Iron Phosphate Catalyzed Asymmetric Cross-Dehydrogenative Coupling of 2-Naphthols with β-Ketoesters

Article abstract of DOI:10.1021/acs.orglett.7b01152

Chiral iron phosphate complexes were successfully exploited for asymmetric cross-dehydrogenative coupling reactions between 2-naphthols and β-ketoester derivatives. On the basis of kinetic studies, it is suggested that iron monophosphate complexes constitute the active catalysts that induce stereoselectivity during the carbon-carbon bond-formation step.

Full text of DOI:10.1021/acs.orglett.7b01152

Letter
pubs.acs.org/OrgLett
Iron Phosphate Catalyzed Asymmetric Cross-Dehydrogenative
Coupling of 2‑Naphthols with β‑Ketoesters
Sachin Narute and Doron Pappo*
Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
S
* Supporting Information
ABSTRACT: Chiral iron phosphate complexes were success-
fully exploited for asymmetric cross-dehydrogenative coupling
reactions between 2-naphthols and β-ketoester derivatives. On
the basis of kinetic studies, it is suggested that iron
monophosphate complexes constitute the active catalysts
that induce stereoselectivity during the carbon−carbon bond-
formation step.
ron catalyzed cross-dehydrogenative coupling (CDC)
partners (Figure 1C). Feng developed a chiral NiII−redox FeII
Ireactions have emerged as powerful methods for synthesiz- heterobimetallic cooperative catalyst system for coupling
between β-ketoesters and xanthene, which follows strategy
A,6 whereas both the iron[salan] and the iron[phosphate]3
complexes, developed by Katsuki5 and Pappo7 (Scheme 1),
ing novel C−C bonds by coupling two C−H bonds under
oxidation conditions.1 The development of this chemistry is
highly important because it offers a great opportunity to
diminish the dependence of synthetic chemistry on traditional
cross-coupling reactions, which heavily rely on prefunctional-
ization of the coupling partners and precious metals. Since the
seminal work of Li, which introduced iron and copper salts as
CDC catalysts,2 most synthetic works have been focused on
identifying suitable oxidation conditions and coupling partners
as well as solving efficiency and selectivity challenges that the
chemistry poses.1a,d,3 However, much less attention has been
devoted to developing asymmetric CDC transformations.4
Even more frustrating is the lack of reliable asymmetric
methodologies based on chiral redox iron complexes.5
Scheme 1. Asymmetric Oxidative Cross-Coupling of 2-
Naphthols by Chiral Iron Phosphate Complexes
The three main strategies for inducing asymmetry in iron−
CDC between two nucleophiles involves (a) a cooperative
catalytic system consisting of an achiral iron redox catalyst,
which generates electrophilic species that react with a chiral
Lewis or an organocatalyst-activated nucleophile (Figure 1A),
(b) a chiral redox iron catalyst that mediates intermolecular
coupling between an associated electrophile and a nucleophile
(Figure 1B), and (c) a chiral redox metal catalyst that mediates
the stereoselective coupling between two associated coupling
respectively, are examples of chiral redox iron catalysts that
mediate the enantioselective oxidative coupling of 2-naphthols
according to strategy B. In this paper, we demonstrate that iron
phosphate complexes can also catalyze asymmetric CDC
between associated β-ketoester and 2-naphthol ligands,
according to strategy C. On the basis of mechanistic studies,
it is suggested that iron monophosphate complexes are the
active catalysts for this transformation and that they become
active only at elevated temperatures to afford chiral polycyclic
hemiacetals in moderate to high enantioselectivity.
Recently, our group developed iron-catalyzed CDC reactions
between 1,3-dicarbonyl compounds with substituted phenols,
affording complex phenolic architectures that were employed in
natural product synthesis.8 In its racemic version [FeCl3·6H2O
Figure 1. Conceptual strategies for asymmetric iron−CDC trans-
formations.
Received: April 17, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01152
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Organic Letters
Letter
(10 mol %), t-BuOO-t-Bu (1.5 equiv), 1,2-dichloroethane
(DCE), 80 °C], the coupling of 2-naphthol 1a with α-
substituted-β-ketoester 2a, which has a chiral auxiliary (R =
(−)-menthyl), afforded a mixture of diastereoisomers 3a (7aR,
10aS) and 3a′ (7aS,10aR) in ca. 46:54 ratio (Table 1, entry
and a cross-coupling product 3a was obtained in low yield and
with poor diastereoselectivity (43:57, entry 3). Our optimiza-
tion study, which included extensive ligand screening (see
Table S1), showed a correlation between the size of the 3,3′-
substituents of the phosphoric acid and the product’s
diastereoselectivity [(4-t-Bu)C6H4 > TMS > Me > H, Table
1, entries 3−6], with the least sterically hindered ligand L4 (R1
= H, BNPH) providing the highest diastereoselectivity (88:12)
and efficiency (89% yield, entry 6). On the other hand, the
coupling with ent-L4 as a ligand afforded a much lower dr
(20:80), suggesting that a mismatched relationship exists
between the ligand and menthyl β-ketoester 2a.
Table 1. Ligand Screening for Stereoselective CDC of β-
a
Ketoesters 2 with 2-Naphthol 1a
Our attempt to enhance the reaction outcome by changing
the electronic properties of the ligand by modifying its 6- and
6′-positions with alkyl and aryl groups (Table 1, entries 8−11
Table S1) was less successful. The enantioselective coupling of
β-ketoesters with smaller alkyl groups, such as 2b (R = Me) and
2c (R = Et), is also possible; however, it requires the use of
phosphate ligands with bulky 3- and 3′-substituents, such as L5
[R1 = 4-(2-naphthyl)C6H4, Table 1, entries 12 and 13]. At least
six steps are needed, starting from (R)-BINOL, to prepare
ligands with a complexity similar to ligand L5, transforming the
structure−selectivity relationship study for the CDC of alkyl β-
ketoesters 2b (R = Me) or 2c (R = Et) to unfavorable in terms
of time and cost. The cross-coupling of other alkyl β-ketoester
derivatives, such as propyl, isopropyl, isopentyl, and cyclohexyl,
with 2-naphthol (1a) using Fe[L3]3 (R1 = Me) as a catalyst
afforded the corresponding coupling products 3d−g with
moderate enantioselectivity (entries 14−17). Therefore, we
decided to demonstrate the potential that iron phosphate
complexes have as catalysts in asymmetric CDC reactions by
studying the oxidative coupling between 2-naphthol derivatives
and β-ketoester 2a.
b
c
entry
ligand
product temp (°C), time (h) yield (%) dr (er)
d
1
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3b
3c
3d
3e
3f
80, 3
80
e
46:54
2
L1
L1
L2
L3
L4
ent-L4
L6
L7
L8
L9
L5
L5
L3
L3
L3
L3
L4
rt, 18
3
50, 18
50, 14
50, 18
50, 16
50, 48
50, 90
50, 18
70, 72
50, 17
50, 14
50, 18
50, 16
50, 14
50, 18
50, 22
50, 72
32
82
91
89
80
70
73
78
85
52
58
73
67
87
73
31
43:57
64:36
83:17
88:12
20:80
75:25
80:20
75:25
80:20
32:68
34:66
37:63
24:76
27:73
23:77
88:12
4
5
6
7
8
9
10
11
12
13
14
15
16
17
3g
3a
f
Further attempts to improve the stereoselectivity by
changing the reaction parameters, such as the solvent, iron
source, and the oxidant, were ineffective (see Table S2). The
use of dioxygen instead of peroxide as a terminal oxidant is
possible, and the desired coupling product, 3a, was obtained
with a comparable diastereoselectivity (dr = 88:12) yet with a
considerably lower reactivity (31% yield, entry 18).
18
a
General reaction conditions: (i) Fe(ClO4)3·H2O (5 mol %), L (15
mol %), CaCO3 (15 mol %) in PhCF3/HFIP (1:1, 1.0 mL) at 50 °C, 3
h; then (ii) 2a (0.1 mmol), 1a (0.15 mmol), t-BuOOt-Bu (0.15
mmol), from rt to 50 °C, 12−28 h. Isolated yields after column
chromatography. dr and er were determined by HPLC (see the SI).
b
c
d
The reaction was carried out using FeCl3·6H2O in DCE as a
solvent.8f BINOL was obtained as the sole product; The reaction was
e
f
Mechanistically, we previously proposed that the enantiose-
lective oxidative homocoupling of 2-naphthol 1a at room
temperature by iron phosphate complexes (Scheme 2)
commences when the peroxide is associated with iron
triphosphate complex I.7 Next, peroxide bond cleavage takes
place and generates a high-valent iron complex. This complex
benefits from exchanging one of the phosphate ligands with a 2-
naphtholate ligand (complex II) that can push the electron
density (SET process) that stabilizes the metal. As a result, a
persistent bounded naphthoxyl radical (III) is being formed.
An oxidative radical−anion coupling with a second nucleophilic
2-naphthol(ate) via a radical−anion coupling mechanism is the
slowest step in this process, which affords (R)-BINOL.7
The above mechanistic scheme changes at 50 °C in the
presence of an anionic−bidentate β-ketoester ligand. Our
nonlinear effect experiments13 (Figure 3A, for details, see the
SI) revealed a linear relationship between the diastereomeric
excesses of the hemiacetal product, 3a, and the enantiomeric
excess of the ligand. This experiment implies that only a single
chiral phosphate ligand participates in the asymmetry-
determining step. Therefore, it is suggested that the
phosphate-to-β-ketoester ligand exchange process seems to be
the chemoselectivity-determining step in the two competitive
carried out using dioxygen atmosphere (1 atm) as the oxidant.
1).8f To develop an asymmetric version of this transformation,
we examined a chiral anion strategy9 based on phosphoric
acids10 derived from BINOLs.11,12 First, conditions used for the
enantioselective oxidative coupling of 2-naphthols catalyzed by
the chiral iron phosphate catalyst Fe[L1]3 [(5 mol %, Figure 2),
Figure 2. Phosphoric acid ligands (L).
t-BuOOt-Bu (1 equiv), PhCF3 (α,α,α-trifluorotoluene)/HFIP
(1,1,1,3,3,3-hexafluoropropan-2-ol, 1:1), room temperature]
were explored.7 Under these conditions, only a homocoupling
process took place, affording enantioenriched (R)-BINOL 4 as
the sole product (entry 2).7 However, when the reaction
temperature was elevated above 50 °C, the selectivity changed
B
DOI: 10.1021/acs.orglett.7b01152
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Organic Letters
Letter
between the two associated ligands (complex VI and then
VII). The catalytic cycle is terminated by the release of the
hemiacetal product, 3, via another ligand exchange process. The
ability of Fe[phosphate]3 complexes to convert into two active
catalysts (II and IV) through a ligand-exchange process that
adjusts the number of anionic phosphate ligands according to
the requirements of the asymmetric transformation is an
important finding for future developments of asymmetric CDC
transformations.
Scheme 2. Proposed Mechanisms for the Oxidative
Homocoupling of 2-Naphthols7 and the CDC of 2-
Naphthols with β-Ketoesters by Iron Phosphates Complexes
The scope of the catalytic system was demonstrated by
coupling various 2-naphthols, 1b−h, with β-ketoester 2a (Table
2). The reaction was found to be efficient for 2-naphthol
Table 2. Asymmetric CDC of 2-Naphthols with 1 β-
a
Ketoester 2a under Optimized Reaction Conditions
b
c
entry
R
product
yield (%)
dr
d
1
H
3a
3h
3i
72
71
79
57
77
63
73
77
89:11
88:12
88:12
82:18
87:13
85:15
89:11
90:10
2
3
4
5
6
7
6-nBu
6-iPr
6-Br
3-Br
3j
3k
3l
6-CO2Me
6-(4-t-Bu)C6H4
6-(3,5-bisCF3)C6H3
3m
3n
d
8
a
General reaction conditions: (i) Fe(ClO4)3.H2O (5 mol %), L4 (15
mol %), CaCO3 (15 mol %) in PhCF3/HFIP (1:1, 1.0 mL) at 50 °C, 3
h; then (ii) 2a (0.1 mmol), 1 (0.15 mmol), t-BuOOt-Bu (0.15 mmol),
b
rt to 50 °C, 14−96 h. Isolated yields of cross-coupling product after
c
column chromatography. dr was determined by HPLC (see the SI).
d
The reaction was carried out on a 3 mmol scale.
derivatives that are substituted at the 6- and 3-positions,
affording the corresponding coupling products 3h−n with good
yields and high diastereoselectivity, ranging from 82:18 to
90:10 (entries 2−8). These results support the premise that
steric and electronic changes at the 2-naphtholic component
have only a minor effect over the stereoselectivity. On the other
hand, as discussed previously, the alkyl ester group in the β-
ketoester has a higher impact over the face selection during the
C−C bond-forming step.
Figure 3. (A) Nonlinear relationship study between the ee of the
ligand (L4) and the de of the product 3a. (B) Enantioselective
homocoupling of 2-naphthol 1a catalyzed by Fe[L1]3 in the presence
or absence of β-ketoester 2b.
To conclude, iron phosphate catalyzed asymmetric CDC
between 2-naphthols 1 and β-ketoester derivatives 2 that
affords polycyclic hemiacetals 3 was developed. On the basis of
our mechanistic investigations, it is postulated that iron bearing
a single phosphate ligand acts as the active redox catalyst in the
stereoselectivity-determining step. Furthermore, our studies
support the premise that the coupling takes place between two
associated ligands via a radical−anion coupling mechanism. We
intend to further explore the reactivity of the iron phosphate
complexes in order to apply them as chiral redox catalysts in
asymmetric CDC transformations.
asymmetric transformations [homocoupling (via complex III,
Scheme 2) vs cross-coupling (via complex IV)]. In order to
probe this key step, the degrees of efficiency and
enantioselectivity of the Fe[L1]3-catalyzed homocoupling of
2-naphthol (1a)7 at room temperature in the presence (circles,
Figure 3) and absence (squares) of a competitive ligand (β-
ketoester 2b) were monitored (for details, see the SI). The
results clearly shows that the addition of β-ketoester 2b to the
reaction at room temperature leads to a significant reduction in
the rate of the homocoupling process (solid blue lines),
whereas the enantiomeric excess of BINOL 4 is only slightly
affected (dashed black lines). It is postulated that complexes II
and IV (Scheme 2) are found in equilibrium at room
temperature and the complex IV is an inferior catalyst
compared with complex II in catalyzing the oxidative
homocoupling of 2-naphthol. Furthermore, complex IV,
which has a single phosphate and a β-ketoester ligand,
undergoes a SET process to form complex V. This complex
becomes reactive only at elevated temperatures (above 50 °C),
mediating intramolecular oxidative radical-anion coupling
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b01152.
Experimental procedures and spectroscopic data of all
new compounds (PDF)
C
DOI: 10.1021/acs.orglett.7b01152
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
4198. (e) Kshirsagar, U. A.; Parnes, R.; Goldshtein, H.; Ofir, R.;
Zarivach, R.; Pappo, D. Chem. - Eur. J. 2013, 19, 13575. (f) Parnes, R.;
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: pappod@bgu.ac.il.
ORCID
Doron Pappo: 0000-0002-8363-8709
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We acknowledge the financial support provided by the Israel
Science Foundation (Grant No. 164/16).
■
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