Catalytic enantioselective reduction of ketones by a chiral gallium complex and catecholborane

Article abstract of DOI:10.1002/(SICI)1521-3773(19990201)38:3<335::AID-ANIE335>3.0.CO;2-T

A catalytically active version of Noyori's famous BINAL reagent Li[A1H(OEt)-(BINOL dianion)] is formed by the combination of LiGaH₄, monothiobinaphthol (MTB), and catecholborane. With this mixture unsymmetrical ketones can be reduced in high enantioselectively. Un = unsaturated unit; R = alkyl.

Full text of DOI:10.1002/(SICI)1521-3773(19990201)38:3<335::AID-ANIE335>3.0.CO;2-T

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afforded by xenon, and oxygen was used for reionization. The CA/CA
mass spectrum of [C,Si,Cl] was obtained by daughter ion selection
[LiAlH(OEt)(BINOL dianion)] (BINOL ˆ 2,2'dihydroxy-
‡
1
,1'-binaphthyl) in stoichiometric amounts to effect an
with B2, and recording the subsequent fragmentations by scanning
E(2).[ All spectra were accumulated and on-line processed with the
AMD/Intectra data system; in general 5 to 50 spectra were averaged
to improve the signal-to-noise ratio.
1
electronically controlled hydride addition to 1 (R ˆ unsatu-
21c]
2
[3]
rated unit, R ˆ alkyl). We have used the principles of
ªhardº and ªsoftº[ Lewis acids and bases to attempt a
catalytic analogue of this reagent. Monothiobinaphthol
4]
[
21] a) R. Srinivas, D. Sülzle, T. Weiske, H. Schwarz, Int. J. Mass Spectrom.
Ion Processes 1991, 107, 368; b) R. Srinivas, D. Sülzle, W. Koch, C. H.
DePuy, H. Schwarz, J. Am. Chem. Soc. 1991, 113, 5970; c) D. Schröder,
H. Schwarz, Int. J. Mass Spectrom. Ion Processes 1995, 146/147, 183;
d) C. A. Schalley, D. Schröder, H. Schwarz, Int. J. Mass Spectrom. Ion
Processes 1996, 153, 173.
[5]
[6]
(
MTB) and LiGaH4 were selected as the ªsoftº ligand/
catalyst combination and catecholborane as a ªhardº terminal
hydride source to promote only the removal of alkoxide
products and not the chiral ligand from 4 (Scheme 1). While a
[
22] A. E. Newkirk, J. Am. Chem. Soc. 1946, 68, 2736.
[
23] In analogy, photolysis of 4 (X ˆ Cl) has been speculated to yield
neutral 1c, see ref. [7].
O
[
[
24] For a definition and examples for distonic ions: R. L. Smith, P. K.
Chou, H. I. Kenttämaa in The Structure and Dynamics of Organic Ions
Un
R2
R2
(Eds.: T. Baer, C. Y. Ng), Wiley, London, 1996, p. 197.
1
H
X
O
H
S
25] According to the experimental data in ref. [19a], the preference for
S
Ga
Un
‡
� 1
Ga
SiF formation is only 16 kcalmol . However, we trust our calculated
O
O
�
1
value of 26.9 kcalmol . A similar discrepancy between calculated and
experimental energetics of SiF was reported earlier, see: E. W.
X
‡
Li
THF)n
(
RO)2B
H
B(OR)2
Li
(
Ignacio, H. B. Schlegel, J. Phys. Chem. 1990, 94, 7439.
O
(THF)n
‡
‡
[
26] While the fragmentations of [Si,C,Cl] leading to [SiCl ‡ C] and to
(R)-3
R2
(
R)-4
‡
Un
[
Si ‡ CCl] may also be attributed to the mere energetic preference of
0
� 1
(R)-2
these exit channels (SDH ˆ 386 and 388 kcalmol , respectively),
f
‡
0
f
compared to those for the formation of [SiC ‡ Cl] (SDH ˆ
�
1
‡
0
f
� 1
4
16 kcalmol ) or [Si ‡ CCl ] (SDH ˆ 405 kcalmol ), of the higher
‡
energy channels only SiC is observed experimentally, but not the
O
‡
OR
lower energy fragment CCl ; data taken from refs. [19a,b,27].
S
O
Ga
Li
B
[
27] A. E. Ketvirtis, D. K. Bohme, A. C. Hopkinson, J. Phys. Chem. 1995,
O
S
99, 16121.
O
H
Li
THF)3
(
6
(
R,R)-5
Scheme 1. Reduction of ketone 1 with a complex of MTB and LiGaH4
Un ˆ unsaturated unit) as well as with the complex 5 and the borate 6.
(
Catalytic Enantioselective Reduction of
Ketones by a Chiral Gallium Complex and
Catecholborane**
range of spectator ligands gave moderate selectivities (X ˆ
OMe (72% ee), 2-S-C H (72% ee), 1,2-HOCH CH SH
10
7
2
2
(
24% ee)), a mixture of LiGaH and two MTB ligands proved
4
Alan Ford and Simon Woodward*
particularly effective for a range of ketones (Table 1); the
mixture was characterized in the solid state as 5, which is
isostructural with the gallium ± BINOL catalysts of Shibasaki.
Practical catalytic reduction of prochiral ketones to secon-
dary alcohols of high enantiopurity is indispensable to many
[7]
These reactions are technically simple, stirring the reaction
is not essential and any cryostat or even a domestic freezer
suffices for cooling. At catalyst loadings below 2.5 mol% the
enantioselectivity falls: At 1 mol% (� 258C) 1a is reduced in
organic preparations. Oxazaborolidine-catalyzed BH reduc-
3
[
1]
[2]
tion and ruthenium-catalyzed transfer hydrogenation are
1
2
effective for the reduction of R C(O)R 1 provided an
appropriate difference, steric or electronic, exists between
the groups. As steric differences cannot always be realized,
alternative catalysts that sense electronic differences between
the groups in addition to steric requirements are highly
desirable. In a seminal paper Noyori described the use of
8
6% ee and with 0.5 mol% (08C) in 72% ee. Pre-storage of
solutions of LiGaH /2MTB at ambient temperature for two
4
Table 1. Enantioselective reduction of ketones 1 by 5 (2.5 mol%) and
catecholborane (1.1 equiv) by method B (see experimental section, unless
otherwise stated).
[
*] Dr. S. Woodward, A. Ford
R1
R2
Ketone
Yield [%]
ee [%]
Department of Chemistry, University of Hull
Kingston-upon-Hull HU67RX (UK)
Fax: (‡44)1482-466410
a
b
c
d
e
f
g
h
i
Ph
Ph
Ph
Ph
Me
Et
nBu
iBu
Me
Me
n-C
Me
Me
89 ± 95[a]
96
80
65
80
95
76
70
60
89 ± 91[a]
93
92
92
87
87
81
75
63
E-mail: s.woodward@chem.hull.ac.uk
[
**] This work was supported by the EPSRC (ROPA GR/K91682). We
also thank Profs. Serafino Gladiali and Davide Fabbri for a generous
gift of 1,1'-bi(2-thionaphthol) and Dr. Simon J. Teat (Daresbury
Laboratory) for solving the structure of compound 5.
4-BrPh
4-MePh
2-furyl
PhCHˆCH
EtCꢀC
6
H
13
Supporting information for this article is available on the WWW
under http://www.wiley-vch.de/home/angewandte/ or from the au-
thor.
[a] Using methods A ± C.
Angew. Chem. Int. Ed. 1999, 38, No. 3
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
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335

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weeks did not affect the ee value for the reduction of 1a,
neither did deliberate addition of water to the reaction (one
equivalent per Ga, to simulate impure ªwetº ketones).
The enantioselectivity in the reduction of 1a shows an
unusual temperature dependence: a maximum ee value is
attained at � 20 to � 158C, both the chemical yield and ee
value decrease steadily at temperatures below this range. At
[4] R. G. Pearson, Coord. Chem. Rev. 1990, 100, 403 ± 425.
[
5] S. M. Azad, S. M. W. Bennett, S. M. Brown, J. Green, E. Sinn, C. M.
Topping, S. Woodward, J. Chem. Soc. Perkin Trans 1 1997, 687 ± 694.
6] A. E. Shirk, D. F. Shriver, Inorg. Synth. 1977, 17, 45 ± 47.
[
[
7] Data for structure 5 are given in the supporting information. BINOL
analogues are known, see a) M. Shibasaki, H. Sasai, T. Arai, Angew.
Chem. 1997, 109, 1290 ± 1310; Angew. Chem. Int. Ed. Engl. 1997, 36,
1
237 ± 1256; b) T. Iida, N. Yamamoto, H. Sasai, M. Shibasaki, J. Am.
Chem. Soc. 1997, 119, 4783 ± 84; c) T. Iida, N. Yamamoto, S.
Matsunaga, H.-G. Woo, M. Shibasaki, Angew. Chem. 1998, 110,
2383 ± 2386; Angew. Chem. Int. Ed. 1998, 37, 2223 ± 2226.
�
788C an 18% yield of racemic alcohol is realized. One
explanation of this behavior is that transmetalation of 4 to 3
(
X ˆ MTB dianion) is slow below � 208C and that an achiral
[8] For related chemistry, see a) C. W. Lindsley, M. DiMare, Tetrahedron
Lett. 1994, 35, 5141 ± 5144; b) A. Arase, M. Hoshi, T. Yamaki, H.
Nakanishi, J. Chem. Soc. Chem. Commun. 1994, 855 ± 856.
catalytic process begins to compete. Support for this hypo-
thesis comes from the observation that added lithium
alkoxides do catalyze catecholborane reduction of 1a via
the borate 6. The use of the MTB ligand is vital to the
reaction; LiGaH with either 1,1'-bi(2-naphthol) or 1,1'-bi(2-
thionaphthol) leads to low selectivities (3 ± 34% ee). The
probable causes are decomplexation of the chiral ligand by
catecholborane and poor lithium coordination. Both the
presence of 6 and removal of the chiral ligand may account,
in part, for the lower enantioselectivities encountered in some
[
9] a) G. Giffels, C. Dreisbach, U. Kragl, M. Weigerding, H. Waldmann, C.
Wandrey, Angew. Chem. 1995, 107, 2165 ± 2166; Angew. Chem. Int. Ed.
Engl. 1995, 34, 2005 ± 2006; b) F. Almqvist, L. Torstensson, A.
Gudmundsson, T. Frejd, Angew. Chem. 1997, 109, 388 ± 389; Angew.
Chem. Int. Ed. Engl. 1997, 36, 376 ± 377.
[
8]
4
[
10] a) Review: C. Girard, H. B. Kagan, Angew. Chem. 1998, 110, 3088 ±
3
127; Angew. Chem. Int. Ed. 1998, 37, 2922 ± 2959; b) D. G. Black-
mond, J. Am. Chem. Soc. 1997, 119, 12934 ± 12937; c) D. Guillaneux,
S. H. Zhao, O. Samuel, D. Rainford, H. B. Kagan, J. Am. Chem. Soc.
1
994, 116, 9430 ± 9439.
[
9]
recent titanium work.
The solid-state structure of the pre-catalyst 5[7] is not
retained in solution during the catalysis. The new species
formed are currently still under investigation. However, the
[
10]
absence of a nonlinear effect in the reduction of 1a by 5
suggests that a mononuclear catalyst with a single active MTB
ligand is responsible for the enantioselection. In Noyoriꢁs
Hydroformylation of Internal Olefins to Linear
Aldehydes with Novel Rhodium Catalysts**
[
3]
BINAL reagent the (R )-ligand gives the (R)-alcohol
a
Lars A. van der Veen, Paul C. J. Kamer, and
Piet W. N. M. van Leeuwen*
because of repulsion between the n electrons of the reagent
and the p electrons of the substrate. The similarity in the ee
value for the reduction of 1a ± d suggests a related electronic
control with 5, but steric factors cannot be ruled out.
Hydroformylation is one of the worldꢁs largest homoge-
neously catalyzed processes in industry, which produces more
than six million tons of aldehydes and alcohols annually.[
Since linear aldehydes are the most desired products a key
issue in this process is the control of regiochemistry. High
selectivities in the hydroformylation of terminal alkenes have
1]
Experimental Section
All operations were performed under argon. A solution of MTB (15 mg,
0
.05 mmol) in THF (5 mL) was treated with LiGaH
solution in Et O) and the mixture stirred (208C, 25 min). The reaction was
cooled to � 208C and catecholborane (1.1 mL of a 1m THF solution,
.1 mmol) and ketone (1.0 mmol) were added. The solution was stirred at
4
(100 mL of a 0.25m
[2]
been reported for both diphosphites and diphosphanes.
2
Selective hydroformylation of internal alkenes, which is of great
interest in industry and in synthetic organic chemistry, on the
other hand is still a relatively unexplored terrain (Scheme 1).
1
�
208C for 18 h (method A), or sealed and stored at � 208C (method B;
�
158C and 4 mol% catalyst for 1d). Alternatively, the catecholborane and
ketone were added at � 788C and the reaction mixture stirred as it warmed
to room temperature overnight (method C). Normal workup procedures
afforded the alcohols 2 as essentially single products (the ee values were
determined by gas chromatography on a chiral column (LIPODEX A or
CYCLODEX B) or the a-methoxy-a-(trifluoromethyl)phenyl acetate
analyzed for the alcohol of 1h).
Scheme 1. The hydroformylation of trans-4-octene to linear and branched
aldehydes.
Received: July 28, 1998 [Z12216IE]
German version: Angew. Chem. 1999, 111, 347 ± 349
Keywords: asymmetric reductions ´ gallium ´ hydrides ´
[*] Prof. Dr. P. W. N. M. van Leeuwen, L. A. van der Veen,
ketones
Dr. P. C. J. Kamer
Institute of Molecular Chemistry
University of Amsterdam
Nieuwe Achtergracht 166, 1018 WV Amsterdam (The Netherlands)
Fax: (‡31)20-525-6456
[
1] a) Review: E. J. Corey, C. J. Helal, Angew. Chem. 1998, 110, 2092 ±
118; Angew. Chem. Int. Ed. 1998, 37, 1987 ± 2012; b) E. J. Corey, R. K.
Bakshi, S. Shibata, C.-P. Chen, V. K. Singh, J. Am. Chem. Soc. 1987,
09, 7925 ± 7926.
2
E-mail: pwnm@anorg.chem.uva.nl
[
**] This work was supported by SON/STW.
1
[
[
2] Review: R. Noyori, S. Hashiguchi, Acc. Chem. Res. 1997, 30, 97 ± 102.
3] R. Noyori, I. Tomino, Y. Tanimoto, M. Nishizawa, J. Am. Chem. Soc.
Supporting information for this article is available on the WWW
under http://www.wiley-vch.de/home/angewandte/ or from the au-
thor.
1984, 106, 6709 ± 6716.
3
36
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
1433-7851/99/3803-0336 $ 17.50+.50/0
Angew. Chem. Int. Ed. 1999, 38, No. 3