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An Yttrium-Based Strong Lewis Acid for the Heterogeneous Catalysis of the DielsЦAlder Reaction.

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(1 51 ;I) I< 11 Sh~iniim.,4</uC i ~ ~ i / / Scci.
~ g r A. 1976. 33.751: b) F. A . Cotton. G
W i l k i i i \ o r i ~ I d i r i i i c (Y/ 1 i i o i f i i i i ; i . C'liiwii.\~ri. Sthed.. Wiley Interscience. New
Yoi-k. 1988
1161 : I ) ] H Weiigi-owus. R . R Schrock. C . S. Day. IMJI:?.C h i ? .1981. .?I), 184: b)
t . A C ottcm. M . Shanp. W. A. WojtcLak. ;bid 1991. 30, 3670; c ) I:. A. Cott o n . M . P 1)iebold. P A . Kibalii. h i d . 1988.37. 799, d ) M . Y. Chiang. S. GamI b ~ i - , ~ ~ ~I:
: i 1.111
.
Bolhuis. O,:Smioi?i~,ln/li~,i
1988. 7. 1864; e) Y Wieistra. S. Gamhmitta. M . Y. C'hiang. i h i d 1988. 7. 1x66. f ) Y. Wielstra. S. Gambnrotta. A.
Mcct\iii:i. d. d e B o c r . i l d . 1989. X. 250: g) Y Wielstr;~.S. Gambai-otta. A . L.
Spch. h i d 1989, ,S. 2948. h) Y. Wielstra. S. Gambarottd, A. L. Spek. h! J. _I.
. h d 1990. 9 . 2142: i ) Y. Wielstra. S. Gambarotta. A. Meetsma. S
Kh.in. ihlcl 1990. 1). X70.
1171 . I ) M Beiiai-<I. M . M . Rohiner. ./. ,4fn Chriu. S w . 1992, //4.4785: b)
~ ~ r ~ ~ i i i ( J f ~ i [ 5, / [1991,
i / l i [ 111. 157. c ) R. L. DeKock. M . A. Teterson. L. E. L
Rc?nold\. I H C'hcii. J. Baerendc. P. Vernooi~a.rhirl. 1993. 12. 2794.
sition (determined by X-ray fluorescence) wiis found to be
82.6 mol % Zr, 15.6 mol YOY, and 1.8 mol '!o S. The physicochemical characterization of the catalyst was carried out by
X-ray powder diffraction, FTIR, potentiometric titrations, temperature-programmed desorption (TPD), scanning electron microscopy (SEM), and N, adsorption techniques. The X-ray
powder diffraction profile of the catalyst shows the formation of
a cubic phase (Fig. 1). The I R spectra of pyridine adsorbed on
the catalyst show absorption bands at 1640, 1605. 1577, 1542,
1490, and 1444cm-' (Fig. 2). The strong absorption bands at
An Yttrium-Based Strong Lewis Acid
for the Heterogeneous Catalysis
of the Diels- Alder Reaction**
Alive Keshavaraja, Vishnumurthy R. Hegde, Bipin
Pandey,* Arumugamangalam V. Ramaswamy,*
Pradeep Kumar, and T. Ravindranathan"
While yttrium has gained considerable prominence in superconductivity research in recent times,[" very little is known
about its application in organic chemistry.['] Indeed, recent efforts by Evans et al.'31to use YI, as a catalyst for stereoselective
reduction in the presence of a suitable auxiliary and by
Kobayashi et al.141to use Y(OTf), (Tf = trifluoromethanesultonyl) as a Diels- Alder catalyst were unsuccessful. Since
yttrium with its vacant d orbitals offers opportunities for forming strong Lewis acid sites in a heterogeneous catalyst, development of such a catalyst is highly desirable for organic synthetic
transformations. Here, we report on the synthesis and characterization of the first yttrium-based strong Lewis acid catalyst
and its applications mainly in Diels-Alder reactions and one
example of a hetero-Diels- Alder reaction. The acidity of the
catalyst has been demonstrated by FTIR s t ~ d i e s , ' ~potentio]
metric titration.["] and temperature-programmed desorption
(TPD)''] of ammonia. whereas its surface area has been determined by t h e N z adsorption (BET) method.[*]
The catalyst was prepared by treating a mixture of aqueous
solutions of yttrium nitrate and zirconyl nitrate (molar ratio
16: 84) with aqueous ammonia (28 YO)under vigorous stirring
until a pH value of 8.5 was achieved and a precipitate was
formed. This precipitate was washed with deionized water. dried
a t 1 1 0 C , treated with 2 N sulfuric acid, dried again at 120°C for
24 h. and subsequently was heated to 500°C at a programmed
heating rate o f 2 'C inin-' and then calcined for 3 h. This resulted in a highly acidic catalytic material, whose chemical compo[*] 111- A . V. Kam~isw:miy. A. Kcsha\vrap
C';i~aly\isI>ivision
N;itioniil C hcmical Laboratory
I'une 41 1 OOK (India)
I ; . I C ~ ~ I X l i l t code + (212) 3.10 233
Dr. B. P;inde>. Dr T. Ravindranathan. V. R . Hegde, Dr. P. Kumar
O r g m i c ( hemistry. Technology Division
N;itional C'hcmical Laboratory
[**I
NCL C'oiiiniunication No. 6049. A. K . and V. R. H. thank the Council of
S c i e i i l i l i c ;and Industrial Research. New Delhi, Government of India for Senior
K c h u r c h bcllownhipa. The authors arc thankful to Dr. S. G. Hegde for FTIR
\ ~ u d i c \and fruitful discussion.
28
-
Fig. I . X-ray powder diffraction pattern of the yttrium-based catalyst prior to (a)
and after (b)sulfation. The diffracto_grdm was recorded on a RiKaku diffractometer.
model D!M,ix. IIIVC. with Ni-filtered Ca,, radiation. I = intensity (arbitrary
units).
t
A
/
1
1800
1700
1600
1500
1400
c- ;[cm-'l
Fig. 2 . FTIR spectrum of pyridine adsorbed on the
recorded on a Nicolet 60 SXB FTIR spcctrometcr. .4
units).
) ttrium-based
=
catalyst.
,absorption (arbitrary
1605 and 1444cm-' indicate the presence of coordinated
pyridine at the Lewis acid sites of the catalyst. The weak absorption at 1542 cm-', attributed to the pyridinium
indicates
the presence of a few Bronsted acid sites. The potentiometric
titration of the acid sites with n-butylamine in nonaqueous
medium (Fig. 3) shows the influence of yttrium in enhancing the
number of acid sites. The amount of n-butylamine consumed
was 7.7 mol equiv g-' for the yttrium-based catalyst compared
to 5.8 mol equiv g - ' for the yttrium-free catalyst. The presence
of very strong acid sites in the catalyst is indicated by the peak
maxima at 530°C in the T P D profile (Fig. 4).l7] The scanning
electron micrograph of the sample shows the presence of uni-
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increasing the reactivity19]and altering the selectivity of DielsAlder reactions.["] Indeed, acceleration of the Diels- Alder reaction is one of the testing grounds for newly discovered Lewis
acids.["] One of the recent arrivals on the scene is LiClO, in
ether,['2] where the strong Lewis acidity of the lithium ion is
presumably responsible for the acceleration of the Diels- Alder
Since Kobayashi et al.L4I reported that Y(OTf), was
unable to accelerate the Diels- Alder reaction between cyclopentadiene 1 and 1,4-naphthoquinone 2 a (Scheme I), we ex-
-n
3b,n = 1, R =CH3
3c,n = 2,R = H
2b,R =CH,
Fig. 3. Potentiometric titration curves of the sulfated catalyst with yttrium (curve a)
and without yttrium (curve b) in CH,CN. For details see ref. [7]. n = number of
molar equivalents of n-butylamine per g.
6
7
9
8
4
10
4
11
12
13a,R=Et
13b,R=H
14a, R - E t
14b, R=H
15
16
Scheme 1. Formulae of the dienes, dienophiles, and Diels-Alder products from
Table 1.
-
T ~ C I
Fig. 4. TPD profile (ammonia) of the yttrium-based catalyst, recorded on a Sorbstar apparatus, Institute of Isotopes, Hungary, with He as the carrier gas, a flow rate
of 50 mLmin-', and a heating rate of 10 K min-' from room temperature to 625 "C.
NH, = amount of ammonia desorbed (arbitrary units).
form-sized (around 0.3 pm) particles (Fig. 5). The surface
area[*] of the sample determined by the BET method was
150 m2g-'. The lattice defects caused by the incorporation of
yttrium in the Zr4+ sites appear to enhance the number and
strength of the Lewis acid sites of the catalyst.
The Diels- Alder reaction and its variants like the intramolecular, hetero-, inverse-electron demand, and asymmetric DielsAlder reactions offer considerable opportunities to organic
chemists for the construction of complex structures. In last few
years there has been a tremendous upsurge in the interest in
amined the reaction of this system with our catalyst as a test
case. As shown in Table 1, stirring the reaction mixture (including our catalyst) at room temperature for 5 h gave exclusively
the endo (100% endo selectivity) product in 93 % yield (entry 1).
For comparison, the reaction without the catalyst under identiTable 1. Diels-Alder reactions [a] catalyzed by an yttrium-based Lewis acid
~~
Entry DieneDieno- Motdr Time Convel- Prophile
ratio
[h] [c] sion[%] duct
[bl
[dl
[el
1
2
3
4
5
6
7
8
9
10
j 1 [k]
Fig. 5. Scanning electron micrograph of the catalyst.
2144
;c) VCH Verlagsgeselhchaft mbH, 0-69451 Weinhem, 199.5
1
1
1
1
[i]
[j]
1
1
1
1
[I]
2a
2b
4
6
2a
2a
9
11
13a
13b
15
2.1
2.1
2.1
2.1
1.2:l
1.2.1
1.2:l
1O:l
2:l
2.1
1.2
5
10
6
8
20
48
8
10
5
20
20
100
100
100
90
85
50
100
45
100
-
85
Yield
[f]
[d
3a
93
3b
5
85
7[h]
8
3e
10
12
14a
14b
16[m]
Ratio
Ref
endo exo-
92
80
65
45
92
66
94
~
66
100:O
96:4
95:s
98:2
-
98:2
99:l
-
95:5
-
-
[41
[15]
[14]
(141
[41
[41
[12]
~ 7 1
[I21
~
1161
[a] Unless otherwise indicated, all reactions were performed in CH2C12(approx.
100 mg of dienophile in 10 mL of CH,Cl,) at room temperature under stirring. The
amount of catalyst taken was half the weight of the dienophile. [b] The molar ratio
of diene to dienophile. [c] Sampling time or duration in hours. [d] The YOconversion of the dienophile, based on recovered material. No destruction of dienophile
was observed. [el The structure of major isomer is given. All the products were
characterized by spectral and analytical data or by comparison with authentic
samples. [fl Yield [YO]of products after column chromatography based on the
recovered starting material. [g] Endo-em ratio was determined by G C by comparisonwithauthenticsamplesofbothisomers[10a-c]. [h] Seeref. [14]. [i] (E)-1,3-Pentadiene. [j] 1,3-cyclohexadiene. [k] The reaction was carried out without any solvent. [l] Acrolein. [m] Only one regio- and stereoisomer was obtained.
0570-0833/95/3419-2144$ lO.OO+ .2S/O
Angew. Chem. Inl. Ed. Engl. 1995, 34, No. f9
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cal conditions gave only 4 % of the endo product 3a. Under
usual thermal Diels- Alder reaction conditions, that is heating
to reflux in benzenc, an endo to exo product ratio of 95: 5 was
obtained.iis'The enhanced endo selectivity and the reaction rate
with our catalyst prompted us to examine the Diels- Alder reaction of I with 2-methylnaphthoquinone 2b. which is known to
be ;I troublesome
however. 2 b was converted quantitatively into 3b and isolated in 85% yield (entry 2). The reaction of I with other demanding and structurally different
dienophiles such as the spiro compounds 4 and 6 gave 5 (92%)
and 7 ( X O ' X ) , respectively (entries 3 and 4). Mention must be
made here that 6 reacts with 1 under a variety of Diels-Alder
conditions such as. heating to reflux in high boiling solvents.
neat heating. sealed tube heating, or ultra sound acceleration,
with or without various well-known Lewis acids like BF,.
TiCI,. etc.. to give 7 in not more than 10% yield.['41The generality of the D i d - - Alder catalysis with the yttrium-based reagent
has been demonstrated with other dienes and dienophiles (entries 5 9). However. our attempts to use our catalyst also for
carboxylic acids as dienophile, instead of the corresponding esters have not been successful (entries 9. 20). The yttrium-based
catalyht has also been found to be useful for hetero-Diels-Alder
reactions (entry 11 ).['"I
A control experiment has shown that for the reactions 6, 7,
and 1 1 without the catalyst, the reaction does not go at all under
identical conditions. Literature evidence is also available for the
failure of the Diels-Alder reactions of 2,Lisl 3.'l4] 4,r1416,[l4I
8.['-' and 1 I
without Lewis acid. The catalyst can be recovered Lifter the rcaction and can be reused; comparable results for
the Diels Alder reaction were obtained in second runs.
As discussed earlier based on the results from the TPD studies
and potentiometric titrations, the lattice defects introduced due
to yttrium increase the number and strength of the active Lewis
acidic sites. Furthermore the oxophilic nature of yttrium enhances the reactivity of oxygen-containing dienophiles, thereby
lowering the L U M O and decreasing the gap between the HOM O of diene and LUMO of dienophile.'i81Surprisingly, in our
reaction. only 20 mol YOexcess of diene is sufficient for the
efficient Diels Alder reaction. which is usually not the case.
Indeed. the diene dimerization of either 1 or 1,3-cyclohexadiene
was not observed with the catalyst.
I n conclusion, a yttrium-based catalyst has been prepared by
a novcl methodology and has been fully characterized by XRD,
SEM, and BET surface area studies. Its Lewis acidity has been
demonstrated by the FTIR spectrum of adsorbed pyridine. the
desorption of' ammonia. and potentiometric titration studies.
The general applicability of the new Lewis acid catalyst for
Diels . Alder reactions has been established with several models.
We are currently investigating its application as a strong Lewis
acid in other organic reactions.
E.ypritniwtd Procrriurr
In ;I typiwl cxpcrinient (Table I , entry I ) . cpclopentadicne I (0.198 g. 0.003mol).
1.4 n:iphthr~quinone(0.158 g. 0.001 mol) 2a. and catalyst (0.075 g. 50% by weight
with rcrpect to dienophile) in dichloromethane (10 mL) was stirred at room temperature lor 5 Ii The pi-ogress of the reaction was monitored by TLC. The catalyst was
lilteicd <)I".thc w h c i i t removed by rotary evaporation. and the residue chroin;itogl-;iphed ovc'i J silica gel colunin by eluling with 5% acetone:petroleum ether
to givc 0 20x3 9 of the mrlo iidduct 3a (93 Yo yield).
German version:
Received: April 29. 1995 [Z 7942 IE]
C / i ~ m1995,
.
107. 2333-2336
Ari,tyii,.
Keywords: Iliels Alder reactions . heterogeneous catalysis .
Lewis acids . yttrium compounds
[I] S. M . Zahurak. D Werder, Nururr, fLorrdorr1 1987. 32Y. 423.
[2] Recently selective oxidation or isobutane by Y,O,-CeFc, has been reported:
W. D. Zhang. D. L. Tang. X P. Zhou, H. L. Wan. K . R . Tsai, J. Chcr72. 'or..
Cheiri. Commurr. 1994, 771
[3] D. A. Evans. S.G . Nelson. M. R. Gagne. A. R . Muci. J l t i i . C ' h m .So<.. 1993.
f f S . 9800.
141 S. Kobayashi. 1 Hachiya. M . Araki. H. Ishitani. P r r d r i v / r o f i Lcrt. 1993. 34.
3755.
IS] N . Mizuno, H. Fuji. H. Igarashi. M. Misono, .I A m ('hcni. Soc. 1992. 114.
7151.
[6] R. Cid. G Pecchi. Appl. Crrrd. 1985, 14, I 5
[7] M. J. F. M. Verhaak. A. J. van Dillen. J W. Geus. App/. ('urn/. 1993. 105. 251.
[8] Determination of specific surface area was carried out h) BET ( B r u n n e r ~Emmett Teller) N 2 adsorption using a Omnisorp 100 CX apparatus.
(91 Review on the catalysis of Diels-Alder reactions. see U Pindur, G. Lutz. C.
Otto, Clzem Rcs. 1993. 93, 741.
[lo] a ) B. Pandey. P V. Dalvi, Aiigeu.. Chern. 1993. 105. 1724: A n g c w C/icwi. / f i r .
E d Engl. 1993.32. 1612: b)W. R. Roush. B. E. Brown. J. Urp. U i r w 1992.57,
3380: c ) B. Pandey, P. V. Dalvi, A. A. Athawale, B. G Pant. P P. Kewale. J.
Chciri. So<,.C'Ii~rn.C'omririm. 1990. 1505
[ l l ] See Section VI-B and X in ref. 191.
[12] P A. Grieco, J. J. Nunes. M. D. Gaul. J. A m C/ieni. S w 1990. 113. 4595.
1131 a ) A. Casasch. C;. Desimoni. G. Faita. A. G. Invernizn, S Laneti. P. P. Righetti, J. A m . C h n . Sol.. 1993.11.?,8002:b) R. M. Pagni, G W. Kabalka. S. Bains,
M. Presco. J. Wilson. J. Bartmess. J Org. Chcm. 1993. 58. 3130
1141 B. Pandey. C U. Dinesh. R . S. Reddy. V. R Hegde. unpublished results
[IS] V. K . Singh. B. N. Raju. P. T Deota. Srrirh. Coiirmun. 1988. 1K. 567.
[16] S. Danishersky. M Bednarski. Tifrciherlrorr L e / / . 1984. 3,721
[I71 E. C. Angell. F. Fringuelli. M. Gno. L. Minuti. A. Taticchi. E. Wenkert. J. Org.
( ' J i r w . 1988. 53. 4325.
1181 Lewis acids are known to lower the LUMO of dienophilea a ) K . N. Houk.
R. W. Strorier. J. A m . Cliern. Soc. 1973. 95. 4094; b ) D . M . Birney. K . N . Houk.
ibrd 1990. 112. 4127.
Enantioselective Reduction of Ketones
with Sodium Borohydride, Catalyzed by
Optically Active (B-Oxoaldiminato)cobalt(rI)
Complexes
Takushi Nagata, Kiyotaka Yorozu, Tohru Yamada,
and Teruaki Mukaiyama *
Dedicated to Profkssor h a r Ugi
on the occasion of'his 65th hirthduy
A wide variety of asymmetric reducing agents have been developed for the enantioselective reduction of prochiral ketones.['] An asymmetric reduction of prochiral ketones catalyzed by chiral oxazaborolidines is noteworthy in this context.[']
Numerous successful applications taking advantage of the catalytic reduction have been shown : for example, various optically
active secondary alcohols such as prostaglandin intermedia t e ~ . [a~g~o ]n i ~ t s , [potassium
~~'
channel blockers,["I and the like
were prepared by the enantioselective reduction with oxazaborolidine catalysts. On the contrary, few examples on the use
of sodium borohydride with chiral metal complex catalysts in
enantioselective reduction have been r e ~ 0 r t e d . I ~ .It was recently shown that optically active p-oxoaldimines form a new
class of effective ligand for aerobic enantioselective epoxidation
["I
Prof. Dr. T. Mukaiyama
Department or Applied Chemistry. Faculty of Science
Science University of Tokyo
Kaguraraka. Shinjukn-ku. Tokyo 162 (Japan)
T. Nagata. Dr. K. Yorozu. Dr. T. Yamada
Basic Research Laboratories for Organic Synthesis
Mitsui Petrochemical Industries
Nagaura. Sodegaura-shi. C'hiba 299-02 (Japan)
Telefax: Int. code + 438:6253YS
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