Enantioselective synthesis of anti homoallylic alcohols from terminal alkynes and aldehydes based on concomitant use of a cationic iridium complex and a chiral phosphoric acid

Article abstract of DOI:10.1021/ja405790t

We report a highly diastereo- and enantioselective synthesis of anti homoallylic alcohols from terminal alkynes via (E)-1-alkenylboronates based upon two catalytic reactions: a cationic iridium complex-catalyzed olefin transposition of (E)-1-alkenylboronates and a chiral phosphoric acid-catalyzed allylation reaction of aldehydes.

Full text of DOI:10.1021/ja405790t

Communication
pubs.acs.org/JACS
Enantioselective Synthesis of Anti Homoallylic Alcohols from
Terminal Alkynes and Aldehydes Based on Concomitant Use of a
Cationic Iridium Complex and a Chiral Phosphoric Acid
Tomoya Miura,* Yui Nishida, Masao Morimoto, and Masahiro Murakami*
Department of Synthetic Chemistry and Biological Chemistry, Kyoto University, Katsura, Kyoto 615-8510, Japan
S
* Supporting Information
Z counterparts. We subsequently searched for catalysts of higher
ABSTRACT: We report a highly diastereo- and enantio-
selective synthesis of anti homoallylic alcohols from
terminal alkynes via (E)-1-alkenylboronates based upon
two catalytic reactions: a cationic iridium complex-
catalyzed olefin transposition of (E)-1-alkenylboronates
and a chiral phosphoric acid-catalyzed allylation reaction of
aldehydes.
activity as well as stereoselectivity and found that cationic
iridium(I) catalysts activated by slowly bubbling H2 directly
through the solution for 5 min10,11 could catalyze the olefin
transposition of the E isomers even at 28 °C with excellent
stereoselectivity. Thus, cationic iridium(I) complexes ([Ir-
(cod)2]X)12 were treated with various monodentate phosphine
ligands (PR3) (P/Ir = 2.5) under a hydrogen atmosphere. (E)-1-
Butenylboronate (E)-2a was readily prepared by dicyclohex-
ylborane-catalyzed cis hydroboration of but-1-yne with pina-
colborane13 and then reacted with benzaldehyde (1a) in the
tereocontrolled C−C bond formation in acyclic systems
presence of the activated iridium catalyst at 28 °C for 16 h (Table
S
bearing multiple stereocenters is one of the most critical
issues in organic synthesis. Allylation of carbonyl compounds
with allylmetal reagents provides a reliable method for the
enantioselective synthesis of homoallylic alcohols,1,2 so it is often
used for the asymmetric synthesis of polyketide natural
products.3 Thus, the development of facile methods for the
synthesis of stereochemically defined γ-substituted allylmetal
species from readily accessible starting materials has received
much attention.4 Here we report a new method for synthesizing
anti homoallylic alcohols in an enantioselective way starting from
terminal alkynes and aldehydes (Figure 1). Mechanistically, a
transition-metal catalyst and an asymmetric organocatalyst work
in relay.5−7
14,15
1). When tetrafluoroborate and PCy3
were used as the
Table 1. Optimization of the Reaction Conditions for the
a
Diastereoselective Allylation of 1a with (E)-2a
b
c
entry
X
ligand
PPh3
yield (%)
anti/syn
1
2
3
4
5
6
7
PF6
PF6
PF6
PF6
OTf
SbF6
BF4
3
0
−
−
>98:2
P(n-Bu)3
PCy3
78
7
P(t-Bu)3
PCy3
−
74
82
91
>98:2
>98:2
>98:2
PCy3
PCy3
a
Conditions: 1a (0.40 mmol), (E)-2a (0.80 mmol), [Ir(cod)2]X (5.0
mol %), and ligand (12.5 mol %) in 1,2-dichloroethane (1,2-DCE) (1
b
c
1
mL) at 28 °C for 16 h. Isolated yields. Determined by H NMR
Figure 1. Synthesis of optically active anti homoallylic alcohols starting
from terminal alkynes.
analysis.
counterion and the ligand, respectively, anti homoallylic alcohol
3aa was isolated in 91% yield with almost complete
diastereoselectivity (anti/syn = >98:2) (entry 7).
We recently reported a convenient method for the
diastereoselective synthesis of anti homoallylic alcohols employ-
ing 1-alkenylboronates as the precursors of allylboron reagents.8
A cationic rhodium(I) complex catalyzes the olefin trans-
position9 of the 1-alkenylboronate to generate a transient (E)-
2-alkenylboronate intermediate, which reacts with a coexisting
aldehyde in a stereospecific way. However, the rhodium(I)-
catalyzed reaction required heating at 90 °C, and the observed
anti/syn ratio depended on the double-bond geometry of the 1-
alkenylboronate precursor. The E isomers, which were more
easily prepared and hence were the preferred precursors, showed
lower diastereoselectivities (85:15 to 96:4) compared with their
Our previously reported rhodium system8 required heating at
90 °C for olefin transposition, and the reaction of 1a with (E)-2a
using [Rh(nbd)(MeCN)2]SbF6−dppm (90 °C, 12 h) afforded
3aa in 86% yield with 89:11 dr. When [Rh(nbd)(MeCN)2]-
SbF6−dppm was activated with H2 in place of [Ir(cod)2]BF4−
PCy3, the reaction failed to occur at 28 °C. Control experiments
were then performed to compare the iridium(I)- and rhodium-
Received: June 10, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja405790t | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
(I)-catalyzed transposition processes in the absence of aldehyde
by 1H NMR monitoring (Scheme 1). In the case of iridium, 14%
Scheme 1. Control Experiments in the Absence of Aldehyde
The asymmetric allylation reaction was applied to other
terminal alkynes having various simple or functionalized
substituents. (E)-1-Alkenylboronates 2b−h were prepared by
cis hydroboration of the corresponding terminal alkynes with
pinacolborane,13 and their reactions with 1a using the activated
[Ir]+-PCy3/(R)-TRIP catalyst system were examined (Table 2).
(E)-1-Alkenylboronates 2b−d having ethyl, isobutyl, and phenyl
groups all exhibited high enantioselectivities as well as high yields
and excellent diastereoselectivities (entries 1−3). Functional
groups such as siloxy, methoxycarbonyl, and chloro groups were
allowed at the terminus of the alkyl chain (entries 4−6). On the
other hand, only modest diastereo- and enantioselectivities were
observed with substrate 2h having a siloxy group at the C3
position (entry 7).
The scope of aldehydes in the reaction with (E)-1-
pentenylboronate (E)-2b was also examined (Table 3). An
electronically and sterically diverse array of aromatic aldehydes
1b−f reacted to give the corresponding anti homoallylic alcohols
3bb−fb in 85−99% yield with excellent diastereoselectivities and
high enantioselectivities (entries 1−5). In addition, α,β-
unsaturated aldehyde 1g gave a comparable result (entry 6).
Worthy of note was the observation that aliphatic aldehydes such
as 3-phenylpropanal (1h) and cyclohexanecarbaldehyde (1i)
also successfully participated in the stereoselective reaction
(entries 7 and 8).
of (E)-2a isomerized after only 1 min at 28 °C, and the E isomer
of 2-butenylboronate 4a was almost completely formed. After 5
h, the E/Z ratio of 4a became constant (ca. 4:1). In the case of the
rhodium catalyst [Rh(nbd)(MeCN)2]SbF6−dppm, the forma-
tion of 4a was barely observed after 1 min even at 90 °C. After 20
min, 9% of (E)-2a isomerized to 4a with an E/Z ratio of ca. 2:1.
Thus, the active iridium catalyst strongly promotes the olefin
transposition at 28 °C, and the generation of (E)-4a is kinetically
favored. We assume that as soon as the (E)-4a is generated, it
immediately reacts with a coexisting aldehyde via a six-membered
chairlike transition state to produce the anti homoallylic alcohol.
The use of cationic iridium(I) complexes rendered it possible
to carry out the allylation reaction at temperatures as low as 28
°C, opening a route to asymmetric synthesis. The Antilla group
reported that the chiral phosphoric acid (R)-TRIP16 catalyzes a
highly enantioselective allylation reaction of 1a with 4a.17 Their
work prompted us to examine the iridium(I)-catalyzed reaction
of 1a with 2a in the presence of (R)-TRIP (eq 1). Anti
homoallylic alcohol 3aa was exclusively obtained in 91% isolated
yield with a high level of enantioselectivity (96% ee).18 This
result suggested that the activated iridium(I) catalyst and (R)-
TRIP did not interfere with each other19 but independently
played their roles as catalysts. A large-scale experiment using 670
mg (6.3 mmol) of 1a also gave a comparable result [1.01 g (91%
yield) of 3aa with 96% ee].
Finally, we examined a one-pot, two-step reaction to
synthesize anti homoallylic alcohols starting from terminal
alkynes (Scheme 2). Terminal alkynes 5a−c were treated with
pinacolborane in the presence of dicyclohexylborane (5.0 mol %)
at room temperature or 0 °C for 6−20 h. After the volatile
materials were removed under reduced pressure, 1,2-DCE
solutions of 1a and the activated [Ir]+-PCy3/(R)-TRIP catalysts
were successively added to the residue. The reaction mixture was
stirred at 28 or 0 °C, and subsequent chromatographic
a
Table 2. Asymmetric Allylation Reactions of 1a with Various (E)-1-Alkenylboronates 2b−h
b
c
d
entry
2
R2
X
Y
T (°C)
t (h)
3
yield (%) (anti/syn)
ee (%)
1
2
3
4
5
6
7
2b
2c
2d
2e
2f
Et
5.0
5.0
10
15
20
20
10
15
10
28
0
17
40
64
20
18
18
44
3ab
3ac
3ad
3ae
3af
90 (>98:2)
86 (>98:2)
83 (>98:2)
85 (>98:2)
86 (>98:2)
92 (>98:2)
97 (92:8)
93
95
88
90
93
93
17
i-Bu
Ph
10
−15
28
(CH2)3OTBS
(CH2)2CO2Me
(CH2)3Cl
7.5
7.5
5.0
7.5
28
2g
2h
28
3ag
3ah
OTBS
28
a
Conditions: 1a (0.40 mmol), (E)-2 (0.80 mmol), [Ir(cod)2]BF4−PCy3 (X mol %, Ir:P = 2:5), (R)-TRIP (Y mol %), and MS 4A (50 mg) in 1,2-
b
c
d
1
DCE (1 mL). Isolated yields (averages of 2 runs). Determined by H NMR analysis. Determined by chiral HPLC.
B
dx.doi.org/10.1021/ja405790t | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
Table 3. Asymmetric Allylation Reactions of Various Aldehydes 1b−i with (E)-2b
b
c
d
entry
1
R1
X
Y
T (°C)
t (h)
3
yield (%) (anti/syn)
ee (%)
1
2
3
4
5
6
7
8
1b
1c
1d
1e
1f
4-NO2C6H4
4-MeOC6H4
4-MeC6H4
2-MeC6H4
2-furyl
5.0
7.5
7.5
10
15
10
20
10
10
20
20
28
28
28
0
20
17
20
72
38
19
64
64
3bb
3cb
3db
3eb
3fb
85 (>98:2)
99 (>98:2)
87 (>98:2)
98 (>98:2)
91 (>98:2)
96 (>98:2)
88 (>98:2)
82 (97:3)
95
92
90
92
92
93
91
88
10
5.0
5.0
28
28
5
1g
1h
1i
PhCHCH
PhCH2CH2
Cy
3gb
3hb
3ib
10
10
5
a
Conditions: 1 (0.40 mmol), (E)-2b (0.80 mmol), [Ir(cod)2]BF4−PCy3 (X mol %, Ir:P = 2:5), (R)-TRIP (Y mol %), and MS 4A (50 mg) in 1,2-
b
c
d
1
DCE (1 mL). Isolated yields (averages of 2 runs). Determined by H NMR analysis. Determined by chiral HPLC.
stereoselectivities observed were similar to those of (E)-1-
Scheme 2. One-Pot Cis Hydroboration/Olefin
alkenylboronates 2. It is noteworthy that olefin transposition
takes place multiple times from a position remote to the boryl
group.20
Transposition/Allylation Reaction
In summary, we have demonstrated that a cationic iridium(I)
complex/chiral phosphoric acid relay system provides a highly
diastereo- and enantioselective route to anti homoallylic alcohols
starting from terminal alkynes and aldehydes. A cationic
iridium(I) complex promotes the olefin transposition of 3- and
5-alkenylboronates as well as (E)-1-alkenylboronates at 28 °C,
thus generating in situ (E)-2-alkenylboronates that are often
laborious to prepare stereoselectively. The asymmetric allylation
reaction catalyzed by a chiral phosphoric acid is compatible with
the cationic iridium(I) complex.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and spectral data for new compounds.
This material is available free of charge via the Internet at http://
pubs.acs.org.
purification afforded the corresponding anti homoallylic alcohols
3aa−3ac in good overall yields with high enantioselectivities.
Accessibility of starting materials is one of the most important
criteria for synthetically useful organic reactions. All of the
starting materials required for the present reaction are readily
available, even from commercial sources.
To further expand the synthetic utility of this relay system, we
employed 3- and 5-alkenylboronates 6a and 6i as the allylboron
precursors (Scheme 3). The reaction proceeded smoothly to give
anti homoallylic alcohols 3aa and 3ai in high yields, and the
AUTHOR INFORMATION
Corresponding Author
tmiura@sbchem.kyoto-u.ac.jp; murakami@sbchem.kyoto-u.ac.
jp
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by MEXT [Grants-in-Aid for Scientific
Research on Innovative Areas 22105005 and 24106718,
Scientific Research (B) 23350041, and Young Scientists (A)
23685019], JST (ACT-C), and Takeda Science Foundation. M.
Morimoto is grateful for the JSPS Research Fellowship for Young
Scientists.
Scheme 3. Asymmetric Allylation Reactions of 1a with 3- and
5-Alkenylboronates 6a and 6i
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Int. Ed. 2011, 50, 11465.
(9) For related olefin transpositions using boryl-substituted substrates,
see: 1-Boryl-3-siloxy-1-alkenes: (a) Yamamoto, Y.; Miyairi, T.; Ohmura,
T.; Miyaura, N. J. Org. Chem. 1999, 64, 296. 2-Boryl-1-silyl-1-alkenes:
(b) Ohmura, T.; Oshima, K.; Suginome, M. Angew. Chem., Int. Ed. 2011,
50, 12501. Triallyloxyboranes: (c) Lin, L.; Yamamoto, K.; Matsunaga,
S.; Kanai, M. Angew. Chem., Int. Ed. 2012, 51, 10275.
NOTE ADDED AFTER ASAP PUBLICATION
■
Due to a production error, this paper was published on the Web
with errors in Scheme 2. The corrected version was reposted on
July 30, 2013.
(10) For activation of [Ir(alkene)n]+ complexes with H2, see:
(a) Baudry, D.; Ephritikhine, M.; Felkin, H. J. Chem. Soc., Chem.
D
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