КИНЕТИКА И МЕХАНИЗМ ^^^^^^^^ ХИМИЧЕСКИХ РЕАКЦИЙ, КАТАЛИЗ
UDC 541.1
KINETICS AND MECHANISM OF INTRAMOLECULAR ALDOL CONDENSATION OF 2,5-HEXADIONE, A DFT AND MP2 STUDY
© 2013 M. R. Zardoost
Department of Chemistry, Qaemshahr Branch, Islamic Azad University, Qaemshahr, Iran E-mail: m1605chemist@yahoo.com Received 18.10.2012
A theoretical study of kinetics and mechanism of intramolecular aldol condensation of 2,5-hexadione was performed using DFT methods at B3LYP and MP2 levels of theory using 6-311g, 6-311g*, 6-311G**, 6-311++G and 6-311++G** basis sets. Equilibrium molecular geometries and harmonic vibrational frequencies of the reactant, transition states, and products were calculated. Two reaction paths were studied. The considered rate constants and activation thermodynamic parameters were calculated. The results indicated a fairly good agreement with experimental data. It was demonstrated that the five member ring is the most stable one for all calculations. These calculations showed that the reaction proceeds through an asynchronous concerted mechanism.
Keywords: intramolecular, aldol condensation, transition state, DFT, MP2, 2,5-hexadione. DOI: 10.7868/S0207401X13100117
1. INTRODUCTION
2,5-Hexadione belongs to a group ofcompounds with numerous applications. So the reactions and properties of these compounds have been studied in various aspects [1—6]. In addition, they are also used as intermediates in the production of natural products [7, 8].
When certain dicarbonyl compounds are treated with base, intramolecular aldol reactions can occur, leading to the formation of cyclic products [9]. In principle, intramolecular aldol reactions can lead to a mixture of products, depending on which enolate ion is formed. For example, 2,5-hexadione might yield either
the five membered ring product 3-methyl-2-cyclopen-tenone or the three membered ring product (2-methyl cyclopropenyl) ethanone. These two paths are demonstrated in Scheme 1. In practice, though, only the cy-clopentenone is formed [9]. Nozire and Crdova studied amino acid catalyzed aldol condensation of acetal-dehyde in aqueous and salt solutions [10]. A typical organic reaction proceeds in a special mechanism. But there may be many proposed mechanisms for a typical organic reaction. Experimental methods of evaluating mechanisms have many instrumental limitations such as
Scheme 1. Intramolecular aldol condensation of 2,5-hexadione
O /
H2?^TCH3
H2C. /CH3
cc \
V O N
O
w
H2C
'CH.
2
H2C * CH3
OH
/
CH3
O^C
/
Ch \
;ch2
ho'
C
I
CH3
O w
/C CH H2C //
нгс"снз
CH3
+
/
о^Я
с
/
H
о—H
\\^CH2 с
+ А
H H
H3C
Table 1. Key geometrical parameters of the reactant, TSs and products at the B3LYP/6-311G** level of theory
Reactant TS1 Intermediate TS2 Products
First path
C1-C2 - 3.139 1.550 1.456 1.343
C1-H15 1.094 1.583 - 2.951 -
C2-O7 1.211 1.282 1.434 1.811 -
O7-H15 - 1.152 0.963 0.969 0.963
C1-H16 1.095 1.085 1.096 1.431 -
O7-H16 - 3.329 - 1.238 0.963
Second path
C2-C11 - 2.525 1.522 1.429 1.304
C2-O7 1.211 1.310 1.412 1.664 -
C11-H17 1.097 2.710 - 2.925 -
C11-H18 1.095 1.081 1.084 1.614 -
O7-H17 - 1.013 0.964 0.970 0.972
O7-H18 - 4.014 - 1.120 0.962
trapping the intermediates or transition states in confirming the mechanism. In contrast, computational methods can make confirming the mechanisms easier, cheaper and exacter with more computational details.
Stéphane Humbel et al. studied intramolecular aldol condensation of 1,6-diketones in acidic medium theoretically, and investigated substituent effect on the reaction [11]. In the present work we investigated the kinetics of the reaction in details. There is a fairly good agreement between our calculations and experimental results.
process. All minimum and transition state structures are verified by vibrational frequency analysis.
The activation energies and Arrhenius factors were computed using equations, which were derived from the transition state theory [25, 26]:
Ea = AH# + RT, (1)
A = (ekBT/h)exp(AS #(T)/r). (2)
2. COMPUTATIONAL DETAILS
The structures corresponding to the reactants, transition states (TSs), intermediates and products of the reaction under investigation were optimized and electronic structures and harmonic vibrational frequencies of all stationary points along the reaction pathway were calculated using Gaussian 03 computational package [12] with DFT and MP2 methods.
Optimization of geometries of the stationary points on the potential energy surfaces were performed using Beck's there-parameter hybrid exchange functional with the correlation functional of Lee, Yang, Parr(B3LYP) [13, 14] with the 6-311g [15, 16], 6-311g* [17, 18], 6-311g** [17, 18], 6-311++g [17, 18], and 6-311++g** [17, 18] basis sets. The synchronous transit guided quasi Newton (STQN) method [19, 20] was used to locate the TSs. The intrinsic reaction coordinate (IRC) method [21, 22] was also applied in order to check and obtain the profiles connecting the TSs to the two associated minima of the proposed mechanism. The natural bond orbital (NBO) analysis [23, 24] was applied to determine the charge changes occurring in the studied
3. RESULTS
Scheme 2 presents the optimized structures of reactants, TSs, intermediate, and products with the selected geometrical parameters at the B3LYP/6-311g**. The data are tabulated in Table 1. Throughout this paper, all inter nuclear distances are in angstroms and all angles are in degrees.
The bond lengths and angles of the two transition states, TS1 and TS2, were calculated using the B3LYP level of the theory along with 6-311g, 6-311g*, 6-311g**, 6-311++g, and 6-311++g** basis sets. According to table 1, the first step of the mechanism is initiated with the C1—H15 bond cleavage for the first path (TS1), and TS1 has a breaking n bond of C2—O7 and forming O7—H15 bond distance of 0.963 A and forming C1—C2 bond distance of 1.550 A. The intermediate of the 2,5-hexadione in the first path, has a breaking C2—O7 bond distance of 1.434 A and breaking C1—H16 bond distance of1.096 A to form the second transition structure (TS2), and TS2 has a forming C1—C2 double bond distance of1.343 A and O7-H16 bond distance of 0.963 A to form products. The Paul-
Scheme 2. Optimized geometries of reactant, TS1, intermediate, TS2 and products for the first path at the B3LYP/6-311G** level
Products
ing relation [27] was used to determine the related partial bond orders, and in the first step of the first path the values of 0.44, and 0.73 were obtained for C1— H15, and O7—H15, respectively. The obtained partial bond orders indicate that 56% of C1—H15 bond is broken, whereas O7—H15 has 73% reaching to the first transition state. For the second step of first path the values of 0.63, 0.53, and 0.57 were obtained for O7-H16, C2-O7, and C1-H16, respectively. The obtained partial bond orders indicate that 47% of C2— O7 bond and 43% C1—H16 is broken, respectively, whereas O7—H16 has 63% reaching to the second transition state. The extent of broken and formed
bonds in transition state shows that a synchronous concerted mechanism has occurred for the aldol condensation of 2,5-hexadione.
In second path the first transition structure has a breaking C2—O7 n bond distance 1.310 A and a forming C2—C11 bond distance of 2.525 angstrom. The second TS, has a breaking C11—H18 bond distance 1.614 A and a forming O7—H18 bond distance of 1.120 A to form products. The Pauling relation [27] was used to determine the related partial bond orders, and in the first step of second path the values of 0.19, and 0.85 were obtained for C11—C2, and O7—C2, respectively. The obtained partial bond orders indicate
that 15% of C2—O7 bond is broken, whereas O7-C2 has 85% reaching to the TS1. For the second step of second path the values of 0.77, and 0.41 were obtained for O7-H18, and C11-H18, respectively. The obtained partial bond orders indicate that 77% of O7-
H18 bond is broken, whereas C11-H18 has 59% reaching to the TS2. The extent of broken and formed bonds in transition state shows that a synchronous concerted mechanism has occurred for second step of the aldol condensation of 2,5-hexadione.
Table 2 shows the charge distribution in the reac-tant, TS1, intermediate, TS2 and the charge difference between TS1 and reactant, and the charge difference between intermediate and TS2 (ACharge) by means of NBO analysis for first path. The results at the TS2 indicate that a small negative charge developed on C2 and C1 which demonstrates C2—C1 bond formation is faster than O7—H15 bond formation. The charge difference shows that electron donor groups at C1 accelerate the reaction.
Table 3 shows the charge distribution in the reactant, TS1, intermediate, TS2, and the charge difference between TS1 and reactant and between intermediate and TS2 (ACharge) by means of NBO analysis for second path. The results at the TS2 indicate that a small negative charge developed on C1 and C2 which demonstrates C2—C1 bond formation is faster than
O7—H17 bond formation. The charge difference shows that electron donor groups at C2 accelerate the reaction.
B3LYP/6-311G** results for the reaction paths are shown in Fig. 1 (for first path) and Fig. 2 (for second path). These figures demonstrate the energy as a function of the reaction coordinate, and represent the minimum energy paths, which connect the reactant to the intermediate through the saddle points.
The imaginary frequency for every transition structure was calculated to confirm the TSs. Tables 4 and 5
present, AGAHASEa, and Arrhenius factor for the two reaction channels. In the first channel the Gibbs free energy barriers are 48.094 and 45.868 kcal • mol-1 at B3LYP/6-311++G**, for the TS1 and TS2 formation, respectively. In the second channel the
Table 2. Distributed NBO charges on the reactant, TSs, and intermediate at the B3LYP/6-311G* level of theory or first path (the numbering of atoms is like that in Scheme 2)
C1 C2 O7 H15 H16
Reactan
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