Theoretical study and synthesis of the reaction between triphenylphosphine, dialkyl acetylenedicarboxylates and 2-aminobenzimidazole, 2-hydroxy-3- nitropyridine or 1,2,3,4-tetrahydrocarbazole

Article abstract of DOI:10.1135/cccc2009523

Triphenylphosphine reacts with dialkyl acetylenedicarboxylates in the presence of heterocyclic compounds, such as 2-aminobenzimidazole, 2-hydroxy-3-nitropyridine or 1,2,3,4- tetrahydrocarbazole to generate stable phosphorus ylides. Some ylides exist in so

Full text of DOI:10.1135/cccc2009523

Theoretical Study and Synthesis
785
THEORETICAL STUDY AND SYNTHESIS OF THE REACTION BETWEEN
TRIPHENYLPHOSPHINE, DIALKYL ACETYLENEDICARBOXYLATES
AND 2-AMINOBENZIMIDAZOLE, 2-HYDROXY-3-NITROPYRIDINE OR
1,2,3,4-TETRAHYDROCARBAZOLE
a2
Sayyed Mostafa HABIBI-KHORASSANIa1,*, Malek Taher MAGHSOODLOU
,
a5
Ali EBRAHIMIa3, Reza HEYDARIa4, Nourollah HAZERI
,
a
b
a
Mohammad Amin KAZEMIAN , Mahmoud NASSIRI , Belghais ADROM
a
and Mohammad ZAKARIANEJAD
a
Department of Chemistry, The University of Sistan and Baluchestan,
P.O. Box 98135-674, Zahedan, Iran; e-mail: smhabibius@ yahoo.com,
1
2
5
3
4
mt_magsoodlou@yahoo.com, ebrahimi@hamoon.usb.ac.ir, heydari@hamoon.usb.ac.ir,
nhazeri@yahoo.com
b
Department of Maritime, The University of Chabahar, Chabahar, Iran
Received October 8, 2009
Accepted March 26, 2010
Published online August 9, 2010
Triphenylphosphine reacts with dialkyl acetylenedicarboxylates in the presence of hetero-
cyclic compounds, such as 2-aminobenzimidazole, 2-hydroxy-3-nitropyridine or 1,2,3,4-
tetrahydrocarbazole to generate stable phosphorus ylides. Some ylides exist in solution as
a mixture of two geometrical isomers as a result of restricted rotation around the carbon–
carbon partical double bond resulting from conjugation of the ylide moiety with the adja-
cent carbonyl group, whereas others occur as a single isomer only. For this reason, the as-
signments of more stable Z- or E-isomers as the major or minor forms were investigated
using theoretical calculations.
Keywords: Heterocyclic compounds; Stable phosphorus ylides; Theoretical calculations.
Trivalent phosphorus compound is known to be a nucleophile, whereas it
behaves as an electron donor toward good electron acceptor either in the
ground or excited state1,2. In recent years, there has been increasing interest
in the synthesis of organophosphorus compounds, i.e., those bearing a car-
bon atom bound directly to a phosphorus atom3–39. This interest has re-
sulted from the recognition of the value of such compounds in variety of
biological, industrial and chemical synthetic use3–7. A large number of
methods has appeared describing novel synthesis of organophosphorus
compounds6,7
.
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© 2010 Institute of Organic Chemistry and Biochemistry
doi:10.1135/cccc2009523

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Habibi-Khorassani et al.:
There are many studies on the reaction between trivalent phosphorus
nucleophiles and α,β-unsaturated carbonyl compounds in the presence of
a proton source such as alcohol or phenol7. In the set of investigations that
have been made on development of new routes in synthesis of stable phos-
phorus ylides20–25, we now describe the reaction between triphenylphos-
phine 1 and dialkyl acetylenedicarboxylates 2 in the presence of hetero-
cyclic compounds 3 for generating the corresponding stable phosphorus
ylide 4 in fairly high yield. An “atoms in molecules” (AIM) analysis40 at
HF/6-31G level of theory has been performed in order to gain a better un-
derstanding of most geometrical parameters of both E-4 (a, c, d, e, f and h)
and Z-4 (a, c, d, e, f and h) phosphorus ylides.
EXPERIMENTAL
Melting points and IR spectra (νmax, cm–1) of all compounds were measured on an
Electrothermal 9100 apparatus and a Shimadzu IR-460 spectrometer, respectively. The 1H,
13C and 31P NMR spectra (δ, ppm; J, Hz) were obtained on a BRUKER DRX-500 Avance in-
strument with CDCl3 as a solvent at 500.1, 125.8 and 202.5 MHz, respectively. The mass
spectra were recorded on a GCMS-QP5050A mass spectrometer operating at an ionization
potential of 70 eV. Elemental analyses for C, H and N was performed using a Heraeus
CHN-O-Rapid analyzer. Heterocyclic compounds 3, dialkyl acetylenedicarboxylates and
triphenylphosphine, purchased from Fluka, were used without further purifications.
Dimethyl 2-(Z-Aminobenzimidazol-1-yl)-3-(triphenylphosphanylidene)butanedioate (4a).
General Synthetic Procedure
To a magnetically stirred solution of triphenylphosphine (0.26 g, 1 mmol) and 2-amino
benzimidazole (0.13 g, 1 mmol) in 10 ml of acetone was added dropwise a mixture of
dimethyl acetylenedicarboxylate (0.14 g, 1 mmol) in 3 ml of acetone at –5 °C over 10 min.
After a few minutes stirring at room temperature the product was filtered, recrystallized and
then washed with cold diethyl ether (3 × 5 ml). Colorless powder obtained as a final prod-
uct, yield 0.52 g (96%), m.p. 208–210 °C. IR (KBr): 1725 and 1700 (C=O); 3100 (NH2). MS
(m/z, %): 537.22 (M+, 3), 506.22 (M – OCH3, 23), 262 (PPh3, 38), 183 (PPh2, 47), 108 (PPh,
23), 77 (Ph, 90). For C31H28N3O4P (537.56) calculated: 69.27% C, 5.25% H, 7.82% N; found:
68.57% C, 5.31% H, 7.95% N.
Dimethyl 2-(2-aminobenzimidazol-1-yl)-3-(triphenylphosphanylidene)butanedioate (4a). Only
isomer of 4a (Z-4a): yield 96%. 1H NMR (500.1 MHz, CDCl3): 3.16 and 3.76 (6 H, 2 s,
3
3
2 OCH3); 5.03 (1 H, d, JPH = 16.4, P=C-CH); 5.60 (1 H, d, JHH = 7.9, CH, C7H6N3); 5.94
3
3
(2 H, bro, NH2); 6.52 (1 H, t, JHH = 7.5, CH, C7H6N3); 6.89 (1 H, t, JHH = 7.5, CH, C7H6N3);
7.25 (1 H, d, JHH = 6.8, CH, C7H6N3); 7.39–7.61 (15 Harom, m, 3 C6H5). 13C NMR (125.8 MHz,
3
1
2
CDCl3): 42.6 (d, JPC = 126.3, P=C); 49.8 and 52.9 (2 s, 2 OCH3); 56.8 (d, JPC = 15.3,
P=C-CH); 106.3, 115.1, 118.0 and 120.6 (4 C, C7H6N3); 125.6 (d, JPC = 92.3, Cipso); 129.0,
1
3
2
(d, JPC = 12.3, Cmeta); 132.4 (Cpara); 133.5 (d, JPC = 9.8, Cortho); 134.3, 141.9 and 154.8 (3 C,
C7H6N3); 171.2 (d, JPC = 14.0, C=O ester); 171.5 (d, JPC = 12.3, P-C=C). 31P NMR (202.5 MHz,
3
2
CDCl3): 23.31 (Ph3P+-C).
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Theoretical Study and Synthesis
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Diethyl 2-(2-aminobenzimidazol-1-yl)-3-(triphenylphosphanylidene)butanedioate (4b). Colorless
powder, yield 0.53 g (95%), m.p. 151–153 °C. IR (KBr): 1700 and 1725 (C=O); 3150 (NH2).
MS (m/z, %): 565 (M+, 3), 520 (M – OCH2CH3, 39), 475 (M – 2 OCH2CH3, 42), 262 (PPh3,
68), 183 (PPh2, 49), 108 (PPh, 26), 77 (Ph, 100). For C33H32N3O4P (565.61) calculated:
70.08% C, 5.70% H, 7.43% N; found: 69.37% C, 5.65% H, 7.31% N.
Minor isomer of 4b (Z-4b): yield 94%. 1H NMR (500.1 Hz, CDCl3): 0.45 and 1.27 (6 H,
3
2 t, JHH = 7.1, 2 OCH2CH3); 3.73 and 4.23 (4 H, 2 m, 2 ABX3 system, 2 OCH2CH3); 5.02
3
3
(1 H, d, JPH = 16.8, P=C-CH); 5.59 (1 H, d, JHH = 7.9, CH, C7H6N3); 5.98 (2 H, bro, NH2);
3
3
6.51 (1 H, t, JHH = 7.5, CH, C7H6N3); 6.88 (1 H, t, JHH = 7.5, CH, C7H6N3); 7.25 (1 H, d,
3JHH = 7.7, CH, C7H6N3); 7.38–7.71 (15 Harom, m, 5 C6H5). 13C NMR (125.8 MHz, CDCl3):
1
2
13.9 and 14.2 (2 s, 2 OCH2CH3); 42.4 (d, JPC = 124.6, P=C); 56.9 (d, JPC = 15.5, P=C-CH);
58.6 and 61.7 (2 s, 2 OCH2CH3); 106.3, 115.0, 117.9 and 120.5 (4 C, C7H6N3); 125.9 (d,
1JPC = 91.7, Cipso); 128.9 (d, JPC = 12.3, Cmeta); 132.3 (Cpara); 133.6 (d, JPC = 9.8, Cortho);
3
2
170.2 (d, JPC = 12.1, C=O); 170.6 (d, JPC = 14.3, P-C=C). 31P NMR (202.5 MHz, CDCl3):
3
2
23.35 (Ph3P+-C).
Di-tert-butyl 2-(2-aminobenzimidazol-1-yl)-3-(triphenylphosphanylidene)butanedioate (4c). Col-
orless powder, yield 0.60 g (97%), m.p. 103–105 °C. IR (KBr): 1744 and 1616 (C=O); 3145
(NH2). MS (m/z, %): 621.31 (M+, 4), 548 (M – OMe3, 32), 262 (PPh3, 66), 183 (PPh2, 71), 108
(PPh, 56). For C37H40N3O4P (621.72) calculated: 71.48% C, 6.49% H, 6.76% N; found:
71.51% C, 6.65 H, 6.71% N.
Only isomer of 4c (Z-4c): yield 97%. 1H NMR (500.1 MHz, CDCl3): 0.96 and 1.54 (18 H,
3
3
2 s, 2 OCMe3); 4.39 (1 H, d, JPH = 17.5, P=C-CH); 5.58 (1 H, d, JHH = 7.8, CH, C7H6N3);
3
3
6.16 (2 H, bro, NH2); 6.49 (1 H, t, JHH = 7.2, CH, C7H6N3); 6.86 (1 H, t, JHH = 7.2, CH,
C7H6N3); 7.23 (1 H, d, JHH = 7.8, CH, C7H6N3); 7.37–7.73 (15 Harom, m, 3 C6H5). 13C NMR
3
1
2
(125.7 MHz, CDCl3): 28.1 and 28.3 (2 OCMe3); 41.9 (d, JPC = 125.1, P=C); 57.7 (d, JPC
=
15.6, P=C-CH); 78.4 and 81.5 (2 OCMe3); 106.2, 114.9, 117.9 and 120.4 (4 C, C7H6N3);
1
3
2
126.4 (d, JPC = 91.5, Cipso); 128.8 (d, JPC = 11.9, Cmeta); 132.2 (Cpara); 133.7 (d, JPC = 9.6,
3
2
C
ortho); 134.6, 141.9 and 155.1 (3 C, C7H6N3); 169.0 (d, JPC = 13.9, C=O); 170.9 (d, JPC
=
11.7, P-C=C). 31P NMR (202.5 MHz, CDCl3): 23.06 (Ph3P+-C).
Dimethyl 2-(2-hydroxy-3-nitropyridine-2-carboxylate-2-yl)-3-(triphenylphosphanylidene)butane-
dioate (4d). Colorless powder, yield 0.51 g (94%), m.p. 156–158 °C. IR (KBr): 1723 and 1614
(C=O). MS (m/z, %): 544 (M+, 3), 485 (M – CO2Me, 41), 367 (M – 2 CO2Me, 27), 262 (PPh3,
60), 183 (PPh2, 53), 108 (PPh, 26), 77 (Ph, 100). For C29H25N2O7P (544.51) calculated:
63.97% C, 4.63% H, 5.14% N; found: 64.05% C; 4.71% H, 5.27% N.
Major isomer of 4d (Z-4d): yield 77%. 1H NMR (500.1 MHz, CDCl3): 3.20 and 3.78 (6 H,
3
2 s, 2 OCH3); 5.65 (1 H, d, JPH = 17.4, P=C-CH); 6.32–7.77 (18 Harom, m, 3 C6H5,
1
C5H3N2O3). 13C NMR (125.8 MHz, CDCl3): 42.9 (d, JPC = 128.7, P=C); 52.2 and 52.9
2
1
(2 OCH3); 60.5 (d, JPC = 17.2, P=C-CH); 102.5 (1 C, C5H3N2O3); 125.5 (d, JPC = 90.3, Cipso);
3
4
129.2 (d, JPC = 12.2, Cmeta); 129.7 (d, JPC = 2.2, Cpara); 132.7 (1 C, C5H3N2O3); 133.3 (d,
2JPC = 9.6, Cortho); 138.2, 144.9 and 153.7 (3 C, C5H3N2O3); 165.2 (d, JPC = 12.8, C=O ester);
3
170.4 (d, JPC = 12.8, P-C=C). 31P NMR (202.5 MHz, CDCl3): 23.97 (Ph3P+-C).
2
Minor isomer of 4d (E-4d): yield 23%. 1H NMR (500.1 MHz, CDCl3): 3.61 and 3.78 (6 H,
3
2 s, 2 OCH3); 5.65 (1 H, d, JPH = 17.2, P=C-CH); 6.23–7.77 (18 Harom, m, 3 C6H5,
C5H3N2O3). 13C NMR (125.8 MHz, CDCl3): 43.1 (d, JPC = 128.7, P=C); 49.5 and 50.6
1
2
1
(2 OCH3); 60.5 (d, JPC = 17.2, P=C-CH); 103.6 (1 C, C5H3N2O3); 125.5 (d, JPC = 90.3, Cipso);
3
4
2
128.9 (d, JPC = 12.1, Cmeta); 131.9 (d, JPC = 2.2, Cpara); 132.1 (d, JPC = 9.8, Cortho); 132.7,
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Habibi-Khorassani et al.:
3
2
137.2, 142.9 and 153.7 (4 C, C5H3N2O3); 165.2 (d, JPC = 12.8, C=O); 170.7 (d, JPC = 17.1,
P-C=C). 31P NMR (202.5 MHz, CDCl3): 24.98 (Ph3P+-C).
Di-tert-butyl 2-(2-hydroxy-3-nitropyridine-2-carboxylate-2-yl)-3-(triphenylphosphanylidene)-
butanedioate (4e). Colorless powder, yield 0.60 g (96%), m.p. 113–115 °C. IR (KBr): 1742 and
1678 (C=O). MS (m/z, %): 575 (M+, 5), 474 (M – CO2C(Me)3, 47), 373 (M – 2 CO2C(Me)3,
20), 262 (PPh3, 80), 183 (PPh2, 73), 108 (PPh, 57). For C35H37N2O7P (628.67) calculated:
66.87% C, 5.93% H, 4.46% N; found: 66.89% C, 6.05% H, 4.57% N.
Only isomer of 4e (Z-4e). 1H NMR (500.1 MHz, CDCl3): 0.99 and 1.55 (18 H, 2 s,
3
2 OCMe3); 5.53 (1 H, d, JPH = 18.3, P=C-CH); 6.33–8.69 (18 Harom, m, 3 C6H5 and
1
C5H3N2O3). 13C NMR (125.8 MHz, CDCl3): 28.1 and 28.1 (2 OCMe3); 43.0 (d, JPC = 124.7,
2
P=C); 60.9 (d, JPC = 18.4, P=C-CH); 78.7 and 81.8 (2 OCMe3); 102.4 (1 C, C5H3N2O3); 126.2
1
3
(d, JPC = 91.7, Cipso); 128.5 (d, JPC = 11.9, Cmeta); 130.1 (1 C, C5H3N2O3); 131.9 (Cpara);
2
3
132.1 (d, JPC = 9.7, Cortho); 134.6, 153.5 and 164.4 (3 C, C5H3N2O3); 168.8 (d, JPC = 5.9,
C=O); 169.3 (d, JPC = 5.9, P-C=C). 31P NMR (202.5 MHz, CDCl3): 23.50 (Ph3P+-C).
2
Dimethyl 2-(1,2,3,4-tetrahydrocarbazole-N-yl)-3-(triphenylphosphanylidene)butanedioate (4f).
Colorless powder, yield 0.54 g (93%), m.p. 102–104 °C. IR (KBr): 1737 and 1616 (C=O).
MS (m/z, %): 575 (M+, 3), 544 (M – OMe, 44), 457 (M – 2 CO2Me, 38), 262 (PPh3, 60), 183
(PPh2, 40), 108 (PPh, 27), 77 (Ph, 100). For C36H34NO4P (575.65) calculated: 75.12% C,
5.95% H, 2.43% N; found: 74.96% C, 6.03% H, 2.51% N.
Major isomer of 4f (Z-4f): yield 61%. 1H NMR (500.1 MHz, CDCl3): 1.72 (4 H, m, 2 CH2);
3
2.73 (4 H, m, 2 CH2); 3.23 and 3.76 (6 H, 2 s, 2 OCH3); 5.04 (1 H, d, JPH = 18.9, P=C-CH);
6.96–7.72 (19 Harom, m, 3 C6H5 and C6H4). 13C NMR (125.8 MHz, CDCl3): 21.2, 22.2, 22.9
1
and 23.5 (4 CH2, C12H12N); 41.9 (d, JPC = 123.21, P=C); 49.2 and 52.6 (2 OCH3); 57.9 (d,
1
2JPC = 15.8, P=C-CH); 109.3, 110.4, 116.8, 118.2 and 120.2 (5 C, C12H12N); 126.1 (d, JPC
=
=
3
2
91.4, Cipso); 127.0 (1 C, C12H12N); 128.5 (d, JPC = 12.3, Cmeta); 131.9 (Cpara); 133.6 (d, JPC
3
9.8, Cortho); 135.9 and 136.0 (2 C, C12H12N); 169.7 (d, JPC = 18.1, C=O ester); 172.9 (d,
2JPC = 16.4, P-C=C). 31P NMR (202.5 MHz, CDCl3): 23.96 (Ph3P+-C).
Minor isomer of 4f (E-4f): yield 39%. 1H NMR (500.1 MHz, CDCl3): 1.72 (4 H, m, 2 CH2);
3
2.73 (4 H, m, 2 CH2); 3.70 and 3.74 (6 H, 2 s, 2 OCH3); 5.01 (1 H, d, JPH = 16.7, P=C-CH);
6.96–7.72 (19 Harom, m, 3 C6H5 and C6H4). 13C NMR (125.8 MHz, CDCl3): 21.3, 22.2, 22.9
1
and 23.5 (4 CH2, C12H12N); 43.8 (d, JPC = 135.7, P=C); 50.4 and 52.2 (2 OCH3); 58.1 (d,
1
2JPC = 15.7, P=C-CH); 109.5, 110.2, 116.9, 118.2 and 120.1 (5 C, C12H12N); 127.5 (d, JPC
=
=
=
3
2
91.1, Cipso); 128.0 (1 C, C12H12N); 128.7 (d, JPC = 12.1, Cmeta); 131.9 (Cpara); 132.1 (d, JPC
3
2
9.9, Cortho); 135.9 and 136.2 (2 C, C12H12N); 169.0 (d, JPC = 12.5, C=O); 169.9 (d, JPC
16.4, P-C=C). 31P NMR (202.5 MHz, CDCl3): 25.14 (Ph3P+-C).
Diethyl 2-(1,2,3,4-tetrahydrocarbazole-N-yl)-3-(triphenylphosphanylidene)butanedioate (4g). Yel-
low powder, yield 0.56 g (92%), m.p. 156–158 °C. IR (KBr): 1723 and 1614 (C=O).
MS (m/z, %): 603 (M+, 3), 530 (M – CO2Et, 38), 457 (M – 2 CO2Et, 31), 262 (PPh3, 50), 183
(PPh2, 73), 108 (PPh, 45), 77 (Ph, 100). For C38H38NO4P (603.70) calculated: 75.60% C,
6.34% H, 2.32% N; found: 75.62% C, 6.40% H, 2.39% N.
Major isomer of 4g (Z-4g): yield 64%. 1H NMR (500.1 Hz, CDCl3): 0.54 and 1.26 (6 H, 2 t,
3JHH = 7.1, 2 OCH2CH3); 1.73 (4 H, m, 2 CH2); 2.71 (4 H, m, 2 CH2); 3.93 and 4.19 (4 H,
3
2 m, 2 ABX3 system, 2 OCH2CH3); 5.05 (1 H, d, JPH = 17.1, P=C-CH); 6.99–7.60 (19 Harom
,
m, 3 C6H5 and C6H4). 13C NMR (125.8 MHz, CDCl31): 14.18 and 14.33 (2 OCH2CH3), 21.33,
2
22.26, 23.02 and 23.57 (4 CH2, C12H12N); 41.49 (d, JPC = 126.1, P=C); 57.91 (d, JPC = 14.6,
P=C-CH); 58.68 and 61.12 (2 OCH2CH3); 109.18, 111.58, 116.73, 118.06 and 120.12 (5 C,
1
3
C
12H12N); 126.50 (d, JPC = 91.7, Cipso); 127.94 (1 C, C12H12N); 128.63 (d, JPC = 12.2, Cmeta);
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Theoretical Study and Synthesis
789
2
131.90 (Cpara); 133.76 (d, JPC = 9.7, Cortho); 136.03 and 136.22 (2 C, C12H12N); 169.92 (d,
2
3JPC = 12.6, C=O); 172.25 (d, JPC = 15.9, P-C=C). 31P NMR (202.5 MHz, CDCl3): 25.03
(Ph3P+-C).
Only isomer of 4g (E-4g): yield 36%. 1H NMR (500.1 MHz, CDCl3): 1.25 and 1.30 (6 H,
3
2 t, JHH = 7.1, 2 OCH2CH3); 1.61 (4 H, m, 2 CH2); 2.66 (4 H, m, 2 CH2); 4.12 and 4.27 (4 H,
3
2 m, 2 ABX3 system, 2 OCH2CH3); 4.97 (1 H, d, JPH = 18.3, P=C-CH); 6.99–7.60 (19 Harom
,
m, 3 C6H5 and C6H4). 13C NMR (125.8 MHz, CDCl3): 14.3 and 14.9 (2 OCH2CH3); 21.3,
22.2, 22.9 and 23.5 (4 CH2, C12H12N); 42.6 (d, JPC = 133.1, P=C); 58.1 (d, JPC = 15.2,
P=C-CH); 60.9 and 61.1 (2 OCH2CH3); 109.2, 111.8, 116.9 118.09 and 119.9 (5 C, C12H12N);
1
2
1
3
127.2 (d, JPC = 91.4, Cipso); 128.2 (1 C, C12H12N); 128.7 (d, JPC = 12.3, Cmeta); 131.9 (Cpara);
2
3
133.8 (d, JPC = 9.9, Cortho); 136.0 and 136.4 (2 C, C12H12N); 170.3 (d, JPC = 17.7, C=O);
172.2 (d, JPC = 15.3, P-C=C). 31P NMR (202.5 MHz, CDCl3): 25.50 (Ph3P+-C).
2
Di-tert-butyl 2-(1,2,3,4-tetrahydrocarbazole-N-yl)-3-(triphenylphosphanylidene)butanedioate
(4h). Colorless powder, yield 0.63 g (95%), m.p. 123–125 °C. IR (KBr): 1717 and 1616 (C=O).
MS (m/z, %): 659 (M+, 4), 558 (M – CO2CMe3, 44), 457 (M – 2 OCMe3, 53), 262 (PPh3, 72),
183 (PPh2, 80), 108 (PPh, 69). For C42H46NO4P (659.81) calculated: 76.46% C, 7.03% H,
2.12% N; found: 76.51% C, 7.12% H, 2.01% N.
Only isomer of 4h (Z-4h). 1H NMR (500.1 MHz, CDCl3): 1.04 and 1.58 (18 H, 2 s,
3
2 OCMe3); 1.69 (4 H, m, 4 CH2); 2.66 (4 H, m, 4 CH2); 4.90 (1 H, d, JPH = 16.1, P=C-CH);
7.28–7.61 (19 Harom, m, 3 C6H5 and C6H4). 13C NMR (125.8 MHz, CDCl3): 21.3, 22.0, 22.9
1
and 23.5 (4 CH2, C12H12N); 28.3 and 28.6 (2 OCMe3); 40.7 (d, JPC = 122.9, P=C); 58.5 (d,
2JPC = 15.9, P=C-CH); 77.4 and 80.2 (2 OCMe3); 108.7, 110.1, 116.6, 117.8 and 120.1 (5 C,
1
3
C
12H12N); 127.7 (d, JPC = 91.1, Cipso); 128.4 (d, JPC = 12.2, Cmeta); 128.6 (1 C, C12H12N);
2
3
131.8 (Cpara); 133.7 (d, JPC = 9.7, Cortho); 135.9 and 136.2 (1 C, C12H12N); 168.3 (d, JPC
=
10.9, C=O); 171.1 (d, JPC = 15.4, P-C=C). 31P NMR (202.5 MHz, CDCl3): 23.77 (Ph3P+-C).
2
RESULTS AND DISCUSSION
The reaction between triphenylphosphine 1 and dialkyl acetylene-
dicarboxylate 2 in the presence of heterocyclic compounds 3, such as
2-aminobenzimidazole, 2-hydroxy-3-nitropyridine or 1,2,3,4-tetrahydro-
carbazole led to stable phosphorous ylides 4 in fairly high yield (Fig. 1).
These reactions were carried out in acetone at ambient temperature and
1
were completed after a few minutes. The H, 31P and 13C NMR spectra of
the crude products clearly indicated the formation of compounds 4a–4h.
No products other than 4a–4h could be detected by NMR spectroscopy. The
structures of compounds 4a–4h were deduced from the elemental analyses,
1
mass, IR, H, 13C and 31P NMR spectra. Although the presence of the 31P nu-
1
cleus has complicated both the H and 13C NMR spectra of 4d, 4f and 4g,
it helps in assignment of the signals by long-range spin–spin couplings
1
1
with the H and 13C nuclei (see Experimental). The H, 13C and 31P NMR
spectra of ylides 4d, 4f and 4g show the mixture of two isomers (see Fig. 1j).
The ylides moieties of these compounds are strongly conjugated with the
adjacent carbonyl group and rotation around the partial double bond in
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790
Habibi-Khorassani et al.:
(E)-4 and (Z)-4 geometrical isomers is slow on the NMR time scale at ambi-
ent temperature. As can be seen, only one geometrical isomer was observed
for ylides 4a–4c, 4e and 4h, presumably, because of both the more plausible
intramolecular hydrogen bonds and the bulky tert-butyl groups (see Fig. 1k).
The assignment of E-4 (d, f and g) and Z-4 (d, f and g) isomers as the major
or minor in phosphorous ylides have been reported previously35–39. The
1H NMR spectrum of 4d exhibited two singlets at δ 3.20 and 3.78 ppm aris-
ing from methoxy group in the Z-isomer, and two singlets at δ 3.61 and
3.78 ppm from that in the E-isomer. The methyl group at δ 3.20 in the Z-
isomer is shielded due to the anisotropic effect of a phenyl group of tri-
phenylphosphine. This effect confirms why the Z-4d and E-4d isomers
FIG. 1
i) The reaction between triphenylphosphine 1, dialkyl acetylenedicarboxylate (2a, 2b or 2c)
and 2-aminobenzimidazole, 2-hydroxy-3-nitropyridine or 1,2,3,4-tetrahydrocarbazole
3
(3a–3h) for generation of stable phosphorus ylides 4a–4h. j) Z- and E-isomers (major and mi-
nor) of stable phosphorus ylides (4d, 4f and 4g) are shown for 2-hydroxy-3-nitropyridine or
1,2,3,4-tetrahydrocarbazole. k) Only one isomer of ylides 4a–4c, 4e and 4h are shown for
2-aminobenzimidazole, 2-hydroxy-3-nitropyridine and 1,2,3,4-tetrahydrocarbazole, respec-
tively
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Theoretical Study and Synthesis
791
could appear as the major and minor forms, respectively, with the percent-
age of both isomers as reported in Experimental. For the 13C NMR spectros-
copy, the anisotropic effect could not be reported for the methoxy group in
the Z-isomer because of the small difference of their chemical shifts.
The singlets for methine protons appeared as two doublets at δ 5.65
(3JPH = 17.4) and δ 5.65 (3JPH = 17.2), respectively for the Z- and E-isomers.
On the basis of the well established chemistry of trivalent phosphorus
nucleophiles3–7, it is reasonable to assume that phosphorus ylide 4 results
from the initial addition of triphenylphosphine to dialkyl acetylene-
dicarboxylates and subsequent protonation of the 1:1 adduct by the hetero-
cyclic compounds to form phosphoranes 4 (see Fig. 1i).
1
The H NMR spectrum of 4a showed two singlets at δ 3.16 and 3.76 ppm
3
arising from methoxy protons and a doublet at δ 5.03 ppm (1 H, d, JPH
=
16.4, P-C-CH) for methine proton. The aromatic protons appeared as
a multiplet at δ 7.39–7.61 ppm. The 13C NMR spectra of 4a displayed 17 dis-
1
tinct resonances in a good agreement with only one isomer. The H and
13C NMR spectra of compounds 4b–4h are similar to those of 4a, except for
the signals from the ester group which appear as characteristic resonance
lines with the corresponding chemical shifts. The structural assignments for
1
compounds 4a–4h were made on the basis of the H and 13C NMR spectra
that were supported by their IR spectra. The carbonyl region of these com-
pounds exhibits absorption bands for each compound. The ester absorption
is at 1744–1614 cm–1, the conjugation of negative charge in ylide moiety
with the adjacent carbonyl group accounting for the reduction in fre-
quency of the carbonyl bands, and allows determination of the ratio be-
tween the Z- and E-isomers.
CALCULATIONS
Recently, different reports have been published on the synthesis of stable
phosphorus ylides from the reaction between triphenylphosphine and reac-
tive acetylenic esters in the presence of N–H, C–H or S–H heterocyclic com-
pounds. These ylides usually exist as a mixture of the two geometrical
isomers, although some ylides exhibit one geometrical isomer. Assignment
of the stability of the two Z- and E-isomers is impossible in phosphorus
1
ylides by experimental methods such as H and 13C NMR and IR spectros-
copy, mass spectrometry and elemental analysis data. For this reason, quan-
tum mechanical calculations have been performed in order to gain a better
understanding of the most important geometrical parameters and also rela-
tive energies of both the geometrical isomers.
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792
Habibi-Khorassani et al.:
In order to determine which is the more stable form of the geometrical
isomers Z-4 (a, c, d, e, f and h) and E-4 (a, c, d, e, f and h) in ylides 4a–4h
(4 (a, c), 4 (d, e) and 4 (f, h) are selected as typical ylides from the different
categories of 4a–4c, 4d–4e and 4f–4h, respectively), first the structures were
optimized at HF/6-31G (gas phase) and HF/6-31G* (involving polarization
functions for hydrogens, solution media, acetone) levels of theory40 by
Gaussian 03 program package41. Also relative stabilization energy of the
two isomers (Figs 2–7) has been calculated at B3LYP/6-311 G(d,p) (both gas
and solution phases) as a result of single point calculations. The relative sta-
bilization energies for both Z-4 (a, c, d, e, f and h) and E-4 (a, c, d, e, f and
h) isomers are reported in Table I, as can be seen, the Z-4a, Z-4c, Z-4d,
FIG. 2
i) Intramolecular hydrogen bonds (dotted lines) in the two E-4a and Z-4a geometrical isomers
of stable ylide 4a. j) A part of molecular map for the two geometrical isomers
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Theoretical Study and Synthesis
793
Z-4e, Z-4f and Z-4h isomers are more stable than the E-4a, E-4c, E-4d, E-4e,
E-4f and E-4h ones (1.00, 2.71, 0.990, 2.44, 1.19 and 2.47 kcal/mol, respec-
tively) by B3LYP solvation data (acetone).
Further investigation was undertaken in order to determine more effec-
tive factors on stability of the two Z- and E-isomers on the basis of AIM cal-
culations42 at HF/6-31G level of theory by the AIM2000 program package43.
In recent years, AIM theory was often applied in the analysis of H-bonds.
In this theory, the topological properties of the electron density distribu-
tion are derived from the gradient vector field of the electron density ∇ρ(r)
and on the Laplacian of the electron density ∇2ρ(r). The Laplacian of the
electron density, ∇2ρ(r), identifies regions of space wherein the electronic
FIG. 3
i) Intramolecular hydrogen bonds (dotted lines) in the two E-4c and Z-4c geometrical isomers
of stable ylide 4c. j) A part of molecular map for the two geometrical isomers
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794
Habibi-Khorassani et al.:
charge is locally depleted (∇2ρ(r) > 0) or built up (∇2ρ(r) < 0)42. Two interact-
ing atoms in a molecule form a critical point in the electron density, where
∇ρ(r) = 0, called the bond critical point (BCP). The values of the charge den-
sity and its Laplacian at these critical points give useful information
regarding the strength of the H-bonds43. The ranges of ρ(r) and ∇2ρ(r) are
3
5
0.002–0.035 e/a0 and 0.024–0.139 e/a0 , respectively, if H-bonds exist44.
The AIM calculation indicates intermolecular hydrogen bond critical points
(H-BCP) for Z-4 (a, c, d, e, f and h) and E-4 (a, c, d, e, f and h) isomers.
Intermolecular H-BCPs along with a part of molecular map are shown in
Figs 2–7 (dotted line). The electron density (ρ), Laplacian of electron den-
sity ∇2ρ(r) and energy density –H(r) are also reported in Tables II–VII. A neg-
ative total energy density at the BCP reflects a dominance of potential
energy density, which is the consequence of accumulated stabilizing elec-
FIG. 4
i) Intramolecular hydrogen bonds (dotted lines) in the two E-4d and Z-4d geometrical isomers
of stable ylide 4d. j) A part of molecular map for the two geometrical isomers
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Theoretical Study and Synthesis
795
tronic charge45. Herein, the numbers of hydrogen bonds in all categories
(Z-4a and E-4a), (Z-4c and E-4c), (Z-4d and E-4d), (Z-4e and E-4e), (Z-4f
and E-4f) and (Z-4h and E-4h) are (5 and 7), (11 and 15), (7 and 6), (13 and
10), (12 and 11) and (15 and 19), respectively. In addition, the values of
electron densities (ρ) are in the ranges (0.0028–0.0294 and 0.0031–0.0250
3
3
au e/a0 ), (0.0030–0.0233 and 0.0029–0.0330 e/a0 ), (0.0054–0.0146 and
0.0065–0.0165 e/a03), (0.0027–0.0143 and 0.0028–0.0189 e/a03),
3
(0.0036–0.0149 and 0.0033–0.0118 e/a0
) and (0.0013–0.0158 and
0.0028–0.0146 e/a03), respectively. Also ∇2ρ(r) are in the ranges
5
(0.028–0.057 and 0.021–0.063 au e/a0 ), (0.0097–0.056 and 0.014–0.063
e/a0 ), (0.026–0.058 and 0.030–0.068 e/a0 ), (0.009–0.056 and 0.012–0.070
5
5
FIG. 5
i) Intramolecular hydrogen bonds (dotted lines) in the two E-4e and Z-4e geometrical isomers
of stable ylide 4e. j) A part of molecular map for the two geometrical isomers
Collect. Czech. Chem. Commun. 2010, Vol. 75, No. 8, pp. 785–805

796
Habibi-Khorassani et al.:
5
e/a05), (0.008–0.059 and 0.012–0.051 e/a0
)
and (0.014–0.051 and
5
0.009–0.058 e/a0 ), respectively. Moreover, the Hamiltonians (–H(r)) are in
the ranges (4.25 × 10–4–17.15 × 10–4 and 11.51 × 10–4–15.90 × 10–4 au),
(3.63 × 10–4–18.59 × 10–4 and 3.55 × 10–4–22.42 × 10–4 au), (1.02 × 10–4–
15.37 × 10–4 and 1.18 × 10–4–18.52 × 10–4 au), (0.958 × 10–4–16.38 × 10–4
and 1.25 × 10–4–18.18 × 10–4 au), (3.95 × 10–4–18.93 × 10–4 and 3.67 × 10–4–
19.77 × 10–4 au) and (4.01 × 10–4–17.42 × 10–4 and 2.97 × 10–4–17.49 × 10–4 au),
respectively (see Tables II–VII).
FIG. 6
i) Intramolecular hydrogen bonds (dotted lines) in the two E-4f and Z-4f geometrical isomers
of stable ylide 4f. j) A part of molecular map for the two geometrical isomers
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Theoretical Study and Synthesis
797
These HBs show ∇2ρ(r) > 0 and H(r) < 0, which, according to classification
of Rozas et al.46 are medium-strength hydrogen bonds. In both ylides
(4a, 4c), the dipole moments for the two E-4a and E-4c isomers (8.25 and
8.18 D, solution media, respectively) are smaller than those for the two
Z-4a and Z-4c ones (12.2 and 11.7 D, respectively); the values of Htot
(= ∑H(r)) for the two E-4a and E-4c isomers (92.1 and 168.2 au, respec-
tively) are higher than those for the two Z-4a and Z-4c ones (55.4 and
120 au, respectively) and the number of hydrogen bonds in E-4a and E-4c
isomers (7, 15) is higher than that of the Z-4a and Z-4c ones (5, 11), respec-
tively. These parameters, as dominate factors on stability, taken altogether,
FIG. 7
i) Intramolecular hydrogen bonds (dotted lines) in the two E-4h and Z-4h geometrical isomers
of stable ylide 4h. j) A part of molecular map for the two geometrical isomers
Collect. Czech. Chem. Commun. 2010, Vol. 75, No. 8, pp. 785–805

798
Habibi-Khorassani et al.:
TABLE I
The relative stabilization energy (kcal/mol) for both Z- and E-isomers of ylides 4a, 4c, 4d,
4e, 4f and 4h, obtained at HF/6-31G, B3LYP/6-311 G(d,p) levels (* 2-aminobenzimidazole,
** 2-hydroxy-3-nitropyridine, *** 1,2,3,4-tetrahydrocarbazole)
Rotational isomer
HF
B3LYP
Z*-4a
E*-4a
0
0
3.05a (1.31)b
2.43a (1.00)b
Z*-4c
0
0
E*-4c
4.89 (3.61)
3.65 (2.71)
Z**-4d
E**-4d
Z**-4e
E**-4e
Z***-4f
E***-4f
Z***-4h
E***-4h
0
0
1.35 (1.02)
1.21 (0.990)
0
0
2.21 (2.23)
0
2.34 (2.44)
0
1.58 (1.26)
0
1.35 (1.19)
0
4.97 (3.36)
2.87 (2.47)
a
b
Vacuum data (gas phase). Solvation data (acetone).
TABLE II
The values of ρ × 103, ∇2ρ × 103 and Hamiltonian –H × 104 for the two Z-4a and E-4a
isomers of ylide 4a calculated at the hydrogen bond critical points. All quantities are in
atomic units
E-4a
ρ × 103
∇2ρ × 103 –H × 104
Z-4a
ρ × 103
∇2ρ × 103 –H × 104
1
2
3
4
5
6
7
7.53
14.9
7.72
7.65
9.03
6.20
7.66
33.7
63.2
27.5
32.2
33.0
21.9
26.9
11.5
14.6
12.4
12.6
15.9
11.6
13.5
1
2
3
4
5
14.2
14.5
7.37
5.35
9.06
51.0
57.2
31.7
18.5
34.5
4.25
10.9
13.1
10.1
17.2
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Theoretical Study and Synthesis
799
TABLE III
The values of ρ × 103, ∇2ρ × 103 and Hamiltonian –H × 104 for the two Z-4c and E-4c
isomers of ylide 4c calculated at the hydrogen bond critical points. All quantities are in
atomic units
E-4c
ρ × 103
∇2ρ × 103 –H × 104
Z-4c
ρ × 103
∇2ρ × 103 –H × 104
1
2
10.9
10.7
4.03
2.96
12.1
13.7
10.9
13.1
14.3
16.4
7.23
8.80
5.58
7.70
8.95
42.5
41.7
15.4
14.0
46.6
52.4
41.3
52.8
63.1
60.7
25.5
36.5
19.7
27.1
33.0
12.0
11.9
1
2
10.2
10.6
5.12
14.0
15.1
12.5
12.1
5.17
6.92
2.92
9.16
39.7
41.6
22.6
55.8
53.8
48.1
46.9
18.0
29.8
9.75
36.0
11.5
12.2
11.3
11.9
3
9.95
8.53
11.8
11.2
4.52
7.73
22.4
3.55
11.2
3
4
4
5
5
3.53
6
6
11.2
11.8
7
7
8
8
9.96
12.7
5.48
18.6
9
9
10
11
12
13
14
15
10
11
13.1
10.8
13.6
16.0
TABLE IV
The values of ρ × 103, ∇2ρ × 103 and Hamiltonian –H × 104 for the two Z-4d and E-4d
isomers of ylide 4d calculated at the hydrogen bond critical points. All quantities are in
atomic units
E-4d
ρ × 103
∇2ρ × 103 –H × 104
Z-4d
ρ × 103
∇2ρ × 103 –H × 104
1
2
3
4
5
6
16.6
15.2
10.7
6.55
7.66
14.4
61.6
67.7
41.1
30.5
35.4
54.1
5.87
17.5
18.5
13.2
14.5
1.18
1
2
3
4
5
6
7
8.36
8.51
9.00
5.50
8.90
14.3
14.7
30.5
31.6
38.2
26.2
40.7
53.7
58.0
15.1
15.4
14.1
12.9
15.3
1.02
11.1
Collect. Czech. Chem. Commun. 2010, Vol. 75, No. 8, pp. 785–805

800
Habibi-Khorassani et al.:
TABLE V
The values of ρ × 103, ∇2ρ × 103 and Hamiltonian –H × 104 for the two Z-4e and E-4e
isomers of ylide 4e calculated at the hydrogen bond critical points. All quantities are in
atomic units
E-4e
ρ × 103
∇2ρ × 103 –H × 104
Z-4e
ρ × 103
∇2ρ × 103 –H × 104
1
2
10.6
12.2
5.40
12.2
12.8
18.9
14.4
2.83
4.19
12.2
41.1
47.5
22.1
47.3
49.1
70.0
53.9
12.8
20.5
45.2
11.7
12.4
14.1
11.8
11.3
1
2
14.3
8.36
6.06
10.3
10.8
13.8
6.52
12.6
12.4
8.76
8.94
8.24
2.76
53.6
38.4
28.6
40.4
42.3
55.5
28.6
48.2
47.3
37.4
34.1
30.6
0.958
15.0
3
3
13.2
11.6
12.3
12.6
12.8
11.3
11.3
14.2
16.4
15.5
4
4
5
5
6
4.15
6
7
1.25
8.04
7
8
8
9
11.5
18.2
9
10
10
11
12
13
9.15
5.35
TABLE VI
The values of ρ × 103, ∇2ρ × 103 and Hamiltonian –H × 104 for the two Z-4f and E-4f
isomers of ylide 4f calculated at the hydrogen bond critical points. All quantities are in
atomic units
E-4f
ρ × 103
∇2ρ × 103 –H × 104
Z-4f
ρ × 103
∇2ρ × 103 –H × 104
1
2
13.1
8.16
3.76
4.06
11.2
4.29
11.4
11.8
3.31
7.93
9.35
50.9
28.9
13.4
14.0
41.5
17.2
41.8
47.3
12.9
37.0
37.3
9.90
14.1
1
2
8.97
4.68
8.31
9.28
4.14
4.74
10.7
3.64
9.43
15.0
7.54
8.86
33.4
20.7
35.5
36.0
14.6
16.2
39.8
16.3
11.4
14.1
17.2
3
8.25
8.24
3
4
4
5
16.6
5
8.70
9.14
16.4
14.5
3.95
6
9.78
3.68
6
7
7
8
11.9
8.74
19.8
11.8
8
8.95
9
9
35.2
58.7
34.9
35.5
10
11
10
11
12
11.7
18.9
11.9
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Theoretical Study and Synthesis
801
make a great stability on E-4a and E-4c in comparison with the Z-4a and
Z-4c forms (the results are summarized in Table VIII). For this reason, it is
possible to see only a single isomer as a lone isomer (E-4a and E-4c) of
ylides 4a and 4c. In addition, in ylide 4h, the same behavior and results
were observed for E-4h isomer as a single isomer (see Table VIII). The theo-
retical results for ylide 4b are similar to ylides 4a, 4c and 4h, for this rea-
son, the relevant data have not been reported for E-4b isomer. Although on
the basis of theoretical calculations (Table I), Z-4a, Z-4b, Z-4c and Z-4h iso-
mers have a stability with respect to the E-4a, E-4b, E-4c and E-4h ones
(1.00, no data, 2.70 and 2.47 kcal/mol, in solution media, respectively) and
TABLE VII
The values of ρ × 103, ∇2ρ × 103 and Hamiltonian –H × 104 for the two Z-4h and E-4h
isomers of ylide 4h calculated at the hydrogen bond critical points. All quantities are in
atomic units
E-4h
ρ × 103
∇2ρ × 103 –H × 104
Z-4h
ρ × 103
∇2ρ × 103 –H × 104
1
2
10.7
8.57
9.64
3.90
4.65
10.7
3.16
2.83
13.9
14.7
8.95
8.22
3.90
6.67
2.90
6.72
10.3
9.86
11.1
43.7
31.0
37.3
14.4
16.2
40.3
12.7
13.3
15.1
17.5
1
2
9.00
8.18
4.86
4.05
9.55
5.20
10.8
4.21
12.6
13.1
9.79
8.49
5.09
11.5
10.1
34.6
35.0
21.4
14.2
37.6
17.9
40.4
17.5
48.2
50.2
36.2
34.3
20.1
44.7
39.7
17.4
14.1
11.6
3
3
4
9.16
9.31
16.5
4
8.48
17.8
9.92
16.5
9.88
5
5
6
6
7
7.89
5.88
7
8
9.76
8
9
53.1
57.4
32.9
33.6
15.8
26.8
11.1
12.6
2.97
11.5
9.34
15.7
5.14
9
11.2
11.3
10
11
12
13
14
15
16
17
18
19
10
11
12
13
14
15
4.01
12.2
12.8
12.1
11.9
9.51
27.3
40.7
38.9
43.2
16.0
11.2
11.9
11.7
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802
Habibi-Khorassani et al.:
seem to be different from the results of predictable properties of the most
important geometrical parameters (Table VIII), perhaps, this different be-
havior is relevant to the huge structures of the ylides 4a, 4b, 4c and 4h in-
volving three large atoms such as the four oxygen, one phosphorus and
three nitrogen (only one nitrogen atom for 4h) and also a very high num-
ber of other atoms (C and H). This point made a limitation in application
of basis set higher than B3LYP/6-311 G(d,p) in a higher performance for
more accurate calculations. On the other hand, same discussion could be
employed for Z-4e (not E-4e) as a lone isomer in ylide 4e. Herein, less di-
pole moment, 9.63, higher Htot = 156 and more hydrogen bonds, 14, as
three dominate factors with respect to E-4e, lead to observe only a single
isomer (Z-4e). In ylides 4d and 4f, two stability factors involving higher
Htot and higher number of hydrogen bonds are relevant to the Z-4d and
Z-4f isomers, whereas stability on E-4d and E-4f has been only emerged
from a result of dipole moment factor. This cause a fairly relative stability
on Z-4d and Z-4f in comparison with E-4d and E-4f isomers in which this
1
is in a good agreement with the experimental results based upon the H,
TABLE VIII
The most important geometrical parameters involving the value of Htot, dipole moment and
number of hydrogen bonds for the two Z- and E-isomers of ylides 4a, 4c, 4d, 4e, 4f and 4h
(* 2-aminobenzimidazole, ** 2-hydroxy-3-nitropyridine, *** 1,2,3,4-tetrahydrocarbazole)
Rotational isomer
Htot, au
Dipole moment, D
No. of hydrogen bonds
Z*-4a
E*-4a
55.4
92.1
120
168
84.9
70.8
156
109
149
123
184
214
8.72a (12.2)b
5.38 (8.25)
8.82 (11.2)
5.68 (8.18)
7.66 (11.1)
8.32 (10.6)
8.04 (9.63)
8.28 (10.7)
8.43 (11.1)
5.75 (7.77)
8.35 (9.11)
6.12 (7.62)
5
7
Z*-4c
11
15
7
E*-4c
Z**-4d
E**-4d
Z**-4e
E**-4e
Z***-4f
E***-4f
Z***-4h
E***-4h
6
14
11
12
11
16
19
a
b
Vacuum data (gas phase). Solvation data (acetone) at B3LYP/6-311 G(d,p) as a result of
single point calculations.
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Theoretical Study and Synthesis
803
13C and 31P NMR data with slightly more experimental abundance percent-
age of 77 and 61% for Z-4d and Z-4f, respectively. Herein, the result for
Z-4g is similar to Z-4f, hence no data have been reported for Z-4g within
the text. In conclusion, theoretical calculations indicated that all ylides in-
volving 4a, 4b, 4c, 4e and 4h can exist as a lone isomer (i.e. E-4a, E-4b,
E-4c, Z-4e and E-4h), while the experimental results show that a single iso-
mer is relevant to the Z-4a Z-4b, Z-4c, Z-4e and Z-4h isomers. On the other
hand, the results for ylides 4d, 4f and 4g are the same in both theoretical
and experimental data in which the Z-4d, Z-4f and Z-4g are major forms in
comparison with E-4d, E-4f and E-4g as minor forms.
CONCLUSIONS
We have prepared the novel phosphorus ylides using a one-pot reaction be-
tween triphenylphosphine and dialkyl acetylenedicarboxylates in the pres-
ence of heterocyclic compounds such as 2-aminobenzimidazole, 2-hydroxy-
3-nitropyridine and 1,2,3,4-tetrahydrocarbazole. The present method has
the advantage that, not only the reaction is performed under neutral condi-
tions, but also the substances can be mixed without any activation or modi-
fications. Heterocyclic compounds containing phosphorus ylides 4a–4h
may be considered as potentially useful synthetic intermediates. In addi-
tion, the assignment of the Z- and E-isomers as a major or minor form
in ylides 4a–4h was undertaken by the theoretical study. The AIM theory
clarified, how the ylides 4a, 4b, 4c, 4e and 4h only appear as a single iso-
mer, whereas 4d, 4f and 4g exist in solution as a mixture of these two iso-
mers, and why either Z- or E-isomer is a more stable form as a major in
solution media.
We gratefully acknowledge financial support from the Research Council of University of Sistan and
Baluchestan, Iran.
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