Tandem Rh-catalysis: Decarboxylative β-keto acid and alkyne cross-coupling

Article abstract of DOI:10.1039/c6cc02522f

Herein, we describe a regioselective Rh-catalyzed decarboxylative cross-coupling of β-keto acids and alkynes to access branched γ,δ-unsaturated ketones. Rh-hydride catalysis enables the isomerization of an alkyne to generate a metal-allyl species that can undergo carbon-carbon bond formation. Ketones are generated under mild conditions, without the need for base or activated electrophiles.

Full text of DOI:10.1039/c6cc02522f

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DOI: 10.1039/C6CC02522F
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COMMUNICATION
β
Tandem Rh-Catalysis: Decarboxylative -Keto Acid and Alkyne
Cross-Coupling
Faben A. Cruz,a Zhiwei Chen,a Sarah I. Kurtoica and Vy M. Donga*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Herein, we describe a regioselective Rh-catalyzed decarboxylative acids with alkynes. We chose alkynes as allyl electrophiles
cross-coupling of –keto acids and alkynes to access branched because they are a common and readily accessible functional
β
, –
γ δ
unsaturated ketones. Rh-hydride catalysis enables the group. Our approach would enable unique access to ketones
isomerization of an alkyne to generate a metal-allyl species that can under mild conditions, without the need for generating
undergo carbon-carbon bond formation. Ketones are generated enolates or the use of activated allylating agents.9-13
under mild conditions, without the need for base or activated
electrophiles.
On the basis of previous studies from Yamamoto,14 Breit,15
and our laboratory,16 we proposed a pathway involving tandem
Rh-catalysis to enable decarboxylative coupling between
acids and alkynes -keto acid
(Scheme 1).17 First,
Rh(I) species combine to generate Rh(III)-hydride
intermediate.18 Insertion of alkyne
into the Rh(III)–H bond
gives Rh-vinyl species . Subsequent -hydride elimination
generates allene and regenerates the Rh(III)-hydride species.
Insertion of allene into the Rh(III)-H bond then forms Rh(III)-
allyl species that can be trapped with a carbon-based
nucleophile.19 Indeed, Breit recently reported the coupling of
1,3-diketones with terminal alkynes.20 In the presence of
-keto
acid , C–C bond formation yields allylated
-keto acid 8 21
Finally, decarboxylation affords the desired ketone
β
-keto
A range of natural processes are driven by the loss of carbon
dioxide, from polyketide synthesis to γ-aminobutyric acid
(GABA) production.1 Various synthetic strategies have emerged
using the formation of CO2 gas as the driving force. Tsuji and
1
2
β
1 and a
a
2
5
β
Saegusa independently reported decarboxylative allylation of β-
6
keto allyl esters.2,3 Shair developed a decarboxylative aldol
using malonic acid half thioesters,4 while Gooßen pioneered
decarboxylative biaryl cross-couplings.5 More recently,
MacMillan and Doyle have used CO2 gas extrusion and
photoredox catalysis to generate a wide range of cross-
couplings, including those that generate Csp2−Csp3 bonds.6
Most relevant to our study, Breit has developed a bioinspired
6
7
β
1
β
.
3
.
To test our mechanistic proposal, we investigated the cross-
coupling of benzoylacetic acid (1a) and 1-phenyl-1-propyne
coupling of
β-keto acids with allenes under Rh-hydride
catalysis.7,8 It occurred to us that by using tandem Rh-catalysis,
(
2a). In the presence of 5 mol% of [Rh(cod)Cl]2 and 10 mol% 1,3-
we could achieve a complementary cross-coupling of
β
-keto
RhIIIHX
RhIIIHX
1
R1
R1
Me
O
R1
RhI
6
2
insertion
1
R2
3
RhI
X
decarboxylation
RhIII
β-hydride
elimination
O
O
insertion
H
RhIII
R1
R2
O
7
O
O
O
O
R1
OH
X = RCO2
RhIII
X
R1
OH
R2
R2
R1
H
1
H
H
5
8
Scheme 1 Proposed decarboxylative β-keto acid and alkyne coupling via tandem Rh-Catalysis.
bis(diphenylphosphino)propane (dppp), the desired branched
–unsaturated ketone 3a was observed in 5% yield with
γ, δ
>20:1 branched to linear regioselectivity (Figure 1). Notably, no
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J. Name., 2013, 00, 1-3 | 1
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group (3j). In addition, electron-deficient para-triVfileuwoArroticmleeOtnhlinyel
and electron-rich para-methoxy substituted rings are tolerated
Figure 1 Ligand Effects on Decarboxylative β-keto Acid and Alkyne
Coupling. a
DOI: 10.1039/C6CC02522F
(
3k and 3l, 63% and 61%, respectively). Finally, β–keto acids
5% [Rh(cod)Cl]2
10% Ligand
with heterocycles (e.g., furan and thiophene) can be used as
carbon pronucleophiles to yield 3m and 3n (90% and 89%,
respectively).
O
Ph
O
O
+
Ph
Me
Ph
Ph
OH
THF (0.5 M)
60 oC, 24 h
3a
1a
2a
Next, we examined the coupling benzoylacetic acid (1a) with
O
various alkynes
were used to alkylate benzoylacetic acid (1a) with >20:1
regioselectivity (3o 3q, 57–75%). In addition, alkynes with
2 (Figure 3). Halogenated 1-aryl-1-propynes
Ph
Ph
4a
–
electron-deficient para-trifluoromethyl and electron-rich para-
methoxy phenyl rings are amenable to alkylating 1a to afford
ketones 3r and 3s (81% and 55%, respectively). Benzoylacetic
acid (1a) can be alkylated using alkynes with aliphatic
substitution in place of aromatic. Aliphatic alkynes present a
challenge as a result of having more than one possible site for
PPh2
PPh2
PPh2
PPh2
Ligand (bite angle)b
yield, 3a:4a
PPh2
PPh2
dppp (92o)
5%, >20:1
dppb (97o)
12%, >20:1
dppe (83o)
No product observed
Me Me
β
-hydride elimination for allene formation. Given this challenge,
we were pleased to find that using alkynes bearing aliphatic
substituents gave the branched ketone product bearing a
terminal olefin. Both free and protected alcohols are tolerated.
A sensitive functional group handle (e.g., the tosyl group)
remains intact under these alkylating conditions (3t, 85%).
Silylated, benzoylated, and benzylated alcohols are all also well-
PPh2
Fe
O
O
PPh2
PPh2
PPh2
PPh2
PPh2
dppf (99o)
Xantphos (105o)
<5%, >20:1
DPEphos (101o)
88% (97%)d, >20:1
18%, >20:1
tolerated (3u, 3w, and 3x, 51–90%). Phthalimide protected
aReaction conditions: 0.1 mmol 1a, 0.1 mmol 2a, 5 mol% [Rh(cod)Cl]2, 10 mol% ligand,
0.2 mL THF (0.5 M), 60 °C, 24 hours. bSee ref 23. cDetermined by GC-FID analysis using
mesitylene as internal standard. dUsing 0.2 mmol 1a, 4 mol% [Rh(cod)Cl]2, 8 mol%
DPEphos, and 2-MeTHF instead.
amines, as well as Boc and Ts protected amines can be installed
on the alkyne coupling partner (3y and 3z, 52% and 59%,
respectively). Acidic N-H bonds are tolerated as shown by the
Figure 2 Branched Selective Decarboxylative Coupling of Alkyne 2a
allyl ester formation was observed despite the precedence for
C–O bond formation between carboxylic acids and alkynes.22
The major by-product observed was acetophenone arising from
decarboxylation of benzoylacetic acid (1a). From further
evaluation of bidentate phosphine ligands, we observed a
relationship between ligand bite angle and reactivity.
Bisphosphine ligands with larger bite angles than dppp, such as
1,4-bis(diphenylphosphino)butane (dppb) and 1, 1’-
bis(diphenylphosphino)ferrocene (dppf), resulted in increased
reactivity. Further increasing the bite angle by use of Xantphos
as a ligand resulted in a dramatic decrease in reactivity. Using
DPEphos provided an optimal bite angle of approximately 101°
for promoting the desired transformation.23 By switching from
THF to 2-MeTHF and increasing the equivalents of benzoylacetic
acid (1a), the catalyst loading can be decreased while increasing
the yield to 97%.
with Various β–keto Acids.a
4% [Rh(cod)Cl]2
8% DPEphos
O
O
O
Ph
+
Ph
Me
R
OH
R
2-MeTHF (0.5 M)
60 oC, 24 h
1
2a
3
O
Ph
O
Ph
O
Ph
Me
iPr
t-Bu
3d, 85%, >20:1
3b, 90%, >20:1b
3c, 80%, >20:1b
O
Ph
O
Ph
PhO2S
Ph
3e, 61%, >20:1
3f, 92%, >20:1
O
Ph
O
Ph
O
Ph
F
Br
Cl
With this protocol in hand, we explored the coupling of
3i, 91%, >20:1
3g, 70%, >20:1
Me Ph
3h, 76% , >20:1
various
β
–keto acids
1
with 1-phenyl-1-propyne (2a). Aliphatic
-hydrogens, were
O
Ph
O
Ph
β
–keto acids, bearing multiple acidic
α
O
alkylated with >20:1 regioselectivity (Figure 2). Primary (3b, 3e,
and 3f), secondary (3c), and tertiary (3d) substitution are all
F3C
Ph
MeO
O
tolerated (61–92%). Notably,
β–keto acids with electron-
3k, 63%, >20:1
3l, 61%, >20:1
3j, 70%, >20:1
withdrawing groups (phenyl and phenylsulfonyl) can be used to
give ketones formally derived from the methyl-ketone dianions
(highlighted in blue, 3e and 3f). β–keto acids bearing aromatic
O
Ph
O
S
rings with a variety of substituents underwent alkylation with
high branched to linear regioselectivity. Halogenated aromatic
rings are well tolerated (3g–3i, 70–91%). Regioselective
3m, 90%, >20:1b
3n, 89%, >20:1
aReaction conditions: 0.4 mmol 1, 0.2 mmol 2a, 4 mol% [Rh(cod)Cl]2, 8 mol% DPEphos,
0.4 mL 2-MeTHF, 60 °C, 24 hours. >20:1 denotes the ratio of 3:4. bReaction ran with 50
mol% benzoic acid.
coupling still occurs when the aromatic ring has an ortho-methyl
2 | J. Name., 2012, 00, 1-3
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formation of ketone 3aa in 82% yield. Notably, using alkynes regioselectivity. We observed deuterium scrambling which
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with free alcohols or amines, as in 3v and 3aa, does not result suggests reversible
in intramolecular cyclization to form the corresponding formation. Initial studies with chiral ligands resulted in
β
-hydride eliminDaOtiIo: n10.10d3u9r/iCn6gCC0a2ll5e2n2eF
tetrahydrofuran or pyrrolidine. These results highlight the high moderate enantioselectivities (up to 54% ee) using
a
chemoselectivity of this protocol. Finally, electrophilic MeOBIPHEP-based ligand.24 These results support the proposed
functionalities can be tolerated as evidenced by the formation role of the Rh-phosphine complex in the key C-C bond
of ketones 3ab–3ae bearing an alkyl bromide, Weinreb amide, formation, however, developing highly enantioselective
ketone, and aldehyde, respectively (46–79%).
variants warrants further efforts.
To provide evidence for the proposed allene intermediate,
we used allene 6a as a substitute for alkyne 2a under standard
4 mol% [Rh(cod)Cl]2
8 mol% DPEphos
O
Ph
O
O
Ph
+
(1)
Ph
3a, 52%, >20:1
Ph
OH
2-MeTHF (0.5 M)
60 °C, 24 h
6a
1a
Figure 3 Branched Selective Decarboxylative Coupling of β–keto
acids 1a with Various Alkynes.a
4% [Rh(cod)Cl]2
8% DPEphos
15%
D
13%
D
O
O
4 mol% [Rh(cod)Cl]2
8 mol% DPEphos
O
R
O
Ph
O
O
+
R
Me
Ph
CD3
(2)
+
Ph
OH
15%
D
Ph
Ph
2-MeTHF (0.5 M)
60 oC, 24 h
Ph
OH
2-MeTHF (0.5 M)
60 °C, 24 h
1a
2
3
D
D
D
1a
2a-d3
8%
18%
Br
Cl
3a-dn, 73%, >20:1
F
O
O
O
Conclusions
This Rh-catalyzed decarboxylative coupling between
Ph
3o, 68%, >20:1
Ph
3q, 57%, >20:1
Ph
3p, 75%, >20:1
β
-keto
CF3
OMe
acids and alkynes provides a complementary approach to
generate ketones, without need for enolate generation and
activated allylic electrophiles. In addition, alkylation at specific
sites can be performed in the presence of multiple reactive sites
due to the directing effect of the carboxylic acid. Our study
contributes to the emerging use of alkynes in various cross-
couplings to generate C–O,25 C–N,26 C–S,27 and C–C bonds.28
Further studies are underway to expand the scope of carbon
pronucleophiles and identify more enantioselective variants for
tandem Rh-catalysis.
O
O
Ph
Ph
3s, 55%, >20:1
3r, 81%, >20:1
OH
OTBS
OTs
O
O
O
Ph
Ph
Ph
3u, 58%, >20:1
3v, 53%, >20:1
3t, 85%, >20:1
OBn
O
OBz
O
NPhth
O
Ph
Ph
3w, 51%, >20:1
Ph
Acknowledgements
3y, 52%, >20:1b
3x, 90%, >20:1b
Funding provided by UC Irvine and the National Institutes of Health
(GM105938). We are grateful to Eli Lilly for a Grantee Award. F.A.C.
is grateful for an NSF Graduate Research Fellowship.
Ts
N
Br
NHTs
O
Boc
O
O
Ph
Ph
Ph
3z, 59%, >20:1
3aa, 82%, >20:1
3ab, 61%, >20:1
O
O
Me
H
Notes and references
O
OMe
H
O
N
H
Me
H
1
2
P. D. van Poelje and E. E. Snell, Annu. Rev. Biochem., 1990, 59,
29.
(a) I. Shimizu, T. Yamada and J. Tsuji, Tetrahedron Lett., 1980,
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J. Am. Chem. Soc., 1980, 102, 6381.
O
O
O
O
Ph
3ac, 79%, >20:1b
Ph
Ph
3ad, 46%, >20:1
3ae, 61%, >20:1b
aReaction conditions: 0.4 mmol 1a, 0.2 mmol 2, 4 mol% [Rh(cod)Cl]1, 8 mol% DPEphos,
0.4 mL 2-MeTHF, 60 °C, 24 hours. >20:1 denotes the ratio of 3:4. bReaction ran with 50
mol% benzoic acid.
3
4
For a review on transition metal-catalyzed decarboxylative
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>20:1 regioselectivity (eq 1). This result suggests the feasibility
of an allene intermediate in the catalytic cycle. To better
understand the proposed β-hydride elimination, we performed
an experiment with deuterated 1-phenyl-1-propyne 2a-d3 (eq
2). Ketone 3a-dn was obtained in 73% yield with high-branched
5
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7
8
For an example of the coupling of
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9
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-keto acid O-H bond may occur
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a
a
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12 For selected examples of branched selective Rh-catalyzed
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Suzuka, and M. Kawatsura, Org. Lett., 2003, 5, 1713; (c) U. 25 For select examples of C–O bond formation from alkynes, see:
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13 For selected examples of branched selective allylic alkylations 28 For select examples of C–C bond formation from alkynes, see:
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14 (a) M. Narsireddy and Y. Yamamoto, J. Org. Chem., 2008, 73,
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4 | J. Name., 2012, 00, 1-3
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