Zirconium-mediated Synthesis and Crystal Structure of 3,6-Diiodo-4,5-dialkyl-phthalic Acid Dimethyl Ester

LI Xu-Dong WANG Hui LI Jun-Qiu MEN Yi-Can QU Hong-Mei

(Key Laboratory of Systems Bioengineering, Ministry of Education, Tianjin Key Laboratory of Biological and Pharmaceutical Engineering, Department of Pharmaceutical Engineering,School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China)

1 INTRODUCTION

Introducing different substituents efficiently to the ring to get desired benzene derivatives has become a research hotspot of organic chemistry. Poly-substituted benzene derivatives are important organic compounds which have great theoretical significance in synthetic chemistry, organic chemistry methodologies and other numerous applications[1-3]. Reppe[4,5]firstly discovered the Ni-catalyzed cyclization of ethyne affording benzene in 1948, which developed a new method to synthesize benzene derivatives.Subsequently, transition metal-mediated cyclotrimerization of alkynes had been extensively studied by Vollhardt, Schore, Yamamoto, et al[6-11].However, when unsymmetrical alkynes were used, a mixture of several benzene derivatives was obtained[12,13]. Therefore, one of the major problems for the reactions is the difficulty in region-selective intermolecular cyclotrimerization with unsymmetrical alkynes to give multi-substituted benzene derivatives[14].

The pioneering studies on zirconocene chemistry from Takahashi, Xi and Liu et al.[15-21]groups were significant because benzene derivatives could be synthesized by intermolecular coupling of three alkynes mediated by zirconocene. Recently, we have reported the zirconocene-mediated synthesis of novel poly-substituted benzene derivatives from alkynes[22,23]. However, there are no reports on the preparation of 3,6-diiodo-4,5-dialkyl-phthalic acid dimethyl esters.

On the basis of our previous studies, a novel series of hexasubstituted benzene derivatives,3,6-diiodo-4,5-dialkyl-phthalic acid dimethyl esters,were synthesized via cycloaddition of two TMS-substituted alkynes and dimethyl acetylenedicarboxylate, which was mediated by zirconocene.After diiodination, three new compounds of 3,6-diiodo-4,5-dimethyl-phthalic acid dimethyl ester(3a), 3,6-diiodo-4,5-dipropyl-phthalic acid dimethyl ester (3b) and 3,6-diiodo-4,5-dibutyl-phthalic acid dimethyl ester (3c) were obtained in high regioselectivity and yields, and their crystal structures were determined by single-crystal X-ray diffraction to confirm the configurations. By this means, specific substituents can be introduced to the benzene ring efficiently by changing the types of alkynes. Being reported as a critical raw material of oligo and polymeric phenylene ethynylene molecules(OPEs)[24], these para-diiodobenzene derivatives are widely used as molecular wires[25]and rigid scaffolds in the construction of nanometric architectures[26,27], dendrimers[28], foldamers[29]and sensors[30–32]. In addition, they can also decorate CBP derivatives applied in organic light-emitting diodes(OLEDs)[33,34].

2 EXPERIMENTAL

2. 1 Materials and instruments

All organic solvents and materials for synthesis were of reagent grade and used without further purification.1H-NMR spectra were acquired on a BRUKER AVANCE III 400MHz and13C-NMR on a 100MHz spectrometer in CDCl3solutions. X-ray diffractions were performed using a Rigaku Saturn CCD area detector diffractometer.

2. 2 Synthesis of 3,6-diiodo-4,5-dimethyl(dipropyl,dibutyl)-phthalic acid dimethyl esters (3a, 3b, 3c)

The title three new compounds were synthesized as shown in Scheme 1.

Scheme 1. Synthetic procedure of 3,6-diiodo-4,5-dialkyl-phthalic acid dimethyl esters

2. 2. 1 Synthesis of 3,6-bis(trimethylsilyl)-4,5-dimethyl(dipropyl,dibutyl)-phthalic acid dimethyl esters (2a, 2b, 2c)

The compounds 2a, 2b, and 2c were synthesized according to our previous work[35], as shown in Scheme 1. A solution of Cp2ZrCl2(365 mg, 1.25 mmol) in 10 mL of THF was cooled to -78 ℃ , and then n-BuLi (1.60 M hexane solution, 1.56 mL,2.50 mmol) was added. After stirring for 15 min,the solution was warmed to –40 ℃ for 30 min and then recooled to –78 ℃ . After 15 min, 1-trimethylsilyl-1-propyne (300 μL, 2.0 mmol) was added to the mixture, and it was warmed to room temperature. After 3 h, the solution was cooled to 0 ℃,and then CuCl (298 mg, 3.0 mmol) and DMAD(0.48 mL, 4.0 mmol) were added. The solution was warmed to room temperature and stirred for 6 h.The mixture was quenched with 3 N HCl and extracted with ethyl acetate. The combined organic phase was washed with water, saturated aqueous NaHCO3solution, and brine. The solution was dried over anhydrous Na2SO4. The solvent was evaporated, and the resulting brown viscous oil was purified by a flash chromatography (silica gel, hexane : ethyl acetate = 5:1 as eluent) to afford the title compounds 1a and 2a as colorless solids. When Dewar benzene 1a was heated in toluene at 100 ℃ for 3 h,benzene 2a was obtained in quantitative yield. So,the title compound 2a was obtained with a total isolated yield of 52%.

2a:1H NMR (CDCl3, Me4Si) δ: 0.32 (s, 18 H),2.37 (s, 6 H), 3.80 (s, 6 H).

Preparation of 3,6-bis(trimethylsilyl)-4,5-dipropyl-phthalic acid dimethyl ester (2b). The synthesis was carried out according to the procedure of 2a with 1-trimethylsilyl-1-pentyne (366 μL, 2.0mmol)as a starting material.

2b: pale-yellow solid; 61% total isolated yield.1H NMR (CDCl3, Me4Si) δ: 0.31 (s, 18 H), 1.01 (t, J =7.2 Hz, 6 H), 1.34～1.44 (m, 4 H), 2.77～2.81 (m, 4 H), 3.80 (s, 6 H).

Preparation of 3,6-bis(trimethylsilyl)-4,5-dibutylphthalic acid dimethyl ester (2c). The synthesis was carried out according to the procedure of 2a with 1-trimethylsilyl-1-hexyne (404 μL, 2.0mmol) as a starting material.

2c: pale-yellow oil; 65% total isolated yield.1H NMR (CDCl3, Me4Si) δ: 0.31 (s, 18 H), 0.95 (t, J =7.2 Hz, 6H), 1.31～1.46(m, 8 H), 2.80～2.84 (m,4H), 3.80 (s, 6 H).

2. 2. 2 Synthesis of 3,6-diiodo-4,5-dimethyl(dipropyl,dibutyl)-phthalic acid dimethyl ester (3a, 3b, 3c)

A solution of 4,5-dimethyl-3,6-bis(trimethylsilyl)phthalic acid dimethyl ester (2a, 367 mg, 1 mmol) in 5 mL of CH2Cl2was cooled to 0 ℃ with stirring, and a solution of ICl in dichloromethane(2.5 mL, 2.5 mmol) was added dropwise over 10 min. The reaction was kept at 0 ℃ for 6 h. The mixture was quenched with 10w% aqueous NaOH solution, and extracted with CH2Cl2. The combined organic layer was dried over MgSO4and evaporated to dryness. The residue was purified by a flash column chromatography (silica gel, hexane:ethyl acetate = 10:1 as eluent) to afford the title compound 3a as white solid (85% isolated yield).

3a:1H NMR (CDCl3) δ: 2.67(s, 6H), 3.91(s, 6H).

3b: colorless solid, 89% isolated yield.1H NMR(CDCl3) δ: 1.07 (t, J = 7.5 Hz, 6 H), 1.50～1.60 (m,4 H), 2.89～2.95 (m, 4 H), 3.89 (s, 6 H);13C NMR(CDCl3) δ: 14.3, 22.9, 42.1, 53.0, 100.4, 138.2,146.1, 167.6.

3c: white solid, 80% isolated yield.1H NMR(CDCl3) δ: 1.01(t, J = 6.4 Hz, 6H), 1.44～1.58(m,8H), 2.92～3.03(m,4H), 3.92(s, 6H).

2. 3 X-ray crystallographic analysis

Compounds 3a, 3b and 3c were recrystallized by hexane and ethyl acetate to give colorless single crystals suitable for X-ray analysis.

Single crystals of the title compounds 3a(0.24mm × 0.12mm × 0.10mm), 3b (0.20mm ×0.18mm × 0.12mm) and 3c (0.26mm × 0.12mm ×0.10mm) were mounted on glass fibers in a random orientation for single crystal diffraction. The data were collected at 133(2) K on a Rigaku Saturn CCD area-detector diffractometer equipped with a graphite-monochromatic Mo-Kα (λ = 0.71073 ?)radiation by using an ω scan mode. The structure was solved by direct methods using SHELXS-97 program[36]and refined by full-matrix least-squares on F2with SHELXL-97 program[37]package. All non-hydrogen atoms were refined anisotropically,and hydrogen atoms were added according to theoretical models. For compound 3a, a total of 6953 reflections were collected in the range of 2.46≤θ≤27.83o (–10≤h≤9, –9≤k≤12, –14≤l≤14),and 3238 were independent with Rint= 0.0351, of which 2644 were observed with I > 2σ(I) and used in the succeeding refinements. The final R = 0.0229 and wR = 0.0547 (w = 1/[σ2(Fo2) + (0.0175P)2+0.0000P], where P = (Fo2+ 2Fc2)/3), S = 1.014,(Δ/σ)max= 0.001, (Δρ)max= 0.730 and (Δρ)min=–0.720 e/?3. For compound 3b, a total of 10618 reflections were collected in the range of 2.07≤θ≤30.03o (–13≤h≤13, –13≤k≤14, –15≤l≤14) by using an ω scan mode with 5339 independent ones(Rint= 0.0354), of which 3971 were observed with I > 2σ(I) and used in the succeeding refinements.The final refinement give R = 0.0428, wR = 0.1072(w = 1/[σ2(Fo2) + (0.0420P)2+ 2.8398P], where P =(Fo2+ 2Fc2)/3), S = 1.115, (Δ/σ)max= 0.004, (Δρ)max= 1.968 and (Δρ)min= –1.604 e/?3. For compound 3c, 55241 total reflections were collected in the range of 1.54≤θ≤27.88o (–34≤h≤34, –11≤k≤11, –33≤l≤33) by using an ω scan mode with 14208 independent ones (Rint= 0.0458), of which 13056 were observed with I > 2σ(I) and used in the succeeding refinements. The final refinement give R= 0.0424, wR = 0.0766 (w = 1/[σ2(Fo2) + (0.0222P)2+ 8.7030P], where P = (Fo2+ 2Fc2)/3), S = 1.154,(Δ/σ)max= 0.003, (Δρ)max= 2.429 and (Δρ)min=–2.289 e/?3.

3 RESULTS AND DISCUSSION

3. 1 Synthesis

The strategy for the synthesis is shown in Scheme 1.The reaction of Cp2Zr(II) species with silylalkynes proceeds with excellent regioselectivity to afford the corresponding 2,5-disilylsubstituted zirconacyclopenta dienes as single products in high yields.Cycloaddition reaction of the 2,5-disilylzirconacyclopentadiene with an internal alkyne can afford para-disilylbenzene. Our group had reported the synthesis of a series of 1,2,4,5-tetrasubstituted benzenes by the removal of introduced silyl groups[22]. Herein, the 3,6-diiodo-4,5-dialkyl-phthalic acid dimethyl esters were obtained by further iodination. Iodine, N-iodosuccinimide and iodine monochloride were demonstrated in this iodination process. When N-iodosuccinimide was used, no reaction occurred. Specially, the reaction proceeds as Scheme 2 when iodine was treated as iodination reagent. A desilylation product was got in 25%NMR yield. It suggests that iodine electric polarization is not strong enough for this iodination reaction. Consequently, when iodine monochloride was employed, the reaction proceeds almost completely with more than 80% isolated yield.

Scheme 2. Iodination of para-disilylbenzene by iodine

3. 2 X-ray crystallographic analysis

The crystal structures of the three compounds were determined and studied. The crystals of the new three compounds are colorless and stable in air at room temperature. Fig. 2 depicts the molecular structures of the title three compounds.

The 3,6-diiodo-4,5-dimethyl-phthalic acid dimethyl ester (3a) crystallizes in the triclinic P1 space group. The 3,6-diiodo-4,5-dipropyl-phthalic acid dimethyl ester (3b) crystallizes in the triclinic Pspace group. And the 3,6-diiodo-4,5-dibutyl-phthalic acid dimethyl ester (3c) crystallizes in the monoclinic P21/c space group.

Selected bond lengths and bond angles for the compound are given in Tables 1～3 and Fig. 2.

Fig 1. 1H NMR and 13C NMR of the three title compounds

Fig. 2. Crystal structures of the title three compounds

For the three title compounds, the bond lengths of C–I were 2.100(3) and 2.108(3) ? for 3a, 2.107(5)and 2.112(5) ? for 3b, and 2.106(4) and 2.110(4) ? for 3c, which are slightly longer than those in 2,5-dibenzoyl-1,4-diiodobenzene (2.091(9) and 2.099(9) ?)[38]. The lengths of C–I and C–O bonds of one of the three compounds agree with the corresponding values in the other two molecules. It seems that the bond lengths are influenced by the steric hindrance of substituents on the benzene ring.

In the benzene ring of compound 3a, the internal angles at the two carbon atoms differ by 2.7°(C(1)–C(2)–C(3) = 118.9(3)° and C(6)–C(1)–C(2) =121.6(3)°), 0.6° (C(1)–C(2)–C(3) = 118.9(3)° and C(1)–C(6)–C(5) = 119.5(3)°) and 2.1° (C(6)–C(1)–C(2) = 121.6(3)° and C(1)–C(6)–C(5) =119.5(3)°). In the benzene ring of compound 3b, the internal angles at the two carbon atoms differ by 2.3° (C(7)–C(6)–C(5) = 119.1(4)° and C(8)–C(7)–C(6) = 121.4(4)°), 0.7° (C(3)–C(8)–C(7) = 120.7(4)°and C(8)–C(7)–C(6) = 121.4(4)°) and 1.6°(C(7)–C(6)–C(5) = 119.1(4)° and C(3)–C(8)–C(7) =120.7(4)°). In the benzene ring of compound 3c, the internal angles at the two carbon atoms differ by 4.2° (C(1)–C(6)–C(5) = 122.0(4)° and C(6)–C(5)–C(4) = 117.8(4)°), 2.0° (C(6)–C(1)–C(2) = 119.8(3)°and C(6)–C(5)–C(4) = 117.8(4)°) and 2.2°(C(1)–C(6)–C(5) = 122.0(4)° and C(6)–C(1)–C(2) =119.8(4)°), which are expected for their different types.

Intermolecular hydrogen bonding is important supramolecular force to link the layers into a 3D supramolecular structure. Non-classical hydrogenbond parameters and symmetry codes for different interactions are given in Table 4 and Fig. 3. In compound 3a, non-classical hydrogen bonding interactions linked by atom C(8) in the molecule at(x, y, z) serve as a hydrogen-bond donor via H(8B),to atom O(2) in the molecule at (–x, –y, 2–z ).Similarly, atom C(11) at (x, y, z) acts as a hydrogen donor via H(11C), to atom O(1) at (1–x, –y, 1–z ).In compound 3b, atom C(10) at (x, y, z) serves as a hydrogen-bond donor via H(10B), to atom O(2) in the molecule at (–x, 1–y, 1–z ). Atom C(13) at (x, y,z) serves as a hydrogen-bond donor via H(13B), to atom O(2) in the molecule at (1–x, 2–y, 2–z ). In compound 3c, atom C(53) at (x, y, z) acts as a hydrogen-bond donor via H(53C), to atom O(1) in the molecule at (x, 1/2–y, –1/2+z).

Fig. 3. Packing diagram of the title compounds, showing the hydrogen bonds (dashed lines)

In addition, in the crystal structure, the neighboring layers are linked by non-classical intermolecular C–H…O hydrogen bonds of ester oxygen atoms with C…O distances of 3.512(4) and 3.320(4)? for compound 3a, 3.224(10) and 3.497(9) ? for compound 3b, and 3.328(6) ? for compound 3c,respectively (Table 4). However, their contribution to the overall lattice energy must be very small. Thus the intermolecular hydrogen bonding plays an important role in stabilizing the structure.

4 CONCLUSION

In conclusion, we provide a method for the preparation of para-diiodobenzene derivatives via cycloaddition of two TMS-substituted alkynes and dimethyl acetylenedicarboxylate with zirconocene and sequentially diiodination in high regionselectivity and yields. Crystal structures were determined and analyzed by single-crystal X-ray diffraction. In addition, these novel compounds can be derived to give a series of compounds which are used as molecular wires, foldamers, sensors and luminescent materials.

Table 1. Selected Bond Lengths (?) and Bond Angles (°) for Compound (3a)

Table 2. Selected Bond Lengths (?) and Bond Angles (°) for Compound (3b)

Angle (°) Angle (°)C(2)–O(1)–C(1) 115.1(4) C(7)–C(6)–C(11) 119.5(4)C(3)–C(4)–I(1) 117.1(3) C(7)–C(8)–C(9) 121.7(4)C(3)–C(8)–C(7) 120.7(4) C(8)–C(3)–C(2) 117.9(4)C(3)–C(8)–C(9) 117.5(4) C(8)–C(3)–C(4) 118.1(4)C(4)–C(3)–C(2) 123.9(4) C(8)–C(7)–C(6) 121.4(4)C(4)–C(5)–C(6) 118.2(4) C(8)–C(7)–I(2) 117.5(3)C(4)–C(5)–C(14) 120.9(4) C(9)–O(4)–C(10) 115.1(5)C(5)–C(4)–C(3) 122.4(4) C(13)–C(12)–C(11) 111.1(5)C(5)–C(4)–I(1) 120.2(3) C(16)–C(15)–C(14) 111.4(5)C(5)–C(6)–C(11) 121.4(4) O(1)–C(2)–C(3) 111.4(4)C(5)–C(14)–C(15) 114.8(4) O(2)–C(2)–O(1) 124.4(5)C(6)–C(5)–C(14) 120.8(4) O(2)–C(2)–C(3) 124.2(5)C(6)–C(7)–I(2) 121.0(3) O(3)–C(9)–O(4) 124.7(5)C(6)–C(11)–C(12) 114.4(4) O(3)–C(9)–C(8) 124.2(5)C(7)–C(6)–C(5) 119.1(4) O(4)–C(9)–C(8) 111.0(4)

Table 3. Selected Bond Lengths (?) and Bond Angles (°) for Compound (3c)

Table 4. Non-classical Hydrogen Bond Geometries (?, °) for Compounds 3a, 3b and 3c

Symmetry codes: (i) –x, –y, 2–z; (ii) 1–x, –y, 1–z; (iii) –x, 1–y, 1–z; (iV) 1–x, 2–y, 2–z; (V) x, 1/2–y, –1/2+z

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