Recyclable chiral dinuclear copper(II) complexes catalyzed asymmetric Friedel-Crafts alkylation of 1-naphthol using N-tosyl aldimine

Article abstract of DOI:10.1016/j.catcom.2015.06.002

A highly efficient dinuclear copper complex catalyzed Friedel-Crafts reaction has been demonstrated for the alkylation of 1-naphthol using N-tosyl aldimine. In this context, various chiral amino alcohol derived Schiff base ligands with different achiral a

Full text of DOI:10.1016/j.catcom.2015.06.002

Recyclable chiral dinuclear copper(II) complexes catalyzed asymmetric
Friedel-Crafts alkylation of 1-naphthol using N-tosyl aldimine
Prathibha Kumari, Ajay Jakhar, Noor-ul H. Khan, Rajkumar Tak, Rukhsana I.
Kureshy, Sayed H.R. Abdi, Hari C. Bajaj
PII:
DOI:
Reference:
S1566-7367(15)00229-0
doi: 10.1016/j.catcom.2015.06.002
CATCOM 4351
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Catalysis Communications
Received date:
Revised date:
Accepted date:
7 April 2015
4 June 2015
5 June 2015
Please cite this article as: Prathibha Kumari, Ajay Jakhar, Noor-ul H. Khan, Rajkumar
Tak, Rukhsana I. Kureshy, Sayed H.R. Abdi, Hari C. Bajaj, Recyclable chiral dinuclear
copper(II) complexes catalyzed asymmetric Friedel-Crafts alkylation of 1-naphthol using
N-tosyl aldimine, Catalysis Communications (2015), doi: 10.1016/j.catcom.2015.06.002
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ACCEPTED MANUSCRIPT
Recyclable chiral dinuclear copper(II) complexes catalyzed asymmetric Friedel-Crafts
alkylation of 1-naphthol using N-tosyl aldimine
Prathibha Kumari,a, b Ajay Jakhar, a, b Noor-ul H. Khan,*a,b Rajkumar Tak, a, b Rukhsana I.
Kureshy,a, b Sayed H. R. Abdi,a, b and Hari C. Bajaj a, b
aDivision of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (CSMCRI), Council of
Scientific & Industrial Reseach (CSIR), G. B. Marg, Bhavnagar 364002, Gujarat (India). Fax: (+91)0278-2566970,
Email: Khan251293@yahoo.in.
bAcademy of Scientific and Innovative Research (AcSIR), (CSIR-CSMCRI), G. B. Marg, Bhavnagar 364 002, Gujarat
(India).
Abstract: A highly efficient dinuclear copper complex catalyzed Friedel-Crafts reaction has
been demonstrated for the alkylation of 1-naphthol using N-tosyl aldimine. In this context,
various chiral amino alcohol derived Schiff base ligands with different achiral and chiral
linkers were synthesized and their copper (II) complexes were generated in situ. One of the
dinuclear copper complex with chiral linker has emerged as an efficient catalyst and affords
the desired arene products in excellent enantioselectivities (ee up to 99%) with very good
yields (up to 98%). The dinuclear catalyst used in this study was recoverable and recyclable
with retention of its catalytic activity.
Key words: Enantioselective, Friedel-Crafts reaction, 1-naphthol, tosyl aldlimine, copper
Schiff base
1. Introduction
Friedel-Crafts reaction is a versatile C-C bond forming reaction in organic chemistry
[1-7]. The asymmetric version of this reaction can straightaway produce optically pure
alkylated arenes, which are important precursors for a broad spectrum of biologically active
compounds and natural products [3]. To accomplish these asymmetric alkylation products,
various aromatic compounds were used as nucleophile with different aldehydes, ketones,
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activated olefins, epoxides and imines as electrophiles. More specifically, asymmetric FC
reaction of phenols [10,11] can directly provide optically active aminomethylphenols having
great utility as pharmaceutically active compounds [12, 13] and as chiral ligands in catalytic
asymmetric transformations [14-16]. The synthesis of chiral amino naphthols largely relies on
a diastereoselective reaction using chiral precursors [17-19], kinetic resolution [20, 21] and
chiral chromatography of racemates [22]. Among the diverse methods reported, asymmetric
Friedel-Crafts reaction of 1-naphthols with imines represents a direct and attractive approach
for the synthesis of optically active amino arylnapthols known as Betti base [23, 24]. The first
asymmetric FC-type reaction of 2-naphthols and aldimines was reported by Hui [25] in 2010
using a stoichiometric amount of a chiral zinc complex. Subsequently, Wang [26] and
Chimni [27] independently developed this catalytic protocol using cinchona alkaloids based
organocatalysts while Kenichi Murai [28], et al. have reported imidazoline derived chiral
organocatalysts. Besides, Hiroaki Sasai [29] et al. have recently reported dinuclear vanadium
complex catalyzed FC reaction of 2-naphthol and tosyl aldimines.
In this direction, earlier we have reported [30] copper based catalytic system for
enantioselective addition of indole on tosyl aldimine. For initial experiments on
enantioselective FC reaction of 1-naphthol with N-(3-nitrobenzylidene)-4-methylbenzene
sulfonamide 4a, we have used in situ generated copper complexes of ligands La and Lb as
catalysts, but the results obtained were not very encouraging as we got the alkylated product
with 15-27% ee and 40 -50% yield [31].
Screening of ligands
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Due to high solubility of the catalyst under homogenous condition recycling of the
catalyst is difficult. To overcome this problem increasing the molecular weight of the catalyst
while creating more number of active sites, is one among the strategy to bring down catalyst
solubility thereby facilitating catalyst recovery and product isolation while increasing catalyst
reactivity. With these points in mind, we designed and synthesized recyclable chiral tridentate
Schiff base ligands with ONO donor atoms possessing different linkers (achiral and chiral)
L1-L7 and their corresponding in situ generated Cu(II) complexes and evaluated their
catalytic efficiency in asymmetric FC reaction of 1-naphthol with tosyl aldimine. Among all
the copper catalysts tested, the dinuclear copper catalyst derived from the ligand L2,
possessing diphenyl amino alcohol chiral moiety and diethyl D-tartrate as chiral linker was
found to be better catalyst for asymmetric Friedel-Crafts reaction of tosyl aldimine with 1-
naphthol. Furthermore, based on UV-vis and kinetics studies we have proposed a plausible
mechanism of the reaction and demonstrated the recyclability of the catalytic system.
2. Experimental
2.1. General Procedure for synthesis of bisaldehyde with trigol linker 2a, 2b, 2c
The compounds 2a, 2b, 2c were synthesized by the reported procedure [32].
2.2. General Procedure for synthesis of Ligand L1 & L3
The compounds L1 & L3 were synthesized by the reported procedure [33].
2.3. General Procedure for synthesis of Ligand L2, L4, L5, L6 & L7
In an oven-dry single-necked round bottom flask, bis-aldehyde 2b (1 mmol) was dissolved in
dry MeOH (10 mL) to which a solution of desired chiral amino alcohol [{(1R, 2S)-2-amino-
1,2-diphenylethanol in the case of ligand L2}/{L-valinol in the case of ligand L4}/{(S)-phenyl
glycinol in the case of ligand L5}/{(S)-phenyl alaninol in the case of ligand L6}/{ (1R,2S)-cis-
1-amino-2-indanol in the case of ligand L7}] (2 mmol) in dry MeOH (5 mL) was added
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dropwise. The resulting solution was stirred at RT and the reaction was monitored on TLC.
Once the reaction was completed, the solvent was removed under reduced pressure and the
yellow solid thus obtained was used as such for in situ complex during catalytic run.
2.4. General procedure for asymmetric Friedel-Crafts reaction of addition of 1-naphthol
to N-sulfonyl aldimine:
Under an N2 atmosphere, Cu(OTf)2 (0.005 mmol), and (1R, 2S)-L2 (0.00275 mmol) were
dissolved in DCM (2 mL) The solution was stirred at room temperature for 2 h, then 4 Å
molecular sieves (100 mg) were added and stirred for another 10 min. Next, tosyl aldimine
(0.20 mmol) and 1-naphthol or 2-naphthol (0.4 mmol) were added, and the reaction mixture
o
was stirred at 25 C. After the reaction was completed, the solvent was removed under
reduce pressure and the residue was purified by column chromatography on silica gel eluted
with ethyl acetate/petroleum ether (1:9) to afford the desired product as white powder.
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Scheme for the synthesis of ligands
3. Results and Discussion
3.1 Optimization of reaction conditions
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The chiral dimeric Schiff base ligands (L1-L7) with different linkers like trigol, piperazine
and diethyl D-(-)-tartrate at C5 position of 3-(tert-butyl)-2-hydroxybenzaldehyde were
synthesized by the condensation of bis-aldehyde 2 with two equivalents of chiral amino
alcohols in dry methanol. Subsequently all the ligands were well characterized by
spectroscopic techniques like 1H NMR, 13C NMR, FT-IR, LC-MS and elemental analysis.
In our initial studies on asymmetric Friedel-Crafts reaction, we screened the catalytic
activity of in situ generated Cu(II) complexes derived from ligands L1-L3 for the reaction of
o
1-naphthol with 4a as model substrate at 25 C. The dinuclear Cu(II) complex generated
from ligand L2 having diethyl D-(-)-tartrate as linker provided better product yield and
enantioselectivity (Table 1, entry 2) as compare to ligands L1 and L3 (Table 1, entries 1 and
3). Therefore, we used diethyl D-(-)-tartrate linker to synthesize a series of dimeric Schiff
base ligands L4-L7 with various chiral amino alcohols. Still, the ligand L2 (Table 1, entry 2)
that was derived from diphenyl amino alcohol was found to be better for enantioselective FC
reaction than rest of the ligands (Table 1, entries 1, 4-7). For the sake of comparision, we
generated mononulear complex of ligand L8 that showed lower catalytic activity as compared
to dinuclear complex generated with ligand L2 (Table 1, entries 2, 8). To carryout further
study, we investigated the effect of various metal sources on the outcomes of the reaction,
which ended up with poor yield and enantioselectivity in the product (Table 1, entries 9-13).
Favourably the dimeric ligand L2 with diethyl tartrate linker worked fairly well with
Cu(II) triflate, for clarity we are referring the active dinuclear catalyst as Cu(II)-L2 (Table 1,
entry 2). Several solvents such as THF, acetonitrile and toluene (Table 1, entries 14-16) were
experimented with, out of these, dichloromethane was found to be the best solvent (ee 75%,
yield 98%, Table 1 entry 2) for this asymmetric FC reaction. Having identified the
appropreiate metal source and solvent, we then varied the catalyst loading from 1.5 mol% to
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Table 1. Optimization of reaction conditions for enantioselective FC reaction of 4a as model
substrate with 1-naphthola
Entry
Ligand
Copper Source
Catalyst
loading
5 mol%
5 mol%
5 mol%
5 mol%
5 mol%
5 mol%
5 mol%
5 mol%
5 mol%
5 mol%
5 mol%
5 mol%
5 mol%
5 mol%
5 mol%
5 mol%
1.5 mol%
2.5 mol%
2.5 mol%
2.5 mol%
Solvent
Yieldb [%]
eec [%]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19d
20e
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OAc)2
CuBr2
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
THF
60
98
85
60
55
65
45
37
50
70
trace
30
40
60
50
45
80
98
98
98
31
75
43
51
32
25
41
42
20
0
L1
L2
L3
L4
L5
L6
L7
L8
L2
L2
L2
L2
L2
L2
L2
L2
L2
L2
L2
L2
CuI
-
5
0
Zn(OTf)2
Sc(OTf)3
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
50
89
31
70
99(-)
99(+)
64(-)
Toluene
acetonitrile
DCM
DCM
DCM
DCM
aReaction Conditions: 4a (0.2 mmol), 1-naphthol (0.4 mmol), ligand L1 – L8 (0.00275 mmol), Cu(OTf)2 (0.005 mmol) in 2
mL DCM, 100 mg of 4 Å MS and reaction was stirred for 24 h. bIsolated yields after flash column chromatography. ee was
c
determined by HPLC (Chiralpak IA).d In ligand L2 when (1S, 2R)-2-amino-1,2-diphenylethanol was used and In ligand L2
e
(1R, 2R)-2-amino-1,2-diphenylethanol was used.
5 mol% (Table 2, entries 2, 17 and 18). The 2.5 mol% was found to be the optimum catalyst
loading to get the best results in terms of enantioselectivity and yield of the alkylated product
(Table 1, entry 18, ee 99%, yield 98%). It is noteworthy to mention that when (1R, 2S)-2-
amino-1,2-diphenylethanol part in L2 was replaced by (1S, 2R)-2-amino-1,2-diphenylethanol,
the product 3a was obtained in the same yield and enantioselectivity but with opposite
configuration of the desired product (yield, 98%; ee 99%, Table 1, entry 19). Moreover, on
further changinging (1R, 2S)-2-amino-1,2-diphenylethanol moiety in L2 with (1R, 2R)-2-
amino-1,2-diphenylethanol we have achieved the product 3a having same configuration with
98% yield but with 64% ee, which is lower than the ligand with mismatch configuration
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(Table 1, entry 20). After screening all the parameters for the enantioselective FC reaction of
o
1-naphthol and 4a as model substrates, 2.5 mol% dinuclear Cu(II)-L2 in DCM at 25 C was
taken as optimum reaction condition.
Table 2. Enantioselective addition of 1-naphthol or 2-naphthol with tosyl adimine with
dinuclear Cu(II)-L2 catalyst under optimized condition.
Entry
X, Y, Z, R
(5a-5k) or (6a-6d)
Yield (%) ee (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
OH, H, H, 3-NO2C6H4
OH, H, H, 4-NO2C6H4
OH, H, H, 2-BrC6H4
OH, H, H, 4-BrC6H4
OH, H, H, 2-ClC6H4
OH, H, H, 4-ClC6H4
OH, H, H, C6H4
OH, H, H, 2-MeC6H4
OH, H, H, 4-MeOC6H4
OH, H, H, 4-MeC6H4
OH, H, H, n-C3H7
OH, H, Cl, 3-NO2C6H4
OH, H, OMe, 3-NO2C6H4
H, OH, 3-NO2C6H4
H, OH, 4-NO2C6H4
H, OH, 4-ClC6H4
98
95
84
95
80
90
82
79
82
80
70
95
85
90
88
85
80
(5a) 99
(5b) 96
(5c) 96
(5d) 90
(5e) 88
(5f) 96
(5g) 92
(5h) 85
(5i) 90
(5j) 92
(5k) 78
(5l) 96
(5m) 93
(6a) 85
(6b) 82
(6c) 78
(6d) 75
H, OH, C6H4
aEnantioselective FC reaction of different tosyl adimine (4a-4k) was performed with 2 equivalents of 1-naphthol or 2-
naphthol in the presence of 2.5 mol% catalyst. Reaction was stirred for 1-3days. bYield of isolated product. cee values were
determined on a Chiralpak IA, IB, IC column.
With this backdrop, to explore the general applicability of the catalyst Cu(II)-L2, FC
alkylation of several aryl and alkyl tosyl aldimines with 1-naphthol were carried out under the
optimized reaction condition. In all the cases, the active dinuclear Cu(II)-L2 catalyst
effortlessly catalyzed the reaction and gave the products in good to excellent yield (78-98%).
The substrate without any substitutions in aryl group of tosyl aldimine gave good yield with
high enantioselectivity (Table 2, entry 7). However, the electron withdrawing substituents at
meta and para position of the substrate provided desired product with excellent
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enantioselectivity in high yields (Table 2, entries 1, 2, 4 and 6). Whereas the electron
withdrawing group at ortho position ended up good enantioselectivity with moderate yields
(Table 2, entries 3 and 5). But the substrates with electron donating group showed only
marginal effect both on yield and enantioselectivity (Table 2, entries 8-10). Besides, the
aromatic aldimines, this catalytic protocol also worked well for aliphatic aldimine (Table 2,
entry 11) and also for the derivatives of 1-naphthol (Table 2, entries 12, 13). Furthermore,
enantioselective FC alkylation of 2-naphthol with tosyl aldimine 4a was carried out under
the above mentioned reaction condition to extend the applicability of the catalytic protocol.
Agreeably, the same catalyst provided desired substituted 2-naphthol derivative in good yield
and enantioselectivity (Table 2, entry 14). Encouraged by these results achieved with 2-
naphthol, we performed FC alkylation with few more tosyl aldimines, which gave good
yields of the alkylated products with moderate to good enantioselectivity (Table 2, entries 14-
17).
3.2 Kinetics of the reaction
We studied the rate of reaction with respect to concentration of catalyst, concentration of 1-
naphthol and concentration of tosyl aldimine which revealed that the reaction is first order
with catalyst and 1-naphthol whereas with electrophile is zero order (kinetics studies were in
detailed discussed in supporting information). Accordingly, the in situ generated complex
Cu(II)-L2 first interact with aldimine to form intermediate (I). Since the rate of reaction is
independent of the substrate (tosyl aldimine), it is assumed that the interaction of substrate
with the catalyst Cu(II)-L2 is the fast step of the reaction and is supported by the
hyperchroism in UV-vis. Spectroscopy (Fig. 5, [c] in supporting information). In the next
step 1-naphthol aligned itself to the activated aldimine from Si-face (Fig 1b, only half unit is
shown for the sake of clarity) to form intermediate (II), which in a slow and rate determining
step form R-enantioenriched product with regeneration of the catalyst.
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Fig. 1 Proposed mechanism of 1-naphthol with tosyl aldimine
3. 3 Recyclability of the catalyst:
Mostly, under homogeneous conditions the catalysts are not recyclable due to their inherent
problem of solubility and thereby separation during the post catalysis workup. However, in
the present study the catalyst recycling experiments were conducted by scaling up reaction by
using 1 mmol (259 mg) of 4a with active catalyst Cu(II)-L2 ( 0.025 mmol, 38 mg) and 2
mmol of 1-naphthol as a nucleophile at 25 °C in CH2Cl2 in the presence of activated 4 Å
molecular sieves under nitrogen atmosphere. After the first catalytic run, the amount of the
solvent was reduced and an excess amount of n-hexane:ethyl acetate (9:1) was added to the
reaction mixture. The catalyst precipitated out from the reaction mixture, the unreacted
substrate and the product remained in the supernatant solvent, which was then separated by
centrifugation and the recovered catalyst was dried in vacuum, stored under dry and inert
atmosphere till used for the subsequent catalytic run. At every post-workup step of catalyst
recycling experiments only 98-99% of the catalyst was recovered but the amounts of the
substrate taken were constant. Consequently, a gradual decline in the product yield was
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apparent which was more pronounce in fourth cycle onwards (Fig 2). The recovered catalyst
worked well for the asymmetric Friedel-Crafts reaction of 1-naphthol with 4a without any
apparent loss in its reactivity and enantioselectivity up to six cycles (Fig. 2). In order to know
the activity of the recycled catalyst we have done the kinetics after every consecutively runs
till up to 3rd run of recyclability process (graphs attached in supporting information).
ee
Yield
98
98
98
99
97
99
97
99
99
95
98
99
100
80
60
40
20
0
1
2
3
4
5
6
Fig. 2 Recyclability of catalyst Cu (II)-L2 using 4a with 1-naphthol.
4. Conclusion
This study demonstrated the chiral dinuclear Cu(II)-L2 complex possessing chiral D-diethyl
tartrate linker catalyzed asymmetric FC reaction of 1-naphthol using various kinds of tosyl
aldimines for the synthesis of Betti base derivatives with good to high enantioselectivity with
the added advantage of recyclability of the catalyst over several runs with the retention of
activity as well as enantioselectivity.
Acknowledgements
CSMCRI approval number is CSIR-CSMCRI – 213/2014. P. Kumari and N. H. Khan are
thankful to UGC and CSIR Indus Magic Project CSC-0123 for financial assistance. P.
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Kumari is thankful to AcSIR for Ph. D. registration. The authors are also thankful to
“Analytical Science Discipline” for providing instrumental facilities.
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Highlights
 Synthesis of chiral Schiff base ligands using different linkers
 First asymmetric F-C reaction of 1-naphthol with tosyl imine using
copper catalyst
 Good catalytic activity in terms of product yields and enantioselectivity
 High degree of recyclability and easy separation of products
 Proposed catalytic mechanism based of Uv-visible spectra and kinetics
studies
15