ISSN 0012�5008, 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.

355

The common method of preparation of polyure�thanes is based on the reaction of terminal groups ofhydroxy�containing oligomers with di� or polyisocy�anates [1]. Isocyanates are rather toxic compoundsand their preparation with the use of phosgene is notso immaculate in environmental context [2].

Among few reactions that can be used for designingnon�isocyanate polyurethanes, the reaction of cyclo�carbonates with primary amines seems to be ratherpromising. Oligomers with cyclocarbonate groups areobtained from epoxide or hydroxy�containing precur�sors [3], and the reaction of cyclocarbonate groupswith amines leads to hydroxyurethanes:

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 targetmodification 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 beenstudied so far. Few kinetic data available in the litera�ture [5, 6] indicate the first order of the reaction in thereactants at their equimolar ratio and the second order

O

CH2

O

HC

O

HCCH2

O NH

OH

O

CH2

HCO

O

NH

HO

~

~~

~~

+ H2N~

in amine when the reaction proceeds with its excess.The latter feature indirectly indicates the possibility ofcatalytic assistance of the second amine molecule inthe opening of the cyclocarbonate ring, although thereis no rigorous proof for this assumption.

In this work, we have studied the mechanism ofaddition of amino group to cyclocarbonate ring in thereaction of ethylene carbonate with methylamine asan example by means of quantum�chemical calcula�tions.

The calculations were performed by the densityfunctional theory (DFT) method using the nonempir�ically generalized gradient approximation and thePBE functional [7, 8] with the TZ2P basis set bymeans of PRIRODA software [9, 10]. Geometry wasoptimized for all initial reactants, stable intermedi�ates, and transition states. The character of revealedstationary points (minimum or saddle point on thepotential energy surface (PES)) was determined bycomputing the eigenvalues of the matrix of secondderivatives of energy with respect to nucleus coordi�nates. The reaction coordinate was calculated todecide whether transition states were involved in agiven transformation. The relative energy values werecorrected 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 carbonatethat correspond to four cyclic isomers 1a–1d withintramolecular hydrogen bond and six open conform�ers 1e–1j (Scheme 1).

CHEMISTRY

Reaction of Cyclocarbonates with Amines as an Alternative Route to Polyurethanes: A Quantum�Chemical Study

of Reaction MechanismM. V. Zabalov, R. P. Tiger, and Academician A. A. Berlin

Received June 30, 2011

DOI: 10.1134/S0012500811120032

Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 119991 Russia

356

DOKLADY CHEMISTRY Vol. 441 Part 2 2011

ZABALOV et al.

Scheme 1.

OH O

O

NMe

H OH N

O

HMe

OO

H O

O

NH

Me OH N

O

O

HMe

HO ON

H

O

Me HO OO

NH

MeHO O

O

NMe

H

HO

ON

O

MeH HO

OO

N

H

Me

HO

OO

N

Me

H

0.9841.823

0.9851.816

1.013

1.012

0.970

1.9341.021

0.9792.061

1.017

1a 1b 1c 1d

1e 1g

1h 1i 1j

1f

Cyclic isomer 1a with hydrogen bond at the carbo�nyl oxygen is most stable. The O–H bond is elongatedby ~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 tothe rest of the molecule. The difference in energybetween 1a and 1b is not significant being 1 kcal/mol.The other isomers with hydrogen bonds (1c, 1d) andthe acyclic conformers without hydrogen bonds (1e–1j) are less stable. One of the acyclic products (1g) is

…

only 2 kcal/mol less stable than 1a. The table showsthat the largest difference between the conformerenergies is 10.5 kcal/mol.

The heat of reaction (ΔH) calculated as the differ�ence between the energies of the initial reactants andproduct 1а, corresponding to the global minimum onthe PES, is –15.1 kcal/mol.

MECHANISM OF THE REACTIONOF METHYLAMINE ADDITION

TO ETHYLENE CARBONATE

It is known from experiment [5, 6] that the reactionof cyclocarbonate with amine can have both the firstand second order in amine. It is reasonable to assumethat a second amine molecule can participate in theprocess by the formation of H�bonded associates andbehave as a catalyst by facilitating proton transfer fromamine to cyclocarbonate through cyclic transitionstates, as occurs in the reactions of aminolysis, hydrol�ysis, and alcoholysis of carbonyl�containing com�pounds [11, 12].

We considered the reactions with both one and twoamine molecules. Two reaction routes were revealed inboth cases.

Route 1: one�stage. For the one�stage route, werevealed (Scheme 2) two cyclic transition states (TS):a four�membered state involving one amine molecule(TS1a, TS1b) and a six�membered state involving twoamine molecules (TS2a, TS2b).

Total electron energy (E) for the initial compounds and theproducts of the reaction of ethylene carbonate with methy�lamine and their relative stability (ΔE)

Compound –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 –

Ethylene carbonate 95.769226 –

DOKLADY CHEMISTRY Vol. 441 Part 2 2011

REACTION OF CYCLOCARBONATES WITH AMINES 357

Scheme 2.

Scheme 2 shows parenthetically the activationenergies (in kcal/mol) for the reactions that proceedthrough the corresponding transition states. Isomer�ism with different orientation of methylamine withrespect to the five�membered ring is possible in suchtransition states. The formation of four isomers is pos�sible theoretically for TS1 and TS2 taking intoaccount the relative disposition of the carbonyl oxy�gen. However, only two of the four isomers occur foreach type, while the other isomers somewhat changeand transform into transition states of another type(TS3 and TS7, see below). The activation energy ofthe reaction that proceeds through TS2 is almost

3 times lower (Scheme 2) than that for the reactionthrough TS1, that is, the participation of a secondamine molecule leads to substantial acceleration of thereaction.

Route 2: two�stage. The two�stage process is morecomplex 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 fromethylene carbonate and methylamine is 6.6 kcal/moland activation energies for the reaction through TS3aand TS3b are very high.

Scheme 3.

It should be noted that transition states TS3a andTS3b are similar to TS1a and TS1b with the only dif�ference that the four�membered ring in TS3a andTS3b is formed via the carbonyl oxygen atom ratherthan the oxygen of the alkoxy group. Further transfor�

mation of 4 to the product may be accomplished in twoways. The first simplest way includes the migration ofthe hydroxy proton to the alkoxy oxygen through TS4with an activation energy of 15.8 kcal/mol(Scheme 4). Like previous transition states, TS4 can

O O

HN

O

HMe

O O

HN

O

HMe

O OH

NH

NH

MeOHH

Me

O OH

NH

NH

MeOHH

Me

OH O

O

NHMe

TS1a (28.3) TS1b (29.7) TS2a (10.8) TS2b (11.0)

O O

O

+ MeNH2

1a

O O

OH

NH

Me

O O

OH

NH

Me

O O

HO NHMeO O

O

+ MeNH2

TS3a (34.6) TS3b (34.7)

4

358

DOKLADY CHEMISTRY Vol. 441 Part 2 2011

ZABALOV et al.

exist as two isomers with different arrangement of themethylamino group relative to the five�membered

ring; however, both isomers have the same energy and,therefore, only one of them is shown in Scheme 4.

Scheme 4.

The second way (Scheme 5) of transformation of4 to product 1a includes initial proton migrationfrom the amide group to the oxygen of the alkoxygroup through TS5 to form intermediate 5 (Z iso�

mer), which is 9 kcal/mol higher on the PES than 4.Then, the hydroxy proton in intermediate 5migrates to the amide nitrogen through TS6 to yieldproduct 1a.

Scheme 5.

The route of transformation of amino alcohol 4 toproduct 1a by Scheme 5 seems to be unreal becausethe activation energy of the reaction through TS5 istoo high. As a whole, the two�stage process of forma�tion and decomposition of intermediate 4 is substan�

tially less favorable than the one�stage process withparticipation of two amine molecules via TS2 andtherefore it does not take place in the reaction. How�ever, situation changes when two amine molecules areinvolved in the two�stage process (Scheme 6).

OH

OMe

O

NH

OH O

O

NHMe

O O

HO NHMe4

TS4 (15.8)

1a

O

N

OH

Me

OH

OH

N

O

HO

Me

ON

OH

Me

HO

O O

HO NHMe4

OH O

O

NHMe

1a

TS5 (40.4)

TS6 (25.3)

5

DOKLADY CHEMISTRY Vol. 441 Part 2 2011

REACTION OF CYCLOCARBONATES WITH AMINES 359

Scheme 6.

The formation of four isomers with differentarrangement of methyl groups is possible for transi�tion states TS7 and TS8 and for intermediates 6 and7. Transition states TS7 are similar to TS2, but thesix�membered ring in TS7 is formed with the carbo�nyl oxygen rather than the alkoxy oxygen (the samedifference was between TS3–TS1). In the aboveconsideration of the similar process with the partic�ipation of one amine molecule, the formation ofintermediate 4 was an endothermic process. Theprocess is exothermic when two amine moleculesproduce solvates 6a–6d. The activation energiesthrough TS7 are much lower than via TS3 and evenlower than via TS2.

The calculations show (Scheme 6) that interme�diates 6 could not immediately rearrange to giveproduct 1a because the second (solvating) aminemolecule occupies an inappropriate position. Themovement of solvating molecule is possible throughdesolvation of 6 to form 4 and a new solvation at theposition necessary for further transformation toproduce intermediate 7. The solvation–desolvationenergies are low and are within 8.7–11.9 kcal/molfor different isomers (6a–6d and 7a–7d). The dif�ference between the energies of intermediates 6 and7 is not higher than 1 kcal/mol, and the energy con�sumed for desolvation will return in the process dueto solvation of the intermediate.

The activation energies of the reaction throughTS8 are lower than those through TS7 and the

highest barrier for transformation in all isomericprocesses is provided just by the first stage (throughTS7). The minimal barrier for the reactionthrough TS7 was 8.3 kcal/mol (TS8c); i.e., thetwo�stage process with participation of two aminemolecules is the most energetically favorable forthis reaction.

ACKNOWLEDGMENTS

This work was supported by the Russian Founda�tion for Basic Research (project no. 11–03–00432�a).

REFERENCES

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OO

O

HN

H

NH

Me

H Me

OO

O

HN

H

NH

Me

H Me

O O

O HN

HN Me

H

H

Me

O O

O

O O

HO NHMe4

OH O

O

NHMe

1a

+ 2MeNH2

TS7a–TS7d TS8a–TS8d(6.8, 10.8, 6.9, 10.8)(8.8, 9.0, 8.3, 8.6)

6a–6d

+ MeNH2

7a–7d

O O

O HN

HN Me

H

H

Me

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ZABALOV et al.

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