Base-free benzylation of 1,3-dicarbonyl compounds using sulfamic acid supported on silica by linker: a combined experimental and theoretical approach

Article abstract of DOI:10.1007/s00706-018-2284-x

Abstract: Sulfamic acid stabilized on the surface of silica by the n-propyl organic group linker which is named silica-bonded N-propylsulfamic acid was applied as an efficient heterogeneous catalyst with good recyclability and reusability for direct benzylation of 1,3-dicarbonyl compounds using secondary aromatic alcohols or styrenes as alkylating agents in high yields and short reaction times. All the reactions were carried out in nitromethane as solvent under an air atmosphere. The catalyst showed reusable feature by six times without a significant loss in its activity. Graphical abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s00706-018-2284-x

Monatshefte für Chemie - Chemical Monthly
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ORIGINAL PAPER
Base-free benzylation of 1,3-dicarbonyl compounds using sulfamic
acid supported on silica by linker: a combined experimental
and theoretical approach
Morteza Karimzadeh1 Hamed Saberi Asl1 Hajar Hashemi1 Dariush Saberi2 Khodabakhsh Niknam1
•
•
•
•
Received: 4 December 2017 / Accepted: 19 August 2018
Ó Springer-Verlag GmbH Austria, part of Springer Nature 2018
Abstract
Sulfamic acid stabilized on the surface of silica by the n-propyl organic group linker which is named silica-bonded N-
propylsulfamic acid was applied as an efficient heterogeneous catalyst with good recyclability and reusability for direct
benzylation of 1,3-dicarbonyl compounds using secondary aromatic alcohols or styrenes as alkylating agents in high yields
and short reaction times. All the reactions were carried out in nitromethane as solvent under an air atmosphere. The catalyst
showed reusable feature by six times without a significant loss in its activity.
Graphical abstract
OH
R4
O
O
S
O
O
O
i
O
Si
NHSO3H
2
O
O
R1
R2
R3
R5
R1
R2
or
+
R4
Nitromethane/Reflux
R
5
3 or
R: R
R
Keywords Heterogeneous catalysis Á Sulfamic acid Á Alcohols Á Styrenes Á 1,3-Dicarbonyls
Introduction
green chemistry is the use of catalytic reactants with higher
selectivity. Therefore, using the biocompatible and green
catalytic systems with the ability to recycle and reuse has
been the focus of researchers. Heterogeneous catalysis has
opened up a new window to chemists as a fundamental
procedure from the standpoint of recovery, reusability, and
product purification, especially in the production of fine
chemicals.
The development of synthesis methods under the condi-
tions in accordance with the framework of green chemistry
is of great interest to chemists. One of the 12 principles of
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s00706-018-2284-x) contains
supplementary material, which is available to authorized
users.
Construction of carbon–carbon bond is among the
important reactions in organic chemistry [1, 2]. One of the
useful types of such reactions is the benzylation of 1,3-
dicarbonyl compounds which leads to the generation of
many bioactive structural motifs such as warfarin, regli-
tazar, and tipranavir [3, 4]. Traditionally, the alkylation of
1,3-dicarbonyl compounds is performed via the reaction of
a 1,3-dicarbonyl with alkyl halides, phosphonates, car-
boxylates, or carbonates, mediated by a base. The common
drawbacks associated with these methods, such as pro-
duction of large amounts of by-products and their limited
& Dariush Saberi
Saberi_d@pgu.ac.ir
& Khodabakhsh Niknam
khniknam@gmail.com
1
Department of Chemistry, Faculty of Sciences, Persian Gulf
University, Bushehr 75169, Iran
2
Fisheries and Aquaculture Department, College of
Agriculture and Natural Resources, Persian Gulf University,
Bushehr 75169, Iran
123

M. Karimzadeh et al.
applications [5–10], has led to attempts to discover the
alternative approaches.
As an eco-friendlier and atom-economical process,
benzylation of 1,3-dicarbonyl compounds using secondary
aromatic alcohols or styrenes as alkylating agents, without
use of base, has been provided. In view of atom efficiency
and avoiding waste, this route was superior to traditional
methods, but the low reactivity of alcohols or styrenes
towards the nucleophiles limited its use to the employment
of a suitable catalyst. However, in the past several years,
different types of catalysts including Lewis acidic metals
such as InCl3 [11], Bi(OTf)3 [12], Ln(OTf)3 (Ln = La, Yb,
Sc, Hf) [13, 14], FeCl3 [15–17], Fe(ClO4)3 [18], InBr3 [19],
homogeneous Brønsted acids such as p-toluenesulfonic
acid [20], phosphotungstic acid (PWA) [21], HClO4 [22],
triflic acid [23], dodecylbenzenesulfonic acid [24], and
heterogeneous Brønsted acids such as TfOH/SiO2 [25],
12-tungstophosphoric acid supported on nano silica (NPW/
SiO2) [26], NaHSO4/SiO2 [27], H-montmorillonite [28],
and HClO4/SiO2 [22], have been used for these transfor-
Fig. 1 Preparation of silica bonded N-propylsulfamic acid (SBNPSA)
optimal reaction conditions. By employment of CH2Cl2 as
the solvent and SBNPSA (60 mg, containing 0.02 mmol of
H?) as the catalyst under reflux conditions, the product 3a
was obtained in 70% yield (Table 1, entry1). The effect of
some organic solvents such as EtOAc, CH3CN, toluene,
EtOH, and CH3NO2 was investigated and the best results
were achieved with CH3NO2, affording the desired product
in 95% yield within 15 min at reflux conditions (Table 1,
entries 2–6). Reducing the reaction temperature to room
temperature resulted in a sharp decline in efficiency
(Table 1, entries 7–9). In the absence of the catalyst, the
reaction did not proceed even after 2 h of reaction time
(Table 1, entry 10). This showed that the catalyst is one
prominent compound in this reaction. Further use of
SBNPSA beyond the above-mentioned amount (0.07) did
not show any further increase in the yield, whereas the
lower use (0.05) resulted in poor yield (Table 1, entries 11
and 12). Therefore, 60 mg of SBNPSA was chosen as the
most efficient amount in the rest of study. Under the
standard reaction conditions, when diphenylmethanol was
replaced by styrene, the same reaction was performed and
the product 3b was formed in 88% yield.
mations. Moreover,
a
ruthenium complex [29],
trimethylsilyl trifluoromethanesulfonate (TMSOTf) [30],
B(C6F5)3 [31], and BF3.OEt2 [32], have also been previ-
ously examined for this transformation. Recently, Xu et al.
has been reported an air-stable bis(isopropylcyclopentadi-
enyl) zirconium perfluorooctanesulfonate for benzylation
of 1,3-dicarbonyl derivatives with alcohols [33].
Undoubtedly, good progress has been made to date, but
some common drawbacks, associated with these methods,
such as air- or moisture-sensitive and failure in recovery of
catalysts along with difficulty in preparation of them, long
reaction time, expensive reagents, and strong acidic con-
ditions are still highly demanded to develop much more
convenient, prompt and recyclable system for the catalytic
benzylation of 1,3-dicarbonyl compounds.
To demonstrate the generality of this method, we
investigated the benzylation of 1,3-diketones with various
secondary aromatic alcohols or styrenes in the presence of
SBNPSA and the results are summarized in Table 2.
According to the obtained data, the reaction of diben-
zoylmethane, acetylacetone, benzoylacetone, ethyl ace-
toacetate, and isopropyl acetoacetate with benzhydrylic
alcohols or styrene derivatives were converted into the
corresponding products in good–excellent yields (85–97%;
Table 2, entries 1–20). Various aromatic alcohols or
styrenes having methyl, methoxy, chloro, and fluoro sub-
stitution group underwent smooth hydroalkylation with
1,3-diketones to afford the corresponding products in
optimum conditions. Generally speaking, the electron-
withdrawing groups on the phenyl ring of benzhydrylic
alcohols or styrenes slightly benefited the yields (Table 2,
entries 4, 5, 10, 11). Reaction of benzoylacetone and ethyl
acetoacetate with unsymmetric benzhydrylic alcohols or
styrenes gave two inseparable diastereomers (Products 3n,
In this regard and in continuation of the research to
develop new catalysts for organic transformations [34–43],
we found that silica-bonded N-propylsulfamic acid
(SBNPSA), which has been previously synthesized in our
research lab, can effectively catalyst the benzylation of 1,3-
dicarbonyl compounds with various secondary aromatic
alcohols or styrenes as a reusable catalyst in short reaction
times.
Results and discussion
SBNPSA was prepared according to our previously
reported procedure [34]. Synthetic route and the structure
of the catalyst are shown in Fig. 1.
Initially, the reaction between diphenylmethanol
(1 mmol) and 1,3-diphenylpropane-1,3-dione (2 mmol)
was carried out, as the model reaction, to establish the
123

Base-free benzylation of 1,3-dicarbonyl compounds using sulfamic acid supported on silica by…
Table 1 Investigated conditions for the benzylation of diphenylmethanol with 1,3-diphenylpropane-1,3-dione using of SBNPSA as the catalyst
Entry
Catalyst/g
Solvent
Temp./°C
Time/min
Yielda/%
1
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
–
CH2Cl2
EtOAc
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
85
25
30
70
60
60
60
25
95
75
40
20
–
2
3
CH3CN
Toluene
Ethanol
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
30
4
30
5
60
6
15
7
30
8
60
40
9
r.t.
120
120
15
10
11
12
Reflux
Reflux
Reflux
0.07
0.05
95
80
15
Reaction conditions: 1a (2.0 mmol), 2a (1.0 mmol), 2.0 cm3 solvent
aIsolated yields
3p, 3q, and 3s). In addition, the reaction of isopropyl
acetoacetate with benzhydrol led to the corresponding
product 3t with excellent efficiency (Table 2, entry 20).
The efficacy of the presented method in the preparation
of aryl-1,3-dione derivatives in comparison with some
previously reported catalysts is illustrated in Table 3. The
results showed that our method is superior in some cir-
cumstances such as catalyst amount, reaction time, or
product yields.
the b-diketone (or in enol form) to yield the addition
product.
The reusability of SBNPSA was investigated in the
model reaction. For each of the repeated reactions, the
catalyst was recovered easily by simple filtration, washed
exhaustively with warm ethanol and dried. The results
showed that a significant loss in its activity was noticeable
after reusing the catalyst six times (Fig. 4).
Based on the results and according to the reported
findings, the proposed mechanism for direct C–C bond
coupling reaction of secondary benzylic alcohols with 1,3-
dicarbonyl compounds in the presence of SBNSA is shown
in Fig. 2. The mechanism of these reactions, catalyzed by
Brønsted acids, was generally thought to involve the for-
mation of a carbocation intermediate and subsequent SN1
attack on the b-dicarbonyl compound to yield the addition
product. Therefore, in this work, SBNPSA protonates the
alcohol and carbocation intermediate 1 would rapidly be
generated by loss of water. Then, the nucleophilic a-carbon
of the b-diketone attacks the carbocation to form the
product. Note that the b-diketone might be preferentially in
an enol tautomeric at high temperature (100 °C). A
mechanism very similar to the addition of b-diketones to
alcohols is proposed as operative for SBNPSA catalyzed
addition of b-diketones to styrenes (Fig. 3). Again, the
crucial species is the carbocation intermediate 1, it attacks
Conclusion
In summary, we have shown that silica-bonded N-propyl-
sulfamic acid is an efficient solid acid catalyst for the direct
coupling of secondary benzylic alcohols or styrene
derivatives with 1,3- dicarbonyl compounds, resulting in
the corresponding product in high yields and short reaction
times. This method represents a simple, clean, and reusable
alternative to the already established use of homogeneous
acid catalyst.
Experimental
All chemicals were purchased from Merck or Fluka
1
Chemical Companies. The H and 13C NMR was run on
Bruker Avance (DRX 500 MHz and 400 MHz)
123

M. Karimzadeh et al.
Table 2 Benzylation of 1,3-dicarbonyl compounds with benzyl alcohol or styrene derivatives in nitromethane under reflux conditions
1,3-
Alcohol or
Entry
1
Product
Time/min Yield/%a
M.p. /°C
228–230
Dicarbonyl
styrene
O
O
O
O
OH
O
O
O
O
O
O
Ph
Ph
Ph
Ph
Ph
25
25
15
95
88
85
Ph
Ph
Ph
Ph
Ph
Ph
(227–229 [25])
124–126
3a
O
2
3
(129–131 [29])
155–157
3b
O
OH
CH3
Ph
O
(154–156 [44])
111–113
3c
O
OH
CH3
O
O
Ph
Ph
4
15
97
Ph
Ph
F
(110–112 [29])
102–104
F
3d
O
O
O
O
O
O
Ph
Ph
5
6
7
15
20
20
95
85
85
Ph
Ph
Ph
Ph
Cl
(107–109 [29])
3e
Cl
OH
CH3
O
O
Ph
Ph
105–107 [14]
105–108
MeO
3f
MeO
O
O
O
O
O
O
Ph
O
Ph
Ph
Ph
H3C
(103–105 [25])
45–47
3g
H3C
O
OH
CH3
H3C
CH3
8
9
15
15
88
92
H3C
CH3
(43–45 [23])
3h
3i
O
O
OH
115–118
O
O
H3C
CH3
H3C
CH3
(117–118 [22])
123

Base-free benzylation of 1,3-dicarbonyl compounds using sulfamic acid supported on silica by…
Table 2 continued
1,3-
Alcohol or
styrene
Entry
10
Product
Time/min Yield/%a
M.p. /°C
Dicarbonyl
O
O
OH
77–79
O
O
CH3
H3C
CH3
15
15
95
95
H3C
CH3
Cl
(77–79 [25])
3j
Cl
O
O
58–60
(57–58 [29])
54–56
O
O
H3C
CH3
11
H3C
CH3
F
3k
F
OH
CH3
O
O
O
O
O
O
H3C
H3C
CH3
CH3
12
13
14
20
20
20
85
88
87
H3C
CH3
MeO
(53–55 [25])
58–60
3l
MeO
H3C
O
O
H3C
CH3
H3C
(57–59 [25])
94–96 [38]
3m
OH
OH
CH3
O
O
152–154
O
O
H3C
Ph
15
20
95
Ph
CH3
(150–152 [25])
82–84
3o
O
O
O
O
H3C
Ph
16
17
18
25
15
15
85
87
92
Ph
CH3
OEt
(97–99 [25])
3p
OH
OH
O
O
O
O
CH3
42–44 [38]
85–87 [44]
H3C
H3C
OEt
OEt
H3C
3q
O
O
O
O
O
O
O
H3C
OEt
OEt
3r
O
19
20
25
25
88
85
Oil [23]
89–91
H3C
OEt
H3C
3s
OH
Conditions: SBNPSA (60 mg, containing 0.02 mmol of ?H), b-dicarbonyl compound (2.0 mmol), alcohol or styrene (1.0 mmol), 2 cm3
nitromethane in reflux conditions
aIsolated yield
123

M. Karimzadeh et al.
Table 3 Comparison between efficiency of SBNPSA with some previously reported catalysts in the synthesis of product 3a
Entry
Catalyst/mol%
Conditions
Time/h
Yield/%
References
1
2
3
4
5
6
PTSA (p-toluenesulfonic acid) (5)
HClO4 (1)
CH3NO2 (100 °C)
Toluene (70 °C)
Solvent-free (70 °C)
CH3NO2 (70 °C)
CH3NO2 (101 °C)
CH3NO2 (100 °C)
0.5
17
83
77
99
97
82
95
[23]
[22]
HClO4–SiO2 (1)
TfOH–SiO2 (3)
H2SO4 (5)
17
[22]
5
[25]
0.08
0.25
[45]
SBNPSA (2)
This work
Fig. 2 Proposed mechanism of the SBNPSA-catalyzed addition of b-diketones to alcohols
instruments in CDCl3. Results are reported in ppm, using
TMS as an internal standard. IR spectra were obtained
using a Shimadzu FT-IR 8300 spectrophotometer. Melting
points were determined on a SMP1 Melting Point apparatus
in open capillary tubes. Reaction progress was followed by
TLC using silica gel SILG/UV 254 plates. Silica bonded N-
propylsulfamic acid (SBNPSA) was prepared according to
our previously reported procedure [34].
considering both polarization function and diffuse function
to give more accurate results. There were no symmetric
limitations for optimized compound (3p), and C1 sym-
metry selected for this compound. Finding global or local
minima and then vibrational assignments were performed
using frequency calculations at the same level of theory
[46–54]. The GIAO method is one of the most usual
methods for computing nuclear magnetic shielding tensors.
The 13C and 1H NMR isotropic shielding values were
obtained by GIAO approach using the optimized structure
achieved by M06-2X/6-311 ??G(2d, p) method. The
natural bond orbital technique applied for optimized
Geometric
optimization
of
1-phenyl-2-(1-
phenylethyl)butane-1,3-dione (3p) performed with Min-
nesota functional at M06-2X/6-311 ??G(2d, p) level of
theory [46–54]. Selection of basis set was done based on
123

Base-free benzylation of 1,3-dicarbonyl compounds using sulfamic acid supported on silica by…
Fig. 3 Proposed mechanism of the SBNPSA-catalyzed addition of b-diketones to styrenes
done using Gauss View 5.0 [57]. Computations were per-
formed by means of GAUSSIAN 09 package program [58]
(see Supporting Information).
General procedure for direct benzylation of 1,3-
dicarbonyl compounds using alcohols or styrenes
as alkylating agents
To a mixture of 1,3-dicarbonyl compound (2 mmol),
alcohol or styrene derivatives (1 mmol), and 0.6 g
SBNPSA, was added 2 cm3 nitromethane as solvent. The
mixture was stirred under reflux conditions and the reaction
was followed by TLC. After completion, the mixture was
filtered, and the remaining was washed with warm ethanol
to separate catalyst and nitromethane was removed under
reduced pressure. Then, the crude products were recrys-
tallized from mixture of dichloromethane and n-hexane.
All the synthesized products were known and characterized
Fig. 4 Recyclability of SBNPSA in benzylation of 1,3-diphenyl-
propane-1,3-dione (2 mmol) with diphenylmethanol (1 mmol) in
nitromethane under reflux conditions. Reaction time: 15 min
structures to find donor–acceptor interactions [55, 56].
Molecule building, viewing capabilities, setting up Gaus-
sian jobs, and finally visualizing Gaussian results were
123

M. Karimzadeh et al.
27. Aoyama T, Miyota S, Takido T, Kodomaric M (2011) Synlett
2011(20):2971
28. Motokura K, Fujita N, Mori K, Mizugaki T, Ebitani K, Kaneda K
(2006) Angew Chem Int Ed 45:2605
by comparison of their spectral and physical data with
those reported in literature.
Isopropyl 2-(diphenylmethyl)-3-oxobutanoate (3t, C20H22O3)
Yield 85%; white solid; m.p.: 89–91 °C; IR: = 708, 1102,
1175, 1418, 1571, 1654, 1700, 1715, 2984 cm-1; 1H NMR
(300 MHz, CDCl3): d = 7.25–7.06 (m, 12H, Ar),
4.79–4.73 (m, 1H, –CH(CH3)2), 4.68 (d, 1H, J = 12 Hz,
CH), 4.43 (d, 1H, J = 12 Hz, COCHCO), 2.01 (s, 3H,
CH3CO), 0.94 (d, 3H, J = 6.3 Hz, –CH3), 0.90 (d, 3H,
J = 6.3 Hz, –CH3) ppm; 13C NMR (75 MHz, CDCl3):
d = 201.5, 167.2, 141.6, 141.4, 128.8, 128.6, 127.8, 127.7,
126.9, 126.7, 68.8, 65.3, 50.7, 29.6, 20.8 ppm; MS (EI,
70 eV): m/z (%) = 310 (M?, 1), 167 (100).
29. Liu PN, Zhou ZY, Lau CP (2007) Chem Eur J 13:8610
30. Theerthagiri P, Lalitha A (2010) Tetrahedron Lett 51:5454
31. Reddy CR, Vijaykumar J, Gree R (2010) Synthesis 21:3715
32. Bisaro F, Prestat G, Vitale M, Poli
2002(11):1823
G (2002) Synlett
33. Zhang X, Qiu R, Zhou C, Yu J, Li N, Yin S, Xu X (2015)
Tetrahedron 71:1011
34. Niknam K, Saberi D (2009) Tetrahedron Lett 50:5210
35. Niknam K, Saberi D (2009) Appl Catal A Gen 366:220
36. Niknam K, Saberi D, Sadegheyan M, Deris A (2010) Tetrahedron
Lett 51:692
37. Nourisefat M, Saberi D, Niknam K (2011) Catal Lett 141:1713
38. Umasish J, Srijit B, Sukhendu M (2007) Tetrahedron Lett
48:4065
Acknowledgements Financial support for this work by the Research
Council of Persian Gulf University, Bushehr, Iran, is gratefully
acknowledged.
39. Hajipour AR, Karimzadeh M, Tavallaei H (2015) J Iran Chem
Soc 12:987
40. Chiniforoshan S, Khalesi SB, Tabrizi L, Hajipour AR, Cher-
mahini AN, Karimzadeh M (2015) J Mol Struct 1082:56
41. Hajipour AR, Karimi H, Karimzadeh M (2014) Monatsh Chem
145:1461
42. Hajipour AR, Karimzadeh M, Ghorbani S (2014) Synlett 25:2903
43. Hajipour AR, Karimzadeh M, Azizi G (2014) Chin Chem Lett
25:1382
44. Yadav JS, Subba Reddy BV, Pandurangam T, Raghavendra Rao
KV, Praneeth K, Narayana Kumar GGKS, Madavi C, Kunwar
AC (2008) Tetrahedron Lett 49:4296
References
1. March J (1992) Advanced organic chemistry, 4th edn. Wiley,
New York
2. Scolastico C, Nicotra F (1999) Current trends in organic syn-
thesis, 1st edn. Plenum, New York
3. Kischel J, Mertins K, Michalik D, Zapf A, Beller M (2007) Adv
Synth Catal 349:865
45. Xia F, Zheng LZ, Pei NL (2012) Tetrahedron Lett 53:2828
46. Becke AD (1993) J Chem Phys 98:1372
4. Narayana V, Varala R, Zubaidha P (2012) Int J Org Chem 2:287
5. Arcadi A, Cerichelli G, Chiarini M, Giuseppe SD, Marinelli F
(2000) Tetrahedron Lett 41:9195
6. Lee HS, Park JS, Kim BM, Gellman SH (2003) J Org Chem 68:1575
7. Dieter RK, Gore VK, Chen N (2004) Org Lett 6:763
8. Arumugam S, Mcleod D, Verkade JG (1998) J Org Chem
63:3677
47. Lee C, Yang W, Parr RG (1988) Phys Rev B 37:785
48. Becke AD (1993) J Chem Phys 98:5648
49. Zhao Y, Truhlar D (2008) Theor Chem Acc 120:215
50. Karimzadeh M, Manouchehri N, Saberi D, Niknam K (2018)
Struct Chem 29:383
51. Hajipour AR, Karimzadeh M, Ghorbani S, Farrokhpour H,
Chermahini AN (2016) Struct Chem 27:1345
9. Adib M, Sheikhi E, Karimzadeh M, Bijanzadeh HR, Amanlou M
(2012) Helv Chim Acta 95:788
52. Hajipour AR, Ghorbani S, Karimzadeh M, Jajarmi S, Chermahini
AN (2016) Comput Theor Chem 1084:67
10. Adib M, Karimzadeh M, Mahdavi M, Sheikhi E, Mirzaei P
(2011) Synlett 2011(6):834
53. Hajipour AR, Chermahini AN, Karimzadeh M, Rezapour M
(2015) Struct Chem 26:159
11. Yasuda M, Somyo T, Baba A (2006) Angew Chem Int Ed 45:793
12. Rueping M, Nachtsheim BJ, Kuenkel A (2007) Org Lett 9:825
13. Noji M, Konno Y, Ishii K (2007) J Org Chem 72:5161
14. Huang W, Wang J, Shen Q, Zhou X (2007) Tetrahedron Lett
48:3969
15. Ahmad R, Riahi A, Langer P (2009) Tetrahedron Lett 50:1490
16. Kischel J, Michalik D, Zapf A, Beller M (2007) Chem Asian J
2:909
17. Yuan Y, Shi Z, Feng X, Liu X (2007) Appl Organomet Chem 21:958
18. Thirupathi P, Kim SS (2010) Tetrahedron 66:2995
19. Vicennati P, Cozi PG (2007) Eur J Org Chem 2007:2248
20. Sanz R, Martinez A, Miguel D, Alvarez-Gutierrez JM, Rodriguez
F (2006) Adv Synth Catal 348:1841
21. Wang GW, Shen YB, Wu XL (2008) Eur J Org Chem 2008:4999
22. Liu PN, Dang L, Wang QW, Zhao SL, Xia F, Ren YJ, Gong XQ,
Chen JQ (2010) J Org Chem 75:5017
54. Hajipour AR, Karimzadeh M, Jalilvand S, Farrokhpour H,
Chermahini AN (2014) Comput Theor Chem 1045:10
55. Reed AE, Weinstock RB, Weinhold F (1985) J Chem Phys
83:735
56. Reed AE, Curtiss LA, Weinhold F (1988) Chem Rev 88:899
57. Ii R, Keith T, Millam J, Eppinnett K, Hovell L, Gilliland R
(2003) GaussView V. 3.09. Gaussian Inc, Wallingford
58. Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA,
Cheeseman JR, Scalmani G, Barone V, Mennucci B, Petersson
GA, Nakatsuji H, Caricato M, Li X, Hratchian HP, Izmaylov AF,
Bloino J, Zheng G, Sonnenberg JL, Hada M, Ehara M, Toyota K,
Fukuda R, Hasegawa J, Ishida M, Nakajima T, Honda Y, Kitao
O, Nakai H, Vreven T, Montgomery JA, Peralta JE, Ogliaro F,
Bearpark M, Heyd JJ, Brothers E, Kudin KN, Staroverov VN,
Kobayashi R, Normand J, Raghavachari K, Rendell A, Burant JC,
Iyengar SS, Tomasi J, Cossi M, Rega N, Millam JM, Klene M,
Knox JE, Cross JB, Bakken V, Adamo C, Jaramillo J, Gomperts
R, Stratmann RE, Yazyev O, Austin AJ, Cammi R, Pomelli C,
Ochterski JW, Martin RL, Morokuma K, Zakrzewski VG, Voth
GA, Salvador P, Dannenberg JJ, Dapprich S, Daniels AD, Farkas
JB, Foresman JV, Ortiz J, Cioslowski DJ (2009) Gaussian 09,
Revision B.01. Gaussian Inc, Wallingford
23. Sanz R, Miguel D, Martinez A, Alvarez-Gutierrez JM, Rodriguez
F (2007) Org Lett 9:2027
24. Shirakawa S, Kobayashi S (2007) Org Lett 9:311
25. Liu PN, Xia F, Wang QW, Ren YJ, Chen JQ (2010) Green Chem
12:1049
26. Rafiee E, Khodayari M, Shahebrahimi S, Joshaghani M (2011) J
Mol Catal A Chem 351:204
123