Structural studies of the low-valent titanium "Solution": What goes on in the pinacol coupling reaction? [3]

Full text of DOI:10.1021/ja001843t

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J. Am. Chem. Soc. 2001, 123, 1503-1504
1503
Scheme 1
Structural Studies of the Low-Valent Titanium
“Solution”: What Goes on in the Pinacol Coupling
Reaction?
Yuko Hashimoto, Utako Mizuno,† Hideki Matsuoka,†
Takafumi Miyahara,‡ Masahiro Takakura,‡
Mamoru Yoshimoto,‡ Koichiro Oshima, Kiitiro Utimoto, and
Seijiro Matsubara*,§
Department of Material Chemistry
and Department of Polymer Chemistry
Graduate School of Engineering, Kyoto UniVersity
Yoshida, Sakyo, Kyoto 606-8501, Japan
Materials and Structures Laboratory
Scheme 2
Tokyo Institute of Technology, 4259 Nagatsuta
Midori-ku, Yokohama 226-8503, Japan
The details of the SAXS apparatus are fully described
elsewhere.8 The sample solution was introduced into a glass
capillary (2 mmφ, Mark) and the top of the capillary was
completely sealed by silicon adhesive. These procedures were
performed in a glove box filled with Ar gas. Figure 1 shows the
SAXS curve thus obtained in double logarithmic scale. Obviously,
the scattering curve consisted of two components, which means
that there are two particles in the “homogeneous” solution. To
estimate the size, shape, and number of each particle component,
a fitting of the experimental curve by the calculated theoretical
curve was performed. We assumed that both of these particles
are spherical. Excellent agreement was obtained as shown in
Figure 1 (solid line) with R1 ) 7.7 Å, R2 ) 85.0 Å, A1 ) 9.6,
and A2 ) 23.0, where Ri is the sphere radius and Ai is the
amplitude of each component in the scattering curve. According
to the basic scattering theory, the scattering intensity is propor-
tional to (1) the square of electron density difference between
particle and solvent, (2) the square of particle volume, and (3)
the number of particles. Since it is fair to assume that both of
these particles have the same electron density, and we already
know the size of each spherical particle, we can calculate the
number of each particle by using Ai values. By simple calculation,
the number ratio of R1/R2 particles was found to be 561/1. An
apparent large contribution of the larger R2 particle to the SAXS
curve is simply due to the larger volume V2: We can find only
one large particle R2 in almost 550 smaller particles R1. Thus,
SAXS measurement clearly showed that there are two kinds of
particles in the solution, and gave us their sizes and numbers with
high accuracy.
ReceiVed May 26, 2000
Low-valent metal salts have been widely used for organic
syntheses.1 Their potential as electron donor and Lewis acid is
suitable for reactions using a carbonyl compound as a substrate:
i.e., Grignard, Barbier, Reformatsky, and pinacol coupling. The
low-valent metal salts are used either as a homogeneous solution
or heterogeneous dispersion depending on the solvent and ligand.
The homogeneous solution often promotes the efficiency of the
reaction, including control of the stereochemistry. For example,
in the low-valent titanium salt-mediated pinacol coupling reac-
tions,2 the homogeneous reaction gave higher diastereoselectivity.3
Recently, we reported an enantioselective pinacol coupling
reaction using titanium(II) chloride (TiCl2)-chiral amine (Scheme
1).4 In the procedure of this reaction, TiCl2 was dissolved into a
mixture of THF and amine and used for the reaction as a solution
that seemed to be homogeneous. However, a solution that is
judged to be homogeneous merely from its appearance cannot in
fact be identified as such.5 To control the reaction pathway, more
detailed information about the solution is necessary. Methods to
analyze the structure of the metal salt in a solution are limited.
EXAFS (Extended X-ray Absorption Fine Structure), which is a
common method, shows mainly the distance between atoms and
its coordination number.6 As we wished obtain more macroscopic
information regarding the metal salt (for example, the aggregation
situation and cluster size), we tried to apply SAXS (Small-Angle
X-ray Scattering) to the structural analysis of our titanium reagent
solution.
Furthermore, we tried to observe these particles contained in
the solution by the AFM (Atomic Force Microscopy) technique
using our ultraflat sapphire (single-crystal Al2O3) stages.9 For
AFM observation of molecules or clusters in air, a solution
containing molecules should be spread on a stage and dried.
During drying of the titanium salt-containing solution, TiCl2-
amine particles would be partly oxidized, aggregated, and adhered
to the flat sapphire stage. During the manipulation, the sizes of
the particles would be changed mainly by aggregation. So, the
following procedure was conducted to suppress the change of
the sizes: under Ar atmosphere, a solution of 1 and 4 in THF
was diluted 3000-fold with benzene, and spread on the ultraflat
sapphire stage. The dilution was expected to prevent cohesion of
salts. Figure 2 shows the AFM image (1 × 1 µm2) of the particles
deposited on the atomically flat sapphire terraces, indicating the
existence of two types of particles. In Figure 3, histograms of
the particles on the AFM image are shown. The height of the
bigger particle on the stage was measured to be 102 ( 13 Å and
the smaller one was 9 ( 2 Å high.
The low-valent titanium which we used in Scheme 1 was
titanium(II) chloride (1), prepared by treatment of titanium(IV)
chloride with hexamethyldisilane according to the reported
procedure (Scheme 2).7 The method affords titanium(II) chloride
and chlorotrimethylsilane, so titanium(II) chloride could be
isolated after the removal of volatile compounds. In our reaction
(Scheme 1), an addition of THF (5.0 mL) and amine (4) (4.0
mmol) to titanium(II) chloride (1) gave a solution that realized
the pinacol coupling of benzaldehyde in 40% ee. First, we measure
SAXS of this THF solution containing TiCl2 (1) and amine 4.
* To whom correspondence should be addressed. Fax: +81(75)7534863.
E-mail: matsubar@mc.kyoto-u.ac.jp.
† Department of Polymer Chemistry, Kyoto University.
‡ Tokyo Institute of Technology.
§ Department of Material Chemistry, Kyoto University.
(1) Rieke, R. D. CRC Critical ReV. Surf. Chem., 1991, 1, 131. ActiVe
Metals; Fu¨rstner, A., Ed.; VCH: Weinheim, 1996.
(2) Fu¨rstner, A.; Bogdanovic, B. Angew. Chem., Int. Ed. Engl. 1996, 35,
2442.
(3) Clerici, A.; Clerici, L.; Porta, O. Tetrahedron Lett. 1996, 37, 3035.
(4) Matsubara, S.; Hashimoto, Y.; Okano, T.; Utimoto, K. Chem. Lett. 1999,
1411.
(5) Aleandri, L. E.; Bogdanovic, B.; Gaidies, A.; Jones, D. J.; Liao, S.;
Michalowicz, A.; Rozie`re, J.; Schott, A. J. Organomet. Chem. 1993, 459, 87.
(6) Lin, Y.; Finke, R. G. Inorg. Chem. 1994, 33, 4891.
(7) Naula, S. P.; Sharma, H. K. Inorg. Synth. 1985, 24, 181,
(8) Ise, N.; Okubo, T.; Kunugi, S.; Matsuoka, H.; Yamamoto, K.; Ishii, Y.
J. Chem. Phys. 1984, 81, 3294.
(9) Yoshimoto, M.; Maeda, T.; Ohnishi, T.; Koinuma, H.; Ishiyama, O.;
Shinohara, M.; Kubo, M.; Miura, R.; Miyamoto, A. Appl. Phys. Lett. 1995,
67, 2615.
10.1021/ja001843t CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/27/2001

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1504 J. Am. Chem. Soc., Vol. 123, No. 7, 2001
Communications to the Editor
Figure 1. SAXS curve of a solution containing TiCl2 (2.0 mmol) and
amine 4 (4.0 mmol) in THF (5 mL). SAXS curve of a solution 1 (2.0
mmol) and 4 (4.0 mmol) in THF (5 mL). The ordinate in the relative
intensity of scattered X-rays and the abscissa is the scattering vector q
where q ) 4π sin(θ)/λ with λ the wavelength of X-ray and 2θ the
scattering angle. The dots are experimental data. Clearly, the curve
consisted of two components, which means that there are two kinds of
particles in the system. The dotted lines are the calculated scattering curves
for spheres by the equation I(q)){3(sinx - x cosx)/x3}2, where x ) qR,
R is the radius of sphere, R1 ) 7.7 Å and R2 ) 85.0 Å. The solid line is
the sum of these calculated scattering curves with the amplitude A1 )
9.6 for the R1 sphere and A2 ) 23.0 for the R2 sphere, i.e. I(q) )
9.6{3(sin(7.7q) - (7.7q) cos(7.7q))/(7.7q)3}2 + 23.0{3(sin(85.0q) -
(85.0q) cos(85.0q))/(85.0q)3}2. The whole scattering profile is excellently
reproduced by this composite curve, which means that there are two kinds
of spherical particles in the system in the “homogeneous” solution. The
experimental curve shown is the scattering only from solute: the solvent
scattering was subtracted from the solution scattering with the transmission
coefficient 0.8115, which was determined by the ratio of the direct beam
intensities from the solution and solvent.
Figure 3. Histograms of particles in the AFM image in Figure 2: (a)
the average of the height for 58 smaller paticles, 9 ( 2 Å, and (b) the
average of the height for 39 bigger paticles, 102 ( 13 Å.
Scheme 3
particle size of TiCl2 species in solution precisely owing to
oxidation and drying manipulation,12 the results after our high
dilution procedure showed the particle sizes in solution with
relatively high accuracy. Thus, it may be possible through AFM
measurements of the metal salt-containing solution to get some
supporting information for particles in solution on an atomic scale.
The large particle (R2) in the SAXS was considered to be a
cluster (5 in Scheme 3); the smaller one (R1) might be a
monomeric amine-titanium-THF complex (6 in Scheme 3)
considering the single-crystal X-ray data of the titaminum-amine
complex.13 The existence of two kinds of particles, which are
supposed to be a cluster (large) and a monatomic titanium-amine
complex (small), will cause a decrease in enantioselectivity.
Addition of another solvent will allow the collapse of the cluster.
Work toward improvement of the enantioselective pinacol
coupling is underway.14 The present structural studies of the active
metal solution by SAXS as well as AFM measurements will give
us useful information to show the actual situation in solution,
which will lead to the precise control of the pinacol coupling
reaction.
Figure 2. AFM image (1 × 1 µm2) of particles prepared from a solution
10
containing TiCl2 (2.0 mmol) and amine 4 (4.0 mmol) in THF.
Interestingly, two sizes of particles (the supposed radius: 4.5
and 51 Å)11 observed from the AFM image are comparable to
the values estimated from the SAXS measurements (R1 ) 7.7 Å
and R2 ) 85.0 Å). Although the AFM results do not show the
Acknowledgment. This work was supported by a Grant-in-Aid from
the Japanese Ministry of Education, Science, Sports, and Culture;
Materials and Structures Laboratory, Tokyo Institute of Technology,
Collaborative Research Projects.
JA001843T
(10) The AFM images were obtained in air, at room temperature using an
AFM (Seiko Instruments SPI 3700). The sapphire stage used in this
measurement has an ultra-smooth surface with atomic steps (2 Å in height)
and atomically flat terraces. The inset shows the height profile along the solid
line (180 nm long) in the AFM image. The contact mode AFM image was
taken using a 100 µm long microfabricated V-shape Si3N4 cantilever with a
spring constant of 0.09 N/m. The curvature radius of the AFM Si3N4 tip was
about 20 nm.
(12) The oxidation and drying procedure changes the size of the particle
more or less. Especially, the drying procedure is supposed to cause the
shrinking of the particles.
(13) Edema, J. J.; Duchateau, R.; Gambarotta, S.; Hynes, R.; Gale, E. Inorg.
Chem. 1991, 30, 154. Oshiki, T.; Kiriyama, T.; Tsuchida, K.; Takai, K. Chem.
Lett. 2000, 334.
(14) With the addition of tetrahydrothiophene to the reaction in Scheme 1,
the product was obtained in 58% ee: Hashimoto, Y.; Matsubara. S.
Unpublished result.
(11) The AFM data showed the heights of the adhered titanium complexes.
The radius was estimated simply by supposing those are spheres.