Enantioselective oxidative coupling of 2-naphthol derivatives by copper-(R)-1,1′-binaphthyl-2,2′-diamine-TEMPO catalyst

Article abstract of DOI:10.1002/adsc.201300513

An efficient chiral copper catalytic system [(R)-(+)-1,1′-binaphthyl- 2,2′-diamine-copper(I) chloride-TEMPO] for the asymmetric oxidative coupling of 2-naphthol derivatives to synthesize enantiomerically enriched BINOL derivatives has been developed with good to excellent enantiomeric excess (up to 97% ee). The addition of a catalytic quantity of 2,2,6,6- tetramethylpiperidin-1-yl oxyl (TEMPO) to the copper-(R)-(+)-1,1′- binaphthyl-2,2′-diamine complex greatly enhanced the reactivity and enantioselectivity of the enantioselective oxidative coupling of 2-naphthol derivatives and also reduced the reaction temperature from 90 °C to room temperature in dichloromethane solvent.

Full text of DOI:10.1002/adsc.201300513

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DOI: 10.1002/adsc.201300513
Enantioselective Oxidative Coupling of 2-Naphthol Derivatives
by Copper-(R)-1,1’-Binaphthyl-2,2’-diamine-TEMPO Catalyst
Santosh Kumar Alamsetti,a Elamvazhuthi Poonguzhali,a Dhandapani Ganapathy,a
and Govindasamy Sekara,*
a
Department of Chemistry, Indian Institute of Technology Madras, Chennai, Tamil Nadu – 600036, India
Fax : (+91)-44-2257-4202; e-mail: gsekar@iitm.ac.in
Received: June 11, 2013; Published online: && &&, 0000
Dedicated to Professor M. Periasamy on the occasion of his 60th birthday.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300513.
Abstract: An efficient chiral copper catalytic
system [(R)-(+)-1,1’-binaphthyl-2,2’-diamine-cop-
was developed by Nakajima using l-proline-derived
diamine-Cu(I) complex.[4] Subsequent improvements
were made by Kozlowski[5] using Cu(I)-1,5-diazadeca-
line or (À)-sparteine complex. Later on, Kim used
Cu(I)-BINAM complex as a catalyst for the oxidative
coupling of 2-naphthols.[6] Independent reports by
Uang, Chen and Gong using a chiral oxovanadium
complex[7] of Schiff bases represented significant
progress in the oxidative coupling of 2-naphthols. In
addition, the chiral ruthenium,[8] iron complexes[9] and
N-heterocyclic carbene copper complexes[10] were re-
ported by other research groups and have added
strength to the field of asymmetric oxidative coupling.
Recently, we have synthesized an enantiopure
model of galactose oxidase (GO) enzyme and this has
been efficiently utilized for the aerobic oxidative ki-
netic resolution (AOKR) of racemic benzoins to syn-
thesize highly enantiomerically enriched benzoins in
the presence of molecular oxygen and 2,2,6,6-tetrame-
thylpiperidin-1-yl oxyl (TEMPO).[11] To demonstrate
the further utility of this enantiopure GO model, the
(R)-BINAM-Cu complex has been examined as a cata-
per(I) chloride-TEMPO] for the asymmetric oxida-
tive coupling of 2-naphthol derivatives to synthesize
enantiomerically enriched BINOL derivatives has
been developed with good to excellent enantiomer-
ic excess (up to 97% ee). The addition of a catalytic
quantity of 2,2,6,6-tetramethylpiperidin-1-yl oxyl
(TEMPO) to the copper-(R)-(+)-1,1’-binaphthyl-
2,2’-diamine complex greatly enhanced the reactivi-
ty and enantioselectivity of the enantioselective oxi-
dative coupling of 2-naphthol derivatives and also
reduced the reaction temperature from 908C to
room temperature in dichloromethane solvent.
Keywords: asymmetric synthesis; BINOL; chiral
copper catalyst; molecular oxygen; oxidative cou-
pling
Axially chiral biaryls constitute a large class of biolog- lyst for the asymmetric oxidative coupling to synthe-
ically important natural products, structurally diver- size enantiopure BINOL and herein we report our
gent chiral auxiliaries and ligands (Figure 1).[1] Partic- preliminary results.
ularly, axially chiral 1,1’-binaphthol (BINOL), which
is stable at high temperature due to the high rotation- oxidative coupling of methyl 3-hydroxy-2-naphthoate
al barrier, has proved itself and its derivatives to be 10 using 5 mol% of Cu(OTf)2 and 5 mol% of (R)-
We started our investigation with enantioselective
AHCTUNGTRENNUNG
excellent chiral ligands in the asymmetric synthe- BINAM in toluene at 908C in the presence of molec-
sis.[1c,d,2] Generally, enantiopure BINOL is synthesized ular oxygen. The reaction took 7 days to furnish 45%
by enzymatic or chemical resolution from the corre- of dimethyl 2,2’-dihydroxy-1,1’-binaphthyl-3,3’-dicar-
sponding racemic BINOL.[2] Asymmetric oxidative boxylate 11 with just 4% enantiomeric excess
coupling of 2-naphthols is the simplest alternative (Table 1, entry 1). When the Cu
ACHTUNGRTEN(NGNU OTf)2 was replaced
route to synthesize chiral BINOL. by CuCl, the enantioselectivity of the oxidative cou-
The first successful asymmetric oxidative coupling pling drastically increased to 97% ee, however the re-
was reported by Brussee[3] using a super stoichiomet- action took 8 days and afforded only 27% of the
ric quantity of Cu(II)-(S)-amphetamine complex. The product (Table 1, entry 2). Then several copper salts
catalytic version of the asymmetric oxidative coupling were screened with chiral BINAM, but all of them
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Figure 1. Axially chiral biaryl-containing biologically active natural products and chiral ligands.
Table 1. Effect of copper salts on the asymmetric oxidative
To improve the yield of the oxidative coupling reac-
tion, several chiral ligands such as BINOL, (À)-spar-
teine and Schiff base ligands were screened with CuCl
in toluene at 908C in the presence of molecular
oxygen (Figure 2). Although in some cases the yields
are slightly improved, all these ligands gave less selec-
tivity than (R)-BINAM-CuCl complex and the (R)-
BINAM-CuCl complex turned out to be the best
choice for the oxidative coupling of 10.
coupling of 2-naphthol derivative 10.
Subsequently the reaction was studied with differ-
ent solvents to increase the yield of the oxidative cou-
pling reaction (Table 2). Among various solvents
screened, toluene was found to be the best solvent in
view of the enantioselectivity, but the setback of low
yield remained unsolved (Table 2, entry 8). Recently,
we have used Cu-BINAM complex in combination
with TEMPO in the presence of molecular oxygen as
an efficient catalytic system for the enantioselective
oxidation of alcohols. Also, the oxidative coupling is
a radical reaction and TEMPO being a radical species
might promote the oxidative coupling. Based on the
above-mentioned facts, we reasoned that the Cu-
BINAM catalyst in combination with a catalytic quan-
tity of TEMPO could be an efficient catalyst for oxi-
dative coupling of 2-naphthols. As per our expecta-
tions, the addition of a catalytic quantity of TEMPO
[a]
Isolated yields.
The % ee was determined by HPLC using a Daicel chir-
[b]
alPAK AD-H column. The absolute configuration of the
product BINOL derivative 11 was determined as (S)
based on the sign of its specific rotation and comparison
with the literature values.[4] See the Supporting Informa-
tion for further details.
were found to be inferior to CuCl in terms of selectiv- (5 mol%) increased the yield from 27% to 80%, but
ity (Table 1, entries 3–9).[12]
the enantiomeric excess was reduced to 56% (Table 2,
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Enantioselective Oxidative Coupling of 2-Naphthol Derivatives
Figure 2. Effect of ligands on the asymmetric oxidative coupling of 10.
chloromethane was found to be the best choice,
Table 2. Effect of solvents on the oxidative coupling of 10.
giving 94% ee and 95% yield in 3 days at room tem-
perature (Table 2, entry 14).
Next, we examined the effect of the ratio of ligand
L1 and CuCl in the asymmetric oxidative coupling of
10. Among the several combinations of Cu salt and
ligand, a 1:2 ratio of copper chloride and ligand L1
provided better selectivity. with a 1:2 ratio of cata-
lysts, 2.5 mol% of CuCl and 5 mol% of ligand L1
gave a maximum of 97% ee and 90% yield in 48 h at
room temperature.
To prove the scope of the (R)-BINAM-CuCl-
TEMPO catalytic system, several BINOL derivatives
were synthesized using the optimized reaction condi-
tions and the results are summarized in Table 3. Dif-
ferent ester groups were tolerated at the C-3 position
of naphthol 10. Interestingly, short-chain aliphatic al-
cohols such as methanol, ethanol and propanol esters
of 2-naphthol derivatives gave good enantioselectivi-
ty.[13] When the chain length of the alcohol of the
ester was increasing, the selectivity was also gradually
decreasing (Table 3, entries 1–6). Enantiomeric excess
did not alter even after replacing the methyl by allyl
or phenyl group (Table 3, entries 7 and 8). Phenyl
rings containing both the electron-releasing groups
such as methyl group[14] and electron-withdrawing
group such as chloro group were well tolerated in this
oxidative coupling reaction (Table 3, entries 9–11).
[a]
Reaction was carried out in a pressure tube filled with
O2.
Reaction was carried out with 5 mol% TEMPO.
Reaction was carried out with 5 mol% TEMPO at room
[b]
[c]
temperature.
entry 13) at 908C. To solve the problems of longer re- Even substitutions on the 2-naphthol skeleton provid-
action time, high reaction temperature and poor ed moderate to very good enantioselectivity for the
enantioselectivity, the reaction was studied with dif- oxidative coupling reaction (Table 3, entries 12–14).
ferent solvents at room temperature. Surprisingly, di- The enantiomeric excesses of the BINOLs were deter-
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Table 3. Substrate scope for the (R)-BINAM-CuCl-
TEMPO-catalyzed oxidative coupling of 2-naphthol deriva-
tives 10.
Figure 3. The single crystal XRD structure of (R)-BINAM-
CuCl complex (30% probability).[16]
mined by HPLC using Daicel chiralPAK AD-H and
OD-H columns.
The single crystal X-ray diffraction structure of the
(R)-BINAM-CuCl complex is shown in Figure 3.[15]
The X-ray structure analysis shows that the Cu com-
plex possesses a square pyramidal geometry with four
nitrogen atoms [from two (R)-BINAM molecules] co-
ordinated with copper and one chloride ion occupies
the axial position. The complex crystallized in mono-
clinic crystal system with space group C2 and unit cell
dimensions
a=26.325(3),
b=10.7088(10),
c=
14.3108(13), b=100.767(4).; Z=4; cell volume=
3963.4(6) ꢁ3.[16]
In this reaction, the copper(I) complex will be oxi-
dized to copper(II) by molecular oxygen. The result-
ing copper(II) complex will oxidize the b-naphthols to
BINOLs through oxidative coupling and the cop-
AHCTUNGTREGpNNUN er(II) complex will be reduced to the copper(I) com-
plex. The reduced copper(I) complex could be oxi-
dized back to complete the catalytic cycle by molecu-
lar oxygen. We assume that the addition of TEMPO
in this oxidative coupling reaction makes the redox
reaction more facile. The TEMPO will oxidize the re-
duced copper(I) complex to the copper(II) complex
and the TEMPO will be reduced to its hydroxy amine
(TEMPOH). This TEMPOH will be easily oxidized
back to TEMPO by molecular oxygen which is some-
what like the established protocol in copper complex-
TEMPO-O2-catalyzed alcohol oxidation reactions.
However, a detailed study to understand the mecha-
nism, application and scope of this catalyst is under-
way.
In summary, we have developed an efficient chiral
copper catalytic system [(R)-BINAM-CuCl-TEMPO]
[a]
Isolated yield.
[b]
The % ees of all the compounds were determined by
HPLC using Daicel chiralPAK AD-H and OD-H col-
umns.
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Enantioselective Oxidative Coupling of 2-Naphthol Derivatives
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for the asymmetric oxidative coupling of 2-naphthol
derivatives to synthesize enantiomerically enriched
BINOL derivatives with good to excellent enantio-
meric excess (up to 97% ee). Addition of TEMPO to
the (R)-BINAM-CuCl complex-catalyzed oxidative
reaction drastically increases the reaction rate and the
selectivity at room temperature in dichloromethane
solvent.
[4] M. Nakajima, I. Miyoshi, K. Kanayama, S. Hashimoto,
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Experimental Section
Typical Experimental Procedure for Asymmetric
Oxidative Coupling
A mixture of (R)-BINAM (14.2 mg, 0.05 mmol) and cop-
per(I) chloride (2.8 mg, 0.025 mmol) in 4 mL of dichlorome-
thane was stirred at room temperature for 10 min, TEMPO
(7.82 mg, 0.05 mmol) was added to the reaction mixture.
After stirring for 5 min, methyl 3-hydroxy-2-naphthoate
(202 mg, 1 mmol) was added and then the reaction mixture
was stirred under an O2 atmosphere (using an O2 balloon).
After completion of the reaction (as monitored by TLC),
the reaction mixture was concentrated and the resulting resi-
due was directly purified on silica gel column chromatogra-
phy using hexanes-ethyl acetate mixture as eluent to obtain
[6] K. H. Kim, D. W. Lee, Y. S. Lee, D. H. Ko, D. C. Ha,
Tetrahedron 2004, 60, 9037–9042.
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9655–9659.
(S)-dimethyl
late; yield: 181 mg (90%); mp 238–2418C; Rf 0.29 (hex-
ACHTUNGTRENNUNG
anes:ethyl acetate=90:10); 1H NMR (400 MHz, CDCl3):
2,2’-dihydroxy-1,1’-binaphthyl-3,3’-dicarboxy-
d=10.75 (s, 2H), 8.70 (s, 2H), 7.96–7.90 (m, 2H), 7.38–7.32
(m, 4H), 7.20–7.15 (m, 2H), 4.06 (s, 6H); 13C NMR
(100 MHz, CDCl3): d=170.7, 154.1, 137.3, 133.0, 129.9,
129.6, 127.3, 124.8, 124.1, 117.1, 114.3, 52.9; IR (neat): n=
3187, 2954, 1676, 1325, 1210, 1076, 727 cmÀ1; HR-MS: m/z=
403.1173 [M+1]+, calcd. for C24H19O6: 403.1182; [a]D25:
À155.5 (c=1 in CHCl3). [lit.[4] [a]2D5: À125.0 (c=1 in THF)].
The enantiomeric excess (% ee) was determined to be 97%
by HPLC (ChiralPAK AD-H column, 10% i-PrOH/hexanes,
1 mLminÀ1): tR (major)=11.3 min, tR (minor)=20.0 min.
Acknowledgements
[11] S. K. Alamsetti, S. Mannam, P. Muthupandi, G. Sekar,
Chem. Eur. J. 2009, 15, 1086–1090.
We thank CSIR, New Delhi (01ACTHNURGTNE(UNG 2378)/10/EMR-II) for the fi-
[12] When NaCl (source for chloride counter ion;
0.05 mmol, 1 equiv. with respect to Cu salt) was added
as additive to CuSO4, the reaction afforded 30% ee and
10% isolated yield in 8 days. Similarly, when NaCl
(source for chloride counter ion; 0.5 mmol, 1 equiv.
with respect to Cu salt) was added to CuSO4, (R)-
(+)-1,1’-binaphthyl-2,2’-diamine and TEMPO in di-
chloromethane solvent at room temperature, the reac-
tion afforded 41% ee and 20% isolated yield in 2 days.
These experimental results show that the addition of
external chloride counter ion (NaCl as additive) to Cu
salt has no significant effect in the selectivity and hence
the effect of the copper salt on the selectivity may be
a solubility factor.
nancial support. SKA thanks CSIR for a fellowship during
his Ph.D. period, EP thanks DST-fast track project (SR/FT/
CS-033/2008) and DGP thanks CSIR for a fellowship.
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[13] A branched alcohol ester (isopropyl ester) gave 45%
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[2] J. M. Brunel, Chem. Rev. 2007, 107, PR1–PR45.
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[14] An ortho-methylphenyl substituted ester gave 63% ee
and 72% isolated yield.
[15] For the X-ray single crystal structure of the racemate
see: G. Zi, L. Xiang, Y. Zhang, Q. Wang, Z. Zhang,
Appl. Organomet. Chem. 2007, 21, 177–182.
[16] CCDC 802244 contains the supplementary crystallo-
graphic data for the (R)-BINAM-CuCl complex in this
paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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Enantioselective Oxidative Coupling of 2-Naphthol
Derivatives by Copper-(R)-1,1’-Binaphthyl-2,2’-diamine-
TEMPO Catalyst
7
Adv. Synth. Catal. 2013, 355, 1 – 7
Santosh Kumar Alamsetti, Elamvazhuthi Poonguzhali,
Dhandapani Ganapathy, Govindasamy Sekar*
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