Synthesis and structure of [2-((L)-menthoxycarbonyl)ethyl]diphenyltin halides

Synthesis

The exchange reaction of compound 1, obtained from the reaction of [2-((L)-menthoxycarbonyl)ethyl]triphenyltin with iodine chloride (Tian et al., 2005a,b), with excess sodium iodide in acetone afforded compound 2 (Scheme 1). In order to react completely, excess of sodium iodide and long reaction time (3 h) were required. Compound 2 is a colorless crystalline solid with a sharp melting point, and soluble in common organic solvents such as benzene, acetone, ethanol, and trichloromethane.

Scheme 1:

Synthesis of compounds 1 and 2.

Spectroscopic analysis

Among the stretching modes, the stretch vibration of carbonyl (C=O) in the 1640–1740 cm^-1 region is known to depend on the nature of coordination of the carbonyl oxygen to metal (Maughan et al., 1981; Paterson et al., 1984). The ν (C=O) band of 1 and 2 appears at ~1670 cm^-1 and exhibits a remarkable red shift compared with that of the free esters (~1740 cm^-1), clearly indicating that the carbonyl is coordinated to the tin atom in compounds 1 and 2 (Maughan et al., 1981). The fact that there is almost no variation of the carbonyl frequency between the pure solid and a solution of trichloromethane for 1 and 2 confirms the presence of the C=O→Sn coordination in non-coordination solvents (Maughan et al., 1981; Paterson et al., 1984).

The ^lH resonance assignments of compounds 1 and 2 are given in the Experimental section. The assignment of the SnCH₂ protons was made from the lower ²J(¹¹⁹Sn-¹H) (76 Hz) compared with the corresponding ³J(¹¹⁹Sn-¹H) (100 Hz) coupling. The ^lH NMR data also support the presence of carbonyl oxygen to tin coordination in 1 and 2. The δ values of the CHO- proton of the menthoxyl moiety show a downfield shift (Δδ=0.22 for 1, 0.31 for 2) compared with that in free menthyl ester MenOCOCH₂CH₂SnPh₃ (δ 4.60) (Tian et al., 2005a,b) because the coordination of carbonyl to tin (CHOC=O→Sn) causes the deshielding of the CHOC=O proton.

Compared with the ¹³C NMR of compound 1 reported previously (Tian et al., 2005a), the ¹³C chemical shift of the SnCH₂ carbon atom of 2 increases (from 14.96 ppm for 1 to 17.36 ppm for 2) and the coupling constant ¹J(¹¹⁹Sn-¹³C) decreases (from 550 Hz for 1 to 492 Hz for 2), which is consistent with the ¹³C NMR of (MeOCOCH₂CH₂)₂SnX₂ (X=Cl, I) (Balasubramanian et al., 1997). The ¹³C δ values of CHO and C=O in 2 are larger (Δδ=4.60 and 5.21, respectively) than those of four-coordinated MenOCOCH₂CH₂SnPh₃ (Tian et al., 2005a), which further prove the coordination of carbonyl to tin (CHOC=O→Sn).

The ¹¹⁹Sn chemical shifts depend on the number and nature of the groups coordinated with the central tin atom (Davies et al., 2008). The ¹¹⁹Sn chemical shift value of compound 2 (-96.6 ppm) is more high field than that of 1 (-74.6 ppm), which is in agreement with the ability of withdrawing electrons of chlorine and iodine. The δ value of 2 is also high field shifted as compared to that of four-coordinated IPh₂SnCH₂CH₂CH₂CH₂SnPh₂I (-55.1 ppm), and similar to that of five-coordinated analog chol-OCOCH₂CH₂SnPh₂I (chol=cholesteryl) (-97.2 ppm) (Buchanan et al., 1998). This indicates that compounds 1 and 2 are five-coordinated in solution.

Structure analysis of 1 and 2

Compounds 1 and 2 both crystallize in the chiral space group P2₁2₁2₁. Their molecular structures are shown in Figures 1 and 2, respectively, and selected bond lengths and angles are given in Table 1. In compound 1, the Sn atom is five coordinated and possesses a distorted trigonal bipyramidal geometry with the trigonal plane defined by the C(1), C(14), and C(20) atoms belonging to two phenyl substituents and the menthoxycarbonylethyl group, respectively, and the axial positions occupied by the Cl(1) and O(1) atoms. The Sn(1) atom is 0.155(2) Å out of the C₃ trigonal plane of the [C₃SnOCl] trigonal bipyramidal polyhedron in the direction of Cl(1), which results in a significant deviation of the C(1)-Sn(1)-Cl(1) (95.00(7)°), C(14)-Sn(1)-Cl(1) (97.90(9)°), and C(20)-Sn(1)-Cl(1) (97.22(7)°) angles from the ideal value of 90°. The three bond angles on the trigonal plane are in the range 115.97(10)°–123.44(10)°, and the bond angle O(1)-Sn(1)-Cl(1) at the axial position is 170.20(4)°. The Sn(1)-O(1) distance of 2.4781(16) Å is in the range of a normal Sn-O coordination bond length (Harston et al., 1991; Buchanan et al., 1996), clearly indicating significant bonding interactions between the Sn atom and the O(1) atom of the carbonyl group. The intramolecular C=O→Sn coordination results in a five-membered chelate ring with a narrow bite angle C(1)-Sn(1)-O(1) (75.21(7)°). The Sn(1)-Cl(1) bond distance (2.4299(7) Å) is longer than that in four-coordinated triorganotin chlorides such as Ph₂ClSn(CH₂)₄SnClPh₂ (2.397(3) Å) (Dakternieks et al., 1999) and Ph₂ClSnMen (2.396(7) Å) (Hofmann et al., 2008), and comparable with that found in the intramolecularly coordinated diphenyltin chlorides containing five-membered chelates such as Ph₂ClSnCH₂CH₂CH₂OMe (2.4412(8) Å) (Lebl et al., 2005) and Ph₂ClSnCH₂-[19]-crown-6 (2.4323(11) Å) (Kuate et al., 2008).

Figure 1:

The molecular structure of 1.

Figure 2:

The molecular structure of 2.

The structure of compound 2 is similar to that of 1, and the tin atom is also five-coordinated and has distorted trigonal bipyramidal geometry with an axial bond angle O(1)-Sn(1)-I(1) being 169.60(6)°. The Sn(1) atom is displaced by 0.346(3) Å from the C₃ trigonal plane of the [C₃SnOI] trigonal bipyramidal polyhedron in the direction of I(1). The intramolecular Sn(1)-O(1) distance (2.512(2) Å) is longer than that of 1 (2.4781(16) Å), which is consistent with the increase of the tin atom Lewis acidity in the sequence I<Cl (Spencer et al., 1990), and shorter than that of an analog MeOCOCH₂CH₂SnIPh₂ (2.55(2) Å) (Harston et al., 1991).

Materials and physical measurements

[2-((L)-menthoxycarbonyl)ethyl]triphenyltin was prepared according to the reported method (Tian et al., 2005a). The other chemicals (Sinopharm Chemical Reagent Company Limited, Shanghai, China) were of reagent grade and were used without further purification. Carbon and hydrogen analyses were obtained using a Perkin Elmer 2400 Series II elemental analyzer (Perkin Elmer, Waltham, MA, USA). Melting points were measured on an XR4 microscopic melting point apparatus (Shanghai Optical Instrument Factory, Shanghai, China). IR spectra were recorded on a Nicolet 470 FT-IR spectrophotometer using KBr disks in the range 4000–400 cm^-1 (Thermo Nicolet Corporation, Madison, WI, USA). ¹H and ¹³C NMR spectroscopic data were collected using a Bruker Avance 300 FT-NMR spectrometer (Bruker Corporation, Switzerland) with CDCl₃ as solvent and Me₄Si as internal standard. ¹¹⁹Sn NMR spectra were recorded in CDCl₃ on a Varian Mercury Vx300 spectrometer (Varian Corporation, USA) using Me₄Sn external reference.

Synthesis of [2-((L)-menthoxycarbonyl)ethyl]diphenyltin chloride (1)

This compound was synthesized according to the literature procedure (Tian et al., 2005a). The oily product 1 was dissolved in dichloromethane-hexane (1:1, v/v), and the solution obtained was left in the refrigerator for 2 days. The resulting colorless crystals were filtered and dried in vacuum. M.p. 48–49°C [literature: oil (Tian et al., 2005a)], yield 58%. Anal. Found: C, 57.80; H, 6.37. Calc. for C₂₅H₃₃ClO₂Sn: C, 57.78; H, 6.40%. IR (KBr, ν, cm^-1): 1667 (C=O), 1229 (C-O). ¹H NMR (CDCl₃, δ, ppm): 0.65 (d, J=6.9 Hz, 3H, CH₃), 0.82–1.89 (m, 15H, Men), 1.75 (t, J=7.5 Hz, ²J(¹¹⁹Sn-¹H)=78 Hz, 2H, CH₂Sn), 2.82 (t, J=7.5 Hz, ³J(¹¹⁹Sn-¹H)=100 Hz, 2H, COCH₂), 4.82 (dt, J_ae=4.3 Hz, J_aa=11.1 Hz, 1H, HCO), 7.33–7.41 (m, 6H, (p-, m-H)-C₆H₅), 7.79–7.84 (m, 4H, ²J(¹¹⁹Sn-¹H)=62 Hz, o-H-C₆H₅).

Synthesis of [2-((L)-menthoxycarbonyl)ethyl]diphenyltin iodide (2)

The solutions of [2-((L)-menthoxycarbonyl)ethyl]diphenyltin chloride (1.04 g, 2 mmol) in acetone (30 mL) and sodium iodide (0.60 g, 4 mmol) in acetone (30 mL) were mixed and heated at reflux for 3 h under stirring, and then cooled to room temperature and filtered. The filtrate was evaporated under reduced pressure by a rotary evaporator. The solid residue was extracted into dichloromethane (20 mL) and filtered. A white solid product was obtained by removal of solvent under reduced pressure, and recrystallized from ethanol and dried in vacuum. Yield 0.98 g (80%), m.p. 72–73°C, [α]D20 -32.8° (c, 0.50, CH₂Cl₂). Anal. Found: C, 48.97; H, 5.33. Calc. for C₂₅H₃₃IO₂Sn: C, 49.13; H, 5.44%. IR (KBr, ν, cm^-1): 1669 (C=O), 1225 (C-O). ¹H NMR (CDCl₃, δ, ppm): 0.61 (d, J=6.8 Hz, 3H, CH₃), 0.83–1.76 (m, 15H, Men), 1.93 (t, J=7.5 Hz, ²J(¹¹⁹Sn-¹H)=76 Hz, 2H, CH₂Sn), 2.79 (t, J=7.5 Hz, ³J(¹¹⁹Sn-¹H)=98 Hz, 2H, COCH₂), 4.91 (dt, J_aa=11.0 Hz, J_ae=4.3 Hz, 1H, HCO), 7.34–7.40 (m, 6H, (p-, m-H)-C₆H₅), 7.78–7.82 (m, ²J(¹¹⁹Sn-¹H)=62 Hz, 4H, o-H-C₆H₅). ¹³C NMR (CDCl₃, δ, ppm): 16.30 [CH(CH₃)₂], 20.80 (CH₃), 22.05 [CH(CH₃)₂], 23.33 (C-3), 26.15 [CH(CH₃)₂], 31.29 (C-5), 34.23 (C-4), 40.74 (C-6), 46.96 (C-2), 78.89 (C-1) (Men), 17.36 [¹J(¹¹⁹Sn-¹³C)=492 Hz, CH₂Sn], 30.78 [²J(¹¹⁹Sn-¹³C)=35 Hz, CH₂CO], 128.59 [³J(¹¹⁹Sn-¹³C)=66 Hz, m-C], 129.45 [⁴J(¹¹⁹Sn-¹³C)=14 Hz, p-C], 136.60 [²J(¹¹⁹Sn-¹³C)=50 Hz, o-C], 139.92 [¹J(¹¹⁹Sn-¹³C)=640 Hz, i-C], 179.68 (C=O). ¹¹⁹Sn NMR (CDCl₃, δ, ppm): -96.6.

Crystal structure determination of compounds 1 and 2

The colorless single crystals were obtained from chloroform-hexane (1:2, v/v) by slow evaporation at room temperature. The intensity data for crystals of compounds were measured at 291(2) K on a Bruker Smart Apex area detector fitted with graphite monochromatized MoK_α radiation (0.71073 Å) using the φ and ω scan technique. The structures were solved by a direct method and refined by a full-matrix least-squares procedure based on F² using SHELXL-97 (Sheldrick, 2008). The non-hydrogen atoms were refined anisotropically, and hydrogen atoms were placed at calculated positions (C-H=0.93 Å for aromatic H atoms, C-H=0.96 Å for methyl H atoms, C-H=0.97 Å for methylene H atoms, and C-H=0.98 Å for methine H atoms) and were included in the refinement in the riding-model approximation. The crystallographic parameters and refinements are summarized in Table 2. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication numbers CCDC 904059 and 904060.