INTRODUCTION:

In the past years, green chemistry have been investigated intensively for their potential applications in synthetic chemistry [1]. Among the 12 principles of green chemistry is safer solvents [2]. For example, the Diels–Alder reaction has been applied to the synthesis of the 8-azabicyclo [4.3.0]nonanone core of cytochalasans[3–12]. The synthesis of (3aR,4S,5S,7aR)-5-methyl-3-oxo-1,3,3a,4,5,7a-hexahydroisobenzofuran-4-carboxylic acid by a tandem Diels-Alder cycloaddition intramolecular lactonization under solventless conditions was reported[13]. This is a Diels-Alder type reaction [14-16]. It is extensive used for organic synthesis. There is no literature about the mechanism of the multi-channel reaction (2E,4E)-6-hydroxyhexa-2,4-dien-1-ylium + furan-2,5-dione . The main purposes of this paper are as follows: (1) to investigate the microscopic mechanism of reaction between (2E,4E)-6-hydroxyhexa-2,4-dien-1-ylium and furan-2,5-dione by way of density functional theory (DFT;(2) compare with the characteristics of chemical reaction channels.(3)to find the major reaction channel.

The mechanism of the multi-channel reaction (2E,4E)-6-hydroxyhexa-2,4-dien-1-ylium + furan-2,5-dione was investigated by density functional theory (DFT).The geometries and the frequencies of reactants, intermediates, transition states, and products were calculated at the B3LYP/6–311G (d) level, obtaining stable structures. The parameters of geometry configuration are shown in Fig. 1 (R1-P4). The vibration analysis and the IRC analysis demonstrated the authenticity of intermediates and transition states, and the reaction processes were confirmed by the changes of charge density at bond-forming critical point (as shown by the numeric value in parentheses in Figs. 1) [17–18]. All calculations were carried out with the Gaussian 03 program [19].

RESULTS AND DISCUSSION:

The calculated energies (E) and relative energies (Erel) of reactants, intermediates, transition states, and products are listed in Table 1. All energies (E) include zero-point energy (ZPE) corrections. Vibration frequencies of reactants, intermediates, and products are positive and those of all transition states have only one imaginary frequency. Figure 2 is a schematic map of energy levels for four reaction channels.

The Reaction Mechanism and Energy Analysis of the Four Reaction channels.

The reaction between for (2E,4E)-6-hydroxyhexa-2,4-dien-1-ylium and furan-2,5-dione the synthesis of IM1A is a bimolecular reaction. R1 collides with R2 to form intermediate IM1A through transition state TS1. In TS1, the bond length and the charge density at bond-forming critical point of C1-C3,C2-C4 are 0.2264nm and 0.0509 a.u. ,0.2241 nm and 0.0536 a.u., respectively, and the activation energy is 55.23KJ.mol^-1. TS1A has six-member ring, consists of C1-C6.The charge density at ring-forming critical point is 0.0132 a.u..The IM1A forms IM2A through transition state TS2A. This is a concerted reaction process with the closing of a ring and the transfer of a hydrogen atom. In TS2A, the bond length and the charge density at bond- forming critical point of C1–O1, O1–H1, and H1–O2 are0.2044 nm and 0.0682 a.u., 0.1394 nm and 0.1019a.u. , and 0.1118 nm and 0.2121 a.u., respectively, and the activation energy is 206.77KJ.mol^-1 .It bas four-member ring ,consists of C1,O1,H1,O2,the charge density at ring-forming critical point is 0.0556 a.u.. It is a rate-controlling step. IM2A has three ring. The IM2A forms P1 through transition state TS3A. This is a concerted reaction process with the opening of a ring and the transfer of a hydrogen atom. In TS3A, the bond length and the charge density at bond- forming critical point of C1-O2,O2-H1 are 0.2299 nm and 0.0396 a.u.,0.1805 nm and 0.0386a.u., respectively, and the activation energy is 83.85KJ.mol^-1. The reaction mechanism of R1+R2→P1 is :R1+R2→TS1 →IM1A→TS2A→IM2A→TS3A→P1. E1 is the activation energy for this channel, and E1 is 206.77KJ.mol^-1.

The reaction mechanism of R1+R2→P2 is similar to that of R1+R2→P1. The IM1B forms IM2B through transition state TS2B. This is a concerted reaction process with the closing of a ring and the transfer of a hydrogen atom. In TS2B, the bond length and the charge density at bond-forming critical point of C1–O1, O1–H1, and H1–O2 are0.2030 nm and 0.0701 a.u., 0.1380 nm and 0.1052a.u. , and 0.1128 nm and 0.2059 a.u., respectively, and the activation energy is210.74KJ.mol^-1 .It bas four-member ring ,consists of C1,O1,H1,O2,the charge density at ring-forming critical point is 0.0568 a.u.. It is a rate-controlling step. The IM2B has three ring. The IM2B forms P2 through transition state TS3B. This is a concerted reaction process with the opening of a ring and the transfer of a hydrogen atom. In TS3B, the bond length and the charge density at bond-forming critical point of C1-O2,O2-H1 are 0.2302 nm and 0.0393 a.u.,0.1787 nm and 0.0402a.u., respectively, and the activation energy is 86.48 KJ.mol^-1. The reaction mechanism of R1+R2→P2 is :R1+R2→TS1 →IM1B→TS2B→IM2B→TS3B→P2. E2 is the activation energy for this channel, and E2 is 210.74 KJ.mol^-1.

The reaction between for (2E,4E)-6-hydroxyhexa-2,4-dien-1-ylium and furan-2,5-dione the synthesis of IM1C is also bimolecular reaction. The IM1C forms P3 through transition state TS2C. This is a concerted reaction process with the closing of a ring and the transfer of a hydrogen atom and opening of a ring. In TS2C, the bond length and the charge density at bond-forming critical point of O1-H1, O2-H1, O1-C1 and C1-O2 are 0.1061 nm and 0.2572 a.u.,0.1414 nm and 0.0872a.u., 0.1725 nm and 0.1330 a.u., and 0.2279 nm and 0.03089 a.u. ,respectively, and the activation energy is 127.17KJ.mol^-1. It bas four-member ring ,consists of C1,O1,H1,O2,the charge density at ring-forming critical point is 0.0341 a.u. . It is a rate- controlling step. The reaction mechanism of R1+R2→P3 is: R1+R2→TS1→IM1C→TS2C→P3. E3 is the activation energy for this channel, and E3 is 127.1KJ.mol^-1.

The configuration of IM1D is similar that of IM1C. The IM1D forms P4 through transition state TS2D. In TS2 D, the bond length and the charge density at bond- forming critical point of O1-H1, O2-H1, C1-O1, and C1-O2 are 0.1062 nm and 0.2564 a.u., 0.1435 nm and 0.0809a.u., 0.1732 nm and 0.1311 a.u., and 0.2311 nm and 0.0380a.u. , respectively, and the activation energy is 133.59KJ.mol^-1. It bas four-member ring ,consists of C1,O1,H1,O2,the charge density at ring-forming critical point is 0.0341 a.u.. It is a rate- controlling step. The reaction mechanism of R1+R2→P4: R1+R2→TS1→IM1D→TS2D→P4. E4 is the activation energy for this channel, and E4 is 133.59KJ.mol^-1. The reaction mechanism of R1+R2→P4 is: R1+R2→TS1→IM1D→TS2D→P4.

The details of the reaction mechanism of the four reaction channels are shown as Fig.3.

TS1

TS2A

TS3A

TS3B

TS2C

TS2D

TS2B

IM1A

IM2A

IM1B

IM2B

IM1C

IM1D

Figure-1 Optimized geometry configurations of various compounds in the reaction: bond length in nanometers, bond angle in degrees, and charge density at the bond-forming critical point in atomic units.

CONCLUSIONS:

The mechanism of the multi-channel reaction (2E,4E)-6-hydroxyhexa-2,4-dien-1-ylium + furan-2,5-dione was investigated by density functional theory (DFT).It is shown that sequence of activation energy of four reaction channels is E2>E1>E4>E3.The activation energy of channel 3 decreased by 83.57 KJ.mo^-1compared with that of channel 2.It is shown that the reaction channel of producing (3aR,4S,5S,7aR)-5-methyl-3-oxo-1,3,3a,4,5,7a-hexahydroisobenzofuran-4-carboxylic acid is the major channel and the others are minor channels. It is identical with McKenzie, et al.’s conclusions [13].

Figure 2 Schematic map of energy levels in the reaction channels

Table 1 Energies (E ) and Relative Energies (E_rel) of Various Species and imaginary frequency of transitions

Species	E+ZPE (a.u.)	Erel (kJ mol⁻¹)	ν (cm⁻¹)	Species	E+ZPE (a.u.)	Erel (kJ mol^-1)	ν (cm⁻¹)
R^t(R1+R2)	-689.088890	0.00		R^t(R1+R2)	-689.088890	0.00
TS1^t(TS1)	-689.067854	55.23	450.8i	TS1^t(TS1)	-689.067854	55.23	450.8i
IM1A^t(IM1A)	-689.136537	-125.10		IM1B^t(IM1B)	-689.132890	-115.52
TS2A^t(TS2A)	-689.057785	81.67	1420.3i	TS2B^t(TS2B)	-689.052622	95.22	1501.3i
IM2A^t(IM2A)	-689.114281	-66.66		IM2B^t(IM2B)	-689.109829	-54.98
TS3A^t(TS3A)	-689.082343	17.19	216.7i	TS3B^t(TS3B)	-689.076892	31.50	224.1i
P1^t(P1)	-689.131894	-112.91		P2^t(P2)	-689.137381	-127.31
R^t(R1+R2)	-689.088890	0.00		R^t(R1+R2)	-689.088890	0.00
TS1^t(TS1)	-689.067854	55.23	450.8i	TS1^t(TS1)	-689.067854	55.23	450.8i
IM1C^t(IM1C)	-689.128362	-103.63		IM1D^t(IM1D)	-689.134860	-120.69
TS2C^t(TS2C)	-689.079949	23.47	425.5i	TS2D^t(TS2D)	-689.083975	12.90	450.1i
P3^t(P3)	-689.137314	-127.14		P4^t(P4)	-689.137933	-128.76

ACKNOWLEDGEMENT:

We thank the Scientific Research Foundation of the Education Department of Sichuan (No. 12ZB120) for financial support.

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Received on 11.09.2013 Modified on 08.10.2013

Asian J. Research Chem. 6(11): November 2013; Page 1054-1059