Asymmetric Direct 1,2-Addition of Aryl Grignard Reagents to Aryl Alkyl Ketones

Article abstract of DOI:10.1021/acs.orglett.5b03379

The enantioselective addition of Grignard reagents to ketones was promoted by a BINOL derivative bearing alkyl chains at the 3,3′-positions. This is the first asymmetric direct aryl Grignard addition to ketones reported to date. A variety of tertiary diaryl alcohols could be obtained in high yields and enantioselectivities without using any other metal source.

Full text of DOI:10.1021/acs.orglett.5b03379

Letter
pubs.acs.org/OrgLett
Asymmetric Direct 1,2-Addition of Aryl Grignard Reagents to Aryl
Alkyl Ketones
Kazuki Osakama and Makoto Nakajima*
Graduate School of Pharmaceutical Sciences, Kumamoto University, 5-1, Oe-honmachi, Chuo-ku, Kumamoto 862-0973, Japan
S
* Supporting Information
ABSTRACT: The enantioselective addition of Grignard reagents
to ketones was promoted by a BINOL derivative bearing alkyl
chains at the 3,3′-positions. This is the first asymmetric direct aryl
Grignard addition to ketones reported to date. A variety of tertiary
diaryl alcohols could be obtained in high yields and enantiose-
lectivities without using any other metal source.
nantiopure tertiary alcohols are important structural motifs
in organic chemistry and are ubiquitous in natural products
aryl Grignard reagents to simple ketones using magnesium
binaphtholate as a chiral ligand, providing tertiary diaryl alcohols
in high yields and enantioselectivities.
E
and pharmaceutical compounds.1 The simplest approach to
constructing these structures involves the enantioselective 1,2-
addition of organometallics to ketones.2,3 The addition of alkyl
groups to ketones has been achieved by developing catalysts
using organozinc,4 titanium,4 and aluminum.5 By contrast, the
enantioselective addition of readily available Grignard reagents
has proven to be more difficult.6 Recently, copper-catalyzed
asymmetric additions of alkyl Grignard reagents were reported.7
Only a few Grignard reagent additions have been achieved
without the use of additional metals, and these reactions have
required more than stoichiometric amounts of a chiral ligand to
achieve a high enantioselectivity.8,9
Initially, we attempted the addition of a THF solution of 2′-
acetonaphthone (1a) to phenylmagnesium bromide (2a) (3.5
equiv) and (R)-BINOL (3a) (100 mol %) as a chiral ligand in
THF at −45 °C (Table 1, entry 1). In our research, 2 equiv of the
Grignard reagent was consumed during deprotonation of the
chiral ligand, and more than 1 equiv of the reagent was essential
for the reaction with a ketone; therefore, we first used 3.5 equiv of
2a. The reaction proceeded smoothly to give the tertiary alcohol
4a in good yield and low ee. The stereoselectivity was improved
by investigating other BINOL derivatives. The desired product
was obtained in moderate enantioselectivity in the presence of
(R)-3,3′-diphenyl-1,1′-binaphthol 3b, which was the best catalyst
identified in our previous alkynylation reactions (entry 2). This
result suggested that the high stereoinduction of tertiary alcohols
was essential for obtaining steric effects at the 3,3′-positions of
the BINOL skeleton. We next tested the 2,6-dimethylphenyl-
substituted BINOL derivative 3c, but no enantioselectivity was
achieved (entry 3).
BINOLs with substituents at the 3,3′-positions are versatile
C2-symmetric chiral ligands; however, few examples have
described the use of compounds bearing alkyl groups in these
positions as asymmetric ligands.18 Therefore, we accepted the
challenge associated with synthesizing these new types of BINOL
ligands. The BINOL derivatives 3d and 3e, bearing benzyl and 1-
methoxy-1-methylethyl groups, provided a poor yield and poor
enantioselectivity (entries 4 and 5); however, the ligand 3f, which
included 2-methoxy-2-methylpropyl groups, yielded an im-
proved enantioselectivity (entry 6). The ligand 3g gave almost
the same result (entry 7). We next introduced bulky aliphatic
chains at the 3,3′-positions of BINOL (entries 8 and 9).
The addition of aryl groups to ketones was first achieved using
an asymmetric reaction of ZnPh2, as reported by Fu et al. in
1998.10 Over the two subsequent decades, several approaches to
obtaining chiral tertiary alcohols have been explored using
arylzinc,11 -titanium,12 -boron compounds,13 and -aluminum.14
These reports showed excellent results; however, they are
disadvantaged in their requirement for nucleophile preparation.
For example, the preparation of arylzinc reagents from excess
ZnEt2 and arylboron compounds required elevated temperatures
and long reaction times. Triarylaluminum compounds were
prepared from AlCl3 and 3 equiv of aryl Grignard reagents in
THF over 12 h. In comparison with these preparations,
asymmetric direct aryl Grignard additions to ketones are very
simple procedures and efficient methods for reduction of other
metal wastes. Although enantioselective catalytic additions of aryl
Grignard reagents with excess amounts of Ti(OiPr)4 were
reported recently,15 to the best of our knowledge, no examples of
enantioselective aryl Grignard addition to simple ketones
without the use of other metals have yet been described.16
We previously reported the enantioselective alkynylation of
lithium acetylide to ketones using lithium binaphtholate as a
catalyst.17 Here, we report the first direct asymmetric addition of
Received: November 25, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03379
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Organic Letters
Letter
a
Table 1. Optimization of the Chiral Ligands
Table 2. Various Simple Ketones Were Tested for the
Preparation of the Tertiary Alcohols
a
entry
1
R1
R2
4
yield (%)
ee (%)
1
2
3
4
5
6
7
1a
1b
1c
1d
1e
1f
2-naphthyl
1-naphthyl
4-MeOC6H4
4-BrC6H4
4-ClC6H4
2-naphthyl
cHex
Me
Me
Me
Me
Me
Et
4a
4b
4c
4d
4e
4f
94
70
95
92
98
97
92
93
96
91
91
86
86
30
1g
Me
4g
a
All reactions were conducted using ketone 1 (0.5 mmol),
phenylmagnesium bromide (2a) (1.75 mmol), and ligand 3i (0.5
mmol) in THF (1.0 mL) at −45 °C.
We next investigated the reactions of various Grignard
reagents with acetophenone (1h) (Table 3). 2-Naphthylmagne-
Table 3. Various Grignard Reagents Were Tested for the
a
Preparation of the Tertiary Alcohols
a
Unless otherwise noted, the reactions were conducted using the
following procedure: A THF solution of 2′-acetonaphthone (1a) (0.5
mmol) was added to a solution of phenylmagnesium bromide (2a)
(1.75 mmol) and ligand 3 (0.5 mmol) in THF (1.0 mL) at −45 °C.
entry
2
R3
4
yield (%)
ee (%)
1
2
3
4
5
6
7
8
9
2b
2c
2d
2e
2f
2-naphthyl
4-MeOC6H4
3-MeOC6H4
2-MeOC6H4
4-MeC6H4
2-MeC6H4
4-FC6H4
ent-4a
ent-4c
4h
80
94
90
97
96
98
83
70
10
94
84
87
6
b
Phenylmagnesium bromide (2a) (1.75 mmol) was added to the
4i
solution of 2′-acetonaphthone (1a) (0.5 mmol) and ligand 3 (0.5
mmol) in THF (1.0 mL) at −45 °C.
4j
88
90
94
97
59
2g
2h
2i
4k
4l
Gratifyingly, we succeeded in improving the enantioselectivity.
The tertiary alcohol was obtained in a 92% yield and 90% ee
using ligand 3i with 3,3-dimethylbutyl groups. Furthermore,
reactions unrelated to the ligand were partially suppressed by
addition of phenylmagnesium bromide (2a) to the mixture of 2′-
acetonaphthone (1a) and the ligand 3i in THF, yielding the
corresponding product in 94% yield and 93% ee (entry 10).19 In
our procedure, the reaction reached completion within 1 h. The
reaction time was shorter than the time required for the
corresponding aryl organometallics, which generally required
over 12 h. Although we used stoichiometric amounts of the chiral
ligand 3i, this ligand was easily recovered and reused.20
The conditions optimized for ketone 1a were applied to the
reactions of other simple ketones 1b−g (Table 2). 1′-
Acetonaphthone (1b) showed a lower chemical yield than 1a,
although the obtained product 4b displayed an excellent
enantioselectivity (entry 2). Although the product 4e displayed
a slightly lower enantioselectivity, the aromatic ketones 1c−e
bearing electron-donating and -withdrawing groups smoothly
reacted to give the tertiary alcohols 4c−e in very high yields and
enantioselectivities (entries 3−5). 2-Naphthyl ethyl ketone (1f),
which included bulkier R2 groups than 1a, reacted with an
acceptable enantioselectivity (entry 6). Cyclohexyl methyl
ketone (1g) gave a high yield with a low ee (entry 7). Therefore,
intermolecular interactions such as π−π stacking between the
aromatic ketone and the Grignard reagent appeared to play a key
role in the transition state of the 1,2-addition.
4-ClC6H4
cHex
ent-4e
ent-4g
2j
a
All reactions were conducted using acetophenone (1h) (0.5 mmol),
Grignard reagent 2 (1.75 mmol), and ligand 3i (0.5 mmol) in THF
(1.0 mL) at −45 °C.
sium bromide (2b) afforded the product ent-4a in good yield
with a high enantioselectivity (entry 1) with an absolute
configuration opposite that of the product 4a derived from the
phenyl addition to 2′-acetonaphthone. The use of 2c and 2d,
which bore electron-donating groups at the 4′-, and 3′-positions
of the aromatic group, slightly decreased the enantioselectivities
of the tertiary alcohols ent-4c and 4h (entries 2 and 3). A
methoxy group at the 2′-position dramatically decreased the ee
(entry 4). This effect was attributed to chelation at the metal
center. The methyl-substituted Grignard reagents 2f and 2g
generated the product in a high yield and acceptable ee (entries 5
and 6). Compounds 2h and 2i, which bore electron-withdrawing
groups, provided a slightly lower chemical yield, but the ee values
of the tertiary alcohols 4l and ent-4e were excellent (entries 7 and
8). A reaction of cyclohexylmagnesium bromide (2j) dramati-
cally decreased the yield of the product, accompanied by a
moderate ee (entry 9). The basicity of the aliphatic Grignard
reagent was stronger than that of the aromatic reagent; therefore,
enolization of the ketone occurred as the main side reaction.
B
DOI: 10.1021/acs.orglett.5b03379
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Organic Letters
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The mechanism underlying the stereoinduction was inves-
tigated by examining the structure of the Mg salt of the ligand
(Table 4). BINOL derivatives have two oxygen atoms; therefore,
Table 5. Reduction of the Amounts of Chiral Ligand 3i and
Phenylmagnesium Bromide (2a)
a
a
Table 4. Investigation of the Structure of the Mg Salt
entry
X
Y
yield (%)
ee (%)
1
2
3
4
5
100
100
50
3.5
3.0
2.0
1.6
1.4
94
90
81
70
68
93
92
90
64
47
30
20
a
Each reaction was conducted using 2′-acetonaphthone (1a) (0.5
mmol), phenylmagnesium bromide (2a) (Y equiv), and ligand 3i (X
mol %) in THF (1.0 mL) at −45 °C.
Our new BINOL derivative 3i is easily recyclable and may
potentially afford catalytic activity. Further investigations toward
achieving BINOL-catalyzed direct Grignard additions are
currently in progress.
a
Ligand 5 was prepared in situ by stirring a 1:1 ratio of 3i and nBu2Mg.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b03379.
■
two compounds, the monomagnesium salt 5 and the
dimagnesium salt 6, were presumed to be present. Mono-
magnesium binaphtholate was used as a good asymmetric
catalyst and could be readily prepared from BINOL and
nBu2Mg.21 We tried to use this species in our Grignard addition;
however, the product was obtained in a moderate yield and a low
ee (entry 1). We attempted a reaction of 1a and 1.5 equiv of 2a in
the absence of ligand 3i, and the tertiary alcohol was obtained in
77% yield (entry 2). This chemical yield was lower than that
obtained in the presence of the ligand. No enantioselectivity was
achieved from the O-methylated BINOL derivative 7 (entry 3).
We next examined the relationship between the ee values of
the ligand 3i and those of the product 4a. No clear nonlinear
effects were observed in the 1,2-addition between 1a and 2a.22
Aggregation has been reported in magnesium binaphtholate;21a,e
however, we assumed that the monomeric species acted as an
asymmetric auxiliary in the transition state.
These results suggested that the dimagnesium salt 6 was
generated in the reaction and coordinated to the Grignard
reagent as a Lewis base to enhance the nucleophilicity.
Furthermore, the magnesium atom of 6 was effective in activating
the ketone, and substituents at the 3,3′-positions of BINOLs
controlled the direction of nucleophilic attack on the prochiral
carbon atom. Although further investigations of the mechanism
are required to fully understand this method, the transition state
appeared to be a monomeric dimagnesium salt 6.
Finally, we examined the effects of the ratio between the chiral
ligand 3i and the Grignard reagent (Table 5). Interestingly, even
50 mol % of 3i gave the product in a good yield and an ee of 90%
(entry 3); however, reducing the amounts of the chiral ligand
tended to reduce the yield and ee (entries 4 and 5). These results
suggested that the complex between the Grignard reagent and 3i
was more reactive than the Grignard reagent alone. Therefore, a
well-designed BINOL may provide a high ee in a catalytic version
of this asymmetric Grignard addition. We are currently
investigating the synthesis of more effective BINOL derivative.
In conclusion, we have demonstrated the first example of
enantioselective direct aryl Grignard additions to aryl alkyl
ketones to afford the tertiary diaryl alcohols. Our protocol is very
simple and does not require other metals or long reaction times.
S
Experimental procedures and spectral data for all new
products (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: nakajima@gpo.kumamoto-u.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology of Japan.
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