Synthesis and Characterization of Stereoisomers of 1,4-Dihydro-1,4

Two different polymorphs of trans-DHDAtBu were obtained as a pale yellow platelet crystal and a colorless crystalline fiber. The colorless crystal has...
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Organometallics 2009, 28, 6109–6113 DOI: 10.1021/om900727c

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Synthesis and Characterization of Stereoisomers of 1,4-Dihydro-1,4-diarsinines Manabu Arita,† Kensuke Naka,*,‡ Yasuhiro Morisaki,† Akiko Nakahashi,† and Yoshiki Chujo*,† †

Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510 Japan, and ‡Department of Chemistry and Materials Technology, Graduate School of Science and Technology, Kyoto Institute of Technology, Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Received August 18, 2009

Novel 1,4-diarsa-2,5-cyclohexadiene, i.e., 1,4-dihydro-1,4-dimethyl-2,3,5,6-tetrakis(tert-butoxycarbonyl)-1,4-diarsinine, was synthesized by radical reaction of pentamethylcyclopentaarsine (cyclo(MeAs)5) and di-tert-butyl acetylenedicarboxylate. Two steroisomers, cis-DHDAtBu and transDHDAtBu, were obtained as crystals, of which structures were determined by X-ray crystallography. The six-membered ring in cis-DHDAtBu has a boat form, and both the methyl groups on the arsenic atoms are in equatorial positions. Two different polymorphs of trans-DHDAtBu were obtained as a pale yellow platelet crystal and a colorless crystalline fiber. The colorless crystal has a boat-shaped sixmembered ring, and the yellow crystal has a nearly flat chair-shaped ring. The double bonds connected with the two arsenic atoms were longer than that of the boat-shaped DHDAtBu. The UV-vis absorption spectra of cis-DHDAtBu and trans-DHDAtBu in CH2Cl2 showed long-wavelength absorption maxima at 272 and 300 nm, respectively. Introduction Since 1,4-diaza-, diphospha-, and diarsa-2,5-cyclohexadienes have two heteroatoms connected with two rigid bridges, they are considered to be more conformationally restricted than general bidentate ligands. Reported 1,4-diaza-1,2 and diphospha-2,5-cyclohexadienes3 are, however, readily oxidized in air or in the presence of metal ions. Similar cyclic organoarsenic compounds, such as 5,l0-dihydroarsanthrene,4 were synthesized, but details about the function were not reported due to their low solubility. We have recently reported the facile synthesis of a novel 1,4diarsa-2,5-cyclohexadiene, i.e., cis-1,4-dihydro-1,4-dimethyl2,3,5,6-tetrakis(methoxycarbonyl)-1,4-diarsinine (cis-DHDAMe), by radical reaction of pentamethylcyclopentaarsine (cyclo-(MeAs)5) and dimethyl acetylenedicarboxylate.5 Their stability toward air and moisture was great enough to allow its handling in air. The 1H NMR spectrum of crude DHDAMe before recrystallization indicated the coexistence of a stereoisomer as a miner product, which might be trans-DHDAMe. Isolation of this isomer, however, failed. The structure of *Corresponding authors. E-mail: [email protected]. (1) Akutsu, H.; Ogasawara, M.; Saburi, M.; Kozawa, K.; Uchida, T. Bull. Chem. Soc. Jpn. 1996, 69, 1223. (2) (a) Bieger, K.; Heckmann, G.; Fluck, E.; Weller, F.; Peters, K.; Peters, E.-M. Z. Anorg. Allg. Chem. 1995, 621, 1981. (b) Davis, M.; Mann, F. G. J. Chem. Soc. 1964, 3770. (c) Ivonin, S. P.; Tolmachev, A. A.; Chernega, A. N.; Pinchuk, A. M. Heteroat. Chem. 2002, 13, 46. (3) Rodrigues, A.; Ferreira, P. M. T.; Monteiro, L. S. Tetrahedron 2004, 60, 8489. (4) Kennard, O.; Mann, F. G.; Watson, D. G.; Fawcett, J. K.; Kerr, K. A. Chem. Commun. 1968, 269. (5) Nakahashi, A.; Naka, K.; Chujo, Y. Organometallics 2007, 26, 1827. r 2009 American Chemical Society

cis-DHDAMe was suitable for a ligand of a dinuclear complex because the direction of the two coordination sites is parallel. Recently, we have reported the stoichiometric synthesis of mono- and dinuclear trans-dichloropalladium(II) complexes.6 The obtained dinuclear complex was insoluble in toluene and n-hexane and partially soluble in CH2Cl2. In this paper, we applied di-tert-butyl acetylenedicarboxylate for the synthesis of 1,4-dihydro-1,4-diarsinine to improve the solubility. We obtained the trans-isomer in addition to cis-1,4-dihydro-1,4-diarsinines (Scheme 1), and their crystal structures are discussed.

Results and Discussion The radical reaction of cyclo-(MeAs)5 and di-tert-butyl acetylenedicarboxylate gave cis-DHDAtBu and transDHDAtBu as a mixture. On the basis of the 1H NMR analysis of the mixture after purification by silica gel chromatography, the yields of cis-DHDAtBu and trans-DHDAtBu were estimated to be 50% and 10%, respectively. Pure cisDHDAtBu was easily obtained as a colorless crystal by recrystallization from the mixture. The yield of isolated cisDHDAtBu was 35%. The structure of cis-DHDAtBu was estimated by 1H and 13C NMR, FAB-MASS spectrometry, and elemental analysis. The stereochemical structure of cisDHDAtBu was determined by X-ray crystallography. The six-membered ring has a boat shape, and both the methyl groups on the arsenic atoms are in an equatorial position, as shown in Figure 1. (6) Naka, K.; Nakahashi, A.; Arita, M.; Chujo, Y. Organometallics 2008, 27, 1034. Published on Web 09/09/2009

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Figure 1. ORTEP diagram of cis-DHDAtBu with thermal ellipsoids shown at the 50% probability level.

The pale yellow filtrate after the recrystallization for cisDHDAtBu was concentrated to obtain a pale yellow residue. The residue was dissolved in CH2Cl2, and methanol was added to give a pale yellow platelet crystal. The structure of the pale yellow platelet crystal was established by 1H and 13C NMR, MASS spectrometry, and elemental analysis. The X-ray crystallography suggests that the six-membered ring was a chair form and was nearly flat, as shown in Figure 2. When methanol as a poor solvent was added to the residue, a different polymorph of trans-DHDAtBu was obtained as a colorless crystalline fiber. A bundle of the ordered fiber of trans-DHDAtBu is a tetragonal prism crystal. X-ray crystallography shows that the six-membered ring was a boat form similar to that of cis-DHDAtBu (Figure 3). One methyl group on the arsenic atom is in an equatorial position, and the other is in a flagpole position. A selection of bond lengths and angles for the three crystals is summarized in Table 1. Six-membered rings containing two double bonds usually adopt boat conformations, since their chair structures usually have large stresses.1 Although the usual bond angle at the sp2-hybridized carbon is 120°, the As-CdC bond angle of the chair-shaped transDHDAtBu extended to 129°. A similar phosphorus compound, trans-dihydrophosphanthrene,1 adopts an asymmetrical boat conformation because the P-CdC angles extend to only 122°. The lone pair on a trivalent arsenic has s character, and the bond angle at the trivalent arsenic is Table 1. Selected Distances (A˚) and Angles (deg) of the Frame of DHDAs cis-DHDAtBu

Figure 2. ORTEP diagram of chair-shaped trans-DHDAtBu with thermal ellipsoids shown at the 50% probability level.

As(2)-As(1) C(2)-C(7) As(1)-C(2) As(1)-C(10) C(7)-As(2)-C(12) C(2)-C(7)-As(2) C(7)-C(2)-As(1) C(2)-C(7)-C(4) C(12)-C(10)-C(17)

3.3705(5) 1.330(4) 1.973(3) 1.967(3) 95.47(11) 121.0(2) 121.3(2) 119.4(2) 119.4(3)

C(10)-C(12) As(2)-C(7) As(2)-C(12) C(10)-As(1)-C(2) C(10)-C(12)-As(2) C(12)-C(10)-As(1) C(7)-C(2)-C(8) C(10)-C(12)-C(9)

1.329(4) 1.962(3) 1.980(3) 94.57(11) 121.2(2) 121.1(2) 123.1(3) 123.7(2)

trans-DHDAtBu (boat) As(1)-As(2) C(12)-C(13) As(1)-C(13) C(13)-As(1)-C(13) C(13)-C(12)-As(2) C(12)-C(13)-C(17)

3.542(1) 1.324(3) 1.970(2) 95.88(14) 124.34(19) 123.5(2)

As(2)-C(12) C(12)-As(2)-C(12) C(12)-C(13)-As(1) C(13)-C(12)-C(7)

1.958(2) 96.21(14) 124.42(19) 124.1(2)

trans-DHDAtBu (chair)

Figure 3. ORTEP diagram of boat-shaped trans-DHDAtBu with thermal ellipsoids shown at the 50% probability level.

As(1)-As(1) C(10)-C(18) As(1)-C(18) C(18)-As(1)-C(10) C(18)-C(10)-As(1) C(10)-C(18)-C(16)

Scheme 1

3.7956(5) 1.339(3) 1.947(3) 101.05(10) 128.9(2) 119.2(2)

As(1)-C(10)

1.956(2)

C(10)-C(18)-As(1) C(18)-C(10)-C(13)

127.36(19) 122.3(2)

Article

Figure 4.

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C NMR spectra of cis-DHDAtBu (a) and trans-DHDAtBu (b) in CDCl3.

Figure 5. 1H NMR spectra of cis-DHDAtBu (a) and trans-DHDAtBu (b) in CDCl3.

close to 90° by its p character. The interior angles around the arsenic atoms of the six-membered ring in the chair-shaped trans-DHDAtBu were extended to 101° from 95° of that in cis-DHDAtBu. The bond angles can extend while the electronic state of a trivalent arsenic becomes close to the sp3 hybrid. The flexibility of the bond angles around the arsenic center is an inherent property in the present organoarsenic compound. In the chair-shaped trans-DHDAtBu, the As-C bonds were shorter (1.952 A˚) than those of cis-DHDAtBu, which

indicates an increase in the π-bonding character of the As-C bonds in the six-memberd ring. In the 13C NMR analysis, the peak assigned to the ethylene carbon for the chair-shaped trans-DHDAtBu shifted downfield compared with that for cis-DHDAtBu (Figure 4), which indicates a lower electron density of the CdC bonds in the chairshaped trans-DHDAtBu than that in cis-DHDAtBu. These observations suggest that the electron polarization proceeded from the six-membered ring to the arsenic atoms in trans-DHDAtBu.

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Arita et al. Table 2. Crystallographic Data and Refinement Parameters cis-DHDAtBu

Figure 6. UV-vis absorption spectra of cis-DHDAtBu (A) and trans-DHDAtBu (B) in CH2Cl2.

The 1H NMR spectra of Figure 5 show that the peak assigned to the methyl groups on the arsenic atom for the chair-shaped trans-DHDAtBu shifted upfield compared with that for cis-DHDAtBu, which may be due to an increase in the electron density of the methyl groups on the arsenic atom. The difference between the boat-shaped transDHDAtBu and cis-DHDAtBu is that the boat-shaped trans-DHDAtBu has the flagpole methyl group. The steric hindrance causes the longer As-As distance (3.542 A˚) than that of cis-DHDAtBu (3.370 A˚). The UV-vis absorption spectra of cis-DHDAtBu and trans-DHDAtBu in CH2Cl2 showed long-wavelength absorption maxima at 272 nm (11 000 M-1 cm-1) and 300 nm (5200 M-1 cm-1), respectively, as shown in Figure 6. Because no pair of peaks with the same area was obtained in the 1H and 13C NMR spectra of trans-DHDAtBu, flipping of the ring structure might occur in the solution. The conjugation system of the flat six-membered ring of the chair-shaped trans-DHDAtBu affects the absorption in the visible range. This suggests that trans-DHDAtBu probably exists mainly as the chair form in the solution. There are at least two reasons why the chair form is expected to be more stable. One is because of the symmetrical structure. Another reason must be because of the conjugation stabilization.

Conclusions We successfully obtained cis and trans forms of 1,4-dihydro1,4-dimethyl-2,3,5,6-tetrakis(tert-butoxycarbonyl)-1,4-diarsinine (DHDAtBu) by the radical reactions of cyclo(MeAs)5 and di-tert-butyl acetylenedicarboxylate. The ring in cis-DHDAtBu has a boat form, and both the methyl groups on the arsenic atoms are in an equatorial position. Two different polymorphs of trans-DHDAtBu were obtained as a pale yellow platelet crystal and a colorless crystalline fiber. The colorless crystal has a boat-shaped six-membered ring, and the yellow crystal has a nearly flat chair-shaped ring. The flexibility of the bond lengths and angles around the arsenic center is an inherent property in the present organoarsenic compound.

Experimental Section Materials. Unless otherwise noted, all reagents and chemicals were purchased from commercial sources and used without further purification. 2,20 -Azobis(isobutyronitrile) (AIBN) was (7) Elmes, P. S.; Middleton, S.; West, B. O. Aust. J. Chem. 1970, 23, 1559.

empirical C26H42As2O8 formula fw 632.44 cryst syst orthorhombic space group Pna21 a (A˚) 19.743(3) b (A˚) 14.6563(17) c (A˚) 10.6223(9) R (deg) 90 β (deg) 90 γ (deg) 90 V (A˚3) 3073.7(6) Z 4 Dcalcd (g cm-3) 1.367 μ (mm-1) 2.216 T (K) 101 reflns collected 28 115 indep reflns 6911 (Rint) (0.0672) obsd reflns 6318 [I > 2σ(I)] params 339 Tmax/Tmin 0.5562/0.2702 residual 0.818/-0.314 density (e A˚-3) R1 wR2 GooF

0.0322 0.0725 1.023

trans-DHDAtBu trans-DHDAtBu (chair) (boat) C26H42As2O8

C26H42As2O8

632.44 orthorhombic Pbca 11.0446(14) 15.6598(13) 17.0855(17) 90 90 90 2955.0(5) 4 1.422 2.305 115 27 098 3367 (0.0899) 2526

632.44 orthorhombic Pnma 20.309(7) 24.889(7) 6.0164(15) 90 90 90 3041.1(16) 4 1.381 2.239 98 24 685 3564 (0.1092) 2486

170 0.6557/0.4592 1.086/-0.528

177 0.6630/0.4006 0.621/-0.747

0.0394 0.0776 1.021

0.0374 0.0618 0.947

recrystallized from methanol. Pentamethylcyclopentaarsine was prepared as described in the literature.7 Equipment. 1H (400 MHz) and 13C (100 MHz) NMR spectra were recorded on a JEOL EX400 spectrometer, and samples were analyzed in CDCl3 using tetramethylsilane as an internal standard. UV-vis spectra were obtained on a Shimadzu UV3600 spectrophotometer, and samples were analyzed in CH2Cl2 at room temperature. Elemental analyses were performed at the Microanalytical Center of Kyoto University. High-resolution mass spectra (HR-MS) were obtained on a JEOL JMS-SX102A spectrometer. All procedures were performed under a nitrogen or argon atmosphere. Synthesis of cis-DHDAtBu. A benzene solution (20 mL) of AIBN (60 mg) was added to a refluxing benzene solution of ditert-butyl acetylenedicarboxylate (5.0 g, 22 mmol) and cyclo-(MeAs)5 (2.0 g 4.4 mmol). Afer the mixture was refluxed for 16 h, the solvent was removed to yield a pale yellow solid. The pale yellow solid was purified by column chromatography (silica gel saturated with CH2Cl2) and was subsequently recrystallized from ethanol to give cis-DHDAtBu as a colorless prism crystal. Yield: 35%. Rf = 0.12 (CH2Cl2). 1H NMR (CDCl3) δ: 1.52 (s, 6H); 1.50 (s, 1H). 13C NMR (CDCl3) δ: 165.35; 144.32; 82.82; 28.18; 7.55. HR-FAB-MS (m/z): calcd for C26H42As2O8, 632.1312; found, 632.1328. Anal. Calcd for C26H42As2O8: C, 49.38; H, 6.69. Found: C, 49.24; H, 6.47. Synthesis of trans-DHDAtBu. The solvent of the pale yellow filtrate of recrystallization of cis-DHDAtBu was removed, and the residue was recrystallized from CH2Cl2 and methanol to give a pale yellow platelet crystal or from methanol to give a colorless microcrystalline fiber. Rf = 0.15 (CH2Cl2). 1H NMR (CDCl3) δ: 1.52 (s, 6H); 1.34 (s, 1H). 13C NMR (CDCl3) δ: 166.18; 148.55; 82.79; 28.08; 7.12. HR-EI-MS (m/z): calcd for C26H42As2O8, 632.1312; found, 632.1311. Anal. Calcd for C26H42As2O8: C, 49.38; H, 6.69. Found: C, 49.36; H, 6.48. X-ray Crystal Structure Analysis. X-ray diffractions were collected on a Rigaku R-AXIS RAPID-F graphite-monochromated Mo KR (λ= 0.71075 A˚) radiation diffractometer with imaging plate. A symmetry-related absorption correction was carried out by

Article using the program ABSCOR.8 The structures were solved via Patterson methods (Dirdif 99.2)9 and refined (SHELXL-97)10 using Yadokari-XG.11 The program ORTEP-III12 was used to generate the X-ray structural diagrams. Crystal data and more information on X-ray data collection are summarized in Table 2.

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from the Ministry of Education, Culture, Sports, Science, and Technology, Government of Japan.

Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research (No. 20350054)

Supporting Information Available: Complete sets of crystallographic data in CIF format for the three reported X-ray crystal structures. This material is available free of charge via the Internet at http://pubs.acs.org.

(8) Higashi, T. ABSCOR. Program for Absorption Correction; Rigaku Corporation: Japan, 1995. (9) Beurskens, P. T.; Beurskens, G.; Bosman, W. P.; de Gelder, R.; Garcia-Granda, S.; Gould, R. O.; Israel, R.; Smits, J. M. M. DIRDIF99.2 program system; Crystallography Laboratory, University of Nijmegen: The Netherlands, 1999.

(10) Sheldrick, G. M. SHELXL-97; University of G€ottingen: G€ottingen, Germany, 1997. (11) Wakita, K. Yadokari-XG. Program for Crystal Structure Analysis; 2000. (12) Burnett, M. N.; Johnson, C. K. ORTEPIII; Report ORNL-6895; Oak Ridge National Laboratory: Oak Ridge, TN, 1996.