Copper-catalyzed divergent oxidative pathways of 2-naphthol derivatives:: Ortho -naphthoquinones versus 2-BINOLs

Article abstract of DOI:10.1039/c6ob01183g

Catalyst-dependent divergent pathways of 2-naphthol derivatives have been investigated. A readily available CuCl₂-DMAP catalyst system promotes the aerobic oxidation of 2-naphthol derivatives to ortho-naphthoquinones whereas switching the catalyst system to Cu(OAc)₂-DBN under an argon atmosphere allows the oxidative coupling of 2-naphthols to 1,1′-bi-2-naphthols (BINOLs) in good to excellent yields.

Full text of DOI:10.1039/c6ob01183g

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DOI: 10.1039/C6OB01183G
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Copper-Catalyzed Divergent Oxidative Pathways of 2-Naphthol
Derivatives: ortho-Naphthoquinones versus 2-BINOLs
H. Y. Kim,a* S. Takizawa,b and K. Oha*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Catalyst-dependant divergent pathways of 2-naphthol derivatives in the presence of 4 mol% [Cu(CH3CN)4]PF6-DBED catalyst
have been investigated. A readily available CuCl2-DMAP catalyst system.7 Thus, there exists a significant gap in our knowledge
system promotes the aerobic oxidation of 2-naphthol derivatives of reaction pathways of 2-naphthol derivatives under aerobic
to ortho-naphthoquinones whereas switching the catalyst system copper catalyst conditions
to Cu(OAc)2-DBN in an argon atmosphere allows the oxidative
coupling of 2-naphthols to 1,1’-bi-2-naphthols (BINOLs) in good to
excellent yields.
Tyrosinases are known to selectively catalyze the oxygenation
of phenol derivatives to ortho-quinones with the help of two
copper atoms through the activation of molecular oxygen.1
Efforts to develop biomimetic aerobic oxidation of phenolic
compounds have revealed that the nature of the metal ligand
significantly influence the activation modes of dioxygen.2
Accordingly, the aerobic oxidation of phenols can be
manipulated to produce either ortho-quinones via oxygenation
or coupling products via oxidative coupling under a diverse
array of synthetic ligand systems.3 In contrast to the successful
development of aerobic oxidation protocols of phenols, 2-
naphthol derivatives under similar copper-catalyzed aerobic
Scheme 1. Catalytic Aerobic Oxidation of 2-Naphthols.
oxidation conditions predominantly lead to the formation of
1,1’-bi-2-naphthols (BINOLs) via an oxidative coupling pathway
(Scheme 1).4 While the first biomimetic copper-catalyzed
The fact that the reaction of 2-naphthol derivatives could lead
to both BINOLs and ortho-naphthoquinones under the copper-
catalyzed aerobic oxidation conditions prompted us to
investigate the electronic effect of copper ligands as well as 2-
naphthol substrates. It is widely known that the change of
arene oxidation potentials could be effected using
substituents, as exemplified in the successful industrial
production of H2O2 using 2-ethylanthraquinone rather than the
parent anthraquinone.8 Also, the biaryl C-C bond formations
are known to be heavily influenced by the electronic interplay
between substrates and catalyst ligand systems.9 Thus, we
anticipated that the reaction pathways of electronically
manipulated 2-naphthol derivatives could be selectively
controlled by fine-tuning of copper-ligand systems. Herein, we
describe the successful implementation of a switch in the
reaction pathway of 2-naphthols on demand to provide either
ortho-naphthoquinones or BINOLs in the presence of
aerobic oxidation of 2-naphthol to ortho-naphthoquinone was
accomplished by Brackman and Havinga as early as in 1955,5
defying the inherent aerobic oxidative coupling pathway of 2-
naphthols to BINOLs under a catalytic amount of copper
catalysts remains a challenging area of research. Consequently,
the aerobic oxidations of 2-naphthol derivatives to ortho-
naphthoquinones typically employ more than stoichiometric
amounts of copper catalysts.6 Recently, the Lumb group
disclosed a copper-catalyzed aerobic oxidation of 2-naphthol
to 4-(naphthalen-2-yloxy)naphthalene-1,2-dione in 71% yield
a. Center for Metareceptome Research, College of Pharmacy, Chung-Ang University,
84 Heukseok-ro, Dongjak, Seoul 156-756, Republic of Korea.
b. The Institute of Scientific and Industrial Research, Osaka University, Mihogaoka,
Ibaraki, Osaka 567-0047, Japan.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Table 1. Optimization of ortho-Naphthoquinoone Formation from 4-Pentyl-2-Naphthol.
copper(II) catalyst systems with readily available amine
ligands.
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DOI: 10.1039/C6OB01183G
Ortho-naphthoquinones, versatile synthetic building blocks in
organic chemistry,10 are typically prepared using the
stoichiometirc amounts of oxidants such as m-CPBA,11
(PhSe(=O))2O,12 Fremy’s radical13 and hypervalent iodine
reagents.14 While the metal-catalyzed oxidation protocols have
been developed in the presence of stoichiometric amounts of
peroxides,15 apart from the above copper catalyst systems
there is a single example of Co(II)-catalyzed aerobic oxidation
of 2-naphthols to ortho-naphthoquinones.16 Puzzled by the
difficulty associated with the ortho-naphthoquinone formation
from the copper-catalyzed aerobic oxidation of 2-naphthols,
we elected 4-substituted-2-naphthol derivatives17 to explore
the mechanistic pathways to BINOLs and ortho-
naphthoquinone under copper catalysis. First, we examined
the use of copper salts in the coordinating solvents (Table 1).
Upon employing a catalytic amount of CuCl2 (30 mol%), the
formation of a trace amount of ortho-naphthoquinone 2a was
observed in EtOH (entry 1). Next, we screened various amine
ligands since the copper(II)-amine complexes had shown broad
versatility in the oxidation4,18 as well as oxidative coupling of
phenolic compounds.5,19 The addition of morpholine
significantly improved the reaction conversion, but
Entry Metal (mol%) Ligand
(mol%)
Solvent Yield
(%)[a]
1
2
CuCl2 (30)
CuCl2 (30)
-
EtOH
EtOH
<5[b]
morpholine
(30)
38[c]
3
4
5
6
7
8
9
10
11
12
13
CuCl2 (30)
CuCl2 (30)
CuCl2 (30)
CuCl2 (30)
CuCl2 (30)
CuCl2 (30)
CuCl2 (30)
CuCl (30)
CuOTf (30)
CuO (30)
Et3N (30)
DABCO (30) EtOH
pyridine (30) EtOH
EtOH
45[c]
NR
23[b]
71
65
NR
DMAP (30)
DMAP (30)
DMAP (30)
DMAP (30)
DMAP (30)
DMAP (30)
DMAP (30)
DMAP (30)
EtOH
MeOH
THF
CH2Cl2 28[b,d]
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
NR
NR
NR
46[c]
24[b,d]
0[b,d]
61
CuBr2 (30)
accompanied with the formation of several by-products 14
containing a BINOL (entry 2). The use of other tertiary amine
Cu(OTf)2 (30) DMAP (30)
Cu(OAc)2 (20) DMAP (30)
15
16
17
CuCl2 (20)
CuCl2 (10)
CuCl2 (10)
CuCl2 (8)
DMAP (10)
DMAP (20)
DMAP (10)
DMAP (8)
DMAP (5)
DMAP (8)
bases suggested that the function of the copper(II)-amine
complexes could be altered (entries 3-4). To our delight, the
use of pyridine led to a clean formation of 2a, albeit in a low
yield of 23% (entry 5). The reaction rate was significantly
enhanced with DMAP, leading to an isolated yield of 2a in 71%
(entry 6). The subsequent solvent study showed that the use
of protic solvents was needed to generate active catalyst
species as well as to retain the sufficient catalytic turnover
activity (entries 7-9). The nature of copper salts was
investigated next (entries 10-15). While the use of CuCl, CuOTf,
and CuO resulted in the completely inactive catalyst species
(entry 10-12), the employment of CuBr2 and Cu(OTf)2 restored
some aerobic oxidation activity for 1a, leading to 24-46% of 2a
(entry 13-14). Interestingly, the use of Cu(OAc)2 changed the
course of reaction to the oxidative coupling, providing a BINOL
as a sole product in 40% yield (entry 15). The ratio of CuCl2 and
DMAP was critical for the aerobic oxidation of 1a; use of 2:1
ratio of copper and amine maintained the yield of 2a (entry
16), but the use of 1:2 ratio mainly led to the formation of
intractable mixtures (entry 17). The catalyst loading could be
lowered down to 10 mol%, providing a comparable isolated
yield of 2a in 68% in 48 h (entry 18).20 However, the reaction
using a balloon of O2 significantly improved the reaction time,
providing 80 % of 2a in 16 h in the presence of 8 mol% of
catalysts (entry 19). A longer reaction time of 36 h was
necessary upon further lowering down the catalyst loading to
5 mol%, providing a slightly reduced isolated yield of 73%
(entry 20). Further a control experiment of the reaction also
confirmed that the oxidation did not proceed in the absence of
a source of dioxygen (entry 21).
<10[c]
68
18
19[e]
80
73
NR
20[e,f] CuCl2 (5)
21[g]
CuCl2 (8)
[a]
[b]
Isolated yields of products after column chromatography.
Incomplete
[c]
[d]
reaction. Several products including a BINOL.
(40%). Reaction using O2 balloon for 16 h. Reaction for 36 h. Reaction
under argon. NR = No Reaction.
BINOL as a major product
[e]
[f]
[g]
The optimized oxidation conditions using an O2 balloon were
further scrutinized using an array of substituted 2-naphthol
derivatives (Scheme 2). The newly developed aerobic oxidation
protocol proved to be general; electronically and sterically
diverse 2-naphthtols smoothly underwent the desired aerobic
oxidation to provide ortho-naphthoquinones
general, 2-naphthols with an electron-donating group at the 4-
position provided the ortho-naphthoquinones 2a-2d in
slightly higher yields as opposed to a phenyl group at the 4-
position (2e-2h). The reaction was also applicable to electron-
deficient 2-naphthol (2m), functionalized 2-naphthols (2o-2q
and 2-naphthol (2r), clearly demonstrating the generality of
the current aerobic oxidation protocol. The use of 1-naphthol
resulted in the formation of para-benzoquinone (2s).
(2a-2q). In
(
)
)
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pronounced solvent effect was revealed inViecwhAlroticrlienOantleinde
DOI: 10.1039/C6OB01183G
solvents (entries 13-15).
Table 2. Optimization of BINOL Formation froom 4-Pentyl-2-Naphthol.
Entry Metal (mol%)
Ligand
(mol%)
Solvent
Yield
(%)[a]
1[b]
2[b]
3
4
5
6
7
8
9
10
11
12
13
14
15
CuCl2 (10)
CuCl2 (10)
CuCl2 (10)
DBU (10)
DBU (10)
DBU (10)
DBU (10)
DBU (10)
DBU (10)
-
DBN (10)
DBN (20)
DBN (10)
DBN (30)
DBN (30)
DBN (20)
DBN (20)
DBN (20)
EtOH
10[c]
25[c]
61[d]
68[d]
55[d]
57[d]
NR
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Cu(OAc)2 (10)
Cu(OTf)2 (10)
Cu(NTf2)2 (10)
Cu(OAc)2 (10)
Cu(OAc)2 (10)
Cu(OAc)2 (10)
Cu(OAc)2 (20)
Cu(OAc)2 (30)
CuOAc (30)
71[d]
76[d,e]
50[d]
78[d]
NR
Scheme 2. Scope of ortho-Naphthoquinone Formation from 2-Naphthols ([a]
[b]
(CH2Cl)2 51[d]
CHCl3
CCl4
Reaction with 2-Naphthol using 30 mol% of CuCl2-DMAP,
Naphthol).
Reaction with 1-
Cu(OAc)2 (10)
Cu(OAc)2 (10)
Cu(OAc)2 (10)
75[d]
50
[a]
[b]
With the objective of finding alternative copper-amine
Isolated yields of products after column chromatography.
Reaction under
[c]
[d]
complexes to induce the oxidative coupling of 2-naphthols, we air. Intractable product formation (no ortho-naphthoquinone). Incomplete
reactions. [e] Yield of 81% in 60 h. NR = Noo Reaction.
investigated the role of other amine bases (Table 2). While the
use of DBU resulted in a quite complex mixture of products, it
The scope and limitation of the oxidative coupling of 2-
was possible to detect a small amount of oxidative coupling
naphthols was further explored under the optimized argon
product 3a in a 10% yield (entry 1). The change of solvent to
atmosphere (Scheme 3). While a variety of 2-naphthols could
CH2Cl2 led to a slightly improved yield of 3a (entry 2), however
be converted into the corresponding BINOLs, a general trend
the most striking mode of the reaction was observed when the
emerged; 2-naphthols with electron-donating groups were
reaction was performed under high diluted oxygen conditions,
better substrates for the current oxidative coupling reaction.
leading to the exclusive formation of 3a in 61% (entry 3). While
Thus, 4-alkyl-2-napthols provided the corresponding BINOLs in
the reaction did not proceed to completion even under the
71-76% (3a
-
3e), however 3,4-diphenyl-2-napthol required
Likewise,
prolonged reaction time, the reaction did not produce any
other byproducts including ortho-naphthoquinone 2a. Other
copper(II) salts could be also used for the oxidative coupling
under argon atmosphere in comparable yields and efficiency
(entries 4-6). After confirming the critical role of DBU (entry 7),
we further optimized the reaction using DBN to 71% of 3a
(entry 8). Next, we investigated the optimal ratio of metal and
base (entries 9-10), and found that the use of a slight excess of
DBN was beneficial to the overall reaction conversion (entry
10). The use of higher catalyst loading only resulted in an
incremental improvement in yield to 78% (entry 11). Finally,
we also tested the use of CuOAc in the oxidative coupling of 1a
without much success (entry 12). While no solvent effect was
observed in common organic solvents (no significant product
formations <10% in DMF, DMSO, C6H6, PhCH3, THF, MeOH), a
more catalyst loading (30 mol%) to give 46% of 3f 21
.
either an aryl substituent or an electron-deficient substituent
on 2-naphthols significantly slowed down the reaction rate,
resulting BINOLs in 46-67% yields (3g
naphthols were also tolerated under the oxidative coupling
conditions to give 54-70% yields of products (3i 3k).
-3h). Functionalized 2-
-
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ortho-naphthoquinone 2a (Table 2, entry 2), it is likely that a
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high molecular oxygen concentrationDOmI: 1a0y.103p9r/oCm6OoBte0118th3Ge
formation of other copper catalyst species that mediate
unknown reaction pathways.25 The oxidation potential
between the catalyst and 2-naphthols seems to play an
important role as the reaction worked well for 2-naphthols
with a more electron-donating group (3a vs 3f in Scheme 3),
thus the reaction of unsubstituted 2-naphthol did not proceed
at all.26
Scheme 3. Scope of BINOL Formation from 2-Naphthols ([a] 30 mol% of Cu(OAc)2-
DBN, [b] 30 mol% of CuCl2-DBN).
The divergent reaction pathways of 2-naphthols were further
scrutinized for the detailed reaction mechanisms. Our studies
revealed that the aerobic oxidation reaction of 2-naphthols
was initially catalyzed by the CuCl2-DMAP catalyst system to
give catechols
4 (Scheme 4a). The resulting catechols 4
spontaneously oxidized upon exposure to air as confirmed in
our control experiments.22 The current oxidation protocol
possesses useful features; 1) no benzylic oxidation of ortho-
naphthoquinones occurs, 2) the use of higher catalyst loading
(i.e. 30 mol%) allows a shorter reaction time to provide
products in quantitative yields, thus minimizing the exposure
of the product under oxidation conditions. The oxidative
coupling of 2-naphthols under the Cu(OAc)2-DBN catalyst
system did not go to completion, resulting in the recovered
starting material, 2-naphthols (20-30%). Our attempts to
improve the reaction conversion using more catalyst loadings
beyond 30 mol% even lowered the reaction conversion to 30-
50% as opposed to 80% using 10 mol% of catalyst (Scheme
4b). It is possible that the active catalysts are inactivated by
the resulting BINOLs.23 In the current coupling reaction
conditions, the solvent, CH2Cl2, acts as one-electron and one-
proton acceptors from copper species and 2-naphthols, just
like oxidant, O2.24 DBN in our optimized conditions should also
act as one proton acceptor. Since the presence of O2 in our
reaction conditions causes the formation of many unidentified
products with only 25% of BINOL 3a without the formation of
Scheme 4. Plausible Mechanistic Pathways of 2-Naphthol Derivatives under
Cu(II)-amine Catalysis.
Conclusions
In summary, we have developed copper(II)-amine catalyst
systems that control the divergent reaction pathways of 2-
naphthol derivatives. Using readily available amine ligands,
either the catalytic aerobic
α-oxidation of 2-naphtols to the
corresponding catechols or the oxidative coupling of 2-
naphthols to BINOLs was selectively achieved. While more
mechanistic studies await, the copper-catalyzed reactions of 2-
naphthols with lower oxidation potentials than 2-naphthol
itself should provide important mechanistic information to
decouple the reaction pathways of different catalyst species.
Additional studies to understand the mechanism of copper
catalysis are currently underway, and our results will be
reported in due course.
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G. Goor, J. Glenneberg, S. Jacobi, Hydrogen Peroxide. In
Acknowledgements
Ullmann’s Encyclopedia of Industrial Chemistry. VWiewileAyrt-icVleCOHn:line
DOI: 10.1039/C6OB01183G
Weinheim, Germany, 2007
.
This work was supported by the National Research Foundation
of Korea (NRF) grants funded by the Korea government (MSIP)
(NRF-2015R1A5A1008958 and NRF-2015R1C1A2A01053504).
We thank Osaka University for an International Collaboration
Grant to ST and KO.
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19 Y. Takizawa, T. Munakata, Y. Iwasa, T. Suzuki, T. Mitsuhashi,
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20 ortho-Naphthoquinones are not generally stable under
prolonged oxidation conditions. The subjection of 2a under
our optimized oxidation conditions for 16 h resulted in the
recovery of 89% 2a that contained 2% of benzylic oxidation
products.
21 The use of 10 mol% of Cu(OAc)2-DBN resulted in the
formation of 3f in 31% in 68 h. A recent 4,4-biaryl-BINOL
synthesis by Okuma utilized 200 mol% of CuCl2 and 80 mol%
of phenylethylamine to give products in 60-80% yields; see K.
Jeganmohan, Org. Lett. 2015, 17, 3042-3045; f) E. Gaster, Y.
Vainer, A. Regev, S. Narute, K. Sudheendran, A. Werbeloff, H.
Shalit, D. Pappo. Angew. Chem. Int. Ed. 2015, 54, 4198-4202;
g) K. Morimoto, K. Sakamoto, T. Ohshika, T. Dohi, Y. Kita,
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M. C. Kozlowski, Chem. Rev. 2013, 113, 6234-6458.
4
For recent reviews, see: a) J. M. Brunel, Chem. Rev. 2007,
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,
107, PR1-PR45; b) M. C. Kozlowski, B. J. Morgan, E. C. Linton,
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22 For an observation of the aerial oxidation of naphthalene-
1,2-diol, see: a) A. Takuwa, O. Soga, J. Chem. Soc. Perkin
5
6
W. Brackman, E. Havinga, Recl. Trav. Chim. Pays-Bas 1955
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23 Control experiments using 20 mol% BINOL 3b additive only
led to the formation of 3a in 12%, confirming the product
inhibition by BINOLs through the sequestration of copper
catalysts.
24 Halomethanes are excellent electron acceptors, see: a) H. M.
Sung-Suh, L. Kevan, J. Phys. Chem. A 1997, 101, 1414-1418;
7
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Journal Name
b) A. Kotsinaris, G. Kyriacou, C. H. Lambrou, J. Appl.
Electrochem. 1998, 28, 613-616; c) J. Mohanty, H. Pal, S. K.
Nayak, S. Chattopadhyay, A. V. Sapre, Chem. Phys. Lett.
2003, 370, 641-646. For use of CH2Cl2 as an oxidant in biaryl
couplings, see; d) T. Takeya, H. Doi, T. Ogata, T. Otsuka, I.
Okamoto, E. Kotani, Tetrahedron 2004, 60, 6295-6310.
25 Reactions under strict anaerobic conditions (using vacuum
degassing) resulted in significantly lower reaction
View Article Online
DOI: 10.1039/C6OB01183G
conversions to 24%, however the use of wet CH2Cl2 did not
influence the outcome of the BINOL formation, leading to
75% yield of 3a
.
26 For different oxidation potentials of cuprous-cupric
complexes with nitrogen ligands, see; B. R. James, R. J. P.
Williams, J. Chem. Soc. 1961, 2007-2019.
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