so immaculate in environmental context [2]. Among few reactions that can be used for designing non isocyanate polyurethanes, the reaction of cyclo carbonates ...
ISSN 00125008, Doklady Chemistry, 2011, Vol. 441, Part 2, pp. 355–360. © Pleiades Publishing, Ltd., 2011. Original Russian Text © M.V. Zabalov, R.P. Tiger, A.A. Berlin, 2011, published in Doklady Akademii Nauk, 2011, Vol. 441, No. 4, pp. 480–484.
CHEMISTRY
Reaction of Cyclocarbonates with Amines as an Alternative Route to Polyurethanes: A QuantumChemical Study of Reaction Mechanism M. V. Zabalov, R. P. Tiger, and Academician A. A. Berlin Received June 30, 2011
DOI: 10.1134/S0012500811120032
The common method of preparation of polyure thanes is based on the reaction of terminal groups of hydroxycontaining oligomers with di or polyisocy anates [1]. Isocyanates are rather toxic compounds and their preparation with the use of phosgene is not so immaculate in environmental context [2].
in amine when the reaction proceeds with its excess. The latter feature indirectly indicates the possibility of catalytic assistance of the second amine molecule in the opening of the cyclocarbonate ring, although there is no rigorous proof for this assumption.
Among few reactions that can be used for designing nonisocyanate polyurethanes, the reaction of cyclo carbonates with primary amines seems to be rather promising. Oligomers with cyclocarbonate groups are obtained from epoxide or hydroxycontaining precur sors [3], and the reaction of cyclocarbonate groups with amines leads to hydroxyurethanes:
In this work, we have studied the mechanism of addition of amino group to cyclocarbonate ring in the reaction of ethylene carbonate with methylamine as an example by means of quantumchemical calcula tions.
~ HC
CH2 O + H2N~
O
OH ~ HC O CH2
NH~ O
HO
O
CH2 O ~ HC O NH ~
Primary or secondary hydroxy groups at the ure thane function favor the hydrolytic stability of poly urethanes due to intra and intermolecular hydro gen bonding and can be also used for the target modification of these polymers and some epoxy res ins [4]. The mechanism of the opening of the cyclocarbon ate ring under the action of amino groups has not been studied so far. Few kinetic data available in the litera ture [5, 6] indicate the first order of the reaction in the reactants at their equimolar ratio and the second order
Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 119991 Russia
The calculations were performed by the density functional theory (DFT) method using the nonempir ically generalized gradient approximation and the PBE functional [7, 8] with the TZ2P basis set by means of PRIRODA software [9, 10]. Geometry was optimized for all initial reactants, stable intermedi ates, and transition states. The character of revealed stationary points (minimum or saddle point on the potential energy surface (PES)) was determined by computing the eigenvalues of the matrix of second derivatives of energy with respect to nucleus coordi nates. The reaction coordinate was calculated to decide whether transition states were involved in a given transformation. The relative energy values were corrected for zero point energy. STRUCTURE OF PROBABLE PRODUCTS OF METHYLAMINE ADDITION TO ETHYLENE CARBONATE Ten minima were found on the PES for the prod ucts of methylamine addition to ethylene carbonate that correspond to four cyclic isomers 1a–1d with intramolecular hydrogen bond and six open conform ers 1e–1j (Scheme 1).
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O N O 0.984
H
O
Me 1.013
H
O
N O 0.985
1.823
H
O 0.970
Me
O
H
1.816
1a
1b
1.934
H 1.021N
O O 0.979
Me
O
O
HO
O
N Me
O
N H
H
O 1e
Me 1g
1f
H N Me
1.017
H 2.061N H Me 1d
O HO
N Me
HO
O
1c
H HO
O
O
H 1.012
HO
HO O
O
O
O
O N Me
O 1h
1i
H
N H 1j Me
Scheme 1.
Cyclic isomer 1a with hydrogen bond at the carbo nyl oxygen is most stable. The O–H bond is elongated by ~10% on account of the Н … О=С– bond (l = 1.82 Å). Isomer 1b differs from 1a only in the arrange ment of substituents at the nitrogen atom relative to the rest of the molecule. The difference in energy between 1a and 1b is not significant being 1 kcal/mol. The other isomers with hydrogen bonds (1c, 1d) and the acyclic conformers without hydrogen bonds (1e– 1j) are less stable. One of the acyclic products (1g) is Total electron energy (E) for the initial compounds and the products of the reaction of ethylene carbonate with methy lamine and their relative stability (ΔE) –E, au
ΔE, kcal/mol
1a
437.961974
0
1b
437.960364
1.0
1c
437.950356
7.3
1d
437.950102
7.4
1e
437.949448
7.9
1f
437.955318
10.5
1g
437.958858
2.0
1h
437.945288
4.2
1i
437.956873
3.2
1j
437.953495
5.3
Methylamine
342.168688
–
95.769226
–
Compound
Ethylene carbonate
only 2 kcal/mol less stable than 1a. The table shows that the largest difference between the conformer energies is 10.5 kcal/mol. The heat of reaction (ΔH) calculated as the differ ence between the energies of the initial reactants and product 1а, corresponding to the global minimum on the PES, is –15.1 kcal/mol. MECHANISM OF THE REACTION OF METHYLAMINE ADDITION TO ETHYLENE CARBONATE It is known from experiment [5, 6] that the reaction of cyclocarbonate with amine can have both the first and second order in amine. It is reasonable to assume that a second amine molecule can participate in the process by the formation of Hbonded associates and behave as a catalyst by facilitating proton transfer from amine to cyclocarbonate through cyclic transition states, as occurs in the reactions of aminolysis, hydrol ysis, and alcoholysis of carbonylcontaining com pounds [11, 12]. We considered the reactions with both one and two amine molecules. Two reaction routes were revealed in both cases. Route 1: onestage. For the onestage route, we revealed (Scheme 2) two cyclic transition states (TS): a fourmembered state involving one amine molecule (TS1a, TS1b) and a sixmembered state involving two amine molecules (TS2a, TS2b). DOKLADY CHEMISTRY
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REACTION OF CYCLOCARBONATES WITH AMINES
O
O H
O
O H
O
N Me H TS1a (28.3)
N
Me
O H O H N N OH Me H H Me TS2a (10.8)
O H
TS1b (29.7)
357
O H O H N N OH Me H H Me TS2b (11.0)
O + MeNH 2
O
O
O
NHMe O H
O 1a
Scheme 2.
Scheme 2 shows parenthetically the activation energies (in kcal/mol) for the reactions that proceed through the corresponding transition states. Isomer ism with different orientation of methylamine with respect to the fivemembered ring is possible in such transition states. The formation of four isomers is pos sible theoretically for TS1 and TS2 taking into account the relative disposition of the carbonyl oxy gen. However, only two of the four isomers occur for each type, while the other isomers somewhat change and transform into transition states of another type (TS3 and TS7, see below). The activation energy of the reaction that proceeds through TS2 is almost
O
O
H O N Me H TS3a (34.6)
3 times lower (Scheme 2) than that for the reaction through TS1, that is, the participation of a second amine molecule leads to substantial acceleration of the reaction. Route 2: twostage. The twostage process is more complex and involves the stage of formation of inter mediate 4, amino alcohol (Scheme 3). It is an endot hermic process, the enthalpy of formation of 4 from ethylene carbonate and methylamine is 6.6 kcal/mol and activation energies for the reaction through TS3a and TS3b are very high.
O
O
O
N
H Me
H TS3b (34.7)
O
HO NHMe 4
O + MeNH 2
O
O
O
Scheme 3.
It should be noted that transition states TS3a and TS3b are similar to TS1a and TS1b with the only dif ference that the fourmembered ring in TS3a and TS3b is formed via the carbonyl oxygen atom rather than the oxygen of the alkoxy group. Further transfor DOKLADY CHEMISTRY
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mation of 4 to the product may be accomplished in two ways. The first simplest way includes the migration of the hydroxy proton to the alkoxy oxygen through TS4 with an activation energy of 15.8 kcal/mol (Scheme 4). Like previous transition states, TS4 can
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exist as two isomers with different arrangement of the methylamino group relative to the fivemembered
O
O
ring; however, both isomers have the same energy and, therefore, only one of them is shown in Scheme 4.
H
H N
O Me TS4 (15.8)
O
O
O
HO NHMe 4
NHMe O H
O 1a
Scheme 4.
The second way (Scheme 5) of transformation of 4 to product 1a includes initial proton migration from the amide group to the oxygen of the alkoxy group through TS5 to form intermediate 5 (Z iso
O
mer), which is 9 kcal/mol higher on the PES than 4. Then, the hydroxy proton in intermediate 5 migrates to the amide nitrogen through TS6 to yield product 1a.
OH
O H
HO
O
N Me TS5 (40.4)
O
N Me O
H TS6 (25.3)
O O
HO NHMe 4
OH
NHMe O H
Me
O N HO
O 1a
5
Scheme 5.
The route of transformation of amino alcohol 4 to product 1a by Scheme 5 seems to be unreal because the activation energy of the reaction through TS5 is too high. As a whole, the twostage process of forma tion and decomposition of intermediate 4 is substan
tially less favorable than the onestage process with participation of two amine molecules via TS2 and therefore it does not take place in the reaction. How ever, situation changes when two amine molecules are involved in the twostage process (Scheme 6). DOKLADY CHEMISTRY
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REACTION OF CYCLOCARBONATES WITH AMINES
O
O
O H
N Me H
H
N TS7a–TS7d (8.8, 9.0, 8.3, 8.6) H Me
O
O
+ 2MeNH2
O
O + MeNH2
O
HO NHMe 4
O
O
O
N
H
H
O H N
H Me
Me
N
O H N
359
O
H Me O H N Me H TS8a–TS8d (6.8, 10.8, 6.9, 10.8)
O
H Me O H N H 7a–7d
O NHMe O
H Me 6a–6d
H
O 1a
Scheme 6.
The formation of four isomers with different arrangement of methyl groups is possible for transi tion states TS7 and TS8 and for intermediates 6 and 7. Transition states TS7 are similar to TS2, but the sixmembered ring in TS7 is formed with the carbo nyl oxygen rather than the alkoxy oxygen (the same difference was between TS3–TS1). In the above consideration of the similar process with the partic ipation of one amine molecule, the formation of intermediate 4 was an endothermic process. The process is exothermic when two amine molecules produce solvates 6a–6d. The activation energies through TS7 are much lower than via TS3 and even lower than via TS2. The calculations show (Scheme 6) that interme diates 6 could not immediately rearrange to give product 1a because the second (solvating) amine molecule occupies an inappropriate position. The movement of solvating molecule is possible through desolvation of 6 to form 4 and a new solvation at the position necessary for further transformation to produce intermediate 7. The solvation–desolvation energies are low and are within 8.7–11.9 kcal/mol for different isomers (6a–6d and 7a–7d). The dif ference between the energies of intermediates 6 and 7 is not higher than 1 kcal/mol, and the energy con sumed for desolvation will return in the process due to solvation of the intermediate. The activation energies of the reaction through TS8 are lower than those through TS7 and the DOKLADY CHEMISTRY
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highest barrier for transformation in all isomeric processes is provided just by the first stage (through TS7). The minimal barrier for the reaction through TS7 was 8.3 kcal/mol (TS8c); i.e., the twostage process with participation of two amine molecules is the most energetically favorable for this reaction. ACKNOWLEDGMENTS This work was supported by the Russian Founda tion for Basic Research (project no. 11–03–00432a). REFERENCES 1. Saunders, J. H. and Frisch, K. C., Polyurethanes, Chemistry and Technology. Part I, Chemistry; Inter science: New York, 1962, Translated under the title Khimiya poliuretanov, Moscow: Khimiya, 1968. 2. Tiger, R.P., Polym. Sci. Ser. B, 2004, vol. 46, nos. 5/6, pp. 142–153. 3. Plate, N.A. and Slivinskii, E.B., Osnovy khimii i tekh nologii monomerov (Foundations of Chemistry and Technology of Monomers), Moscow: MAIK “Nauka/Interperiodika,” 2002. 4. Shapovalov, L.D., Figovskii, O.L., and Kudryavtsev, B.B., Vopr. Khim. Khim. Tekhnol., 2004, no. 1, pp. 231–236. 5. Garipov, R.M., Sysoev, V.A., Mikheev, V.V., Zagidullin, A.I., Deberdeev, R.Ya., Irzhak, V.I., and Berlin, A.A., Dokl. Phys. Chem., 2003, vol. 393, nos. 1– 3, pp. 289–292.
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6. Nemirovski, V.B. and Skorokhodov, S.S., J. Polym. Sci., 1967, vol. 16, pp. 1471–1473.
10. Laikov, D.N. and Ustynyuk, Yu.A., Russ. Chem. Bull. Int. Ed., 2005, vol. 54, pp. 820–826.
7. Perdew, J.P., Burke, K., and Ernzerhoff, M., Phys. Rev. Lett., 1996, vol. 77, pp. 3865–3868. 8. Ernzerhoff, M. and Scuseria, G.E., J. Chem. Phys., 1999, vol. 110, pp. 5029–5036.
11. Litvinenko, L.M. and Oleinik, N.M., Mekhanizm deistviya organicheskikh katalizatorov (Mechanism of Action of Organic Catalysts), Kiev: Naukova Dumka, 1984.
9. Laikov, D.N., Chem. Phys. Lett., 1997, vol. 281, pp. 151–156.
12. Tiger, R.P., Doctoral (Chem.) Dissertation, Moscow: IKhF AN SSSR, 1980.
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