Efficient and selective hydrogenation of C-O bonds with a simple sodium formate catalyzed by nickel

Article abstract of DOI:10.1039/c7cc08709h

A Ni-catalyzed hydrogenation of C-O compounds with sodium formate is developed. Various esters, i.e. aryl, alkenyl, benzyl pivalates, and even the aryl ethers, were efficiently reduced with a loading of nickel catalysts down to 0.5 mol%. Reactive functional groups such as C-C double bonds, carbonyl, CN, MeS and halogen groups are tolerable. This reaction can be used for the modification of complex molecules and carried out at a large scale.

Full text of DOI:10.1039/c7cc08709h

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Efficient and Selective Hydrogenation of C-O Bonds with a Simple
Sodium Formate Catalyzed by Nickel
Xiaoxiang Xi,a Tieqiao Chen,*a,b Ji-Shu Zhang,a and Li-Biao Hanc
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A Ni-catalyzed hydrogenation of C-O compounds with sodium be efficiently synthesized starting from simple phenoxides
formate is developed. Various esters, i.e. aryl, alkenyl, benzyl through two steps (Scheme 1b). It was reported that
pivalates, and even the aryl ethers, were efficiently reduced with a hydrosilanes and aluminum hydrides were able to
loading of nickel catalyst down to 0.5 mol%. Reactive functional hydrogenate the C-O bonds.11-13 Recently, Hartwig reported
groups such as C-C double bonds, carbonyl, CN, MeS and halogen the reduction of aryl ethers with hydrogen gas. However, an
groups are tolerable. This reaction can be used for the excess amount of a strong base t-BuONa was required.9a,12 Due
modification of complex molecules and carried out at a large scale. to the harsh conditions, in those catalytic system, only limited
substrates with a relatively robust functional group like amide,
Organic compounds bearing C-O bonds are abundant in both
nature and synthetic fields. Recently, the utilization of these
compounds through metal-mediated C-O activation has drawn
great attention.1 For instance, like organohalides, the once
recognized “inert” phenoxides have been successfully used in
amine, ester, methoxy and CF3 groups were demonstrated. To
the best of our knowledge, there is no general catalytic
hydrogenation of C-O compounds with a safe and readily
available reductant which is compatible with a wide range of
reactive functional groups.
the
metal-catalyzed
cross-coupling
reactions
with
organometals (Mg, Zn, B etc.)2-5 and Z-H compounds (Z = C, N,
P)6-8 to generate the corresponding carbon-carbon and carbon-
heteroatom bonds, respectively (Scheme 1a). However,
despite a big progress has been achieved, the C-O activation
reactions, that are efficient enough to be used as a practically
useful method in the laboratory (not to mention industry), are
quite limited because these reactions generally require a large
amount of the metal catalysts (usually 5-20 mol%).9 Therefore,
the development of an efficient C-O bond activation with a low
loading of the catalyst has been a big challenge.
a) T r ansition metal-catalyzed cross-couplings
cat. M
+
R or Z
O
R'
R-M or Z-H
reactive coupling reagents
high loading catalysts (over 5 mol%, generally 10-20 mol%)
b) As r emovable pr otecting gr oups (C-O hydr ogenation)
cat. M
R-X
5-10 mol% Ni
O
R'
O
R'
H
aluminohydrides
or hydrosilanes
R
R
c) This work: hyd rogenation of C-O bonds with read ily available HCOONa
cat. Ni (dwon to 0.5 mol%)/HCOONa
R
H
R
R'O
R = aryl, alkenyl
benzyl
R' = carboxyl, methyl
low catalyst loading
cheap reductant
easy scale-up
mild conditions
wide functional tolerance
applied to complex structures
The hydrogenation of C-O bonds is of great interest,
considering not only the pure academic curiosity but also its
great synthetic value. Because a phenolic group can facilitate
the introduction of a functional group to the benzene ring via
such as C-H activation, ortho-metalation and Friedel-Crafts-
type reaction etc.,10 and thus various aromatic compounds can
Scheme 1 Transition metal-catalyzed transformation of C-O compounds.
Herein, we report an efficient hydrogenation of C-O bonds
under mild conditions by employing the cheap, readily
available sodium formate HCOONa as the reductant. Various
esters can be converted to the corresponding hydrocarbons in
high yields with a low loading of nickel catalyst (down 0.5
mol%) (Scheme 1c). This reaction is quite general, aryl, alkenyl,
benzyl pivalates and even aryl methyl ether all could be
reduced. Moreover, functionalities such as C-C double bonds,
carbonyl, CN, MeS, F, amide and aldehyde groups are tolerable
under the reaction conditions.14 Mechanistic studies showed
that this transformation might proceed via a Ni(0)/Ni(II)
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Table 2 Ni-catalyzed transfer hydrogenation of C-O compounds via C-O activation.a
process involving oxidative addition, ligand exchange,
decarboxylation and reductive elimination.
cat Ni/dcype
R'O
R
H
R
In the presence of 5 mol% Ni(COD)2 and 10 mol% dcype
(1,1'-bis(dicyclohexylphosphino)ethane), naphthyl pivalate 1a
reacted with HCOONa readily in toluene, producing 2a nearly
quantitatively (Table 1, entry 1). This reaction can be
conducted at lower loadings of the catalyst, and with 1 mol%
nickel catalyst, 2a was also quantitatively obtained (Table 1,
entries 2-4). The ratio of Ni/P is crucial to this reaction. Only 16%
yield of 2a was obtained with Ni/P = 1/2; whereas no reaction
took place in the absence of the phosphine ligand (Table 1,
entries 5 and 6). The choice of a suitable phosphine ligand is
also pivotal for this reaction, since no reaction was observed
with other phosphine ligands such as PCy3, PPh3, 1,1-
HCOONa, toluene, heat, 24 h
1
2
Naphthyl pivalates:
OPiv
OPiv
OPiv
MeO
NC
1a to 2a, 97%
1b to 2b, 96%
1c to 2c, 95%
OPiv
OPiv
OPiv
OHC
NHC(O)Bu-t
1f to 2f, 86%
OMe
1d to 2d, 76%
OPiv
1e to 2e, 83%b,c
Opiv
Phenyl pivalates:
Ph
OPiv
bis(diphenylphosphino)methane
bis(diphenylphosphino)ethane
(dppm),
1,2-
N
(dppe), 1,3-bis(diphenyl-
1g to 2g, 81%b,c,d
1h to 2h, 87%b,c
MeO
1i to 2i, 95%
phosphino)propane (dppp), 1,6-bis(diphenylphosphino)hexane
(dpph) and 1,1’-bis(diphenylphosphino)ferrocene (dppf) (Table
1, entries 7-13). In addition to Ni(COD)2, the easily handled
NiCl2 and Ni(acac)2 also showed high catalytic activity (Table 1,
entries 14 and 15). However palladium and rhodium tested
were not effective under similar reaction conditions (Table 1,
entries 16 and 17). As for the solvent, the hydrogenation also
proceeded smoothly in dioxane, but poorly in DMF and DMAc
(Table 1, entries 18-20).
O
H
N
OPiv
Me2N
OPiv
OPiv
OPiv
t-Bu
MeO
1k to 2k, 72%c,e
1j to 2j, 92%b
1l to 2l, 70%b,c
O
PhO
OPiv
NC
OPiv
1m to 2m, 88%b,c
1n to 2n, 65%b,c
1o to 2o, 96%b,c
Alkenyl pivalates:
OPiv
OPiv
OMe
Ph
Table 1 Optimization of the reaction conditions.a
F
OPiv
1p to 2i, 84%b,c
1q to 2p, 91%b,c
1r to 2q, 89%b,c
OPiv
OPiv
PivO
1u to 2t, 72%
(Z/E = 7:3)c
1s to 2r, 70%c,f
1t to 2s, 63%c,f
OPiv
Benzyl pivalates:
OPiv
PhO
OPiv
1w to 2v, 90%b
1x to 2w, 71%b,c
1v to 2u, 97%
MeO
Aryl ether:
OPiv
OMe
MeS
OPiv
MeO
1z
to 2y, 91%b,c
1aa to 2a, 63%c,h,i
1y to 2x, 68%b,c,g
OMe
OMe
O
O
OMe
1ad to 2o, 52%c,h
N
1ab to 2z, 65%c,h
1ac to 2g, 52%c,h
a Reaction conditions: 0.5 mmol 1a, 1.0 mmol HCOONa, metal catalyst, phosphine
ligand (M/P = 1:4) and 1 mL solvent were heated in a 10 mL glass tube at 120 oC
for 24 h. b GC yield using n-tridecane as an internal standard. c Ni/P = 1:2.
a
Reaction conditions: 0.5 mmol 1, 1 mmol HCOONa, 1 mol% Ni(COD)2/dcype
(Ni/P = 1:4) and 1 mL toluene were heated in a 10 mL glass tube at 120 oC for 24
h. 2 mol% Ni(COD)2. 140 oC. Ni/P = 1:10. 10 mol% Ni(COD)2. 5 mol%
b
c
d
e
f
g
h
Ni(COD)2. Ni/P = 1:8. 10 mol% Ni(COD)2, 20 mol% carbene.HCl (for structure,
see SI), 20 mol% t-BuONa. i The yield is 7% in the absence of HCOONa.
This Ni-catalyzed hydrogenation was
a rather general
reaction. A variety of pivalates and ethers readily reacted with
sodium formate under the present reaction conditions to
the product 2c in 95% yield (Table 2, 1c). Noticeably, even the
reactive aldehyde group was tolerable under the current
reaction conditions (Table 2, 1d). N-Fused quinolinyl and
polycyclic phenanthrenyl pivalates were also reduced readily
by sodium formate, giving the corresponding hydrogenated
products in high yields (Table 2, 1g and 1h).
produce the corresponding hydrocarbons
2 in good to
excellent yields. Thus, naphthyl pivalates bearing 6-methoxy,
4-methoxy and 4-amide groups all generated the expected
hydrogenative products in high yields (Table 2, 1b, 1e and 1f).
Substrate with an electron-withdrawing cyano group also gave
2 | J. Name., 2012, 00, 1-3
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Importantly, phenyl pivalates, which usually show low
reactivity in C-O bond activation,15 were also proved to be
good substrates. Both electron-rich and electron-deficient
ones were hydrogenated by sodium formate (Table 2, 1i-1q). A
fluoro functionality could also survive in the present catalytic
system (Table 2, 1q). It was noted that the ortho-substituted
pivalates also showed high reactivity (Table 2, 1p and 1q),
indicating that the corresponding hydrocarbons can be easily
obtained from the corresponding phenoxides via transition
metal-catalyzed ortho-C-H activation and subsequent removal
of the C-O group by using the present reaction.
Scheme 3 Mechanistic consideration.
More surprisingly, under the current reaction conditions,
even alkenyl pivalates could be selectively reduced to the
As for the mechanism, we isolated complex A1 generated
via oxidative addition of 1a to Ni(0) complex in the presence of
dcype.5f,6b,16 This complex was thought to be one of the
catalytic intermediates. Indeed, a catalytic amount of A1 (1
mol%) catalyzed the hydrogenation of 1a with sodium formate
to produce 2a quantitatively (Scheme 3a). A stoichiometric
reaction of complex A1 with sodium formate also took place
readily to give 2a in 63% yield. The deuterated experiment was
also performed, confirming that the hydrogen was originated
from sodium formate (Scheme 3b).
corresponding alkenes in good to high yields (Table 2, 1r-1u).
To the best of our knowledge, this is the first hydrogenation of
alkenyl pivalates via C-O activation. Since these compounds
are derivatives from ketones, this reaction provided an
efficient method for converting ketones to alkenes.
The substrate scope was not limited to the sp2C-O
compounds described above, benzyl pivalates also reacted
with sodium formate catalyzed by nickel, producing the
corresponding hydrocarbons in good to high yields (Table 2,
1v-1z). Methoxy, phenoxy and even the highly coordinated
thiomethyl groups were all well tolerated in the present
catalytic system. By switching the phosphine ligand to a
carbene ligand, aryl methyl ethers were also hydrogenated
(Table 2, 1aa-1ad).
a) Selective removal of a C-O bond from a complex molecules
O
O
H
H
2 mol% Ni(COD)2, 4 mol% dcype
H
H
O
H
H
1 mmol HCOONa
1 mL toluene, 120 oC, 24 h
PivO
PivO
H
Scheme 4 A proposed mechanism. Ligands were omitted for clarity.
1ae, 0.5 mmol
2aa, 86%
O
2 mol% Ni(COD)2, 4 mol% dcype
OMe
OMe
Therefore, although a Ni(0)/Ni(I) process could not be
completely ruled out,17 we prefer a Ni(0)/Ni(II) catalytic cycle
as shown in Scheme 4.18 At first, C-O bond oxidatively added to
1 mmol HCOONa
1 mL toluene, 140 oC, 24 h
NHPiv
2ab, 80%
NHPiv
1af, 0.5 mmol
b) Gram-scale (10 mmol) reaction
H
H
OPiv 0.5 mol% Ni(COD)2, 2 mol% dcype
Ni(0) complex forming complex
exchange with sodium formate generating
decarboxylation of produced the hydridonickel species
Reductive elimination of species yielded the hydrogenative
product and regenerated the Ni(0) complex.
A
, followed by a ligand
B. Subsequent
20 mmol HCOONa
10 mL toluene, 140 oC, 24 h
2a, 1.24 g
97% isolated yield
1a
B
C.
2.28 g, 10 mmol
C
Scheme 2 Application of the current hydrogenation of C-O bonds.
2
To demonstrate its synthetic value, this reaction is
applicable to remove a C-O bond from a complex molecule
(Scheme 2a). For example, the C-O bond in 1ae, an estrogen,
was removed readily, yielding the corresponding product 2aa
in 86% yield. A derivative of amino acid 1af could also be used
as the substrate to generate 2ab in 80% yield under similar
reaction conditions. This reaction can be carried out readily in
a relatively large scale. Thus, by simply heating a mixture of 10
mmol 1a, 2 equiv.s sodium formate in 10 mL toluene in the
presences of 0.5 mol% Ni(COD)2 and 2 mol% dcype for 24 h, 1a
was almost completely converted to 2a. Analytically pure 2a
was obtained in 97% isolated yield (Scheme 2b) by simply
passing the mixture through a short silica gel column using
petroleum ether as the eluent.
Conclusion
In summary, we have developed an efficient Ni-catalyzed
hydrogenation of C-O compounds with the readily available
sodium formate under mild reaction conditions. This reaction
can be conducted at a low loading of nickel catalyst (down to
0.5 mol%). Naphthyl, phenyl, alkenyl and benzyl pivalates,
even aryl ethers all could be reduced via C-O cleavage. The
reaction is easily conducted in gram-scales and is applicable to
modify a complex molecule with delicate functional groups.
Further studies on the mechanism of this nickel catalyzed
hydrogenation of C-O bonds are underway in this laboratory.
Partial financial supports from NFSC (21403062, 21373080),
and HNNSF 2015JJ3039 are gratefully acknowledged.
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6792; (e) J. Cornella, E. P. Jackson and R. Martin, Angew.
Chem. Int. Ed., 2015, 54, 4075.
Selected examples on cross coupling with amines: (a) M.
Notes and references
1
For recent reviews on C-O activation, see: (a) D.-G. Yu, B.-J. Li
and Z.-J. Shi, Acc. Chem. Res., 2010, 43, 1486; (b) B. M.
Rosen, K. W. Quasdorf, D. A. Wilson, N. Zhang, A.-M.
7
Tobisu, S T. Himasaki and N. Chatani, Chem. Lett., 2009, 38
,
710; (b) S. D. Ramgren, A. L. Silberstein, Y. Yang and N. K.
Garg, Angew. Chem. Int. Ed., 2011, 50, 2171; (c) M. Tobisu, A.
Yasutome, K. Yamakawa, T. Shimasaki and N. Chatani,
Tetrahedron, 2012, 68, 5157; (d) T. Shimasaki, M. Tobisu and
N. Chatani, Angew. Chem. Int. Ed., 2010, 49, 2929; (e) J. Li
and Z.-X. Wang, Org. Lett., 2017, 19, 3723; (f) H. Yue, L. Guo,
X. Liu and M. Rueping, Org. Lett., 2017, 19, 1788.
Resmerita, N. K. Garg and V. Percec, Chem. Rev., 2011, 111
1346; c) B.-J. Li, D.-G. Yu, C.-L. Sun and Z.-J. Shi, Chem. Eur. J.,
2011, 17 1728; (d) M. Tobisu and N. Chatani, Top.
,
,
Organomet. Chem., 2013, 44, 35; (e) T. Mesganaw and N. K.
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K. Muto and K. Itami, Eur. J. Org. Chem., 2013, 2013, 19; (g) J.
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9
Cross coupling with P-H compounds: (a) J. Yang, T. Chen and
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Xiao, T. Chen and L.-B. Han, J. Org. Chem., 2016, 81, 3911.
To the best of our knowledge, only five examples on C-O
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Ed., 2015, 54, 8600; (k) H. Zeng, Z. Qiu, A. Domínguez-
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510; (l) S. Z. Tasker, E. A. Standley and T. F. Jamison, Nature,
2014, 509, 299.
Selected examples on cross coupling with Grignard reagents:
(a) E. Wenkert, E. L. Michelotti and C. S. Swindell, J. Am.
Chem. Soc., 1979, 101, 2246; (b) J. W. Dankwardt, Angew.
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Leiendecker, C.-C. Hsiao, L. Guo, N. Alandini and M. Rueping,
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TMSCH2Li, 1 mol%); (d) C. Zarate, M. Nakajima and R. Martin,
J. Am. Chem. Soc., 2017, 139, 1191 (Silylation with Me3Si-
BPin, 1-10 mol%); (e) L.-G. Xie and Z.-X. Wang, Chem. Eur. J.,
2011, 17, 4972 (with a special nickel catalyst for coupling
with Grignard reagents, 2.5-10 mol%).
2
10 (a) V. Snieckus, Chem. Rev., 1990, 90, 879; (b) G. H. Posner
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3
4
Selected examples on cross coupling with organozincs: (a) B.-
J. Li, Y.-Z. Li, X.-Y. Lu, J. Liu, B.-T. Guan and Z.-J. Shi, Angew.
Chem. Int. Ed., 2008, 47, 10124; (b) C. Wang, T. Ozaki, R.
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11 (a) P. Alvarez-Bercedo and R. Martin, J. Am. Chem. Soc.,
2010, 132, 17352; (b) M. Tobisu, K. Yamakawa, T. Shimasaki
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Liu, J. Jia and M. Rueping, ACS Catal., 2017, 7, 4491.
Selected examples on cross coupling with organoborons: (a)
M. Tobisu, T. Shimasaki and N. Chatani, Angew. Chem. Int.
Ed., 2008, 47, 4866; (b) K. W. Quasdorf, X. Tian and N. K.
Garg, J. Am. Chem. Soc., 2008, 130, 14422; (c) K. W.
Quasdorf, M. Riener, K. V. Petrova and N. K. Garg, J. Am.
Chem. Soc., 2009, 131, 17748; (d) C. Zarate, R. Manzano and
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D.-G. Yu, S.-F. Zheng, Z.-H. Wu and Z.-J. Shi, Chem. Eur. J.,
2011, 17, 786; (f) L. Guo, X. Liu, C. Baumann and M. Rueping,
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130, 14468.
12 A. G. Sergeev and J. F. Hartwig, Science, 2011, 332, 439.
13 In the presence of 2 equiv.s t-BuONa and 20 mol%
nickel/carbene catalyst, aryl methyl ethers could be
converted to arenes in toluene at 160 oC without external
reductant, see: M. Tobisu, T. Morioka, A. Ohtsuki and N.
Chatani, Chem. Sci., 2015, 6, 3410.
14 Those groups can be reduced by hydrosilanes and H2
catalyzed by nickel. For examples, see: (a) N. Barbero and R.
Martin, Org. Lett., 2012, 14, 796; (b) A. Dey, S. Sasmal, K.
Seth, G. K. Lahiri and D. Maiti, ACS Catal., 2017, 7, 433; (c) H.
Yue, L. Guo, S.-C. Lee, X. Liu and M. Rueping, Angew. Chem.
Int. Ed., 2017, 56, 3972; (d) T. Patra, S. Agasti, A. Modak and
D. Maiti, Chem. Commun., 2013, 49, 8362; (e) T. Patra, S.
Agasti, Akanksha and D. Maiti, Chem. Commun., 2013, 49, 69.
15 Compared with naphthalene and higher π-extended
aromatic substrates, anisole derivatives usually showed
lower reactivity. This issue has been detailed described in a
recent Chatani’s account, see Ref. 1h.
16 K. Muto, J. Yamaguchi, A. Lei and K. Itami, J. Am. Chem. Soc.,
2013, 135, 16384.
17 Prof. Martin proposed a Ni(0)/Ni(I) process for C-O reduction
by hydrosilane, see: J. Cornella, E. Gomez-Bengoa and R.
Martin, J. Am. Chem. Soc., 2013, 135, 1997.
5
Selected examples on cross coupling with other
organometals: (a) H. Ogawa, Z.-K. Yang, H. Minami, K.
Kojima, T. Saito, C. Wang and M. Uchiyama, ACS Catal., 2017,
7, 3988; (b) X. Liu. C.-C. Hsiao, I. Kalvet, M. Leiendecker, L.
Guo, F. Schoenebeck and M. Rueping, Angew. Chem. Int. Ed.,
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Am. Chem. Soc., 2017, 139, 1191; (d) C. Zárate and R. Martin,
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6
Selected examples on cross coupling with reactive
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Yamaguchi and K. Itami, Angew. Chem. Int. Ed., 2014, 53,
6791; (c) E. Koch, R. Takise, A. Studer, J. Yamaguchi and K.
18 For some mechanistic studies on C-O activation by using
density functional theory (DFT) calculations, see: (a) H. Xu, K.
Muto, J. Yamaguchi, C. Zhao, K. Itami and D. G. Musaev, J.
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K. N. Houk, J. Am.Chem. Soc., 2014, 136, 2017; (c) Q. Lu, H.
Yu and Y. Fu, J. Am. Chem. Soc., 2014, 136, 8252.
Itami, Chem. Commun., 2015, 51, 855; (d) K. Muto, T.
Hatakeyama, J. Yamaguchi and K. Itami, Chem. Sci., 2015, 6,
4 | J. Name., 2012, 00, 1-3
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