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Catalytic Hydrogenation with Rhodium Complexes Containing dipamp-pyrphos Hybrid Ligands.

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[I] a ) A . Hassner in Azidrs and Nitrenes (Ed.: E . F. V. Scriven), Academic
Press, Orlando, FL, USA, 1984, pp. 35-94; b) E. F. V. Scriven. K. Turnbull, Chem. Rev. 1988,XX. 297.
[2] C. J. Moody in Studies in Natural Producis Chemistry, Vol. I (Ed.: A. U.
Rahman), Elsevier, Amsterdam, 1988, pp. 163- 185.
[3] a ) K. Isomura, M. Okada, H. Taniguchi, Chem. Lett. 1972, 629; b) H.
Hemetsberger, I. Spira. W. Schoenfelder. J. Chen?.Rex / S ) 1977, 247; c ) K.
Friedrich, G. Boeck, H. Fritz, Tetrahedron Lett. 1978,3327;d) C. J. Moody,
C. W. Rees. J. A. R. Rodrigues, S. C. Tsoi, J. Chem. Res. ( S ) 1985. 238;
e) M. P. Sammes in The Chemistrj of Herrrocyclrc Conipound.y, Vol. 4X ( / j
Pyrroles (Ed.: R. A. Jones), Wiley, New York, 1990. pp. 619-620.
[4] a) C. Vogel. P. Delavier, Tetrahedron Lett. 1989, 30, 1789; b) C. Vogel, P.
Delavier, P. G. Jones, D. Doring, ihid. 1991. 32, 1409.
[5] Z. Arnold, A. Holy, Collect. Czech, Chem. Commun. 1961, 26, 3059- 3073.
[6] H. Neumann. D. Seebach, Chem. Ber. 1978, ill, 2785.
171 Organrkum, 18th ed., Dtsch. Verlag der Wissenschaften, Berlin, 1990,
pp. 327-329.
[S] D. Knittel, Synrhesis 1985, 186.
H-indol-2[9] Crystallographic data for methyl 3-B-styryl-4.5,6.7-tetrahydro-l
carboxylate 5b: monoclinic, space group P2,/c, u =1069.2(4). h =
1466.1(3), c =1924.5(6) pm. fi = 97.350)'. V = 2.992 nm3, Z = 8 (two independent molecules). Data collection: colorless prism 0.4 x 0.3 x 0.2 mm.
T = - 130 "C, Mo,. radiation (E. =71.073 pm. 28,,, = 50"). Stoe-STADI-4
diffractometer, 120489 reflections. Structural refinement: Siemens
SHELXTL PLUS software program, F anisotropic, H atoms with a riding
model, W R = 0.075 for 2730 reflections > 3o(F). Further details of the crystal structureinvestigation may be obtained from the Fachinformationszentrum Karlsruhe, GeSellSchdft fur wissenschafthche Information mbH.
W-76344 Eggenstein-Leopoldshafen (FRG), on quoting the depository
number CSD-57154. the names of the authors. and the journal citation.
Catalytic Hydrogenation with Rhodium Complexes
Containing dipamp-pyrphos Hybrid Ligands""
By Ulrich Nagel* and Thomas Krink
The hydrogenation of olefins with rhodium phosphane
complexes as catalysts['] under homogeneous conditions is
well established, and chiral complexes and prochiral
ole fin^[^] are known to give rise to mixtures of enantiomers
with various compositions. In related work a series of catalysts has been developed for the hydrogenation of dehydroamino acids.[41 Among other empirically determined
properties, efficient catalysts should have C, symmetry.[51
The development of P- and C-chiral bisphosphane ligands16'
with additional ether groups['] allows a systematic investigation of the influence of the symmetry on the activity and
enantioselectivity of the catalyst.
The introduction of one ortho substituent into one of each
pair of phenyl groups of diop was described recently.[*] In
parallel work we succeeded in synthesizing ligands that can
be considered combinations of the the two "classic" ligands
employed successfully in hydrogenation, dipamp and pyrphos (Fig.
In our case the similarity is quite profound,
since both dipamp and pyrphos are 1,2-bisphosphanes.
Replacement of one phenyl group on each of the phosphorus atoms in pyrphos with an ortho-anisyl group leads to the
mixture of diastereomers 1 a-c."'] The synthesis of these
ligands, their purification, and the preparation of the cata-
Prof. Dr. U. Nagel, T. Krink
lnstitut fur Anorganische Chemie der UniversitHt
Auf der Morgenstelle 18
0-72076 Tubingen (FRG)
Telefax: Int. code (7071)292436
[**I Enantioselective Catalysis, Part 10. This research was supported by the
Volkswagen-Stiftung and the Fonds der Chemischen Industrie. We thank
Priv.-Doz. Dr. B. Koppenhoefer, Tiibingen, for valuable suggestions for
the analysis of enantiomers and J. M. Briody. Maynooth, for the measurement of some of the enantiomeric excesses. Part 9: [l].
Verlagsgesellschufi mhH. 0-694Sl Weinheim. 1993
p h 2 p C N B ~ ~
Fig. 1. The structures of dipamp and pyrphos, and their combination to give
1 a, 1 b, 1c. The stars mark stereogenic centers. An = 2-methoxyphenyl (anisyl),
Boc = tprr-butoxycarbonyl.
3a, 3b, 3c
Fig. 2. The structures of catalysts 2 , 3 a , 3b, and 3c. The spatial arrangement
of the aryl groups in 3a. 3b, and 3c is shown in Figure 3.
lysts are described in ref.[l]; in this communication we report
on the results in asymmetric hydrogenation.
Whereas 2 and 3a,b have C, symmetry, 3c is asymmetric
(Fig. 3).[' 'I By comparing 2 with 3a,b, it is possible to examine the influence of the additional ether groups while retaining the C , symmetry. The consequences of a reduction in
symmetry are revealed by a comparison of the activity and
enantioselectivity of 3c with that of 2 and 3a,b.
, ,, ,
I +'p-ph
Fig. 3. The spatial arrangement of the aryl groups in 3a. 3b, and 3c. Only the
important portions of the molecules are shown (view along the C , axis in 3a and
3 b).
Since both the enantioselectivity and the rate of the catalysis depend on many external parameters, we varied pressure
and temperature. The results of these investigations are
shown in Figures 4-7 for the hydrogenation of Z-cc-acetamidocinnamic acid to (S)-acetylphenalanine with catalysts 2
and 3a-c.
The introduction of the additional methoxy groups in catalyst 2 has rate-increasing (3a and 3c) and rate-decreasing
$ lO.00+ ,2510
Angew. Chem. In[. Ed. Engl. 1993, 32, N o . 7
/ 3a
P [bar1
Fig. 4. Reaction rates for the hydrogenation of Z-a-acetamidocmnamic acid
with catalysts 2, 3a, 3b, and 3c at 25°C and at different H, pressures.
TO =turnover.
100 7
P Ibarl
Fig. 5. ee Values of the (S)-N-acetylphenylalanineobtained with catalysts 2,
3a, 3b, and 3c at different H, pressures (reaction conducted at 25.0"C).
P [bar1
Fig. 6. Reaction rates for the hydrogenation of Z-a-acetamidocinnamic acid
with catalysts 2, 3a, 3b, and 3c at 50°C and at different H, pressures.
namic acid also depend on ligand symmetry. A direct comparison of C , symmetry with asymmetry (C, symmetry) is
possible with compounds 3a, 3b, and 3c, since these are
stereoisomers with the same empirical formula. No comparisons of this type have been previously reported. Remarkably, the results from the reaction with asymmetric
diastereomer 3c are almost as good as, and in part even
better than, those with 3a.["I At least in this case, C, symmetry is not essential for high enantiomeric excesses. Whereas the parent compound 2 provides constant enantiomeric
excesses over the entire pressure range, the ee vaIues obtained with the catalysts 3a, 3b, and 3c decrease to different
extents. At pressures over 40 bar 2 is the most selective catalyst; in the low-pressure range 3a and 3c are definitely superior to the parent compound 2. The hybrid ligand in catalyst
3a (matched pair) combines the partial structures R-pyrphos
(which gives (S)-N-acetylphenylalanine in 97 YOee) and Sdipamp (which give the S isomer in 95 YOee) and provides the
S isomer in up to 99 % ee. The unsymmetric hybrid ligand in
3c from R-pyrphos and rneso-dipamp yields the S isomer in
up to 98 % ee but is remarkably superior to the matched pair
3a at higher pressures. In each case the pyrphos structure
element dominates the selectivity. In the optimization of a
Iigand the isoinversion principle[13] helps to only a limited
extent. Our measurements show that the selectivities obtained at 25 "C are higher than those at 50 " C ; however, the
differences are not particularly distinct.
Experimental Procedure
The preparation of the catalysts 2[6] and 3a-c[l] were described previously.
The hydrogenations were conducted in an autoclave with a stirrer with 510 pmol catalyst, 5- 10 mmoi substrate, and 50 mL methanol. The reaction
mixture was initially under argon, which was removed by evacuation, and
hydrogen was introduced. The hydrogenation started when stirring started. The
H, consumption could be determined at any point in the reaction with a resolution of +0.01 mmol. The hydrogenation reaction has three stages: 1) Saturation phase: when the stirrer is started hydrogen is stirred into the solution (ca.
3 min). 2) Induction phase: in low-pressure hydrogenations (up to 10 bar) the
rate of the hydrogenation initially increases only slowly (probably until all of
the cyclooctadiene (cod) of the catalyst is hydrogenated). 3) Hydrogenation
phase: the rate of the hydrogenation is constant at the selected catalyst/substrate ratio. Workup: The solvent was removed by distillation, and the crude
product was dissolved in aqueous NaOH solution and extracted three times
with dichloromethane and diethyl ether. The aqueous phase was acidified to pH
3 with Z N HCI. The N-acetylphenylalanine formed was extracted with ethyl
acetate. Analysis: A solution of N-acetylphenylalanine in methanol was converted to the corresponding methyl ester by treatment with HCl/methanol.
Baseline separation of the enantiomers was achieved by gas chromatography
(GC: Chrompack CP 9000 with a flame-ionization detector; integration:
Chrompack integrator with the Mosaic program for data evaluation; capillary
column coated with (S)-chirasil-Val, standards for calibration prepared from
the racemate and a sample of (S)-N-acetylphenylalanine(99.95% ee)).
100 1
Received: January 27,
Revised version: March 12, 1993 [Z5835IE]
German version: Angew. Chem. 1993, 105, 1099
P [bar]
Fig. 7. ee Values of the (S)-N-acetylphenylalanineobtained with catalysts 2,
3a, 3b, and 3c at different H, pressures (reaction conducted at 5 0 . 0 T ) .
effects (3b) because of the effect on ligand symmetry and the
resulting relative position to rhodium. The enantiomeric excesses obtained for the hydrogenation of Z-a-acetamidocinAngen. Chem. Int. Ed. Engl. 1993, 32, No. 7
[I] U. Nagel, T. Krink, Chem. Ber., 1993, 126, 1091-1100.
[2] J. A. Osborn, F. H. Jardine, G. Wilkinson, J. Chem. SOC.A 1966, 1711.
[ 3 ] Reviews: K. E. Konig in Asymmetric Synthesis, Vol. 5 (Ed.: J. D. Morrison), Academic Press, New York, 1985, pp. 71 - 101 ; H. Brunner in The
Chemistry of the Metal-Carbon Bond, Vol. 5 (Ed.: E R. Hartley). Wiley,
Chichester, 1989, pp. 109-146; H. Brunner, Top. Stereochem. 1988, 18,
129-247; I. Ojima, N. Clos, C. Bastos, Tetrahedron 1989,45,6901-6939;
R. Noyori, M. Kitamura in Modern Synthetic Methods, Vol. 5 (Ed.: R.
Scheffold), Springer, Berlin, 1989, pp. 115-198; R. Noyori, Chem. SOC.
Rev. 1989, 18, 187-208.
H. B. Kagan in Asvmmetric Synthesis, Vol. 5 , (Ed.: J. D. Morrison), Aca.141_
demic Press, New York, 1985, p. 1-39; H. B. Kagan, M. Sasaki in The
Chemistry of Organophosphorus Compounds, Vol. 1 (Ed.: F. R. Hartley),
Wiley, Chichester, 1990, pp. 51-102.
[5] J. K. Whitesell, Chem. Rev. 1989,89,15S1-1590, M. 3. Burk, J. Am. Chem.
Soc. 1991, 113, 8518-8519 and references therein. For more on C,-sym-
0 VCH Verlagsgesellschaft mhH, 0-69451 Weinheim, 1993
0570-0833/93/0707-l053$lO.OO+ .25/0
metric ligands as chiral inductors see also: 0. Reiser, Angew. Chem. 1992,
According to a b initio calculations, the strain energy of
105, 576;Angew. Chem. Int. Ed. Engl. 1992,31,547.
(SiH), polyhedra increases with the number of three-mem161 U.Nagel, B. Rieger, Chem. Ber. 1988. 121, 1123-1131; U. Nagel, B.
bered rings in the polyhedron framework (see Scheme 1).[” 3l
Rieger, Organomerallics 1989,8, 1534-1538; U. Nagel. B. Rieger, A.
Bublewitz, J Organomet. Chem. 1989,370, 223-239.
[71 U.Nagel, A. Bublewitz, Chem. Ber. 1992, 125, 1061-1072;[l].
[XI K. Burgess, M. J. Ohlmeyer, K. H. Whitmire, Organometdics 1992,
I I, 3588-3600. diop = 2,3-O-isopropylidene-2,3-dihydroxy-l,4-b1s(dipheny1phosphino)butane.
191 B. D. Vineyard, W. S. Knowles, M. J. Sabacky, G. L. Bachman, D. J.
Weinkauff, J. Am. Chem. SOC.1977,99,5946-5952;U. Nagel, E.Kinzel,
J. Andrdde, G. Prescher, Chem. Ber. 1986,119,3326-3343.
[lo] The pyrrolidine nitrogen atom is protected by the Boc group, which can be
removed easily.
[I11 Rotamers of 2 c exist because of the partial double bond character of the
(391kJ mol- 1 239.6pm) (476kJ mol- I ; 237.5 pm) (590kJ mo, - 1 231.4pm)
amide bond of the Boc group; they are evident in the 31P{’H) NMR
Scheme 1. (SiH). Polyhedra; calculated strain energies and Si-Si distances in
[12]Recently the review appeared in this journal “On Quantifying Chirality”:
parentheses [2, 31.
A. B. Buda, T. Auf der Heyde, K. Mislow, Angew. Chem. 1992, 104,
1012-1031;Angew. Chem. Int. Ed. Engl. 1992,31,989-1007.Mislowetal.
attempt to give measures of chirality for geometric shapes. A conclusion of
their investigations is “that the maximum chirality is associated with minOne expects the difficulty of the syntheses of these fascinatimum symmetry.. _ ”
ing polyhedral comp0unds[~1to follow the same trend. In1131 H.Buschmann, H.-D. Scharf, N. Hoffmann, P. Esser, Anxew. Chem. 1991,
103,480-518; Angew. Chem. Int. Ed. Engl. 1991,30,414.BY using the
deed, until now only hexahedro-octasilane derivatives (Si
isoinversion principle one can predict that for this type of catalysis a
substituents: s ~ M ~ , ~C B
M ~ ,,C H M ~ ,2, , 6 - ~ t , c , ~have
catalyst that has been optimally tailored to the substrates will provide the
been synthesized.[51(For the heavier homologue germanium,
best ee values at 49°C and 2.3 bar.
(tBu,Si),Si, : The First Molecular Silicon Compound with a Si, Tetrahedron**
By Nils Wiberg,* Christian M . M . Finger,
and Kurt Polborn
Dedicated to Professor Heinrich Noth
on the occasion of his 65th birthday
In a recently published review on the cluster chemistry of
the heavier elements in the fourth main group (Si, Ge, Sn),
which is still in its infancy, S. Masamune et al. state in their
concluding remarks that the syntheses of tetrasilatetrahedrane 1, disilyne 2, and 1,1,l-pentasilapropellane3 are currently the greatest challenges for silicon chemists.”] We report here on our achievement of one of these goals-the
synthesis of a molecular silicon compound with a Si, tetrahedron (see
with regard to the question of a disilyne).
1 (here R = SitBu,)
a triprismo-hexagermane with CH(SiMe,), substituents on
the germanium atoms has also been generated.c6’) Since compounds with a tetrahedral framework constructed of atoms
of one element exist for most of the elements near silicon in
the periodic table, boron,”’ aluminum,[*]
carbon,[”] phosphorus (P,), and arsenic (As,), it is especially
puzzling that neither a tetrahedra-tetrasilane nor a tetrahedro-germane have been synthesized previously.
A tetrahedro-tetrasilane Si,R, should be more stable than
either a hexahedro-octasilane or a triprismo-hexasilane if it
has sterically demanding R groups (see
we chose to synthesize the target tetrahedron molecule 1 with
tri-tert-butylsilyl groups (R = SitBu,, “supersilyl”) (for
more on the name supersilyl see comment[81). Since Siztetrahedra are anionic components in alkaline and alkaline
earth silicides such as NaSi, KSi, and BaSi, , it was reasonable to try to give these “molecular freedom” (e.g.
Siz- + 4 R X --* Si,R, + 4X-). All attempts at derivatization have failed thus far (one reason for this is the high
reducing ability of the silicides; see comment[”, 12]).
Another imaginable approach, the dehalogenation of the
previously unknown tBu,Si-SiCI, with sodium, did not
provide access to 1, although the analogous reaction of the
less bulky tBuMe,Si-SiBr, led to hexahedro-octasilane (Si
substituents = SiMe,tBu).[’] Treatment of tBu,Si-SiC1,
with sodium in benzene at 80 “C did not give 1 ; instead NaCl
formation and hydrogen uptake led to a number of products
including 1,2-bis(supersilyl)disilane 4,which can be brominated easily to provide tetrabromobis(supersily1)disilane 5,
and the very interesting tris(supersily1)cyclotrisilane 6.[’
t Bu,
Institut fur Anorganische Chemie der Universitat
Meiserstrasse I, D-80333Miinchen (FRG)
Telefax: Int. code + (89)5902-451
Sterically Overloaded Silicon Compounds, Part 6, Silicon Compounds,
Part 96.This research was supported by the Deutsche Forschungsgemeinschaft. We thank Dr. J. Evers (Institut fur Anorganische Chemie der Universitlt Miinchen) for the measurement and interoretation of various rotating crystal and Wemenberg photographs, and Dr. 0 . Seligmann
(Institut fur Pharmazeutische Biologie der Universitat Munchen) for
recording a FAB mass soectrum. Part 5 and 95:J. Kovacs, G.Baum. G.
Fritz, D. Fenske, N. Wiberg, H. Schuster, K. Karaghiosoff, Z . Anorg.
Allg. Chem. 1993,619, 453.
, ,H
t Bu,Si-SiEr,-SiBr,-Si
H, / \ H
‘Sit Eu,
At this Doint we found that 1 (R = SitBu,)
“ _ can be obtained according t o Equation (a) by the reaction of 5 with
t S u 3 ~ i ~ a . [ i ~~~~~~~d
4,the precursor
Verlagsgesellschaft mbH, 0-694.51 Wemheim. 1993
[*] Prof. Dr. N. Wiberg, DipLChem. C. M. M. Finger, Dr. K. Polborn
2 5 + 41Bu,SiNa
1 (R
$ 1 O . O O i .ZSjO
+ 41Bu,SiBr + 4NaBr
Angew. Chem. In[. Ed. Engl. 1993,32. No. 7
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hybrid, containing, catalytic, rhodium, hydrogenation, dipamp, complexes, pyrphos, ligand
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