Cobalt-Catalyzed Allylic C(sp3)-H Carboxylation with CO2

Article abstract of DOI:10.1021/jacs.7b02775

Catalytic carboxylation of the allylic C(sp³)-H bond of terminal alkenes with CO₂ was developed with the aid of a Co/Xantphos complex. A wide range of allylarenes and 1,4-dienes were successfully transformed into the linear styrylacetic acid and hexa-3,5-dienoic acid derivatives in moderate to high yields, with excellent regioselectivity. The carboxylation showed remarkable functional group tolerability, so that selective addition to CO₂ occurred in the presence of other carbonyl groups such as amide, ester, and ketone. Since styrylacetic acid derivatives can be readily converted into optically active γ-butyrolactones through Sharpless asymmetric dihydroxylation, this allylic C(sp³)-H carboxylation showcases a facile synthesis of γ-butyrolactones from simple allylarenes via short steps.

Full text of DOI:10.1021/jacs.7b02775

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Communication
Cobalt-Catalyzed Allylic C(sp3)–H Carboxylation with CO2
Kenichi Michigami, Tsuyoshi Mita, and Yoshihiro Sato
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b02775 • Publication Date (Web): 11 Apr 2017
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Cobalt-Catalyzed Allylic C(sp3)–H Carboxylation with CO2
Kenichi Michigami, Tsuyoshi Mita,* Yoshihiro Sato*
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Faculty of Pharmaceutical Sciences, Hokkaido University, Sapporo 060ꢀ0812, Japan
Supporting Information Placeholder
ABSTRACT: A catalytic carboxylation of the allylic C(sp3)–H
bond of terminal alkenes with CO2 was developed with the aid of
a cobalt/Xantphos complex. A wide range of allylarenes and 1,4ꢀ
dienes were successfully transformed into the linear styrylacetic
acid and hexaꢀ3,5ꢀdienoic acid derivatives in moderate to high
yields, with excellent regioselectivity. The carboxylation showed
remarkable functional group tolerability, so that selective addition
to CO2 occurred in the presence of other carbonyl groups such as
amide, ester, and ketone. Since styrylacetic acid derivatives can
be readily converted into optically active γꢀbutyrolactones through
Sharpless asymmetric dihydroxylation, this allylic C(sp3)–H carꢀ
boxylation showcases a facile synthesis of γꢀbutyrolactones from
simple allylarenes via short steps.
boxylation, since nucleophilic allylmetal species are necessary for
the reaction with CO2. We therefore envisaged the generation of a
lowꢀvalent allylmetal species, some of which are generated
through the cleavage of allylic C(sp3)–H bonds by lowꢀvalent
alkylmetal complexes (e.g., alkylꢀCo(I) and Rh(I)) with the reꢀ
lease of an alkane.11,12 Although the reactivities of these allylmetꢀ
al complexes have not been investigated, we hypothesized that a
lowꢀvalent allylmetal complex shows high nucleophilicity toward
CO2 on the basis of the precedented transition metalꢀcatalyzed
carboxylation,13 thus furnishing β,γꢀunsaturated carboxylic acids
(eq 2).
Scheme 1. Strategy for the Generation of a Nucleophilic Alꢀ
lylmetal Complex
Transition metalꢀcatalyzed direct functionalization of C–H
bonds has been recognized as a potent and efficient platform for
straightforward organic synthesis to not only construct carbon
frameworks but also introduce various heteroatoms (B, N, O, Si,
halogens, etc.) without the need for preꢀfunctionalization of the
poorly reactive C–H bonds.1 However, catalytic functionalization
of C(sp3)–H bonds has seen much less development because of
the inherent low reactivity and lability of the C(sp3)–M (transition
metal) complexes generated by C–H bond cleavage.2 Besides,
most of the reported methods for catalytic C–H addition to carꢀ
bonyl electrophiles are restricted to highly reactive carbonyl comꢀ
pounds such as aldehydes, imines, αꢀketocarbonyl compounds,
and isocyanates.3 Hence, the development of catalytic caboxylaꢀ
tion of C(sp3)–H bonds with the much less reactive carbon dioxꢀ
ide (CO2) would be an innovative milestone in the fields of C–H
activation and fixation chemistry of CO2, an abundant, inexpenꢀ
sive, and nontoxic C1 feedstock.4 Although catalytic carboxylaꢀ
tion of acidic C–H bonds through deprotonation by basic metal
complexes has been well studied,5 there are only a few examples
of C–H carboxylation apart from acidꢀbase mechanism.6 In addiꢀ
tion, most of the past studies demonstrate the carboxylation of
C(sp)–H and C(sp2)–H bonds. Catalytic direct carboxylation of
C(sp3)–H bonds is still in its nascent stage, with only a few reꢀ
ported methods for methane carboxylation7 and UV irradiation
methods.8,9 Hence, catalytic C(sp3)–H carboxylation with CO2
under mild conditions holds a formidable challenge that would
open new horizons to the synthesis of aliphatic carboxylic acids.
Based on the above hypothesis, we extensively investigated the
reaction conditions for the carboxylation of allylarene 1a under
CO2 atmosphere (1 atm, closed), using various metal salts, ligꢀ
ands, and alkylated reagents.14 A combination of Co(acac)2,
Xantphos, and AlMe3 provided the corresponding transꢀ
styrylacetic acid methyl ester 2a in 32% yield, together with oleꢀ
fin isomers in 41% yield after treatment with CH2N2 (Table 1,
entry 1). Both Co(III) and Co(I) complexes were competent cataꢀ
lysts (entries 2 and 3) for this reaction; however, other transition
metal salts such as Cr(II), Mn(I), Fe(III), Rh(I), Ir(I), Ni(II), and
Cu(I) salts did not promote carboxylation but 1a was recovered,
and in some cases, olefin isomers were observed (entries 4ꢀ10).15
Notably, the reaction offered extremely high linear/branch selecꢀ
tivity, so that only the linear carboxylated product 2a was obꢀ
tained. Encouraged by this result, we examined the effects of
various ligands in the presence of Co(acac)2. However, DPEphos,
DPPF, DPPP, and 2,2’ꢀbpy afforded the olefin isomerization
product 3a as the major product (entries 11ꢀ14). The reaction was
strongly influenced even by subtle structural differences between
Xantphos and DPEphos, implying that the oxygen atom in
Xantphos might play a crucial role as a hemilabile ligand in this
carboxylation (vide infra).
To this end, we initially targeted terminal alkenes as substrates
for C(sp3)–H carboxylation. Allylic C(sp3)–H functionalization
using highꢀvalent Pd(II) and Rh(III) catalysts has been widely
studied, which allows direct substitution of the allylic C–H bond
by a variety of nucleophiles through the generation of electroꢀ
philic η3ꢀallylmetal intermediates (Scheme 1, eq 1).10 However,
these methods do not meet the requirements of our planned carꢀ
Next, the reaction conditions were further screened under the
Co(acac)2/Xantphos system. When the amount of AlMe3 was
reduced to 1.5 equiv, the yield of 2a increased to 45% (entry 15).
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The concentration of 1a also affected the efficiency, in which
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2
3
4
lower concentration suppressed olefin isomerization to afford the
desired 2a in 58% yield (entry 16). The addition of 1 equiv of CsF
further accelerated the carboxylation to provide 2a in 71% yield
(entry 17).16
Table 1. Condition Screening
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yields (%)a
entry metal salt
Ligand
3ab
34/7
58/9
54/12
ꢀ/ꢀ
2a
32
21
22
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
45
58
71
4a
13
ꢀ
5
ꢀ
1a
ꢀ
ꢀ
4
99
98
86
67
93
69
89
ꢀ
ꢀ
ꢀ
4
ꢀ
3
2
1
2
Co(acac)2
Co(acac)3
Xantphos
Xantphos
3
CoCl(PPh3)3 Xantphos
4
CrCl2
Xantphos
Xantphos
Xantphos
5
6
MnBr(CO)5
Fe(acac)3
ꢀ/ꢀ
ꢀ
9/2
ꢀ
7
[Rh(cod)Cl]2 Xantphos
25/6
5/ꢀ
ꢀ
ꢀ
8
9
[Ir(cod)Cl]2
Ni(acac)2
CuI
Co(acac)2
Co(acac)2
Co(acac)2
Co(acac)2
Co(acac)2
Xantphos
Xantphos
Xantphos
DPEphos
DPPF
27/3
6/2
ꢀ
ꢀ
10
11
12
13
14
15c
74/3
87/4
74/12
90/ꢀ
26/5
11/3
17/3
ꢀ
7
10
ꢀ
15
9
1
Figure 1. Substrate scope of allylarenes (0.4 mmol). Isolated
DPPP
a
yields are shown. 1ꢀ11% of olefin isomer (α,βꢀunsaturated carꢀ
2,2’ꢀbpy
Xantphos
Xantphos
Xantphos
boxylic acid) was observed.14 b2 equiv of AlMe3 was used.
16c,d Co(acac)2
The efficient and highly regioselective carboxylation of alꢀ
lylarenes led us to examine the feasibility of using 1,4ꢀdienes,
which are assumed to show similar reactivity to allylarenes, as
substrates. Indeed, a wide range of 1,4ꢀdiene derivatives were
applicable to the cobaltꢀcatalyzed carboxylation, and hexaꢀ3,5ꢀ
dienoic acid derivatives were obtained in good to high yields
(Figure 2).
17c,d,e Co(acac)2
Reaction conditions: allylarene 1a (0.2 mmol), catalyst (10 mol% of the
metal), ligand (20 mol%), AlMe3 (0.6 mmol), DMA (1.0 mL, 0.2 M), CO2
(1 atm, closed), 12 h. aYields were determined by1H NMR analysis using
b
1,3,5ꢀtrimethoxybenzene as an internal standard. Ratio of E/Z isomers.
c1.5 equiv of AlMe3 was used. d0.05 M in DMA (4.0 mL). e1 equiv of CsF
(0.2 mmol) was added.
With the optimal conditions in hand, we next explored the subꢀ
strate scope of the allylic C–H carboxylation of allylarenes under
the Co(acac)2/Xantphos/AlMe3 system (Figure 1). Methyl esterifiꢀ
cation was unnecessary except in the case of 1o, and a series of
linear carboxylic acids were obtained with excellent regioselectivꢀ
ity. Additionally, a back extraction/crystallization sequence was
effective for their purification process especially in case the prodꢀ
uct is a solid.14 Electronꢀneutral allylarenes 1aꢀ1d, including the
orthoꢀsubstituted 1c, gave the corresponding linear carboxylic
acids 5aꢀ5d in moderate to good yields. A variety of functionalꢀ
ized allylarenes bearing trifluoromethyl groups (1e and 1f) and
alkoxy groups (1gꢀ1i) were tolerated under the present reaction
conditions. Remarkably, preꢀtreatment of 1j containing a phenolic
hydroxy group with 0.5 equiv of AlMe3 afforded an aluminum
aryloxide, and subsequent carboxylation with 1.5 equiv of AlMe3
furnished the corresponding carboxylic acid 5j in 72% yield. The
reaction proceeded selectively with CO2 when using substrates
containing amide (1k), ester (1l), and ketone (1m) carbonyls,
which are generally reactive toward nucleophiles than is CO2 and
incompatible with the carboxylation using strongly nucleophilic
organomagnesium and organolithium reagents. Furthermore, inꢀ
dole (1n) and quinoline (1o) derivatives were carboxylated to
afford the products in 84% and 63% yields, respectively.
Figure 2. Substrate scope of 1,4ꢀdienes and a simple alkene (0.4
a
mmol). Isolated yields are shown unless otherwise noted. Yields
were determined by 1H NMR analysis using 1,3,5ꢀ
trimethoxybenzene as an internal standard.
Compounds 6aꢀ6c possessing different substitution patterns on
the internal alkene moieties (6b and 6c contain alkene regioisoꢀ
mers) exhibited different reactivities for the carboxylation of CO2,
but there was no significant change in the E/Z ratios of the interꢀ
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nal alkenes.14 1,4ꢀDienes containing a cyclohexenyl moiety (6d)
3a and 4a were unreactive under the optimized carboxylation
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2
3
4
5
6
7
8
and gemꢀdiphenyl moiety (6e) afforded the corresponding linear
carboxylic acids 7d and 7e in 78% and 57% yields, respectively.
The more structurally complicated bicyclo[2.2.2]octaneꢀbased
1,4ꢀdiene 6f bearing ketone and dimethyl ketal moieties also unꢀ
derwent carboxylation; after careful work up using aq. citric acꢀ
id/NaCO3 buffer and methyl esterification, the corresponding
ester 8f was obtained in 41% yield, with 46% recovery of 6f. This
result indicated the dimethyl ketal moiety could be tolerated under
the cobaltꢀcatalyzed carboxylation conditions. Furthermore, carꢀ
boxylation of a simple terminal alkene 6g provided the carboxylic
acids 7g, 7g’, and 7g”. Even though the yield and regioselectivity
of the reaction using 6g were low, this result suggests the feasibilꢀ
ity of extending the protocol to less reactive allylic C–H bonds.
conditions. While further inꢀdepth investigation is required to
unveil the exact role of CsF in this carboxylation, its beneficial
effect on the yield might be attributed to the interaction with CO2,
which allows for the dissolution of more CO2 into the reaction
system rather than the generation of the tetracoordinate fluoroꢀ
aluminate complex.16,22
The cobaltꢀcatalyzed allylic C–H carboxylation provides a new
synthetic strategy for linear styrylacetic acid derivatives from
allylarenes, which could not be achieved by simple deprotonation
n
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using a strong base (e.g., BuLi), followed by the reaction with
CO2.23 Since styrylacetic acid derivatives have nonꢀconjugated
alkene and carboxylic acid moieties, a variety of transformations
can be expected. In particular, styrylacetic acid derivatives are key
synthons of various functionalized γꢀarylbutyrolactones, which
are structural motifs found in many natural products (Figure 4).24
Indeed, several transformations of styrylacetic acid derivative 5i
into γꢀbutyrolactones could be achieved (Scheme 2). Carboxylic
acid 5i derived from the commercially available safrole 1i was
readily converted into antiꢀβꢀiodoꢀ and antiꢀβꢀhydroxyꢀγꢀlactone
9i and 10i in good yields by treatment with Oxone®/KI25 and diꢀ
methyldioxirane (DMDO),26 respectively. Besides, Sharpless
asymmetric dihydroxylation of the corresponding ester 2i furꢀ
nished the optically active synꢀβꢀhydroxyꢀγꢀbutyrolactone 11i in
80% yield with 99% ee.27 These protocols provide a stereodiverꢀ
gent synthesis of γꢀarylbutyrolactones from allylbenzene derivaꢀ
tives via short steps. Furthermore, 5h was stereoselectively conꢀ
verted into a similar lactone 11h, which is a key intermediate in
the synthesis of tricyclic pharmacophores, through the oxaꢀPictetꢀ
Spengler reaction and subsequent oxidation.28
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Figure 3. Proposed Catalytic Cycle.
Figure 4. γꢀArylbutyrolactoneꢀBased Bioactive Natural Products.
Scheme 2. Synthesis of γꢀArylbutyrolactones
Based on the obtained results, including significant ligand efꢀ
fects, we propose a plausible reaction pathway in Figure 3. Since
cobalt catalysts with different valences (Co(I), Co(II), Co(III))
equally promoted the carboxylation, a lowꢀvalent methylcobalt(I)
species I would first be generated from the cobalt salt, Xantphos,
and AlMe3.17 Dialkylcobalt(II) is known to undergo disproporꢀ
tionation to form Co(I) and Co(III),18 and the latter further proꢀ
duces Co(I) through reductive elimination. Next, the alkene in the
substrate would coordinate to cobalt, thus facilitating the cleavage
of the adjacent allylic C–H bond to produce η3ꢀallylcobalt species
III. At this stage, the oxygen atom in the Xantphos ligand might
coordinate to cobalt, which would assist the tautomerization of η3ꢀ
allylcobalt III to η1ꢀallylcobalt complexes IV and IV’.19 The rigid
backbone of Xantphos might work as a hemilabile ligand to proꢀ
mote the tautomerization; in contrast, in the case of DPEphos,
which has a more flexible backbone, the oxygen atom is reluctant
to coordinate to the cobalt center due to the free rotation of the
Ar–O–Ar bond. Subsequently, two different ways are possible.
Reductive elimination of methane from complex IV would lead to
a lowꢀvalent allylcobalt species V,11 which is stabilized by an aryl
or alkenyl ligand.20 Subsequently, C–C bond formation with CO2
would proceed at the γꢀposition21 to produce cobalt carboxylate
VI. Transmetalation between VI and AlMe3 would furnish the
linear carboxylated product along with the regeneration of
methylcobalt species I.
In summary, we have developed a catalytic direct allylic
C(sp3)–H carboxylation by using the Co(acac)2/Xantphos/AlMe3
system, which enabled highly regioselective transformation of
allyl groups into linear 3ꢀbutenoic acid motifs with good functionꢀ
al group tolerance. The carboxylated products, styrylacetic acids,
could be readily converted into γꢀarylbutyrolactones. Further reꢀ
search into the synthetic application of the protocol is ongoing,
and the results will be reported in due course.
On the other hand, reductive elimination of complex IV’ would
generate the olefin isomerization product 3 and methylated prodꢀ
uct 4. This reductive elimination step would be irreversible, since
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14063. (b) Ishida, N.; Masuda, Y.; Uemoto, S.; Murakami, M. Chem. Eur.
J. 2016, 22, 6524. (c) Seo, H.; Katcher, M. H.; Jamison, T. F. Nat. Chem.
DOI: 10.1038/NCHEM.2690.
ASSOCIATED CONTENT
Supporting Information
1
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8
(9) We have developed a formal C(sp3)–H carboxylation through cataꢀ
lytic C–H silylation followed by fluorideꢀmediated CO2 incorporation: (a)
Mita, T.; Michigami, K.; Sato, Y. Org. Lett. 2012, 14, 3462. (b) Mita, T.;
Michigami, K.; Sato, Y. Chem. Asian J. 2013, 8, 2970. Very recently, a
sequential method involving aluminateꢀmediated allylic C(sp3)–H deproꢀ
tonation and Cu(I)ꢀcatalyzed carboxylation of allylic alcohol derivatives
has been reported. See: Ueno, A.; Takimoto, M.; Hou, Z. Org. Biomol.
Chem. 2017, 15, 2370.
Supporting Information Available. The information is available
free of charge via the Internet at http://pubs.acs.org.
Supplemental data, experimental procedures, and characterizaꢀ
tions.
AUTHOR INFORMATION
Corresponding Author
(10) For recent examples of Pd(II)ꢀcatalyzed allylic C(sp3)–H functionꢀ
alizations, see: (a) Chen, M. S.; White, M. C. J. Am. Chem. Soc. 2004, 126,
1346. (b) Young, A. J.; White, M. C. J. Am. Chem. Soc. 2008, 130, 14090.
(c) Kondo, H.; Yu, F.; Yamaguchi, J.; Liu, G.; Itami, K. Org. Lett. 2014,
16, 4212. (d) Ammann, S. E.; Liu, W.; White, M. C. Angew. Chem., Int.
Ed. 2016, 55, 9571. For Rh(III)ꢀcatalyzed variants, see: (e) Cochet, T.;
Bellosta, V.; Roche, D.; Ortholand, J.ꢀY.; Greiner, A.; Cossy, J. Chem.
Commun. 2012, 48, 10745. (f) Shibata, Y.; Kudo, E.; Sugiyama, H.;
Uekusa, H.; Tanaka, K. Organometallics 2016, 35, 1547 and references
therein.
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tmita@pharm.hokudai.ac.jp; biyo@pharm.hokudai.ac.jp
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENTS
(11) (a) Klein, H.ꢀF.; Helwig, M.; Braun, S. Chem. Ber. 1994, 127,
1563. (b) Urtel, H.; Meier, C.; Eisenträger, F.; Rominger, F.; Joschek, J.
P.; Hofmann, P. Angew. Chem., Int. Ed. 2001, 40, 781. (c) Beck, R.;
Flörke, U.; Klein, H.ꢀF. Organometallics 2015, 34, 1454.
This work was financially supported by GrantsꢀinꢀAid for Scienꢀ
tific Research (C) (No. 26410108), GrantsꢀinꢀAid for Scientific
Research (B) (No. 26293001) from JSPS, and also by JST ACTꢀC
(No. JPMJCR12YM). K.M. thanks JSPS for a fellowship (No.
14J08052).
(12) For C(sp2)–H addition to aldimines catalyzed by a lowꢀvalent coꢀ
balt species, see: Gao, K.; Yoshikai, N. Chem. Commun. 2012, 48, 4305.
(13) For recent examples of catalytic carboxylation via the insertion of
CO2 into C(sp3)–M bonds, see: (a) León, T.; Correa, A.; Martin, R. J. Am.
Chem. Soc. 2013, 135, 1221. (b) Liu, Y.; Cornella, J.; Martin, R. J. Am.
Chem. Soc. 2014, 136, 11212. (c) Moragas, T.; Cornella, J.; Martin, R. J.
Am. Chem. Soc. 2014, 136, 17702. (d) Nogi, K.; Fujiihara, T.; Terao, J.;
Tsuji, Y. Chem. Commun. 2014, 50, 13052. (e) Zhang, S.; Chen, W.ꢀQ.;
Yu, A.; He, L.ꢀN. ChemCatChem 2015, 7, 3972. (f) Börjesson, M.; Moraꢀ
gas, T.; Martin, R. J. Am. Chem. Soc. 2016, 138, 7504. (g) Moragas, T.;
Martin, R. Synthesis 2016, 48, 2816. (h) Moragas, T.; Gaydou, M.; Martin,
R. Angew. Chem., Int. Ed. 2016, 55, 5053.
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(14) For the detail of condition screening, see the SI.
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(16) CsF accelerated Niꢀcatalyzed hydrocarboxylation of alkynes,
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factor for efficient carboxylation (see the SI).
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n
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57:43, see the SI). Besides, no carboxylated product was observed from
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1,4ꢀdiene
6d
by
the
same
protocol,
suggesting
that
(8) For carboxylation of C(sp3)–H bonds under UV irradiation, see: (a)
Masuda, Y.; Ishida, N.; Murakami, M. J. Am. Chem. Soc. 2015, 137,
Co(acac)2/Xantphos/AlMe3 catalytic system is advantageous in terms of
both regioselectivitity as well as reactivity.
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(24) For the total synthesis of (+)ꢀNicotlactone A, see: (a) Krishna, P.
(27) (a) Wang, Z. M.; Zhang, X.ꢀL.; Sharpless, K. B.; Sinha, S. C.;
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droxylation of β,γꢀunsaturated carboxylates, see: (b) Neumeyer, M.;
Brückner, R. Eur. J. Org. Chem. 2016, 5060.
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M.; Song, S.; Xu, Y.; Miao, Z.; Zhang, A. RSC Adv. 2015, 5, 27502.
R.; Prabhakar, S.; Sravanthi, C. Tetrahedron Lett. 2013, 54, 669. For (+)ꢀ
Frenolicin B, see: (b) Zhang, Y.; Wang, X.; Sunkara, M.; Ye, Q.; Ponoꢀ
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5566. For (+)ꢀClavilactone B, see: (c) Takao, K.; Nanamiya, R.; Fukushiꢀ
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