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The Use of a Ten-Membered Tetraphosphole Macrocycle to Increase the Lifetime of a Palladium Catalyst.

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I51 a) E. Differding, R. W. Law, Tetrahedron Lett. 1988,29,6087-6090; b) Hdv.
chim. Act0 1989,72,1248-1252; c) E. Differding, G. M. Riiegg, Tetrahedron
Lett. 1991,32,3815-3818; d) E. Differding, H. Ofner, Synlett 1991,187-189;
e) E. Differding, R. 0 . Duthaler, G. M. Ruegg, C. Schmit, ibid. 1991, 395396; f) E. Differding, G. M. Riiegg, R. W. Lang, Tetrahedron L e f t . 1991, 32,
1779-1782; g) E. Differding, M. Wehrli, ibid. 1991, 32, 3819-3822
[6] M. M. Kabat, Tetrahedron: Asymmetry 1993, 4, 1417-1420.
[7] a) D. Enders, B. B. Lohray, Angew,. Chem. 1987,99,359-360; Angew. Chem.
Int. Ed. Engl. 1987, 26, 351-352; b) ibid. 1988, 100, 594-596 and 1988, 27,
581 -583; c) B. B. Lohray, D. Enders, ffelv. chim.Acra 1989, 72, 980-984;
d) D. Enders, B. B. Lohray, F. Burkamp, V. Bhushan, R. Hett, Liebig.$ Ann.
1996, 189-200.
[8] a) D. A. Evans in Asvmmetric Synthesis, Vol. 3 (Ed.. J. D. Morrison), Academic Press, New York, 1984, pp: 1- 110; b) C. H. Heathcock in Asymmetric
Synthesis, Vol. 3 (Ed.: J. D. Morrison), Academic Press, New York, 1984,
pp. 111-212.
Suitable crystals were obtained from pentane at -20°C. C,,H,,OFSi,
M , = 244.43, crystal dimensions about 0.3 x 0.3 x 0.5 mm, orthorhombic,
V=1513.7A3,
P2,2,2(no 18),a=19.753(l),b =10.916(6),~=7.0199(9).&
Z = 4, peals,,=1.073 gem-'. Data were collected with an Enrdf-Nonius
CAD4 four-circle diffractometer with graphite-monochromated Cu,, radiation ( 2 =1.54179 A). The structure was solved with direct methods (Gensin,
Gentan, from Xta13.2) [lo]. Some of the hydrogen positions could be localized,
and the remainder were calculated. There were 1461 observed reflexions
[1>2u(I)], and 145 parameters refined; R = 0.095, R , = 0.061; min./max.
residual electron density -1.7/ + 0.6e k’.The configuration at C F was
determined by using the well-known configuration at CSi. Crystallographic
data (excluding structure factors) for the structures reported in this paper
have been deposited with the Cambridge Crystallographic Data Centre
as supplementary publication no. CCDC-100579. Copies of the data can be
obtained free of charge on application to The Director, CCDC, 12 Union
Road, Cambridge CBZlEZ, UK (fax: int. code +(1223)336-033; e-mail:
deposit @,chemcrys.cam.ac.uk).
S. R. Hall, H. D. Fldck, J. M. Stewart, XTAL3.2 Reference Manual, Universities of West Australia, Genf, and Maryland, 1992.
For the preparation of the software used, see Ball &Stick Ver. 2.2: A. Falk, N.
Miiller, G. Schoppel, L. Webb, Linz (Austria), Stafford (UK).
D. Enders, D. Ward, J. Adam, G. Raabe, Angew. Chem. 1996,108,1059- 1062;
Angew. Chem. Int. Ed. Engl. 1996, 35,981-984.
D. Parker, Chem. Rev. 1991, 91, 144-1457.
All compounds gave appropriate spectroscopic data (IR, MS, NMR) and
elemental analyses.
mixture of the various stereoisomers of each macrocyle behaves
as the single stereoisomer that is the best adapted to coordinate
to any given transition
center.
In view of the importance of palladium catalysts in organic
synthesis, we decided to study the properties of the palladium
complexes of one of the most accessible tetraphosphole macrocyc1es7 the ten-membered tetraphosphane
The
Of
with [PdCl,(PhCN),] in dichloromethane immediately affords
the red 1:1 complex 2 (Scheme l), which was characterized by
X-ray crystal structure analysis (Figure 1).
’.
phw
Ph
Ph
/
Ph
Me
Scheme
’.
Me Me
Ph
Me
2, M = Pd; 3, M = Pt
1
c11
C13
The Use of a Ten-Membered Tetraphosphole
Macrocycle to Increase the Lifetime of a
Palladium Catalyst
FranGois Mercier, Frank Laporte, Louis Ricard,
Franqois Mathey,* Marc Schroder, and Manfred Regitz
Transient [PdL,] complexes (L = phosphane) are the active
intermediates in a number of reactions and processes[’] such as
the Heck reaction[’’ and the Stille cross-coupling reaction.r31In
some cases, the catalyst is lost at the end of the process as an
insoluble precipitate, especially when the reaction is performed
at relatively high temperature as in the case of the Heck reaction.14] Recently we described a series of readily accessible tetraphosphole macrocycles of variable ring sizes (9- 16 atoms).r51
The phosphorus atoms in these macrocycles are characterized
by a low inversion barrier (about 16 kcal mol-’) because of
their incorporation into phosphole [email protected]]This means that a
[*I Prof. Dr. F. Matbey, Dr. F. Laporte, Dr. F. Mercier, Dr. L. Ricard
Laboratoire “Het&roelementset Coordination” URA 1499
DCPH Ecole Polytechnique
F-91128 Palaiseau Cedex (France)
Fax: Int. code + 16933 3990
e-mail : [email protected]
Prof. Dr. M. Regitz, Dip].-Chem. M. Schroder
Fachbereich Chemie der Universitat
Erwin-Schrodinger Strasse, D-67663 Kaiserslautern (Germany)
2364
0 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1997
Figure 1. Crystal structure of 2. Phenyl groups have been omitted for clarity. Important distances [A] and angles [“I: Pd-CI 2.344(2), Pd-CI’ 2.346(2), Pd-P2
2.261(2), Pd-P2 2.264(2), Pd . . . P1 3.78; CI-Pd-CI‘ 89.0(1), CI-Pd-P2 89.18(6),
CI’-Pd-P2 89.16(6), P2-Pd-PT 93.2(1)
The structure shows a diagonal chelation of the palladium
atom. The metal resides in a square-planar coordination environment with a cis arrangement of the phosphorus and chlorine
atoms. The Pd . . .PI separations are relatively short (3.79 A).
The angle between the lone pair at Pl(P1) and the Pl(P1)-Pd
bond is calculated to lie around 72”. Thus, some through-space
interaction probably exists between the noncoordinated phosphorus atoms and palladium. The 31P NMR spectrum reveals
the presence of two nonequivalent P atoms that show a large
coupling (2J(P,P) = 79 Hz). The analogous platinum complex 3
is similar (’J(P,P) = 65 Hz), but additional Pt-P couplings are
seen (‘J(Pt,P) = 3149 Hz). A simulation also indicates some
coupling between the Pt and the noncoordinated P atoms
(J(Pt,P)z 127 Hz).
The structural characteristics of the Pd” complex 2 suggested
that some stabilization of a [Pdo(l)] intermediate could be
achieved because of the cradle conformation of 1in the complex
and because of the through-space interactions of the noncom-
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Angew. Chem. Int. Ed. Engl. 1997,36, No. 21
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plexed lone pairs ofelectrons of 1with the 14-electron palladium
center. To test this hypothesis, we decided to compare the performance of 1 against the well-studied trifurylphosphane ligand
Fu,P in the palladium-catalyzed Stille cross-coupling reaction.['.*] Two types of catalytic systems were used for the comparison. The first, classical one used a mixture of one equivalent
of Pd(OAc), with four P donors (corresponding to one molar
equivalent for 1 ) . In the second system, two equivalents of CuI
were added.r81The lifetime of the catalyst was evaluated by
running the coupling of PhI with CH,=CHSnBu, until the reaction stopped, then recharging the used catalyst with further
batches of the same reagents. This process was repeated several
times. The results are collected in Table 1. An inspection of these
Table 1. Stille coupling 01' iodobenzene with tributylvinyltin.
Entry
1
2
3
4
5a
5b
6
7
Phosphane Catalyst (mol%)
T["C]
t[h]
Fu,P
50
50
80-90
80-90
80-90
80-90
80-90
60
60
60
60
60
60
60
3
3
3
3
3
3
3
1
Fu,P
1
Fu,P
Fu,P
1
Fu,P
Fu,P
8a
1
1
1
8b
8c
1
~~
1
~~
~
Pd (5)
Pd (5)
Pd(l)
Pd(1)
Pd/Cu (1)
Pd/Cu(l)
Pd/Cu (1)
Pd/Cu(0.25)
Pd/Cu(0.25)
PdpZu(0.25)
PdiCu(0 25)
PdiCu(0.25)
PdiCu(0.25)
Pd/Cu(0.25)
5
18
5
20
40
24
24
Conversion[%][a]
100
85
100 [b]
95
100 [b]
66
100
85
85
50
90
100
94
95
~-~
[a] Determined by gas chromatography. [b] Pd precipitate observed.
data clearly shows that the macrocycle 1 gives a slightly less
active catalyst than with Fu,P (compare entries 1 and 2 as well
as 7 and 8a), which is consistent with the suspected higher
stability of the [Pd(l)] system. In contrast, the lifetime of the
[Pd(l)] catalyst is much longer than that of the [Pd(Fu,P),]
species. Indeed. a precipitate is often observed at the end of the
catalytic cycle with Fu,P (entries 3 and 5a) and the activity of
the catalyst drops sharply with time (entry 7). Recycling the
catalyst is possible with 1, even at low concentration (entries
8 a-c) but this is not possible with Fu,P (entries 5 a, b). Consequentiy, the [Pd( l)] catalyst is much more cost-effective than
[PdFu,P),l.
Similar results to those for the Stille coupling have been obtained in the Pd-catalyzed allylation of the malonate anionr9]
(Table 2). In this case, the palladium complex of 1 is the best
catalyst, especially a t low concentration (entries 1 and 6). It is
noteworthy that diagonal complexation of two of the four lone
pairs of electrons of 1 by a [Mo(CO),]['] fragment drastically
decreases the efficiency of the catalyst (Table 2, entries 1 and 5 ) .
Table 2. Palladium-catalyzed reaction of PhCH=CHCH,OAc
(COOMej, [a].
with NaCH-
Entry
Phosphane
Pd[mol%]
I[h]
Conversion Is/.][b]
1
2
3
4
5
6
7
1
0.5
FuJ'
Ph,P
(Ph,PCH,),
1 Mo(CO),
I
(PhzPCHdz
0.5
0.5
0.5
05
0 05
0 05
3
3
3
3
3
48
48
95
60
10
98
72
100
93
[a] In THF at room temperature. [b] Traces of (PhCH=CHCH,),C(CO,Me), are
always formed.
AnZen. Chem. I n / . Ed Eng/.
1997,36,No. 21
This observation confirms the positive effect of the noncomplexed P atoms of 2 on the catalytic activity.
Finally, a comparison has been drawn with the palladacycles
proposed by Herrmann et
for the Heck reaction
(Scheme 2). After 48 h, we observed a 89 YOconversion; a 96 YO
II%Pd
~
CH3C02Na
DMA, 140T
Y
n
B
u
\ /
Scheme 2
conversion was achieved with the catalyst from Herrmann et
However, our catalyst was still active, since quantitative
conversion was observed after additional heating for 48 h. Thus,
the extraordinary resistance of our system toward degradation
is clearly established.
Experimental Section
2: 1 (0.39 g, 0.5 mmol) was dissolved under argon in CH,CI, (20 mLj and a solution
of [PdCI,(PhCN),] (0.20 g, 0.5 mmol) in CH,CI, ( 5 mL) was added dropwise. After
the mixture had been stirred at room temperature for 15 min, the solvent was
evaporated under vacuum, and the residue was extracted with hot C,H, (2 x 1 mL)
The extracts were cooled to room temperature and allowed to stand to give dark red
monocrystals of 2. Yield: 0.43 g (90%). "P NMR (CD,CI,). 6 = -11.50, 22.76
(AB system, 'J(A,B) =79.4 Hz); 'HNMR (CD,CI,): 6 = 1.82 (s, 6 H ; CH,), 1.94
(s, 6 H ; CH,), 2.07 (d, 4J(H,P) = 3.1 Hz, 6 H ; CH,), 2.11 (br., 4 H ; CH,), 2.19 (d,
4J(H,P) =1.4Hz, 6 H ; CH,), 6.60-7.90(m, 20H; Ph); "C N M R (CD,CI,):
6 =13.3, 15.52, 15.71, 17.64 (s; CH,), 21.97 (pseudo-t; CH,); elemental analysis
(%): calcd for 2-O.SC6H,: C 64.35, H 5.20, P 12.53, CI 10.76; found: C 64.47, H
5.35, P 13.20, CI 10.67.
3: 1 (0.039 g, 0.05 mmol) was dissolved under argon in CH,CI, (2 mL) and a solution of [PtCl,(cod)l (0.020 g, 0.05 mmol) in CH,CI, ( 5 mL) was added dropwise.
The mixture was heated for 15 min at 40 "C. After the solution had been allowed to
cool, it was concentrated under vacuum, and Et,O (5 mL) was added. The precipitate was washed with Et,O and dried under vacuum. Yield 0.040g (80%).
'IP NMR ([D,]DMSO): 6 = - 8.50, 9.17 (AB system, 'J(P,,Pt) = 3149 Hz,
'J(P,,Pt) =127Hz. 'J(P,,P,) = 650 Hz); 'HNMR ([D6]DMSO):d'=1.34(s, 6 H ;
CH,), 1.46 (s, 6 H ; CH,), 1.61 (s, 6 H ; CH,), 1.73 (s, 6 H ; CH,). 2.05 (d,
,J(H,P)=1.7 Hz, 4 H ; CH,), 6.11-7.40 (m. 20H; Ph); I3C NMR ([D,]DMSO):
6 =14.17. 15.84, 16.04, 16.34 (s; CH,), 20.0 (pseudo-t; CH,).
Stille coupling: An appropriate amount of catalyst was preformed under argon in
a dry schlenk flask equipped with a magnetic stirring bar in freshly degassed DMF
(3 mL) at room temperature. It was prepared from either Pd(OAc), (1 equiv) and
tri-2-furylphosphane (4 equiv) or from Pd(OAc), (1 equiv) and 1 (1 equiv) by stirring at 40°C for 15 min. Iodobenzene (0.112 mL, 1 mmol) was added to the preformed catalyst and, after a period of 15 min at room temperature, tributylvinyltin
(0.292 mL, 1 mmol) was introduced. Thereafter, in specific cases Cul(2 equtv) was
added. The flask was then immersed in a thermostated bath, and the evolution of
the crude reaction mixture was monitored by GC on a Varian 3400 apparatus
equipped with a WCOT (25 m, 0.25 mm) capillary column, CP-SIP-SCB stationary
phase between 40'C and 220°C.
Palladium catalyzed allylic substitution: A mineral oil dispersion of NaH (60%
NaH, 1.25 mmol) was washed free of oil with dry n-pentane (2 x 5 mL). The oil-free
NaH was suspended in T H F (4 mL), stirred, cooled to 0°C. and treated dropwise
with a solution of dimethyl malonate (1.2 equiv) After the reaction was complete,
the resulting solution of sodium dimethyl malonate was transferred by cannula
under argon into a 50 mL flask containing l-acetoxy-3-phenyl-2-propene
(1.2 mmol). and the catalyst, which had been previously prepared by mixing solutions containing [Pd(dba),] (1 equiv, dba = dibenzylidenacetone) i n THF ( I mL)
and the phosphane (4 equiv) in THF ( I mL,) and stirring under argon at room
temperature (in the cases of 1 and l.Mo(CO),, the catalyst was prepared as above
from tetraphosphole (1 equiv); the reaction was stirred for 15 min at 50 "C). The
reaction mixture was stirred at room temperature and the degree of conversion was
calculated from the crude reaction mixture by 'H NMR spectroscopy. The work up
involved dilution in glacial acetic acid, extraction with Et,O, and washing with
brine. Subsequent evaporation of the solvents gave the crude product as a yellow oil,
which was purified by chromatography on alumina by using hexane/ethyl acetate
(80120)
Heck coupling: Aryl bromide ( 5 mmol), N,N-dimethyl acetamide (DMA) ( 5 mL),
n-butyl acrylate (5.5 mmol), and anhydrous sodium acetate (0.5 g) were placed
8 WILEY-VCH Verlag GmbH, D-69451 Wetnheim, 1997
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under argon in a 50 mL double-necked flask equipped with a reflux condenser. The
flask was heated at 140°C and the solution of freshly prepared catalyst (1 equiv,
1 mL DMA) (see above), was added rapidly. The conversion of the aryl bromide
was monitored by G C on 50 fiL samples of the crude mixture dissolved in acetone
(2 mL).
r
1
Received: March 11, 1997
Revised version: June 23, 1997 [Z10299IE]
German version: Angew. Chem. 1997, 109, 2460-2462
Keywords: homogeneous catalysis
platinum
- P ligands - palladium -
[l] J. Tsuji, Palladium Reugenrs and Cafaiysts. Innovations in Organic Synfhesis,
Wiley, Chichester, 1995.
[2] A. de Meijere, F. E. Meyer, Angew. Chem. 1994, 106,2473; Angew. Chem. Int.
Ed. Engl. 1994, 33, 2379.
[3] J. K. Stille, Angew. Chem. 1986,98, 504; Angeu,. Chem. Int. Ed. Engl. 1986,25,
508.
[4] W A. Herrmann, C. Brossmer, K. Ofele, C.-P. Reisinger, T. Priermeier, M.
Beller, H. Fischer, Angew. Chem. 1995,107,1989; Angew. Chem. Int. Ed. Engl.
1995,34, 1844.
[5] F. Laporle, F. Mercier, L. Ricard, F. Mathey, J. Am. Chem. SOC.1994, 116,
3306.
[6] W Egan, R. Tang, G. Zon, K. Mislow, J. Am. Chem. Sac. 1971, 93, 6205.
Theoretical study: L. Nyulaszi, J. Phys. Chem. 1995, 99, 586.
[7] V. Farina, B. Krishnan, J. A m . Chem. Soc. 1991, 113, 9585.
[8] V. Farina, S. Kapadia, B. Krishnan, C. Wong, L. S . Liebeskind, J. Org. Chem.
1994,59, 5905.
[9] For a recent review of this type of reaction, see ref.1, pp. 290-422.
[lo] Crystallographic data (excluding structure factors) for the structures reported
in this paper have been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication no. CCDC-100247. Copies of the data
can be obtained free of charge on application to The Director, CCDC,
12 Union Road, Cambridge CB2 lEZ, UK (fax: int. code t(1223) 336-033;
e-mail: [email protected]).
B
Scheme 1. The working hypothesis.
afford the enantiomerically enriched ketone 2 with regeneration
of A. To realize this new concept, the chiral transition metal
catalyst should be capable of preferentially activating the enol
ether over the coexisting water and product ketone. Based on
these considerations, we selected chiral cationic Pd complexes as
the catalyst. Shibasaki et al. recently reported that the chiral
cationic Pd complex prepared from [PdCl,{(R)-binap)]
(binap = 2,2’-bis(diphenylphosphany1)-I,l’-binaphthyl)
and
AgOTf (1 : 1) in the presence of 4-A molecular sieves (MS) in wet
DMF serves as an efficient asymmetric catalyst for aldol reactions due to the intermediacy of a chiral Pd e n ~ l a t e . [ ~ I
Therefore, we focused on the two chiral cationic Pd species
generated from [PdCl,((R)-binap}] (3a) or [PdCl,{(R,R)-diop}]
(3b) and silver salts for the protonation of cyclic enol ether l a
with water (Scheme 2). Whereas the Pd species generated by
Asymmetric Catalytic Protonation of Silyl Enol
Ethers with Chiral Palladium Complexes**
Masaharu Sugiura and Takeshi Nakai*
The enantioselective protonation of prochiral enolates with a
chiral proton source has attracted much attention as a general
methodology for the asymmetric synthesis of a-substituted carbony1 compounds.“’ While most of the methods reported so far
require the use of more than a stoichiometric amount of a chiral
Brarnsted acid, the catalytic use of a chiral acid, combined with
a stoichiometric amount of an achiral proton source, has also
been successful.[’] Disclosed here is a conceptually new approach to the asymmetric catalytic protonation of enol silyl
ethers, which employs water as the proton source and a chiral
palladium complex as the catalyst.
Scheme 1 illustrates our working hypothesis.[31Chiral transition metal complex A, if coordinatively unsaturated, should activate silyl enol ether 1 and promote attack by water onto the
silyl group to afford chiral metal enolate B and a silanol. Enolate B would then be diastereoselectively protonated by water to
[*] Prof. T. Nakai, Dr. M. Sugiura
Department of Chemical Technology
Tokyo Institute of Technology
Meguro-ku, Tokyo 152 (Japan)
Fax: Int. code +(3)5734-2885
e-mail: [email protected]
[**I This work was supported by a JSPS Grant for the “Research for the Future”
Program.
2366
0 WILEY-VCH Verlag GmbH, D-69451 Weinhelm, 1997
(4-binap
(R, R)-diop
Scheme 2. Protonation of cyclic enol ether l a by water in the presence of chiral,
cationic Pd complexes generated from [PdCl,{(R)-binap)] (3a) or [PdCI,((R)-diop]]
(3b) and silver salts.
mixing 3a and two equivalents of AgPF, showed high catalytic activity but low enantioselectivity (Table 1, entry I), that
generated by the method of Shibasaki et
provided higher
enantiomeric excesses; however, the reproducibility was very
poor (entry 2). We therefore suspected that a contaminant plays
a vital role in this reaction. Significantly enough, when a
small amount of diisopropylamine-a possible contaminant of
la-was added to the catalyst solution, the enantioselectivity
remarkably improved to 79 % ee; the reproducibility was also
very good (entry 3).c51 Analogous use of the diop complex 3b
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