A new method for the generation of titanium(III) complexes and its application in pinacol coupling reactions

Article abstract of DOI:10.1002/adsc.200505180

A new method for the in situ generation of various Ti(III) complexes is presented. Cyclohexadienyl-Ti(IV) derivatives, which are readily prepared from the corresponding lithiated cyclohexadienes, afford the corresponding Ti(III) complexes upon thermal C-Ti-bond homolysis. The Ti(III) compounds generated using this novel method have successfully been used in the reductive dimerization of benzaldehyde. In particular, TiBr₃, TiCp ₂Cl and Ti(O-i-Pr)₃ have been generated via this approach. Moreover, the method also offers an entry to new chiral Ti(III) complexes as documented by the preparation of Ti(III)CpTADDOLate.

Full text of DOI:10.1002/adsc.200505180

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DOI: 10.1002/adsc.200505180
A New Method for the Generation of Titanium(III) Complexes
and its Application in Pinacol Coupling Reactions
Christoph Alexander Knoop, Armido Studer*
Organisch-Chemisches Institut, Fachbereich Chemie, Corrensstrasse 40, 48149 Münster, Germany
Fax: (þ49)-251-833-6523, e-mail: studer@uni-muenster.de
Received: April 28, 2005; Accepted: August 8, 2005
Abstract: A new method for the in situ generation of
various Ti(III) complexes is presented. Cyclohexa-
dienyl-Ti(IV) derivatives, which are readily prepared
from the corresponding lithiated cyclohexadienes, af-
ford the corresponding Ti(III) complexes upon ther-
Reduction of activated alkyl halides have been achieved
using Ti(III).[9] Moreover, Ti(III) complexes have suc-
cessfully been used in pinacol reactions.[10,11] Even enan-
tioselective couplings have recently been reported.[12]
Most of the Ti(III) complexes studied have been gener-
ated from the corresponding Ti(IV) compounds and a
coreducing reagent.[13]
Recently, we published our results on the use of
cyclohexadienyl-Ti compounds in ionic allylations
(Scheme 1).[14] We assumed that these compounds may
also offer a clean entry to Ti(III) complexes upon simple
thermal Ti-carbon bond homolysis. In contrast to estab-
lished methods for the generation of Ti(III) derivatives,
a coreducing reagent is not necessary in our approach.[15]
Since the starting cyclohexadienyl-Ti(IV) compounds
are readily prepared by transmetalation using the ap-
propriate Ti(IV) complex, new unexplored Ti(III) re-
agents should be available via this route. In this commu-
nication we report first results on the use of cyclohexa-
dienyl-Ti(IV) derivatives as precursors for Ti(III) com-
plexes. Moreover, these reducing reagents will be ap-
À
mal C Ti-bond homolysis. The Ti(III) compounds
generated using this novel method have successfully
been used in the reductive dimerization of benzalde-
hyde. In particular, TiBr3, TiCp2Cl and Ti(O-i-Pr)3
have been generated via this approach. Moreover,
the method also offers an entry to new chiral Ti(III)
complexes as documented by the preparation of
Ti(III)CpTADDOLate.
Keywords: radical chemistry; reductive cyclization;
stereoselective reductions; synthetic methods; titani-
um
Radical chemistry has gained increasing importance plied in pinacol coupling reactions.
during the last 30 years.[1] In contrast to ionic chemistry,
In order to suppress the ionic-type chemistry of cyclo-
many functional groups are tolerated under radical con- hexadienyl-Ti compounds (Scheme 1, on the left) we de-
ditions. Several methods for the clean generation of rad- cided to block the allylic positions with bulky tert-butyl
icals have been reported to date. However, most of the groups. In addition, the bulky substituents should allow
radical reactions are conducted using toxic trialkyltin a highly regioselective metalation for steric reasons.
hydrides. To circumvent the use of toxic tin compounds, Birch reduction of readily available 1,3-di-tert-butylben-
many research groups are currently looking at environ- zene[16] afforded cyclohexadiene 1 in 42% yield. Along
mentally benign radical processes.[2] Electron-transfer with 1, the isomeric 1,4-cyclohexadiene, which was sep-
reagents have successfully been used in this context. In arated by chromatography, was formed in <5% yield.
particular, samarium diiodide (SmI2) has been found The metalation of diene 1 was studied first. As a test re-
to be a highly efficient electron-transfer reagent for con- action to prove our concept the pinacol reaction of ben-
ducting various radical processes.[3] However, SmI2 is zaldehyde was investigated. To this end, diene 1 was de-
rather expensive and is readily oxidized on air.
protonated with various strong bases at À788C in ethe-
As an alternative to SmI2, Ti(III) complexes have
been studied by various groups.[4–11] Nugent and Rajan-
Babu showed that titanocene(III) chloride can be ap-
plied to reductively open epoxides.[5] The b-titanoxy rad-
icals thus generated can be used in radical cyclizations
and intermolecular addition reactions. Gansäuer[6,7] lat-
er showed that the epoxide openings can be performed
Scheme 1. Cyclohexadienyl-Ti(IV) compounds in ionic ally-
using catalytic[8] amounts of titanocene(III) chloride. lations and as sources for Ti(III) complexes.
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Generation of Titanium(III) Complexes
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results. Reactions in dimethoxyethane and diethyl ether
did not work (entries 3 and 4). Importantly, without
transmetalation the pinacol coupling product was not
formed, clearly showing that the coupling reaction oc-
curs via the suggested intermediate 2 (entry 5).[19] Using
titanocene dichloride as transmetalation reagent, diol 3
was formed in 32% yield (dr¼2.5:1, entry 6). A higher
selectivity was obtained if the reaction mixture was al-
lowed to warm to À158C (dr¼6.3:1, entry 7). Transme-
talation at lower temperature (À1008C) did not provide
a better result (entry 8). The selectivities obtained using
in situ generated Cp2TiCl are disappointingly low. Based
on literature reports, far higher selectivities were ex-
pected for these reactions (97:3).[13] We assumed that
the strongly coordinating TMEDA may disrupt well-or-
dered rigid transition states for the coupling reaction. In
fact, amine-free generation of Cp2TiCl provided diol 3 in
Scheme 2. Reductive coupling of benzaldehyde.
real solvents (Scheme 2). Transmetalation with a Ti(IV) excellentselectivity (>95:5, entry 9). The yield could be
complex furnished the cyclohexadienyl-Ti-derivative 2. improved upon increasing the amount of Ti(III) precur-
After addition of benzaldehyde, the reaction mixture sor (2 equivs., 56%, entry 10).
was allowed to warm to room temperature.
The first experiment was conducted with s-BuLi/tetra- as Ti(III) precursor (entry 11). Probably, transmetala-
methylethylenediamine (TMEDA) as base (1equiv., tion did not proceed because the typical color change oc-
Coupling product 3 was not identified with Ti(NEt3)4
60 min, À788C, Table 1, entry 1). Transmetalation was curring after Li-Ti exchange was not observed. Reaction
performed with Ti(O-i-Pr)4 (1equiv., À788C, 60 min) with TiCpCl3 did not provide diol 3 (entry 12). In situ
and benzaldehyde (1equiv.) was added. We were
generated TiBr3 provided the pinacol product in 37%
pleased to observe that the desired reductive coupling yield (dr¼1.7:1, entry 13).
of benzaldehyde to give diol 3 occurred. Moreover, ionic
Currently it is not clear why only moderate yields are
Ti(IV)-mediated allylation of benzaldehyde was com- obtained in pinacolizations using reagents of type 2.
pletely suppressed by the bulky tert-butyl groups. How- Competing reduction of intermediately formed ketyl
ever, the yield was not satisfactory (15%).[17] The diaster- radical anion is not a problem since only trace amounts
1
eoisomeric ratio was determined by H-NMR spectro- of benzyl alcohol were identified as side product. The cy-
scopy (4:1). Incomplete deprotonation may be a reason clohexadienyl radical generated as by-product during
À
for the moderate yield. Therefore, reaction was repeat- Ti C bond homolysis probably undergoes dimerization
ed using Schlosser base,[18] however, a similar result was or disproportionation. Indeed, the disproportionation
obtained (entry 2). Increasing the reaction time for de- products 1,3-di-tert-butylbenzene and cyclohexadiene
protonation and transmetalation did not provide better 1 were identified after the reactions. We assume that re-
Table 1. Reductive coupling under various conditions.
Entry
Base
Ti(IV) derivative
Solvent
Yield [%]
dr [rac:meso]
1
2
3
4
5
s-BuLi/TMEDA
s-BuLi/KO-t-Bu
s-BuLi/KO-t-Bu
s-BuLi/KO-t-Bu
s-BuLi/TMEDA
s-BuLi/TMEDA
s-BuLi/TMEDA
s-BuLi/TMEDA
s-BuLi/KO-t-Bu
s-BuLi/KO-t-Bu
s-BuLi/TMEDA
s-BuLi/TMEDA
s-BuLi/TMEDA
Ti(O-i-Pr)4
Ti(O-i-Pr)4
Ti(O-i-Pr)4
Ti(O-i-Pr)4
–
TiCp2Cl2
TiCp2Cl2[b]
TiCp2Cl2
TiCp2Cl2
TiCp2Cl2
Ti(NMe2)4
TiCpCl3
THF
THF
DME
Et2O
THF
THF
THF
THF
THF
THF
THF
THF
THF
15
15
–
–
–
32
24
22
32
56
–
4: 1
1.5: 1
–
–
–
2.5: 1
6.3: 1
2.0: 1
>95: 5
>95: 5
–
6
7[a]
8
9
10[c]
11
12
13
–
37
–
TiBr4
1.7: 1
[a]
Reaction mixture was allowed to warm to À158C.
Transmetalation at À1008C.
[b]
[c]
2 equivalents of the Ti(III) precursor were used.
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action of TiX3 with the cyclohexadienyl radical to form
À788C provided a solution containing titanium(III)
HTiX3 is not occurring since benzyl alcohol expected triisopropoxide which was successfully used for the re-
as product in that case[20] was formed in only trace ductive dimerization of benzaldehyde. Diol 3 was isolat-
amounts, as already mentioned. We also tested dihy- ed in 40% yield (Scheme 3, Table 2, entry 1). Using the
droanthracene (4) in the lithiation, transmetalation cou- same protocol TiCp2Cl (entry 2) and TiBr3 (entry 3)
pling sequence. However, diol 3 was not formed.
were generated and used in the pinacolization of benzal-
We then decided to use cyclohexadiene 5, which is dehyde. Switching to Schlosser base provided similar re-
readily prepared from commercially available resorcin sults (compare entry 1with 4 and entry 2 with 5). The
dimethyl ether, as precursor for Ti(III) compounds. As- yield could be increased upon using two equivalents of
sisted by the methoxy substituents lithiation can readily the Ti(III) precursor, however, selectivity decreased
be achieved.[21] Transmetalation of the lithiated cyclo- (entry 6). With the s-BuLi/KO-t-Bu/Ti(O-i-Pr)4-system
hexadiene derived from 5 will afford the desired other aldehydes were tested. Hence, reaction with tolyl-
Ti(IV) derivative 6 which can be used in ionic allylations. aldehyde provided the corresponding diol 8 (R4 ¼p-
Indeed, treatment of 6 (R1 ¼R2 ¼R3 ¼O-i-Pr) with ben- MeC6H4) in 65% yield with moderate selectivity (entry
zaldehyde at À788C provided alcohol 7.[22] However, 8). A lower yield was observed for the reaction with
we assumed that Ti(III) complexes can be liberated p-methoxybenzaldehyde (entry 9). 2-Naphthaldehyde
from 6 upon warming to room temperature. To this provided a slightly better selectivity (entry 10).
end, Li-6 readily generated by deprotonation using s-
Except for the TiBr3 case, all the other Ti(III) com-
BuLi/TMEDA, was transmetalated with Ti(O-i-Pr)4. plexes generated via cyclohexadiene 5 provided lower
The reaction mixture was subsequently allowed to selectivities than using Ti(III) compounds deriving
warm to room temperature. Renewed cooling to from metalated dienes of type 2. We assume that the
chelating methoxy substituents may still coordinate to
À
the Ti(III) complex after Ti C bond homolysis and
this in turn may lead to decreased selectivities. There-
fore, enantioselective coupling was studied using a chiral
Ti(IV) derivative derived from cyclohexadiene 1.
The TADDOL-derived[23] Ti(IV) compound 9 was
readily prepared from 1 as described above using
CpTiClTADDOLate in the transmetalation.[14] Addi-
tion of benzaldehyde and warming to room temperature
provided diol 3 with 48% yield in a moderate diastereo-
selectivity (dl:meso¼3:1, Scheme 4). The enantiomeric
excess of dl-3 was determined by chiral HPLC (ee¼
56%). To the best of our knowledge this is the first report
on a chiral Ti(III)-TADDOLate.[24] Although the selec-
tivity is not high in the present case, we could show that
our approach is useful for the study of new chiral Ti(III)
compounds. Work along this line is underway.
We showed that cyclohexadienyl-Ti(IV) compounds,
which are readily prepared from the corresponding lithi-
Scheme 3. An alternative approach to Ti(III) complexes.
ated cyclohexadienes by transmetalation using estab-
Table 2. Reductive dimerization of various aldehydes using cyclohexadienes of type 6.
Entry
Base
Ti(IV) derivative
R4
Yield [%]
dr [rac :meso]
1
2
3
s-BuLi/TMEDA
s-BuLi/TMEDA
s-BuLi/TMEDA
s-BuLi/KO-t-Bu
s-BuLi/KO-t-Bu
s-BuLi/KO-t-Bu
s-BuLi/KO-t-Bu
s-BuLi/KO-t-Bu
s-BuLi/KO-t-Bu
s-BuLi/KO-t-Bu
Ti(O-i-Pr)4
TiCp2Cl2
TiBr4
Ti(O-i-Pr)4
TiCp2Cl2
TiCp2Cl2
TiBr4
Ti(O-i-Pr)4
Ti(O-i-Pr)4
Ti(O-i-Pr)4
Ph
Ph
Ph
Ph
Ph
Ph
Ph
p-MePh
p-MeOPh
2-naphthyl
40
411
20
36
42
60
18
65
19
1.8: 1
3: 1
>95: 5
1: 1
12: 1
4
5
6[a]
7
1: 1.4
>95: 5
1.9: 1
2.6: 1
8
9
10
315.6: 1
[a]
2 equivalents of the Ti(III) precursor was used.
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Generation of Titanium(III) Complexes
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1.03 mmol) were added. The solution was stirred at this tem-
perature for 45 min. TiCp2Cl2 (257 mg, 1.03 mmol) was added.
Stirring was continued for another 60 min at À788C. After ad-
dition of benzaldehyde the reaction mixture was allowed to
warm to room temperature over 0.5 h and was stirred for addi-
tional 15 h. The reaction was stopped upon addition of NH4Cl
(aqueous saturated) and HCl (1M). The aqueous layer was ex-
tracted with tert-butyl methyl ether (MTBE, 3 times). The com-
bined organic layers were dried over MgSO4 and the solvents
were removed under vacuum. Purification by FC (pentane/
MTBE, 3/2) afforded diol 3; yield: 56%.
Scheme 4. Coupling using a chiral Ti(III) complex.
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft for funding
(STU 280/3–2).
À
lished Ti(IV) chemistry, undergo thermal Ti C bond ho-
molysis to generate the corresponding Ti(III) com-
plexes. These Ti(III) compounds can be used in the re-
ductive dimerization of benzaldehyde. Since many
Ti(IV) compounds can be used in the transmetalation
process, this new approach offers an entry to Ti(III)
complexes which have not been prepared to date. For
example, Ti(III)CpTADDOLate was successfully pre-
pared by this method. Variation of the ligands in the pre-
cursor Ti(IV) complexes will allow one to tune the reac-
tivity ofthe corresponding Ti(III)complexes. Moreover,
our approach will open the door to new chiral Ti(III)
complexes.
References and Notes
[1] a) W. B. Motherwell, D. Crich, Free Radical Chain Reac-
tions in Organic Synthesis, Academic Press, London,
1992; b) P. Renaud, M. P. Sibi, Radicals in Organic Syn-
thesis, Vols. 1& 2, Wiley-VCH, Weinheim,
2001;
c) S. Z. Zard, Radical Reactions in Organic Synthesis,
Oxford University Press, Oxford, 2003; d) H. Togo, Ad-
vanced Free Radical Reactions for Organic Synthesis,
Elsevier, 2004.
[2] a) P. A. Baguley, J. C. Walton, Angew. Chem. 1998, 110,
3272–3283; Angew. Chem. Int. Ed. 1998, 37, 3072–
3082; b) A. Studer, S. Amrein, Synthesis 2002, 835–849.
[3] a) H. B. Kagan, J. L. Namy, Tetrahedron 1986, 42, 6573–
6614; b) G. A. Molander, C. R. Harris, Chem. Rev. 1996,
96, 307–338; c) T. Skrydstrup, Angew. Chem. 1997, 109,
355–358; Angew. Chem. Int. Ed. 1997, 36, 345–347;
c) G. A. Molander, C. R. Harris, Tetrahedron 1998, 54,
3321–3354; d) A. Krief, A.-M. Laval, Chem. Rev. 1999,
99, 745–777; e) H. B. Kagan, Tetrahedron 2003, 59,
10351–10372; f) D. J. Edmonds, D. Johnston, D. J. Proct-
er, Chem. Rev. 2004, 104, 3371–3403.
Experimental Section
General Remarks
All reactions were carried out in dried glassware under an ar-
gon atmosphere. Tetrahydrofuran (THF) was freshly distilled
from potassium under argon. Diethyl ether (Et2O) was freshly
distilled from K/Na under argon. All other solvents and re-
agents were purified according to standard procedures or
were used as received from Aldrich or Fluka. 1H NMR spectra
were recorded on a Bruker AMX 400, AC 300, ARX 300 or an
ARX 200. Chemical shifts d are given in ppm relative to CHCl3
at 7.26 ppm as external standard. Diastereomeric ratios were
[4] a) A. Gansäuer, S. Narayan, Adv. Synth. Cat. 2002, 344,
465–475; b) A. Gansäuer, T. Lauterbach, S. Narayan,
Angew. Chem. 2003, 115, 5714–5731; Angew. Chem.
Int. Ed. 2003, 42, 5556–5573.
1
determined using H NMR spectroscopy or HPLC. Enantio-
meric excess was measured by HPLC analysis using a Baker-
bond Standard (DNBPG, covalent, 5-mm, 4.6ꢀ250 mm) col-
umn (eluent: hexane/i-PrOH, 98/2 with addition of 0.1% acetic
acid, flow rate 1mL min À1). TLC was performed using Merck
silica gel 60 F254 plates; detection with UV or dipping into a
solution of KMnO4 (1.5 g in 400 mL H2O, 5 g NaHCO3), fol-
lowed by heating. Flash column chromatography (FC) was per-
formed using Merck or Fluka silica gel 60 (40–63 mm) applying
a pressure of about 0.4 bar.
[5] a) W. A. Nugent, T. V. RajanBabu, J. Am. Chem. Soc.
1988, 110, 8561–8562; b) T. V. RajanBabu, W. A. Nu-
gent, J. Am. Chem. Soc. 1989, 111, 4525–4527; c) T. V.
RajanBabu, W. A. Nugent, M. S. Beattie, J. Am. Chem.
Soc. 1990, 112, 6408–6409; d) T. V. RajanBabu, W. A.
Nugent, J. Am. Chem. Soc. 1994, 116, 986–997.
[6] A. Gansäuer, A. Barchuk, D. Fielenbach, Synthesis 2004,
2567–2573.
[7] S. Fuse, M. Hanochi, T. Doi, T. Takahashi, Tetrahedron
´
Lett. 2004, 45, 1961–1963; A. Fernandez-Mateos, L. Ma-
´
toes Buron, R. Rabanedo Clemente, A. I. Ramos Silvo,
Typical Procedure
´
R. Rubi Gonzalez, Synlett 2004, 1011–1014.
[8] A. Fürstner, A. Hupperts, J. Am. Chem. Soc. 1995, 117,
Cyclohexadiene 1 (180 mg, 0.94 mmol) was dissolved in THF.
The solution was cooled to À788C and KO-t-Bu (16 mg,
1.03 mmol) followed by s-BuLi (817mL, 1.26 M in hexane,
4468–4475.
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Christoph Alexander Knoop, Armido Studer
COMMUNICATIONS
[9] a) C. L. Cavallaro, J. Schwartz, J. Org. Chem. 1995, 60,
7055–7057; b) S. G. Davies, S. E. Thomas, Synthesis
1984, 1027–1029; c) J. D. Parrish, D. R. Shelton, R. D.
Little, Org. Lett. 2003, 5, 3615–3617.
[14] F. Schleth, A. Studer, Angew. Chem. 2004, 116, 317–319;
Angew. Chem. Int. Ed. 2004, 43, 313–315; b) F. Schleth,
T. Vogler, K. Harms, A. Studer, Chem. Eur. J. 2004, 10,
4171–4185.
[10] T. Mukaiyama, T. Sato, J. Hanna, Chem. Lett. 1973,
1041–1044.
[15] Very recently, Waymouth reported on the generation of
À
Ti(III) via Ti O bond homolysis: K.-H. Huang, J. H.
[11] a) H. G. Raubenheimer, D. Seebach, Chimia 1986, 40,
12–13; b) Y. Zhang, T. Liu, Synth. Commun. 1988, 18,
2173–2178; c) A. Clerici, L. Clerici, O. Porta, Tetrahe-
dron Lett. 1996, 37, 3035–3038; d) A. Gansäuer, Chem.
Commun. 1997, 457–458; e) T. A. Lipski, M. A. Hilfiker,
S. G. Nelson, J. Org. Chem. 1997, 62, 4566–4567; f) A.
Gansäuer, M. Moschioni, D. Bauer, Eur. J. Org. Chem.
1998, 1923–1927; g) A. Gansäuer, D. Bauer, Eur. J.
Org. Chem. 1998, 2673–2676; h) T. Hirao, B. Hatano,
M. Asahara, Y. Muguruma, A. Ogawa, Tetrahedron
Lett. 1998, 39, 5247–5250; i) M. Bandini, P. G. Cozzi, S.
Morganti, A. Umani-Ronchi, Tetrahedron Lett. 1999,
40, 1997–2000.
[12] a) A. Bensari, J.-L. Renaud, O. Riant, Org. Lett. 2001, 3,
3863–3865; b) A. Chatterjee, T. H. Bennur, N. N. Joshi, J.
Org. Chem. 2003, 68, 5668–5671; c) Y.-G. Li, Q.-S. Tian,
J. Zhao, Y. Feng, M.-J. Li, T.-P. You, Tetrahedron: Asym-
metry 2004, 15, 1707–1710.
[13] Electrochemical reductions: R. J. Enemaerke, J. Larsen,
T. Skrydstrup, K. Daasbjerg, J. Am. Chem. Soc. 2004,
126, 7853–7864; R. J. Enemaerke, J. Larsen, G. J. Hjol-
lund, T. Skrydstrup, K. Daasbjerg, Organometallics
2005, 24, 1252–1262.
Han, A. P. Cole, C. B. Musgrave, R. M. Waymouth, J.
Am. Chem. Soc. 2005, 127, 3807–3816.
[16] J. R. Cozort, J. F. Outlaw Jr., A Hawkins, S. Siegel, J.
Org. Chem. 1983, 48, 4190–4197.
[17] For an alternative approach to titanium(III) isopropox-
ide see: E. A. Matiushenkov, N. A. Sokolov, O. G. Kulin-
kovich, Synlett 2004, 77–80.
[18] M. Schlosser, Modern Synthetic Methods, Vol. 6, (Ed.: R.
Scheffold), VCH, Weinheim, 1992, pp. 227–271.
[19] Very recently, Rossi presented SET processes using lithi-
ated cyclohexadienes: S. E. Vaillard, A. Postigo, R. A.
Rossi, J. Org. Chem. 2004, 69, 2037–2041.
[20] S. C. Berk, K. A. Kreutzer, S. L. Buchwald, J. Am. Chem.
Soc. 1991, 113, 5093–5095.
[21] A. Studer, S. Amrein, F. Schleth, T. Schulte, J. C. Walton,
J. Am. Chem. Soc. 2003, 125, 5726–5733.
[22] F. Schleth, unpublished results, University of Münster.
[23] D. Seebach, A. K. Beck, A. Heckel, Angew. Chem. 2001,
113, 96–142; Angew. Chem. Int. Ed. 2001, 40, 92–138.
[24] Dimethyl tartrate was used as a ligand in TiCl3-mediated
pinacol couplings, see ref.[11c]
.
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Adv. Synth. Catal. 2005, 347, 1542 – 1546