A computational study on the substituent effects and product stereoselectivity of the intermolecular...

A computational study on the substituent effects and product stereoselectivity

of the intermolecular formal aza-[3C3] cycloaddition reaction between

vinylogous amides and a,b-unsaturated imine cations

Yan Wang, De-Cai Fang *, Ruo-Zhuang Liu **

College of chemistry, Beijing Normal University, 19th Xin Wai St., Beijing 100875, China

Received 23 March 2006; received in revised form 31 May 2006; accepted 1 June 2006

Available online 10 June 2006

Abstract

The substituent effects and product stereoselectivity of the title reaction has been studied using density functional theory (DFT) calculations at

the B3LYP/6-31G(d,p) level of theory. It was found that the substituents do not perturb the mechanism of intermolecular formal aza-[3C3]

cycloaddition. Our calculations also show that methyl or benzyl groups on the N atom of vinylogous amide favor the addition step, but alkyl

substituents on the either N atom or terminal C atom of a,b-unsaturated imine cation have opposite effects. Alkyl substituents on the N atom of

a,b-unsaturated imine cation may lower the activation barriers for elimination of amide. The steric interaction between two substituents leads to

the formation of major product both thermodynamically and kinetically.

q 2006 Elsevier B.V. All rights reserved.

Keywords: [3C3] Cycloaddition; Vinylogous amide; a, b-Unsaturated imine cation; DFT calculations; Substituent effects; Stereoselectivity

1. Introduction

The reactions of vinylogous amides with a,b-unsaturated

iminium salts leading to 1,2-dihydropyridines [1–8] (Scheme 1)

have attracted intensive attention of experimental chemists in

recent years as these reactions may be used to synthesize various

piperidinyl heterocycles, representing useful fundamental

synthetic building blocks. This type of reaction may be charac-

terized as a formal aza-[3C3] cycloaddition reaction [9–13] due

to the fact that each portion provides three atoms to form a new

six-membered heterocycle in either intermolecular [1–4] or

intramolecular [5–7] fashion. Experimental chemists have

also assumed that intermolecular reactions involve a sequence

that consists of C-1,2-addition to an iminium salt followed by

b-elimination giving an 1-azatriene and a 6p-electron electro-

cyclic ring-closure of 1-azatriene as shown in Scheme 1,

proceeding with high stereoselectivity (Scheme 2). In previous

work, a simple model of the above intermolecular reaction has

been studied theoretically [14] (Scheme 3). However, the

substituent effect and stereoselectivity are still unreported.

0166-1280/$ - see front matter q 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.theochem.2006.06.004

* Corresponding authors. Tel./fax: C86 10 58805422.

E-mail addresses: [email protected] (D.-C. Fang), [email protected]

(R.-Z. Liu).

In this article, 12 differing substituted modes, as shown in

Scheme 4, are reported to elucidate the substituent effect and

stereoselectivity.

2. Computational methods

All calculations were carried out with Gaussian 03 suite of

programs [15]. The structures of reactants, complexes, tran-

sition states, intermediates and products were optimized at

density functional theory (DFT) B3LYP level using the

6-31G(d,p) basis set. The stationary points were characterized

by frequency calculations in order to verify that minima and

transition structures (TS) have zero and one imaginary

frequency, respectively. Zero-point energy (ZPE) corrections

were included without scaling. The solvent effects were

considered by performing B3LYP/6-31G(d,p) single-point

calculations on the gas-phase optimized geometries using the

self-consistent reaction field (SCRF) method based on the

polarizable continuum model (PCM) [16–25], in which

toluene was used as solvent to mimic experimental conditions

(3Z2.379).

3. Results and discussions

As shown in Scheme 3, there are three generalized steps for

the reactions investigated herein. The first step is the addition

Journal of Molecular Structure: THEOCHEM 770 (2006) 169–178

www.elsevier.com/locate/theochem

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mailto:[email protected]

Scheme 1.

Y. Wang et al. / Journal of Molecular Structure: THEOCHEM 770 (2006) 169–178170

reaction between two reactants 1 and 2 with formation of a

cationic intermediate INT1. The second step is the removal of

HC and subsequent 1,4-elimination of HNR12 (NH3 in parent

reaction) from INT1 to form a molecular intermediate INT3

promoted by the nucleophilic attack of AcOK in the reaction

medium. The final step is the subsequent isomerization of

INT3 and electrocyclic ring-closure yielding the final

product. The differing substituent groups in varied positions

do not change the above mechanism, however stereoselectivity

for the final products is evident, rationalized in the following

discussion.

3.1. Substituent effect on the addition step

In this section, because trans-isomer of 2 is more stable

than cis-one (4.9–8.8 kcal/mol), only trans-isomer of 2 was

considered. The addition step proceeds via a transition state

Scheme

(TS1) connecting INT1 directly with complex (COM1)

between 1 and 2. Three substitution sites investigated

herein are as follows: N atom of 1, N atom of 2 and terminal

C atom of 2. The inclusion of larger substituents leads to

more complicated structures of COM1, as shown in Fig. 1,

along with the values of intermolecular distance for H/O.

Three types of structures for COM1 have been located in the

12 cases considered, in which two conformers are stabilized

in the form of intermolecular ionic [26] C–H/O hydrogen

bonding [27–30], denoted as COM1-X and COM1-Y,

respectively. Another type of complexes is produced in the

form of intermolecular ionic N–H/O hydrogen bonding

denoted as COM1-Z. Due to the fact that N–H/O type

hydrogen bonding is usually stronger than C–H/O, N–H/O hydrogen bonded complexes, for reactions h and l, can be

optimized. Extensive exploratory searches on the complexes

for reactions b, d and f located only one C–H/O hydrogen

bonded complex, as indicated in Fig. 1. The energy

difference for two different C–H/O hydrogen bonded

complexes is very small, usually less than 0.3 kcal/mol, so

it is not important to differentiate them. By comparing the

bond length of H/O in Fig. 1 with energy changes in

Table 1, it is clear that the longer H/O is, the weaker the

intermolecular stabilization of the complex is. The molecular

graphs of TS1 and INT1, obtained with the AIM2000

program [31,32], are depicted in Fig. 2, from which it can

be found that inclusion of methyl or benzyl groups at the N

atom of 1 may shorten the intermolecular distance for H/O

of COM1, but also increases the length of the C–C bond

forming in TS1. Opposite effects are observed for alkyl

groups at the N or terminal C atom of 2.

The computed relative energies of stationary points for the

addition step are listed in Table 1, from which one may observe

2.

Scheme 3.

Y. Wang et al. / Journal of Molecular Structure: THEOCHEM 770 (2006) 169–178 171

that the substituted groups affect the relative energies of TS1

and INT1 more significantly than that of COM1. The methyl

or benzyl groups of R2 can lower the relative energies of

COM1, TS1 and INT1, while opposite effects are seen for

Scheme

alkyl groups of R1 or R3. In addition, methyl or benzyl groups

of R2 can give an early and more stable transition state along

the reaction path, while alkyl groups of R1 or R3 have inverse

effects. When all three substituent positions are other than

4.

Fig. 1. Three conformations of COM1, including the intermolecular distances (A) for H/O optimized at the B3LYP/6-31G(d,p) level of theory and eight structures

of INT3.


hydrogen atoms, the overall result is an elevation of relative

energies for COM1, TS1 and INT1.

For reactions a and f, the relative energies of SCRF-

B3LYP/6-31G(d,p) single-point calculations are very close

to those of the optimized geometries in solution phase at

same level (see Table 1). Thus, the solvent effects were

characterized by SCRF-B3LYP/6-31G(d,p) single-point

calculations on the gas-phase optimized geometries,

employing the relatively simple polarizable continuum

model (PCM) [16–25]. The relative energies in toluene

together with relative free energies of stationary points for

reactions a and f only either in gas phase or in solution

phase are also summarized in Table 1 from which one may

observe that the solvent makes ionic reactant 2 more stable

and leads to the relative energies of COM1, TS1 and

INT1 in toluene being higher than corresponding results in

gas phase, which is similar to the parent reaction (without

any substituted groups). The changes of the relative

energies of COM1, TS1 and INT1 are 8–14, 5–13 and

6–15 kcal/mol, respectively. In consequence, the inclusion

of solvent is not in favor of addition step, both thermo-

dynamically and kinetically. From Table 1, it can also be

found that the influences of the substituted groups and the

solvent toluene on relative free energies are similar to

those on relative energies although the magnitude of

relative free energies is different somewhat from that of

relative energies, i.e. the trend of change of relative

energies is the same as that of relative free energies.

Table 1

The B3LYP/6-31G(d,p) computed relative energies with ZPE correction and relative free energies (DE and DG298 K, kcal/mol) of stationary points for addition step

COM1-X COM1-Y COM1-Z TS1 INT1

DE DG DE DG DE DE DG DE DG

a K26.5 K26.3 K16.8 K4.8 7.9 K14.2 K1.7

(K14.8)a (K14.8) (4.1) (K5.4)

[K13.6]b [K4.1] [5.1] [17.5] [K3.0] [9.5]

b K23.5 7.7 K1.2

(K14.5) (12.3) (3.7)

c K27.8 K27.5 K8.5 K19.7

(K15.2) (K15.1) (2.1) (K8.2)

d K24.6 3.1 K6.1

(K14.9) (10.2) (0.8)

e K28.5 K28.4 K11.1 K21.6

(K15.2) (K15.2) (1.6) (K8.7)

f K22.9 K13.6 6.1 19.1 K1.5 12.1

(K13.6) (12.6) (5.6)

[K12.2] [K1.8] [13.5] [26.7] [7.1] [20.6]

g K25.1 K24.8 0.3 K8.1

(K14.7) (K14.3) (9.3) (0.1)

h K30.3 4.2 K4.3

(K18.6) (10.0) (1.3)

i K23.1 K23.2 9.8 1.1

(K15.3) (K15.4) (15.0) (6.0)

j K21.7 K21.8 11.9 3.2

(K14.5) (K14.7) (16.4) (7.4)

k K21.9 K21.8 10.4 4.1

(K14.0) (K14.2) (15.6) (8.9)

l K28.9 6.3 K1.7

(K18.0) (11.5) (3.0)

a The data in parentheses are for solution phase single-point calculations.b The data in square bracket are optimized results in solution phase.


In addition, the rate constants of addition reactions a, j and k

in toluene solution were calculated approximately according to

the transition state theory (TST) with POLYRATE program

[33]. The calculated results are shown in Table 2, from which

one can see that the rate constants for reactions j and k at room

temperature of 298.15 K, are quite small, but when the

temperature is raised to 423.15 K (experimental temperature),

the reactions can proceed in moderate rate.

3.2. Substituent effect on the elimination of HC and HNR12

From the previous work, one may observe that the elimin-

ation of NH3 has two pathways, which has no substantial

difference, so here only one situation is considered for

simplicity. The nucleophilic attack of the O atom in AcOK

on H atom of C1 in INT1 forms complex COM2, corre-

sponding to a molecular complex between AcOH and INT2.

The combination of one anion (AcOK) with cationic inter-

mediate INT1 is barrier-less. There exists a novel

intramolecular N–H/N hydrogen bond in INT2, which is

relevant to HNR12 elimination. The removal of HNR1

2 from

INT2 is finalized via TS2 by way of an 1,4-elimination to form

an intermolecular hydrogen bonded complex COM3 between

HNR12 and INT3 on the reaction profile. The geometries of

INT2 and TS2 are given in Fig. 2 together with the values of

the bond lengths directly involved in the reaction mechanism.

The computed relative energies of stationary points for HC

and HNR12 elimination are listed in Table 3, from which one

can see that the energies of the optimized stationary points are

below those of AcOKCINT1. The energy barriers for

the subsequent removing of HNR12 range from 18.1 to

24.8 kcal/mol, in which that of reaction f is the smallest.

Such barriers are easily overcome due to the larger exothermic

effect for the formation of COM2. For example, more than 130

(in gas phase) or 70 kcal/mol (in toluene solution) of energy is

released (see Table 3).

The energy of AcOK in solution is lowered by much more

than those of other stationary points, thus the relative energies

of neutral molecules are raised noticeably in toluene solvent.

However, solvent effect does not have a noticeable influence on

the energy barrier of HNR12 elimination, as neutral molecules

are stabilized to approximately uniform extent by less polar

solvent.

3.3. Substituent effect on isomerization and cyclization

Following the two steps above, the remaining INT3 has

only eight different forms (Fig. 1), of which the former five will

yield two identical products and hence only the latter three

INT3j, INT3k and INT3l are discussed here (see Scheme 5). It

must be indicated that the methyl group in cyclohexane chair

conformation is in either equatiorial or axial form, but only

Fig. 2. Molecular graphs of some optimized stationary points in the gas phase (obtained with the AIM2000 program). The bond lengths directly involved in the

reaction are given in angstrom and the nuclear repulsion energies are given in atomic unit.

Table 2

The rate constants k (cm3 molK1 sK1) of addition reaction a, j and k in toluene

solution

298.15 K 423.15 K

a 3.49!104 6.91!105

j 6.68!10K6 6.34!10K2

k 2.64!10K6 1.84!10K2


equatiorial form will be discussed in this section as it is more

stable for a bulky group to be in the equatiorial position. It was

also found that toluene as solvent has only negligible influence

on the geometries and relative energies of isomerization and

final cyclization, due to only neutral species being involved in

these steps. Therefore, only gas phase results are reported for

reactions j, k and l herein.

Table 3

The B3LYP/6-31G(d,p) computed relative energies (DE, kcal/mol) with ZPE correction of stationary points for HC and HNR12 elimination step

INT1CAcOK COM2 INT2CAcOH TS2CAcOH COM3CAcOH INT3CHNR12CAcOH

a 0.0 K139.1 K132.9 K108.1 K121.3 K119.3

(0.0)a (K76.7) (K73.1) (K47.0) (K57.8) (K55.8)

[0.0]b [K74.4] [K72.0] [K47.8] [K59.8] [K58.9]

b 0.0 K136.6 K132.6 K111.0 K120.3 K117.5

(0.0) (K77.1) (K75.6) (K52.9) (K60.6) (K58.6)

c 0.0 K132.9 K126.5 K102.8 K117.3 K114.5

(0.0) (K73.6) (K70.0) (K44.5) (K56.6) (K53.5)

d 0.0 K131.7 K127.8 K107.3 K116.9 K114.2

(0.0) (K74.1) (K72.5) (K50.4) (K58.5) (K56.3)

e 0.0 K131.6 K124.9 K102.2 K116.3 K113.9

(0.0) (K72.7) (K68.9) (K44.3) (K56.4) (K53.4)

f 0.0 K130.0 K126.1 K108.0 K117.2 K114.4

(0.0) (K72.9) (K71.5) (K51.9) (K60.0) (K57.8)

[0.0] [K70.5] [K70.2] [K52.0] [K61.1] [K59.6]

g 0.0 K129.2 K125.4 K105.7 K115.7 K112.6

(0.0) (K72.7) (K71.2) (K49.9) (K58.8) (K55.9)

h 0.0 K136.4 K130.1 K105.8 K122.1 K118.5

(0.0) (K75.4) (K71.7) (K46.3) (K60.3) (K56.7)

i 0.0 K129.2 K125.4 K105.1 K116.8 K113.4

(0.0) (K72.6) (K71.1) (K49.7) (K59.9) (K56.9)

j 0.0 K128.5 K124.7 K104.3 K117.1 K112.7

(0.0) (K72.3) (K70.8) (K49.2) (K60.7) (K56.9)

k 0.0 K128.8 K125.6 K101.9 K114.3 K110.5

(0.0) (K72.9) (K72.4) (K47.9) (K58.5) (K55.2)

l 0.0 K135.5 K129.1 K105.0 K121.3 K117.7

(0.0) (K75.1) (K71.5) (K46.1) (K60.1) (K56.6)

a The data in parentheses are for solution phase single-point calculations.b The data in square bracket are optimized results in solution phase.


After INT3j, the isomerization may take place in two

different directions, generating two different gauche inter-

mediates shown in Fig. 2, from which one may see that

the nuclear repulsion energy of INT4j 0 is about 14 a.u.

less than that of INT4j. The dihedral angle C4–C3–C2–

C1 in INT4j and INT4j 0 are K61.6 and 48.08, respect-

ively, which results from the differing steric repulsions of

substituents at the C1 and N7 atoms, while the corre-

sponding values for model reaction are only K29.0 and

30.08. The relative energies of INT4j and INT4j 0 are

increased by 3.6 and 2.6 kcal/mol, respectively, when

compared to the parent reaction. The difference of

nuclear repulsion energy between two TSs of the cycliza-

tion process is also about 14 a.u., leading to TS4j 0 being

approximately 4 kcal/mol lower than TS4j.

The potential energy surfaces obtained are shown in Fig. 3,

together with the relative free energies of stationary points for

reaction a either in gas phase or in solution phase, from which

one can realized that the relative free energies are very close to

relative energies for reaction a, i.e. the entropy factor is not

crucial for this process. From Fig. 3, it can also be seen that the

forming gauche intermediates are much less stable than the

trans intermediate INT3j. With the transition state theory

(TST), the reaction rate constants of isomerization from

INT3j to INT4j or INT4j 0, at an experimental temperature

of 423 K, are 1.896!107 and 2.758!107 sK1, respectively,

and those for the cyclization processes are 1.072!105 and

5.964!106 sK1, i.e. these two processes are comparable.

However, the processes of conversion from INT4j or INT4j 0

to INT3j are much faster, and the rate constants are 1.621!1012 and 7.011!1011 sK1, i.e. the concentrations of INT4j and

INT4j 0 in the reaction system are very small. Therefore, such

two-step reaction processes might be simplified to be one-step

in order to calculate the overall reaction rate constant from

INT3j to final products. The ratio of rate constants for

generating 3j and 3j 0 is 187.1, a little larger than the experi-

mental one. Such difference may be due to the DFT

approximation errors. If the energy of TS4j were lowered by

2 kcal/mol, the calculated ratio would be within experimental

one.

As for the reaction k, the difference for the relative

energies of two transition states TS4k and TS4k 0 are

comparable to their counterparts in reaction j (see Fig. 3)

and the energy difference for the final products is also

approximately 5 kcal/mol. The calculated overall rate

constants for the formation of two different products are

14.07 and 922.1 sK1, respectively, with our calculations

representing the experimental observed stereoselectivity. In

order to further characterize the intrinsic bases of high

stereoselectivity, a reaction (denoted as reaction l) with H

atom to replace methyl on N atom in reaction j has been

designed. From Fig. 3, it is clear that the final products

Scheme 5.


become more stable, almost equivalent to those of the

model reaction, indicating that the interaction between R2

and R3 groups not only makes final products unstable

relative to INT3 but also makes the energy difference

between two products increase.

4. Conclusions

According to the above discussions, the following con-

clusions may be drawn:

1. The substituted groups on three sites do not change the

overall reaction mechanisms.

2. The addition step is favored by substituting 1 with methyl

or benzyl groups while opposite effects are observed by

substituting 2 with alkyl groups at N and/or terminal C

atoms. Alkyl substituent groups on the N atom of the a,b-

unsaturated imine cation can lower energy barrier for the

elimination of HNR12 by approximately 3 kcal/mol or

perhaps more, while substituents at other positions

change the barrier only slightly, usually less than

1 kcal/mol.

3. The substituted groups on 1 and 2 can make the

isomerization and cyclization steps slightly less favorable

thermodynamically, and they can also lead to larger

energy difference between the two final products. The

formation of the major product is favored not only

thermodynamically but also kinetically.

4. Our calculated rate constants accounts for the experimen-

tally observed ratio of final products.

Fig. 3. Potential energy profile of the isomerization and cyclization step for reactions a, j, k and l. The data in parentheses are relative free energies DG298 K in gas

phase and the data in square bracket are relative free energies in solution phase.


Acknowledgements

Authors L.R.Z. and F.D.C. express thanks to the Major State

Basic Research Development Programs (Grant no.

2004CB719900) for support and author F.D.C. thanks the

National Natural Science Foundation of China (no.

20372011) and the Program for New Century Excellent

Talents in University (NCET) for support.

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A computational study on the substituent effects and product stereoselectivity of the intermolecular...

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