Atom-economic synthesis of new imidazoles and benzimidazoles with thiophosphoryl and hydroxyl hroups

Article abstract of DOI:10.1007/s10593-005-0150-3

Nucleophilic addition of secondary phosphine sulfides to an aldehyde of the azole series occurs under noncatalytic conditions in THF at room temperature and enables the preparation in high yield of imidazoles and benzimidazoles with thiophosphoryl and hyd

Full text of DOI:10.1007/s10593-005-0150-3

Chemistry of Heterocyclic Compounds, Vol. 41, No. 3, 2005
ATOM-ECONOMIC SYNTHESIS OF NEW
IMIDAZOLES AND BENZIMIDAZOLES WITH
THIOPHOSPHORYL AND HYDROXYL GROUPS*
N. I. Ivanova, L. V. Baikalova, N. A. Konovalova, I. A. Zyryanova, A. I. Albanov,
L. M. Sinegovskaya, V. A. Kukhareva, A. A. Tatarinova, N. D. Avseenko,
B. G. Sukhov, N. K. Gusarova, and B. A. Trofimov
Nucleophilic addition of secondary phosphine sulfides to an aldehyde of the azole series occurs under
noncatalytic conditions in THF at room temperature and enables the preparation in high yield of
imidazoles and benzimidazoles with thiophosphoryl and hydroxyl groups, which are promising building
blocks for organic synthesis and polydentate ligands for metal complexes.
Keywords: bis(2-phenylethyl)phosphine sulfide, 2-[bis(2-phenylethyl)thiophosphorylhydroxymethyl]-
1-ethyl(vinyl)imidazoles, hydrochlorides of 2-[bis(2-phenylethyl)thiophosphorylhydroxymethyl]-1-
ethyl(vinyl)imidazoles, 2-formyl-1-organylimidazoles, 2-formyl-1-organylbenzimidazoles, hydrothio-
phosphorylation.
The reaction of accessible aldehydes of the azole series [1] with PH acids has been accomplished for the
first time in the example of secondary phosphine oxides, which add under mild conditions to 2-formyl-1-
organylimidazole and 2-formyl-1-organylbenzimidazole giving 2-(diorganylphosphorylhydroxymethyl)-1-
organylimidazoles in practically quantitative yield [2-4]. The latter readily form biomimetic complexes with
transition metal salts [5, 6].
The aim of the present work is the study of the principles of the processes of interaction of functional
aldehydes with PH acids, and also the development of a convenient general approach to the synthesis of new
chiral azoles with hydroxyl and phosphorus-containing groups. Therefore the reaction of 1-organyl-2-formyl-
imidazoles 1a,b and 2-formyl-1-organylbenzimidazoles 1c,d with bis(2-phenylethyl)phosphine sulfide has been
carried out and investigated. The phosphine sulfide is readily obtained from the available bis(2-
phenylethyl)phosphine [7] and elementary sulfur.
Information on the interaction of aldehydes with secondary phosphine sulfides is limited to studies on
the addition of dialkyl- and diarylphosphine sulfides to aliphatic and aromatic aldehydes in the presence of bases
[8, 9], and also to formaldehyde and chloral under noncatalytic conditions [8, 10].
_______
* Dedicated to Academician V. I. Minkin on his 70th birthday.
__________________________________________________________________________________________
A. E. Favorsky Irkutsk Institute of Chemistry, Irkutsk 664033, Russia; e-mail: n_iva@irioch.irk.ru.
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 365-371, March, 2005. Original article
submitted September 30, 2004.
0009-3122/05/4103-0317©2005 Springer Science+Business Media, Inc.
317

It turned out that the hydrothiophosphorylation of aldehydes 1a-d by bis(2-phenylethyl)phosphine
sulfide proceeds readily at room temperature in THF with the formation of 2-[bis(2-
phenylethyl)thiophosphorylhydroxymethyl]-1-ethyl(vinyl)imidazoles and the corresponding benzimidazoles 2a-
d, the yield of which was 65-82%.
R3
R3
S
S
N
N
P(CH2CH2Ph)2
P(CH2CH2Ph)2
+
H
R2
R2
H
CH
N
N
C
O
OH
R1
R1
1a–d
2a–d
1, 2 a R1 = Et, R2 = R3 = H, b R1 = Vin, R2 = R3 = H; c R1 = Et, R2 = R3 = (CH) 4;
d R1 = Vin, R2 + R3 = (CH)4
In the 1H and 13C NMR spectra of compounds 2a-d the most characteristic signals were of the CH group
31
of the OCHP=S fragment. The vicinal coupling constant of P–C–1H in the α-hydroxyphosphine sulfides was
small [10], consequently the splitting of the resonance signal of the proton, present as a narrow singlet at
4.9-5.2 ppm, was not always observed. Other characteristic signals were the broadened singlet of the hydroxyl
group proton at 4.5-5.8 ppm (1H NMR) and the doublet at ~68 ppm (1JPC ~58 Hz) (13C NMR). The
nonequivalence of the signals of the two phenylethyl groups in the 1H and 13C NMR spectra of
thiophosphorylazoles 2a-d is caused by the presence of the chiral carbon atom in the OCHP=S fragment. The
resonance of the protons of each of the CH2 groups of the two phenylethyl fragments of PhCH2CH2P(S) in the
1H NMR spectrum gives four multiplets, two of which are superimposed ( high field multiplets for the CH2P
protons and low field for the CH2Ph protons), which was confirmed by C–H correlation spectroscopy. In the
benzimidazole derivative 2c the difference of the chemical shifts of the two anisotropic protons of the NCH2CH3
group was significantly greater than in the imidazole derivative 2a, consequently the resonance of these protons
for 2c is displayed as two doublets of quartets.
The absorption band at 3130-3150 cm-1 in the IR spectra (KBr, nujol) of thiophosphorylazoles 2a-d
belongs to the stretching vibration of the associated OH group. An intense νOH band at 3420-3442 cm-1 was
observed in the spectra of solutions in low polarity CCl4 (conc. ~10-1-10-4 M). However at higher dilution (to 10-4
M) in CHCl3 and CH2Cl2 this band disappears, which indicates its belonging to the vibrations of an OH group
participating in a simple intermolecular hydrogen bond, probably OH···N.
Disappearance of the indicated band is accompanied by the appearance in the IR spectra of solutions of
compounds 2a-d of a band for free OH groups (3607 cm-1) and a band for νOH at ~3530 cm-1, probably
characterizing an intramolecular OH···S bond.
The ionization constants of compounds 2a-c, and the ionization constants of some of their oxygen
analogs, viz. 2-[bis(2-phenylethyl)phosphorylhydroxymethyl]-1-ethylimidazole (3a) and 2-[bis(2-phenylethyl)-
phosphorylhydroxymethyl]-1-vinylimidazole (3b) were determined by potentiometric titration with perchloric
acid in acetonitrile to assess the electron-donating ability of compounds 2a-c.
The data obtained indicate that the azoles with thiophosphoryl functions 2a-c, like their analogs with
phosphine oxide fragments 3a,b, are "monoacidic" bases in acetonitrile with a center of protonation at the N-3
atom of the azole ring [11]. The increased basicity (by ~2 pKa units) of phosphorylazoles 3a,b compared with
the thiophosphorylazoles 2a-c is possibly linked with the higher stabilization of the cation formed on titration of
phosphorylazoles 3a,b with perchloric acid. This is the result of an intramolecular hydrogen bond with the
oxygen atom which is more electronegative than the sulfur atom in the corresponding thiophosphorylazoles
2a-c.
318

TABLE 1. Values of pKa of Thiophosphorylazoles 2a-c and
Phosphorylazoles 3a,b (in Acetonitrile)
Compound
рKa
13.21
11.95
11.63
15.41
14.56
2a
2b
2c
3a
3b
H ...X
R3
+
N
P(CH2CH2Ph)2
R2
CH
OH
N
R1
X = O, S
We note that the secondary phosphine sulfides are less reactive than their oxygen analogs in reactions
with aldehydes of the azole series. The hydrophosphorylation of formylimidazole 1a with bis(2-
phenylethyl)phosphine oxide proceeds quantitatively in 1 h [3], while complete reaction of this aldehyde with
bis(2-phenylethyl)phosphine sulfide requires 24 h. These data are in agreement with the data of [10] on the lower
reactivity of dimethylphosphine sulfide in comparison with dimethylphosphine oxide in reactions with carbonyl
compounds.
The functional thiophosphorylimidazoles 2a,b and thiophosphorylbenzimidazoles 2c,d interact readily
with gaseous hydrogen chloride (diethyl ether, room temperature) forming the corresponding stable
hydrochlorides 4a-d of composition 1:1 in yields up to 99%.
R3
R2
S
+
NH
_
Cl
22-24°C
1 h
P(CH2CH2Ph)2
2a–d
HCl
+
CH
OH
N
R1
4a–d
1
A comparative analysis of the H NMR spectra of the initial compounds 2a-d and the salts 4a-d
synthesized from them showed the presence of a low field displacement of the signals of the protons in radical
R1: up to δ 0.40 ppm for the CH2 protons of the ethyl substituent and up to δ 0.57 ppm for the protons of =CH2
of the vinyl fragment. These data, and also the significant low-field displacement of the proton signal of the
(S)PCHO (up to δ 0.72 ppm), indicate the protonation of the N-3 atom of the imidazole ring, similar to that
observed for hydrochlorides obtained from functionally substituted 1-vinyl(ethyl) imidazoles [12].
The data of the IR spectra of hydrochlorides 4a-d (KBr, nujol), particularly the presence of a broad
+
intense band for the stretching vibrations of the ammonium cation NH at 2400-3100 cm-1 and the high
frequency displacement of the absorption bands of the heterocycle (1523→1581, 1496→1518 cm-1), also
indicates that the reaction of thiophosphorylazoles 2a-d with HCl proceeds with the participation of only the
pyridine-type nitrogen atom of the azole ring.
319

A low frequency displacement is observed for the doublet absorption band in the IR spectra of
hydrochlorides 4a-d characterizing the stretching vibrations of the S=P fragment [13] participating in H bonds at
556, 566 (2a), 550, 556 (4a); 558, 568 (2b), 550, 556 (4b); 550, 568 (2c), 547, 557 (4c); 540, 565 (2d), 539, 560
(4d). In addition, a broad low intensity absorption band was present in the spectra of hydrochlorides 4a-d for an
associated OH group with two maxima at ~3450 and ~3530 cm-1. The doublet disappears in CHCl3 solution at
high dilution and a band for a free OH group appears at a frequency of ~3610 cm-1. These data indicate that the
hydrochlorides 4 being investigated form associates through intermolecular hydrogen bonds involving the sulfur
and chlorine heteroatoms (NH···Cl and OH···S).
The hydrothiophosphorylation of aldehydes of the azole series is therefore a simple atom-economic
method of synthesizing new polyfunctional chiral heterocyclic compounds. These are highly reactive building
blocks for organic synthesis, precursors of optically active amphiphilic ligands for enantioselective processes,
and also convenient models for solving fundamental problems of coordination chemistry.
EXPERIMENTAL
1
The H, 13C, and 31P NMR spectra were obtained on a Bruker DPX 400 (400, 101, and 161 MHz
respectively), internal standard was HMDS (1H NMR), and deuterochloroform (13C NMR), and external standard
85% H3PO4 (31P NMR). The IR spectra were recorded on a Bruker IFS 25 spectrometer in KBr disks and in
nujol.
The pKa values were determined by potentiometric titration in absolute acetonitrile with a precision of
±0.03 by the method described in [14], the standard base was diphenylguanidine of pKa = 17.90 [15]. The
titration was carried out at 20°C in an EV-74 universal ionomer with glass and silver chloride electrodes. The
titrant was 0.1 N HClO4.
The synthesis of 2-[bis(2-phenylethyl)phosphorylhydroxymethyl]-1-ethylimidazole (3a) from bis(2-
phenylethyl)phosphine oxide and 1-ethyl-2-formylimidazole was described in [3], 2-[bis(2-
phenylethyl)phosphorylhydroxymethyl]-1-vinylimidazole (3b) was obtained analogously from bis(2-
phenylethyl)phosphine oxide and 2-formyl-1-vinylimidazole.
Preparation of 2-[Bis(2-phenylethyl)thiophosphorylhydroxymethyl]-1-ethyl(vinyl)imidazoles and
-benzimidazoles (2a-d) (General Procedure). A mixture of one of the aldehydes 1a-d (1.6 mmol) and bis(2-
phenylethyl)phosphine sulfide (1.5 mmol) in THF (9 ml) was purged with argon and stirred at room temperature
for 24 h. The solvent was evaporated in vacuum, the viscous residue was kept for 72-80 h at room temperature to
crystallize, then washed with small portions of ethanol (7 × 1 ml), and recrystallized from hexane. A light yellow
to light brown powder was obtained, soluble in chloroform, acetone, and DMSO.
2-[Bis(2-phenylethyl)thiophosphorylhydroxymethyl]-1-ethylimidazole (2a). Yield 80%;
mp 127-128°C. 1H NMR spectrum (CDCl3), δ, ppm (J, Hz): 1.38 (3H, t, 3JHH = 7.1, CH3); 2.14, 2.34, 2.52 (4H,
2
3
m, CH2P); 2.53, 2.74, 2.98 (4H, m, PhCH2); 4.13, 4.19 (2H, qq, JAB = 13.9, JHH = 7.1, NCH2); 4.94 (1H, s,
13
PCH); 6.93, 6.98 (2H, both s, H-4, H-4); 7.13-7.25 (10H, m, Ph). C NMR spectrum (CDCl3), δ, ppm (J, Hz):
15.90 (CH3); 27.91 (d, 1JPC = 45.4, CH2P); 28.20 (d, 2JPC = 2.3, PhCH2); 28.01 (d, 2JPC = 2.3, CH2Ph); 30.47 (d,
1
1JPC = 45.4, CH2P); 42.13 (NCH2); 67.89 (d, JPC = 58.7, PCH); 120.42 (C-5), 126.25, 126.42 (C-p), 127.69
3
3
(C-4), 128.31, 128.47, 128.53, 128.56 (C-m, C-o), 141.01 (d, JPC = 14.9, Cipso), 140.14 (d, JPC = 14.9, Cipso),
142.79 (C-2). 31P NMR spectrum (CDCl3), δ, ppm: 57.01. IR spectrum (KBr and nujol, cm-1): 3150 (νOH); 3134,
3107, 3060, 3024 (ν=CH phenyl, imidazole rings); 2976, 2930, 2907, 2840 (νC-H); 1600, 1581, 1523, 1496, 1487,
1452 (νC=C,C=N phenyl, imidazole rings); 1073 (δCOH); 566, 556 (νP=S). Found, %: C 65.99; H 6.88; N 7.08;
P 7.84; S 8.21. C22H27N2OPS. Calculated, %: C 66.31; H 6.83; N 7.03; P 7.77; S 8.05.
2-[Bis(2-phenylethyl)thiophosphorylhydroxymethyl]-1-vinylimidazole (2b). Yield 63%;
1
mp 118-119°C. H NMR spectrum (CDCl3), δ, ppm (J, Hz): 2.16, 2.36, 2.51 (4H, m, CH2P); 2.65, 2.79, 2.98
3
(4H, m, PhCH2); 4.57 (1H, br. s, OH); 4.95 (1H, d, JHH = 8.9, =CH2, cis); 4.97 (1H, s, PCH); 5.23 (1H, d,
320

3JHH = 15.7, =CH2, trans); 7.02 (1H, s, H-5); 7.22 (1H, s, H-4); 7.11-7.29 (10H, m, Ph); 7.39 (1H, dd, 3JHH = 8.9,
13
2
3JHH = 15.7, =CH, cis, trans). C NMR spectrum (CDCl3), δ, ppm (J, Hz): 28.25 (d, JPC = 2.3, PhCH2); 28.19
2
1
1
1
(d, JPC = 2.3, PhCH2); 28.57 (d, JPC = 45.0, CH2P); 30.53 (d, JPC = 45.0, CH2P); 67.85 (d, JPC = 57.0, PCH);
103.59 (=CH2); 117.74 (C-5); 126.47, 126.56 (C-p); 128.39, 128.34 (C-4); 128.53, 128.61, 128.68 (C-m, C-o),
3
3
130.16 (=CH), 140.78 (d, JPC = 14.1, Cipso), 141.64 (d, JPC = 14.1, Cipso), 142.87 (C-2). 31P NMR spectrum
(CDCl3), δ, ppm: 59.40. IR spectrum (KBr and nujol, cm-1): 3150 (νOH); 3140, 3106, 3085, 3058, 3021 (ν=CH
,
2
ν=CH, phenyl, imidazole rings); 2943, 2907, 2843 (νC–H); 1641 (νC=C vinyl group); 1601, 1582, 1524, 1496, 1483,
1452 (νC=C
,
phenyl, imidazole rings); 1079 (δCOH); 568, 558 (νP=S). Found, %: C 67.04; H 6.49; N 7.17;
C=N
P 7.61; S 8.21. C22H25N2OPS. Calculated, %: C 66.64; H 6.36; N 7.07; P 7.81; S 8.08.
2-[Bis(2-phenylethyl)thiophosphorylhydroxymethyl]-1-ethylbenzimidazole (2c). Yield 82%;
mp 131-132°C. 1H NMR spectrum (CDCl3), δ, ppm (J, Hz): 1.24 (3H, t, CH3, 3JHH = 7.1); 2.22, 2.46, 2.56 (4H,
m, CH2P); 2.67, 2.77, 3.09 (4H, m, PhCH2); 4.19 (1H, dq, NCH2, 3JHH = 7.1, 2JAB = 13.9); 4.58 (1H, dq, NCH2);
5.08 (1H, d, 2JPH = 4.2, PCH); 5.73 (1H, br. s, OH); 7.09-7.33 (13H, m, H-7, H-8, H-9, Ph); 7.67 (d, 3JHH = 7.2,
2
H-6). 13C NMR spectrum (CDCl3), δ, ppm (J, Hz): 14.62 (CH3), 28.14 (d, JPC = 2.3, PhCH2), 28.17 (d,
2JPC = 2.3, PhCH2), 28.10 (1JPC = 45.0, CH2P), 30.40 (d, 1JPC = 45.0, CH2P), 40.08 (NCH2), 68.14 (d, 1JPC = 57.0,
PCH), 110.25 (C-6), 119.31 (C-9), 122.57 (C-7), 123.17 (C-8), 126.25, 126.49 (C-p), 128.32, 128.46, 128.60,
3
3
128.62 (C-m, C-o), 135.09 (C-5), 140.66 (d, JPC = 13.8, Cipso), 140.90 (d, JPC = 13.8, Cipso), 141.71 (C-4),
149.57 (C-2). P NMR (CDCl3), δ, ppm: 58.83. IR spectrum (KBr and nujol, cm-1): 3100 (νOH); 3103, 3082,
31
3061, 3025 (ν=CH phenyl, benzimidazole rings); 2982, 2900, 2870, 2849 (νC-H), 1602, 1496, 1454 (νC=C
,
C=N
phenyl, benzimidazole rings); 1073 (δCOH); 568, 550 (νP=S). Found, %: C 69.87; H 6.62; N 6.68; P 6.81; S 7.04.
C26H29N2OPS. Calculated, %: C 69.62; H 6.52; N 6.25; P 6.91; S 7.13.
2-[Bis(2-phenylethyl)thiophosphorylhydroxymethyl]-1-vinylbenzimidazole (2d). Yield 65%;
mp 97-98°C. 1H NMR spectrum (CDCl3), δ, ppm (J, Hz): 2.24, 2.46, 2.64 (4H, m, CH2P); 2.69, 2.79, 3.04 (4H,
3
m, PhCH2); 4.70 (1H, br. s, OH); 5.16 (1H, s, PCH); 5.33 (1H, d, JHH = 8.6, =CH2, cis); 5.33 (1H, d,
3JHH = 15.6, =CH2, trans); 7.10-7.35 (12H, m, H-7, H-8, Ph); 7.46 (1H, dd, =CH), 7.56 (1H, d, 3JHH = 7.3, H-9);
3
13
2
7.69 (1H, d, JHH = 7.3, H-6). C NMR spectrum (CDCl3), δ, ppm (J, Hz): 28.08 (d, JPC = 2.3, PhCH2); 27.95
2
1
1
1
(d, JPC = 2.3, PhCH2); 28.31 (d, JPC = 45.0, CH2P); 30.45 (d, JPC = 45.0, CH2P); 67.79 (d, JPC = 55.8, PCH);
109.74 (=CH2); 111.68 (C-6); 119.62 (C-9); 123.24 (C-7); 123.98 (C-8); 126.23, 126.37 (C-p); 128.19, 128.35
3
3
(C-o); 128.37, 128.49 (C-m), 129.79 (=CH); 134.23 (C-5); 140.45 (d, JPC = 13.7, Cipso), 140.59 (d, JPC = 13.7,
C
ipso); 141.91 (C-4); 149.40 (C-2). 31P NMR spectrum (CDCl3), δ, ppm: 59.23. IR spectrum (KBr and nujol,
cm-1): 3100 (νOH); 3085, 3057, 3025 (ν=CH2, ν=CH phenyl, imidazole rings; 2946, 2927, 2900, 2864 (νC–H); 1641
(νC=C vinyl group); 1602, 1582, 1497, 1483, 1454 (νC=C,C=N phenyl, benzimidazole rings; 1074 (δCOH); 555, 540
(νP=S). Found, %: C 69.83; H 6.81; N 6.24; P 6.83; S 7.06. C26H27N2OPS. Calculated, %: C 69.93; H 6.09;
N 6.27; P 6.94; S 7.19.
Preparation of Hydrochlorides of 2-[Bis(2-phenylethyl)thiophosphoryl-hydroxymethyl]-1-ethyl-
(vinyl)imidazoles and -benzimidazoles (4a-d) (General Procedure). A mixture of one of the
thiophosphorylazoles 2a-d (0.5 mmol) and absolute ether (25 ml) was saturated with dry hydrogen chloride gas
and stirred at room temperature for 1 h, continuously passing dry HCl through the reaction mixture.
Hydrochlorides 4a,b were isolated by rubbing the obtained oil in fresh portions of absolute ether (3 × 25 ml)
until a powder formed, which was dried in vacuum. Hydrochlorides 4c,d were separated by filtration at once,
then dried in vacuum.
Hydrochloride of 2-[Bis(2-phenylethyl)thiophosphorylhydroxymethyl]-1-ethylimidazole (4a). Yield
81%; mp 106-109°C. Found, %: C60.32; H 6.46; Cl 8.31; N 6.55; P 6.91; S 7.74. C22H28ClN2OPS. Calculated,
%: C 60.76; H 6.44; Cl 8.17; N 6.44; P 7.13; S 7.36.
Hydrochloride of 2-[Bis(2-phenylethyl)thiophosphorylhydroxymethyl]-1-vinylimidazole (4b). Yield
93%; mp 124-126°C. Found, %: C 60.79; H 5.80; Cl 8.23; N 6.51; P 6.96; S 7.63. C22H26ClN2OPS. Calculated,
%: C 61.04; H 6.01; Cl 8.21; N 6.47; P 7.17; S 7.40.
321

Hydrochloride of 2-[Bis(2-phenylethyl)thiophosphorylhydroxymethyl]-1-ethylbenzimidazole (4c).
Yield 99%; mp 157-159°C. Found, %: C 64.39; H 6.09; Cl 7.45; N 5.89; P 6.07; S 7.01. C26H39ClN2OPS.
Calculated, %: C 64.40; H 6.19; Cl 7.33; N 5.78; P 6.40; S 6.60.
Hydrochloride of 2-[Bis(2-phenylethyl)thiophosphorylhydroxymethyl]-1-vinylbenzimidazole (4d).
Yield 94%; mp 120-122°C. Found, %: C 64.55; H 6.02; Cl 7.71; N 5.88; P 5.99; S 6.25. C26H28ClN2OPS.
Calculated, %: C 64.66; H 5.80; Cl 7.36; N 5.80; P 6.42; S 6.63.
The work was carried out with financial support by the President of the Russian Federation of Advanced
Scientific Schools (grant No. NSh-2241.2003.3), and also within the framework of the Program for Fundamental
Investigations of the Russian Academy of Sciences 2004 (state contract No. 21-3 from 01.04.2004).
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