Theoretical study of the mechanism of high-pressure induced 1,3-dipolar cycloadditions of azides with electron-rich olefins

Journal of Molecular Structure: THEOCHEM 821 (2007) 145–152 www.elsevier.com/locate/theochem

Theoretical study of the mechanism of high-pressure induced 1,3-dipolar cycloadditions of azides with electron-rich olefins Ji-Cai Fan, Jun Liang, Yun Wang, Zhi-Cai Shang

*

Department of Chemistry, Zhejiang University, Hangzhou 310027, China Received 11 May 2007; received in revised form 30 June 2007; accepted 5 July 2007 Available online 14 July 2007

Abstract The high-pressure induced 1,3-dipolar cycloadditions of azides with electron-rich olefins have been studied by means of density functional theory (DFT) method. It is shown that high-pressure could induce the 1,3-dipolar cycloaddition of azides not only with electrondeficient olefins but also with electron-rich ones. The results derived from the theoretical calculations also indicate that the concerted mechanism is both kinetically and thermodynamically preferred to the stepwise one. In addition, the solvent effects on the stability of the products and transition states are taken into account, and the comparison of the calculated results between the 1,3-dipolar cycloadditions of azides with electron-deficient olefins and electron-rich ones is employed. Ó 2007 Elsevier B.V. All rights reserved. Keywords: High-pressure; DFT; Concerted mechanism; Solvent effect; Comparison

1. Introduction The 1,3-dipolar reaction as a versatile method for preparing five-membered heterocyclic compounds is a classical reaction in organic chemistry and has been studied extensively. These cycloadditions have been utilized for the preparation of compounds that are of fundamental importance in diverse fields of chemistry [1]. In regard to the mechanism of the 1,3-DC reactions, two extreme theories were proposed: one is the concerted mechanism with simultaneous formation of the two new sigma bonds in the transition state, suggested by Huisgen [2,3]; the other is put forward by Firestone: that is, the stepwise mechanism containing two continuous processes, through which a diradical transition state is present [4–6]. The utility of 1,3-DC reactions in synthetic organic chemistry is well established, however, considerable controversy still surrounds the mechanism of these reactions[2,4,6,7]. A multiplicity of experimental data, such as stereospecificity, *

Corresponding author. Tel.: +86 0571 87952379; fax: +86 0571 87951895. E-mail address: [email protected] (Z.-C. Shang). 0166-1280/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2007.07.004

activation entropy, activation energy, solvent effects, periselectivity, reactivity of the dipolarophiles, and regioselectivity, are now available [8–11]. For nitrile oxide cycloadditions, experimental data have been interpreted either as supportive of a concerted mechanism [3,7,12,13] or in favor of a stepwise one with diradical intermediates [4–6]. A number of theoretical investigations of the 1,3-DC reactions have appeared in the literature, all the ab initio calculations predict the existence of a synchronous transition state for the 1,3-DC reactions, although they do not rule out the possibility of a two-step pathway since highly unsymmetrical regions of the potential energy surface have not been explored adequately. Semiempirical calculations, on the other hand, consistently support a two-step mechanism going through a very unsymmetrical transition structure in a cyclo conformation, then forming a diradical which closes to the products after passing over a second, lower energy transition state. However, ab initio calculations on the reaction of formonitrile oxide with acetylene predict a concerted mechanism at the molecular orbital level [14–16], but a stepwise mechanism after inclusion of extensive electron correlation [17].

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The reaction of alkyl azides with a variety of olefins, leading to the formation of triazolines was first reported by Wolff [18], subsequently, the work of several investigators studied it in some detail [19–21]. For instance, the pioneering efforts of Huisgen [22] and L’abbe [23] established that azide cycloadditions with acrylates and related electron-deficient olefins are generally stereo- and regioselective and tend to be very sluggish. High-pressure [24] as an effective factor in 1,3-DC of nitrones [25], nitronates [26], and diazomethane [27], has also been noticed. Moreover, Dauben reported that electrondeficient arylsulfonyl azides are capable of reacting with electron-rich silylenol ethers at 15 kbar [28]. On the basis of these findings, Anderson et. al. [29] performed the 1,3DC of azides with electron-deficient olefins at 12 kbar at room temperature and obtained relatively good results. Albeit numerous theoretical studies of the mechanism of 1,3-DC have been carried out, the systems selected mostly are fulminic acid or nitrones with acetylene or ethylene [17,30–34], few are azides. Moreover, in these studies attentions were approximately focused on the problem of regioselectivity [14,15,35–44]. As a consequence, a detailed understanding of this reaction at a reliable quantum chemical level should be of great significance, therefore, we did some theoretical research on the high-pressure induced 1,3-DC of azides with electron-deficient olefins to compare with the experimental results and found out that high-pressure can indeed induce the 1,3-DC of azides with electrondeficient olefins [45], meanwhile, the mechanism of the reaction was also investigated. For the importance of cycloaddition in preparing fivemembered heterocyclic compounds and at the same time, in order to extend the application of high-pressure in the 1,3-DC of azides with olefins, more attention should be paid both in experimental and theoretical aspects. With this in mind, the purpose of our work is to explore whether high-pressure could also accelerate the 1,3-DC of azides with electron-rich olefins and in particular, to interpret the mechanism of the reaction from a theoretical point of view. Besides, solvent effects on the reaction are also considered. From Scheme 1 we can see that two isomers are formed in this reaction: for isomer 1, the substituents are

on C4 of the product and for isomer 2, the substituents are on C5. 2. Theoretical methods All the electronic structure calculations were performed by means of the GAUSSIAN 98 program packages [46]. All structures were optimized by density functional theory (DFT) methods, using Becke’s three-parameter (B3) [47,48] exchange functional along with the Lee–Yang–Parr (LYP) [49] nonlocal correlation functional (B3LYP) [50]. The standard split-valence double-n basis set with polarization functions on heavy atoms, 6-31G*, was adopted, meanwhile 6-31+G(d,p) basis set was also adopted for one of the systems to check the stability of the calculated results at the B3LYP/6-31g* level. Open-shell computations were based on an unrestricted formalism. Energy minima and first-order saddle points were determined by analytical computation of vibrational frequencies and by viewing the motion of the imaginary vibrational mode for the transition states. The transition states were further confirmed by the intrinsic reaction coordinate (IRC) procedures. Solvent effects were considered using the polarizable continuum model (PCM) with the permittivities of 32.63, 24.55, 8.93, and 4.34 for CH3OH, C2H5OH, CH2Cl2, and (CH3CH2)2O, respectively. This approach had proven to be able to provide a reasonably good description of polarization effect of the solvent. 3. Results and discussion The barrier height (values including zero-point-energies) of the concerted and stepwise mechanisms for 1,3-DC of azides with electron-rich olefins calculated at the B3LYP/ 6-31g* and B3LYP/6-31+g(d,p) levels are summarized in Table 1. From Table 1 it is clear that the concerted cycloadditions possess barrier height that are about 47.9–53.5 kcal/ mol lower than the corresponding stepwise ones, this gives a strong indication that the concerted mechanism is kinetically favored with respect to the stepwise one. The experimental barrier height for 1,3-DC of azides with electron-

R' +RN3 Me

N2 N1

N3 4

5

R

N2 N1

+ Me

R

R' isomer 1

nBu -1: nBu -2: nBu -3: nBu -4:

Scheme 1.

4

Me

isome r 2

R=Ph, nBu; EWG= H, Me , OMe, NH2 Ph -1: R=Ph , R '=H; Ph -2: R =Ph , R' =Me; Ph -3: R=Ph , R '=OMe; Ph -4: R =Ph , R' =NH2

R'

5

N3

R=nBu, R=nBu, R=nBu, R=nBu,

R'=H; R'=Me; R'=OMe; R'=NH2

J.-C. Fan et al. / Journal of Molecular Structure: THEOCHEM 821 (2007) 145–152 Table 1 Calculated barrier height (kcal/mol) for the concerted and stepwise mechanisms Speciesa

Ph-1b Ph-1c Ph-2 Ph-3 Ph-4 n-Bu-1 n-Bu-2 n-Bu-3 n-Bu-4 a b c

Concerted

Stepwise

DEa,1 (isomer 1)

DEa,1 (isomer 2)

DEa,2 (isomer 1)

DEa,2 (isomer 2)

18.7 20.5 20.4 14.5 15.4 20.9 22.6 16.6 18.2

19.7 21.4 21.5 21.4 22.4 21.0 22.9 22.0 24.3

72.2 72.7 72.7 67.8 67.4 71.3 71.9 67.0 68.8

72.5 73.1 73.7 70.5 71.6 71.3 73.3 70.5 72.2

See Scheme 1 for the definition of Ph-1  Ph-4 and n-Bu-1  n-Bu-4. Calculated at the B3LYP/6-31g(d) level. Calculated at the B3LYP/6-31+g(d,p) level.

rich olefins are unknown now, however, from Ref. [6], typical reaction barriers obtained experimentally for a variety of 1,3-DC lie in the range of 8.0–18.0 kcal/mol. Table 1 shows that the barrier height for the concerted mechanism approximately fall into that range, but for the stepwise one, the barriers amount to about 67.0–73.7 kcal/mol. Furthermore, the barrier height of isomer 1 is smaller than isomer 2, which means isomer 1 is more easily obtained and would be the predominant products of the reaction. The optimized structures of the transition states (TSs) at the B3LYP/6-31g* level are depicted in Figs. 1 and 2 (Figs. 1 and 2 are the two products of the reaction between azide and methacrylate, where R = Ph, R 0 = H). They all have single imaginary frequencies, and their single imaginary frequencies are in correspondence with the stretching vibrations of C4–N3 and C5–N1 bonds for TSA and TSC, but for TSB and TSD, they are in correspondence with the stretching vibration of C4–N3 bond.

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Selected geometry parameters of the TSs calculated at the B3LYP/6-31g* as well as B3LYP/6-31+g(d,p) level are shown in Table 2. As can be seen from the results calculated at the B3LYP/6-31g* level: bond lengths of C4–N3 and C5–N1 (atoms labeling is given in Fig. 1) for the con˚ , respeccerted mechanism in Fig. 1A are 2.197 and 2.121 A ˚, tively, whereas these lengths are 2.184 and 3.978 A respectively in Fig. 1B. On the other hand, for TSA, the C5–C4–H6–H7dihedral angle is very close to C4–C5–C8– H9 (162.3° vs 156.2°), and hence the hybridization of C4 and C5 changes from sp2 to nearly sp3; but for TSB, the dihedral angles of C5–C4–H6–H7 and C4–C5–C8–H9 are 147.0° and 171.6°, respectively, which means that, in the first step, only the C4–N3 bond forms. In addition, while the hybridization of C4 changes from sp2 to nearly sp3, the hybridization of C5 remains unchanged. From Fig. 1B we can also see: since the N1, N2, N3 of azide group and C4, C5 of the olefin are in an extended conformation not a cyclo one, before the diradical close to the final five-membered ring in the second step, the C5–C4 bond has to rotate to make the closure feasible and this needs to pass an energy barrier, so this pathway calls for more energy than the concerted one. A comparison between Fig. 2C and D reveals the same result. Although the concerted mechanism is favored, the formation of the two new bonds may proceed unevenly. We believe that our calculated results are in accord with a process that the bond formation of C5–N1 has progressed to a greater extent than C4–N3, consequently, a charge imbalance in the transition state and suggests an initial electrophilic attack by the terminal nitrogen, therefore partial positive charge is developed on C4. So, if the electron-giving substituents were at C4, it could distribute the positive charge and therefore make the transition state stable, then the conformation having an electron-giving substituent at

Fig. 1. Optimized transition structures of the two mechanisms for isomer 2.

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Fig. 2. Optimized transition structures of the two mechanisms for isomer 1.

Table 2 Optimized geometry parameters for the two TSsa Variable

Ab

Ac

Bb

Bc

Variable

Cb

Cb

Db

Dc

C5–C4–H6–H7 C4–C5–C8–H9 N1–N2–N3 N2–N3–C4 N3–C4–C5 C4–C5–N1 C5–N1–N2 N3–C4–H6 N3–C4–H7 N1–C5–C8 N1–C5–H9 N1–N2 N2–N3 N3–C4 C4–C5 C5–N1 C5–C8

162.3 156.2 135.1 97.8 101.3 101.9 103.1 91.7 92.4 98.6 89.6 1.178 1.274 2.197 1.385 2.121 1.511

162.5 155.8 134.7 97.8 101.2 101.9 103.5 91.5 92.6 98.9 89.7 1.180 1.275 2.195 1.388 2.110 1.512

147.0 171.6 153.8 115.3 110.6 63.1 60.4 96.1 92.5 69.5 142.5 1.160 1.267 2.184 1.464 3.978 1.497

147.7 172.1 153.4 115.2 110.8 60.9 59.6 95.2 92.5 71.7 143.1 1.160 1.267 2.187 1.464 4.035 1.498

C5–C4–C6–H7 C4–C5–H8–H9 N1–N2–N3 N2–N3–C4 N3–C4–C5 C4–C5–N1 C5–N1–N2 N3–C4–C6 N3–C4–H7 N1–C5–H8 N1–C5–H9 N1–N2 N2–N3 N3–C4 C4–C5 C5–N1 C4–C6

161.8 155.6 135.0 97.3 97.0 106.0 103.3 100.6 87.6 92.3 92.7 1.180 1.276 2.261 1.386 2.071 1.505

162.0 155.4 134.5 97.4 96.7 106.1 103.7 101.0 87.4 92.2 93.2 1.182 1.277 2.262 1.389 2.059 1.504

145.7 179.9 154.0 116.5 103.8 66.4 57.1 102.0 94.4 64.2 150.3 1.161 1.272 2.183 1.462 3.936 1.507

146.4 177.6 153.3 115.7 103.6 74.3 62.6 102.4 93.3 52.4 153.7 1.162 1.272 2.198 1.463 3.699 1.507

a b c

Angles are in degrees and distances in angstroms. Calculated at the B3LYP/6-31g(d) level. Calculated at the B3LYP/6-31+g(d,p) level.

C4 of the product, namely isomer 1, is more stable than the electron-giving substituent at C5 of the product (isomer 2), and thus the major product of this cycloaddition. This matches our calculation results in Table 1 very well. Chandra et. al. [51] used local softness as approach to regiochemistry in 1,3-DC reactions of phenyl azide with styrene and phenylacetylene, and obtained the same regioselectivity of the product as well. Inspired by both experimental [29] and our theoretical results [45] that 1,3-DC reaction of azides with electrondeficient olefins could be induced by high-pressure, we are wondering whether high-pressure could also promote

the 1,3-DC reaction of azides with electron-rich olefins. Therefore, the Gibbs free energies (G) of the reactants, products as well as transition states at different pressures were calculated. Since the cycloadditions of azides with electron-deficient olefins mainly proceed in a room temperature due to the instability of triazolines, all calculations of the cycloadditions of azides with electron-rich olefins were carried out at 298.15 K too. Table 3 collects the calculated results. As can be seen from Table 3, with the increase in pressure, the relative free energies of all the reactions decrease, which means that the reaction is more feasible at a high-

J.-C. Fan et al. / Journal of Molecular Structure: THEOCHEM 821 (2007) 145–152

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Table 3 The relative Gibbs free energiesa (kcal/mol) of the reaction for the two TSs at different pressures Species

Concerted 1 atm

Ph-1b,m Ph-1b,n Ph-1c,m Ph-1c,n Ph-2b Ph-2c Ph-3b Ph-3c Ph-4b Ph-4c n-Bu-1b n-Bu-1c n-Bu-2b n-Bu-2c n-Bu-3b n-Bu-3c n-Bu-4b n-Bu-4c a b c m n

0.5 0.5 1.0 7.1 7.6 0.1 0.6 5.9 6.3

Stepwise 10 atm

50 atm

100 atm

1 atm

10 atm

50 atm

100 atm

1.4 1.4 1.0 0.8 1.2 0.5 1.4 5.7 1.4 6.0 1.4 1.5 1.2 0.6 1.4 4.5 1.4 4.8

2.4 2.4 1.9 1.8 2.4 1.4 2.4 4.7 2.4 5.1 2.3 2.3 2.3 1.7 2.3 3.6 2.3 4.0

2.7 2.7 2.2 2.2 2.7 1.5 2.7 4.4 2.7 4.8 2.7 2.7 2.7 2.0 2.7 3.3 2.7 3.6

51.0 50.1 51.7 50.3 49.4 51.7 51.7 54.0 50.6 54.0 48.2 48.1 47.9 48.2 48.6 52.1 48.5 51.6

49.6 48.7 50.2 49.0 48.0 50.4 50.3 52.6 49.1 52.6 46.7 46.6 46.6 47.0 47.2 50.7 47.1 50.2

48.7 47.7 49.3 48.0 47.0 49.4 49.3 51.7 48.2 51.7 45.9 45.8 45.6 45.9 46.3 49.8 46.3 49.3

48.4 47.3 49.0 47.6 46.6 49.0 49.1 51.0 47.9 51.4 45.5 45.4 45.2 45.5 45.9 49.4 45.9 48.9

The Gibbs free energy is relative to that of the corresponding concerted transition state of isomer 1 at 1 atm. The isomer 1. The isomer 2. Calculated at the B3LYP/6-31g(d) level. Calculated at the B3LYP/6-31+g(d,p) level.

pressure. At the same time, the calculated barrier height of 1,3-DC reaction of azides with electron-rich olefins for the concerted mechanism is 14.5–24.3 kcal/mol (Table 1), approximately the same with the electron-deficient ones (18.0–21.0 kcal/mol) [45], which indicates that the 1,3-DC of azides with electron-rich olefins may can also be induced by high-pressure from theoretical point of view. Meanwhile, the relative Gibbs free energies of the reaction for the formation of the transition states of isomer 1 are lower than that of isomer 2, that is to say, isomer 1 is more easily formed relative to isomer 2, namely, isomer 1 should be the predominant product of the reaction, which is in accordance with the calculated results in Table 1. For all the products, the barrier height of the reaction from the concerted mechanism is smaller than from diradical one,

which means the reaction occurs more easily by the concerted pathway. The entropy of the reaction at different pressures is also calculated and collected in Table 4. The results indicate that the entropy of the reaction is negative and with the increase of pressure, the entropy of the reaction increase, so high-pressure is expected to enhance the rate of the 1,3-DC reaction of azides with electron-rich olefins. Solvent is an important factor in organic reaction, it has some effects on the chemical equilibria, rate as well as the mechanism of the chemical reaction. The experimental [29] and our calculated results of the 1,3-DC reaction of azides with electron-deficient olefins both show that solvent has an important effect in the reaction, so we calculated the solvent effect on all the products and the transition states to

Table 4 The entropy (kcal/K) of the reaction at different pressures Species Ph-1a Ph-1b Ph-2 Ph-3 Ph-4 n-Bu-1 n-Bu-2 n-Bu-3 n-Bu-4 a b

Isomer 1

Isomer 2

1 atm

10 atm

50 atm

100 atm

1 atm

10 atm

50 atm

100 atm

0.047 0.047 0.049 0.050 0.049 0.049 0.050 0.052 0.050

0.043 0.042 0.028 0.045 0.044 0.045 0.029 0.047 0.045

0.039 0.039 0.025 0.042 0.041 0.045 0.025 0.046 0.042

0.038 0.038 0.023 0.041 0.040 0.040 0.024 0.043 0.041

0.045 0.046 0.044 0.047 0.045 0.049 0.050 0.049 0.050

0.041 0.041 0.023 0.043 0.041 0.044 0.028 0.045 0.045

0.038 0.038 0.020 0.040 0.038 0.044 0.025 0.044 0.042

0.036 0.036 0.018 0.038 0.036 0.040 0.024 0.040 0.040

Calculated at the B3LYP/6-31g(d) level. Calculated at the B3LYP/6-31+g(d,p) level.

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explore whether the polar solvent or apolar one is good for the 1,3-DC reaction with electron-rich olefins. From Tables 5 and 6 we can conclude that as decreasing the solvent polarity, the solvation energies of both products and transition states decrease, which clearly indicates that the products and the transition states are both stabilized when the polarity of the solvent increases. Therefore, increasing the polarity of the solvent can facilitate the formation of the products and at the same time stabilize the transition state, that is to say, polar solvent such as methanol and ethanol is favored in 1,3-DC reaction of azides with electron-rich olefins. Meanwhile, it also shows that the solvation energies of both product and transition state of isomer 1 are much larger than that of isomer 2, which implies that solvent plays a more important role in stabilizing isomer 1 than isomer 2. Nguyen et al. [52] estimated the

solvent effect on the 1,3-DC of N2O to acetylene and their results also show: solvent can influence the regioselectivity on the reaction [53,54], and when increasing the polarity of the solvent, all structures tend to increase their stabilization. The solvation energy is considered as the change in Gibbs energy when an ion or molecular is transferred from a vacuum (or the gas phase) into a solvent, so the solvation energy can be predicted by Eq. (1), where l and r are dipole moment and radius of solute in a solvent modeled as a uniform dielectric with relative permittivity er, eo is permittivity of vacuum [55]. DG0solv ¼ G0vapor  G0solvation NA er  1 l2  ¼ Q  4 e0 2er þ 1 r3

ð1Þ

Table 5 Solvation energies (kcal/mol) of the products in four different solvents Species Ph-1a Ph-1b Ph-2 Ph-3 Ph-4 n-Bu-1 n-Bu-2 n-Bu-3 n-Bu-4 a b

Isomer 1

Isomer 2

CH3OH

C2H5OH

CH2Cl2

(C2H5)2O

CH3OH

C2H5OH

CH2Cl2

(C2H5)2O

8.7 10.2 8.3 9.5 12.9 7.4 7.3 8.0 11.6

8.5 9.9 8.1 9.2 12.5 7.2 7.0 7.9 11.2

7.2 8.3 6.9 7.8 10.5 6.1 6.0 6.7 9.4

5.4 6.3 5.2 6.0 8.0 4.7 4.6 5.0 7.1

8.5 10.0 7.8 8.4 10.7 7.0 6.4 7.0 9.2

8.3 9.7 7.7 8.1 10.4 6.8 6.3 6.7 9.1

7.0 8.2 6.5 6.8 8.9 5.8 5.3 5.7 7.8

5.3 6.1 4.8 5.1 6.8 4.4 4.0 4.3 6.0

Calculated at the B3LYP/6-31 g(d) level. Calculated at the B3LYP/6-31+g(d,p) level.

Table 6 Solvation energies (kcal/mol) of the two TSs in four different solvents Species a,m

Ph-1 Ph-1a,n Ph-1b,m Ph-1b,n Ph-2a Ph-2b Ph-3a Ph-3b Ph-4a Ph-4b n-Bu-1a n-Bu-1b n-Bu-2a n-Bu-2b n-Bu-3a n-Bu-3b n-Bu-4a n-Bu-4b a b m n

Concerted

Stepwise

CH3OH

C2H5OH

CH2Cl2

(C2H5)2O

CH3OH

C2H5OH

CH2Cl2

(C2H5)2O

5.6 6.6 5.3 5.3 5.2 4.9 6.4 6.0 12.5 8.9 3.8 3.6 3.7 3.2 7.4 4.7 9.7 7.1

5.4 6.5 5.1 5.2 5.1 4.7 6.3 5.8 12.0 8.6 3.8 3.5 3.6 3.1 7.2 4.6 9.4 7.0

4.7 5.5 4.3 4.4 4.3 4.0 5.3 4.9 10.2 7.3 3.2 3.0 3.1 2.6 6.2 3.9 8.0 5.9

3.5 4.2 3.3 3.4 3.3 3.0 4.1 3.7 7.7 5.6 2.4 2.3 2.3 2.0 4.5 3.0 6.0 4.5

6.1 7.1 5.6 6.6 5.9 5.3 7.8 5.9 11.3 10.8 4.5 4.2 4.2 3.9 5.9 4.7 10.8 8.0

5.8 6.8 5.4 6.4 5.7 5.2 7.3 5.7 10.9 10.4 4.4 4.1 4.1 3.8 5.6 4.6 10.5 7.8

4.9 5.7 4.6 5.4 4.8 4.4 6.1 4.9 8.8 8.8 3.7 3.4 3.5 3.3 4.7 3.9 8.7 6.6

3.7 4.3 3.5 4.1 3.6 3.3 4.5 3.7 6.4 6.5 2.8 2.6 2.6 2.5 3. 6 2.9 6.3 4.9

The isomer 1. The isomer 2. Calculated at the B3LYP/6-31g(d) level. Calculated at the B3LYP/6-31+g(d,p) level.

J.-C. Fan et al. / Journal of Molecular Structure: THEOCHEM 821 (2007) 145–152 Table 7 The dipole moments (Debye) for the concerted and stepwise mechanisms Speciesa Ph-1b Ph-1c Ph-2 Ph-3 Ph-4 n-Bu-1 n-Bu-2 n-Bu-3 n-Bu-4 a b c

Concerted

Stepwise

l-1 (isomer 1)

l-1 (isomer 2)

l-2 (isomer 1)

l-2 (isomer 2)

2.0 2.2 2.2 2.4 3.7 2.6 2.6 2.1 3.3

1.6 1.7 1.6 1.0 1.3 2.4 2.3 1.0 2.2

2.5 2.6 2.8 3.5 3.8 2.8 2.9 3.4 4.1

2.3 2.4 2.3 2.5 3.2 2.8 2.8 3.1 2.8

See Scheme 1 for the definition of Ph-1  Ph-4 and n-Bu-1  n-Bu-4. Calculated at the B3LYP/6-31g(d) level. Calculated at the B3LYP/6-31+g(d,p) level.

The difference in the Gibbs energy of solvation for isomer 1 and isomer 2, is then given by Eq. (2) assuming that they have the same size.  2  N A er  1 l l2  31  32 DDG0solv ¼  Q ð2Þ 4 e0 2er þ 1 r r The dipole moments (l) for the concerted and stepwise mechanism of the products are collected in Table 7, and er is 32.63, 24.55, 8.93, and 4.34 for CH3OH, C2H5OH, CH2Cl2, and (CH3CH2)2O, respectively. From these data and the two equations we can find out why for both the concerted and the diradical mechanisms, the solvation energies of isomer 1 are much larger than isomer 2. To validate the rationality of the B3LYP/6-31g* results, the B3LYP/6-31+g(d,p) calculations have also been performed on a simple system where R = Ph, R 0 = H and the results are collected in Tables 1–7. It can be seen that the differences of the results calculated with the two computational methods are quite limited, in particular, the differences of barrier heights predicted at the two levels are never over 2 kcal/mol. This gives a strong indication that basis set effects on these systems under investigation are very marginal. From our calculated results of the 1,3-DC reaction of azides with both electron-deficient [45] and electron-rich olefins we can conclude: on one side, concerted mechanism is preferred for this two types of reaction, and high-pressure cannot only induce the cycloaddition with electrondeficient olefins but maybe also electron-rich ones from theoretical point of view; On the other side, for the reaction with electron-deficient olefins, isomer 2 is the predominant products, but for electron-rich olefins, isomer 1 is the major products in the reaction. Meanwhile, for both electron-deficient and electron-rich olefins, increasing the polarity of the solvent can facilitate the formation of the products and at the same time stabilize the transition state, and the solvation energy of isomer 1 is smaller than isomer 2 in the 1,3-DC reaction of azides with electron-deficient olefins, but for electron-rich ones the result is contrary. All these differences are for the reason that one type of substituents

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on olefins are electron-withdrawing groups, the other are electron-giving ones, but they are in accordance with each other essentially. 4. Conclusions In this work, the 1,3-dipolar cycloadditions of azides with electron-rich olefins at high-pressure have been investigated by means of theoretical calculations at the B3LYP/ 6-31g* level, and at the same time, the calculation method was justified by taking a larger basis set (6-31+g(d,p)) for one system to check the stability of the result. The calculation results show in one hand, the reaction is in preference to the concerted mechanism ascribed to a lower energy barrier relative to that of the stepwise one; in the other hand, the Gibbs free energies of the reaction decrease with the increase in pressure, which indicates that high-pressure could induce the 1,3-DC of azides with electron-rich olefins from theoretical point of view. Moreover, for the concerted mechanism, isomer 1 has been found to be more stable than isomer 2, and therefore should be the major product of the reaction. The effect of solvent on the products and transition states has been considered using the PCM model. It is shown that solvent plays an important role in the reaction, that is, increasing the polarity of the solvent will enhance the interactions between solvent and products as well as solvent and transition states, at the same time, the solvation energy of isomer 1 is larger than isomer 2. References [1] (a) R.R. Kumar, S. Perumal, H.B. Kagan, R. Guillot, Tetrahedron 12380 (2006) 62; (b) P.K. Kalita, B. Baruah, P.J. Bhuyan, Tetrahedron Lett. 7779 (2006) 47; (c) M.P. Sibi, L.M. Stanley, T. Soeta, Adv. Synth. Catal. 2371 (2006) 348; (d) W.J. Choi, Zh. D. Shi, K.M. Worthy, L. Bindu, R.G. Karki, M.C. Nicklaus, R.J. Fisher Jr., T.R. Burke, Bioorg. Med. Chem. Lett. 5262 (2006) 16. [2] R. Huisgen, Angew. Chem. Int. Ed. Engl. 565 (1963) 2. [3] R. Huisgen, J. Org. Chem. 2291 (1968) 33. [4] R.A. Firestone, J. Org. Chem. 2285 (1968) 33. [5] R.A. Firestone, J. Org. Chem. 2181 (1972) 37. [6] R.A. Firestone, Tetrahedron 3009 (1977) 33. [7] R. Huisgen, J. Org. Chem. 403 (1976) 41. [8] J.M. Lluch, J. Bertran, Tetrahedron 2601 (1979) 35. [9] H. Suga, A. Funyu, A. Kakehi, Org. Lett. 97 (2007) 9. [10] S.A. Popov, V.A. Reznikov, J. Heterocycl. Chem. 293 (2006) 43. [11] M.T. Nguyen, A.K. Chandra, T. Uchimaru, S. Sakai, J. Phys. Chem. A 105 (2001) 10943. [12] A. Padwa (Ed.), 1,3-Dipolar Cycloaddition Chemistry, vol. I, WileyInterscience, New York, 1984, pp. 1–176. [13] For an excellent review of nitrile oxide cycloadditions, see: P. Caramella, P. Grunanger, in: A. Padwa (Ed.), 1,3-Dipolar Cycloaddition Chemistry, vol. I, Wiley-Interscience, New York, 1984, pp. 291–391. [14] D. Poppinger, Aust. J. Chem. 465 (1976) 29. [15] D. Poppinger, J. Am. Chem. Soc. 7468 (1975) 97. [16] A. Komornicki, J. Goddard, H.F. Schaefer III., J. Am. Chem. Soc. 1763 (1980) 102.

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