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Enantioselective Catalysis with Chiral Phosphine Oxide Preligands.

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Angewandte
Chemie
Ligand Design
Enantioselective Catalysis with Chiral Phosphine Oxide
Preligands
Natalia V. Dubrovina and Armin Brner*
Keywords:
asymmetric catalysis · iridium · P ligands ·
palladium · phosphanes
Another new chiral ligand! The search
for new construction principles for
enantiomerically pure trivalent phosphorus compounds for enantioselective
metal catalysis continues unabated. This
statement may well give rise to astonishment since the number of chiral P
ligands in the literature has in the
interim entered the thousands.[1] Both
academic and economic reasons are
responsible for this phenomenon. Since
the first use of chiral phosphines in Rh
hydrogenation catalysts more than
30 years ago[2] new metal-catalyzed reactions that can be controlled by chiral P
ligands have been continuously discovered.[3, 4] Numerous applications of enantioselective syntheses in the pharmaceutical industry as well as in the
production of chiral fragrances and
agrochemicals also now bear witness to
the considerable potential of this methodology in the chemical industry.[5] Important are a high enantioselectivity and
productivity of the catalyst. Unfortunately theory has thus far offered scant
help as to which substituents on the
trivalent phosphorus atom give rise to
the desired catalytic properties. This
uncertainty has hitherto challenged the
creativity of the catalyst chemist, and as
a consequence has thrust many a “treas[*] Prof. Dr. A. B"rner
Fachbereich Chemie
Universit't Rostock
Albert-Einstein-Strasse 3a
18059 Rostock (Germany)
E-mail: armin.boerner@ifok.uni-rostock.de
Dr. N. V. Dubrovina, Prof. Dr. A. B"rner
Leibniz-Institut f<r Organische Katalyse
Universit't Rostock e.V.
Buchbinderstrasse 5/6
18055 Rostock (Germany)
Fax: (+ 49) 381-466-9324
Angew. Chem. Int. Ed. 2004, 43, 5883 –5886
ure” of almost forgotten phosphorus
chemistry into the light of day. Meanwhile the palette of phosphorus ligands
ranges from electron-rich alkylphosphines[6] , through phosphinites,[7] and
phosphonites,[8] to electron-deficient
phosphites.[9] The replacement of oxygen substituents in P-OR compounds
with amino groups made further electronic and steric variants available.[10]
The number of choices is extended
by the division into monodentate,[11]
bidentate,[1] and tridentate[12] ligands,
which in turn can coordinate with the
catalytically active metal center with a
wide range of bite angles.[13] Each newly
discovered ligand class offers the guarantee of its patenting, an important
aspect for an exclusive industrial exploitation in the future.
From a preparative viewpoint many
trivalent phosphorus compounds are
affected by one problem: they are subject to oxidation. It is therefore frequently necessary to work in an inert gas
atmosphere, not exactly an advantage
for routine use and scale-up. Repeatedly
transient protection of the phosphorus
atom is advised. However, only the BH3
protective group, which when required
can be removed in situ shortly before
coordination to the metal center, has
achieved any significance.[14] A disadvantage is that the P BH3 adduct is
unstable towards a series of acids and
Lewis bases. Moreover, commonly used
deprotecting reagents such as strongly
basic amines also attack hydrolyzable
P O bonds.
Significantly more robust is the protection of the phosphorus atom in the
form of the P=O bond. Removal of the
“oxygen protective group” takes place
by reduction, which is normally carried
DOI: 10.1002/anie.200460848
out with silanes in a separate reaction
step prior to catalysis. However, in this
way the problem of handling oxidationsensitive phosphorus compounds is
merely postponed, not solved.
One way out is offered by phosphine
oxides (1; Scheme 1), about which will
Scheme 1. Pre-established tautomerization
equilibrium for the generation of a P-ligand–
metal complex.
be reported here. These phosphorus
derivatives can be synthesized simply
by, for example, a P–C coupling reaction
with phosphine halides and subsequent
hydrolysis of a remaining P X bond.[15]
They are stable to oxidation and inert to
water. Naturally compounds of type 1
are not suitable to coordinate sufficiently strongly to soft transition metals
through the hard oxygen atom. Their
potential as ligands arises in the first
instance through the tautomerism illustrated in Scheme 1 in which the pentavalent phosphine oxides 1 are in chemical equilibrium with the trivalent phosphinous acids 2. As early as 1968 Chatt
and Heaton pointed out that this equilibrium which lies far to the left-hand
side can be displaced in favor of 2 by
coordination to a suitable metal center.[16] In catalysis the term precatalyst is
used for stable precursors of a catalyst,
we therefore logically propose the term
“preligand” for compounds of type 1.
Roundhill and co-workers utilized
the displacement of this tautomeric
equilibrium (Scheme 1) to construct
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5883
Highlights
the Pt hybrid complex 6 by reaction of
the platinum tetrakisphosphine complex
4 with diphenylphosphine oxide
(5)(Scheme 2).[17] One interesting detail
of the ligand coordination is the intramolecular hydrogen bond in the backbone that leads to the formation of a
quasi-chelate which confers additional
conformational stability on the complex.[18]
In 1986 van Leeuwen et al. demonstrated for the first time the catalytic
potential of preligands, namely in the Ptcatalyzed hydroformylation of olefins
with terminal and internal double
bonds.[19] Comparable catalysts were
used later by Parkins for the hydrolysis
of nitriles.[20] Although these results already indicated the value of this method
for the in situ generation of trivalent P
Scheme 3. Solid-phase synthesis of preligands
resolution of the racemate.
In principle these methods
can be used for both tautomeric forms. Of potential
for resolution of the racemate are, for example, salt
formation or esterification
of the phosphinous acid
with chiral alcohols.[26]
The groups of Feringa
and de Vries isolated the
enantiomerically
pure
SPOs 10–12 on the 100 mg
scale by preparative HPLC
on chiral phases.[27] The
new, monodentate preligands were used in the Ircatalyzed enantioselective
hydrogenation of prochiral
acetophenone imines, for
The work of Li had already established the ready
accessibility of SPOs with a
stereogenic
phosphorus
atom, whose most prominent
Scheme 2. Construction of a Pt complex with a phosphiarea of application should be
nous acid as ligand by complexation of isomerized phosenantioselective
catalysis.
phine oxides.
An old problem of synthetic
chemistry could also be a which up to 83 % ee was obtained with
ligands, it is only since 2001 that pre- step closer to a solution: the synthesis phosphine oxides of type 10.
Whereas the enantiomerically pure
ligands have been used to a greater of ligands with a stereogenic phosphorus
extent in homogeneous catalysis. Since atom is still more difficult than that of P preligands in the above example had to
this time they have been abbreviated to ligands with a chiral carbon back- be prepared by tedious HPLC separa“SPOs” (secondary phosphine ox- bone,[25] , even though significant im- tion, Hamada and co-workers recently
ides).[21]
provements have been achieved recent- succeeded in synthesizing the enantiomerically pure diaminophosphine oxide
The first highlight point was LiCs use ly.[6]
of SPOs in Pd-catalyzed cross-coupling
Two possibilities for the synthesis of 15 by highly diastereoselective in situ
reactions with non-activated aryl chlor- enantiomerically pure SPOs, which for- hydrolysis of the triaminophosphine 14
ides.[22] The syntheses of the preligands tunately are highly stable towards epi- (Scheme 4).[28] The starting material for
received particular impetus through the merization, appear especially promis- this conversion, the triamine 13, is
use of combinatorial methods. An ex- ing: the diastereoselective generation of accessible in a few steps from l-aspartic
tensive palette of phosphine oxides 9 a the stereogenic phosphorus atom and acid. The formation of 15, which was
was prepared by the addition of
Grignard reagents or metal alkyl compounds to the heterogenized dichlorophosphinamines 7 a and hydrolysis of
the products 8 a (Scheme 3). Secondary
phosphine oxides (9 b) with four different substituents on the phosphorus center were accessed by the use of monoalkylchlorophosphines (7 b).
Shortly afterwards Wolf and Lerebours too used SPOs in Pd-catalyzed
coupling reactions of arenes.[23] At the
same time they reported the high stability of the catalysts to hydrolysis in Stille
couplings in water as solvent.[24]
Scheme 4. In situ synthesis of a diamidophosphorus acid silyl ester ligand. Bn = benzyl.
5884
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 5883 –5886
Angewandte
Chemie
isolated in 60 % yield as the main
product of the hydrolysis on moist
SiO2, could be monitored by 31P NMR
spectroscopy.
The preligand 15 was tested in the
Pd-catalyzed enantioselective alkylation
of unsymmetrical diketones in the presence of bis(trimethylsilyl)acetamide
(BSA).[29, 30] Surprisingly, the best results
were obtained with an excess of four
equivalents of BSA. Detailed investigations showed that in the presence of PdII,
complete tautomerization with subsequent coordination of the phosphorus
acid 16 to the metal ion did not, as
expected, take place; the displacement
of the equilibrium is first enforced by in
situ O,N silylation with BSA. The ligand
17 subsequently coordinates at the palladium center. This compound could
also be prepared and isolated in a
comparison experiment by silylation of
15.[31] Thus, at the end of this new and
fascinating reaction sequence a nitrogen
analogue of ligand classes recently well
established in enantioselective homogeneous catalysis is obtained: that of the
chiral, monodentate triesters of phosphorus acid (phosphites)[9] and diesters
of amidophosphorus acid (amidophosphites).[32, 33]
It remains to add that high yields and
up to 94 % ee were achieved with ligand
17 in the alkylation of cinnamyl acetates
with b-substituted cycloalkanones. Kinetic investigations confirmed that two
monodentate ligands coordinate at the
Pd center. The exocyclic aminoethyl
group which was already essential in
the diastereoselective formation of the
preligand 15 has possibly a directing
effect upon the attacking nucleophile
within the context of an attractive
secondary interaction.[34]
Published Online: October 21, 2004
[1] Ligand collections are found in: H.
Brunner, W. Zettlmeier, Handbook of
Enantioselective Catalysis with Transition Metal Compounds, Vol. II, VCH,
Weinheim, 1993; J. Seyden-Penne, Chiral Auxiliaries in Asymmetric Synthesis,
Wiley, New York, 1995; D. Laurenti, M.
Santelli, Org. Prep. Proced. Int. 1999, 31,
245 – 294; W. Tang, X. Zhang, Chem.
Rev. 2003, 103, 3029 – 3069; P. J. Guiry,
C. P. Saunders, Adv. Synth. Catal. 2004,
346, 497 – 637.
Angew. Chem. Int. Ed. 2004, 43, 5883 –5886
[2] L. Horner, H. BMthe, H. Siegel, Tetrahedron Lett. 1968, 9, 4023 – 4026; W. S.
Knowles, M. J. Sabacky, Chem. Commun. 1968, 1445 – 1446; L. Horner, H.
Siegel, H. BMthe, Angew. Chem. 1968,
80, 1034 – 1035; Angew. Chem. Int. Ed.
Engl. 1968, 7, 942 – 943; T. P. Dang, H. B.
Kagan, Chem. Commun. 1971, 481.
[3] Comprehensive Asymmetric Catalysis
(Eds.: E. N. Jacobsen, A. Pfaltz, H.
Yamamoto), Springer, Berlin, 1999; Catalytic Asymmetric Synthesis (Ed. I. Ojima), Wiley, New York, 2000.
[4] Phosphines are increasingly being used
not only as ligands but also as organocatalysts in enantioselective syntheses.
See, for example: E. Vedejs, O. Daugulis, N. Tuttler, J. Org. Chem. 2004, 69,
1389 – 1392, and references therein.
[5] J.-P. GenÞt, Acc. Chem. Res. 2003, 36,
908 – 918; Asymmetric Catalysis on Industrial Scale (Eds.: H. U. Blaser, E.
Schmidt), Wiley-VCH, Weinheim, 2004.
[6] In respect of more recent work see:
K. V. L. CrOpy, T. Imamoto, Tetrahedron
Lett. 2002, 43, 7735 – 7737; G. Hoge, H.P. Wu, W. S. Kissel, D. A. Pflum, D. J.
Greene, J. Bao, J. Am. Chem. Soc. 2004,
126, 5966 – 5967.
[7] See, for example: M. Clochard, E.
Mattmann, F. Mercier, L. Ricard, F.
Mathey, Org. Lett. 2003, 5, 3093 – 3094.
[8] M. T. Reetz, A. Gosberg, R. Goddard,
S.-H. Kyung, Chem. Commun. 1998,
2077 – 2078.
[9] See, for example: M. DiOguez, A. Ruiz,
C. Claver, Dalton Trans. 2003, 2957 –
2963; Z. Hua, V. C. Vassar, I. Ojima,
Org. Lett. 2003, 5, 3831 – 3834; H.
Huang, Z. Zheng, H. Luo, C. Bai, X.
Hu, H. Chen, Org. Lett. 2003, 5, 4137 –
4139; I. Gergely, C. HegedMs, H. GulyPs,
Q. SzRllRsy, A. Monsees, T. Riermeier, J.
Bakos, Tetrahedron: Asymmetry 2003,
14, 1087 – 1090; A. Korostylev, A. Monsees, C. Fischer, A. BRrner, Tetrahedron:
Asymmetry 2004, 15, 1001 – 1005.
[10] F. Agbossou-Niedercorn, U. Suisse, Coord. Chem. Rev. 2003, 242, 145 – 158;
N. V. Dubrovina, V. I. Tararov, Z. Kadyrova, A. Monsees, A. BRrner, Synthesis 2004, 2047 – 2051.
[11] Reviews: F. Lagasse, H. B. Kagan,
Chem. Pharm. Bull. 2000, 48, 315 – 324;
I. V. Komarov, A. BRrner, Angew.
Chem. 2001, 113, 1237 – 1240; Angew.
Chem. Int. Ed. 2001, 40, 1197 – 1200.
[12] P. Barbaro, C. Bianchini, G. Giambastiani, A. Togni, Eur. J. Inorg. Chem.
2003, 4166 – 4172.
[13] P. W. N. M. van Leeuwen, P. C. J. Kamer, J. N. H. Reek, P. Dierkes, Chem. Rev.
2000, 100, 2741 – 2769.
[14] Review: M. Ohff, J. Holz, M. Quirmbach, A. BRrner, Synthesis 1998, 1391 –
1415.
www.angewandte.org
[15] H.-J. Kleiner, Phosphorverbindungen in
Houben-Weyl, Methoden der Organischen Chemie, 4. ed. (Ed. M. Regitz),
Thieme, Stuttgart, 1982, p. 240 – 245.
[16] J. Chatt, B. T. Heaton, J. Chem. Soc. A
1968, 2745 – 2757; see also: K. R. Dixon,
A. D. Rattray, Can. J. Chem. 1971, 49,
3997 – 4004; E. Lindner, B. Schilling,
Chem. Ber. 1977, 110, 3266 – 3271.
[17] W. B. Beaulieu, T. B. Rauchfuss, D. M.
Roundhill, Inorg. Chem. 1975, 14, 1732 –
1734.
[18] H-bond stabilized quasi chelate ligands
were used recently in Rh-catalyzed
hydroformylation: B. Breit, W. Seiche,
J. Am. Chem. Soc. 2003, 125, 6608 –
6609.
[19] P. W. N. M. van Leeuwen, C. F. Roobeek, R. L. Wife, J. H. G. Frijns, J.
Chem. Soc. Chem. Commun. 1986, 31 –
33; P. W. N. M. van Leeuwen, C. F. Roobeek, J. H. G. Frijns, A. G. Orpen, Organometallics 1990, 9, 1211 – 1222.
[20] Hydrolysis of nitriles: T. Ghaffar, A. W.
Parkins, Tetrahedron Lett. 1995, 36,
8557 – 8660; for a later use in the amidation of nitriles see: C. J. Cobley, M.
van den Heuvel, A. Abbadi, J. G. de Vries, Tetrahedron Lett. 2000, 41, 2467 –
2470.
[21] Occasionally the abbreviation POP is
also used which is intended to describe
the tautomeric equilibrium between a
P=O and a P compound.
[22] G. Y. Li, Angew. Chem. 2001, 113, 1561 –
1564; Angew. Chem. Int. Ed. 2001, 40,
1513 – 1516.
[23] C. Wolf, R. Lerebours, J. Org. Chem.
2003, 68, 7077 – 7084.
[24] C. Wolf, R. Lerebours, J. Org. Chem.
2003, 68, 7551 – 7554.
[25] K. M. Pietrusiewicz, M. Zabłoka, Chem.
Rev. 1994, 94, 1375 – 1411.
[26] For racemate cleavage of secondary
phosphine oxides see: J. Drabowicz, P.
Lyzwa, J. Omelanczuk, K. M. Pietrusiewicz, M. Mikołajczyk, Tetrahedron:
Asymmetry 1999, 10, 2757 – 2763; F.
Wang, P. L. Polavarapu, J. Drabowicz,
M. Mikołajczyk, J. Org. Chem. 2000, 65,
7561 – 7565.
[27] X.-b. Jiang, A. J. Minnaard, B. Hessen,
B. L. Feringa, A. L. L. Duchateau,
J. G. O. Andrien, J. A. F. Boogers, J. G.
de Vries, Org. Lett. 2003, 5, 1503 – 1506.
[28] T. Nemoto, T. Matsumoto, T. Masuda, T.
Hitomi, K. Hatano, Y. Hamada, J. Am.
Chem. Soc. 2004, 126, 3690 – 3691.
[29] Review: M. T. El Gihani, H. Heaney,
Synthesis 1998, 357 – 375.
[30] Reviews: E. J. Corey, A. Guzman-Perez,
Angew. Chem. 1998, 110, 402 – 415;
Angew. Chem. Int. Ed. 1998, 37, 388 –
401; J. Christoffers, A. Mann, Angew.
Chem. 2001, 113, 4725 – 4732; Angew.
Chem. Int. Ed. 2001, 40, 4591 – 4597.
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5885
Highlights
[31] For the preparation of analogous chiral
compounds and their reactions see: V. J.
Blazis, K. J. Koeller, C. D. Spilling, J.
Org. Chem. 1995, 60, 931 – 940; A.
De la Cruz, K. J. Koeller, N. P. Rath,
C. D. Spilling, I. C. F. Vasconcelos, Tetrahedron 1998, 54, 10 513 – 10 524.
[32] M. van den Berg, A. J. Minnaard, E. P.
Schudde, J. van Esch, A. H. M. de Vries,
5886
J. G. de Vries, B. L. Feringa, J. Am.
Chem. Soc. 2000, 122, 11 539 – 11 540; S.
Doherty, E. G. Robins, I. PPl, C. R.
Newman, C. Hardacre, D. Rooney,
D. A. Mooney, Tetrahedron: Asymmetry
2003, 14, 1517 – 1527.
[33] Recently the use of a chiral monodentate diamidophosphite as ligand in Pdcatalyzed enantioselective alkylation
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
was reported: V. N. Tsarev, S. E. Lyubimov, A. A. Shiryaev, S. V. Zheglov, O. G.
Bondarev, V. A. Davankov, A. A. Kabro, S. K. Moiseev, V. N. Kalinin, K. N.
Gavrilov, Eur. J. Org. Chem. 2004, 2214 –
2222.
[34] Reviews: M. Sawamura, Y. Ito, Chem.
Rev. 1992, 92, 857 – 871; A. BRrner, Eur.
J. Inorg. Chem. 2001, 327 – 337.
Angew. Chem. Int. Ed. 2004, 43, 5883 –5886
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