fachinformationsrentrum
Karlsruhe. D-76344 Eggenstein-Leopoldshafen. on quoting the depository
number ('SD-58430.
I f triethylamiiie is used instead of N,N-diethylaniline. aromatiration predominates.
G. Stork. R . Mook, Jr., S . A. Biller. S . D. Rychnovsky. J. A m . Chmi. SOC.
1983. IfJC. 3741 -3742.
For intramolecular radical addition to cyclic 1.3-dienes. see: C. E. Schwartz.
D. P Curran. J. At??. Chem. SOC.1990, ff2. 9272-9284.
For corresponding reductive cyclization reaction of a-bromoacetals and a
monooletin unit. see: C. Hackmann. H. J. Schdfer, Telruhedron 1993,49,45594574.
The use ol'inethqllithium is essential here. For deprotonation of other sulfones
(see e p. 6) successfully applied bases such as LDA o r rrBuLi [7.9] d o not cause
conversion of 9.
A ratio I I : 12 of3.7.1 is reproducibly obtained ifall resultant diastereomers of
10 arc used. Preliminary mechanistic studies show that the reyioselectivity of
thia rcductive sultone cleavage is highly dependant upon the relative configuration of diastereomers 10 at the acetal carbon atom.
A. K. M Anisurzaman. R. L. Whistler. Curholrydr. Res. 1978. 61. 511-518.
E . Kcinnii. D Eren. J. Org. Chrrn. 1986. 51, 3165- 3169.
We thank Priv.-Dor. Dr. M . Gobel. Universitit Frankfurt. for sending us a
sample of authentic (+)-ivangulin.
M. Stiles. H. Finkbeiner. J. A m . Chem. SOC.1959, 81. 505-507.
M . M . Murta. M. B. M. de Azevedo, A E. Greene. J. Org. Chem. 1993. 58,
7537 7541.
A thorough G G M S analysis of the crude product confirms that no additional
meth! Icii;itioii products are formed apart from 1.
Increase in the Enantioselectivity of Asymmetric
Hydrogenation in Water Influenced by
Surfactants or Polymerized Micelles**
micelles.['] In the asymmetric hydrogenation of methyl (Z)-2-Nacetylaminocinnamate (If), the substrate exclusively used up to
now, the enantioselectivity is lower in water than in other solvents (particularly alcohols) for all of the catalysts investigated
in the absence of surfactants and is increased relative to the
blank by the addition of amphiphilic compounds. Thus it
seems important to determine whether this is only an
apparent increase in enantioselectivity, caused. for example, by
suppressing the influence of unspecific catalyst impurities (perhaps metallic rhodium) that reduce the selectivity. as a consequence of the considerably shorter reaction times relative to the
blanks.
2
Our investigations into the hydrogenation of a large number
of substrates (la-n) with the catalyst 2 conclusively favor a real
increase in selectivity, presumably by optimizing the conformation of the catalyst, which is concentrated on the micelle surface,
in the transition state. By adding sodium dodecylsulfate (SDS)
in substoichiometric amounts, the enantioselectivity of the hydrogenation of the esters la-h increases to 96-98 % ee ( S ) - N acyl-a-amino acid ester, almost independent of the wide-ranging
solubility of the substrates (Table 1). It corresponds to a sevenfold increase of the average relative enantioselectivity Q['] over
the blank in water without surfactant:
The doubled relative enantioselectivity
Arvind Kumar, Gunther Oehme," Jean Pierre Roque,
Manuela Schwarze, and Rudiger Selke*
The hydrogenation of sparingly soluble compounds using
slightly soluble complexes of rhodium(1) biphosphanes or
biphosphinic acid esters as catalysts in water can be considerably accelerated by the addition of solubilizing agents that form
[*I
Prof. Dr. P. Selke. Dr. A. Kumar."' M. Schwarze
Ma~-Pl;inck-Gesellschaft
Arbeitsgruppe "Asymmetrische Katalyse" an der Universitit
Buchbinderstrasse 5-6. D-18055 Rostock (FRG)
TeleFdx: Int code + (381)46693-24
Prof Dr G Oehme
Institut fur Organische Katalyseforschung a n der Universitat Rostock. e. V.
Prof Dr. J. P.Roque
Universite Montpellier 11, Laboratoire de Chimie Organique Physique
Montpellier (France)
[ '1
[**I
Permanent address: Central Drug Research Institute, Lucknow (India)
Carbohydrate Phosphinites as Chiral Ligands for Asymmetric Syntheses Catalyred by Complexes. Part 11. This work was supported by the Fonds der
Chemischen Industrie. We thank H. Burneleit and P. Barthelemy for preparative work. and Dr. C. Fischer and K. Kortus for the chromatographic determination o f the enantioselectivity. Part 10: [7]
Ailgell,
Chcvm In(. Ed. E n d . 1994. 33, N o . 21
shows that the reported enantiomeric excess in methanol (see
ref. [3]) is also clearly exceeded, although the reaction rate is
generally higher in methanol. The experimentally determined
enantiomeric ratio-here SIR--is used as the value q.
The lower enantioselectivities of hydrogenations in methanol
without surfactants also supports a real increase in selectivity
caused by micelle-forming compounds in water, because, due to
the high reaction rate in methanol, the explanation of an influence of less active, unspecific catalyst impurities cannot be applicable.
Strongly in favor ofour postulate o f a real increase in selectivity influenced by surfactants is the high Q value of 5.2 for the
completely water-soluble In, as here the reaction rate is also
high in the blank. The increase in activity of about one order of
magnitude can be explained by the solubilizing influence of the
surfactant on the catalyst. For the acids li-rn and ester In, no
clearcut increase in the enantioselectivity over the corresponding blanks in methanol by SDS addition can be detected
( Q = 1.2k 0.3).
,C VCH Verlugsgesdlschuft rnbH, D-69451 Weinheim, 1994
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Table 1. Hydrogenation of the ester substrates la-h and I n and the acid substrates l i - m with and without the addition of wrfactant.
L ["a] [a]
Substrate
I , ,
R'
R2
R'
R3
bin1
blank
inH20
+5Omg
SDS
Ph
Me
Me
Me
Ph
Me
Me
Me
Me0
Me0
Me0
Me0
Me0
H
Me0
Me0
H
Ph
Me
Me
Me
Me
Me0
HO
H
H
Me0
AcO
Me0
Me0
H instecid of arvl
1 4
11 0
24 0
40 0
40 0
X l
A
Me
Me
H instead of aryl
100 0
32
3
la
Ih
C2H,0H
Me
lc
Id
le
If
Ig
Et
Me
CIH,OH
Ih
C,H,OH
Ii
lj
Ik
II
lm
H
H
H
In
Me
Me
Me0
Me0
Me0
AcO
HO
H
HO
HO
07
1.6
1.9
2.0
3.1
21 5
23.0
31.0
1170
2250
2600
1500
390
41 5
429
Hydrogenation [b]
"h cc ( S )
blank
+50mg
inH,O
SDS
58
151
5x
49
20
6
14
-
21
-
61
13
29
18
-
95.6
96.5
91.2
98.1
96.1
97.2
97.3
11.2
82.0
83.0
81.4
82.8
2.4
1.2
1.1
2.4
3.1
2.0
I .'i
-
1.2
6.9
9.1
-
6.4
6.9
0:
H
Q, [cl
Q
7.4k13
Q':
2.1
1.6
1.3
1.1
1.2
0.9
Q': 1.2
I .o
96.3
96.3
-
78.6
95.1
95.6
95.9
5.1
16.8
95.1
5.2
-
k 0.6
*
0.3
[a] Solubility of 1 as the percentage of 1 mmol which dissolves in 15 mL water at 25 C. [bl Hydrogenatlon conditions: 1 mmol Substrate, 0.01 mmol catalyst 2 , 15 mL solcent.
25 C. 0.1 MPa Hi. [c] Calculated using the values for "hcr in methanol from ref. [ 3 ] .
Particularly interesting was the introduction of the new precatalyst 4,recently synthesized by our group [Eq. (a)] .[41 Essen-
(cod)acac
from the analogous benzylidene compounds as a consequence
of contamination by the initial hel late.'^] In contrast, due to its
good solubility. the hydrogenation times in the aqueous blanks
are surprisingly short with 4, obtained from the anisylidene
compound 3. The complete conversion of the substrate 1 to the
methyl ester of N-acetylphenylalanine in 1.5 h with a half-life
I i:2 of 33 min warranted the assumption that unspecific hydrogenations d o not falsify the blank result of the enantioselectivity
of 61 .I Yoee ( S )(Table 2). The addition of SDS also with 4 raises
the selectivity considerably (94% "e ( S ) , Q =7.8).
The reason for the influence of the amphiphile on the
stereoselectivity is not easy to explain, as an influence on the
conformation of the catalyst in the transition state cannot be
identified spectroscopically. A change in the mechanism of the
catalyst at the micelle surface is not expected. The Eyring plot
obtained by varying the temperature does not display any point
of inflection. Very probably the surface-active substances are
effective in their micellar form, as the critical micelle concentration must be approached for the maximal increase in the activity
and enantioselectivity. However, the effect can be achieved with
less modifier when polymerized micelles are used. These are
formed by irradiating m-undecenyl sulfate as depicted in Equation (b) and are effective at considerably lower concentrations
- H3CO-@CH0
3
tial for the synthesis was the support of the hydrolysis by the
electronic influence of the methoxy function of the anisylidene
protective group. Only products that clearly showed lower activities in water (with similar enantioselectivity) were obtained
'2
CH2=CH(CHd-OSO,Na
-
:-irradiation
~~~
I
(b)
(CH,-CH(CH,),-OSO,Nii),
11 =
I
40-60
Table 2. Comparison of the sensitivity of the catalysts derived from 2 and 4 to the addition of surfactants, 1 mmol substrate If.
Surfactants
[Rh(Ph-fi glup-OH)(cod)]+2
Solvents
4 = S'R
[inlnl
% "I'
(3)
37
11
1,
[dl
-
0.1 3 mmol
MeOH
H,O
2
I, L
H,O [c]
MeOH [c]
-
-
3
94.8
H2O
390
6
83.4
97.1
68
6.1
H,O
6
95.7
46
41
[Rh(Me-a-glup-OH)(cod)] 4
Yo LT
y =S R
Q [bl
H,O [c]
McOH [c]
(S)
+
Q Ibl
[min]
3
73.9
67
61.1
94.0
4.1
-
-
1.8
33
2
32
7.8
4.9
1.2
4
93.5
30
7.2.
4.5
-
-
-
SDS
0.13 mol equlv
S 0 , N a groups
in polymerized micelles
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due to their stability. To utilize the polymer to its full effect
(Fig. 1 ) the precatalyst is first dissolved in methanol, which is
subsequently completely removed. This preformation in
methanol also has a positive effect on the monomeric SDS due
to the better solubility. The polymer effect here, however, does
not increase the reactivity as much as in the case of acetal hydrolysis."]
ed four times with boiling T H F until only the pyridine hydrochloride remained The
pooled solutions were subsequently reduced under vacuum to 50 mL. The tai-get
compound crystallized out at 0 "C and was obtained in a very pure form in low yield
after recrystallization in anhydrous. oxygen-free tolueneitriethylamine (70i30):
10.8g (33.3%), m.p. 175-178'C; [3]i5 = +86.9 ( c = 2 in CHCI,): "P NMR
(101.2 MHz, [D,]pyridine, 25 C): d =113.6 (d. '.J(P.P) = 2.5 Hz). 116.5 (d.
'J(P.P) = 2.5 Hz); 13C NMR (62.8 MHz, [DJpyridine. 25 C): 6 = 54.5. 54.8
5 ) . 99.5 (d. 'J(C.P) = 6 HL: C I ) . 101.3 ( C 7 )
(CH,:a-CH,), 62.7 ( C b ) ,68.7 (C2
and arene-C signals: correct elemental analysis (C,H.P); MS (70 eV). iu;: 680
( [ M 'I).
Synthesis of 4: [Rh'(cod)(acac)] (1.55 g, 5 mmol) and 5 (3.4 g, 5 mmol) were dis-
solved in T H F ( 5 mL) to afford the chelate. and 4 0 % aqueous HBF, (10 mmol)
,
100
404
0.00
m'
0.05
added. The yellow solution turned red. After 28 h. of which 10 h were at 65 ' C, the
solution was filtered to remove impurities. Oxygen-free diethyl ether (40 mL) were
stirred into the filtrate. and the complex separated out as an orange syrup. The
remaining yellow solution was decanted, and diethyl ether (40 mL) uas again added
to the oil. The product 4 crystallized out: 4.3 g (89.7%); "P NMR (101.2 MHz.
CDCI,, 25 ' C ) : 6 = 132.7 (dd. 'J(P.Rh) =178. 'J(P,P) = 27 HI), 137.3 (dd.
'J(P.Rh) =181, *J(P,P) = 27Hzj; "CNMR(62.8 MHz,CDCI,.25 C ) : d = 27.9.
28.9. 30.7, 30.9 (CH, of cod), 55.1 (CH,), 60.9 (C6). 68.7, 70.7. 76.2 ('J(C.P) =
8 Hz), 80.8 ('J(C,P) = 9 Hz) (C2.3.4.5). 98.3 (d. -'J(C.P) = 4 Hz: C1). 101.5, 102.2
(CH of cod). 128.5-134.6 (C-phenylj: LSI-MS (positive, matrix: sulfolane): m::
773 [A4 ' - BFJ. 665 [773 cod].
a
1
0
0.10
0.15
inmol O S 0 3 N a
0.20
Received: May 24, 1994 [Z6965IE]
German version. Angrw. Chrm. 1994, 106, 2272
+
Fig. 1. Hydrogenation of 1 mmol I f in 15 mL H 2 0 in the presence of micelle-forming siibstxnces with 0.01 mmol precatalyst 4 (for conditions see Table 1). Open
symhols: i , ?: filled symbols: %re (5'): circles: SDS as micelle-forming substance:
squires: polymeri~ccdmicelles with OS0,Na groups.
The function of the precatalyst 4 is determined by the axial
aglycone. As we were recently able to demonstrate, induction of
chirality in hydrogenation reactions decreases with an increasing number of axial substituents on the pyranose ring in
analagous rhodium(1) chelate complexes of type 3, which are
protected by a benzylidene group in the 4,6-position of the carbohydrate residue.l" Thus we have now also demonstrated this
decrease in optical induction for the first of the conformatively
less rigid analogues carrying an axially oriented substituent
without the inflexibility imposed by the dioxane ring (Table 2).
The enantioselectivity of such conformatively labile seven-membered chelates was shown to be very sensitive to the influences
of a modifier.[' ')I We coiijecture that this is a key to direct
optical induction over a wide range.
[I]G. Oehme. E. Paetzold. R. Selke, J. Mol. Curril. 1992, 7/. L1: I . Grassert. E .
Paetzold. G. Oehme, Tetruherfron 1993, 49. 6605.
[2] Tosubstantiate theuseEulnessofthevaluePforcomparison oftheselectivitysee
ref. [3].
[3] R. Selke. C. Facklam. H. Foken. D. Heller. 7i.rruliedron A.n.minrfrj, 1993, 4.
369.
[4] In ref. [3] the formulas are erroneously given for the complex derived from
methyl-n-u-glucopyranoside, which at that time was not known
[5] R. Selke. P. Barthelemy. J. P. Roque, ,.C/7iru/ Reacfions in Heterojimeous C'otu/w i s " . ChiCat-Symposium, Briissel, Oktoher 1993.
[6] B. Andre, B. Boyer. G. Lamaty. J. P. Roque, Tetrahedron L m . 1991, 32. 18x1.
[7] R. Selke. M. Schwarze, H. Baudisch. I. Grassert, M. Michalik. G. Oehme. N .
Stall. B. Costisella. J. Moi. Cutal. 1993. 84. 223.
[S] D. Seebach et al. have a fascinating example from (taddo1)titanium coordination
chemistry, in which the induction range extends from 90% PC ( R ) to over 90%
ee (S) product: D . Seebdch, D. A. Plattner, A. K. Beck, Y. M. Wang, D. Hunzicker, H e h . Chim.Actu 1992, 75.2171. Results, deductions. and literature about
the dependence of the enantioselectivity o n the ligand conformation in
analogous catalyst chelate complexes can also he found here.
[9] Investigations have. for example, produced an inversion of the route of induction for a precatalyst analogous to 3 with a 4.6-0-benzylidene protective group
upon changing to apolar solvents: R. Selke. J. Prakr. Clwin. 1987. 329. 717.
E.uperiniental Procedure
The experimental procedures for the hydrogenation. the synthesis of the precatalyst
2 and the substrates. the derivatization of the hydrogenated products, and the
determination of the enantioselectivity are described or referred to in ref. [3], and the
synthesia of polymerized micelles in ref. [6]. With polymerized micelles the formation of the catalyst was performed in methanol. For all experiments of Figure l and
Table ?-but not fur those of Table 1-a homogenous solution was made from the
precatalyst and the micelle-forming compounds under argon in methanol ( 5 mL) by
stirring for 1 h. The solbent was then removed under V B C U U ~ ,the dry residue
dispersed in water ( I 5 mL) under argon by stirring for 1 h, the substrate added.
stirred for 10 min. the argon atmosphere replaced with hydrogen, and the hydrogenation started by stirring. The hydrogenated products were isolated by exhaustive
extraction with dichloromethane. For the acid hydrogenation products and the ester
from If. the results of the determination of selectivity were additionally checked in
double experiments by conversion in methanol to exclude the possibility of enantiomer enrichment during isolation. For this, the hydrogenation suspension was
quantitatively evaporated to dryness. and the residue dissolved in methanol. This
pi-ocedure wis ;~lsoapplied to all experiments with the precatalyst 4. For the enantiomeric excesses determined by gas chromatography. the standard deviation w a ~
less than 1 '$4w; similar results were also obtained for the derivatized hydrogenation products of la, e. h. and i , which were separated by HPLC. To determine the
substrate solubility. a small excess of substrate was stirred in water (100 mL) at
25 C for 1 h. the filtrate evaporated on a watch glass, dried under vacuum. and the
residue weighed.
Synthesis of 5. the glucopyranoside ligand of 3 : A solution of methyl-4.6-O-anisylidene-a-u-~lucopyranoside(21.78 g. 32 mmol) in pyridine (20 mL) was added to
Ph,PCI (15.53 g. 70.4 nimol) in T H F (100 mL) within 5 miin with vigorous stirring.
Pyi-idinc hydrochloride precipitated out. The next day the solid residue wasextractA I ? , L ~ I IC'hi~ni
.
l i l t . Ed. €n,qI. 1994. 33. N o . 21
Towards One-Dimensional Carbon Wires
Connecting Single Metal Centers: A Cumulenic
C, Chain that Mediates Charge Transfer
between Rhenium and Manganese Termini **
Weiging Weng, Tamas Bartik, and John A. Gladysz*
Compounds in which linear elemental carbon chains span
two transition metals, [L,MC,M'Lk], exhibit a variety of unusual physical and chemical properties."] However, only complexes
with x = 1-4 have been isolated to date. We have been particularly interested in charge transfer and electron delocalization
across the wire-like C,
and have sought to study
such phenomena in longer-chain compounds. We reported earlier that C, complexes can be prepared by methoxide group ab["I Prof. Dr. J. A. Gladysz.
[**I
Dr. W. Weng, Dr. T. Bartik
Department of Chemistry, University of Utah
Salt Lake City, UT 84112 (USA)
Telefax: Int. code +(801)581-8433
We thank the National Science Foundation for support of this research
VCH Verlugs,qesellschuft m h H , 0-69451 Weinheirn, 1994
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