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Asymmetric DielsЧAlder Reactions Catalyzed by a Chiral Iron Lewis Acid.

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Asymmetric Diels-Alder Reactions Catalyzed
by a Chiral Iron Lewis Acid**
E r n s t Peter Kiindig,* Bernadette Bourdin, and
G e r a l d Bernardinelli
Very impressive results have recently been reported for enantioselective Diels-Alder reactions catalyzed by chiral Lewis
acids.['. 21 Main group elements dominate in this chemistry, and
aluminum- and boron-based catalysts with chiral ligands have
produced the best results to date. We report here on a highly
enantioselective D i e k A l d e r reaction catalyzed by a Lewis acid
derived from an easily prepared and well-defined transition
metal complex.
complexes apart, only three exTitaniumL2"]and
amples of asymmetric transition-metal catalysts for the DielsAlder reaction have been reported. An Fe"'-based Lewis acid,
containing a C,-chiral bis(oxazo1ine) ligand and generated in
situ, catalyzed the reaction between cyclopentadiene and 3-acryloyl-I ,3-oxazolidin-2-one to give the cycloadduct with 86 YOee
and 99 : 1 end0ie.w selectivity.[31 Lower enantioselectivities
( < 6 0 % ) were observed for the same reaction with another Fe"'
system with bis(su1foxide) l i g a n d ~ , [which
had also been generated in situ. The reactions between cyclopentadiene and the
dienophiles acrolein, methacrolein, and methyl methacrylate,
catalyzed by a chiral Lewis acid based on zirconium, also gave
only modest enantioselectivi tiesc5]
Achiral transition-metal catalysts for the Diels-Alder reaction were investigated by several groups.[6- *I The complex
[CpFe(CO),(thf)]BF, was one of a series of catalysts studied by
Hersh et al. They found that the Diels-Alder reaction between
acroleins and cyclopentadiene was accelerated in the presence of
the iron(I1) complex. The catalytic activity. however, was significantly greater than expected on the basis of stoichiometrically
determined rate constants.['"] This suggested that another catalyst. a Br~lnstedor Lewis acid impurity, was present and cast
some doubt on the role of the iron complex in this process.
According to these authors, the best evidence for involvement of
the iron center would be to demonstrate that a chiral analogue
is capable of asymmetric induction in this reaction. We here
provide this evidence.
The Lewis acidity of the d6 iron(11) center in the complex
described above arises as a result of the overall positive charge
of the complex and the presence of the two C O acceptor ligands.
This effect is, however, attenuated by the donor ligand Cp, and
[CpFe(CO)]'+ is a weak Lewis acid as a result. The most direct
way to asymmetrically modify this complex is by substituting
one or both carbonyl ligands by a chiral jigand. It is readily
apparent that Lewis acidity will only be preserved if the electronic characteristics of the new ligand resemble those of CO. In
the search for a catalyst with chirality at the metal center, Hossain et al. found that substitution of one of the two CO ligands
by PPh, completely suppressed activity in the Diels-Alder reaction. Some activity remained, however, with the less electronrich complex [CpFe(CO){P(OMe),}(thf)]BF,.[8h1
We recently reported the synthesis of both enantiomers of
several new C,-chiral phosphorus ligands that. to a degree.
Prof. Dr. E. P. Kundig. B. Bourdin
Departement de Chimie Organique. Unibcrsitk de Genebe
30 Quai Ernest Anserinct. CH-1211 Gent've (Swirzerland)
Telefax: Int. code (22)329-6102
Dr. G. Bernardinelli
Laboratoire de Crystallographic, Universite de Geneve (Switzerland)
We thank the Swisa National Science Foundation for support of this work
(FNS grant 30-32673.91)
emulate the bonding Characteristics of C0.[91We envisaged replacing the two cis carbonyl ligands of various transition-metal
catalysts with these new ligands and using the resulting complexes in asymmetric reactions. For the investigations reported
here, we chose to modify the Lewis acid [CpFe(CO),]+ with the
fluorophenyl ligand (R,R)-2.
Complex [CpFe(L)Me] (3) was readily prepared by photolytic
ligand substitution in 1 (Scheme 1). Treatment of a CH,CI,
hv, PhCH3,69%
CHaCN : 6
CH,=CH-CHO : 7
... be\
>90 %
Scheme 1. Synthesis of the cliiral Lewis acid 4 and i t s complexes with CO, acctonitrile. acrolein. and crotonaldehyde.
solution of 3 with HBF, at -78 "C presumably generated the
coordinatively unsaturated cationic intermediate 4. Reaction of
4 with CO in situ afforded the carbonyl complex 5,and reaction
with MeCN gave complex 6 . The X-ray structure analysis of 6
shows the spatial arrangement of the four bulky pentafluorophenyl groups in the complex (Fig.
Fig. 1. Crystal structure
N. % = O , @ =
or rdc-6. The
( R . R ) enantiomer
P. @ = F
The deep green solution of 4 in CH,CI, turned dark red when
acroleiii was added. Addition of hexane resulted in precipitation
of a brown solid 7, which was found to be the acrolein complex
with an t i 1 M-O=C interaction.["] Full characterization
proved difficult because of the instability of 7 in solution and its
high reactivity towards Lewis bases.
In the presence of 5 mol % of (R,R)-7and 2.5 mol% of 2,6-dirert-butylpyridine, inethacrolein reacted with cyclopentadiene
to give (1 S,2R,4S)-2-methylbicyclo[2.2.l]hept-5-ene-2-carbaldehyde (10) in 62 %i yield, 90 % ee. and with an e.xo/enrlo formyl
selectivity of 971'3. This and further examples of asymmetric
Table 1 Asymmetric Diels-Alder reactions catalyzed by (R.R)-7 [a].
e C H 0
T [ ,C]/l [h]
Product [b]
eso,,endo [c]
Yield [d]
ee [%I
84 [el ( R )[fl
e w : 64
94 [gl
[a] The i-c;ic~ionswere carried out in freshly distilled CH,CI, (1 M solution) with 5 m o l % of catalyst, 2.5 mol'X of 2.6-di-rrr.r-butylpyridine, and equimolar portions ofdiene
and dienophile (unless otherwise noted). [b] The structure shown is the major product. [c] The terms eso and endu refer to the position of the formyl group. [d] Isolated yield
after flash chromatography. [el Determined by GC analysis after conversion to the acetal derivative of (2R,4R)-pentanediol [14]. [fl The configuration was assigned by
conipariiig thc sign of [XI" with that in the literature: ( R ) - 9 [I a]. (R)-10 [l a.141. [g] Determined from the ' H N M R spectrum after conversion to the acetal derivative of
(2X.4R)-p~nt;inediol.[h] Determined from the ' H N M R spectrum in the presence of the chiral shift reagent Eu(hfc), . [i] Assignment after conversion to norhornen-'-one and
comparison of the sign of[^], with that in the literature [13 a.211. [j] Determined by GC analysis on a chiral column (MN FS-Lipodex E). [k] A single regioisomer was observed.
[l] 5 equii of diene wab used; yield with 1 equiv wap 28%. [in] wi&,-Formyl assignment is based on ' H N M R and GC comparison.
Diels-Alder reactions between r,B-enals and dienes are listed
in Table 1 . 2.6-Di-tert-butylpyridine was added to scavenge
residual acid impurities. In the absence of a scavenger. the rate
of reaction was found to vary; this is consistent with Hersh's
findings.["'] More importantly, enantioselectivities were lower
(20-60Y0 c ~ and
not reproducible. This behavior is expected if
a competing achiral catalyst is present."']
The best results were obtained with the very reactive
dienophile r-bromoacrolein. Corey et al. used this compound
for the synthesis of versatile cycloaddition products.[", 13] As
was observed with other Lewis acid catalysts, high e x ) selectivity was found for a-substituted enals. Yields were lower with
other dienophiles, but enantioselectivities remained quite high.
Thus, while the ee of the endo product 9 from the reaction with
acrolein matched the best literature result,"41 yield and exo/endo
selectivity were low. (E)-Crotonaldehyde did not react. Higher
reaction temperatures proved impracticable because of slow decomposition of the catalyst above -20°C. The yield for the
reaction of r-bromoacrolein (entry 8) with cyclohexadiene was
improved from 28 to 88 YOby increasing the proportion of diene
from one equivalent to five. It is noteworthy that a single enantiomer of the nzdo-formyl diastereomer was formed in this reaction is and that diastereoselectivity is good.
A transition state, based on the solid state structure of 6, that
accounts for the observed stereoselectivity is shown in Figure 2.
The diene approaches the olefin si face of the s-trans conformer
Fig. 2. Model of the transition state for the Diels-Alder reaction catalyzed by 7. A
partial structure of complex 6. the coordinated a-substituted enal. and the cyclopentadiene are shown.
of the coordinated enal. The re face is shielded by a pentafluorophenyl ring of the ligand (R,R)-2.
In conclusion, despite the limitations that can be attributed to
the weak Lewis acidity["] and low thermal stability of 7, these
first results are highly encouraging. They confirm the hypothesis
that chiral ligands that model the bonding properties of CO are
attractive for asymmetric catalysis. Further studies will focus on
understanding the transition state in these reactions and on
improving the catalytic system for the Diels-Alder reaction and
other Lewis acid catalyzed processes." 'I
E.xper irnent al Pro crdure
(R.R)-3.A NL-saturated solution of1 [I61 (0.291 g, 1.5 mmol) and ( R , R ) - 2(1.249 g.
1 .5 mmol) [9] in toluene (90 mL) was irradiated at 15 C with a 125 W high-pressure
mercury lamp equipped with a pqrex filter. A slow stream of N, was maintained
throughout the reaction, and progress was monitored by IR analysis (ICO) of
samplcs taken from the reaction. When it11 starting material had been consumed. the
mixture h a s filtered through Celite. and volatiles were removed under vacuum.
Recrystalliration from hexane at -20 C afforded ( R . R ) - 3(1.004 p. 69 "A) Enantiomeric purity w a s >Y9"% a s ineasured by HPLC (Daicel OD. eluent: hcrane:
~PrOH400:1).M.p.141 C(decomp.):[r];'= + 1 . 8 3 ( i = 0.2inCHC1,). ' H N M R
(400 MHz. C,D,, 21 C): d = 0.37 (hr. t. 'J(H.P) = 6.5 Hr, 3H: Me). 1 12- 1.76
im, 6 H ; CH,), 3.87-4.02 (in. 1 H: OCH). 3.97 (s. 5 H : C,H,), 4.21 4.35 (m, I H :
OCH); " P N M R (162 MHr. C,D,. 21 C. H,PO,)
5 -156.81 (AB(bd).
'J(P.P) = 8 2 H r : P , ) , I58.13(AB(bd),'J(P.P) = X2Hz:P2): '.'CNMR(100MHr.
CD,CI,, 21 C ) : 6 = - 25.3 (hr. t . 'J(C,P) = 28 Hr: CH,). 18.8 (CH,). 2X.X (d.
J(C.P) = 7 Hr: CH,). 2Y.2 (d. J ( C . P ) = 4 Hz, CH,). 80.0 (OCHj, 81.8 id.
J(C:P) = 5 H7: OCH). 84.9 (C,H5), 136.4- 137.5
C,,,,,,). 140.4-145 2 (m, C,,,,,,). 146 0 147.0 (m. C
Anal.. calcd for C,,H,,,F,,FeO,P, (966.28): C 43.50. H 1.67; found: C 43.36. H
(R.R)-7:To a stirred solution of(R.Rj-3 (0.581 g, 0.6 mmol) in CH,CI, (60 mL) at
- 78 C was added HBF, (90 pL of ii 54% solution in ether. 1 equiv). After 15 inin
at - 75 C. the green solution was treated with acrolem (210 pL. 5 equiv) and a n
immediate color change to red wiis observed. The reaction mixturc was allowed to
warm up to - 20 C over a period of 3.5 h. iind then concentrated to about 10 m L .
The product was precipitated by addition of hexane (20 mL). isolated. and thcn
washed several times with the Same solvent. A red-hrmn powder (R.R)-7 was
obtained in 90% yield (0.591 g). ' H N M R (400 MHr. CD,CI,. -20 C): 6 = 9.79
(br.s. 1 H ) . 7 . 0 9 - 6 . 8 3 ( b r . d . 1 H . J = 1 5 H z ) . 6 . 8 2 - 6 . 6 0 ( i n . lH).6.35--6.10(m,
I H). 4.47 (s. j H ) . 4.23-4.02 (m. I H ) , 3.90 3.70 (m,1 H ) . 2.15 1.25 im. 6 H ) :
" P N M R ( 1 6 2 M H r . CD,CI,, -20 C. H,PO,): f i = l 5 6 . l 5 (AB(hd). J = 7 9 H r .
P,), 159.55 (AB(hd). J =79 Hz. Pz).
Cycloaddition reactions. To freshly distilled CH,CI, ( 2 mL) was added at -40 C
(R.R)-7 (1 10 mg. 0.1 mmolj and 2.6-di-1~~rr-butylpyridiiie
(12 pL. 0.05 mmol). The
resulting dark red solution was then treated with methacrolein (170 pL. 2.0 nimol).
After the mixture had been stirred for 15 min. freshly distilled cyclopentadiene
(170 pL, 2 0 mmol) was slowly added ( 5 min). The reaction mixture was stirred at
-20 C for 20 h. The catalyst w'as precipitated by addition of hexane. and wished
three times with the same solvent. The combined organic phase was exposed to air
to remove traccs of residual catalyst by oxidation. Filtration through Celite. solvent
removal. and flash chromatography oii silica gel (eluent 1iexane:ether 10J) gave 10
in 62% (174 mg) with a n r i i h ' ~ ~iratio
of 3/97 (determined by GC before chromatography). The oc of c u - 1 0 was 90%.
Received. April 9, 1994 [26835IE]
German version: Aiigiw. C'heni. 1994. 106. 1931
[9] E. P. Kiindig. C. Dupre. B. Bourdin. A. Cunningham, Jr.. D. Pons. Hclr. C h i .
Actrr 1994, 77. 421.
[10] Crystallographic data for rac-6. Red crystals were grown from a CH,CI,
hexane solution. (C,,H,,,NOIP,F,,,Fc)(C~,H~~)(PF,).
M , = 1223.4: / I = 0.582
min-'. F(O00) = 2440.
monoclinic, P2,:n. Z = 4.
ir =11.893(5). /r=16.213(8). c = 23.402(8)A./~=91.09(1) , V=4512(3),&'.
from 25 reflections (14<20<26 1. Cell parameters and intensitie? were me3sured at rooiii temperature on a Nonius CAD4 diffractorneter hith graphitcmonochromated Mo,, radiation (j.= 0.71069
w - 2 0 scans. scan width
0.25 tgO. and scan speed 0.02-0.14 per s. Two referencc reflections
ineasured mery 100 retlections showed variation less than 3 2o(I).
11 </i<11. O<X<16. 0 < / < 2 3 : 4978 measured reflections, 4838 unique
reflections of ivhich 2862 wcrc observable (IFil >40(&l)). Data were corrected
for Lorenti and polariration effects but not for absorption. The structure w a y
solved by direct methods (MULTAN 87 [17]). all other calculations used
XTAL [I81 system and ORTEP [I91 programs. Atomic scattering factors and
iinornaloiis dispersion terms were taken from ref. [20]. Full-marrrx Icastsquares refinement b i d o n F with weighting 1 gave fins1 values
R = 1i.K = 0.010 for 656 variables. Hydrogen atoms
positions. Both the PF, and hexane molecules arc disordered. The fluorine
a t o m of PF; were found at 13 sites uith population parameters ranging
between 0.25 and 1 ; the hexane molecules were entirely disordered. The disoriduul R factors. Further details 01' the crystal
ilable on request from the Director of the Cambridge Crystallographic Data Centre, 12 Union Road. GB-Cambridge
CB21EZ ( U K ) . on quoting the full journal citation.
[ I l l Complex 7 is stable i n the solid state and can he handled in air. I t reacts with
CH,CN to give 6 quantitatively. In CH,CI,. 7 decomposes gradually above
-20 C. The ' H NMR spectrum showed 7 to he a 1 . 1 complex ofacrolein with
4. Although signals were broad. the pattern is consiatent with an M-O=C'
rr-hound acrolt'in. Thc IR spectrum of the more stahle crotonaldehyde complex 8 showed an absorption at 1604 cm- (in KBr). Compared to free (€)-crotonaldehyde. thc carhonyl hand in the complex is shifted by 88 em-' to lower
uavenumbers. This is consistent with the proposed mode ofcoordination oftlie
Lewis acid. For important references on Lewis acid-carhonyl complexation,
see a) Y.-H. Huang, J. A. Gladysz, J CI7wii. Ediu. 1988.65.298;h) S. Shambayat^, S. L. Schreiher i n Cbmpri.hen.~rwOrgmft. Si.n//resrs. KI/. I (Eds. B. M.
Trost. I . Fleming), Pergnmon. Oxford. 1991. pp. 283-324: c) S. E. Denmark.
N G. Almstead. J h i . ( ' h i w i . So(,. 1993. I l i , 3133. and references therein.
[I21 Control experiments with the chiral ligand only (no iron) in combination with
HBF, or BF, gave. at best, very ION yields of racemic Diels-Alder products.
1131 a ) E.J. Corey. T-P. Loh. 1 An7. Chcm. Soc. 1991. ll.?. 8966; h) E. J Corey. C.
L. C>win, J. Or,y. Clicwi. 1992. 57. 7372.
[I41 t i . Furuta. S. Shimiru. Y Miwii. H. Yamamoto. J. Or,p. Chmi. 1989. 54, 1481
[IS] Low Lewis acidity should be an asset in the hetero-Diels-Alder reaction and in
the Mukaiyama reaction. For report? on the catalysis of the hetcro-DidAlder reaction by [CpRu(PPh,),(C,H,)]'
and [CpRu((S,S)-chiraphos).
(C2HA)]'(25% c c ) . see J. W Faller, C. J. Smart, li,rrrr/ier6-ow Lerr. 1989. 30.
11x9, and o f t h e Mukaiyama aldol reaction by [CpFe(dppe)]+. see T. Bach. D.
N. A. Fox. M. T. Reetr. J. C/ieiii.Soc. Chem. Coinniuil. 1992,1634. For a review
of transition metal Lewis acids see W. Beck, K. Siinkel Chcm. Rcv 1988. 8 K .
[16] a j C. Roger, M.-J. Tudoret. V. Guerchais. C. Lapinte. J O r ~ u i i o n i e r .CIwni.
1989. 365.347. h) T. S. Piper. G . Wilkinson. .I. Iiiorz. N i d . Clirm. 1956.3, 104.
1171 P. Main. S.J. Fiske. S. E. Hull. L. Lessinger. G Gerinain, I-P. Declercq.
M. M. Woolfson. 4 Sjsfeni O / Cornpurer Progrnms for rhe Autoniaric Solurinn
of C r j s t d Srrrr( lures froni X-Ru? Diffruction D u m , University of York. England, and Louvain-la-Neuve. Belgium. 1987.
[18] XTAL3.2 C:.srr's.~~unrru/(Eds.:
S.R . Hall. H. D. Flack, J M. Stewart),Universities of Western Australia and Maryland. 1992.
1191 C. K. Johnson, O R T E P I I ; Rrporr ORNL-5138. Oak Ridge National Lahoratory, Oak Ridge. TN, 1976.
[20] Inri~riiationul7uhlr?/or .Y-ru) ( ' r ~ , r a / / r r R r f l p h j ,&J/ IV. Kynoch, Birmingham.
[21] L. A. Paquette. C. W Doeckc. F. R. Kearney, A. F. Drake. S.F. Mason. J. Am.
Clirm. Soc. 1980. 102. 7228.
[l] For the first successful asymmetric Leivis acid catalyzed Diels- Alder reaction,
see a) S. Hashimoto, N. Komeshima. K. Koga. J. Chen?.Soc. Chrin. Commim.
1979. 437: for recent reviews. see b) U . Pindur. G . Lutr. C. Otto. C h m . Rev.
1993. 93. 741. c ) H. B. Kagan. 0. Riant. Chrni. Rrw 1992, Y3. 1007: d ) K.
Narasaka. S ~ ~ I I / I 1991,
C . ~ I 1.
[2] a) See [Ic]. and references therein: b) J. M. Hawkins. S. Loren, M. Namhu. J
Am. Chem. Soc. 1994.116. 1657. c) K. Ishihara. H . Yamamoto, ihrd. 1994. 116.
1561: d ) J. Bao. W. D. Wulff. A. L. Rheingold. h i d . 1993. 115. 3814; e ) K.
Ishihara. 0 Gao. H. Yamamoto. ;hid. 1993, 115. 10412; I ) J. Org C h ~ m1993,
.TH. 6917: g) E. J. Corey, T. D. Roper. K. Ishihara. G. Sarakinos. X W n / i d r m
Lrrr. 1993. 34. 8399: h ) E. J. Corey. Z . Wang. ihid 1993.34.4001 ; I ) E J. Corey.
T.-P. Loh. ihid. 1993, 34. 3979: j) K . Tanaka. H. Uno. H. Osuga. H. Suzuki.
ZJrruiiedron' A.s.vmnierrj 1993. 4. 629; k) D. A. Evans, T. Lectka. S. J. Miller.
Z,rraher/ron L e r r . 1993, 34, 7027; I) D. A. Evans. S. J. Miller. T. Lectka. J. Ain.
Chein. S o i . 1993. 115. 6460; m j E J. Corey. T.-P. Loh. T. D. Roper. M. D.
Arimioara. M C . Noe, ihid 1992. 114, 8290; n ) E.1. Corey. S. Sarshar. J.
Bordner, rhid L992. 114. 7938. 01 K. Nawsaka. 1. Yaiiianioto, ~ ~ r r i r l ? ~ ~ ~ / r i i . o n
1992. 4K. 5743.
[3] E. J. Corey. N. Imai. H.-Y. Zhang. J hi. C/w7i Soc. 1991, 113, 728.
[4] N . Khiar. I. FernindeL. F. Alcudia. E,trcihedroii Lert. 1993. 34. 123.
[5] Y . Hong, B. A. Kuntz, S. Collins. Orguiionieru/lics 1993, 12, 964.
[h] a) P. V. Bonnesen. C. L. Puckett. R. V. Honeychuck. W. H. Hersh. .I A i i i .
Sot.. 1989. I l l , 6070: h) R. V. Honeychuck. P. V. Bonnesen. 1. Farahi.
W. H. Hersh. J. Org. C7iein. 1987, S 2 . 5293.
[7] a ) W. Odenkirk. A. L. Rheingold. B. Bosnich. J. h i . Cheiii. Soc. 1992. 114,
6392, h) T. K. Hollis, W. Odenkirk. N. P. Robinson. J. Wheban. B Bosnich,
Z,/rahcdron1993.49, 5415; c ) T K. Hollis. N. P. Robinson, B. Bosnich. J. A m
Clicm. So<. 1992, 114. 5464.
181 a ) A. K. Sahii. M . M. Hossain. ~ ~ r r n h r r l r oLi(i2 r t . 1993. 34. 3833, h) A. S. 01son. W. .I. Seirz. M. M. Hossain. ibrd 1991. 3.5299.
VCH Vc.rlu~.~,pr.scllse/iufi
nihH. 0-69451 Weinhim, fYY4
OS70-0X33:Y4!/818-IK58.R /(I.Ofl+ .25G
A n g e ~ t .Clieni. l i i r .
Ed. € i i ~ I1994,
33. NO 18
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acid, chiral, asymmetric, dielsчalder, reaction, iron, lewis, catalyzed
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