Iridium-catalyzed enantioselective allylation of silyl enol ethers derived from ketones and α,β-unsaturated ketones

Article abstract of DOI:10.1039/c5cc07221b

The unified Ir-catalyzed enantioselective allylic substitution reactions of silyl enol ethers derived from ketones and α,β-unsaturated ketones with branched, racemic allylic alcohols are described. This transformation is catalyzed by the Carreira system and proceeds without fluoride, and with high ee and b:l ratio. The synthetic utility of this method was illustrated by the concise enantioselective total synthesis of marine natural products calyxolane A, B and by the assignment of the absolute configuration of calyxolane A.

Full text of DOI:10.1039/c5cc07221b

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COMMUNICATION
Iridium-Catalyzed Enantioselective Allylation of Silyl Enol Ethers
Derived from Ketones and α,β-Unsaturated Ketones
Xiao Liang,ab Kun Weia* and Yu-Rong Yanga*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Claisen
1,4-addition
O
O
O
The unified Ir-catalyzed enantioselective allylic substitution
reactions of silyl enol ethers derived from ketones and α,β-
unsaturated ketones with branched, racemic allylic alcohols is
described. This transformation is catalyzed by the Carreira system
and proceeds with fluoride-free, high ee and b:l ratio. The
synthetic utility of this method was illustrated by the concise
enantioselective total synthesis of marine natural products
calyxolane A, B and assignment of absolute configuration of
calyxolane A.
R1
R1
R4
R2
R1
O
R2
R1
R2
OTMS
Ph
Ph
Ir catalyst
O
O
P
N
+
R2
OBoc
R1
R2
CsF, ZnF2
R
(
L1
)-
Hartwig, 2005
R3
O
R3 OTMS
Ir catalyst
+
R2
Hartwig, 2014
OCO2Me
R4
R2
KF, 18-c-6
R1
R1
The development of highly efficient and catalytic, selective
methods for the construction of new stereocenters is of paramount
importance in organic synthesis.1 The asymmetric alkylation of
unstabilized ketone enolates is one of the challenges. In this regard,
introduction of the stereocenter at the β position of the ketones is
less developed than that of the α position.2 Although classical 1,4-
addition or Claisen rearrangement is theoretically workable to
provide the enantioselective homoallylic ketone, experimentally, its
catalytic asymmetric process is not trivial.3 In 2005, Hartwig’s group
first reported Ir-catalyzed reaction of ketone silyl enol ethers with
linear, achiral allylic carbonates.4 Phosphoamidite ligand (R)-L1 was
used in the presence of metal fluorides to introduce the homoallylic
Scheme 1 Catalytic Enantioselective Preparation of Homoallylic
Ketone.
Ir catalyst
O
P
N
OH
O
O
OTMS
Carreira ligand
+
R2
R1
R2
R1
fluoride-free
?
S
(
L
)-
Carreira ligand
this work
Scheme 2 Design Plan
reaction9 of the branched, racemic allylic alcohols with silyl enol
ethers derived from ketones or α,β-unsaturated ketones (Scheme
2), yielding the same products reported by Hartwig et al.4,5 This
alternative method might be noteworthy, because the secondary
allylic alcohols are usually much easily prepared than allylic
carbonates,10 by addition of the aldehydes with vinyl Grignard
reagents. Meanwhile, the net loss of water molecule in the reaction
is more environmentally benign. Plus, the promoters usually being
acidic in Carreira systems could activate the silyl enol ethers and the
secondary allylic alcohols simultaneously, thus avoiding exploring
fluoride sources for different silyl enolates of ketones or
unsaturated ketones, one of key issues of Hartwig’s works, and
making a unified and simple manipulation for both ketone and
unsaturated ketone silyl enol ethers. However, the potential side
reactions, such as nucleophilic addition of the ketone product, re-
formation of an enolate of the ketone product followed by the
ketones with
a high branched-to-linear regioselectivity and
excellent enantioselectivity (Scheme 1). Very recently, the same
group expanded substrate scope of the reaction to α,β-unsaturated
ketones.5 In Hartwig’s elegant works,4,5,6g the judicious choice of
metal fluoride promoters such as CsF, ZnF2 or KF is essential.
The recent pioneering and systematic works from Carreira’s
group showcase the Ir-catalyzed enantioselective cross-coupling of
various nucleophiles and electrophilic, branched, racemic allylic
8
alcohols.7 The Ir-(P, olefin) complex, i.e. (S)-L, catalyzes the
reactions giving diverse synthetically useful chiral building blocks
with superb enantioselectivity. Inspired by these seminal
achievements of Carreira’s group, we envision a new
reaction with
a second allylic electrophile, monosubstitued
allyllation leading to linear products, and much unstablized
property of silyl enolates of α,β-unsaturated ketones make this
work challenging. Here we wish to report our results.
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Table 1 Optimization of Ir-Catalyzed Allylation of Ketone Silyl Enol
Ethera
yield
entry
1
promoter
Zn(OTf)2
Yb(OTf)3
Sc(OTf)3
Bi(OTf)3
TFA
solvent
DME
t (h)
12
8.5
2.5
2.5
2
b:lc
ee (%)d
80
(%)b
78
>50:1
>50:1
>50:1
>50:1
>50:1
>50:1
>50:1
>50:1
>50:1
>50:1
>50:1
>25:1
15:1
2
DME
76
81
70
30
60
73
60
83
70
75
73
35
76
3
DME
89
4
DME
60
5
DME
>99.5
85
6
DME
13
4
BF3⋅Et2O
Zn(OTf)2
In(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
7
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
Toluene
1,4-dioxane
THF
63
aAll reaction were carried out on 0.2 mmol scale under standard conditions
(see Table 1 for details). bIsolated yields after purification by preparative TLC.
cRatio of b:l in brackets as determined by 1H NMR integration of the crude
reaction mixture. dEnantiomeric excess determined by HPLC (chiralpak AD-H or
chiralcel OD-H); absolute stereochemistry was assigned by analogy.4
8
2
80
9
3
94
10
11
12
13
3
91
2
83
2
86
With the optimized conditions for the allylation of ketone silyl
enol ether in hand, we evaluated the scope of the substrates. As
shown in Table 2, the reaction proceeded well with a range of
substrates. A number of substituted aryl vinyl carbinols all furnished
CH3CN
2.5
82
aReaction conditions: 1a (0.15 mmol, 1.5 equiv), 2a (0.10 mmol, 1.0 equiv),
[{Ir(cod)Cl}2] (3% equiv), (S)-L (12% equiv), promoters (10% equiv), solvent
(0.4 mL). bIsolated yield after purification by preparative TLC. cRatio of
branched (3a) to linear (4a) determined by 1H NMR integration. eDetermined
by HPLC (chiralpak AD-H).
homoallylic
ketones
in
good
yields
and
excellent
enantioselectivities. Different electronically demanding substituting
groups such as -OMe, CF3 and halogens are compatible in the
conversions. Other aromatic and heteroaromatic systems such as
naphthalene, thiophene and furan could be successfully employed.
Due to the acidic promoter used, the reaction didn’t take place in
the substrate of pyridine substituted vinyl carbinol (not shown).
Reactions of the aliphatic carbinol also occurred in high
enantiomeric excess and regioselectivity, and the isolated yields
were unfluctuating comparing the aromatic substrates (3k, 3l).
Similarly, the aliphatic silyl enol ether reacted with high ee and b:l
ratio (3m). The branched-to-linear selectivities drop in a limited
cases (3c, 3j), nevertheless, they are generally high enough for
practical manipulation in organic synthesis. The absolute
configuration of the reaction products was determined to be (S) by
comparison of the specific rotation with literature data.4
Our study of the Ir-catalyzed allylation of ketone silyl enol ether
began by evaluating the reaction of aryl vinyl carbinol with silyl enol
ether derived from methyl acetophenone. Considering that choice
of acidic promoters and solvents is one key factor, we started to
screen the promoters. Fixing DME as the solvent, referring to
Hartwig’s conditions, we tried Zn(OTf)2 as a promoter first and
found it was effective, leading to the isolation of branched 3a in
78% yield and 80% ee after 12 h (Table 1, entry 1). The excess silyl
enol ether (1.5 equiv used) turned into the starting methyl
acetophenone. The crude 1H NMR suggested high regioselectivity
(b:l ratio). Next, we continued to test other Lewis acids Yb(OTf)3,
Sc(OTf)3, Bi(OTf)3, In(OTf)3, BF3 etherate and Brønsted acid TFA. We
observed that using of Sc(OTf)3 did significantly improve the
reaction outcome, the isolated yield and the 89% ee (entry 3) and
shorter reaction time being competitive among the acids. Although
TFA delivered the best ee, the yield is impractically low caused by
the decomposition of silyl enol ether (entry 5). After screening the
solvents, 1,2-dichloroethane in the presence of Sc(OTf)3 was
identified as the optimal conditions, leading to 94% ee and 83%
isolated yield (entry 9).
Considering there are more potentially valuable functional
groups generated in α,β-unsaturated ketones, in contrast to the
ketones, we elected to expand the substrates into the silyl enol
ether derivatives of α,β-unsaturated ketones. In fact, prior to
recent report by Hartwig in 2014, reactions of α,β-unsaturated
ketones as the nucleophile are less common because the enolate
can undergo aldol condensation or Michael additions with another
α,β-unsaturated ketone. Gratifyingly, the optimized reaction
conditions employed with ketone are directly applicable to those of
Table 2 Substrate Scope of Ir-Catalyzed Allylation of Ketone Silyl
Enol Ethera,b,c,d
the α,β-unsaturated ketones.11 Table
3 shows the scope of
allylation of silyl enol ether derivatives of the α,β-unsaturated
ketones with carbinols under the conditions used for the ketone. In
all cases, the allylation products were obtained with >90% ee. The
branched-to-linear selectivities were also high enough for practical
use. For example, aliphatic carbinols reacted to yield products with
a super ee and >25:1 regioselectivity (6i, 6j). Reaction of the
2 | J. Name., 2012, 00, 1-3
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O
aliphatic substituted silyl enol ether also occurred with selectivities
OH
S
S
akin to the aromatic substrates under these conditions (6k).
(R)-7
calyxolane B
O
[
]
44
Table
3
Substrate Scope of Ir-Catalyzed Allylation of α,β-
8a: 73% (12:1)
O
(S)-L
Unsaturated Ketone Silyl Enol Ethera,b,c,d
S
OH
R
S
OH
3b
7
(S)-
73% (>25:1), 94% ee
Ph
ent-calyxolane A
2b
+
1. O3
8b: 65% (20:1)
[
]
40
OH
2. Et3SiH
85%
O
OTMS
(R)-7
(S)-7
O
S
R
Ph
L
(R)-
1b
R
calyxolane A
8c: 76% (11:1)
[
] +23
3b'
OH
O
74% (>25:1), 91% ee
R
R
8d: 66% (16:1)
ent-calyxolane B
] +52
[
Scheme
3 Unified, Catalytic Enantioselective Synthesis of
Calyxolane A and B
In summary, we have developed the unified and catalytic
enantioselective allylic substitution reactions of ketone or α,β-
unsaturated ketone silyl enol ethers with branched, racemic
carbinols to give homoallylic ketones or α,β-unsaturated ketones.
Notable features of these reactions include the absence of metal
fluoride activators, the mild reaction conditions i.e. the shorter
reaction time without heating, high regioselectivities and
enantioselectivities with a broad range of silyl enol ethers and
carbinols. The synthetic utility of this method was illustrated by the
concise synthesis of calyxolanes. Studies to extend these methods
to synthesis of natural products are ongoing.
We are indebted to Professors J. F. Hartwig and E. M. Carreira for
their works that inspired us to design ours. Y.-R. Y. thanks the NSFC
(21472200) and The Hundred Talents Program of CAS for financial
support.
aAll reaction were carried out on 0.2 mmol scale under standard conditions
(see Table 1 for details). bIsolated yields after purification by preparative TLC.
cRatio of b:l in brackets as determined by 1H NMR integration of the crude
reaction mixture. dEnantiomeric excess determined by HPLC (chiralpak AD-H or
chiralcel OD-H); absolute stereochemistry was assigned by analogy.5
Finally, in an effort to demonstrate the practicality and utility of
this catalytic process, we examined the application to the total
synthesis of marine natural products calyxolanes (Figure 1).12
Calyxolane A and B are rare 1,3-diphenylbutanoid compounds
isolated from the marine sponge Calyx podatypa collected in Puerto
Rico by Rodriguez and co-workers in 1997. Due to the scanty
samples of natural source, their bioactivities and absolute
stereochemistry are still unknown. We envision that our Ir-
Catalyzed allylation of ketone silyl enol ether would provide the
most direct pathway delivering these compounds.13 By using (S)-L
or (R)-L as ligands, Ir-catalyzed allylation reaction provided
homoallylic ketones which were subjected to CBS reduction14
employing (R) and (S) catalysts 7 respectively, as shown in Scheme
4, yielding two pairs of homoallylic carbinols 8a-8d. After ozonolysis
and reduction of the intermediate lactols, all of the diastereomeric
1,3-diphenylbutanoids were obtained in highly convergent manner.
Comparison with the reported optical rotations in isolation paper,
absolute configuration of calyxolane A could be assigned as Figure
1.15
Notes and references
1
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Ph
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S
R
N
B
Me
(S)-7
calyxolane A
] +34.6
calyxolane B
[
[ ] 0
Figure 1
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2
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5
6
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For systematic publications by Carreira’s group, see: (a) M.
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8
For
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a
review on chiral olefins as steering ligands in
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ü
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Racemic substitution reaction of allylic alcohols and
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