Catalytic, highly enantio- and diastereoselective pinacol coupling reaction with a new tethered bis(8-quinolinolato) ligand

Article abstract of DOI:10.1021/ja045430u

A new class of chiral ligand, tethered bis(8-quinolinol) (TBOxH), is developed. Its chromium complex, TBOxCrCl (3 mol %), effectively catalyzes the pinacol coupling reaction of aromatic aldehydes at room temperature with high yield (up to 94%), high diast

Full text of DOI:10.1021/ja045430u

Published on Web 09/28/2004
Catalytic, Highly Enantio- and Diastereoselective Pinacol Coupling Reaction
with a New Tethered Bis(8-quinolinolato) Ligand
Norito Takenaka, Guoyao Xia, and Hisashi Yamamoto*
Department of Chemistry, The UniVersity of Chicago, 5735 South Ellis AVenue, Chicago, Illinois 60637
Received July 29, 2004; E-mail: yamamoto@uchicago.edu
Scheme 1. Syntheses of TBOxH and TBOxCrCla
Chiral 1,2-diols are structural motifs often found in various
important natural products1 and have also proven valuable as chiral
ligands2 and auxiliaries3 in stereoselective organic syntheses. This
class of diols has been successfully prepared in enantioenriched
forms by resolution of racemic diols,4 by Sharpless asymmetric
dihydroxylation of alkenes,5 and by asymmetric transfer hydrogena-
tion of 1,2-diketones.6 Arguably, the most direct method to prepare
1,2-diols is a reductive coupling of simple aldehydes (eq 1).7
However, the identification of a catalytic asymmetric pinacol
coupling reaction has remained a challenge for organic chemistry
since not only enantioselectivity but also diastereoselectivity (dl
-70 °C, followed by oxidation with tetrachloro-1,2-benzoquinone, 76%.
vs meso) needs to be controlled in a single bond-forming event.
High stereoselectivity in pinacol coupling reaction has remained
elusive even through stoichiometric protocols.8 To date, efforts
a Reaction conditions: (a) tBuLi, 7-tert-butyl-8-methoxy-quinoline, THF,
(b) BBr3, CH2Cl2, 95%. (c) THF, reflux, followed by air oxidation, 99%.
Table 1. Reactions of Various Aromatic Aldehydes
toward this goal have focused on the use of chiral low-valent
titanium catalysts.9-11 Described herein are the development of a
new chiral tethered bis(8-quinolinolato) (TBOx)12 chromium catalyst
and its application to the highly enantio- and diastereoselective
pinacol coupling reaction of aromatic aldehydes, as well as the first
example of an aliphatic aldehyde.
entry
Ar
time (h)
yield (%)
dl:meso
ee (%)
1
2
3
4
5
6
7
8
9
Ph
10
18
12
12
10
9
20
14
14
94
93
93
92
91
94
89
92
88
98:2
98:2
97:3
98:2
97:3
97:3
92:8
96:4
97:3
97
98
97
97
98
98
95
98
95
o-MePh
p-MePh
m-MeOPh
p-BrPh
p-ClPh
p-CF3Ph
1-naph
2-naph
Simple chromium(II) salts such as CrCl2 and Cr(ClO4)2 have
been reported to promote radical-mediated pinacolic coupling
reaction either as a stoichiometric reagent13 or as a catalyst14 in
the presence of a stoichiometric co-reductant. Fu¨rstner et al. found
that the reducing ability of Cp2Cr toward aromatic aldehydes
exceeded that of CrCl2 during the course of his study on chromium-
catalyzed Nozaki-Hiyama-Kishi (NHK) reaction.15 The observed
difference between the two was attributed to a Cp ligand that is
more electron-rich than a chloride ligand. An electron-rich Cr(II)
complex would, therefore, be an attractive candidate as a catalyst
for the pinacol coupling reaction. Given the low flexibility for
structure modification of chiral Cp-based ligands, our attention was
eventually turned to 8-quinolinol as a metal template. The develop-
ment of an achiral template into an asymmetric chiral ligand was
successfully achieved in two steps from 2,2′-diiodo-1,1′-binaphthyl16
and 7-tert-butyl-8-methoxy-quinoline. Its Cr(III) complex was
synthesized according to the literature17 (Scheme 1). To the best
of our knowledge, however, no optically active chiral chromium
complexes have been studied as a catalyst for pinacol coupling
reaction.
(TBOxCr(III)Cl), co-reducing agent (Mn), the product scavenger
(TESCl), and aldehyde were mixed in CH3CN under an atmosphere
of Ar at room temperature. Significantly, the reaction is effectively
catalyzed with 3 mol % of the catalyst, which represents the lowest
catalyst/substrate ratio for an asymmetric catalytic pinacol coupling
reaction.11a The isolated crude silyl ethers were treated with aqueous
HCl in THF to afford diols in high yields and excellent enantio-
and diastereoselectivities (Table 1).
The present catalyst system proved to be quite insensitive to
changes in steric effect (entries 2 and 8 in comparison to 1) and
electron withdrawing substituents on the aromatic ring such as Br,
Cl, and CF3 (entries 5, 6, and 7), which significantly lowered
enantioselectivities in all cases of the previously reported catalytic
pinacol coupling reactions.11d-f
The synthesis of pinacols from aliphatic aldehydes requires the
use of a more efficient redox system than does the corresponding
reductive coupling of aromatic systems.7b,11f,19 To our surprise, the
scope of the present method was found not to be limited to aromatic
aldehyde derivatives, as cyclohexanecarboxaldehyde underwent
pinacolization (44% yield, dl:meso ) 93:7, 84% ee). This represents
In catalytic pinacol coupling reactions, the role of chlorosi-
lane is generally explained as cleaving the metal-oxygen bond
of a putative pinacol formed to recycle a catalyst.10,15a,b How-
ever, interestingly, TESCl provided better stereoselectivities
than TMSCl in our system (vide infra).18 Thus, the precatalyst
9
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J. AM. CHEM. SOC. 2004, 126, 13198-13199
10.1021/ja045430u CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
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Figure 1. X-ray crystal structure of [TBOx(EtOH)2Cr]Cl.
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that it adopts octahedral coordination. The X-ray structure of rac-
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in a cis-â configuration (Figure 1). A crystal structure of cis-â
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In summary, a new class of chiral ligand, TBOx, was developed,
and the X-ray crystal structure of its chromium complex was
determined. TBOxCrCl was shown to efficiently catalyze the
asymmetric pinacol coupling reactions of both aromatic and
aliphatic aldehydes. Studies are currently underway to elucidate
the mechanism and scope of this pinacol coupling reaction.
Furthermore, the applications of TBOx in other asymmetric catalysis
will be reported in due course.
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Acknowledgment. Support of this research was provided by
the National Science Foundation (CHE-0412060) and by the
University of Chicago. We thank Dr. Ian M. Steele for the X-ray
structure determination.
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Supporting Information Available: Experimental procedures,
spectral data for all new compounds, and crystallographic data. X-ray
crystallographic file in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
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