Chiral tetrafluorobenzobarrelenes as highly efficient ligands for the rhodium-catalyzed asymmetric 1,4-addition of arylboronic acids

Article abstract of DOI:10.1246/cl.2008.860

New C₂-symmetric chiral diene ligands bearing a tetrafluorobenzobarrelene framework and (-)-menthyl groups as chiral auxiliaries were prepared through [4 + 2] cycloaddition of 1,4-bis((-)-menthoxymethyl) benzene with tetrafluorobenzyne. The die

Full text of DOI:10.1246/cl.2008.860

860
Chemistry Letters Vol.37, No.8 (2008)
Chiral Tetrafluorobenzobarrelenes as Highly Efficient Ligands for the Rhodium-catalyzed
Asymmetric 1,4-Addition of Arylboronic Acids
Takahiro Nishimura,ꢀ Makoto Nagaosa, and Tamio Hayashiꢀ
Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502
(Received May 26, 2008; CL-080531; E-mail: thayashi@kuchem.kyoto-u.ac.jp)
New C2-symmetric chiral diene ligands bearing a tetra-
Cl
OR*
fluorobenzobarrelene framework and (ꢁ)-menthyl groups as
chiral auxiliaries were prepared through [4 + 2] cycloaddition
of 1,4-bis((ꢁ)-menthoxymethyl)benzene with tetrafluoroben-
zyne. The diene ligands realized the rhodium-catalyzed asym-
metric addition of arylboronic acids to ꢀ,ꢁ-unsaturated carbonyl
compounds giving ꢁ-arylcarbonyl compounds in high yields
with high enantioselectivity.
NaH
DMF
+
=
R*O
OH
O
Cl
R*O
2: 72%
(−)-menthol
F
F
F
F
F
F
Pentafluorobenzene
BuLi
+
F
F
Heptane
0 °C−rt, 12 h
R*O
R*O
OR*
OR*
Recent progress in rhodium-catalyzed asymmetric reactions
using chiral diene ligands has opened up new possibilities for de-
signing ligands for transition metals.1–4 The chiral diene ligands
developed by us2 and other groups3 have frequently displayed
higher activity and enantioselectivity than chiral phosphine
ligands in catalytic asymmetric reactions. Enantiomerically pure
dienes have been prepared by use of commercially available
chiral building blocks,3a–3d by asymmetric synthesis of chiral
intermediates,2a,3f–3h or by resolution using chiral HPLC.2b–2i
Those ligands generally require multistep synthesis and thus,
the development of readily accessible chiral diene ligands is
highly desirable. Here, we wish to report the new C2-symmetric
tetrafluorobenzo[2.2.2]octatrienes (tetrafluorobenzobarrelenes;
tfb) 15 as chiral diene ligands, which are prepared in two steps
from commercially available reagents (Chart 1). Diastereo-
and enantiomerically pure dienes substituted with (ꢁ)-
menthoxymethyl groups as chiral auxiliaries, are isolated by
column chromatography on silica gel. Their application to
rhodium-catalyzed asymmetric conjugate arylation, which dis-
plays very high enantioselectivity, is also described.
(
R
,
R
)-1
(
S
,
S
)-1
8%
8%
[RhCl((
R
,
R
)-1)]2
)-1)]2
(
R
,
or
R
)-1
93%
[RhCl(coe)2]2
+
or
Hexane
50 °C, 2 h
[RhCl((S,S
(S,S
)-1
78%
4 M KOH (4 equiv to Rh)
Acetone, rt, 1 h
[RhCl((
R
,R
)-1)]2
[Rh(OH)((
R,R)-1)]2
93%
Scheme 1. Synthesis of chiral tfb ligands.
O
F
F
O
Cl(1)
Cl(2)
F
Rh(1)
F
Rh(2)
F
F
O
F
O
F
The C2-symmetric tfb ligands were prepared through a
straightforward pathway (Scheme 1). 1,4-Bis((ꢁ)-menthoxy-
methyl)benzene (2)6 was prepared in 72% yield by the etheration
of 1,4-bis(chloromethyl)benzene with (ꢁ)-menthol. The [4 + 2]
cycloaddition of 2 with tetrafluorobenzyne according to a known
procedure5b,5c gave a 1 to 1 mixture of diastereomers, (R,R)-1
and (S,S)-1. It should be noted that these dienes were prepared
in only two steps and that each diastereomer was isolated by
column chromatography on silica gel, although their yields
Figure 1. ORTEP illustration of complex [RhCl((R,R)-1)]2
with thermal ellipsoids drawn at the 50% probability level.
corresponding diene–rhodium complexes, [RhCl((R,R)-1)]2 or
[RhCl((S,S)-1)]2.8 The hydroxorhodium complex [Rh(OH)-
((R,R)-1)]2 was also prepared by the reaction of [RhCl((R,R)-
1)]2 with aqueous KOH in acetone at room temperature for
1 h. The X-ray crystal structure of the rhodium complex
[RhCl((R,R)-1)]2 determined the relative and absolute configura-
tion of the diene (1R,4R)-1 as shown in Figure 1.9 The steric
difference between a bulky menthoxymethyl group and a small
hydrogen atom creates a good chiral environment around the
rhodium.
The new tfb–rhodium complexes displayed very high cata-
lytic activity and enantioselectivity in asymmetric 1,4-addi-
tion.10 Thus, treatment of 2-cyclohexen-1-one with phenylbor-
onic acid (2 equiv) and KOH (50 mol %) in 1,4-dioxane/H2O
in the presence of [RhCl((R,R)-1)]2 (3 mol % of Rh) at 30 ꢂC
7
were not satisfactory. Treatment of [RhCl(coe)2]2 with (R,R)-
1 or (S,S)-1 in hexane at 50 ꢂC for 2 h gave high yields of the
F
F
F
F
F
F
F
F
O
O
O
O
(
R
,
R
)-1
(S,S)-1
Chart 1.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.8 (2008)
861
tioselective than other chiral ligands in the rhodium-catalyzed
asymmetric 1,4-addition of arylboronic acids to ꢀ,ꢁ-unsaturated
carbonyl compounds.
O
O
Rh complex
(3 mol % Rh)
+
PhB(OH)2
*
KOH (50 mol %)
1,4-Dioxane/H2O (10/1)
30 °C, 1 h
Ph
In summary, we have developed new C2-symmetric diene
ligands having a tetrafluorobenzobarrelene framework, which
were prepared in two steps from commercially available re-
agents. Two diastereomers bearing (ꢁ)-menthyl groups as chiral
auxiliaries were separated by a simple column chromatography
on silica gel. These ligands realized the rhodium-catalyzed
highly enantioselective 1,4-addition of arylboronic acids to
ꢀ,ꢁ-unsaturated carbonyl compounds.11
[RhCl((
[RhCl((
[RhCl((
R
,
R
S
S
)-1)]2
)-1)]2
)-3)]2
97%, 99% ee (
94%, 99% ee (
100%, 96% ee (
R
S
S
)
)
)
Ph
S
,
Ph
S,
(S,S)-3
Scheme 2. Rhodium-catalyzed asymmetric 1,4-addition of
phenylboronic acid to 2-cyclohexen-1-one.
O
O
[Rh(OH)((
R,R)-1)]2
(3 mol % Rh)
References and Notes
+
RB(OH)2
KOH (50 mol %)
1,4-Dioxane/H2O (10/1)
30 °C, 1 h
1
2
For reviews, see: a) J. B. Johnson, T. Rovis, Angew. Chem., Int. Ed.
2008, 47, 840. b) C. Defieber, H. Grutzmacher, E. M. Carreira, Angew.
Chem., Int. Ed. 2008, 47, 4482.
R
¨
R = C6H5
98%, 99% ee (
R
R
R
R
R
)
)
)
)
)
95%, 99% ee (
92%, 99% ee (
89%, 98% ee (
R
R
R
R
)
)
)
)
4-CF3C6H4
4-FC6H4
2-Naphthyl
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4-MeC6H4 94%, 99% ee (
4-MeOC6H4 97%, 99% ee (
4-ClC6H4
3-ClC6H4
95%, 99% ee (
91%, 99% ee (
(E
)-
t
-BuCH=CH 86%, 84% ee (
O
O
O
Ph
C5H11
Ph
Ph
99%, 94% ee (
85%, 99% ee (
OMe
R
)
88%, 96% ee (
R
)
S
)
OMe
O
O
O
3
a) C. Fischer, C. Defieber, T. Suzuki, E. M. Carreira, J. Am. Chem. Soc.
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tBuO
Ph
H
Ph
Ph
97%, 98% ee (
99%, 99% ee (
R
)
R
)
80%, 99% ee (
[10 °C, 6 h in THF/MeOH]
R
)
2005, 127, 10850. e) F. Lang, F. Breher, D. Stein, H. Grutzmacher,
¨
¨
Organometallics 2005, 24, 2997. f) T. Noel, K. Vandyck, J. Van der
¨
Scheme 3. Asymmetric 1,4-addition catalyzed by [Rh(OH)-
((R,R)-1)]2.
Eycken, Tetrahedron 2007, 63, 12961. g) Z.-Q. Wang, C.-G. Feng,
M.-H. Xu, G.-Q. Lin, J. Am. Chem. Soc. 2007, 129, 5336. h) S. Helbig,
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M. A. Grundl, J. J. Kennedy-Smith, D. Trauner, Organometallics
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J. P. N. Brewer, H. Heaney, Tetrahedron Lett. 1965, 6, 4709. b)
J. P. N. Brewer, I. F. Eckhard, H. Heaney, B. A. Marples, J. Chem.
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Uff, Tetrahedron 1969, 25, 25.
Y. Inoue, N. Yamasaki, T. Yokoyama, A. Tai, J. Org. Chem. 1992, 57,
1332.
A. Van Der Ent, A. L. Onderdelinden, R. A. Schunn, Inorg. Synth.
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For examples of the synthesis of rhodium–tetrafluorobenzobarrelene
complexes, see: a) M. A. Esteruelas, L. A. Oro, Coord. Chem. Rev.
1999, 193–195, 557. b) D. M. Roe, A. G. Massey, J. Organomet.
for 1 h gave (R)-3-phenylcyclohexanone in 97% yield with 99%
ee (Scheme 2). The use of [RhCl((S,S)-1)]2 gave the 1,4-addition
product of opposite absolute configuration (S) with the same
high enantioselectivity (99%), indicating that the stereochemical
pathway is decided mainly by the chiral diene moiety with little
influence of the (ꢁ)-menthyl group. The enantioselectivity
observed with (R,R)-1 or (S,S)-1 is higher than that with 2,5-
dibenzylbicyclo[2.2.2]octadiene [(S,S)-3],2b–2f which is one of
the most effective diene ligands for rhodium-catalyzed asym-
metric reactions. The high enantioselectivity of 1 may be
ascribed to the steric bulkiness of menthoxymethyl group.
The reaction scope of the present enantioselective arylation
catalyzed by rhodium/(R,R)-1 is broad (Scheme 3). For exam-
ple, in the presence of [Rh(OH)((R,R)-1)]2 (3 mol % of Rh),
the 1,4-addition of arylboronic acids substituted with several
functional groups to 2-cyclohexen-1-one gave the corresponding
3-arylcyclohexanones in high yields with excellent enantioselec-
tivity (98–99% ee). The addition of (E)-3,3-dimethyl-1-butenyl-
boronic acid gave the corresponding 3-alkenylcyclohexanone in
86% yield with 84% ee. The asymmetric addition of phenylbor-
onic acid to 2-cyclopenten-1-one, 2-cyclohepten-1-one, and
3-nonen-2-one is also catalyzed by rhodium/(R,R)-1 with high
enantioselectivity. The introduction of a 4-methylphenyl or
4-methoxyphenyl group onto the ꢁ-phenyl-substituted enone,
enoate, and enal2d all proceeded with high enantioselectivity
(98–99% ee) to give the corresponding arylation products which
contain the diaryl-substituted stereogenic carbon center. These
results demonstrate that the tfb 1 presented here is not less enan-
4
5
6
7
8
´
Chem. 1971, 28, 273. c) R. Uson, L. A. Oro, R. Sariego, M.
Valderrama, C. Rebullida, J. Organomet. Chem. 1980, 197, 87.
Crystal data for [RhCl((R,R)-1)]2: C68H92Cl2F8O4Rh2, Mr ¼ 1402:14,
9
˚
˚
space group P21 (No. 4), a ¼ 10:382ð2Þ A, b ¼ 17:034ð4Þ A, c ¼
18:749ð5Þ A, V ¼ 3314:1ð13Þ A , Z ¼ 2, Dcalcd ¼ 1:405 g/cm3, T ¼
3
˚
˚
123 K, R1 ¼ 0:0383, wR2 ¼ 0:0644, GOF = 0.822.
10 For recent reviews of asymmetric conjugate additions, see: a) T.
Hayashi, K. Yamasaki, Chem. Rev. 2003, 103, 2829. b) T. Hayashi,
Bull. Chem. Soc. Jpn. 2004, 77, 13. c) S. Darses, J.-P. Genet, Eur. J.
Org. Chem. 2003, 4313. d) K. Fagnou, M. Lautens, Chem. Rev.
2003, 103, 169. e) J. Christoffers, G. Koripelly, A. Rosiak, M. Rossle,
Synthesis 2007, 1279.
¨
11 Supporting Information is available electronically on the CSJ-Journal
Web site, http://www.csj.jp/journals/chem-lett/.
Published on the web (Advance View) July 12, 2008; doi:10.1246/cl.2008.860