Nickel-catalyzed asymmetric Grignard cross-coupling of dinaphthothiophene giving axially chiral 1,1′-binaphthyls

Article abstract of DOI:10.1021/ja0282588

Asymmetric cross-coupling of dinaphtho[2,1-b:1′,2′-d]thiophene with ArMgBr (Ar = Ph, 4-MeC₆H₄, 4-MeOC₆H₄) proceeded withhigh enantioselectivity in THF at 20 °C in the presence of 3 mol % of a nickel catalyst gen

Full text of DOI:10.1021/ja0282588

Published on Web 10/17/2002
Nickel-Catalyzed Asymmetric Grignard Cross-Coupling of
Dinaphthothiophene Giving Axially Chiral 1,1′-Binaphthyls
Toyoshi Shimada, Yong-Hwan Cho, and Tamio Hayashi*
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo, Kyoto 606-8502, Japan
Received August 22, 2002
Scheme 1
Axially chiral biaryls represented by 1,1′-binaphthyls are known
to create effective chiral environments for asymmetric reactions,1
and growing attention has been paid to their synthesis by means of
asymmetric catalysis. One of the most promising methods is the
nickel- or palladium-catalyzed asymmetric cross-coupling,2 which
can be grouped into two types according to the mode of generation
of the axial chirality. One is the cross-coupling of arylmagnesium3
or -boron reagents4 with aryl halides, where the axial chirality is
generated at the formation of the biaryl skeleton. The other is the
enantioposition-selective cross-coupling of achiral biaryl ditriflates,
where one of the enantiotopic triflates participates in the cross-
coupling.5 Here we wish to report the third type of catalytic
asymmetric cross-coupling, that is, the nickel-catalyzed reaction
of dinaphtho[2,1-b:1′,2′-d]thiophene (1) with the Grignard reagents
giving axially chiral 1,1′-binaphthyls (Scheme 1), where the axial
chirality is generated at the cleavage of the carbon-sulfur bond in
the thiophene ring.
It has been reported that the carbon-sulfur bond cleavage takes
place in thiophene and benzothiophene in the reaction with the
Grignard reagents under the catalysis by a nickel complex to give
the cross-coupling products.6 On the other hand, the dinaph-
Table 1. Nickel-Catalyzed Asymmetric Cross-Coupling of 1 with
thothiophene 1, readily accessible from binaphthol,7 is regarded as
the Grignard Reagents 2a
an achiral molecule because of the rapid flipping at ambient
ligand
R in
temp
C)
time
(h)
yield of
% ee of
temperature,8 and it is expected to become an axially chiral molecule
once the thiophene ring undergoes the ring opening. We examined
several reaction conditions, mainly chiral ligands of the nickel
catalyst, for high catalytic activity and high enantioselectivity in
the asymmetric cross-coupling of dinaphthothiophene 1 with
4-methylphenylmagnesium bromide (2a). It was found that the
oxazoline-phosphine (S)-4 containing an isopropyl substituent9 is
the most enantioselective of the ligands examined (Table 1). Thus,
the reaction of 1 with 2 equiv of the Grignard reagent 2a in the
entry
(eq to Ni)
RMgX (2)
(
°
3 (%)b
3c (config)
1d (S)-4 (1.5)
4-MeC6H4
4-MeC6H4
4-MeC6H4
20
20
20
20
20
20
20
20
20
0
120
24
30
12
8
10
10
24
30
24
24
77
97
90
88
86
93
72
92
96
54
97
95 (S)
95 (S)
86 (R)
3 (S)
1 (S)
14 (S)
2 (S)
95 (S)
93 (S)
54 (R)
68 (R)
2
3
(S)-4 (1.5)
(R)-5 (1.5)
4
(S)-(R)-6 (3.0) 4-MeC6H4
5
6
7
8
9
10
11
(S)-7 (1.5)
(S)-8 (3.0)
(R)-9 (1.5)
(S)-4 (1.5)
(S)-4 (1.5)
(S)-4 (1.5)
(S)-8 (3.0)
4-MeC6H4
4-MeC6H4
4-MeC6H4
Ph
4-MeOC6H4
Me
10
presence of 3 mol % of a nickel catalyst generated from Ni(cod)2
and ligand 4 in THF11 at 20 °C for 120 h gave, after aqueous
workup, 77% isolated yield of (S)-2-mercapto-2′-(4-methylphenyl)-
1,1′-binaphthyl (3a), whose enantiomeric purity was determined
to be 95% ee by HPLC analysis with a chiral stationary phase
column (entry 1 in Table 1). Use of a large excess (10 equiv) of
the Grignard reagent 2a gave 97% yield of (S)-3a within 24 h (entry
2). The phenyl-substituted oxazoline-phosphine (R)-5 was the
second best, giving (R)-3a of 86% ee (entry 3). Other chiral
phosphine ligands 6-9 including those used successfully for other
types of asymmetric cross-coupling3-5 were much less enantiose-
lective than the oxazoline-phosphine ligands for the present reaction
(entries 4-7). The reaction with the phenyl (2b) and 4-methox-
yphenyl (2c) Grignard reagents also proceeded with high enanti-
oselectivity in the presence of the nickel catalyst of (S)-4 to give
Me
10
a The reaction was carried out with 10 equiv of Grignard reagent in the
presence of 3 mol % of Ni(cod)2/ligand in THF unless otherwise noted.
b Isolated yield. c Determined by HPLC analysis of the phenylcarbamate
ester of thiol 3 with chiral stationary phase columns: Chiralpak AD (3a,
3c, 3d) (hexane/2-propanol ) 90/10) and Chiralcel OD-H (3b) (hexane/2-
propanol ) 90/10). d Reaction with 2 equiv of 2a.
the corresponding binaphthylthiols 3b and 3c with over 93%
enantioselectivity (entries 8 and 9). For the introduction of the
methyl group, MOP ligand (S)-812 was more effective than the
oxazoline-phosphine (S)-4 in terms of both catalytic activity and
enantioselectivity (entries 10 and 11).
According to the catalytic cycle generally accepted for the nickel-
catalyzed cross-coupling,13 the present reaction involves nickela-
cycle 10 which is formed by oxidative addition of dinaph-
thothiophene 1 to a nickel(0) species14 (Scheme 2). Transmetalation
of the aryl or alkyl group from the Grignard reagent to 10 leads to
* To whom correspondence should be addressed. E-mail: thayashi@
kuchem.kyoto-u.ac.jp.
9
13396
J. AM. CHEM. SOC. 2002, 124, 13396-13397
10.1021/ja0282588 CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
racemization much faster than the catalytic cross-coupling reaction,
and, after the asymmetric carbon-carbon bond formation, the
racemization does not take place. Studies on catalytic asymmetric
synthesis of various kinds of axially chiral biaryls using this type
of methodology are in progress.
Acknowledgment. This work was supported in part by a Grant-
in-Aid for Scientific Research from the Ministry of Education,
Science, Sports, and Culture, Japan.
Supporting Information Available: Experimental procedures,
spectroscopic and analytical data for the products (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
Scheme 3 a
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a Reaction conditions: (a) MeI, K2CO3, acetone, room temperature, 99%;
(b) mCPBA, CH2Cl2, 0 °C, 92%; (c) EtMgBr, THF, room temperature; (d)
I2, room temperature, 67%; (e) (i) B(OMe)3, (ii) 10% HCl, (iii) pinacol,
benzene, reflux 48%; (f) Ph2PCl, room temperature, 62%; (g) Ni(acac)2,
MeMgI, THF, 50 °C, 76%.
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thiophenium salt has been measured to be ∆Gq ) 48 kJ mol-1 at -33
°C. Assuming that 1 has a similar energy barrier, we calculated the half-
life for racemization to be shorter than 0.004 s: Fabbri, D.; Dore, A.;
Gladiali, S.; De Lucchi, O.; Valle, G. Gazz. Chim. Ital. 1996, 126, 11.
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little slower.
(11) THF is a solvent of choice because the dinaphthothiophene 1 is not soluble
in other solvents (benzene, toluene, or diethyl ether) usually used for the
Grignard cross-coupling.
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diorganonickel intermediate 11, the reductive elimination from
which gives the cross-coupling product. The dependency of the
enantioselectivity on the Grignard reagent observed here may
indicate that the stereochemical outcome is determined at or after
the transmetalation step.
The axially chiral cross-coupling products 3 are versatile
intermediates for further transformation, the mercapto group being
replaced by some functional groups by way of the methylsulfinyl
group. Thus, the 2-methylsulfinylbinaphthyl 12, obtained by
methylation of the mercapto group in (S)-3b15 (95% ee) followed
by oxidation of the sulfide with peracid, was allowed to react with
ethylmagnesium bromide in THF16 to generate a binaphthylmag-
nesium bromide, treatment of which with electrophiles gave iodide
(S)-13, boronate (R)-14, and phosphine (S)-1515,17 without racem-
ization (Scheme 3). The substitution of the methylsulfinyl group
with an alkyl group is also possible by nickel-catalyzed cross-
coupling. For example, the cross-coupling of 12 with the methyl
Grignard reagent proceeded in refluxing THF to give a high yield
of the methylation product 16.
In conclusion, we have described a new efficient route to axially
chiral 1,1′-binaphthyls, which has been realized by nickel-catalyzed
asymmetric cross-coupling of dinaphthothiophene 1 with the
Grignard reagents. This asymmetric reaction is formally classified
as a dynamic kinetic resolution of the starting chiral substrate.18
The dinaphthothiophene 1 is a chiral molecule but undergoes
(14) The oxidative addition of a dibenzothiophene to a nickel(0) species forming
a nickelacycle complex has been reported: Vicic, D. A.; Jones, W. D. J.
Am. Chem. Soc. 1999, 121, 7606.
(15) The absolute configuration of the cross-coupling product 3b was
determined by comparison of the optical rotation value ([R]20 -137 (c
D
1.0, chloroform)) of the MOP ligand 15 obtained here with that ([R]20
D
+145 (c 1.0, chloroform)) of the authentic sample prepared from (R)-
binaphthol (ref 12).
(16) Hoffmann, R. W.; Ho¨lzer, B.; Knopff, O.; Harms, K. Angew. Chem., Int.
Ed. 2000, 39, 3072.
(17) The MOP ligand 15 is useful as a chiral ligand for the palladium-catalyzed
asymmetric hydrosilylation of 1,3-dienes (ref 12).
(18) Similar asymmetric ring-opening reactions of biaryl lactones have been
studied extensively by Bringmann: Bringmann, G.; Breuning, M.; Tasler,
S. Synthesis 1999, 525.
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