Pinacol coupling of aromatic aldehydes and ketones in ethyl acetate mediated by TiCl4-Mg

Article abstract of DOI:10.1080/00397910500522090

Titanium tetrachloride in ethyl acetate can be reduced by Mg powder to the corresponding low-valent titanium complexes, which can reduce some aromatic aldehydes and ketones to the corresponding pinacols in 38-85% yields within 15-60 min at rt with stirring. Copyright Taylor & Francis Group, LLC.

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Pinacol Coupling of Aromatic Aldehydes
and Ketones in Ethyl Acetate Mediated by
TiCl4‐Mg
Ke Wang a , Shu‐Xiang Wang a , Ming‐Zhu Gao b & Ji‐Tai Li a
a College of Chemistry and Environmental Science, Hebei University,
Baoding, Hebei Province, China
b Lucky Film Corporation, China
Version of record first published: 16 Aug 2006.
To cite this article: Ke Wang , Shu‐Xiang Wang , Ming‐Zhu Gao & Ji‐Tai Li (2006): Pinacol Coupling of Aromatic
Aldehydes and Ketones in Ethyl Acetate Mediated by TiCl4‐Mg, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 36:10, 1391-1399
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Synthetic Communicationsw, 36: 1391–1399, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500522090
Pinacol Coupling of Aromatic Aldehydes
and Ketones in Ethyl Acetate Mediated
by TiCl4-Mg
Ke Wang and Shu-Xiang Wang
College of Chemistry and Environmental Science, Hebei University,
Baoding, Hebei Province, China
Ming-Zhu Gao
Lucky Film Corporation, China
Ji-Tai Li
College of Chemistry and Environmental Science, Hebei University,
Baoding, Hebei Province, China
Abstract: Titanium tetrachloride in ethyl acetate can be reduced by Mg powder to
the corresponding low-valent titanium complexes, which can reduce some aromatic
aldehydes and ketones to the corresponding pinacols in 38–85% yields within
15–60 min at rt with stirring.
Keywords: Ethyl acetate, low-valent titanium, pinacol, pinacol coupling
INTRODUCTION
One of the significant units in nature is the pinacols, which occur widely in many
biologically active molecules, such as Taxol,[1a] (2)-grayanotoxin III,[1b] and so
on. Recently, pinacols have also been used as the chiral ligands, auxiliaries[2] as
well as versatile synthetic intermediates.[3] Pinacol coupling of a carbonyl
Received in Japan September 13, 2005
Address correspondence to Ji-Tai Li, College of Chemistry and Environmental
Science, Hebei University, Baoding 071002, Hebei Province, China. Fax: 86-32-
5079628; E-mail: orgsyn@mail.hbu.edu.cn
1391

1392
K. Wang et al.
compound is one of the most effective routes for the formation of C–C bonds and
the generation of vicinal diols in one step.[4] It has been employed in the key steps
of several total syntheses of natural products.[5] Recent efforts have focused on
the development of new reagents and reaction systems to improve the reactivity
of the reagents and diastereoselectivity of the products.
Since the first report in 1973 by Mukaiyama et al. of pinacol coupling
reactions mediated with a titanium reagent,[6] low valent titanium has
attracted increasing attention. In 1982, Clerici and Porta reported pinacol
coupling of aromatic aldehydes and ketones promoted by aqueous titanium
trichloride in basic media.[7] The reaction was completed in a few minutes,
but the reducing power of the Ti3þ/Ti4þ system is strongly pH dependent,
and the method has some limitations with respect to some aromatic
aldehydes and ketones. Clerici et al. again reported pinacolization of
aromatic aldehydes mediated by titanium trichloride in dichloromethane in
1996.[8] The reaction was completed in high dl-stereoselectivity, but
aromatic aldehydes bearing an electron-donating group showed lower reactiv-
ity. In 2000, Enders and Ullrich reported asymmetric pinacol coupling of
aromatic aldehydes under homogeneous conditions with TiCl2 in the
presence of enantiopure amines or hydrazines afforded 1,2-diols in
moderate to excellent yields with good dl-diastereoselectivities.[9] In 2001,
Yamamoto et al. reported diastereoselective pinacol coupling of aldehydes
promoted by monomeric titanocene(III) complex Cp2TiPh.[10] Five aromatic
aldehydes gave desired pinacol in 54–96% yields within 1–4 h. In 2004,
Matiushenkov et al.[11] reported the titanium(III) isopropoxide prepared by
the reaction of titanium(IV) isopropoxide with one equivalent of the
Grignard reagent transformed the aldehydes and the aromatic ketones into
the corresponding pinacols in good yields. However, in spite of their
potential utility, some of the reported methods suffer from drawbacks such
as longer reaction times, expensive catalysts, and harsh terms. Our laboratory
has also reported the pinacol coupling of aromatic aldehydes and ketones
using TiCl4–THF–Al, TiCl4–THF-Zn,[12] and TiCl4–THF–Mg[13] in
dichloromethane under ultrasound irradiation. For most of those methods,
dichloromethane or tetrahydrofuran was chosen as the solvent. However,
they are volatile and toxic.
Ethyl acetate is a widely applied reagent; it is cheap, less poisonous, and
easily obtained. Additionally, ethyl acetate has good solubility to TiCl4. So
far, we haven’t found any reports about the pinacol coupling of aromatic
aldehydes and ketones using ethyl acetate as solvent. Herein, we report the
Scheme 1.

Pinacol Coupling of Aromatic Aldehydes and Ketones
1393
result of the pinacol coupling of aromatic aldehydes and ketones mediated by
TiCl4–Mg using ethyl acetate as solvent at room temperature (Scheme 1).
RESULTS AND DISCUSSION
The effect of the reaction conditions on pinacolization is summarized in
Table 1. When the molar ratio of PhCHO–TiCl4–Mg was 1:2:4, the yield
of 1,2-diphenyl-1,2-ethanediol (2a) was 61%. By increasing the amount of
Mg powder and changing the molar ratio of PhCHO–TiCl4–Mg to 1:2:5
and 1:2:6, the yields of 2a increased to 71% and 75%. By changing the
amount of TiCl4 and increasing the molar ratio of PhCHO–TiCl4–Mg to
1:2.5:6 and 1:3:6, the yields of 2a increased to 84% and 85% respectively.
These results showed that increasing the amount of TiCl4 and Mg powder
had a significant effect on the yield of the 1,2-diol.
From these results the reaction conditions we chose were aldehyde
(1 mmol), TiCl4 (2.5 mmol), Mg (6 mmol), and MeCOOEt (5 mL). Using
this reaction system, we did a series of experiments for pinacol coupling of
aromatic aldehydes and ketones. The results are listed in Table 2.
The coupling of most aromatic aldehydes mediated by TiCl4–Mg in
MeCOOEt was carried out in good yields for a short time. For example,
using the present system under stirring at rt within 15–45 min, 2a, 2d, 2h,
2f, and 2j were obtained with 84%, 80%, 79%, 81% and 81% yields respect-
ively. Whereas 2a, 2d, and 2h were prepared in 71%, 58%, and 61% yields
respectively with TiCl4–Et3N in CH2Cl2 under stirring for 5 h,[14] 2f and 2j
were obtained with 75% and 65% yields respectively using titanium–Schiff
base complexes under stirring for 2 days.[15]
Li et al.[16] reported that aromatic aldehydes bearing an electron-donating
group and some ketones showed no reactivity in TiCl4–Mg–THF system.
For example, 4-methoxybezaldehyde (1j) and acetophenone (1 m) afforded
no pinacols monitored by TLC. However, by using TiCl4–Mg–MeCOOEt
system, 2j and 2 m were prepared in 81% and 71% yields.
Table 1. Effect of the reaction conditions on pinacolization of
benzaldehyde
Isolate yield, %
Molar ratio of
Entry
PhCHO–TiCl4–Mg
Time, min
2
3
1
2
3
4
5
1:2:4
1:2:5
1:2:6
1:2.5:6
1:3:6
20
20
20
20
20
61
71
75
84
85
2
3
2
2
2

1394
K. Wang et al.
Table 2. Pinacolization of aromatic aldehydes and ketones mediated by TiCl4–Mg–
MeCOOEt
Isolated
yield, %
Temperature, Time,
min
Entry
Substrate
C6H5CHO
8C
2
3
dl/mesoa
a
b
c
d
e
f
22
23
23
24
25
26
25
25
26
25
25
25
26
25
20
15
20
20
25
15
25
45
40
40
40
60
60
60
84
75
83
80
83
81
75
79
78
81
79
38
71
47
2
2
68/32
37/63
60/40
56/44
29/71
74/26
53/47
46/54
39/61
77/23
77/23
57/43
78/22
—
2-ClC6H4CHO
3-ClC6H4CHO
4-ClC6H4CHO
2,4-Cl2C6H3CHO
3-BrC6H4CHO
4-BrC6H4CHO
4-CH3C6H4CHO
2-CH3OC6H4CHO
4-CH3OC6H4CHO
3,4-(OCH2O)C6H3CHO
C6H5CH55CHCHO
PhCOCH3
1
1
2
2
g
h
i
2
3
4
j
2
k
l
3
6
m
n
trace
0
PhCOPh
1
aThe ratio of dl/meso is determined by H NMR.
Improved diastereoselectivity has been observed in this system. When 1a
is a substrate, the ratio of dl and meso of 2a is 3/2 in the TiCl4–Al system
using Et2O as solvent.[17] In the present system, the ratio of dl and meso of
2a is 68/32.
As shown in Table 2, benzaldehyde and the aromatic aldehydes with
electron-withdrawing substituents in the benzene ring (1b–1 g) had high
reactivity. Under stirring at rt, 1b–1 g afforded 2b–2 g within 15–25 min.
In contrast, the aromatic aldehydes with electron-donating substituents in
the benzene ring (1h–1 k) showed lower reactivity. Compounds 1 h–1 k
needed a little longer reaction time (40–45 min) to afford 2 h–2 k under
stirring at rt. Additionally, aromatic ketones showed lower reactivity than
aromatic aldehydes; the yields of 2 m and 2n were only 71% and 47%
under stirring at rt for 60 min. However, aliphatic aldehydes showed lower
reactivity; for example, isobutyraldehyde and phenylacetaldehyde afforded
no significant conversion via TLC observation.
In addition, the position of substituents in the benzene ring has some
effects on the ratio of dl and meso. When 1c, 1f, 1j, and 1 k are substrates,
the ratios of dl and meso of the 1, 2-diols are 60/40, 74/26, 77/23, and
77/23 respectively. However, when 1b, 1e, and 1i are substrates, the ratios
of dl and meso of the 1,2-diols are 37/63, 29/71, and 39/61 respectively.
Based on these experimental results together with corresponding
literatures,[18] a plausible single-electron transfer (SET) mechanism was

Pinacol Coupling of Aromatic Aldehydes and Ketones
1395
proposed as indicated in Scheme 2. The initial radical A was generated via
a SET process between low-valent titanium and ArCHO. Next, two possible
key coupling species B and C led to the formation of corresponding dl- and
meso-products. dl-Pinacols were preferentially formed by an internal
carbon–carbon coupling of “titanium-bridged” intermediate B whose
oxygen atoms of the two radicals were linked side by side to the low-valent
titanium species, and the groups of the two radicals were located anti each
other to minimize the steric interaction. However, groups with a lone pair
of electrons (like an alkoxyl group, chloride atom) in ortho- position could
coordinate with low-valent titanium and destroy the “titanium bridge.” In
addition, because of the sterically crowded environment, the formation of E
was more favorable than that of D (as shown in Scheme 3) and consequently
decreased the dl-selectivity.
CONCLUSION
In summary, we have found an efficient and convenient method for the
preparation of pinacols from some aromatic aldehydes and ketones using
TiCl4–Mg–MeCOOEt with stirring. The main advantage of the present
procedure is the milder reaction conditions, inexpensive catalyst, and
operational simplicity.
EXPERIMENTAL
Liquid aldehydes were distilled prior to use. IR spectra were recorded on a
Bio-Rad FTS-40 spectrometer (KBr). MS were determined on a VG-7070E
Scheme 2.

1396
K. Wang et al.
Scheme 3.
spectrometer (EI, 70 eV). 1H NMR spectra were measured on a Bruker Avance
400 (400 MHz) spectrometer using TMS as the internal standard and CDCl3 as
a solvent. Elemental analysis was measured on a Heraeus (CHN, Rapid)
analyzer.
Ethyl acetate was purified using this method: a 500-mL round flask was
charged with 300 mL of ethyl acetate, 30 mL of acetic anhydride, and 3
drops of concentrated sulfuric acid; then the solution was heated under
refluxing to remove ethanol and water. After 4 h, the solution was distilled,
and the distillate was dried over anhydrous potassium carbonate. Then the
distillate was distilled again to obtain anhydrous ethyl acetate.
General Procedure
A 50-mL, two-neck, round flask was charged with MeCOOEt (4 mL) and
TiCl4 (2.5 mmol) under a nitrogen atmosphere. The two-neck, round flask
was put into ice water, and then Mg powder (6 mmol) was added. The
reaction mixture was stirred and turned violet–black immediately. After
10 min, the mixture was heated to room temperature. Then the desired
aldehyde (1, 1 mmol) in 1 mL of MeCOOEt was added, and the mixture
was stirred at rt for a period as indicated in Tables 1 and 2 (the reaction
was followed by TLC). After the completion of the reaction, the resulting sus-
pension was quenched with 10 mL of 10% K2CO3 and extracted with ethyl
acetate (3 ꢀ 15 mL). The combined organic layers were washed with
saturated aqueous NaHCO3 solution and brine, dried over anhydrous
magnesium sulfate for 12 h, and filtered. Ethyl acetate was evaporated
under reduced pressure to give the crude product, which was separated by
column chromatography on silica (200–300 mesh) and eluted with
petroleum ether or a mixture of petroleum ether and diethyl ether. The
authenticity of the product was established by 1H NMR, MS, and IR
spectral data.

Pinacol Coupling of Aromatic Aldehydes and Ketones
Data
1397
2a. 1H NMR: d2.73 (2H, s, OH, meso), 3.41 (2H, s, OH, dl), 4.65 (2H, s, CH,
dl), 4.80 (2H, s, CH, meso), 7.09–7.31 (20H, m, Ph-H) ppm. IR (KBr)
n
max:3200–3480 cm21
.
2b. 1H NMR: d3.24 (4H, s, OH), 5.34 (2H, s, CH, dl), 5.58 (2H, s, CH, meso),
7.14–7.28 (16H, m, Ph-H) ppm. m/z (%): 282 (1), 165 (47), 141 (89), 113
(13), 107 (14), 77 (100), 51 (38). IR (KBr) nmax:3100–3500 cm21
.
2c. 1H NMR: d 2.93 (2H, s, OH, meso), 3.53 (2H, s, OH, dl), 4.54 (2H, s, CH,
dl), 4.73 (2H, s, CH, meso), 6.85–7.25 (16H, m, Ph-H) ppm. m/z (%): 263
(1.2), 251 (1.6), 178 (4.6), 165 (4.6), 141 (100), 113 (23.8), 77 (71.0). IR
(KBr) nmax:3260–3318 cm21
.
2d. 1H NMR: d 2.62 (2H, s, OH, meso), 3.18 (2H, s, OH, dl), 4.58 (2H, s, CH,
dl), 4.80 (2H, s, CH, meso), 6.99–7.30 (16H, m, Ph-H) ppm. m/z (%): 276
(14), 249 (32), 155 (100), 111 (8). IR (KBr) nmax:3380–3420 cm21
.
1
2e. H NMR: d 3.40 (4H, s, OH), 5.18 (2H, s, CH, dl), 5.48 (2H, s, CH,
meso), 7.12–7.55 (12H, m, Ph-H) ppm. m/z (%): 352 (1), 305 (1.4),
233 (10), 175 (100), 145 (10), 111 (25), 77 (15). IR (KBr)
n
max:3320–3400 cm21
.
1
2f. H NMR: d 2.82 (2H, s, OH, meso), 3.38 (2H, s, OH, dl), 4.55 (2H, s,
CH, dl), 4.73 (2H, s, CH, meso), 6.91–7.43 (16H, m, Ph-H) ppm. m/z (%):
325 (6), 186 (16), 157 (8), 107 (7), 77 (100), 51 (13). IR (KBr) nmax:3200–
3500 cm21
.
2g. 1H NMR: d 2.44 (2H, s, OH, meso), 3.00 (2H, s, OH, dl), 4.61 (2H, s, CH,
dl), 4.82 (2H, s, CH, meso), 6.97–7.44 (16H, m, Ph-H) ppm. Anal. calcd. for
C14H12O2Br2: C, 45.16; H, 3.23; found: C, 45.19; H, 3.24. IR (KBr)
n
max:3100–3600 cm21
.
2h. 1H NMR: d 2.33 (6H, s, CH3, dl), 2.37 (6H, s, CH3, meso), 4.63 (2H, s, CH,
dl), 4.74 (2H, s, CH, meso), 7.01–7.18 (16H, m, Ph-H) ppm. m/z (%): 242
(1.2), 195 (6), 121 (100), 107 (12), 77 (13). IR (KBr) nmax:3280–3450 cm21
.
2i. 1H NMR: d 3.67 (6H, s, CH3O, dl), 3.71 (6H, s, CH3O, meso), 5.06 (2H, s,
CH, dl), 5.27 (2H, s, CH, meso) 6.76–7.23 (16H, m, Ph-H) ppm. Anal. calcd.
for C16H18O4: C, 70.06; H, 6.61; found: C, 70.02; H, 6.63. IR (KBr)
n
max:3130–3640 cm21
.
2j. 1H NMR: d 3.77 (6H, s, CH3O, dl), 3.81 (6H, s, CH3O, meso), 4.61 (2H, s,
CH, dl), 4.73 (2H, s, CH, meso) 6.75–7.28 (16H, m, Ph-H) ppm. m/z (%): 302
(1), 284 (2.5), 268 (5.0), 255 (11.8), 151(100), 123(32), 93 (77.1), 65 (39.0).
IR (KBr) nmax:3100–3600 cm21
.
2k. 1H NMR: d 4.59 (2H, s, CH, dl), 4.67 (2H, s, CH, meso), 5.95 (4H, s, CH2,
dl), 5.98 (4H, s, CH2, meso), 6.56–6.86 (12H, m, Ph-H) ppm. m/z (%): 302 (1),

1398
K. Wang et al.
284 (2.5), 268 (5.0), 255 (11.8), 151 (100), 123 (32), 93 (77.1), 65 (39.0). IR
(KBr) nmax:3100–3600 cm21
.
2l. 1H NMR: d2.65 (4H, s, OH), 4.29 (2H, d, CH, dl), 4.45 (2H, d, CH, meso),
6.28 (2H, m, –CH55CH–), 6.73 (2H, m, –CH55CH–), 7.18–7.41 (20H, m,
Ph-H) ppm. m/z (%): 282 (1), 266 (15), 221 (12), 177 (24), 162 (26), 151
(30), 135 (23), 120 (70), 85 (38), 77 (17), 57 (100). IR (KBr) nmax:3300–
3500 cm21
.
1
2m. H NMR: d 1.53 (6H, s, CH3, dl), 1.62 (6H, s, CH3, meso), 2.44 (2H, s,
OH, meso), 2.75 (2H, s, OH, dl), 7.22–7.29 (20H, m, Ph-H) ppm. m/z (%):
225 (4), 206 (4), 181 (32), 165 (9), 121 (100), 105 (12), 77 (11), 43 (80). IR
(KBr) nmax:3100–3600 cm21
.
1
2n H NMR: d 3.06 (2H, s, OH, dl), 7.18–7.50 (20H, m, Ph-H) ppm. m/z
(%): 184 (17), 165 (8), 105 (100), 77 (74), 43 (3). IR (KBr) nmax:3100–
3600 cm21
.
ACKNOWLEDGMENTS
The project was supported by Natural Science Foundation of Hebei Province,
China (203107).
REFERENCES
1. (a) Nicolaou, K. C.; Yang, Z.; Liu, J. J.; Ueno, H.; Nantermet, P. G.; Guy, R. K.;
Claiborne, C. F.; Renaud, J.; Couladouros, E. A.; Paulvannan, K.; Sorensen, E. J.
Total synthesis of taxol. Nature 1994, 367, 630–634; (b) Kan, T.; Hosohawa, S.;
Nara, S.; Oikawa, M.; Ito, S.; Matsuda, F.; Shirahama, H. Total synthesis of
(2)-grayanotoxin III. J. Org. Chem. 1994, 59 (19), 5532–5534.
2. Kim, Y. H. Dual enantioselective control in asymmetric synthesis. Acc. Chem. Res.
2001, 34 (12), 955–962.
3. (a) Masamune, S.; Choy, W. Advances in stereochemical control: The 1,2- and
1,3-diol systems. Aldrichim. Acta 1982, 15, 47; (b) Pietruszka, J.; Witt, A.
1,1,2,2-Tetraphenyl-1,2-ethanediol: A new protecting group for the synthesis of
cyclopropylboronic esters. Synlett 2003 (1), 91–94.
4. (a) Gansauer, A. Pinacol coupling of aromatic aldehydes catalysed by a titanocene
complex: a transition metal catalysed radical reaction. Chem. Commun. 1997 (5),
457; (b) Wirth, T. “New” reagents for the “old” pinacol coupling reaction. Angew.
Chem., Int. Ed. Engl. 1996, 35, 61; (c) Robertson, G. M. Pinacol coupling
reactions. In Comprehensive Organic Synthesis; Trost, B. M., Ed.; Pergamon:
New York, 1991; (d) McMurry, J. E. Carbonyl-coupling reactions using
low-valent titanium. Chem. Rev. 1989, 89 (7), 1513–1524.
5. (a) Corey, E. J.; Danheiser, R. L.; Chandrasekaran, S.; Siret, P.; Keck, G. E.;
Gras, J. L. Stereospecific total synthesis of gibberellic acid. A key tricyclic
intermediate. 1978, 100 (25), 8031–8034; (b) Swindell, C. S.; Fan, W. M.;
Klimko, P. G. Pinacol closure of oxygenated taxane skeletal at C-1-C-2 with

Pinacol Coupling of Aromatic Aldehydes and Ketones
1399
stereoinduction by oxygen substituents at C-9 and C-10. Tetrahedron Lett. 1994,
35 (28), 4959–4962.
6. Mukaiyama, T.; Sato, T.; Hanna, J. Reductive coupling of carbonyl compounds
to pinacols and olefins using titanium (?) chloride and zinc. Chem. Lett. 1973
(10), 1041–1044.
7. Clerici, A.; Porta, O. Pinacolic coupling of aromatic aldehydes and ketones
promoted by aqueous titanium trichloride in basic media. Tetrahedron Lett.
1982, 23 (34), 3517–3520.
8. Clerici, A.; Clerici, L.; Porta, O. A highly dl-stereoselective pinacolization of
aromatic aldehydes mediated by titanium trichloride in dichloromethane. Tetrahe-
dron Lett. 1996, 37 (17), 3035–3038.
9. Enders, D.; Ullrich, E. C. Asymmetric pinacol coupling of aromatic aldehydes
with TiCl2/enantiopure amine or hydrazine reagents. Tetrahedron: Asymmetry
2000, 11 (19), 3861–3865.
10. Yamamoto, Y.; Hattori, R.; Miwa, T.; Nakagai, Y. I.; Kubota, T.; Yamamoto, C.;
Okamoto, Y.; Itoh, K. Diastereoselective inter- and intramolecular pinacol
coupling of aldehydes promoted by monomeric titanocene(III) complex
Cp2TiPh. J. Org. Chem. 2001, 66 (11), 3865–3870.
11. Matiushenkov, E. A.; Sokolov, N. A.; Kulinkovich, O. G. Alkylative reduction
of titanium(IV) isopropoxide with EtMgBr: Convenient method for the generation
of subvalent titanium alkoxide reagents and their reactivity in pinacol coupling
reactions. Synlett 2004 (1), 77–80.
12. Li, J. T.; Yang, J. H.; Li, T. S. Reductive coupling of aromatic aldehydes using low
valent titanium complexes under ultrasound irradiation. Chin. J. Org. Chem. 2003,
23 (12), 1428–1431.
13. Li, J. T.; Chen, Y. X.; Li, T. S. Pinacol coupling of aromatic aldehydes and ketones
by TiCl4–Mg–THF under ultrasound irradiation. J. Chem. Research 2005 (6),
361–363.
14. Periasamy, M.; Srinivas, G.; Karunakar, G. V.; Bharathi, P. Reductive coupling
of aromatic aldehydes and imines by the low valent titanium species generated
in the reaction of TiCl4 with Et3N. Tetrahedron Lett. 1999, 40 (42), 7577–7580.
15. Bandini, M.; Cozzi, P. G.; Morganti, S.; Umani-Ronchi, A. Highly diastereoslec-
tive pinacol coupling of aldehydes catalyzed by titanium–Schiff base complexes.
Tetrahedron Lett. 1999, 40 (10), 1997–2000.
16. Li, T. Y.; Wang, L. P.; Zhang, T.; Li, Y. X.; Wang, Z. M. Studies on the stereo-
selectivity of pinacol coupling reaction mediated by TiCl4-Mg. Chem. J. Chin.
Univ. 2000, 21, 1391.
17. Hu, Y. L.; Du, Z. Y.; Wang, J. X.; Xi, Y. S.; Gu, S. J. Reductive coupling of
aldehydes, ketones and aroyl chlorides induced by LVT prepared from TiCl4/Al
system. Synth. Commun. 1998, 28 (18), 3305–3315.
18. (a) Mukaiyama, T.; Yoshimura, T.; Igarashi, K.; Kagayama, A. Efficient diastereo-
selective pinacol coupling reaction of aliphatic and aromatic aldehydes by using
newly utilized low valent titanium bromide and iodide species. Tetrahedron
2001, 57 (13), 2499–2506; (b) Shi, L.; Fan, C. A.; Tu, Y. Q.; Wang, M.;
Zhang, F. M. Novel and efficient Ni-mediated pinacol copling of carbonyl
compounds. Tetrahedron 2004, 60 (12), 2851–2855.