Scholarly article on topic 'Aromatic heterocycles XII. Semiempirical PM3 study of Diels-Alder cycloaddition reaction of substituted phosphabenzenes'

Aromatic heterocycles XII. Semiempirical PM3 study of Diels-Alder cycloaddition reaction of substituted phosphabenzenes Academic research paper on "Chemical sciences"

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Academic research paper on topic "Aromatic heterocycles XII. Semiempirical PM3 study of Diels-Alder cycloaddition reaction of substituted phosphabenzenes"

Central European Science Journals

Central European Journal of Chemistry

CEJC 2(1) 2004 34-51

Aromatic heterocycles XII. Semiempirical PM3 study of Diels-Alder cycloaddition reaction of substituted phosphabenzenes

Liliana Pacureanu1 *, Mircea Mracec1, Zeno Simon1

Institute of Chemistry of Romanian Academy, 24 Mihai Viteazul Avenue, 300223 Timisoara, Romania

Abstract: We report the results of a semiempirical PM3 study of the 1,4 cycloaddition reaction of substituted A 3 -phosphabenzenes with alkynes. The influence of the nature, position and steric hindrance of substituents on the reaction energy is studied. Except for some values, the results are in reasonable agreement with experimental observations and electronic effects of substituents.

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Keywords: Diels-Alder cycloaddition reaction, A3-phosphabenzene, 1-phosphabarrelene, semi-empirical PM3 method

1 Introduction

Reactivity and coordinating properties of aromatic phosphorus heterocycles differ from those of pyridine [1-8]. In the last few years, interest in electronic structure and reactivity of A3-phosphabenzenes have known increased development [1, 3, 9-11].

A3-phosphabenzenes react with nucleophilic reagents and radicals [12]. The Diels-Alder cycloaddition reaction of A3-phosphabenzenes take place at 1,4 positions giving 1-phosphabarrelenes without 1,2 cycloaddition to the double bond P=C. The sp2-hybridized phosphorus atom confers to A3-phosphabenzenes good n-acceptor and poor a-donating properties. In contrast, pyridine is good a donor [1], [3], [7], [8].

Ab initio calculation emphasized no important perturbation of aromaticity due to the replacement of the CH unit by the phosphorus atom, and the resonance energy of A3* E-mail:

Received 21 July 2003; accepted 2 October 2003

phosphabenzene as determined from SCF/3-21G* calculation is 88% of benzene resonance energy [13]. Nucleus independent chemical shift criteria (NICS) calculated at B3LYP/6-31G* level showed a value of -10.2 for phosphabenzene, which is close to NICS calculated for benzene: -11.5 [14].

In phosphabenzene, the more electropositive phosphorus atom bears a positive charge. The converse situation is that of pyridine where the nitrogen atom is negatively charged. The lone pair orbital is the HOMO in pyridine where in A3-phosphabenzene the lone pair is the third occupied level while the LUMO of phosphabenzene is lower in energy conferring very interesting coordinating properties [10], [15].

The substitutedA3-phosphabenzenes [16-18] react with highly reactive dienophiles such as hexafluorobutyne-2, benzyne, dicyanoacetilene at positions 1,4 to form 1-phos-phabarrelenes [19-22]. Even the o-complexes of 2-pheny-4,5-dimethyl-A3-phosphabenzene with tungsten pentacarbonyl undergo 1,4 cycloaddition reaction [22].

The 1,4 Diels-Alder cycloaddition reaction of A3-phosphabenzenes occurs easily if the diene character of phosphabenzene is well pronounced. The stabilizing factor for normal Diels-Alder reactions is the interaction HOMO diene - LUMO dienophile, thus the electron withdrawing substituents of the dienophile and the electron releasing substituents of the diene will increase the reaction rate [23].

Mârkl and Lieb observed the influence of the nature of the substituent group on the 1,4 cycloaddition reaction of A3 -phosphabenzenes with hexafluorobutyne-2 (Figure 1) [21]. The cycloaddition reaction occurs more readily with 2,6-ditertbuthyl-4-methyl A3-

Fig. 1 The reaction of A3-phosphabenzenes with alkynes.

phosphabenzene and 2,6-dimethyl-4-phenyl-A3-phosphabenzene than electron poor 2,4,6-triphenyl-A3-phosphabenzene (Table 4).

Mârkl, Lieb and Martin [24] obtained benzophosphabarrelenes starting from 2,4,6-triphenyl and 2,4,6-tritertbuthyl A3-phosphabenzene and arynes. Better yield was obtained with the more electron rich 2,4,6-tritertbuthyl A3-phosphabenzene (Table 4). The reactions are depicted in Figure 2.

1-Phosphabarrelenes are highly stable, and do not undergo scission or retro Diels-Alder reaction with split off alkynes in thermal or ionization conditions [22].

This study was undertaken to find out the influence of the position, nature and steric hindrance of the substituents of the A3-phosphabenzene on the 1, 4 Diels-Alder cycloaddition reaction using the semiempirical PM3 method.

Fig. 2 The reaction of A3-phosphabenzenes with benzyne.

The 1,4-Diels-Alder cycloaddition reaction of phosphabenzene with hexafluorobutyne-2 is thermodynamically controlled as M. Mracec et al [9] have shown, in concordance with Woodward-Hoffman rules [25], [26]. The number of n electrons involved in the reaction and symmetry predictions prevail, thus the reaction is thermally allowed [25], [26].

2 Method

Molecular geometry calculation and full optimization of reagent and product molecules were performed with the Hyper-Chem 5.11 program [27]. First we proceed to optimize all molecules by molecular modeling using force field MM+. The re-optimization of all molecules was carried out with semiempirical approximation PM3-RHF for the F operator [28]. The method of accelerate convergence with a convergence limit of 10_5 SCF was chosen. The molecules were considered in vacuum and the optimization algorithm Polack-Ribiere conjugated gradient with 0,01Kcal/Amol RMS gradient was used.

3 Results and discussion

The data resulted from PM3 calculations are shown in Table 3, Table 4 and Table 5. Special attention was paid to the charges of the centers of reactions: the carbon atom at position 4 and the phosphorus atom in order to establish the stronger [1], [4] dipole; this is more important in the case of polarisable dienophiles.

From Table 3 we observed that the alkyl substituents decrease the energy of reaction. The lowest values of reaction energies were obtained for 2,6 dialkyl substituted A3-phosphabenzenes with a group at position 4 without steric hindrance. On the contrary electron withdrawing groups at position 2,6 of A3-phosphabenzenes, and a group producing steric hindrance at position 4 will increase the reaction energy. Alkyl substituents at position 4 of the 2,6-phenyl A3-phosphabenzenes influence the reaction energy. Thus, methyl and ethyl substituents decrease the reaction energy, while isopropyl and tert-butyl groups increase the reaction energy. The phenyl electron withdrawing group increases the heat of reaction with respect to methyl and ethyl groups (Figure 3).

The lowest reaction energy of 2,4,6-trimethyl phosphabenzene is due to minimal steric hindrance. Electron withdrawing groups phenyl, isopropyl and tertbutyl at position 4 increase the reaction energy. We have no explanation for the higher value of the 4-ethyl derivative (Figure 4).

Tert-butyl substituents at position 2,6 decrease the reaction energy, especially if position 4 bears a stronger electron releasing group, without steric hindrance. Phenyl substituents do not influence the reaction energy as much as tert-butyl groups do (Figure 5).

The computed values of reaction energy of 2,4,6-trimethyl-A3-phosphabenzene, 2,4,6-triethyl-A3-phosphabenzene and 2,4,6-triisopropyl-A3- phosphabenzene are lower than those of 2,4,6-tritolyl and 2,4,6-tribenzyl-A3-phosphabenzene, which is in agreement with the electronic effects of substituents. In the case of phosphabenzene, the absence of steric hindrance decreases the reaction energy.

In agreement with experimental data the reaction energy of 2,6-ditertbutyl-4-methyl-A3-phosphabenzene is lower than the reaction energy of 2,4,6-triphenyl-A3-phosphabenzene and 2,6-dimethyl-4-phenyl-A3-phosphabenzene.

The data listed in Table 4 shows that alkyl substituents, except ethyl derivatives, decrease the reaction energy of phosphabenzenes with benzyne. The lower values are obtained in the case of reaction of 2,6-ditertbutyl-A3-phosphabenzenes which bear methyl and isopropyl substituents at position 4. Tertbutyl and phenyl group at position 4 increase the reaction energy (Figure 6).

In the case of 2,6-diphenyl-phosphabenzenes, the electron releasing groups methyl, ethyl, and isopropyl decrease the reaction energy in comparison with 2,4,6-triphenylphos-phabenzene. A tertbutyl group at position 4 also increase the reaction energy (Figure 7).

The reaction energy in the case of 2,6-dimethyl substituted phosphabenzenes is increased by tertbutyl and phenyl substituents at position 4, but again the heat of reaction of ethyl derivatives is a little higher than those of methyl derivatives (Figure 8).

The high thermodynamic stability of benzophosphabarrelenes observed experimentally is in agreement with the heat of formation obtained by PM3 calculation. The reaction energies of phosphabenzenes substituted with methyl groups at position 2,6 are lower than those of phosphabenzenes substituted with phenyl groups at the same position but higher than those of tert-butyl substituted phosphabenzenes. 2,4,6-Triethylphospha-benzene and 2,6methyl-4-ethyl-A3-phosphabenzene presents inexplicable higher values for the reaction energy.

2,4,6-Triisopropyl-A3-phosphabenzene presents lower reaction energy while 2,4,6-tritolyl and 2,4,6-tribenzyl-A3-phosphabenzenes have higher values of the reaction energy in agreement with electronic effects of substituents. The reaction energy of 2,4,6-tritertbutyl-A3-phosphabenzene is lower than the reaction energy of 2,4,6-triphenyl-A3-phosphabenzene as experimental data shows. In consequence the results of PM3 calculation, except some values for ethyl derivatives, are in agreement with the experimental data and the electronic effects of substituents.

Atomic charges listed in Table 5 emphasized that the electron withdrawing group phenyl decreases the atomic charge at C4, while electron releasing groups methyl, ethyl,

isopropyl, and tert-butyl increase the atomic charge at C4, thus the [1], [4] dipole is strengthen by alkyl groups and consequently the reactions energy of these compounds are lower. The positive charge of the phosphorus atom increases in the case of phenyl substituted phosphabenzenes in position 2,4 and 6. The obtained values for phosphorus charges of 0.471 and for carbon at position 4 of -0.148 are reasonable in comparison with those obtained with MP2/6-31G* method of 0.55 for phosphorus and -0.25 for C4, respectively [13].

The energy of the HOMO obtained by PM3 calculation is -9.073eV for A3-phosphaben-zene and -8.64eV for 2,4,6-tritertbutyl-A3-phosphabenzene. The absolute values are close to the values of n ionization energies of the HOMO orbital given by photoelectron spectra of phosphabenzene of 9.12 eV [10] and 8.6eV for 2,4,6-tritertbutyl-A3-phosphabenzene

4 Conclusion

The results obtained by PM3 calculation, except for some values, are in good agreement with the available experimental data. Alkyl substituents decrease reaction energies, but steric hindrance influences the reaction energy, especially if bulky substituents such as tert-butyl and isopropyl groups are situated at position 4. Electron withdrawing groups such as phenyl and benzyl increase reaction energies, particularly at position 4. The values of heat of formation of 1-phosphabenzobarrelenes confirm the highly thermodynamic stability of these compounds.

Atomic charges and HOMO energies resulting from PM3 calculation show reasonable values compared with those obtained by ab initio calculation and with ionization energies obtained from photoelectron spectra.


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Compound R2=R6 R4 Yield [%]

3a C6H5 C6H5 53

3b C(CHa)3 CH3 89

3c CH3 C6H5 41

a: R2=R4=R6=C6H5 b: R2=R6=C(CH3)a ; R4=CHa c: R2=R6=CH3 ; R4= C6 H5.

Table 1 The yields of 1,4 cycloaddition reaction of A3-phosphabenzenes with hexafluorobutyne-2 [21].

Compound R2—R4—R6 R Yield [%]

6a C6H5 H 15

6b C6H5 Cl 17

6c C(CH3)3 H 69

6d C(CH3)3 Cl 67

a: R2 =R4=R6 =C6 H5 R=H b: R2 =R4=R6 =C6H5 R=Cl c: R2 =R4=R6 =C(CH3)a R=H d: R2 =R4=R6 =C(CHa)a R=Cl.

Table 2 The yields of 1, 4 cycloaddition reaction of A3-phosphabenzenes with arynes [24].

No Phosphabenzene AH/ (kcal/mol) AH/ (kcal/mol) barrelene ah*r (kcal/mol)

1 2,4,6-triphenyl-phosphabenzene 127.910 -145.820 -19.949

2 2,6-diphenyl-4methyl-phosphabenzene 94.365 -181.469 -22.053

3 2,6-diphenyl-4ethyl-phosphabenzene 89.715 -186.612 -22.546

4 2,6-diphenyl -4isopropyl-phosphabenzene 85.108 -184.389 -15.716

5 2,6-diphenyl-4tbutyl-phosphabenzene 80.652 -186.438 -13.309

6 2,6-diphenyl-4benzyl-phosphabenzene 122.935 -148.733 -17.887

7 2,4,6-trimethyl-posphabenzene 24.291 -254.678 -25.188

8 2,6-dimethyl-4ethyl-phosphabenzene 18.011 -257.208 -21.438

9 2,6-dimethyl-4isopropyl-phosphabenzene 15.110 -261.544 -22.873

10 2,6-dimethyl-4tbutyl-phosphabenzene 9.067 -257.530 -12.816

11 2,6-dimethyl-4phenyl-phosphabenzene 56.107 -216.992 -19.318

12 2,4,6-tritbutyl-phosphabenzene -8.071 -277.422 -15.570

13 2,6-ditbutyl-4methyl-phosphabenzene 5.969 -274.533 -26.721

14 2,6-dibutyl-4ethyl-phosphabenzene -3.331 -277.684 -20.572

15 2,6-ditbutyl-4isopropyl-phosphabenzene -3.429 -281.455 -24.245

16 2,6-ditbutyl-4phenyl-phosphabenzene 39.204 -236.829 -22.252

17 2,4,6-triethyl-phosphabenzene 10.332 -265.176 -21.727

18 2,4,6-triisopropyl-phosphabenzene 9.492 -272.643 -28.354

19 2,4,6-tribenzyl-phosphabenzenez 110.025 -138.175 5.581

20 2,4,6-tritolyl-phosphabenzene 99.419 -174.380 -20.018

21 phosphabenzene 43.050 -240.115 -29.384

*AHf of hexafluorobutyne-2 is -253.781 kcal/mol

Table 3 The heats of formation for reagents, products and reaction energies for the reaction of substituted A3-phosphabenzenes with hexafluorobutyne-2 as resulted from PM3 calculation.

No Phosphabenzene AH°f (kcal/mol) AH/ (kcal/mol) barrelene ahr (kcal/mol)

1 2,4,6-triphenyl-phosphabenzene 127.910 164.559 -93.099

2 2,6-phenyl-4methyl-phosphabenzene 94.365 128.023 -96.090

3 2,6-pheny-l4ethyl-phosphabenzene 89.715 124.145 -95.318

4 2,6-phenyl -4isopropyl-phosphabenzene 85.108 120.388 -94.468

5 2,6-phenyl-4tbutyl-phosphabenzene 80.652 117.466 -92.934

6 2,6-phenyl-4benzyl-phosphabenzene 122.935 158.364 -94.319

7 2,4,6-trimethyl-phosphabenzene 24.291 57.384 -96.655

8 2,6-methyl-4ethyl-phosphabenzene 18.011 53.520 -94.239

9 2,6-methyl-4isopropyl-phosphabenzene 15.110 49.775 -95.083

10 2,6-methyl-tbutyl-phosphabenzene 9.067 46.927 -91.888

11 2,6-methyl-4phenyl-phosphabenzene 56.107 93.900 -91.955

12 2,4,6-tritbutyl-phosphabenzene -8.071 26.902 -94.775

13 2,6-tbutyl-4methyl-phosphabenzene 5.969 37.613 -98.104

14 2,6-tbutyl-4ethyl-phosphabenzene -3.331 33.731 -92.686

15 2,6-tbutyl-4isopropyl-phosphabenzene -3.429 29.794 -96.525

16 2,6-tbutyl-4phenyl-phosphabenzene 39.204 73.875 -95.077

17 2,4,6-triethyl-phosphabenzene 10.332 45.650 -94.430

18 2,4,6-triisopropyl-posphabenzene 9.492 35.853 -103.387

19 2,4,6-tribenzyl-phosphabenzene 110.025 146.210 -93.563

20 2,4,6-tritolyl-phosphabenzene 99.419 136.268 -92.899

21 phosphabenzene 43.050 75.106 -97.724

*AH; of benzyne is 129.748 kcal/mol

Table 4 PM3 calculation of the heat of formation for reagents, products and reaction energies for substituted A3-phosphabenzenes with benzyne.

No Phosphabenzene Atomic charge at P Atomic charge at C4 HOMO [eV] LUMO [eV]

1 2,4,6-triphenyl-phosphabenzene 0.465 -0.042 -8.355 -1.341

2 2,6-phenyl-4methyl-phosphabenzene 0.444 -0.091 -8.441 -1.160

3 2,6-phenyl-4ethyl-phosphabenzene 0.450 -0.096 -8.467 -1.161

4 2,6-phenyl -4isopropyl-phosphabenzene 0.452 -0.093 -8.473 -1.152

5 2,6-phenyl-4tbutyl-phosphabenzene 0.448 -0.082 -8.455 -1.133

6 2,6-phenyl-4benzyl-phosphabenzene 0.455 -0.093 -8.478 -1.174

7 2,4,6-trimethyl-posphabenzene 0.422 -0.095 -8.589 -0.825

8 2,6-methyl-4ethyl-phosphabenzene 0.433 -0.097 -8.675 0.078

9 2,6-dimethyl-4isopropyl-phosphabenzene 0.432 -0.094 -8.670 -0.776

10 2,6-dimethyl-4tbutyl-phosphabenzene 0.430 -0.083 -8.659 -0.755

11 2,6-dimethyl-4phenyl-phosphabenzene 0.449 -0.046 -8.448 -1.065

12 2,4,6-tritbutyl-phosphabenzene 0.423 -0.082 -8.647 -0.765

13 2,6-ditbutyl-4methyl-phosphabenzene 0.417 -0.095 -8.607 -0.792

14 2,6-ditbutyl-4ethyl-phosphabenzene 0.423 -0.100 -8.654 -0.795

15 2,6-ditbutyl-4isopropyl-phosphabenzene 0.424 -0.096 -8.665 -0.789

16 2,6-ditbutyl-4phenyl-phosphabenzene 0.441 -0.044 -8.428 -1.068

17 2,4,6-triethyl-phosphabenzene 0.433 -0.101 -8.704 -0.798

18 2,4,6-triisopropyl-posphabenzene 0.450 -0.096 -8.467 -1.161

19 2,4,6-tribenzyl-phosphabenzene 0.437 -0.091 -8.772 -0.881

20 2,4,6-tritolyl-phosphabenzene 0.452 -0.038 -8.230 -1.286

21 phosphabenzene 0.471 -0.148 -9.073 -0.793

Table 5 The PM3 calculation for atomic charges, and HOMO and LUMO energy of trisubsti-tuted -A3-phosphabenzenes.

25 n 20 --AH, 15 -

kcal/mol 10 -

Me B iPr tBu Ph


Fig. 3 Reaction energy as function of substituent at position 4 of 2,6-diphenyl-A3-phosphabenzenes.

-AH, 20- 1 1 r,


0\w—u—J—u—X —

Me B iPr tBu Ph substituent

Fig. 4 Reaction energy as function of substituent at position 4 of 2,6-dimethyl-A3-phosphabenzenes.

-AH, r.

kcal mol -j q-


Me Et iPr tBu Ph substituent

Fig. 5 Reaction energy as function of substituent at position 4 of 2,6-ditetrtbuthyl-A3-phosphabenzenes.

98 11 96-

-AH, 94- 11"

kcal/mol 92-NI 1 9088 fi Jl Jl Jl Jl f

Me Et iPr tBu Ph substituent

Fig. 6 Reaction energy as function of substituent at position 4 of 2,6-ditertbutyl-A3-phosphabenzenes.


Me Et iPr tBu Ph su bstituent

Fig. 7 Reaction energy as function of substituent at position 4 of 2,6-diphenyl-A3-phosphabenzenes.

-AH, kcal/m ol

100" 80" 60" 40" 20" 0

Me Et iPr tBu substituent

Fig. 8 Reaction energy as function of substituent at position 4 of 2,6-dimethyl-A3-phosphabenzenes.