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Formation of a Hexagonal Columnar Mesophase by N-Acylated Poly(ethylenimine).

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-
and 70% yields. respectively. It should he noted that the salts of ( + )-la and
( - ) - l a crystallized alternately from the same solution! From these salts, ( + ) la, [a]:' = 62.4 (c =1.11 in CHCI,). m.p. 95--96C and ( - - ) - l a . [a]? =
( + ) - l a and ( - ) - l a , ' H N M R (300MHz, CDCI,. 20-C, TMS): d =7.41 (m. IH),
- 62.4 (c = 1.I 1 in CHCI,) were recovered quantitatively. Foi- the rcsolution of
7 23-7.39 (m. I H ) . 7.00&7.21 (m, 2H). 5.00 (hr. IH). 4.30 (m. 2H), 4.22 (dd.
Ib. (+-)-lbwas allowed to react with L-dihenroyltartaric anhydride 10 give a
J = 5 . 0 . 11.4Hz, I H ) . 3.67-3.75 (m, 2H). 3.00 (m.lH): "C NMR (75.4MHz.
diastereomeric mixture of amides. from which the (+)-amlde was separated by
CDCI,. 20 C): d =355.5. 131.3. 129.4. 121.6. 318.5. 117.3, 72.8. 65 2. 57.3. 40.7:
crystallization from ethyl acetate and hexdne. Simple alk;iline hydrolysis afHRMS (El): i i ~ ; : c;ilcd for C,,H,,NO, (M') 177.0790, found 177.0787.
forded ( - ) - l b , m.p. 85-86 C, [a];" = -11.1 (i,=1.14 i n CHCI,) in 80%
( + ) - l b and ( - b i b . ' H N M R (CDCI,). d =7.38 (m. 1H). 7.20 (m, lH), 6.93 (m,
2H).4.90(br.1H).4.50(d.J=6.6Hz,IH),4.20(dd,J=5,9Hz.IH),3.82(dd. overall yield. Optically pure ( + ) - l b , m.p. 85-86 C , [ x ] ~ "= + 11.1 ( < = 1.33 in
CHCI,) was obtained in 80% yield from the (+)-lb-rich mother liquor after
J = 7 . 8 , 1 2 Hz, I H ) . 2 . 5 2 ( m . 1H). 1.44(s. 3H). 1.29(s.3H); '3CNMR(CDCI,):
removing ( & ) - l b .m.p. 120 C. (20%) which first crystallircd out from ethcr.
(5 =155 1. 131.0. 129.2. 121 4, 119.2, 117.1. 84.8.63.5. 57.7.48.1. 28.6. 21.3; HRMS
17) The (+)-salt of l a with (+)-10-camphorsulfonic acid and 'in aldol adduct of
( E l ) : i i i : z calcd for C,,H,,NO, 205.1103, found 205.1099,
( +)-2b were used for X-ray crystallographic analysis. The ahwlute configura(-)-2a. [z];" = - 293 (c, = 1.40 in CHCI,) and ( + ) - 2 a , [aE0 = + 294 ( c = 1.06 in
tion of the diol derived from the aldol is knouzn. Detailed information will be
CHCI,). m.p 76. 77 C ; I R (Nujol): I,,,, = I660 cm- ; ' H NMR (CDCI,): 6 = 7.67
published in a full paper.
( m . I H ) . 7 . 1 8 ( m . IH).7.@@(m.lH).6.86(m,1H),5.44(d,J=8.1Ha.1H),4.30
[XI a ) F. M. Cordero. A. Brandi. F. DeSarlo. G. Viti. J Clietii So<.C h < w Cotm
(dd, J = 4 8 . 11 3 Hz. 1H). 4.00 (m. 2H). 3.86 (dd. J = 9, 11.3 Hr. I H ) . 3.15 (m.
niun. 1994. 1047: h) R. V. Hoffman, N. K . Nayyar, W. ('hen. J. Or,y. C h ~ ~ i i .
IH). 2.40-260 (m. 2H). 1.18 (I, J = 7 . 5 HL. 3H); "C NMR (CDCI,+):6 =177.5.
1993, 58, 2355.
154.9. 131.5, 128.9. 122.1, 121.8. 116.9.71.4.65.3, 52.3,40.4,26.1. 8.6:HRMS(EI)
[Y] D. A. Evans, R. L. Dow. T. L. Shih, J. M. Takacs, R. J Zkihler. J. A m Chon.
m z calcd for C,,H,,NO, 233.1052, found 233.1052.
SOC.1990, 113. 5290.
(-)-2b. [a];" = - 267 (L, = 1.09 in CHCI,) and ( + ) - 2 b ; [I];"= + 267 (c = 1.03 in
[lo] T. D. Penning. S. D. Djuric, R. A. Haiick. V. J. Kalish. M. Miyashiro. B. RowCHCI,); m.p. 73-74 C ; IR (Nujol): i',,, =1665, 1580 c m - ' ; ' H N M R (CDCI,):
ell. s. s. Yu. Synfh. Commun. 1990. 20, 307.
6 =7.84(m. lH).7.17(m,lH),6.97(m,1H),6.86(m,1H),5.42(d.J=7.5Hz,1H).
i i . 57. 1643.
I l l ] M. R . Hale, A. H. Hoveyda, .I Org. C h ~ ~ 1992,
4.27(dd.J=5,11.3H~,1H),3.81(t.J=10.8Hz,1H),2.40-2.70(m.3H).1.40(s.
[I21 S . A. Fiila. Ph. D. thesis. Massachusetts Institute of Technology. Cambridge.
3H). 1.27 (5. 3H). 1.18 (I.J = 7 . 2 Hz. 3H); l3C NMR (CDCI,): 6 =177.7. 154.6,
MA, USA. 1994.
132.1. 128.8. 122.0. 121.9, 116.6,84.3,64.6. 53.6,46.8, 26.6. 25.4. 19.7. 8.7;HRMS
[I31 a ) S. Terashima, S. Yamada. Chem. Phurni Bid/. 1968. 115,1953;h) L. E. Over(El) mi: calcd for C,,H,,NO, 261.1316, found 261.1316.
man, L. A. Robinson, J. Zahlocki, J. A m . C/ic,in.Soc. 1992. /14. 368. c) M. A.
Brimhle. Arist. J Clwm. 1990, 43, 1035; d ) P. A. Levene, R. E. Marker. J. Bid.
Chiwz. 1935, 110. 299.
[I41 a ) M. V. Rangaishenvi. B. Singaram. H. C. Brown. J, O r x . Cheni. 1991, 56,
3286: h) T. Hayashi. Y. Matsumoto. Y. Ito, Cham. Lerr. 1987. 2037.
Table 3 Selected phywal and spectroscoplc data for 1a.b. and 2a.b
+
'
(+)-2b
+
, n ~ & ~ ~
BnO&OTI
2) LiAIH4
l l a , 70 % (>98 %de)
10
(-)-2b
+
Formation of a Hexagonal Columnar
Mesophase by N-Acylated Poly(ethy1enimine)**
/
12
O x o
Hartmut Fischer, Surya S. Ghosh, Paul A. Heiney,
Nicholas C. Maliszewskyj, Thomas Plesnivy,
Helmut Ringsdorf,* and Markus Seitz
13,?3 % 0 9 6 %de)
14: R=CH,OH, 80 %
15: R=CHO, 85 %
16: R=COCH,CH,,70 %
Scheme 3. Severel examples to illustrate the use of isoxazolidine auxiliaries.
constitute important steps in our synthesis of polyketide natural
products.
Received' October 25. 1994 [Z7431 IE]
Germdn version Angeu Cheiii 1995. 107. 869
-
-
Keywords: alkylation asymmetric syntheses chiral auxiliary *
isoxazolidines
[l] D. A Evans, M. D. Ennis, D. J. Marthre. J. A m . C h e m Soc. 1982, 104, 1737.
121 W. Oppolrrr. R. Moretti. S. Thomi. Rtruhedron Lett. 1989, 311. 5603.
[3] See for example S. E. Drewes. D. G. S. Malissar, G. H. 0. Roos, C l i ~ mBer.
.
1993. 126. 2663. and references therein.
.
177.
[4] a ) M. Larchebeque. E. Ignatova. T. Luvigny, J. Orgunomer. C h ~ n 1979.
5 . b) A. (i Myers. B. H. Yang. H. Chen. J. L. Gleasen. J A m . Chwn. So[,.1994,
116. 9361
[ S ] Oppolzer ct al. described a synthesis of (+-)-la (W. Oppolzer, K . Keller. Tefruhivlron L i d i t . 1970, 1117). which has been much improved in the present study.
Chein. CornSee also B. S. Orlek. P. G. Sammes, D. J. Walker, J Chiwi. SCJC.
1111111. 1993. 1412.
[6] A solution of ( k )-la (1 5 g) and ( + )-10-camphorsulfonic acid (23 g) in acetone
(200 m L ) provided. upon six recrystallizations, (+)-salt. [z];" = 61.5
(<, =1.10 i n MeOH) and (-)-salt. [a];" = -13.6 ( c =1.15 in MeOH) in 8 0 %
+
The macroscopic properties of self-organizing systems are
strongly governed by the molecular structure of their constituent
units. Nevertheless, in many cases the same supramolecular
structures can be constructed with a wide variety of building
blocks. In particular, hexagonal columnar mesophases (&) are
formed by various molecular structures."] The constituent unit of
the liquid-crystalline phase may consist of relatively rigid macromolecules with flexible side chains, for example poly(dialky1silane) or substituted poly(glutamate)s.[2
In addition, diskshaped molecules exhibit predominantly this type of columnar
arrangement (discotic hexagonal columnar mesophases, Dh).[61
Macrocyclic oligoamines (azacrowns) can also act as such central
cores, provided that the conformational dynamics of the macrocycle is reduced by N-acylation with aromatic acidsL7- '] or by
complexation with transition metal ions."'. l 3 l In general, the
'
[*] Prof. H. Ringsdorf. T. Plesnivq, M. Seitz
Institut fur Organische Chemie der Universitit
J. J. Becher-Weg 38-22. D-55099Mainz (Germany)
Telefax: Int. code (6131)393145
+
[**I
Dr. H. Fischer
University of Technology. Polymer Chemistry & Technology
Eindhoven (The Netherlands)
S. S. Ghosh, Prof. P. A. Heiney, N. C . Maliszewskyj
Department of Physics and Laboratory for Research
on the Structure of Matter
University of Pennsylvania. Philadelphia. PA 19104 (USA)
Work done at the University of Pennsylvania was supported by National Science Foundation grants D M R MRL 92-20668 and DMR 93-15341
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stability of the columnar mesophase formed by these molecules
decreases with increasing ring size and thus increasing flexibility.
Recently even open chain N-acylated oligoamines of comparable structure were found to exhibit liquid-crystalline phases.[l4J
In this paper the scope of these systems containing N-acylated
ethylenimine as the basic -in principal nonmesogenic-structural unit will be extended to a linear polymer, which will be
compared to a macrocyclic and an open chain analogue (Fig. 1 ) .
R
bonding is no longer favorable. Therefore, the N-acylated linear
poly(ethy1enimine) 3 was not expected to exhibit mesomorphic
properties.
Nevertheless, an enantiotropic liquid-crystalline phase was
detected for the polymer 3. Slow cooling from the isotropic phase
into the mesophase gave a texture characteristic for hexagonal
mesophases with homeotropic regions. This texture is remarkably similar to those of the low molecular weight compounds 1
and 2. In addition, preliminary studies on contact preparations
show miscibility of 2 and 3 over the whole concentration range
within the LC state, which indicates that both compounds have
the same mesophase structure.
WT'n
,c=0
.R
R
2
1
c I08 D, I54 i
c 99 (9,93) i
3
g 61 Qh 120 i
OClOHZI
R =
+&,
Fig. I . Structure and phase behavior of 1,2. and 3. The transition temperatures are
given in "C (c: crystalline: D,: discotic hexagonal columnar; i: isotropic; g: glassy:
'ph: hexagonal columnar: phase transition temperatures from the first cooling and
second heating curves of the DSC measurements, scan rate 10 K min-').
For the macrocycle 1 the hexagonal columnar structure of the
mesophase, which is still a matter of controversy for the cyclic,
'
unambiguously
low molecular weight compounds,['4- 'I is
demonstrated by the hexagonal X-ray diffraction pattern
(Fig. 2).['*l
Fig. 3 . Texture of the N-acylated linear poly(ethy1enimine) 3 under the polarizing
microscope a t 116 'C, obtained after cooling from the isotropic phase to the given
temperature (cooling rate: 0.2 K m i n - I ) .
In accord with results from polarizing microscopy, the DSC
heating trace (Fig. 4) shows a peak at 120 "C corresponding to the
transition from a liquid-crystalline to an isotropic phase ( A H + - ,
= 1.2 kJ mol-' per repeating unit). In addition, one glass transition i s found at 61 "C and another transition at about 80°C.L'g1
Fig. 2 . X-ray picture (flat-plate camera) of a magnetically aligned monodomain of
macrocycle 1 at 150'C (cf. [ 1 8 ] ) . The low-angle region is shown magnified in the
right picture.
In the oligoamide 2, the open-chain analogue of 1, the intrinsic
discoid structure is lost. Nevertheless, a monotropic mesophase
of presumably hexagonal columnar structure can be found as
indicated by polarization microscopy and differential scanning
calorimetry (DSC) . This mesophase formation can, in principle,
be explained by intramolecular hydrogen bonds between the
terminal amide units which provide a disklike molecular conformation in the open-chain compound 2, too.[141The higher conformational flexibility of this system relative to that of macrocycle 1 is reflected by the monotropic nature of the mesophase.
The formation of hexagonal columnar mesophases by N-acylated oligoamines is therefore not restricted to macrocyclic, and
thus intrinsic discoid, structures. In longer open-chain polymeric amides a discoid conformation by intramolecular hydrogen
796
Q VCH Verlugsgesellschujt mbH, 0-69451 Wemhaitn, 1995
Fig. 4. DSC thermogram of the N-acylated poly(ethy1enimine) 3; a) first cooling h)
second heating trace; scan rate: 10 Kmin-I. As a guide for the eye, the second
transition a t about 80°C is emphasized by two straight lines 1191.
The X-ray diffraction pattern of the polymer 3 at 85°C
(Fig. 5) shows three reflections in the small-angle region with
1/2 characteristic of a
lattice spacings in the ratio of 1 :
hexagonal columnar arrangement. The hexagonal lattice parameter was determined to be 35.6 k . In comparison, for the macrocyclic compound 1 the intercolumnar distance a t 120 "C is 33.2 A.
057U-O833/95107U7-~79(,
$ lO.OO+ .25,'0
I/D:
Anymt,. Chem. Inl. Ed. Engl. 1995, 34, No. 7
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I
7 IIT
I 110'
Ct'
/I
0 35
01
04
045
I
Finally, as the constituent unit is identical in the polymeric
and the low molecular weight compounds, the described systems
bridge the gap between classical discotic mesophases (that is.
hexagonal columnar mesophases formed by disk-shaped molecules) and thermotropic liquid-crystalline columnar phases
formed by linear polymers.
Experimental Procedure
The synthesis of the compound 1 (2) was performed by acylation ofthe commercial10-tetraazacyclododecane (triethylenetetramine tetrahydrochloride) with 3.4-bisdecyloxybenzoyl chloride according to known proccdures 18. 91.
II
"
0
'
~
"
0.4
"
"
"
"
0.8
Q/A'
'
"
'
'
I .2
I .h
D
Fig. 5. X-ray dill'raction pattern of 3 obtained at 85 'C after cooling from the
isotropic state. Units are Q = 4nsinH:L = 2s/d. Inset shows scattering near the
(110) and (200) peak?.
All investigated systems- macrocycle 1 and open-chain
oligomer 2, as well as the linear polymer S e x h i b i t the same
supramolecular structure in the mesophase (Fig. 6). Whereas in
the azacrown derivative 1 the discoid geometry is given by the
2
Synthesis of 3: 3,4-bisdecanoxybenroyl chloride (4.52 g, 10 iiimol), linear poly(ethylenimine) (0.4 g. 9.3 mmol; prepared according to described procedures
[24, 251; degree of polymerization estimated to about 22) and .Y.h;-dimethylaminopyridine (1.3 g, 11 mmol. DMAP) were stirred for 3 days at 70 C in 150 mL of
absolute N.N-dimethylformamide ( D M F ) under an argon atmosphere. The white
precipitate was separated and dried for two days under high ~ a c u u mto remove the
solvent. The crude product was suspended in dichloromethane and washed three
times with water. After drying with anhydrous sodium sullate the solution uras
concentrated. and the polymer precipitated in cold acetone This procedure was
repeated (purity of the product checked by TLC). and the polymer finally recry%&
lized from acetone to yield 1.4 g of a white powder. (Additional product was obtained by concentration of the filtrate of the reaction mixture. However, the product
obtained in this way was rather impure and had to be extenaively purified.)
According to elemental analysis and ' H N M R spectroscopy, the polymer is almost
quantitatively acylated ( > 9 5 % ) . ' H N M R (200 MHz. CDCI,): 6 = 0.85 (t. 6H:
CH,(CH2),0). 1.1-1.4. 1.5-1.9(br., 32H; CH,(CH2),CH,0), 3.3 3.6(br.. 4H:
CH,-N-CH,). 3.8-4.0 (br., 4 H ; CH,(CH,),CH,O), 6.6 ~ 6 . 8(br., 4 H : aromatic
ring protons). Elemental analysis: calcd for (C,,H,,NO,), (corresponds to 100%
substitution; M(repeat unit) = 459.72): C 75.77. H 10.74. N j.05; found: C 75.40.
H 10.63, N 2.94. GPC (CHCI,; polystyrene standard): M , = 10500 (calculated for
P , , = 2 2 : M,,=10114).
Received: September 30, 1994 127368 IE]
German version: Angew (.hem 1995. 107. 879
Keywords: liquid crystals . poly(ethy1enimine)s . polymers
Fig. 6 E-oimatioii of hexagonal columnar phases by macrocyclic 1. open-chain
oligoineric 2. and polymeric ethylenimine derivatives 3. As a chiral center is lacking,
both left- and right-handed helices are formed by the polymer.
molecular structure, in the open-chain oligoamide 2 this geometry can perhaps be achieved by means of hydrogen bonding.
Conclusions about the molecular arrangement of the polymer 3
are speculative. However, based on the same mesophase behavior of the compounds under discussion, a possible molecular
structure of the polymer could consist of a helical polymeric
main chain (Fig. 6) radially surrounded by the side chains. This
view is supported by the larger intercolumnar distance of the
polymer 3 relative to that of the azacrown derivative 1; for an
extended polymer chain a smaller distance was expected.
Although the structure of 3 is not necessarily perfectly helical,
the system may resemble that of poly-L-glutamates with long
alkyl chain substituents, for which hexagonal columnar mesophases have indeed been observed.1201However, whereas in the
synthetic polypeptide the unsubstituted polymeric chain is helical.
in the substituted poly(ethy1enimine) 3 the formation of a helical
structure of the main chain may be solely induced by the steric
requirements of the 3,4-bisdecyloxybenzoyl substituents.1211
This
side group has thus an important structural influence on the
polymeric main chain, since the hexagonal columnar structure is
in striking contrast to the lamellar structures that have been observed in linear poly(ethy1enimines) acylated with benzoyl derivatives,['?. 231
[ I ] G. Ungar. Po/jiner. 1993. 34, 2050.
[2] A. J. Lovinger, F. C. Schilling. F. A. Bovey. J. M. Zeigler. Ma~rornol~,culr.\
1986. 19. 2660.
(31 P. Weber, D. Guillon, A. Skoulios. R. D. Miller, J. P/ir\. 1989, 50, 793.
(41 P. Weber. D. Guillon. A. Skoulios. R. D. Miller, Liq. CIT.SI.1990. 8. 825.
[5] Hexagonal columnar structures have even been observed lor polymers without
side branches. A comparative discussion of the formation of hexagonal phases
by polymeric systems is provided by Ungar in [l].
[6] S. Chandrasekhar, Liy. Crvsr. 1993, / 4 . 3.
[7] J.-M. Lehn. J. Malthete. A. M. Levelut. J. Cheni. Soc. ( h r r i i . C'oninirrii. 1985.
1794.
[8] C. Mertesdorf. H. Ringsdorf. Liq. C r w . 1989, 5. 1757
[Y] G. Lattermann, L f y . C r w . 1989. 6. 619.
[lo] D. Tatarsky. K. Banerjee. W. T. Ford, Clieni. Marer. 1990. 2, 138.
1111 C. Mertesdorf. H. Ringsdorf. Mulec. Eng. 1992, 2, 189.
[12] A. Liebrnann, C. Mertesdorf, T. Plesnivy. H. Ringsdorf, J. H. Wendorff,
Aiigew C/ien7. 1991, 103. 1358-1361; Angew. Clzen7. I I I I E d : Dig/ 1991. 30,
1375~-1377.
[13] G. Lattermann, S. Schmidt, R. Kleppinger. J. H Wendorff, Adv. Murw. 1992.
4. 30.
[I41 U. Stebani. G. Lattermann. M. Wittenberg. R. Festag. J. H. Wendorff. Ado.
M u t e r . 1994, 6. 572.
[I51 S. H. J. Idziak. N. C. Maliszewskyj. P. A. Hemey. J. P. McCauley. Jr.. P. A.
Sprengeler. A. B. Smith I l l , J. Am. Chein. So<. 1991, 1 / 3 , 7666.
1161 S. H. J. Idriak. N. C. Maliszewskyj, G. B. M. Vaughan. P. A. Heiney. C.
Mertesdorf. H. Ringsdorf, J. P. McCauley, Jr.. A. B. Smith H I . J. U i w . Soc.
Chem. Commun. 1992. 98.
1171 J. Malthete. A.-M. Levelut. J.-M. Lehn, J C/fmi.Soi. ( ' / i i w i . Conitinm 1992.
1434.
[I81 Aracrown 1 has a high tendency to align on glass surfhces. In addition. a
magnetic tield of about 0.8 T has been applied perpendicular to the glass
capillary. Goniometer scans confirm the hexagonal structure of the mesophase
by the observance of(100) and (1 10) reflections. the corresponding distances of
which are in the ratio of 1 : 11113. However, the ( I 10) reflection is weak and can
only be observed by prolonged measuring times.
1191 Based on preliminary dielectric spectroscopy measurements theglass transition
at 61 ' C corresponds to mobility of the side groups. More extensive DSC
studies on tempered samples showed that the second glass transition may arise
from the melting of crystalline regions.
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[20] J. Watanabe, Y. Takashina. Macroinokculca 1991. 24, 3423.
1211 This kind of shape-induced formation of supramolecular structures IS to some
extent comparable with the results of Percec et al. on polymers with triangular
structural unitscovalently attached to a polymeric chain which, with respect to
the formation of a columnar structure, resemble the Tabacco Mosaic Virus: V.
Percec, J. Heck, G. Johansson, D. Tomaros. M . Kawasumi, J M u c r o n d . Sci.
Puw Appl. Chrin. 1994. A31 (8). 1031.
[22] M. Litt, E Rahl. L. G. Roldan, J. J'o/rni. Sr,i. A-2 1969. 7, 463.
[23] For the sake ofcompleteness, i t should be noted that helical structures already
have been found in anhydrous linear poly(ethylen1mine). which according to
X-ray studies exists as double-stranded helical chains. The unsubstituted linear
polymer has hygroscopic properties and is readily transformed to the linear
structure by absorption of water: Y. Chatani, T. Kobataka, H . Tadokoro, R.
Tanaka. Macrori?oleculc.s 1982. 15. 170. As a reference system for the described
polydmide, however, the mentioned lamellar N-benroylated poly(ethy1enimine) [22] has to be considered.
(241 M. J. Han. J. Y. Chang. Y.Y Lee. Mat.tonio/tcu/es 1982. 15. 255.
[25] K. M. Kem. J. Po/),ni.Sci. P o / j m . Chrn?. 1979. 17, 1977.
Scheme 1. Chiral Lewis acids derived from square-planar Cu" complexes and their
associated coordination complexes with complementary one- and two-point binding dienophiles.
C,-Symmetric Cationic Copper(r1) Complexes
as Chiral Lewis Acids: Counterion Effects in the
Enantioselective Diels-Alder Reaction**
David A. Evans," Jerry A. Murry, Peter von M a t t ,
Roger D. Norcross, and S c o t t J. Miller
We have recently documented the utility of chiral bis(oxazoline)copper(n) complexes I (oxazoline = dihydrooxazole) in the
catalysis of asymmetric group transfer reactions such as cyclo41 In addition, we have
propanation[" *I and a~iridination.1~.
demonstrated that the C,-symmetric bis(oxazoline)copper(rI)
triflate complex 1 a (triflate = trifluoromethanesulfonate = OTf)
is also capable of functioning as an effective chiral Lewis acid in
the Diels-Alder reaction with two-point binding N-acylimide
In an effort to broaden the utility of Lewis acidic
dien0phi1es.I~~
chiral Cu" complexes, we have prepared Cu" complexes with
tridentate bis(oxazolinyl)pyridiner6] (pybox) ligands, [Cull(pybox)X,] (2) and probed their reactivity as chiral Lewis acids
(Scheme 1 ) . In conjunction with this investigation, we have uncovered dramatic counterion effects that strongly influence the
reactivity of these Lewis acids. In this communication, we document the scope of these catalysts in enantioselective Diels-Alder
reactions with unsaturated aldehyde and imide-derived dienophiles.
We began with an investigation of [Cu"(pybox)] complexes
which we anticipated would possess a preferred square-planar
coordination geometry with a single accessible coordination site
for carbonyl-derived dienophiles such as a,P-unsaturated aldehydes. Indeed, we have found that [Cu"(pybox)] complexes 2
serve as effective chiral Lewis acids with aldehyde-derived
dienophiles. For example, the cycloaddition of methacrolein
with cyclopentadiene catalyzed by (pybox)Cu(OTf), (2a)
( 5 mol%, CH,CI,, - 20 "C) afforded the desired cycloadducts
with good exo:endo selectivity (96: 4) and enantioselectivity
[*I
Prof. D. A. Evans, J. A. Murry, P. von Matt, R. D. Norcross. S. J. Miller
Department of Chemistry, Harvdrd University
Cambridge, MA 02138 (USA)
Telefax: Int. code (617)495-1460
Financial support was provided by the National Science Foundation and the
National Institutes of Health. Fellowships from the National Institutes of
Health (J.A.M.). Swiss National Science Foundation, and Ciba-Geigy Jubil:dumsstiftung(P. v. M.), NATO (R. D. N.), and the National Science Foundation (S.J.M.) are gratefully acknowledged. The NIH BRS Shared Instrument
Grant Program 1 S10 RROl748-01Al is acknowledged for providing N M R
facilities.
+
[**I
798
((-;#
VCH V r r l ~ i ~ . s ~ ~ ~ s ~ m
~ /h/ H
s r. h0-69451
uJt
Weinhciin. 199s
(exo adduct: 85 YOee).['] However, extended reaction times
(120 h) were required for complete conversion.
In an effort to increase catalyst reactivity, the cationic complexes 2b-2d were preparedr6]and evaluated. As illustrated in
Figure 1, a large counterion effect was observed in these reactions.
For example, the cycloaddition of methacrolein with cyclopen-
100
=
'
X =SbFs
tblFig. 1 . Plot of the conversion [YO]
as a function of the reaction time [h] for the
Diels-Alder reaction of methacrolein with cyclopentadiene catalyzed by 2 at
-2O'C.
tadiene that required 120 hours for complete conversion with
catalyst 2 a (X = OTf) was complete in 8 hours with catalyst 2d
(X = SbF,). This comparison reveals that the counterion structure dramatically affects catalyst efficiency and that the "noncoordinating" counterions SbF,, PF,, and BF,, strongly differ
in degree of interaction with the Lewis acidic Cu" center.[*'
From the results of the preceding study, the cationic
(pybox)Cu(SbF,), complex (2 d) was determined to be the catalyst of choice, and its optimized performance in the Diels-Alder
reactions of cyclopentadiene with representative aldehyde
dienophiles is given in Table 1 . The tert-butyl ligand 2 was originally chosen in analogy with the acrylimide results and provides
enantioselectivities comparable to those reported in the literat ~ r e .Importantly,
~~]
we have also found that the (S)-benzyl-pybox ligand, derived from L-phenylalanine, provides comparable
~~S?O-OK33~95~0707-(1?9X
S 10.00f .25/0
Angew. Chem. Int. Ed. EngI. 1995, 34. N o . 7
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