aromatic heterocycles xii semiempirical pm3 study of diels alder cycloaddition reaction of substituted phosphabenzenes

CEJC 2(1) 2004 34{51 Aromatic heterocycles XII Semiempirical PM3 study of Diels-Alder cycloaddition reaction of substituted phosphabenzenes Liliana Pacureanu1 Ô , Mircea Mracec1, Zeno Simon1 Institute of Chemistry of Romanian Academy, 24 Mihai Viteazul Avenue, 300223 Timisoara, Romania Received 21 July 2003; accepted October 2003 Abstract: We report the results of a semiempirical PM3 study of the 1,4 cycloaddition reaction of substituted ả -phosphabenzenes with alkynes The inđuence 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 eơects of substituents đ c Central European Science Journals All rights reserved Keywords: Diels-Alder cycloaddition reaction, ¶ empirical PM3 method -phosphabenzene, 1-phosphabarrelene, semi- Introduction Ô Reactivity and coordinating properties of aromatic phosphorus heterocycles di®er from those of pyridine [1-8] In the last few years, interest in electronic structure and reactivity of ¸3 -phosphabenzenes have known increased development [1, 3, 9{11] ¸3 -phosphabenzenes react with nucleophilic reagents and radicals [12] The DielsAlder cycloaddition reaction of ¸3 -phosphabenzenes take place at 1,4 positions giving 1phosphabarrelenes without 1,2 cycloaddition to the double bond P=C The sp2 -hybridized phosphorus atom confers to á3 -phosphabenzenes good ẳ-acceptor and poor ắ-donating properties In contrast, pyridine is good ¾ 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 ¸3 E-mail: lilypac 99@yahoo.com L Pacureanu et al / Central European Journal of Chemistry 2(1) 2004 34{51 35 phosphabenzene as determined from SCF/3-21G* calculation is 88% of benzene resonance energy [13] Nucleus independent chemical shift criteria (NICS) calculated at B3LYP/631G* 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 ¸3 -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 substituted¸3 -phosphabenzenes [16{18] react with highly reactive dienophiles such as hexa°uorobutyne-2, benzyne, dicyanoacetilene at positions 1,4 to form 1-phosphabarrelenes [19{22] Even the ¾-complexes of 2-pheny-4,5-dimethyl-¸3 -phosphabenzene with tungsten pentacarbonyl undergo 1,4 cycloaddition reaction [22] The 1,4 Diels-Alder cycloaddition reaction of ¸3 -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] MÄarkl and Lieb observed the in°uence of the nature of the substituent group on the 1,4 cycloaddition reaction of ¸3 -phosphabenzenes with hexa°uorobutyne-2 (Figure 1) [21] The cycloaddition reaction occurs more readily with 2,6-ditertbuthyl-4-methyl ¸3 - Fig The reaction of ¶ -phosphabenzenes with alkynes phosphabenzene and 2,6-dimethyl-4-phenyl-¸3 -phosphabenzene than electron poor 2,4,6triphenyl-¸3 {phosphabenzene (Table 4) MÄarkl, Lieb and Martin [24] obtained benzophosphabarrelenes starting from 2,4,6triphenyl and 2,4,6-tritertbuthyl ¸3 -phosphabenzene and arynes Better yield was obtained with the more electron rich 2,4,6-tritertbuthyl ¸3 {phosphabenzene (Table 4) The reactions are depicted in Figure 1-Phosphabarrelenes are highly stable, and not undergo scission or retro DielsAlder reaction with split o® alkynes in thermal or ionization conditions [22] This study was undertaken to ¯nd out the in°uence of the position, nature and steric hindrance of the substituents of the ¸3 -phosphabenzene on the 1, Diels-Alder cycloaddition reaction using the semiempirical PM3 method 36 L Pacureanu et al / Central European Journal of Chemistry 2(1) 2004 34{51 Fig The reaction of ¶ -phosphabenzenes with benzyne The 1,4-Diels-Alder cycloaddition reaction of phosphabenzene with hexa°uorobutyne2 is thermodynamically controlled as M Mracec et al [9] have shown, in concordance with Woodward-Ho®man rules [25], [26] The number of ¼ electrons involved in the reaction and symmetry predictions prevail, thus the reaction is thermally allowed [25], [26] 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 ¯eld 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 PolackRibiere conjugated gradient with 0,01Kcal/º Amol RMS gradient was used Results and discussion The data resulted from PM3 calculations are shown in Table 3, Table and Table Special attention was paid to the charges of the centers of reactions: the carbon atom at position 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 we observed that the alkyl substituents decrease the energy of reaction The lowest values of reaction energies were obtained for 2,6 dialkyl substituted ¸3 -phosphabenzenes with a group at position without steric hindrance On the contrary electron withdrawing groups at position 2,6 of ¸3 {phosphabenzenes, and a group producing steric hindrance at position will increase the reaction energy Alkyl substituents at position of the 2,6-phenyl ¸3 {phosphabenzenes in°uence 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) L Pacureanu et al / Central European Journal of Chemistry 2(1) 2004 34{51 37 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 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 bears a stronger electron releasing group, without steric hindrance Phenyl substituents not in°uence the reaction energy as much as tert-butyl groups (Figure 5) The computed values of reaction energy of 2,4,6-trimethyl-¸3 -phosphabenzene, 2,4,6triethyl-¸3 -phosphabenzene and 2,4,6-triisopropyl-¸3 - phosphabenzene are lower than those of 2,4,6-tritolyl and 2,4,6-tribenzyl-¸3 -phosphabenzene, which is in agreement with the electronic e®ects 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-methyl3 ¸ {phosphabenzene is lower than the reaction energy of 2,4,6-triphenyl-¸3 -phosphabenzene and 2,6-dimethyl-4-phenyl-¸3 {phosphabenzene The data listed in Table 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-¸3 {phosphabenzenes which bear methyl and isopropyl substituents at position Tertbutyl and phenyl group at position 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-triphenylphosphabenzene A tertbutyl group at position 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-Triethylphosphabenzene and 2,6methyl-4-ethyl-¸3 -phosphabenzene presents inexplicable higher values for the reaction energy 2,4,6-Triisopropyl-¸3 -phosphabenzene presents lower reaction energy while 2,4,6-tritolyl and 2,4,6-tribenzyl-¸3 {phosphabenzenes have higher values of the reaction energy in agreement with electronic e®ects of substituents The reaction energy of 2,4,6-tritertbutyl¸3 -phosphabenzene is lower than the reaction energy of 2,4,6-triphenyl-¸3 -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 e®ects of substituents Atomic charges listed in Table emphasized that the electron withdrawing group phenyl decreases the atomic charge at C4, while electron releasing groups methyl, ethyl, 38 L Pacureanu et al / Central European Journal of Chemistry 2(1) 2004 34{51 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 The obtained values for phosphorus charges of 0.471 and for carbon at position 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 ¸3 {phosphabenzene and -8.64eV for 2,4,6-tritertbutyl-¸3 {phosphabenzene The absolute values are close to the values of ¼ 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-¸3 {phosphabenzene [15] 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 in°uences the reaction energy, especially if bulky substituents such as tert-butyl and isopropyl groups are situated at position Electron withdrawing groups such as phenyl and benzyl increase reaction energies, particularly at position The values of heat of formation of 1-phosphabenzobarrelenes con¯rm 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 References [1] C Elschenbroich, M Nowotny, A Behrendt, K Harms, S Wocadlo and J Pebler: \Pentakis(´ -phosphinine)iron: synthesis, structure and mode of formation", J Am Chem Soc., Vol 116, (1994), pp.6217{6219 [2] A.J Ashe III and J.C Colburn: \Molybdenum-Carbonyl Complexes of the Group Heterobenzenes", J Am Chem Soc., Vol 99, (1977), pp 8099{8100 [3] C Elschenbroich, J Koch, J Kroker, M.Winsch, W Massa, G Baum and G Stork: \´ -Coordination von unsubstituierten pyridine ´ -benzol(´ -pyridin) chrom und bis(´ -pyridin)chrom", Chem Ber., Vol 121, (1988), pp 1983 [4] K Dimroth, R Thamm und H Kaletsch: \6¼-Komplexe von pyridine-derivaten, synthesen und reactionen\, Z Naturforsch.,Vol 39b, (1984), pp 207{212 [5] P Rosa, N Mezailles, L Ricard, F Mathey, P Le Floch, and Y Jean: \ Dianionic Iron and Ruthenium(2-) Biphosphinine Complexes: A Formal d10 Ruthenium Complex", Angew Chem Int Ed Engl , Vol 40, (2001), pp 1251{1255 [6] P Le Floch and F Mathey: \Transition Metals in Phosphinine Chemistry",Coord Chem Rev., Vol 179-180, (1998), pp.771{791 L Pacureanu et al / Central European Journal of Chemistry 2(1) 2004 34{51 39 [7] C Elschenbroich, S Voss, O Schiemann, A Lippek and K Harms: "´ -Coordination of phosphinine to chromium, molybdenum and tungsten", Organometallics, Vol 17, (1998), pp 4417{4424 [8] C Elschenbroich, M Nowotny, A Behrendt, W Massa, S Wocadlo:" Tetrakis(´ phosphabenzene)nikel", Angew Chem Int Ed Engl., Vol.31, (1992), pp 1343{1345 [9] M Mracec, M Mracec, and Z Simon: \ X Aromatic Heterocycles PM3 and HMO Study on the Diels-Alder Reactions of ¸3 {Heterobenzenes of The 15th Group", Revue Roumaine de Chimie, Vol.45, (2000), pp 1021{1025 [10] C Batich, E Heilbronner, V Hornung, A.J Ashe III, D.T Clark, K.T Cobley, D Kilcast and I Scanlan: \Photoelectron spectra of Phosphabenzene, Arsabenzene and Stibabenzene", J Am Chem Soc., Vol 95, (1973), pp 928 [11] A.J Ashe III: \The group V heterobenzenes", Acc Chem Res., Vol 11, (1978), pp 153 [12] G.MÄarkl and A Merz: \Zur unsetzung des phosphabenzolsystem mit carbenen und carbenoiden\, Tetrahedron Lettter, (1971), pp 1269{1273 [13] L Nyulaszi and G Keglevich: "Study on the aromaticity and reactivity of chlorophosphinine", Heteroat Chem., Vol 5, (1994), pp 131{137 [14] G.Frisson, A.Sevin, N.Avarvari, F Mathey and P Le Floch: \The CH by N replacement e®ects on the aromaticity and reactivity of phosphinines", J.Org.Chem., Vol 64, (1999), pp 5524{5529 [15] H.Oehling, W SchÄafer and A Schweig: \Sequence of highest occupied molecular orbital in the phosphorins system", Angew Chem.Int.Ed.Engl., Vol 10, (1971), pp 656{657 [16] G.MÄarkl:\ 2,4,6-Triphenylphosphabenzol", Angew Chem., Vol 78, (1966), pp 907 [17] K Dimroth: Phosphorus Carbon Double Bonds, Springer-Verlag, Berlin, 1973 [18] G MÄarkl: \Phosphabenzol uns arsabenzol", Chem Unserer Zeit, Vol 16, (1982), pp 1939 [19] G MÄarkl: \Aromatic Phosphorus Heterocycles", Phosphorus and Sulfur, Vol 3, (1977), pp 77 [20] A.J.III Ashe and M.D Gordon: "Bismabenzene Diels-Alder Reaction of Group V Heteroaromatic with Hexa°uorobutyne", J Amer Chem Soc., Vol 94, (1972), pp 7596 [21] G MÄarkl and F Lieb: \Substituierte 1-Phosphaberrelenes", Angew Chem., Vol 80, (1968), pp 702 [22] M Alcaraz and F Mathey: \Accroisment de la reactivite des phosphorines en tant que dienes et philodienes par complexation du phosphore\, Tetrahedron, Vol 25, (1984), pp 207 [23] R Stutmann: \A simple model for substituent e®ects in cycloaddition reaction", Tetrahedron Lett., Vol 29, (1971), pp 2721{2724 [24] G MÄarkl, F Lieb and C Martin: \Substituierte benzo-phosphabarrelenes zur umsetzung von phosphabenzolen mit arinen\, Tetrahedron Lett., Vol 13, (1971), pp 1249 [25] R.B Woodward and R Ho®mann: The conservation of orbital symmetry, Chemie Verlag, Weinheim, 1970 40 L Pacureanu et al / Central European Journal of Chemistry 2(1) 2004 34{51 [26] R Ho®mann, R.B Woodward: \Selection rules for concerted cycloaddition reaction" J Am Chem Soc., Vol 87, (1965), pp 2046{2048 [27] Hyper Chem Professional for Windows, Release 5.11, Hypercube Inc., Gainesville, Fl 32601, USA, 1997 [28] M.J.S Dewar, E.G Zoldbush, E.F Haley and J.J.P Steward: \AM1, a new general purpose quantum mechanical molecular model", J Am Chem Soc., Vol 107, (1985), pp 3902{3909 L Pacureanu et al / Central European Journal of Chemistry 2(1) 2004 34{51 Compound R2=R6 R4 Yield [%] 3a C6 H5 C H5 53 3b C(CH3 )3 CH3 89 3c CH3 C H5 41 a: R2=R4=R6=C H5 b: R2=R6=C(CH )3 ; R4=CH3 c: R2=R6=CH3 ; R4= C6 H5 Table The yields of 1,4 cycloaddition reaction of ¶ [21] -phosphabenzenes with hexa®uorobutyne-2 41 42 L Pacureanu et al / Central European Journal of Chemistry 2(1) 2004 34{51 Compound R2 =R4 =R6 R Yield [%] 6a C H5 H 15 6b C H5 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 =C6 H5 R=Cl c: R2 =R4 =R6 =C(CH3 )3 R=H d: R2 =R4 =R6 =C(CH3 )3 R=Cl Table The yields of 1, cycloaddition reaction of ¶ -phosphabenzenes with arynes [24] L Pacureanu et al / Central European Journal of Chemistry 2(1) 2004 34{51 No 43 Phosphabenzene ¢Hf (kcal/mol) ¢Hf (kcal/mol) barrelene ¢H*R (kcal/mol) 2,4,6-triphenyl-phosphabenzene 127.910 -145.820 -19.949 2,6-diphenyl-4methyl-phosphabenzene 94.365 -181.469 -22.053 2,6-diphenyl-4ethyl-phosphabenzene 89.715 -186.612 -22.546 2,6-diphenyl -4isopropyl-phosphabenzene 85.108 -184.389 -15.716 2,6-diphenyl-4tbutyl-phosphabenzene 80.652 -186.438 -13.309 2,6-diphenyl-4benzyl-phosphabenzene 122.935 -148.733 -17.887 2,4,6-trimethyl-posphabenzene 24.291 -254.678 -25.188 2,6-dimethyl-4ethyl-phosphabenzene 18.011 -257.208 -21.438 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 *¢Hf of hexa°uorobutyne-2 is -253.781 kcal/mol Table The heats of formation for reagents, products and reaction energies for the reaction of substituted ả -phosphabenzenes with hexađuorobutyne-2 as resulted from PM3 calculation 44 L Pacureanu et al / Central European Journal of Chemistry 2(1) 2004 34{51 Phosphabenzene ¢Ho f (kcal/mol) ¢Hof (kcal/mol) barrelene ¢HR (kcal/mol) 2,4,6-triphenyl-phosphabenzene 127.910 164.559 -93.099 2,6-phenyl-4methyl-phosphabenzene 94.365 128.023 -96.090 2,6-pheny-l4ethyl-phosphabenzene 89.715 124.145 -95.318 2,6-phenyl -4isopropyl-phosphabenzene 85.108 120.388 -94.468 2,6-phenyl-4tbutyl-phosphabenzene 80.652 117.466 -92.934 2,6-phenyl-4benzyl-phosphabenzene 122.935 158.364 -94.319 2,4,6-trimethyl-phosphabenzene 24.291 57.384 -96.655 2,6-methyl-4ethyl-phosphabenzene 18.011 53.520 -94.239 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 No *¢Hf of benzyne is 129.748 kcal/mol Table PM3 calculation of the heat of formation for reagents, products and reaction energies for substituted ¶ -phosphabenzenes with benzyne L Pacureanu et al / Central European Journal of Chemistry 2(1) 2004 34{51 No Phosphabenzene 45 Atomic charge at P Atomic charge at C4 HOMO [eV] LUMO [eV] 2,4,6-triphenyl-phosphabenzene 0.465 -0.042 -8.355 -1.341 2,6-phenyl-4methyl-phosphabenzene 0.444 -0.091 -8.441 -1.160 2,6-phenyl-4ethyl-phosphabenzene 0.450 -0.096 -8.467 -1.161 2,6-phenyl -4isopropyl-phosphabenzene 0.452 -0.093 -8.473 -1.152 2,6-phenyl-4tbutyl-phosphabenzene 0.448 -0.082 -8.455 -1.133 2,6-phenyl-4benzyl-phosphabenzene 0.455 -0.093 -8.478 -1.174 2,4,6-trimethyl-posphabenzene 0.422 -0.095 -8.589 -0.825 2,6-methyl-4ethyl-phosphabenzene 0.433 -0.097 -8.675 0.078 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 The PM3 calculation for atomic charges, and HOMO and LUMO energy of trisubstituted -¶ -phosphabenzenes 46 L Pacureanu et al / Central European Journal of Chemistry 2(1) 2004 34{51 Fig Reaction energy as function of substituent at position of 2,6-diphenyl-¶ phosphabenzenes { L Pacureanu et al / Central European Journal of Chemistry 2(1) 2004 34{51 Fig Reaction energy as function of substituent at position of 2,6-dimethyl-¶ phosphabenzenes 47 { 48 L Pacureanu et al / Central European Journal of Chemistry 2(1) 2004 34{51 Fig Reaction energy as function of substituent at position of 2,6-ditetrtbuthyl-¶ phosphabenzenes { L Pacureanu et al / Central European Journal of Chemistry 2(1) 2004 34{51 Fig Reaction energy as function of substituent at position of 2,6-ditertbutyl-¶ phosphabenzenes 49 { 50 L Pacureanu et al / Central European Journal of Chemistry 2(1) 2004 34{51 Fig Reaction energy as function of substituent at position of 2,6-diphenyl-¶ phosphabenzenes { L Pacureanu et al / Central European Journal of Chemistry 2(1) 2004 34{51 Fig Reaction energy as function of substituent at position of 2,6-dimethyl-¶ phosphabenzenes 51 - ... the semiempirical PM3 method 36 L Pacureanu et al / Central European Journal of Chemistry 2(1) 2004 34{51 Fig The reaction of ¶ -phosphabenzenes with benzyne The 1,4 -Diels- Alder cycloaddition reaction. .. the ¾-complexes of 2-pheny-4,5-dimethyl-¸3 -phosphabenzene with tungsten pentacarbonyl undergo 1,4 cycloaddition reaction [22] The 1,4 Diels- Alder cycloaddition reaction of ¸3 -phosphabenzenes. .. 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