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Chemical fixation of CO2 to cyclic carbonates using Al(III) β-aminoalcohol based efficient catalysts: An experimental and computational studies
Accepted Manuscript Title: Chemical fixation of CO2 to cyclic carbonates using Al(III) -aminoalcohol based efficient catalysts: An experimental and computational studies Author: Shailesh Verma Mrinal Kanti Si Rukhsana I. Kureshy Mohd Nazish Manish Kumar Noor-ul H. Khan Sayed H.R. Abdi Hari C. Bajaj Bishwajit Ganguly PII: DOI: Reference:
S1381-1169(16)30086-3 http://dx.doi.org/doi:10.1016/j.molcata.2016.03.018 MOLCAA 9817
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date: Revised date: Accepted date:
3-1-2016 26-2-2016 5-3-2016
Please cite this article as: Shailesh Verma, Mrinal Kanti Si, Rukhsana I.Kureshy, Mohd Nazish, Manish Kumar, Noor-ul H.Khan, Sayed H.R.Abdi, Hari C.Bajaj, Bishwajit Ganguly, Chemical fixation of CO2 to cyclic carbonates using Al(III) rmbeta-aminoalcohol based efficient catalysts: An experimental and computational studies, Journal of Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.03.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Chemical fixation of CO2 to cyclic carbonates using Al(III) βaminoalcohol based efficient catalysts: An experimental and computational studies
Shailesh Verma,a,b Mrinal Kanti Si,b, Rukhsana I. Kureshy,*a,b c Mohd Nazish,a,b Manish Kumar, a,b Noor-ul H. Khan,a,b Sayed H. R. Abdi,a,b Hari C. Bajaj,a,b Bishwajit Ganguly*b,c
a
Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research
Institute (CSMCRI), Bhavnagar- 364 002, Gujarat, India. Fax: +91-0278-2566970; E-mail: [email protected]. b
Academy of Scientific and Innovative Research (AcSIR), Central Salt and Marine Chemicals
Research Institute (CSMCRI), Council of Scientific & Industrial Research (CSIR), G. B. Marg, Bhavnagar- 364 002, Gujarat, India. c
Discipline Computation and Simulation Unit (ADACIF), CSIR-CSMCRI, ), G. B. Marg,
Bhavnagar- 364 002, Gujarat, India. E-mail: [email protected]
* Corresponding author
Graphical abstract
Highlights
Al (III) β-aminoalcohol based complexes as active catalysts.
Cycloaddition reaction of CO2 and epoxides to give commercially important cyclic carbonates.
With the help of DFT calculations the role of co-catalyst was established.
Abstract A series of Al(III) unsymmetrical β-aminoalcohol based complexes 1-6 were synthesized via metalation of the corresponding ligands 1’-6’ those were prepared by the reaction of benzylamine with readily available epoxides viz., styrene oxide, 1,2-epoxy-3-phenoxypropane, 4-tertbutylphenyl glycidyl ether, 4-chlorophenyl glycidyl ether, glycidyl 2-methylphenyl ether and 1,2-epoxyhexane. Among these complexes, the complex 2 was found to be most effective in the cycloaddition of aryloxy/aliphatic terminal epoxides with CO2 under atmospheric pressure to get corresponding cyclic carbonates with high conversion and selectivity (up to >99%) in the presence of tetrabutylammonium bromide as a co-catalyst. The DFT calculations revealed the important role played by counter-ion in the co-catalyst during cycloaddition reaction of CO2 with the substituted epoxides.
Keywords: Cyclic carbonates; Al(III) β-aminoalcohol complexes; β-aminoalcohol; Gaussian 09; benzyl amine.
1. Introduction The chemistry of carbon dioxide (CO2) utilization has gathered significant attention not only because CO2 is one of the potent greenhouse gases, but also as an inexpensive, abundant, non-toxic, non-flammable and bio-renewable C1 resource.1 However, thermodynamic stability of CO2 (∆H0f = -394 kJ mol-1) poses a major hurdle in its utilization in chemical transformations. Nevertheless, this barrier has been crossed with the advent of efficient catalytic systems required to convert CO 2 into valuable chemicals.2 The reaction of CO2 with epoxide is one of the most studied reactions, and depending upon the reaction condition, the choice of substrate epoxide and catalyst one can produce polymeric carbonates or cyclic carbonates.3 These organic carbonates are having several industrial applications as polar aprotic solvents, engineered plastics, masked diols and as one of the constituents in lithium ion batteries.4 A number of active catalysts/initiators, such as quaternary ammonium/phosphonium salts,5 metal complexes,6 phthalocyanine,7 ionic liquids,8 metal oxides9 and immobilized molecular catalysts10 have been reported to impart moderate to good activities in cycloaddition reaction of CO2 with epoxides for the production of cyclic carbonates. Among several metal complexes, salen ligands in combination with transition metal ions such as chromium,11 cobalt,12 magnesium13 and zinc14 have shown their worth in the synthesis of cyclic carbonates. In particular, aluminium salen complexes15 have received good attention in recent times, largely due to less environmental impact of aluminium, high earth abundance and significantly high catalytic activity. Recently, some binary16 and bimetallic17 symmetric aluminium-salen complexes have produced interesting results, but the use of unsymmetrical aluminium complexes for this reaction has not been explored much. Recently, Styring et al.18 reported unsymmetrical aluminium salen complex for the synthesis of styrene carbonate with fairly good success. To take this concept forward and to make catalyst synthetic protocol simpler herein, we report unsymmetrical aluminium complexes derived from readily accessible amino-alcohol based ligands to catalyse cycloaddition of epoxides to CO2 at atmospheric pressure. The most striking feature of the present ligand system lies in its single step synthesis. These ligands can be conveniently synthesized from the ring opening of easily available epoxides with a nucleophile e.g. benzylamine. Coincidently, the catalysts developed for the present study are derived from the epoxides those are used as substrates as well. Further, it is worthwhile to mention that the present system (as in most previously reported systems) role of a suitable additive/co-catalyst is crucial. Among several additives screened in
the present system, tetra butyl ammonium bromide (TBAB) was found to be the best. To gain insight into the reaction mechanism for the formation of cyclic carbonates with substituted epoxides and CO2, we performed DFT calculations using B3LYP/6-31G* level of theory.20g The calculated results showed the role of counter ion (Br-) to stabilize the phenyl substituted epoxide transition state compared to the other substituted epoxides. Such a stabilization of the transition state helps to reduce the activation barrier in phenyl substituted epoxide. 2. Experimental Section 2.1. Methods and Materials Diethylaluminum chloride, benzylamine and epoxides viz., styrene oxide 7a, phenyl glycidyl ether 7b, 4-chlorophenyl glycidyl ether 7c, 4-tert-butylphenyl glycidyl ether 7d, glycidyl 2methylphenyl ether 7e, 2-benzyloxirane 7f, glycidyl isobutyl ether 7g, tertbutylglycidyl ether 7h, epichlorohydrin 7i, 1,2-epoxyhexane 7j, 1,2-epoxyoctane 7k, 1,2-epoxydecane 7l, 1,2epoxydodecane 7m and 1,2-epoxy-5-hexene 7n were purchased from Aldrich Chemicals and were used as received. All the solvents mentioned in the current study were purified by known techniques before use. Microanalysis of the ligands and catalysts was carried out on a Perkin Elmer 2400 CHNS. 1H,
13
C&
27
Al NMR spectra were recorded on Bruker 200 MHz or 500
MHz spectrometer at ambient temperature using CDCl3 and CD3OD as solvent and TMS as internal standard. Yields were determined by comparing the peak area ratio of product to substrate in the 1H NMR spectrum of an aliquot of the crude reaction mixture. FTIR spectra were recorded on a Perkin Elmer Spectrum GX spectrophotometer as KBr pellet. High resolution mass spectra were obtained with a LC-MS (Q-TOFF), Model make Ultra flex TOF/TOF, Burker Daltonics, Germany instruments. For product purification, flash chromatography was performed using silica gel of 60-200 mesh size procured from SD FineChemicals Limited, Mumbai (India).
2.2. General procedure for the synthesis of β-aminoalcohol ligands 1’-6’ A solution of an appropriate epoxide (2 mmol) and benzyl amine (2 mmol) were taken in a 5 mL reaction vial to which silica gel (20 mg) was added and the reaction mixture was stirred
with the help of magnetic bar at room temperature for 4 to 6 h. The resulting mass was diluted with 20% of ethyl acetate in hexane, filtered and cooled to -25 OC. A white solid precipitated out was filtered and the solid thus obtained was dried under vacuum and used as such without further purification. 2.3. General procedure for the synthesis of aluminium β-aminoalcohol complexes 1-6 In a flame dried 50 mL RBF, the above synthesized ligands 1’-6’ (2.0 mmol) were taken in dichloromethane (20 mL) under nitrogen atmosphere to which a solution of diethylaluminium chloride (0.9 M) in toluene (2.2 mmol, 2.4 mL) was added slowly at 0 OC over 30 minutes and the resulting solution was stirred at room temperature for 7 h. The completion of the reaction was checked on TLC. After completion of the reaction the solvent was removed on a rotavapour and a white solid thus obtained was dissolved in methanol and passed through celite pad for removing excess of aluminium. On removal of methanol on rota-vapour, the desired complexes 1-6 were obtained as white solid in quantitative yields. These complexes were characterised by 1H, 13C, 27Al NMR, microanalysis, LCMS and HRMS. 2.4. Typical procedure for synthesis of cyclic carbonates from epoxides and CO2 using Al(III) β-aminoalcohol complexes The cycloaddition reactions were carried out in a dry 5 mL reaction vial equipped with a magnetic bar on an automated stirrer with heating mantle. The reaction vial was charged with appropriate amount of homogeneous catalyst 1-6 (1.0 mol%) and TBAB (1.0 mol%) as cocatalyst and closed with a septum. An appropriate epoxide 7a-n (10 mmol) was introduced to this vial with the help of a syringe. The vial was purged with nitrogen followed by CO2 and the reaction mixture was allowed to stir at a specified temperature for a specified time at around atmosphere pressure of CO2. The conversion and selectivity of the product was determined by 1
H NMR by taking an aliquot from the reaction mass at a regular interval.19
3. Results and discussion We have started our investigation with the synthesis of amino-alcohol ligands 1’-6’ by the ring opening reaction of several aromatic, aryloxy and aliphatic epoxides with benzyl amine (Scheme 1). After due characterization with suitable physico-chemical techniques (data is
given in supporting information), these ligands (1’-6’) were complexed with diethyl aluminium chloride in dichloromethane to provide unsymmetrical aluminium complexes 1-6 (Scheme 1).
Aluminium complexes 1-6 were suitably characterized and to start with complex 1 (1 mol %) was used as catalyst in the cycloaddition of CO2 to styrene epoxide 7a taken as a representative substrate at ~1 atmospheric pressure (balloon pressure) and data are given in Figure 1. However, the complex 1 (Scheme 1) alone was found to be ineffective in catalyzing the cycloaddition
reaction
(Figure
1,
entry
1),
tetrabutylammonium bromide (TBAB, 1 mol%)
however,
addition
of
a
co-catalyst
made a significant improvement in the
reaction outcome (Figure 1, entry 3).
It is noteworthy to mention that TBAB alone has given the product styrene carbonate 7’a in trace amounts (~5%) under identical reaction condition (Figure 1, entry 2), which confirms that both aluminium complex and the co-catalyst is essential for the reaction. Hence, in our subsequent catalytic reactions to evaluate the efficacies of remaining complexes 2–6 (Scheme 1) in the cycloaddition of CO2 to styrene epoxide we maintained the above mentioned reaction condition (Figure 1, entries 4-8). Among these, complex 2 (derived from phenyl glycidyl ether) (Scheme 1) has shown the best performance in the cycloaddition of styrene oxide to give the desired product 7’a in 99% yield (entry 4). Any substituent in the phenyl group of the catalyst, as in representative examples 3-5 was detrimental to the catalytic-performance and gave the product 7’a in relatively lower yields (Figure 1, entries 5-7, yields, 67-87%). Further, the unsymmetrical aluminium complex 6 generated from aliphatic epoxide e.g., 1,2-epoxyhexane was found to be less reactive for this reaction and gave the product 7’a in poor yield (42%, Figure 1 entry 8). As complex 2 was found to be the best among these (entry 4), it was considered for the optimization of other reaction parameters viz. catalyst loading, co-catalyst loading, and temperature variation in the cycloaddition reaction of CO2 with styrene oxide 7a as a representative substrate. The catalyst loading was varied over 0.25 mol% to 1.25 mol% (Table 1, entries 1-5) for this reaction with a co-catalyst loading of 0.25 mol% and found that 1 mol% of the catalyst loading is optimum (Table 1, entry 4). Next, we varied co-catalyst loading from 0.25 mol% to 1.25 mol% with optimized catalyst loading of 1 mol% (Table 1, entries 4, 6-8) and observed an increase in the formation of cyclic carbonate with increase in co-catalyst loading. Among these results 1 mol% catalyst and 1 mol% co-catalyst loading was
found to be optimum (Table 1, entry 8). Thereafter, the effect of temperature on the performance of this protocol was evaluated over a temperature range of 30–120 0C (Table 1, entries 8, 10-12), where 90 0C was considered optimum to give conversion of epoxide to cyclic carbonate (>99%) in 12 h (entry 8). Although excellent results in terms of formation of cyclic carbonate were achieved with tetrabutylammonium bromide as an additive for this reaction, we tested the efficacy of other co-catalysts viz., tetrabutylammonium fluoride TBAF, tetrabutylammonium chloride TBAC, and tetrabutylammonium iodide TBAI (Table 1, entries 8, 13-15) as well, but none of these matched the performance of TBAB (Table 1, entry 8). Further, on conducting the catalytic reaction at higher pressure (10 atm) under the above optimised reaction condition, similar conversion ( >99%) was obtained in 8 h (entry 16) thereby improvement in TOF value.
Having established that the complex 2 is the most active catalyst at 1 mol% loading, in combination with TBAB (1 mol%) (Table 1, entry 8), next we used this catalyst for cycloaddition of various other aromatic/aliphatic aryloxy and aliphatic terminal epoxides as substrates viz., styrene oxide 7a, phenyl glycidyl ether 7b, 4-chlorophenyl glycidyl ether 7c, 4tert-butylphenyl glycidyl ether 7d, glycidyl 2-methylphenyl ether 7e, 2-benzyloxirane 7f, glycidyl isobutyl ether 7g, tertbutylglycidyl ether 7h, epichlorohydrin 7i, 1,2-epoxyhexane 7j, 1,2-epoxyoctane 7k, 1,2-epoxydecane 7l, 1,2-epoxydodecane 7m and 1,2-epoxy-5-hexene 7n under the above optimized reaction conditions and CO2 at around atmospheric pressure. The data in Table 2 revealed that un-substituted aromatic epoxides 7a, 7f and aryloxy epoxide 7b gave excellent conversion (>99%) to cyclic carbonate (entries 1, 2, 6) as compared to the substituted substrate molecules 7c-7e (conversion 90-95%) (entries 3-5). However, aliphatic epoxides 7g and 7h (entries 7, 8) gave only moderate conversions and among them sterically less demanding epoxide 7g bearing isobutyl group gave better results than tertiary butyl bearing group 7h. On conducting the cycloaddition reaction of terminal epoxides with CO2 where epichlorohydrin 7i (entry 9) gave excellent conversion to cyclic carbonate (98%) while for other terminal epoxides 7j-7m the increase in carbon chain length on the epoxide moiety favored the formation of cyclic carbonates (entries 10-13). However, for 1, 2-epoxy-5-hexene 7n the conversion to cyclic carbonate was moderate (62%) (entry 14). It is important to note that the results obtained with present protocol particularly in terms of catalyst loading and activity for similar substrate 7a are far better (TON 99, TOF 8.3 h-1) than other reported dissymmetric aluminium complex18 used as a catalyst which gave 41 TON and 0.85 h-1 TOF in
48 h at higher temperature (110 0C) in the presence of TBAB as co-catalyst in dichloromethane as solvent (entry 15).
The formation of cyclic carbonate was confirmed by recording stepwise FT-IR spectra (Figure 2) (a) initial reaction mass, which contained styrene oxide 7a, complex 2 and TBAB as cocatalyst and (b) after 12 h of reaction in presence of CO2 at 1 atmospheric pressure, where a new broad and strong band appeared at 1700 cm-1 inferring the formation of cyclic carbonate 7’a
3.1 Screening of catalyst 2 for its efficacy in other organic transformations Besides the cycloaddition reaction of CO2 with epoxides, attempts were made to explore the use of catalyst 2 (most active) in the hydrocyanation of 2-nitrovinylcyclohexene with TMSCN as cyanide source in presence of 4-phenyl pyridine N-oxide and achieved 40% yield of the respective product (Table 3, entry 1). The catalyst 2 was also tested for its utility in epoxide ring opening reaction of terminal aryloxy and cyclic epoxides with aniline as a representative nucleophile. Moderate to excellent conversion of β-aminoalcohol was achieved in 6 h (Table 3, entries 2,3).
3.1. Computational results The experimental results revealed that un-substituted aromatic epoxide 7a gave excellent conversion (>99%) to cyclic carbonate (entry 1 Table 2). However, moderate conversion (72%) was achieved with aliphatic epoxides 7j (entry 10) and the epoxide 7n the conversion to cyclic carbonate is less (62%) (entry 14 Table 2). We have performed the quantum chemical DFT calculations to examine the effect of substitutions in epoxide conversion to cyclic carbonate. The mechanistic studies for the conversion of epoxide to cyclic carbonate have been discussed
earlier with Al-catalyst.20 The Proposed catalytic cycle for the formation of cyclic carbonate with present catalyst is given in Scheme 2. For computational simplicity, we have modeled the substitution -CH2-O-Ph of catalyst 2 (Scheme 1) with hydrogen atom in all cases. The epoxide coordinate to the aluminium (III) present in catalyst to form complex A which is attacked by nucleophile Br- present in co-catalyst (n-Bu4N+Br-) to form intermediate B forming of C-Br bond and breaking of C-O bond takes place. Further CO2 reacts with B to form intermediate C, which undergoes ring closure reaction finally yields cyclic carbonate D followed by the regeneration of the catalyst and co-catalyst (Scheme 2).
We have employed the B3LYP/6-31G(d) level of theory to examine the formation of cyclic carbonate D. Reports to prepare cyclic carbonates from CO2 and epoxides using magnesium (II) as catalyst was also examined computationally using B3LYP/6-31G(d) level of theory.21g In this study, we have considered different substituted epoxide substrates (7a, 7j and 7n) to examine the role of substituents on the formation of the cyclic carbonate from corresponding epoxides. The catalysts and the substrate 7a first form a complex (R), which leads to a transition state (TS-1), where the Br- attacks the C2 carbon atom (E, Scheme 2) of the epoxide ring. The calculated activation barrier has been found to be 6.5kcal/mol. In this step the transitional state TS-1 has been located, where the C2-O1 bond is breaking and carbon-bromine bond is forming. The imaginary frequency associated with C2-O1 bond is -374.22 cm-1. The intermediate IN-1 goes to another intermediate IN-2 through the transition state (TS-2), where carbon-dioxide is inserted with the help of the catalyst. The calculated activation barrier was 4.2 kcal/mol. The ring closure takes place via transition state (TS-3) to form the product (P). The SN2 type reaction takes place in (TS-3), where the carbonate oxygen attacks to the carbon atom containing Br, which eliminates the Br- to regenerate the co-catalyst.
The next example considered was the epoxide substituted with n-butane instead of phenyl ring 7a using the same catalyst. The potential energy surface calculated with this substrate also
showed very similar trend as observed with the substrate 7a. The calculated results show that the ring closure step is the rate determining step in this case. The computed activation barrier is 20.1 kcal/ mol (Figure 4). The higher barrier for the formation of cyclic carbonate with 7j compared to 7a corroborates the lower activity observed in the former case. The role of the counter ion Br- seems to be important to govern the activation barrier with these substrates. The substituted n-butane chain is away from the leaving Br- ion in (TS-3), and hence the stabilization of this TS is not possible as observed with the substituted phenyl group (7a). The overall reaction is exergonic in nature. The potential energy surface calculated with 1-butene substituted to the epoxide ring 7n is similar to that of 7j. These results have suggested that the effect of substituents attached to the epoxide substrate can govern the activity with the catalyst used in this study. The counter ion (Br-) of the co-catalyst seems to be responsible to induce the change in the activity with the substrates examined here. 3.2. Computational Methods The ground state and transition state geometries were fully optimized using B3LYP/6-31G (d) level of theory.20g,22,23 The stationary points were characterized by frequency calculations in order to verify that the transition structures possess only one, imaginary frequency. The ground state geometries were characterized with no imaginary frequency. We have performed the intrinsic coordinate (IRC) analysis to verify that each saddle point links two desired minima. All calculations were performed with the Gaussian 09 suite of program.23 4. Conclusions This manuscript revealed a simple two step synthesis of Al(III) unsymmetrical β-aminoalcohol based complexes. These complexes showed excellent catalytic activity in the cycloaddition of epoxides with CO2 at atmospheric pressure. High conversion to cyclic carbonates was achieved for a wide range of substrates. The DFT calculations performed with B3LYP level of theory revealed the Br- ion of co-catalyst controlled the rate determining step in the formation of cyclic carbonates examined here. The calculated results were found to be in good agreement with the experimental results obtained in the formation of cyclic carbonate using substituted epoxides with CO2. The aluminium complex was also found to be moderately active in the hydrocyanation reaction of 2-nitrovinyl cyclohexene with TMSCN and quite active in the epoxide ring opening reaction with amine.
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19. (a) G. A. Luinstra, G. R. Haas, F Molnar, V Bernhart, R. Eberhardt, B. Rieger, Chem. Eur. J. 11, (2005), 6298-2678. (b) C. Beattie, M. North, P. Villuendas, C. Young, J. Org. Chem. 78, (2013), 419-426. 20. (a) J.-Q. Wang, K. Dong, W.-G. Cheng, J. Sun, S.-J., Zhang, Catal. Sci. Technol. 2, (2012), 1480–1484 (b) F. Chen, X. Li, B. Wang, T. Xu, Shi-Lu Chen, P. Liu, C. Hu, Chem. Eur. J. 18, (2012), 9870-9876; (c) S. Foltran, R. Mereau, T. Tassaing, Catal. Sci. Technol. 4, (2014), 1585-1597 21. (a) A. D. Becke, Phys. Rev. A 38, (1988), 3098-3100; (b) K. R. Roshan, A C. Kathalikkattil, J. Tharun, D. W. Kim, Y. S. Wonb, D. W. Park, Dalton Trans. 43, (2014), 2023-2031; (c) C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37, (1988), 785-789; (d) T. Ema, Y. Miyazaki, J. Shimonishi, C. Maeda, J.-Y. Hasegawa, J. Am. Chem. Soc. 136, (2014), 15270-15279; (e) A. Chen, T. Zhao, H. Gao, L. Chen, J. Chen, Y. Yu, Catal. Sci. Technol., DOI: 10.1039/c5cy01024a; (f) T. Ema, K. Fukuhara, T. Sakai, M. Ohbo, F.-Q. Baic, J-Y Hasegawa, Catal. Sci. Technol. 5, (2015), 2314-2321. 22. (a) Hehre, W. J. Radom, L. Schleyer, P.V.R. Pople, J. A. Ab Initio Molecular Orbital Theory, Wiley: New York, 1988. (b) A.-L. Girard, N. Simon, M. Zanatta, S. Marmitt, P. Gonçalves, J. Dupont, Green Chem. 16, (2014), 2815-2825. 23. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, et al., Gaussian09, Revision 401 B01, Gaussian, Inc., Wallingford, CT, 2010.
96
100
99 % Conversion 85 79
80
67
60 42 40 20 0
5
0 Co-catalyst
0
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
Catalyst
1
0
1
2
3
4
5
6
Entry
1
2
3
4
5
6
7
8
Figure 1 Reaction condition: Synthesis of styrene carbonate using complexes 1-6 Substrate 7a (10 mmol), Catalyst (1 mol%), Co-catalyst TBAB (1 mol%), 1 atm CO2 pressure, Conversion was determined by 1H NMR19
30
Initial reaction mass (a) After 12 hour of reaction (b)
T%
25
20
15
10
2500
2000
1500
wave number (cm
1000 -1
500
)
. Figure2. FT-IR Spectra of (a) initial reaction mass of styrene oxide and (b) after 12 h
Figure 3.The potential energy profile for production of cyclic carbonate from phenyl substituted epoxide 7a and carbon dioxide at the B3LYP/6-31G (d) level. The relative free energy difference is given in Kcal/mol.
Figure 4. The potential energy profile for production of cyclic carbonate from n-butane substituted epoxide (7j) and carbon dioxide at the B3LYP/6-31G(d) level. The relative free energy difference is given in Kcal/mol.
20.1 11.6
6.8
+ Figure 5.The potential energy profile for production of cyclic carbonate from 1-butene substituted epoxide (7n) and carbon dioxide at the B3LYP/6-31G (d) level. The relative free energy difference is given in Kcal/mol.
SCHEME 1 Synthesis of unsymmetrical Al Complexes 1-6
Scheme 2. The Proposed mechanistic scheme for conversion of epoxide and CO2 into cyclic carbonate using catalyst and co-catalyst.
Table 1 Optimization of reaction conditions for the synthesis of styrene carbonate 7’a using complex 2
Catalyst Co(mol %) catalyst (mol %)
Cocatalyst
Temp. (0C)
bConv.
1
0.25
0.25
TBAB
90
20
80
6.7
2
0.50
0.25
TBAB
90
52
104
8.7
3
0.75
0.25
TBAB
90
69
92
7.7
4
1
0.25
TBAB
90
78
78
6.5
5
1.25
0.25
TBAB
90
74
59
4.9
6
1
0.50
TBAB
90
82
82
6.8
7
1
0.75
TBAB
90
88
88
7.3
8
1
1
TBAB
90
96
96
8
9
1
1.25
TBAB
90
90
72
6
10
1
1
TBAB
30
23
23
1.9
11
1
1
TBAB
60
55
55
4.6
12
1
1
TBAB
120
96
96
8
13
1
1
TBAF
90
50
50
4.2
14
1
1
TBAC
90
71
71
5.9
15
1
1
TBAI
90
89
89
7.4
Entry
cTON
dTOF
(h-1)
(%)
f
16 1 1 TBAB 90 99 99 12.4 b Reaction condition: Substrate 7a (10 mmol), Catalyst 2, 1 atm CO2 pressure, Conversion was determined by 1H NMR,19 cTON was determined by [mol of product]/[mol of catalyst], dTOF was determined by ([mol of product]/[mol of catalyst])/ [Time] unit h-1, fReaction conducted at 10 atm CO2 pressure for 8 h
Table 2.Scope of various epoxides for catalytic cycloaddition reaction with complex 2
Entry
Substrate
Conv. b (%)
TONc
TOFd
990.57
99
8.25
990.57
99
8.25
950.57
95
7.92
901
90
7.50
931
93
7.75
991.52
99
8.25
500.57
50
4.17
1
7a 2
7b 3
7C 4
7d 5
7e 6 7f 7 7g
8 441
44
3.67
980.57
98
8.17
721
72
6.00
841
84
7.00
881
88
7.34
980.57
98
8.17
621
62
5.17
90
41
0.85
7h 9 7i 10 7j 11
7k 12
7l 13 7m 14 7n 15e
7a Reaction condition: Substrates 7a-n (10 mmol), Catalyst 2 (1 mol%), TBAB as co-catalyst (1 mol%), 1 atm CO2 pressure, bConversion
was determined from 1H NMR,19 cTON was
determined by [mol of product]/[mol of catalyst],
d
TOF was determined by ([mol of
product]/[mol of catalyst]) / [Time] unit h-1, 19 eThe reaction was conducted with the optimal catalyst from reference18
Table 3 Data for screening of catalyst 2 for hydrocyanation and epoxide ring opening reaction
Entry
Substrate
Temp.
Time
cYield
(h) a
1
RT
9
40
b
2
RT
6
99
b
3
RT
6
80
Condition; aThe hydrocyanation reaction of (2-nitrovinyl) cyclohexane (0.16 mmol) was performed With catalyst 2, 9 h and 4-PPNO (10 mol%) in toluene using TMSCN (0.25 mmol) as a source of the cyanide, bthe ring opening of epoxides (glycidyl isobutyl ether and cyclopentene oxide) (2 mmol), aniline (2.2 mmol), catalyst 2 (1.5 mol %), solvent free condition, cIsolated yield
24.
S1381-1169(16)30086-3 http://dx.doi.org/doi:10.1016/j.molcata.2016.03.018 MOLCAA 9817
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date: Revised date: Accepted date:
3-1-2016 26-2-2016 5-3-2016
Please cite this article as: Shailesh Verma, Mrinal Kanti Si, Rukhsana I.Kureshy, Mohd Nazish, Manish Kumar, Noor-ul H.Khan, Sayed H.R.Abdi, Hari C.Bajaj, Bishwajit Ganguly, Chemical fixation of CO2 to cyclic carbonates using Al(III) rmbeta-aminoalcohol based efficient catalysts: An experimental and computational studies, Journal of Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.03.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Chemical fixation of CO2 to cyclic carbonates using Al(III) βaminoalcohol based efficient catalysts: An experimental and computational studies
Shailesh Verma,a,b Mrinal Kanti Si,b, Rukhsana I. Kureshy,*a,b c Mohd Nazish,a,b Manish Kumar, a,b Noor-ul H. Khan,a,b Sayed H. R. Abdi,a,b Hari C. Bajaj,a,b Bishwajit Ganguly*b,c
a
Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research
Institute (CSMCRI), Bhavnagar- 364 002, Gujarat, India. Fax: +91-0278-2566970; E-mail: [email protected]. b
Academy of Scientific and Innovative Research (AcSIR), Central Salt and Marine Chemicals
Research Institute (CSMCRI), Council of Scientific & Industrial Research (CSIR), G. B. Marg, Bhavnagar- 364 002, Gujarat, India. c
Discipline Computation and Simulation Unit (ADACIF), CSIR-CSMCRI, ), G. B. Marg,
Bhavnagar- 364 002, Gujarat, India. E-mail: [email protected]
* Corresponding author
Graphical abstract
Highlights
Al (III) β-aminoalcohol based complexes as active catalysts.
Cycloaddition reaction of CO2 and epoxides to give commercially important cyclic carbonates.
With the help of DFT calculations the role of co-catalyst was established.
Abstract A series of Al(III) unsymmetrical β-aminoalcohol based complexes 1-6 were synthesized via metalation of the corresponding ligands 1’-6’ those were prepared by the reaction of benzylamine with readily available epoxides viz., styrene oxide, 1,2-epoxy-3-phenoxypropane, 4-tertbutylphenyl glycidyl ether, 4-chlorophenyl glycidyl ether, glycidyl 2-methylphenyl ether and 1,2-epoxyhexane. Among these complexes, the complex 2 was found to be most effective in the cycloaddition of aryloxy/aliphatic terminal epoxides with CO2 under atmospheric pressure to get corresponding cyclic carbonates with high conversion and selectivity (up to >99%) in the presence of tetrabutylammonium bromide as a co-catalyst. The DFT calculations revealed the important role played by counter-ion in the co-catalyst during cycloaddition reaction of CO2 with the substituted epoxides.
Keywords: Cyclic carbonates; Al(III) β-aminoalcohol complexes; β-aminoalcohol; Gaussian 09; benzyl amine.
1. Introduction The chemistry of carbon dioxide (CO2) utilization has gathered significant attention not only because CO2 is one of the potent greenhouse gases, but also as an inexpensive, abundant, non-toxic, non-flammable and bio-renewable C1 resource.1 However, thermodynamic stability of CO2 (∆H0f = -394 kJ mol-1) poses a major hurdle in its utilization in chemical transformations. Nevertheless, this barrier has been crossed with the advent of efficient catalytic systems required to convert CO 2 into valuable chemicals.2 The reaction of CO2 with epoxide is one of the most studied reactions, and depending upon the reaction condition, the choice of substrate epoxide and catalyst one can produce polymeric carbonates or cyclic carbonates.3 These organic carbonates are having several industrial applications as polar aprotic solvents, engineered plastics, masked diols and as one of the constituents in lithium ion batteries.4 A number of active catalysts/initiators, such as quaternary ammonium/phosphonium salts,5 metal complexes,6 phthalocyanine,7 ionic liquids,8 metal oxides9 and immobilized molecular catalysts10 have been reported to impart moderate to good activities in cycloaddition reaction of CO2 with epoxides for the production of cyclic carbonates. Among several metal complexes, salen ligands in combination with transition metal ions such as chromium,11 cobalt,12 magnesium13 and zinc14 have shown their worth in the synthesis of cyclic carbonates. In particular, aluminium salen complexes15 have received good attention in recent times, largely due to less environmental impact of aluminium, high earth abundance and significantly high catalytic activity. Recently, some binary16 and bimetallic17 symmetric aluminium-salen complexes have produced interesting results, but the use of unsymmetrical aluminium complexes for this reaction has not been explored much. Recently, Styring et al.18 reported unsymmetrical aluminium salen complex for the synthesis of styrene carbonate with fairly good success. To take this concept forward and to make catalyst synthetic protocol simpler herein, we report unsymmetrical aluminium complexes derived from readily accessible amino-alcohol based ligands to catalyse cycloaddition of epoxides to CO2 at atmospheric pressure. The most striking feature of the present ligand system lies in its single step synthesis. These ligands can be conveniently synthesized from the ring opening of easily available epoxides with a nucleophile e.g. benzylamine. Coincidently, the catalysts developed for the present study are derived from the epoxides those are used as substrates as well. Further, it is worthwhile to mention that the present system (as in most previously reported systems) role of a suitable additive/co-catalyst is crucial. Among several additives screened in
the present system, tetra butyl ammonium bromide (TBAB) was found to be the best. To gain insight into the reaction mechanism for the formation of cyclic carbonates with substituted epoxides and CO2, we performed DFT calculations using B3LYP/6-31G* level of theory.20g The calculated results showed the role of counter ion (Br-) to stabilize the phenyl substituted epoxide transition state compared to the other substituted epoxides. Such a stabilization of the transition state helps to reduce the activation barrier in phenyl substituted epoxide. 2. Experimental Section 2.1. Methods and Materials Diethylaluminum chloride, benzylamine and epoxides viz., styrene oxide 7a, phenyl glycidyl ether 7b, 4-chlorophenyl glycidyl ether 7c, 4-tert-butylphenyl glycidyl ether 7d, glycidyl 2methylphenyl ether 7e, 2-benzyloxirane 7f, glycidyl isobutyl ether 7g, tertbutylglycidyl ether 7h, epichlorohydrin 7i, 1,2-epoxyhexane 7j, 1,2-epoxyoctane 7k, 1,2-epoxydecane 7l, 1,2epoxydodecane 7m and 1,2-epoxy-5-hexene 7n were purchased from Aldrich Chemicals and were used as received. All the solvents mentioned in the current study were purified by known techniques before use. Microanalysis of the ligands and catalysts was carried out on a Perkin Elmer 2400 CHNS. 1H,
13
C&
27
Al NMR spectra were recorded on Bruker 200 MHz or 500
MHz spectrometer at ambient temperature using CDCl3 and CD3OD as solvent and TMS as internal standard. Yields were determined by comparing the peak area ratio of product to substrate in the 1H NMR spectrum of an aliquot of the crude reaction mixture. FTIR spectra were recorded on a Perkin Elmer Spectrum GX spectrophotometer as KBr pellet. High resolution mass spectra were obtained with a LC-MS (Q-TOFF), Model make Ultra flex TOF/TOF, Burker Daltonics, Germany instruments. For product purification, flash chromatography was performed using silica gel of 60-200 mesh size procured from SD FineChemicals Limited, Mumbai (India).
2.2. General procedure for the synthesis of β-aminoalcohol ligands 1’-6’ A solution of an appropriate epoxide (2 mmol) and benzyl amine (2 mmol) were taken in a 5 mL reaction vial to which silica gel (20 mg) was added and the reaction mixture was stirred
with the help of magnetic bar at room temperature for 4 to 6 h. The resulting mass was diluted with 20% of ethyl acetate in hexane, filtered and cooled to -25 OC. A white solid precipitated out was filtered and the solid thus obtained was dried under vacuum and used as such without further purification. 2.3. General procedure for the synthesis of aluminium β-aminoalcohol complexes 1-6 In a flame dried 50 mL RBF, the above synthesized ligands 1’-6’ (2.0 mmol) were taken in dichloromethane (20 mL) under nitrogen atmosphere to which a solution of diethylaluminium chloride (0.9 M) in toluene (2.2 mmol, 2.4 mL) was added slowly at 0 OC over 30 minutes and the resulting solution was stirred at room temperature for 7 h. The completion of the reaction was checked on TLC. After completion of the reaction the solvent was removed on a rotavapour and a white solid thus obtained was dissolved in methanol and passed through celite pad for removing excess of aluminium. On removal of methanol on rota-vapour, the desired complexes 1-6 were obtained as white solid in quantitative yields. These complexes were characterised by 1H, 13C, 27Al NMR, microanalysis, LCMS and HRMS. 2.4. Typical procedure for synthesis of cyclic carbonates from epoxides and CO2 using Al(III) β-aminoalcohol complexes The cycloaddition reactions were carried out in a dry 5 mL reaction vial equipped with a magnetic bar on an automated stirrer with heating mantle. The reaction vial was charged with appropriate amount of homogeneous catalyst 1-6 (1.0 mol%) and TBAB (1.0 mol%) as cocatalyst and closed with a septum. An appropriate epoxide 7a-n (10 mmol) was introduced to this vial with the help of a syringe. The vial was purged with nitrogen followed by CO2 and the reaction mixture was allowed to stir at a specified temperature for a specified time at around atmosphere pressure of CO2. The conversion and selectivity of the product was determined by 1
H NMR by taking an aliquot from the reaction mass at a regular interval.19
3. Results and discussion We have started our investigation with the synthesis of amino-alcohol ligands 1’-6’ by the ring opening reaction of several aromatic, aryloxy and aliphatic epoxides with benzyl amine (Scheme 1). After due characterization with suitable physico-chemical techniques (data is
given in supporting information), these ligands (1’-6’) were complexed with diethyl aluminium chloride in dichloromethane to provide unsymmetrical aluminium complexes 1-6 (Scheme 1).
Aluminium complexes 1-6 were suitably characterized and to start with complex 1 (1 mol %) was used as catalyst in the cycloaddition of CO2 to styrene epoxide 7a taken as a representative substrate at ~1 atmospheric pressure (balloon pressure) and data are given in Figure 1. However, the complex 1 (Scheme 1) alone was found to be ineffective in catalyzing the cycloaddition
reaction
(Figure
1,
entry
1),
tetrabutylammonium bromide (TBAB, 1 mol%)
however,
addition
of
a
co-catalyst
made a significant improvement in the
reaction outcome (Figure 1, entry 3).
It is noteworthy to mention that TBAB alone has given the product styrene carbonate 7’a in trace amounts (~5%) under identical reaction condition (Figure 1, entry 2), which confirms that both aluminium complex and the co-catalyst is essential for the reaction. Hence, in our subsequent catalytic reactions to evaluate the efficacies of remaining complexes 2–6 (Scheme 1) in the cycloaddition of CO2 to styrene epoxide we maintained the above mentioned reaction condition (Figure 1, entries 4-8). Among these, complex 2 (derived from phenyl glycidyl ether) (Scheme 1) has shown the best performance in the cycloaddition of styrene oxide to give the desired product 7’a in 99% yield (entry 4). Any substituent in the phenyl group of the catalyst, as in representative examples 3-5 was detrimental to the catalytic-performance and gave the product 7’a in relatively lower yields (Figure 1, entries 5-7, yields, 67-87%). Further, the unsymmetrical aluminium complex 6 generated from aliphatic epoxide e.g., 1,2-epoxyhexane was found to be less reactive for this reaction and gave the product 7’a in poor yield (42%, Figure 1 entry 8). As complex 2 was found to be the best among these (entry 4), it was considered for the optimization of other reaction parameters viz. catalyst loading, co-catalyst loading, and temperature variation in the cycloaddition reaction of CO2 with styrene oxide 7a as a representative substrate. The catalyst loading was varied over 0.25 mol% to 1.25 mol% (Table 1, entries 1-5) for this reaction with a co-catalyst loading of 0.25 mol% and found that 1 mol% of the catalyst loading is optimum (Table 1, entry 4). Next, we varied co-catalyst loading from 0.25 mol% to 1.25 mol% with optimized catalyst loading of 1 mol% (Table 1, entries 4, 6-8) and observed an increase in the formation of cyclic carbonate with increase in co-catalyst loading. Among these results 1 mol% catalyst and 1 mol% co-catalyst loading was
found to be optimum (Table 1, entry 8). Thereafter, the effect of temperature on the performance of this protocol was evaluated over a temperature range of 30–120 0C (Table 1, entries 8, 10-12), where 90 0C was considered optimum to give conversion of epoxide to cyclic carbonate (>99%) in 12 h (entry 8). Although excellent results in terms of formation of cyclic carbonate were achieved with tetrabutylammonium bromide as an additive for this reaction, we tested the efficacy of other co-catalysts viz., tetrabutylammonium fluoride TBAF, tetrabutylammonium chloride TBAC, and tetrabutylammonium iodide TBAI (Table 1, entries 8, 13-15) as well, but none of these matched the performance of TBAB (Table 1, entry 8). Further, on conducting the catalytic reaction at higher pressure (10 atm) under the above optimised reaction condition, similar conversion ( >99%) was obtained in 8 h (entry 16) thereby improvement in TOF value.
Having established that the complex 2 is the most active catalyst at 1 mol% loading, in combination with TBAB (1 mol%) (Table 1, entry 8), next we used this catalyst for cycloaddition of various other aromatic/aliphatic aryloxy and aliphatic terminal epoxides as substrates viz., styrene oxide 7a, phenyl glycidyl ether 7b, 4-chlorophenyl glycidyl ether 7c, 4tert-butylphenyl glycidyl ether 7d, glycidyl 2-methylphenyl ether 7e, 2-benzyloxirane 7f, glycidyl isobutyl ether 7g, tertbutylglycidyl ether 7h, epichlorohydrin 7i, 1,2-epoxyhexane 7j, 1,2-epoxyoctane 7k, 1,2-epoxydecane 7l, 1,2-epoxydodecane 7m and 1,2-epoxy-5-hexene 7n under the above optimized reaction conditions and CO2 at around atmospheric pressure. The data in Table 2 revealed that un-substituted aromatic epoxides 7a, 7f and aryloxy epoxide 7b gave excellent conversion (>99%) to cyclic carbonate (entries 1, 2, 6) as compared to the substituted substrate molecules 7c-7e (conversion 90-95%) (entries 3-5). However, aliphatic epoxides 7g and 7h (entries 7, 8) gave only moderate conversions and among them sterically less demanding epoxide 7g bearing isobutyl group gave better results than tertiary butyl bearing group 7h. On conducting the cycloaddition reaction of terminal epoxides with CO2 where epichlorohydrin 7i (entry 9) gave excellent conversion to cyclic carbonate (98%) while for other terminal epoxides 7j-7m the increase in carbon chain length on the epoxide moiety favored the formation of cyclic carbonates (entries 10-13). However, for 1, 2-epoxy-5-hexene 7n the conversion to cyclic carbonate was moderate (62%) (entry 14). It is important to note that the results obtained with present protocol particularly in terms of catalyst loading and activity for similar substrate 7a are far better (TON 99, TOF 8.3 h-1) than other reported dissymmetric aluminium complex18 used as a catalyst which gave 41 TON and 0.85 h-1 TOF in
48 h at higher temperature (110 0C) in the presence of TBAB as co-catalyst in dichloromethane as solvent (entry 15).
The formation of cyclic carbonate was confirmed by recording stepwise FT-IR spectra (Figure 2) (a) initial reaction mass, which contained styrene oxide 7a, complex 2 and TBAB as cocatalyst and (b) after 12 h of reaction in presence of CO2 at 1 atmospheric pressure, where a new broad and strong band appeared at 1700 cm-1 inferring the formation of cyclic carbonate 7’a
3.1 Screening of catalyst 2 for its efficacy in other organic transformations Besides the cycloaddition reaction of CO2 with epoxides, attempts were made to explore the use of catalyst 2 (most active) in the hydrocyanation of 2-nitrovinylcyclohexene with TMSCN as cyanide source in presence of 4-phenyl pyridine N-oxide and achieved 40% yield of the respective product (Table 3, entry 1). The catalyst 2 was also tested for its utility in epoxide ring opening reaction of terminal aryloxy and cyclic epoxides with aniline as a representative nucleophile. Moderate to excellent conversion of β-aminoalcohol was achieved in 6 h (Table 3, entries 2,3).
3.1. Computational results The experimental results revealed that un-substituted aromatic epoxide 7a gave excellent conversion (>99%) to cyclic carbonate (entry 1 Table 2). However, moderate conversion (72%) was achieved with aliphatic epoxides 7j (entry 10) and the epoxide 7n the conversion to cyclic carbonate is less (62%) (entry 14 Table 2). We have performed the quantum chemical DFT calculations to examine the effect of substitutions in epoxide conversion to cyclic carbonate. The mechanistic studies for the conversion of epoxide to cyclic carbonate have been discussed
earlier with Al-catalyst.20 The Proposed catalytic cycle for the formation of cyclic carbonate with present catalyst is given in Scheme 2. For computational simplicity, we have modeled the substitution -CH2-O-Ph of catalyst 2 (Scheme 1) with hydrogen atom in all cases. The epoxide coordinate to the aluminium (III) present in catalyst to form complex A which is attacked by nucleophile Br- present in co-catalyst (n-Bu4N+Br-) to form intermediate B forming of C-Br bond and breaking of C-O bond takes place. Further CO2 reacts with B to form intermediate C, which undergoes ring closure reaction finally yields cyclic carbonate D followed by the regeneration of the catalyst and co-catalyst (Scheme 2).
We have employed the B3LYP/6-31G(d) level of theory to examine the formation of cyclic carbonate D. Reports to prepare cyclic carbonates from CO2 and epoxides using magnesium (II) as catalyst was also examined computationally using B3LYP/6-31G(d) level of theory.21g In this study, we have considered different substituted epoxide substrates (7a, 7j and 7n) to examine the role of substituents on the formation of the cyclic carbonate from corresponding epoxides. The catalysts and the substrate 7a first form a complex (R), which leads to a transition state (TS-1), where the Br- attacks the C2 carbon atom (E, Scheme 2) of the epoxide ring. The calculated activation barrier has been found to be 6.5kcal/mol. In this step the transitional state TS-1 has been located, where the C2-O1 bond is breaking and carbon-bromine bond is forming. The imaginary frequency associated with C2-O1 bond is -374.22 cm-1. The intermediate IN-1 goes to another intermediate IN-2 through the transition state (TS-2), where carbon-dioxide is inserted with the help of the catalyst. The calculated activation barrier was 4.2 kcal/mol. The ring closure takes place via transition state (TS-3) to form the product (P). The SN2 type reaction takes place in (TS-3), where the carbonate oxygen attacks to the carbon atom containing Br, which eliminates the Br- to regenerate the co-catalyst.
The next example considered was the epoxide substituted with n-butane instead of phenyl ring 7a using the same catalyst. The potential energy surface calculated with this substrate also
showed very similar trend as observed with the substrate 7a. The calculated results show that the ring closure step is the rate determining step in this case. The computed activation barrier is 20.1 kcal/ mol (Figure 4). The higher barrier for the formation of cyclic carbonate with 7j compared to 7a corroborates the lower activity observed in the former case. The role of the counter ion Br- seems to be important to govern the activation barrier with these substrates. The substituted n-butane chain is away from the leaving Br- ion in (TS-3), and hence the stabilization of this TS is not possible as observed with the substituted phenyl group (7a). The overall reaction is exergonic in nature. The potential energy surface calculated with 1-butene substituted to the epoxide ring 7n is similar to that of 7j. These results have suggested that the effect of substituents attached to the epoxide substrate can govern the activity with the catalyst used in this study. The counter ion (Br-) of the co-catalyst seems to be responsible to induce the change in the activity with the substrates examined here. 3.2. Computational Methods The ground state and transition state geometries were fully optimized using B3LYP/6-31G (d) level of theory.20g,22,23 The stationary points were characterized by frequency calculations in order to verify that the transition structures possess only one, imaginary frequency. The ground state geometries were characterized with no imaginary frequency. We have performed the intrinsic coordinate (IRC) analysis to verify that each saddle point links two desired minima. All calculations were performed with the Gaussian 09 suite of program.23 4. Conclusions This manuscript revealed a simple two step synthesis of Al(III) unsymmetrical β-aminoalcohol based complexes. These complexes showed excellent catalytic activity in the cycloaddition of epoxides with CO2 at atmospheric pressure. High conversion to cyclic carbonates was achieved for a wide range of substrates. The DFT calculations performed with B3LYP level of theory revealed the Br- ion of co-catalyst controlled the rate determining step in the formation of cyclic carbonates examined here. The calculated results were found to be in good agreement with the experimental results obtained in the formation of cyclic carbonate using substituted epoxides with CO2. The aluminium complex was also found to be moderately active in the hydrocyanation reaction of 2-nitrovinyl cyclohexene with TMSCN and quite active in the epoxide ring opening reaction with amine.
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96
100
99 % Conversion 85 79
80
67
60 42 40 20 0
5
0 Co-catalyst
0
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
Catalyst
1
0
1
2
3
4
5
6
Entry
1
2
3
4
5
6
7
8
Figure 1 Reaction condition: Synthesis of styrene carbonate using complexes 1-6 Substrate 7a (10 mmol), Catalyst (1 mol%), Co-catalyst TBAB (1 mol%), 1 atm CO2 pressure, Conversion was determined by 1H NMR19
30
Initial reaction mass (a) After 12 hour of reaction (b)
T%
25
20
15
10
2500
2000
1500
wave number (cm
1000 -1
500
)
. Figure2. FT-IR Spectra of (a) initial reaction mass of styrene oxide and (b) after 12 h
Figure 3.The potential energy profile for production of cyclic carbonate from phenyl substituted epoxide 7a and carbon dioxide at the B3LYP/6-31G (d) level. The relative free energy difference is given in Kcal/mol.
Figure 4. The potential energy profile for production of cyclic carbonate from n-butane substituted epoxide (7j) and carbon dioxide at the B3LYP/6-31G(d) level. The relative free energy difference is given in Kcal/mol.
20.1 11.6
6.8
+ Figure 5.The potential energy profile for production of cyclic carbonate from 1-butene substituted epoxide (7n) and carbon dioxide at the B3LYP/6-31G (d) level. The relative free energy difference is given in Kcal/mol.
SCHEME 1 Synthesis of unsymmetrical Al Complexes 1-6
Scheme 2. The Proposed mechanistic scheme for conversion of epoxide and CO2 into cyclic carbonate using catalyst and co-catalyst.
Table 1 Optimization of reaction conditions for the synthesis of styrene carbonate 7’a using complex 2
Catalyst Co(mol %) catalyst (mol %)
Cocatalyst
Temp. (0C)
bConv.
1
0.25
0.25
TBAB
90
20
80
6.7
2
0.50
0.25
TBAB
90
52
104
8.7
3
0.75
0.25
TBAB
90
69
92
7.7
4
1
0.25
TBAB
90
78
78
6.5
5
1.25
0.25
TBAB
90
74
59
4.9
6
1
0.50
TBAB
90
82
82
6.8
7
1
0.75
TBAB
90
88
88
7.3
8
1
1
TBAB
90
96
96
8
9
1
1.25
TBAB
90
90
72
6
10
1
1
TBAB
30
23
23
1.9
11
1
1
TBAB
60
55
55
4.6
12
1
1
TBAB
120
96
96
8
13
1
1
TBAF
90
50
50
4.2
14
1
1
TBAC
90
71
71
5.9
15
1
1
TBAI
90
89
89
7.4
Entry
cTON
dTOF
(h-1)
(%)
f
16 1 1 TBAB 90 99 99 12.4 b Reaction condition: Substrate 7a (10 mmol), Catalyst 2, 1 atm CO2 pressure, Conversion was determined by 1H NMR,19 cTON was determined by [mol of product]/[mol of catalyst], dTOF was determined by ([mol of product]/[mol of catalyst])/ [Time] unit h-1, fReaction conducted at 10 atm CO2 pressure for 8 h
Table 2.Scope of various epoxides for catalytic cycloaddition reaction with complex 2
Entry
Substrate
Conv. b (%)
TONc
TOFd
990.57
99
8.25
990.57
99
8.25
950.57
95
7.92
901
90
7.50
931
93
7.75
991.52
99
8.25
500.57
50
4.17
1
7a 2
7b 3
7C 4
7d 5
7e 6 7f 7 7g
8 441
44
3.67
980.57
98
8.17
721
72
6.00
841
84
7.00
881
88
7.34
980.57
98
8.17
621
62
5.17
90
41
0.85
7h 9 7i 10 7j 11
7k 12
7l 13 7m 14 7n 15e
7a Reaction condition: Substrates 7a-n (10 mmol), Catalyst 2 (1 mol%), TBAB as co-catalyst (1 mol%), 1 atm CO2 pressure, bConversion
was determined from 1H NMR,19 cTON was
determined by [mol of product]/[mol of catalyst],
d
TOF was determined by ([mol of
product]/[mol of catalyst]) / [Time] unit h-1, 19 eThe reaction was conducted with the optimal catalyst from reference18
Table 3 Data for screening of catalyst 2 for hydrocyanation and epoxide ring opening reaction
Entry
Substrate
Temp.
Time
cYield
(h) a
1
RT
9
40
b
2
RT
6
99
b
3
RT
6
80
Condition; aThe hydrocyanation reaction of (2-nitrovinyl) cyclohexane (0.16 mmol) was performed With catalyst 2, 9 h and 4-PPNO (10 mol%) in toluene using TMSCN (0.25 mmol) as a source of the cyanide, bthe ring opening of epoxides (glycidyl isobutyl ether and cyclopentene oxide) (2 mmol), aniline (2.2 mmol), catalyst 2 (1.5 mol %), solvent free condition, cIsolated yield
24.