The first optically active and sterically regular poly(1,1′-bi-2-naphthol)s: Precursors to a new generation of polymeric catalysts

Full text of DOI:10.1021/jo9608634

5200
J . Org. Chem. 1996, 61, 5200-5201
Sch em e 1. P r ep a r a tion of th e Ch ir a l
Th e F ir st Op tica lly Active a n d Ster ica lly
Regu la r P oly(1,1′-bi-2-n a p h th ol)s:
P r ecu r sor s to a New Gen er a tion of
P olym er ic Ca ta lysts
P olybin a p h th yl (R)-4
Qiao-Sheng Hu, Xiao-Fan Zheng, and Lin Pu*
Center for Main Group Chemistry and Department of
Chemistry, North Dakota State University,
Fargo, North Dakota 58105
Received May 13, 1996
The application of optically active 1,1′-bi-2-naphthols,
(R)-1 and (S)-1, in asymmetric organic reactions has
attracted very extensive attention.1 These molecules and
their derivatives have demonstrated excellent chiral
A chiral binaphthyl monomer (R)-2 is synthesized from
the reaction of 6,6′-dibromo-1,1′-bi-2-naphthol, (R)-3, with
chloromethyl methyl ether in the presence of sodium
hydride.7,8 The specific optical rotation of (R)-2 is [R]D )
+23.1° (c ) 1.0, THF). In the presence of a catalytic
amount of nickel(II) chloride and excess zinc, (R)-2 is
polymerized to generate an optically active polybinaph-
thyl (Scheme 1).9 The best polymerization condition we
have found so far is to heat a mixture of (R)-2, nickel(II)
chloride (10 mol %), zinc (3 equiv), triphenylphosphine
(40 mol %), and bipyridine (10 mol %) in DMF under
nitrogen at 85-90 °C for 24 h. After the insolubles are
filtered away, (R)-4 is purified by precipitation of its
methylene chloride solution with methanol several times.
Gel permeation chromatography (GPC) analysis of (R)-4
shows Mw ) 6000 and Mn ) 3800 (PDI ) 1.6).10 Its
specific optical rotation is [R]D ) -301.9° (c ) 1.0, THF).
(R)-4 is soluble in common organic solvents such as THF,
methylene chloride, and chloroform and has been char-
induction in a number of organic transformations when
used either as chiral auxiliaries or as chiral ligands.1-3
In our laboratory, we are interested in synthesizing chiral
conjugated polybinaphthyls4 and developing sterically
regular polymeric binaphthyl-based catalysts to carry out
organic reactions. Using polymer-supported catalysts in
industrial processes has several advantages, such as
durable catalytic activity and easy recovery of the
catalysts.5,6 Traditionally, polymeric chiral catalysts are
prepared by attaching chiral metal complexes to an
achiral and sterically irregular polymer backbone.5,6 In
these systems, the catalytic sites are randomly oriented
along the polymer chain, which makes it very difficult
to systematically modify the microenviroment of the
catalytic centers to optimize the reactivity and the
stereoselectivity of the catalysts. Herein, we report our
synthesis and characterization of the first optically active
and sterically regular poly(1,1′-bi-2-naphthol). We have
shown that this polymer can be used to prepare a new
generation of polymeric catalysts where the catalytic
centers are highly organized along the polymer chain.
This polymeric catalyst has exhibited greatly enhanced
catalytic activity over the corresponding monomeric cat-
alyst when used in the Mukaiyama aldol condensation.
1
acterized by spectroscopic methods including H and 13C
NMR.
We have prepared another chiral binaphthyl monomer
(R)-5 from the reaction of (R)-3 with acetic anhydride.
(R)-5 is polymerized in the presence of 10 mol % nickel-
(II) chloride and excess zinc to generate (R)-6 (Scheme
2). This polymer is also soluble in methylene chloride,
chloroform, and THF. GPC analysis of (R)-6 shows its
molecular weight Mw ) 6400 and Mn ) 3600 (PDI ) 1.8).
The specific optical rotation of (R)-6 is [R]D ) -353° (c )
0.5, THF). When a mixture of the THF solution of (R)-6
and aqueous potassium hydroxide is heated at reflux, the
ester functions of the polymer are readily hydrolyzed and
an optically active polybinaphthol (R)-7 is obtained. (R)-7
is found to be soluble in basic water solution and
insoluble in regular organic solvents. It is purified by
dissolution in aqueous potassium hydroxide solution
followed by precipitation with hydrogen chloride. The
specific optical rotation of (R)-7 is [R]D ) -139.8° (c )
0.5, 0.5 M aqueous KOH). (R)-7 gives a relatively well-
(1) (a) Rosini, C.; Franzini, L.; Raffaelli, A.; Salvadori, P. Synthesis
1992, 503. (b) Whitesell, J . K. Chem. Rev. 1989, 89, 1581. (c)
Bringmann, G.; Walter, R.; Weirich, R. Angew. Chem., Int. Ed. Engl.
1990, 29, 977.
(2) (a) Takaya, H.; Ohta, T.; Noyori, R. in Catalytic Asymmetric
Synthesis; Ojima, I., Ed.; VCH: New York, 1993; p 1. (b) Ishihara, K.;
Kurihara, H.; Yamamoto, H. J . Am. Chem. Soc. 1996, 118, 3049. (c)
Maruoka, K.; Itoh, T.; Shirasaka, T.; Yamamoto. J . Am. Chem. Soc.
1988, 110, 310.
(3) (a) Sasai, H.; Tokunaga, T.; Watanabe, S.; Suzuki, T.; Itoh, N.;
Shibasaki, M. J . Org. Chem. 1995, 60, 7388. (b) Terada, M.; Mikami,
K. J . Chem. Soc., Chem. Commun. 1995, 2391.
(4) (a) Hu, Q.-S.; Vitharana, D.; Liu, G.; J ain, V.; Wagaman, M. W.;
Zhang, L.; Lee, T.; Pu, L. Macromolecules 1996, 29, 1082. (b) Hu, Q.-
S.; Vitharana, D.; Liu, G.; J ain, V.; Pu, L. Macromolecules 1996, 29,
5075. (c) Ma, L.; Hu, Q.-S.; Musick, K.; Vitharana, D.; Wu, C.; Kwan,
C. M. S.; Pu, L. Macromolecules 1996, 29, 5083.
(5) Blossey, E. C.; Ford, W. T. in Comprehensive Polymer Science.
The Synthesis, Characterization, Reactions and Applications of Poly-
mers; Allen, G., Bevington, J . C., Eds.; Pergamon Press: New York,
1989; Vol. 6, p 81.
(6) Pittman, C. U., J r. In Comprehensive Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press:
Oxford, 1983; Vol. 8, p 553.
(7) (R)-3 is prepared from the bromination of (R)-1: Sogah, G. D.
Y.; Cram, D. J . J . Am. Chem. Soc. 1979, 101, 3035.
(8) An efficient resolution of 1,1′-bi-2-naphthol has been developed
in this laboratory: Hu, Q.-S.; Vitharana, D. R.; Pu, L. Tetrahedron:
Asymmetry 1995, 6, 2123.
(9) (a) Wang, Y.; Quirk, R. P. Macromolecules 1995, 28, 3495. (b)
Phillips, R. W.; Sheares, V. V.; Samulski, E. T.; DeSimone, J . M.
Macromolecules 1994, 27, 2354.
(10) THF was used as the eluting solvent in the GPC analysis of
the polymers in this paper, and polystyrene standards were used. A
laser light scattering study on another rigid and sterically regular
polybinaphthyl prepared in our laboratory shows that the actual
molecular weights of the binaphthyl-based polymers are about 1.4-
2.5 times higher than the data obtained from GPC analysis.4c
S0022-3263(96)00863-8 CCC: $12.00 © 1996 American Chemical Society

Communications
Sch em e 2. P r ep a r a tion of th e Ch ir a l
J . Org. Chem., Vol. 61, No. 16, 1996 5201
chloride. We have discovered that the resulting hetero-
geneous polymeric aluminum catalyst (R)-8 is an excel-
lent catalyst for the Mukaiyama aldol condensation of
benzaldehyde, 9, with 1-phenyl-1-[(trimethylsilyl)oxy]-
ethylene, 10.13 In the presence of 16 mol % (based on
the monomeric unit) of (R)-8, 11 is produced quantita-
tively from this reaction in less than 3.5 h at -78 °C.
However, under similar conditions, when 16 mol % of the
monomeric aluminum complex (R)-12, prepared from the
reaction of (R)-1 with diethylaluminum chloride (0.8
equiv) in methylene chloride solution, is used as the
catalyst, the rate of the Mukaiyama condensation is very
slow. After 3.5 h, only ca. 5% conversion is observed.
Thus, from the monomeric catalyst to the polymeric
catalyst, there is a dramatic increase of the catalytic
activity. In solution, (R)-12 may exist as oligomers
through the stable Al-O-Al bridging bonds to fill the
empty p orbitals of the aluminum(III) atoms, which
greatly reduces the catalytic activity of the complex.14
However, in the polymeric complex (R)-8, formation of
the Al-O-Al bridging bonds is very difficult, and the
vacant p orbitals of the aluminum atoms in the polymeric
complex will be available to activate the carbonyl group
of 9 in the condensation reaction, leading to the excellent
catalytic activity. Neither (R)-8 nor (R)-12 shows enan-
tioslectivity in this reaction. We are currently exploring
the reaction conditions and modifying the structure of
the polymeric catalyst, e.g., by introduction of substitu-
ents to the 3,3′-positions of the binaphthol units in the
polymer, in order to develop a class of enantioselctive
polymeric catalysts.
P olybin a p h th yl (R)-6 a n d th e Ch ir a l
P oly(1,1′-bi-2-n a p h th ol) (R)-7
Sch em e 3. P r ep a r a tion of a P olybin a p h th yl
Alu m in u m (III) Ca ta lyst (R)-8 a n d Its Ap p lica tion
in th e Mu k a iya m a Ald ol Con d en sa tion
In summary, the first optically active and sterically
regular poly(1,1′-bi-2-naphthol)s have been synthesized
through the hydrolysis of chiral polybinaphthyls that
contain ester or acetal functional groups. These chiral
polybinaphthols are soluble in basic water solution and
have been characterized by a number of spectroscopic
methods. The Lewis acid complex made from the poly-
binaphthol shows greatly enhanced catalytic activity over
the corresponding monomeric complex when applied in
the Mukaiyama aldol condensation. Because of the
stereoregularity of the main chain chiral polybinaphthol,
the catalytic centers in the polymeric catalyst are highly
organized along the polymer chain, which makes it
possible to systematically modify the microenvironment
of the catalysts and to tune their reactivity and stereo-
selectivity. Work along this line is in progress.
1
resolved H NMR spectrum, which is consistent with the
polymer structure. The UV spectrum of the polymer in
0.5 M aqueous KOH solution displays λmax ) 274, 340
nm. In the same solution, the circular dichroism spec-
trum of (R)-7 shows positive and negative Cotton ef-
Ack n ow led gm en t. Partial support for this work
from the donors of the Petroleum Research Fund,
administered by the American Chemical Society, is
gratefully acknowledged. We also thank Professor
Philip Boudjouk and Professor Mukund P. Sibi at NDSU
for their help.
fects: [θ]252 ) 2.45 × 103, [θ]276 ) -2.57 × 103, [θ]336
)
-1.30 × 103, and [θ]364 ) 9.06 × 103, [θ]377(sh) ) 6.62 ×
103.11 In the presence of hydrochloric acid, (R)-4 also
undergoes facial hydrolysis to generate the optically
active polybinaphthol.
Su p p or tin g In for m a tion Ava ila ble: Detailed experi-
mental procedures and characterizations involving (R)-2, (R)-
4, (R)-5, (R)-6, and (R)-7 (3 pages).
Treatment of the polybinaphthol (R)-7 with diethyl
aluminum chloride produces a novel polymeric Lewis acid
complex (R)-8 (Scheme 3).12 In (R)-8, the aluminum
centers are expected to be highly organized along the
rigid and sterically regular polymer chain. When the
reaction of (R)-7 with 0.8 equiv of diethylaluminum
chloride is carried out in deuterated methylene chloride,
J O9608634
(13) Selected examples on Lewis acid-catalyzed Mukaiyama aldol
condensations: (a) Reetz, M. T.; Kyung, S.-H.; Bolm, C.; Zierke, T.
Chem. Ind. 1986, 824. (b) Mikami, K.; Matsukawa, S. J . Am. Chem.
Soc. 1994, 116, 4077. (c) Furuka, K.; Maruyama, T.; Yamamoto, H. J .
Am. Chem. Soc. 1991, 113, 1041. (d) Parmee, E. R.; Hong, Y.; Tempkin,
O.; Masamune, S. Tetrahedron Lett. 1992, 33, 1729.
1
after 3 h, the H NMR spectrum of the reaction mixture
shows the complete consumption of diethyl aluminum
(14) Selected references on Al-O-Al bridging bonds in aluminum
alkoxide and aryloxide complexes: (a) Oliver, J . P.; Kumar, R.; Taghiof,
M. In Coordination Chemistry of Aluminum; Robinson, G. H., Ed.; VCH
Publishers: New York, 1993; p 167. (b) J effery, E. A.; Mole, T. Aust.
J . Chem. 1968, 21, 2683. (c) Pasynkiewicz, S.; Starowieyski, K. B.;
Skoweonska-Ptasinska, M. J . Organomet. Chem. 1973, 52, 269.
(11) The molar ellipiticity is calculated on the basis of the concentra-
tion of the monomer unit of (R)-7.
(12) Maruoka, K.; Itoh, T.; Shirasaka, T.; Yamamoto, H. J . Am.
Chem. Soc. 1988, 110, 310.