Highly Regio- and Enantioselective Copper-Catalyzed Reductive Hydroxymethylation of Styrenes and 1,3-Dienes with CO2

Article abstract of DOI:10.1021/jacs.7b10149

Herein, we report a highly regio- and enantioselective copper-catalyzed reductive hydroxymethylation of styrenes and 1,3-dienes with 1 atm of CO₂. Diverse important chiral homobenzylic alcohols were readily prepared from styrenes. Moreover, a variety of 1,3-dienes also were converted to chiral homoallylic alcohols with high yields and excellent regio-, enantio-, and Z/E-selectivities. The utility of this transformation was demonstrated by a broad range of styrenes and 1,3-dienes, facile product modification, and synthesis of bioactive compounds (R)-(-)-curcumene and (S)-(+)-ibuprofen. Mechanistic studies demonstrated the carboxylation of phenylethylcopper complexes with CO₂ as one key step.

Full text of DOI:10.1021/jacs.7b10149

Communication
pubs.acs.org/JACS
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX-XXX
Highly Regio- and Enantioselective Copper-Catalyzed Reductive
Hydroxymethylation of Styrenes and 1,3-Dienes with CO2
Yong-Yuan Gui,†,‡ Naifu Hu,†,‡,§ Xiao-Wang Chen,† Li−Li Liao,† Tao Ju,† Jian-Heng Ye,† Zhen Zhang,∥
Jing Li,† and Da-Gang Yu*,†
†Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Sichuan University, Chengdu
610064, P. R. China
∥School of Pharmacy and Bioengineering, Chengdu University, Chengdu 610106, P. R. China
S
* Supporting Information
the efficient Ru-catalyzed hydroformylation/reduction with CO2
and H2.4 In 2015, Fujihara and Tsuji reported an elegant Cu-
ABSTRACT: Herein, we report a highly regio- and
enantioselective copper-catalyzed reductive hydroxymethy-
lation of styrenes and 1,3-dienes with 1 atm of CO2.
Diverse important chiral homobenzylic alcohols were
readily prepared from styrenes. Moreover, a variety of
1,3-dienes also were converted to chiral homoallylic
alcohols with high yields and excellent regio-, enantio-,
and Z/E-selectivities. The utility of this transformation was
demonstrated by a broad range of styrenes and 1,3-dienes,
facile product modification, and synthesis of bioactive
compounds (R)-(−)-curcumene and (S)-(+)-ibuprofen.
Mechanistic studies demonstrated the carboxylation of
phenylethylcopper complexes with CO2 as one key step.
catalyzed reductive hydroxymethylation of allenes with CO2 to
give homoallylic alcohols.5 The asymmetric catalytic synthesis of
chiral higher alcohols with CO2, however, remains unresolved.
The catalytic asymmetric transformation of CO2 is a well-
known challenge, especially for processes involving C−C bond
formation.6 One reason is that CO2 is difficult to activate and
utilize under mild conditions due to its thermodynamic stability
and kinetic inertness. Most reported examples involve CO2
insertion into epoxides or alcohols to form chiral carbonates
via enantioselective C−O bond formation.7 The only successful
examples of enantioselective C−C bond formation with CO2
were reported by Mori and Marek.8 Taking into consideration
our continuous interest in CO2 transformations9 and enlighten-
ment by the impressive achievements in CuH chemistry,10,11 we
have now realized the first enantioselective Cu-catalyzed
reductive hydroxymethylation of alkenes with CO2 (Scheme
1c) to form a series of chiral homobenzylic/allylic higher
alcohols, many of which can act as key intermediates in chemical
synthesis and as important precursors for medicines or other
bioactive compounds.12 Notably, high yields and excellent regio-,
enantio-, and Z/E-selectivities were observed in these reactions.
To begin our investigation, we employed 4-phenylstyrene 1a
as the model substrate and Cu(OAc)2 as the catalyst under 1 atm
of CO2 (Table 1). The reaction was first tested with different
chiral bisphosphine ligands in cyclohexane (see Supporting
Information (SI) for more information). Among them, (R)-
DTBM-SEGPHOS was found to be the best choice in terms of
enantioselective control, affording 2a with 98:2 er (entry 1). The
yield was increased drastically to 75% when the reaction
temperature was elevated to 60 °C (entry 2). Although
subsequent solvent screening did not improve the results
(entries 3−5), further optimization showed that adding one
equiv of CsOAc and lowering the concentration could further
enhance the yield to 78% without diminishing the enantiose-
lectivity (entry 6). To our delight, evaluation of silanes
demonstrated that (EtO)3SiH showed excellent behavior for
this transformation to produce 2a in 94% yield and 98:2 er
(entries 7−9).13 The absolute configuration of 2a was
determined as (S) by X-ray crystallography of its p-nitro-
arbon dioxide (CO2) is an important and appealing C1
C
building block in chemical synthesis because of its
nontoxicity, abundance, availability, and sustainability.1 Over
the past several decades, extensive efforts have been devoted to
the transformation of CO2 into fuels and high value-added
organic chemicals.2 Among these efforts, the catalytic reduction
of CO2 to produce methanol has received considerable attention
because methanol is both a liquid fuel and a bulk chemical
(Scheme 1a).3 Furthermore, the reduction of CO2 to form higher
alcohols through C−C bond formation is highly appealing
(Scheme 1b) since these products are versatile intermediates in
chemical synthesis. Starting with alkenes, several groups realized
Scheme 1. Reduction of CO2 To Produce Alcohols
Received: September 28, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.7b10149
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
a
Table 1. Reaction Optimization
Table 2. Substrate Scope of Styrene Derivatives
b
c
entry
silane
solvent
yield
er
1
Me(MeO)2SiH
Me(MeO)2SiH
Me(MeO)2SiH
Me(MeO)2SiH
Me(MeO)2SiH
Me(MeO)2SiH
MePh2SiH
CyH
CyH
DMF
CPME
EA
20%
75%
37%
65%
72%
78%
f
98:2
99:1
80:20
98:2
95:5
99:1
d
d
d
d
d
d
d
d
2
3
4
5
6
7
8
9
,
,
,
,
e
e
e
e
CyH
CyH
CyH
CyH
CyH
PMHS
64%
94%
f
91:9
98:2
(EtO)3SiH
d,e,g
10
(EtO)3SiH
a
b
c
0.4 mmol scale. Isolated yield. Enantiomeric ratio (er) determined
d
e
f
by chiral HPLC. 60 °C. CyH (0.2 M) with 1.0 equiv of CsOAc. Not
detected. N2 atmosphere (no CO2). CyH = cyclohexane. DMF =
g
N,N-dimethylformamide. CPME = cyclopentyl methyl ether. EA =
ethyl acetate. PMHS = polymethylhydrosiloxane.
benzene-sulfonate derivative; an ORTEP diagram of this
derivative is provided in Table 1.14
With optimized reaction conditions in hand, we first
investigated the substrate scope of styrene derivatives (Table
2). For styrenes bearing a biphenyl backbone (2b−2g), a
heteroarene such as furan (2h) or thiophene (2i), or a fused
aromatic ring (2j), the desired products 2b−2j could be obtained
readily in both good yields (up to 96%) and excellent
enantioselectivities (up to 99:1 er). Simpler styrenes, bearing
either electron-donating groups (2l, 2n-2p) or a electron-
withdrawing group (2m, 2r, 2s) were well tolerated, affording the
homobenzylic alcohol in good yields and enantioselectivities.
Polysubstituted styrene derivatives (2t−2x) were also proved to
be suitable for our reaction, delivering the desired products in
excellent enantioselectivities (from 97:3 er to >99.5:0.5 er) and
good yields (from 77% to 91%). For some cases (1g, 1i, 1j),
Me(MeO)2SiH showed similar behavior to (EtO)3SiH. Many
functional groups, including halides, amine, ether, and ester, were
well tolerated, which provided huge opportunities for further
transformations.
Next, we explored the substrate scope of 1,3-dienes to yield an
array of chiral homoallylic alcohols. Compared to styrene
derivatives, there are extra challenges for 1,3-dienes including (1)
selectivity between 1,2- and 1,4-addition of the Cu−H species to
the diene,15a (2) control of the Z/E selectivity for the remaining
alkene,15b−d and (3) monohydroxymethylation versus over-
reduction or mutlihydroxymethylation.15e With these challenges
in mind, we further fine-tuned our reaction conditions and
successfully converted a battery of (E)-1-phenyl-1,3-butadienes 3
to the desired 1,4-addition products 4 with high yields and
excellent regio-, enantio-, and Z-selectivities (Table 3).
Substrates with electron-rich (3d−3e) or mildly electron-
withdrawing substituents (3b−3c) at the para position, as well
as those with meta- (3f) or ortho-substituents (3i, 3g, and 3h), all
provided good yields (from 63% to 91%) and good to excellent
enantioselectivities (er from 90:10 to 97:3). In all these cases,
>20:1 regioselectivity for 1,4-addition and >20:1 diastereose-
lectivity for the (Z)-alkene isomer were observed.
a
0.4 mmol scale. Isolated yields provided. er determined by chiral
b
c
HPLC. Me(MeO)2SiH was used as silane reagent. er of 2n was
determined from its sulfonate derivative. See SI for more details.
To shed light on the mechanism of this reaction, several
experiments were carried out (Figure 1). First, silyl formate 5
could be obtained in moderate yield from (EtO)3SiH and CO2
via copper catalysis. However, the reaction of 5 with 1o failed to
give product 2o, demonstrating that the silyl formate may not be
the active intermediate (see SI for more details). The
unproductive and competitive hydrosilylation of CO2 may also
explain the requirement for a large excess of silane to obtain high
conversion. Next, the stoichiometric reaction between 1m,
precatalyst mixture of Cu(OAc)2 and L3,13c and (EtO)3SiH
generated L3-ligated phenylethylcopper complex 611v in 50%
yield. This organocuprate species underwent reaction with CO2
smoothly to give the corresponding carboxylate 7 (see SI for
more details). Based on these preliminary results and previous
reports,5,10,11 we propose the following possible mechanism.
First, L*CuH A is generated in the presence of silanes.
Subsequently, alkenes insert into the Cu−H bond with high
regioselectivity and enantioselectivity to form species B, which
B
DOI: 10.1021/jacs.7b10149
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
a
Table 3. Substrate Scope of 1,3-Dienes
Scheme 2. Product Transformations and Applications
a
Reagents and conditions: (a) Ph3P, 1H-imidazole, I2, Et2O/MeCN, 4
a
0.5 mmol scale. Isolated yields provided. er determined by chiral
h; (b) Ph3P, CBr4, THF, r.t., 2 h; (c) TsCl, DABCO, DCM, r.t.,
overnight; (d) KOtBu, Ph2PH, −78 °C−r.t., overnight; (e) p-
MeC6H4SNa, EtOH; 40 °C, 16 h; (f) NaClO2, TEMPO, NaClO,
MeCN/phosphate buffer pH 6.7, 35 °C, 36 h. See SI for more
information.
b
HPLC. Absolute configuration of 4a was determined as (R,Z) by
analogy to a compound with known absolute configuration and
reported optical rotation. See SI for more details.
lation reaction is highly useful for synthetic and medicinal
chemistry.
In summary, we developed the first highly regio- and
enantioselective reductive hydroxymethylation of styrenes and
1,3-dienes with CO2. Diverse chiral homobenzylic/allylic
alcohols were generated from styrenes/1,3-dienes in excellent
regio- and enantioselectivity. These transformations feature mild
reaction conditions, broad substrate scope, and good functional
group tolerance. Moreover, the desired chiral products are not
only versatile building blocks in synthetic chemistry but also
important intermediates for the synthesis of bioactive com-
pounds such as (R)-(−)-curcumene and (S)-(+)-ibuprofen.
More detailed mechanistic studies and the development of
related enantioselective C−C bond formations with CO2 are
underway.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b10149.
Figure 1. Mechanistic studies and proposed mechanism.
Sulfonate derivative of 2a (CIF)
Detailed experimental procedures, spectral data, and
analytical data (PDF)
then reacts with CO2 to give copper carboxylate C. Further
reduction of C by hydrosilane produces copper alkoxide D.6,15
Finally, σ-bond metathesis of D with hydrosilane regenerates
species A and provides silyl ether E, which is converted to alcohol
2 after desilylation.
To demonstrate the utility of this transformation, several
applications toward the preparation of synthetic intermediates
and bioactive compounds were studied (Scheme 2). Initially, a
variety of nucleophiles reacted smoothly with 2a to generate the
corresponding chiral compounds 8−11 in good yields. More-
over, 2n could be easily transformed to a natural product (R)-
(−)-curcumene 12 following reported procedures.16 Lastly, 2o
was oxidized to the corresponding acid 13, an important
commercial drug (S)-(+)-ibuprofen, in excellent yield and er
value. This result further demonstrates that the hydroxymethy-
AUTHOR INFORMATION
■
Corresponding Author
*dgyu@scu.edu.cn
ORCID
Da-Gang Yu: 0000-0001-5888-1494
Present Address
§Fachbereich Chemie, Philipps-Universitat Marburg, 35037
̈
Marburg, Germany.
Author Contributions
‡These authors contributed equally.
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/jacs.7b10149
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
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ACKNOWLEDGMENTS
■
Dedicated to Professor Qi-Lin Zhou on the occasion of his 60th
birthday. We thank Prof. Ke Zheng and Prof. Jason J Chruma
(Sichuan University) for valuable help and suggestions. Financial
support was provided by the National Natural Science
Foundation of China (21502124), the “973” Project from the
MOST of China (2015CB856600), the “1000-Youth Talents
Program”, and the Fundamental Research Funds for the Central
Universities.
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DOI: 10.1021/jacs.7b10149
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