Improved synthesis and crystal structure of TiCl3(tmeda)(thf): A highly stereoselective pinacol coupling reagent for aromatic aldehydes

Article abstract of DOI:10.1246/cl.2000.334

A monomeric titanium(III) complex, TiCl₃(tmeda)(thf) (1), has been prepared by a reaction of TiCl₄ with TMEDA, Zn, and a catalytic amount of PbCl₂ in an almost quantitative yield. The solid state structure of 1 is revealed by X-ray crystallographic analysis. The complex 1 couples aromatic aldehydes under mild conditions to give 1,2-diols in good to excellent yields with high d/-selectivities.

Full text of DOI:10.1246/cl.2000.334

334
Chemistry Letters 2000
Improved Synthesis and Crystal Structure of TiCl3(tmeda)(thf):
A Highly Stereoselective Pinacol Coupling Reagent for Aromatic Aldehydes
Toshiyuki Oshiki, Takayuki Kiriyama, Kazuaki Tsuchida, and Kazuhiko Takai*
Department of Applied Chemistry, Faculty of Engineering, Okayama University, Tsushima, Okayama 700-8530
(Received December 22, 1999; CL-991088)
1.6 All attempts to prepare 1 by the ligand exchange reaction of
TiCl3(thf)3 with TMEDA under various conditions failed. The
proposed method for the titanium(III) complex has the advan-
tage of high yield and offers a simple procedure for preparation
of Ti(III)–TMEDA complexes.4
A monomeric titanium(III) complex, TiCl3(tmeda)(thf) (1),
has been prepared by a reaction of TiCl4 with TMEDA, Zn, and
a catalytic amount of PbCl2 in an almost quantitative yield.
The solid state structure of 1 is revealed by X-ray crystallo-
graphic analysis. The complex 1 couples aromatic aldehydes
under mild conditions to give 1,2-diols in good to excellent
yields with high dl-selectivities.
The complex 1 is a paramagnetic Ti(III) d1 complex which
is confirmed by ESR spectra (g = 1.9502). The IR spectral data
for 1 is identical to those in the literature.4 The UV/vis spec-
trum of 1 in THF showed two weak d-d transition bands at 659
nm (ε = 13) and 416 nm (ε = 11).7,8
Figure 1 shows the molecular structure of 1, which was
determined by X-ray crystallographic analysis.9 The titanium
center is in a slightly distorted octahedral environment;
TMEDA coordinated in a cis-fasion as a bidentate ligand and
three chloride ligands located at meridional positions. The THF
ligand coordinated at trans to one of the nitrogen atoms N(1)
of the TMEDA(∠O—Ti—N(2) = 172.1(1)°).
The pinacol coupling reaction mediated by early transition
metal reagents is one of the most effective methods for con-
structing a vicinal diol moiety. Among the various transition
metal reagents, low-valent titanium systems have been shown
to be effective and have the advantage of a highly diastereose-
lective formation of dl-diols.1,2 However, there are few reports
on the reaction using isolated low-valent titanium reagents, and
so it is difficult to ascertain the major active species.3
In our ongoing interest in the development of a pinacol
coupling reagent, we focused our attention on the preparation of
a low-valent titanium complex possessing a TMEDA (=
N,N,N',N'-tetramethylethylenediamine) as a ligand. The
TMEDA ligand allows electrons from the aliphatic bidentate
amine to be donated to the titanium center, thus enhancing the
reducing ability of the titanium complex. Moreover, by ligating
aliphatic amines, the solubility to common organic solvents will
increase, and then, the coupling reaction of carbonyl compounds
is expected to proceed in homogeneous phase. Gordon and
Wallbridge reported the preparation of TiCl3(tmeda)(thf) 1 by
the reduction of TiCl4 with LiBH4 in THF. However, the yield
was only 25% and its molecular structure was not given.4 We
report here an improved synthetic method for the titanium(III)-
TMEDA complex 1, and describe its molecular structure. In
addition, we reveal that the complex 1 couples aromatic aldehy-
des in high yields with excellent diastereoselectivity.
Treatment of TiCl4 with TMEDA in CH2Cl2-THF followed
by addition of Zn powder and a catalytic amount of PbCl2 at 25
°C afforded 1 in an almost quantitative yield based on TiCl4
(Eq. 1).5
The bond distances between Ti—Cl of 1 (2.355(2)-
2.369(1) Å) are longer than those observed in Ti(2,4-(SiMe3)2-
2,4-C2B4H4)(tmeda)Cl (2.307(2) Å)10 and are comparable with
that of Cp*TiCl (2.363(1) Å).11 The Ti—N distance of 1 is
shorter than those observed in Ti(II)–TMEDA complexes.12
These results correspond with the ionic radius of titanium
(Ti(IV) = 0.605 Å and Ti(II) = 0.86 Å).13 The bond angle of
N(1)—Ti—N(2) is very similar to those of the reported titani-
um–TMEDA complexes.10,12,14
The non-cyclopentadienyl complex 1 was found to be
effective for stereoselective pinacol coupling of aromatic alde-
hydes. The results are summarized in Table 1. Reaction of
benzaldehyde with 2 equiv. of 1 at 25 °C for 24 h afforded the
corresponding diol in 92% yield with almost complete diastere-
oselectivity (dl / meso = >99 / <1) (Entry 1).15 These high
yields and stereoselectivities are comparable to that for the pre-
Two equiv. of TMEDA were necessary for the formation
of 1; we could not obtain 1 when one equiv. of TMEDA was
used. A catalytic amount of PbCl2 accelerated the formation of
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
335
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3
4
5
D. Gordon and M. G. H. Wallbridge, Inog. Chim. Acta, 111, 77
(1986).
Preparation of 1: To a solution of TiCl4 (0.76 g, 4.0 mmol) in a
mixed solvent of CH2Cl2 (1.6 mL) and THF (6.5 mL) was gradu-
ally added TMEDA (1.2 mL, 8.0 mmol) at 0 °C. After stirring at
25 °C for 20 min, zinc powder (Merck, < 60 µm, assay min 95%)
(0.30 g, 4.5 mmol) and a catalytic amount of PbCl2 (6.5 mg,
0.023 mmol) were both added in one portion. The color of the
reaction mixture changed to dark blue, and the reaction mixture
was then additionally stirred for 30 min. The resulting suspen-
sion was centrifuged to remove inorganic materials. Hexane (6.0
mL) was added to the dark blue supernatant and stored at –20 °C
overnight. Complex 1 (1.36 g, 99% yield) was obtaind as blue
crystals: mp 129—133 °C (dec); ESR (solid, 25 °C): g = 1.9502.
IR (nujol/CsI): 861, 356, 328, 296 cm–1. UV/vis (THF): λ max
(ε), 659(13), 416(11) nm. Anal. Calcd for C10H24Cl3N2OTi: C,
35.05; H, 7.07; N, 8.18%. Found: C, 34.85; H, 7.07; N, 8.09%.
K. Takai, T. Kakiuchi, Y. Kataoka, and K. Utimoto, J. Org.
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6
7
viously reported titanium mediated intermolecular pinacol cou-
pling of aromatic aldehydes.2a–e,k,3b,c,16 For example, the yield
was 80% (Entry 4), compared to that for the TiCl3 in CH2Cl2
system.3b The reaction in toluene proceeded rapidly; the alde-
hyde was completely consumed within 1 h, although the yield
decreased to 83% (Entry 5). On the other hand, protic solvents
led to low stereoselectivities (Entries 6—8).
The coupling reaction of functionalized aromatic aldehydes
also proceeds in high yields with excellent diastereoselectivi-
ties. The corresponding diols were obtained without losses of
ether (Entries 9—11), halide (Entries 14 and 15), cyano (Entry
16), and ester groups (Entry 17) bound to the aromatic rings.
Although 1-dodecanal, an aliphatic aldehyde, was completely
consumed when reacted with 1 within 24 h at 25 °C, many
unidentified products were detected and no trace of the cou-
pling product was observed.
R. J. H. Clark, J. Lewis, D. J. Machin, and R. S. Nyholm, J.
Chem. Soc., 1963, 379.
8
9
B. J. Hathaway and D. G. Holah, J. Chem. Soc., 1965, 537.
Measurement was made on a Rigaku AFC5R diffractometer at
the X-ray Laboratory of Okayama University. Crystallographic
calculation was performed on SGI workstation at Venture
Business Laboratory, Graduate School of Okayama University.
Crystallographic Data for 1: C10H24Cl3N2OTi, fw = 342.57, mon-
oclinic, space group, P21/n (No. 14), a = 11.00(1) Å, b =
12.372(9) Å, c = 12.356(8) Å, β = 101.47(6)°, V = 1648(2) Å3, Z
= 4; dcalcd = 1.380 g cm–1, 3936 reflections, 2θmax = 55.0°,
2θ–ω scan, 25 °C), R1 = 0.124 (all data) and wR2 = 0.205 (all
data) for 154 variables.
10 S. Hosmane, Y. Wang, H. Zhang, K.-J. Lu, J. A. Maguire, T. G.
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15 Typical procedure is described for the coupling reaction of ben-
zaldehyde mediated by 1. To a solution of 1 (0.69 g, 2.0 mmol) in
THF (10 mL) was added dropwise a solution of benzaldehyde
(0.11 g, 1.0 mmol) in THF (1 mL). After being stirred at 25 °C for
24 h, the reaction mixture was quenched by adding dilute aq.
H2SO4. The organic layer was separated and the products were
extracted with ether. The crude products obtained were subjected to
column chromatography (ethyl acetate–hexane, 1 : 3) to give 1,2-
diphenyl-1,2-ethanediol (0.10 g, 92% yield, dl / meso = >99 / <1).
16 For some other transition metal mediated stereoselective coupling
of aromatic aldehydes, see: a) V: T. Hirao, B. Hatano, Y.
Imamoto, and A Ogawa, J. Org. Chem., 64, 7665 (1999). b) Nb:
J. Szymoniak, J. Besançon, and C. Moïse, Tetrahedron, 48, 3867
(1992).
The detailed pathway for the formation of 1 from TiCl4
and the mechanism of the pinacol coupling reaction mediated
by 1 is currently under investigation.
We thank Dr. Chiaki Ishii, Faculty of Engineering, Chiba
University, for measurement of the ESR spectra of 1.
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