HETEROCYCLES

Paper | Regular issue | Vol. 78, No. 3, 2009, pp. 657-668
Received, 12th September, 2008, Accepted, 22nd October, 2008, Published online, 23rd October, 2008.
DOI: 10.3987/COM-08-11547

■ Novel Reactions of Steric Encumbered 1,4-Dilithio-1,3-butadiene with Group 14 Electrophiles: Formation and Structure of Stable Dihydroxygermole

Masaichi Saito,* Michio Nakamura, and Tomoyuki Tajima

Saitama University, Shimo-okubo, Sakura-ku, Saitama 338-8570 , Japan

Abstract

Reactions of 1,4-dilithio-1,3-butadiene 1 having bulky silyl ligands at the 1,4-positions with group 14 electrophiles were examined. Reactions of 1,4-dilithio-1,3-butadiene 1 with tetraethoxygermane gave diethoxygermole 5, which was hydrolyzed to give stable dihydroxygermole 16. The X-ray diffraction analysis of dihydroxygermole 16 revealed intermolecular hydrogen bonds. The reaction of 1 with stannous chloride was also examined.

INTRODUCTION
1,4-Dilithio-1,3-butadiene, which can be prepared from phenylacetylene with lithium1 and 1,4-diiodo-1,3-butadiene with alkyllithium reagents,2 is often employed as the starting compound for the synthesis of metal-containing carbocyclic π-systems, for example, group 14 metalloles.3 In the course of our studies on the synthesis of reactive species with a 1,3-butadiene skeleton,4 we became interested in the preparation of 1,4-dilithio-1,3-butadienes having bulky ligands at the 1,4-positions. Very recently, we reported on the novel 1,4-dilithio-1,3-butadiene 1 that has bulky silyl substituents in the 1,4-positions (Scheme 1).5 On the other hand, the germinal 1,1-diols of the carbon versions are so unstable that the equilibrium is completely shifted from the 1,1-diols to the corresponding ketones and water. In contrast, 1,1-diols of silicon analogs are stable enough to be isolated and many examples have already appeared.6 With regard to the germanium analogs, although some recent examples of stable 1,1-dihydroxygermane have been already reported,7 the chemistry of 1,1-dihydroxygermane remains largely unexplored. In efforts to functionalize 1,4-dilithio-1,3-butadienes, we examined 1,4-dilithio-1,3-butadiene 1 with some group 14 electrophiles. Here we report on the formation and structure of a stable dihydroxygermole, a novel type of 1,1-dihydroxygermanes.

RESULTS AND DISCUSSION
(a) Reaction of 1,4-Dilithio-1,3-butadiene 1 with Chlorosilanes. Hexachlorodisilane was considered be a useful reagent for the preparation of 1,4-dimetalla-cyclohexa-1,3-dienes, which could be potential precursors of six-membered reactive species having two heavier group 14 atoms. 1,4-Dilithio-1,3-butadiene 1, prepared in situ by the reduction of phenylsilylacetylene 3 (Scheme 1),8 was treated with hexachlorodisilane in toluene at low temperature. However, the resulting mixture was complex; the 1H NMR spectrum showed a broad signal at 1.0-1.6 ppm, and only 1,3-butadiene 4 (vide infra) was identified. Although the reaction of 1 with tetrachlorosilane was also examined, the reaction was so complex that no identifiable products were obtained. We concluded that chlorosilanes were very reactive toward 1 and/or the expected products of dichlorosilane derivatives were unstable under the reaction conditions used.

(b) Reaction of 1,4-Dilithio-1,3-butadiene 1 with Tetraethoxygermane. Since the reactions of 1 with chlorosilanes were complex, we next examined the reaction of 1 with commercially available tetraethoxygermane. Tetraethoxygermane is thought to be less reactive than tetrachlorogermane, because ethoxide is a poor leaving group compared with chloride. After treatment of 1 with excess tetraethoxygermane in ether at rt, diethoxygermole 5 was obtained in 10% yield, together with a mixture containing 1,3-butadiene 4, cis-dihydrodibenzopentalene 6a and dibenzopentalene 7 (Scheme 2). The yield of 5 was improved to 26%, by heating the reaction mixture under reflux after the treatment of 1 with tetraethoxygermane in ether at rt (Scheme 2). In these reactions, no compounds derived from a byproduct, ethynyllithium 8 (vide infra), were obtained. The structure of 5 was confirmed by NMR spectroscopy and elemental analysis. Compounds, 4 and 6, were alternatively prepared by treatment of the reaction mixture formed after the reduction of phenylsilylacetylene 3 with H2O (Scheme 3).9 The stereochemistry of 4 was determined to be the Z,Z-form, by comparing its 1H NMR spectrum with that of analogous to that of the corresponding 1,4-diiodo derivative,5 while that of 6 was finally confirmed by X-ray diffraction analysis. Although the 1H NMR spectrum of the crude product showed the formation of triisopropylsilylacetylene (9), derived from the corresponding ethynyllithium 8, it could not be isolated because of its relatively low boiling point11 and the yield of 9 could not be determined. The formation of 5, upon using an excess of tetraethoxygermane suggests that ring closure of the intermediate formed from 1 and an equivalent of tetraethoxygermane should be faster than further addition of tetraethoxygermane to the intermediate.

(c) Reaction of 1,4-Dilithio-1,3-butadiene 1 with Stannous Chloride. To prepare the 1,4-trichlorostannyl derivative 10, 1,4-dilithio-1,3-butadiene 1 was treated with stannous chloride at low temperature and the reaction mixture then treated with carbon tetrachloride at −50 ºC. The reason for using the carbon tetrachloride was because the expected chlorostannylene intermediate 11 would react with carbon tetrachloride to give 10.12 However, 1-chloro-1-silylethynylstannole 12 was obtained in 10% yield (Scheme 4). In order to elucidate the mechanism for the formation of 12, the reaction was monitored using 119Sn NMR spectroscopy. After treatment of 1, generated from 1 in a mixture with 2 and 8, with stannous chloride, the 119Sn NMR spectrum of the resulting mixture in C6D6 revealed two signals at −32 ppm and −110 ppm. The former signal was assigned to 12, suggesting that compound 12 was already formed from the reaction of 1 and stannous chloride. The latter signal was located in a region similar to those of the 1-aryl-substituted tetraphenylstannole anions,13 and was therefore assigned to the stannole anion 13. A possible mechanism for the formation of 12 is proposed in Scheme 5. Reaction of 1 with stannous chloride afforded the stannylidene intermediate 14, which further reacted with ethynyllithium 8 to afford the stannole anion 13. Since the final product 13 was already formed after treatment of 1, generated from 1 in a mixture with 2 and 8, with stannous chloride, as evidenced by 119Sn NMR analysis, it was proposed that the stannole anion 13 reacted with the remaining stannous chloride to afford 12. However, since the stannole anion 13 is also observed in the reaction mixture, the reaction pathway in which the stannole anion 12 reacts with carbon tetrachloride to give 12 would be also involved. Alternatively, ethynyllithium 8 would react with stannous chloride to afford chlorostannylene intermediate 15, which further reacted with 1 to give the stannole anion 13 via another stannylene intermediate 16 (Scheme 6).

(d) Formation of Dihydroxygermole 17 from Diethoxygermole 5. Heating diethoxygermole 5 and sodium hydroxide in THF-H2O under reflux gave dihydroxygermole 17, stable under aerobic conditions, in 79% yield (Scheme 6), the structure of which was established by NMR and IR spectroscopy, and finally confirmed by X-ray diffraction analysis. In the 1H NMR spectrum, a signal assignable to hydroxyl protons was observed at 1.67 ppm. In IR spectra measured both in solution (CCl4) and in the solid state (KBr), two broadened absorptions were observed at about 3400 and 3650 cm-1. The former absorption was assigned to the stretching vibration of hydroxy bonds involved in intermolecular hydrogen bonds,14 while the latter absorption was assigned to the stretching vibration of hydroxy bonds without intermolecular hydrogen bonds.7c,15 While 1,1-dihyrdoxy-2,3,4,5-tetraphenylgermole was so unstable that it underwent trimerization via dehydration to afford the corresponding cyclotrigermoxane,16 the bulky triisopropylsilyl groups prevent the dihydroxygermole 17 from oligomerization.

(e) X-Ray Diffraction Analysis of Dihydroxygermole 17. The molecular structure of 17 is shown in Figure 1. The geometry of the germole ring, with alternation of the C−C bonds, is quite similar to those reported previously.16,17 The Ge−O bond length (1.774(2) Å) in 17 is quite similar to those of tBu2Ge(OH)2 (av. 1.778 Å),14 but slightly shorter than those of (2,6-MesC6H3)2Ge(OH)2 (av. 1.790 Å).7a In the unit cell, two molecules face each other with an O−O distance of about 2.96 Å (Figure 2), suggesting hydrogen-bonding interactions. This is slightly longer than those found in tBu2Ge(OH)2 (2.775-2.849 Å).14

SUMMARY
Reactions of 1,4-dilithio-1,3-butadiene 1 having bulky silyl ligands at the 1,4-positions with tetrachlorosilane and hexachlorodisilane gave complex mixtures. However, 1,4-dilithio-1,3-butadiene 1 was treated with tetraethoxygermane to give diethoxygermole 5, which was hydrolyzed to give stable dihydroxygermole 17. The molecular structure of 17 was confirmed by X-ray diffraction analysis, which revealed intermolecular hydrogen bonds. Reaction of 1,4-dilithio-1,3-butadiene 1 with stannous chloride followed by treatment of the reaction mixture with carbon tetrachloride gave chloroethynylstannole 12, which was derived from ethynyllithium 8. The ethynyllithium 8 was formed by reduction of 3 with lithium as a byproduct, although the generation efficiency could not be estimated.

EXPERIMENTAL
General Procedures: All experiments were performed under an argon atmosphere. Diethyl ether (Et2O), THF, benzene and benzene-d6 were distilled over sodium/benzophenone. Lithium dispersion was used for reduction of phenylsilylacetylene 3. 1H NMR (400 MHz), 13C NMR (101 MHz) and 119Sn NMR (149 MHz) spectra were recorded on a Bruker DRX-400 or a Bruker DPX-400 spectrometer in CDCl3 or benzene-d6. The multiplicities of signals in 13C NMR given in parentheses were deduced from DEPT spectra. Wet column chromatography (WCC) was carried out with Kanto silica gel 60N. Preparative gel permeation chromatography (GPC) was carried out on an LC-918 (Japan Analytical Ind. Co., Ltd.) with JAIGEL-1H and -2H columns with chloroform (CHCl3) as the eluant. Infrared spectra were measured at rt on a JASCO FT/IR-460 plus. All the melting points were determined on a Mitamura Riken Kogyo MEL-TEMP apparatus and were uncorrected. Elemental analyses were carried out at the Microanalytical Laboratory of Molecular Analysis and Life Science Center, Saitama University.

Reaction of 1,4-Dilithio-1,3-butadiene 1 with Hexachlorodisilane. A mixture of lithium (22 mg, 3.12 mmol) and phenyl(triisopropylsilyl)acetylene (3) (206 mg, 0.80 mmol) in Et2O (1.5 mL) was stirred at rt for 4 h. To the resulting mixture was added Et2O (10 mL), and the remaining lithium was filtered off through a glass filter. After removal of ether in vacuo, to the residue was added toluene (5.0 mL) and hexachlorodisilane (0.10 mL, 0.53 mmol) at −85 ºC. After the mixture was warmed to rt over 17 h, removal of volatile substances and materials insoluble in benzene followed by concentration of the filtrate afforded a crude product (188 mg). The 1H NMR spectrum of the crude product revealed a broadened signal at 1.0-1.6 ppm and only 1,3-butadiene 4 was identified.
Reaction of 1,4-Dilithio-1,3-butadiene 1 with Tetrachlorosilane. A mixture of lithium (43 mg, 6.24 mmol) and phenyl(triisopropylsilyl)acetylene (3) (548 mg, 2.12 mmol) in Et2O (2.1 mL) was stirred at rt for 4 h. The remaining lithium was filtered off through a glass filter. After removal of Et2O in vacuo, to the residue was added toluene (5.0 mL) and the resulting solution was added to a toluene (3.0 mL) solution of tetrachlorosilane (3.0 mL, 26.1 mmol) at −105 ºC. After the mixture was warmed to rt over 15 h, removal of volatile substances and materials insoluble in CH2Cl2 followed by concentration of the filtrate afforded a crude product (674 mg). The 1H NMR spectrum of the crude product was so complicated that no products could be identified.
Reaction of 1,4-Dilithio-1,3-butadiene 1 with Tetraethoxygermane. (a) at room temperature. A mixture of lithium (47 mg, 6.73 mmol) and phenyl(triisopropylsilyl)acetylene (3) (595 mg, 2.30 mmol) in Et2O (2.3 mL) was stirred at rt for 5 h. The resulting mixture was added to an Et2O (5.0 mL) solution of tetraethoxygermane (1.56 mL, 7.03 mmol) at −85 ºC. After the reaction mixture was warmed to rt over 15 h, removal of volatile substances and materials insoluble in CH2Cl2 afforded a crude product (254 mg). The crude product was subjected to WCC (eluant; hexane : ethyl acetate = 10 : 1) to give 1,1-diethoxy-3,4-diphenyl-2,5-bis(triisopropylsilyl)germole (5) (75 mg, 10%) together with a mixture containing 1,3-butadiene 4, cis-dihydrodibenzopentalene 6a and dibenzopentalene 7.5 5: 1H NMR(CDCl3): δ 0.88-0.96(m, 6H), 1.01(d, J=7 Hz, 36H), 1.26(t, J=7 Hz, 6H), 3.89(q, J=7 Hz, 4H), 6.81-6.84(m, 4H), 6.94-6.97(m, 6H); 13C NMR(CDCl3): δ 13.56(d), 18.45(q), 19.30(q), 59.09(t), 126.37(d), 126.61(d), 129.04(d), 131.21(s), 142.12(s), 167.35(s). Anal. Calcd for C38H62GeO2Si2: C, 67.15; H, 9.19. Found: C, 66.28; H, 9.40.
(b) Heating under reflux. A mixture of lithium (60 mg, 8.66 mmol) and phenyl(triisopropylsilyl)acetylene (3) (741 mg, 2.87 mmol) in Et2O (2.8 mL) was stirred at rt for 4 h. The resulting mixture was diluted by Et2O (6.0 mL) and then added to Et2O (4.0 mL) solution of tetraethoxygermane (1.92 mL, 8.66 mmol) at −85 ºC. After the reaction mixture was heated under reflux for 14 h, removal of volatile substances and materials insoluble in CH2Cl2 afforded a crude product (1.768 g). The crude product was subjected to WCC (eluant; hexane : EtOAc = 20 : 1) to give 1,1-diethoxy-3,4-diphenyl-2,5-bis(triisopropylsilyl)germole (5) (253 mg, 26%) together with a mixture containing 1,3-butadiene 4, cis-dihydrodibenzopentalene 6a and dibenzopentalene 7.5
Alternative Preparation of 1,3-Butadiene 4 and Dihydrodibenzopentalenes 6. A mixture of lithium (177 mg, 25.4 mmol) and phenyl(triisopropylsilyl)acetylene (3) (2.212 g, 8.56 mmol) in Et2O (8.6 mL) was stirred at rt for 6 h. After treatment of the reaction mixture with H2O (10 mL), the organic layer was extracted with Et2O, dried over anhydrous magnesium sulfate and concentrated to give a crude product (1.635 g). The 1H NMR spectrum of the crude product revealed the formation of triisopropylsilylacetylene.11 The crude product was purified by WCC (eluant; hexane), GPC and recrystallization to give (Z,Z)-2,3-diphenyl-1,4-bis(triisopropylsilyl)-1,3-butadiene (4) (761 mg, 34%), cis-5,10-dihydro-5,10-bis(triisopropylsilyl)dibenzo[a,e]pentalene (6a) (123 mg, 6%) and trans-5,10-dihydro-5,10-bis(triisopropylsilyl)dibenzo[a,e]pentalene (6b) (13 mg, 0.6%). 4: mp 102-103 ºC(recrystallized from hexane+EtOH). 1H NMR(CDCl3): δ 0.59-0.70(m, 6H), 0.80(d, J=7 Hz, 36H), 5.33(s, 2H), 7.21-7.30(m, 10H); 13C NMR(CDCl3): δ 11.90(d), 18.90(q), 126.95(d), 127.29(d), 129.47(d), 129.77(d), 142.99(s), 161.47(s). Anal. Calcd for C34H54Si2: C, 78.69; H, 10.49. Found: C, 78.68; H, 10.74. 6a: mp 140-141 ºC(recrystallized from hexane+EtOH). 1H NMR(CDCl3): δ 1.07(d, J=7 Hz, 36H), 1.44(sept, J=7 Hz, 6H), 3.82(s, 2H), 7.08(dd, J=8, 8 Hz, 2H), 7.24(dd, J=8, 8 Hz, 2H), 7.51(d, J=8 Hz, 2H), 7.53(d, J=8 Hz, 2H); 13C NMR(CDCl3): δ 12.29(d), 19.22(q), 19.24(q), 36.01(d), 120.00(d), 122.79(d), 124.38(d), 124.99(d), 141.62(s), 151.10(s), 153.19(s). Anal. Calcd for C34H52Si2: C, 79.00; H, 10.14. Found: C, 79.21; H, 10.34. 6b: mp 278 ºC(recrystallized from hexane+EtOH). 1H NMR(CDCl3): δ 0.88(d, J=7 Hz, 18H), 0.91(d, J=7 Hz, 18H), 1.31(sept, J=7 Hz, 6H), 4.14(s, 2H), 7.11(ddd, J=1, 8, 8 Hz, 2H), 7.24(ddd, J=1, 8, 8 Hz, 2H), 7.51(d, J=8 Hz, 2H), 7.57(d, J=8 Hz, 2H); 13C NMR(CDCl3): δ 11.81(d), 18.50(q), 18.51(q), 35.32(d), 120.25(d), 122.79(d), 124.20(d), 124.74(d), 140.81(s), 149.72(s), 150.66(s). Anal. Calcd for C34H52Si2: C, 79.00; H, 10.14. Found: C, 77.96; H, 10.11.
Reaction of 1,4-Dilithio-1,3-butadiene 1 with Stannous Chloride. A mixture of lithium (72 mg, 10.3 mmol) and phenyl(triisopropylsilyl)acetylene (3) (893 mg, 3.46 mmol) in Et2O (3.6 mL) was stirred at rt for 4 h. The resulting mixture was diluted by Et2O (7.2 mL) and added to a THF (6.0 mL) solution of stannous chloride (1.094 g, 5.77 mmol) at −85 ºC and the reaction mixture was then warmed to −50 ºC over 2 h. After treatment of the resulting mixture with CCl4 (4.0 mL) at −50 ºC, the reaction mixture was warmed to rt over 10 h. Removal of volatile substances and materials insoluble in hexane afforded a crude product (708 mg). The crude product was subjected to GPC to afford 1-chloro-3,4-diphenyl-2,5-bis(triisopropylsilyl)-1-triisopropylsilylethynylstannole (13) (137 mg, 10%). 13: 1H NMR(CDCl3): δ 0.82-0.93(m, 6H), 0.99(d, J=4 Hz, 18H), 1.01(d, J=4 Hz, 18H), 1.12(br s, 21H), 6.79-6.83(m, 4H), 6.90-6.96(m, 6H); 13C NMR(CDCl3): δ 11.22(d), 13.96(d, J(Sn-C)=55 Hz), 18.50(q), 19.48(q), 19.66(q), 112.73(s, J(Sn-C)=448, 473 Hz), 118.33(s, J(Sn-C)=73 Hz), 126.39(d), 126.50(d), 126.71(d), 129.23(d), 129.46(d), 134.48(s, J(Sn-C)=270, 282 Hz.), 142.59(s, J(Sn-C)=176, 184 Hz), 166.60(s, J(Sn-C)=115 Hz); 119Sn NMR(CDCl3): δ -31.6. Anal. Calcd for C45H73ClSi3Sn: C, 63.40; H, 8.63. Found: C, 63.40; H, 8.81.
Formation of Dihydroxygermole 16 from Diethoxygermole 5. A mixture of diethoxygermole 5 (63 mg, 0.084 mmol) and sodium hydroxide (336 mg, 8.41 mmol) in THF (5.0 mL) and H2O (5.0 mL) was heated under reflux for 9 h. After being cooled to rt, the organic layer was extracted with CH2Cl2 and dried over anhydrous magnesium sulfate. After removal of the solvent, a crude product (63 mg) was purified by WCC (eluant; hexane : EtOAc = 10 : 1) to give 1,1-dihydroxy-3,4-diphenyl-2,5-bis(triisopropylsilyl)germole (16) (42 mg, 79%). 16: mp 133.5-134.5 ºC (recrystallized from hexane). 1H NMR(CDCl3): δ 0.85-0.95(m, 6H), 1.00(d, J=7 Hz, 36H), 1.67(s, 2H), 6.81-6.83(m, 4H), 6.92-6.98(m, 6H); 13C NMR(CDCl3): δ 13.23(d), 19.36(q), 126.50(d), 126.59(d), 129.07(d), 130.95(s), 141.55(s), 167.14(s). IR (CCl4): 3645(OH), 3350-3550(br, OH) cm-1; IR (KBr): 3641(OH), 3200-3600(br, OH) cm-1. Anal. Calcd for C34H54GeO2Si2: C, 65.49; H, 8.73. Found: C, 65.01; H, 8.80.
X-Ray Crystallography of 6a and 17. Data were collected at on Bruker SMART APEX diffractometer fitted with Mo-Kα radiation (λ = 0.71073 Å). The structures were solved by direct-methods (SHELXS-97)18 and refinement (non-hydrogen atoms with anisotropic displacement parameters, H atoms in their calculated positions and with a weighting scheme of the form w = 1/[σ2(Fo2) + (aP)2 + bP] where P = (Fo2 + 2Fc2)/3) was by full-matrix least-squares procedures on F2 (SHELXL-97).19 Crystal data are listed in Table 2 and molecular structures were drawn with ORTEP-II.20 6a: C34H52Si2, FW=516.94; 103 K; crystal system, monoclinic; space group, P21/n; a=7.5220(5), b=22.3043(15), c=18.2951(12) Å; β=92.865(2) º; V=3065.6(4) Å3; Z=4; Dcalc=1.120 g cm-3; R1=0.057 (I > 2σ(I), 6174 reflections), wR2=0.140 (for all reflections) for 7291 reflections and 337 parameters; GOF=1.107. CCDC-700912 contains the supplementary crystallographic data for this compound. 17 C34H54GeO2Si2, FW=623.54; 103 K; crystal system, orthorhombic; space group, I222; a=7.9861(14), b=12.904(2), c=33.827(7) Å; V=3485.9(11) Å3; Z=4; Dcalc=1.188 g cm-3; R1=0.034 (I > 2σ(I), 2802 reflections), wR2=0.083 (for all reflections) for 3167 reflections and 232 parameters; GOF=1.063. CCDC-700913 contains the supplementary crystallographic data for this compound. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (internat.) +44-1223/336-033; Email: deposit@ccdc.cam.ac.uk).

ACKNOWLEDGEMENTS
This work was partially supported by Grants-in-Aid for Young Scientists (B), No. 17750032 (M. S.) and No. 20038010 (M. S.) in Priority Areas "Molecular Theory for Real Systems" from the Ministry of Education, Culture, Sports, Science and Technology of Japan. M. Saito acknowledges a research grant from the Sumitomo Foundation.

References

1. a) L. I. Smith and H. H. Hoehn, J. Am. Chem. Soc., 1941, 63, 1184; CrossRef b) A. G. Evans, J. C. Evans, P. J. Emes, and T. J. Phelan, J. Chem. Soc. B, 1971, 315; CrossRef c) Z. Csürös, P. Caluwe, and M. Szwarc, J. Am. Chem. Soc., 1973, 95, 6171; CrossRef d) Z. Xi, Eur. J. Org. Chem., 2004, 2773. CrossRef
2. a) H. Fang, C. Zhao, G. Li, and Z. Xi, Tetrahedron, 2003, 59, 3779; CrossRef b) Z. Wang, C. Wang, and Z. Xi, Tetrahedron Lett., 2006, 47, 3779.
3. For a review, see: J. Dubac, A. Laporterie, and G. Manuel, Chem. Rev., 1990, 90, 215. CrossRef
4. a) M. Saito, R. Haga, and M. Yoshioka, Chem. Commun., 2002, 1002; CrossRef b) M. Saito, R. Haga, and M. Yoshioka, Chem. Lett., 2003, 32, 912; CrossRef c) M. Saito, R. Haga, and M. Yoshioka, Phosphorus, Sulfur Silicon Relat. Elem., 2004, 179, 703; CrossRef d) M. Saito, R. Haga, M. Yoshioka, K. Ishimura, and S. Nagase, Angew. Chem. Int. Ed., 2005, 44, 6553; CrossRef e) M. Saito and M. Yoshioka, Coord. Chem. Rev., 2005, 249, 765; CrossRef f) R. Haga, M. Saito, and M. Yoshioka, J. Am. Chem. Soc., 2006, 128, 4934; CrossRef g) R. Haga, M. Saito, and M. Yoshioka, Chem. Eur. J., 2008, 14, 4068. CrossRef
5. M. Saito, M. Nakamura, T. Tajima, and M. Yoshioka, Angew. Chem. Int. Ed., 2007, 46, 1504. CrossRef
6. Recent examples, see : a) S. Kondo, T. Harada, R. Tanaka, and M. Unno, Org. Lett., 2006, 8, 4621; CrossRef b) T. F. Anderson, M. A. J. Statham, and M. A. Carroll, Tetrahedron Lett., 2006, 47, 3353; CrossRef c) L. Nielsen, K. B. Lindsay, J. Faber, N. C. Nielsen, and T. Skrydstrup, J. Org. Chem., 2007, 72, 10035; CrossRef d) T. Sasamori, M. Kobayashi, N. Nagahora, Y. Sugiyama, and N. Tokitoh, Silicon Chem., 2007, 3, 199. CrossRef
7. Recent examples, see: a) L. Pu, N. J. Hardman, and P. P. Power, Organometallics, 2001, 20, 5105; CrossRef b) T. Matsumoto, Y. Nakaya, and K. Tatsumi, Organometallics, 2006, 2, 4835; CrossRef c) Y. Sugiyama, T. Sasamori, Y. Hosoi, Y. Furukawa, N. Takagi, S. Nagase, and N. Tokitoh, J. Am. Chem. Soc., 2006, 128, 1023. CrossRef
8. 1,4-Dilithio-1,3-butadiene 1 could not be isolated from the reaction mixture containing the dilithium dibenzopentalenide as a byproduct. See ref. 5.
9. The preferable formation of a cis-isomer of 6 can be reasonably explained by attack of the second water molecule from the less hindered site, which is opposite to the triisopropylsilyl group of the intermediary lithium 5-hydrodibenzopentalenide. A similar reaction was also reported in ref. 10.
10. M. Saito, M. Nakamura, and T. Tajima, Chem. Eur. J., 2008, 14, 6062. CrossRef
11. T. K. Jone and S. E. Denmark, Helv. Chim. Acta, 1983, 66, 2397. CrossRef
12. M. Saito, H. Hashimoto, T. Tajima, and M. Ikeda, J. Organomet. Chem., 2007, 692 , 2729. CrossRef
13. R. Haga, M. Saito, and M. Yoshioka, Eur. J. Inorg. Chem., 2007, 1297. CrossRef
14. J. Beckmann, K. Jurkschat, and M. Schürmann, Eur. J. Inorg. Chem., 2000, 939. CrossRef
15. Although each of the hydroxy groups of the single crystal are involved in intermolecular hydrogen-bonding interactions, an absorption assignable to the stretching vibration of hydroxy bonds without intermolecular hydrogen bonds was reasonably observed, because the IR spectrum of a powdered sample was measured. The absorption was slightly weaker than that found in solution.
16. M. C. Godelie, M. C. Jennings, and K. M. Baines, Main Group Met. Chem., 2001, 24, 823.
17. There are 26 examples of X-ray diffraction data for germoles, deposited in CCDC. Recent examples of 2,3,4,5-tetraphenyl derivatives, see: a) U. Losehand and N. W. Mitzel, J. Chem. Soc., Dalton Trans., 2000, 1049; CrossRef b) S. D. Goodwin, P. Wei, B. C. Beck, J. Su, and G. H. Robinson, Main Group Chem., 2000, 3, 137; CrossRef c) S. J. Toal, H. Sohn, L. N. Zakarov, W. S. Kassel, J. A. Golen, A. L. Rheingold, and W. C. Trogler, Organometallics, 2005, 24, 3081, and ref. 16. CrossRef
18. G. M. Sheldrick, Acta Crystallogr., 1990, A46, 467. CrossRef
19. G. M. Sheldrick, SHELXL-97, Program for Crystal Structure Refinement; Göttingen University: Göttingen, Germany, 1997.
20. C. K. Johnson, ORTEP-II. Report ORNL-5138. Oak Ridge National Laboratory, Tennessee, USA, 1976.