Synthesis and characterization of novel organotin-phosphorous compounds II.

New organotin substituted alpha-anilinomethylphosphonates were prepared and were characterized by FT-IR, 1H- and 13C-NMR spectroscopy and elemental microanalysis.

1. Introduction

Many organometallic compounds exhibit interesting antitumour activity against several human cancer cell lines, and organotin(IV) compounds are a widely studied class of metal-based antitumour drugs [1]. The considerable interest in recent decades in the use of organotin compounds as reagents [2] or intermediates [3] in organic synthesis has led to the preparation of many new organotin compounds [4]. Organotins have been used in industrial and agricultural applications [5,6], as plastic stabilizers and catalysts, antifouling paints, molluscicides, fungicides [7] and disinfectants [8]. The introduction of biocidal organotin groups into phosphorous compounds could possibly enhance their biocidal activities [9]. The present study aimed to develop a new technique for the preparation of a new organotin substituted α-anilinomethylphosphonates.

2. Results and Discussion

A series of novel tri-n-butylstannyl phenyl substituted methylphosphonates IVa-c (Figure 1) have been prepared via a Schiff base intermediate.

Figure 1

The structure of Compounds IVa-c.

The active alkyltin groups were first attached to the meta position of the benzene ring [9,10] and the resulting aldehyde was then allowed to react with aniline and with a selected substituted aniline [9] such as p-chloroaniline, m-trifluoromethylaniline, and m-methoxyaniline giving a highly substituted Schiff base (imine) which is reacted directly with a phosphate ester such as diphenyl phosphate to give solid producs IVa-c (Scheme 1).

Scheme 1

Synthesis of organotin-phosphorus compounds.

The titled compounds were isolated in almost quantitative yield (Table 1). The stoichiometry of the compounds was established by elemental analysis (Table 2) which showed agreement with the calculated values; further evidence was obtained from the corresponding 13C-, 1H-NMR and FT-IR spectra.

Table 1

Physical properties of the title compounds.

Compound	R	Melting point (°C)	Yield (%)
IVa	4-Cl	81-82	87
IVb	3-CF3	87.5	58
IVc	3-OCH3	50-53	53

Table 2

Elemental microanalysis of the title compounds.

Compound	Calculated			Measured
	%C	%H	%N	%C	%H	%N
II	57.43	8.26	-	57.63	8.33	-
III	61.09	8.72	-	61.18	8.64	-
IVa	60.14	6.41	1.90	60.02	6.37	1.93
IVb	59.09	6.13	1.81	58.79	6.21	1.87
IVc	62.14	6.86	1.91	62.21	6.93	1.94

2.1. 13C-NMR Spectra

Generally, the assignments of the 13C-NMR resonances for tin-phosphorous compounds (Table 3) in which there are more than three benzene rings are much more difficult than for the starting compounds (I, II, benzaldehyde, aniline, p-chloroaniline, m-trifluoromethylaniline, m-methoxyaniline, and diphenyl phosphite) (Table 4), because excluding the quaternary carbons, the 13C chemical shifts for the ring carbons in 1, 2 and 3 and its derivatives are spread over 18 ppm, compared with the 13C of the starting material in which only one benzene ring is found. Assignments of the 13C chemical shifts of rings 1, 2 and 3 were based on comparisons with reported values [9,11,12,13,14].

Table 3

13C-NMR data of the titled compounds.

R group	δ (ppm)
	Sn-CH2-			-CH2CH2-			-CH3			C0		C1		C2		C3	C4	C5	C6
4-Cl	29.0			27.8			9.6			59.2		136.0		136.5		146.8	143.3	128.2	129.5
				13.6						53.0
3-CF3	29.0			27.3			9.6			59.4		136.4		137.0		142.4	142.4	128.3	129.5
				13.6						53.2
3-OCH3	29.0			27.3			9.6			60.1		134.3		136.3		142.4	142.4	127.9	128.3
				13.5						54.0
R group	δ (ppm)
	C'1		C'2			C'3			C'4		C'5		C'6
4-Cl	147.4		113.9			135.0			118.1		129.7		112.2
3-CF3	146.4		115.1			139.0			119.3		129.0		120.8
3-OCH3	142.6		91.9			153.8			120.4		153.8		91.9
R group	δ (ppm)
	C''1	C''2,6a			C''3,5a			C''4
4-Cl	150.4	120.7			130.2			127.3
	149.8	120.5			130.2
3-CF3	150.5	120.6			129.5			127.9
	150.1	120.4			129.5
3-OCH3	150.5	120.7			130.9			127.8
	150.4	120.6			130.9

a two non-equivalent carbons.

Table 4

13C NMR data of the starting compounds.

Compound	δ (ppm)
	Sn-CH2-	-CH2CH2-		-CH3			C0		C1	C2	C3	C4	C5	C6
Sn-(CH2-CH2-CH2-CH3)3	27.8	26.8		13.6			-		-	-	-	-	-	-
		18.0
III	29.1	27.4		9.7			192.2		135.6	137.7	143.6	142.6	128.4	129.5
		13.7
Compound		δ (ppm)
		C'1	C'2		C'3			C'4		C'5	C'6
4-Chloroaniline		147.7	114.8		134.7			118.2		130.3	113.2
3-Trifluoromethaneaniline		146.4	115.8		138.9			119.3		129.7	112.2
3-Methoxyaniline		143.3	98.8		152.9			119.0		152.9	98.8
Compound		δ (ppm)
		C''1	C''2,6a			C''3,5a		C''4
Diphenyl phosphite		149.3	120.8			131.8		127.6
		149.0	120.5			131.8

a two non-equivalent carbons.

The quaternary carbons C-1, C-3, C'-1, C'-3, C'-4, C'-5, and C''-1 are readily identified since they are less intense and almost invariant in position compared with other signals, as a result of long relaxation times of the quaternary carbons [9,15].

For example, the 13C-NMR spectrum of diphenyl-1-(4-chloroanilino)-1-[3-(tri-n-butylstannyl)-phenyl]methyl phosphonate (IV) in CDCl3, shows that the 13C signal of C0 (for the numbering refer to Figure 1) appears at δ 53.15 and 59.32 ppm, which indicates a clear spin-spin coupling between 31P and 13C.

The ring carbon values were confirmed by using the substituent chemical shift (SCS) effect of the ‑SnBu3n, -N-R, and (Ph-O)2P(O)-CH groups on the ring carbons, in comparison with the parent compounds. However, it has been found that the SCS effects for the above mentioned groups are additive in all positions. It is worth noting that the 13C chemical shift of C0 in which the phosphorous atom appears directly bonded to it generally appears as a doublet centered at δ 55.5 ppm. As expected the carbon of C0, appears as a single peak coupled with the adjacent 31P atom (I = 0.5, 100%).

13C spin-spin coupling constants involving 31P have been determined during the rapid growth of organophosphorous chemistry, particularly for biological molecules such as the nucleotides, phospholipids, and the titled compounds which contain phosphorous. 13C-31P spin-spin interactions have frequently been used as a probe and also used to identify carbons near the phosphorous atom. The 13C-NMR signal of the imine group (–C=N) of compound III (Scheme 1) shows only one signal at δ 161.0 ppm, which indicates the existence of only one isomer.

2.2. 1H-NMR Spectra

The assignments of the 1H Spectra for tin-phosphorous compounds were summarized in (Table 5).

Table 5

1H-NMR data of the titled compounds.

Compound	δ (ppm)
	3(-(C H2)3CH3)	Ar-H	-O-CH-O-	-O(C H2)2O-	-C HO
II	1.2 (m, 27H)	7.5 (m, 4H)	5.8 (s, 1H)	4.1 (d, 4H)	-
III	1.2 (m, 27H)	7.7 (m, 4H)	-	-	10.3 (s, 1H)
Compound	δ (ppm)
	3(-(CH2)3CH3)	-P-CH-N-	-NH-	Ar-H
IVa	0.6-1.8 (m, 27H)	4.7 (m, 1H)	5.4 (d, 1H)	6.5-6.7 (m, 18H)
IVb	0.6-1.7 (m, 27H)	4.8-5.0 (d, 1H)	5.3-5.4 (m, 1H)	6.6-7.6 (m, 18H)
IVc	0.6-1.6 (m, 27H)	3.8 (s, 1H)	5.4 (d, 1H)	6.25-7.6 (s, 18H)

2.3. FT-IR Spectra

The structures of the titled compounds were further confirmed by their FT-IR spectra as shown in (Table 6).

Table 6

FT-IR data of the titled compounds.

Compound	Wavenumber (cm-1)
	-(CH2)3CH3		Aromatic ring	P-O-Aryl	-P=O	C-O-C		-C=O
	C-H stretching	C-H bending	C=C stretching	Stretching
II	2860, 2910, 2940	1350, 1370	1420, 1455	-	-	1080	-
III	2840, 2910, 2950	1360	1450	-	-	-	1700
IVa	2900, 2940	1290	1480	1180	1200	-	-
IVb	2840, 2900, 2960	1340	1480	1100	1200	-	-
IVc	2920, 2940	1300	1470	1040	1200	-	-

Generally, the conversion of compound II into compound III was confirmed by the disappearance of the peak at 1080 cm-1 and the appearance of a peak at 1700 cm-1 which was assigned to C=O stretching. Moreover the formation of compounds IV was confirmed by peaks at 760, 1150 and 1260 cm-1 which were assigned to C-Cl, C-F and -O-CH3 stretching, respectively. In addition, the formation of compounds IV was confirmed via the FT-IR spectra by the disappearance of peaks at 1700 cm-1 and the appearance of peaks at 3290, 3310, 3320 cm-1 and at 1580 cm-1 assigned to N-H stretching and bending, respectively.

3. Experimental

3.1. Instruments

1H- and 13C-NMR spectra were recorded on a Jeol JNM FX-100 spectrometer operating in the Fourier Transform mode. All the spectra were recorded at ambient temperature. The compounds were dissolved in CDCl3 (concentration 50–100 mg in 2 mL solvent using a 10 mm diameter NMR tube). Chemical shift data were determined relative to the internal standard TMS. Melting points were determined using a Melt-temp melting point apparatus and are uncorrected, as were the boiling points. Elemental analysis were performed at M-H-W Laboratories (Phoenix, AZ, USA). FTIR spectra were recorded on Perkin Elmer 883 (Research Center, College of Science, King Saud University). Thin-layer chromatography (TLC) was performed using the ascending technique with silica gel 60F 254 precoated aluminium sheets.

3.2. Methods

Organotin substituted α-anilinomethyl phosphonates such as diphenyl-1-(4-chloroanilino)-1-[3-(tri-n-butylstannyl)-phenyl]methyl phosphonate (IV) was prepared as follows:

3.2.1. Preparation of 2-[3-(tri-n-butylstannyl)]1,3-dioxolane (II)

In a 500 mL three-neck round bottom flask equipped with two dropping funnels, a reflux condenser, and magnetic stirrer, a suspension of magnesium turnings (1.5 g, 60 mmol) in dry tetrahydrofuran (10 mL) was heated under gentle reflux. A solution of 2[(3-bromophenyl)] 1,3-dioxolane (8 g, 35 mmol) and 1,2-dibromoethane (4.5 g, 24 mmol) in dry tetrahydrofuran (100 mL) was prepared and 5 mL of this solution was added to the magnesium. Several drops of 1,2-dibromoethane were further added to initiate a vigorous reaction. The remaining dioxolane/1,2-dibromoethane solution was added dropwise under refluxing conditions, with occasional stirring, at the same time (tri-n-butylstannyl chloride (10 g, 30 mmol) in dry tetrahydrofuran (30 mL) was added. After the additions were completed, the remaining 1,2-dibromoethane was added in portions to destroy the remaining magnesium metal. After the reaction was completed (1–2 h) the mixture was stirred for 30 min at 50 °C and then allowed to cool with continuous stirring for 40 min. The mixture was hydrolyzed and washed with saturated ammonium chloride solution. The organic layer was separated and the aqueous layer was extracted twice with benzene (70 mL), and the combined organic layers were dried over MgSO4. The solvents were stripped off using a rotavapor and the remaining liquid was fractionally distilled twice under reduced pressure to give 9.93 g (73%) yield) of the desired compound II as a colorless liquid (b.p. 145–147 °C/0.2 mmHg).

3.2.2. Preparation of 3-(tri-n-butylstannyl)-benzaldehyde (III)

3-(Tri-n-butylstannyl)-benzaldehyde was prepared by dissolving 2-[3-(tri-n-butylstannylphenyl)]- 1,3-dioxolane in THF (100 mL) and water (50 mL) containing p-toluenesulfonic acid (1 g). This solution was gentle refluxed under an inert atmosphere. After 48 h, the organic layer was separated and the aqueous layer was extracted twice with portions of benzene (50 mL), and the combined organic layers were dried over MgSO4. The solvents were stripped off and the remaining liquid was fractionally distilled under reduced pressure to give a yield of 89–93% of III as a colorless compound (b.p. 140–142 °C/0.07 mmHg).

3.2.3. Preparation of organotin substituted α-anilinomethyl phosphonates IV

3-(tri-n-Butylstannyl)-benzaldehyde (III, 1 equiv.) was placed in a 25 mL conical flask and then one equivalent of substituted aniline was added. The mixture was gently heated on a hotplate, with occasional stirring. Upon slightly cooling an equivalent of diphenyl phosphite was added, and the resulting mixture was heated for a short time with continuous stirring, until the viscosity of the media increased. After a few mL of methanol were added, the solution was stored in a refrigerator for several hours until a precipitate was formed, which was filtered and recrystallized from methanol twice.

4. Conclusions

A series of novel tri-n-butylstannyl phenyl substituted methyl phosphonated were prepared in a good yield via Schiff bases and their structures were confirmed by FT-IR, 1H- and 13C-NMR spectroscopy and elemental microanalysis.