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A simple and efficient synthesis of boron–imidazole macrocycles and their crystal structures
Tetrahedron Letters 53 (2012) 4526–4528
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
A simple and efficient synthesis of boron–imidazole macrocycles and their crystal structures Waldemar Iwanek a,⇑, Agata Iwanek a, Krzysztof Woz´niak b, Maura Malin´ska b a b
Department of Environmental Protection and Modelling, The Jan Kochanowski University in Kielce, S´wie˛tokrzyska 15, 25-406 Kielce, Poland Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warszawa, Poland
a r t i c l e
i n f o
Article history: Received 6 February 2012 Revised 22 May 2012 Accepted 8 June 2012 Available online 23 June 2012
a b s t r a c t A simple and selective method for the synthesis of boron–imidazole macrocycles is described and their X-ray structures are presented. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Imidazoles Boron Macrocycles Crystallographic structures
The properties of boron as a Lewis acid are often used in the synthesis of macrocyclic compounds.1,2 Usually, such a synthesis is performed via one of three routes: (1) by formation of a boron–alkoxy or ester covalent bond between a boron atom and an oxygen atom;3–10 (2) by formation of a co-ordinate bond between a boron atom and a nitrogen atom;11,12 or (3) by modification of a macrocyclic platform, most often via formation of a boron-nitrogen co-ordinate bond. In the first two cases, the starting materials are appropriately selected boron compounds and monomeric units. In the latter case, modification of the already existing macrocyclic compound is achieved by formation of its derivative with boron compounds.13–15 Boron–imidazole macrocycles are known in the literature. However, they have been prepared in a complicated way, that is, from trimethylsilylimidazole and dichloroborane.16,17 This method was not tetramer-specific, rather it led to the formation of boron–imidazole oligomers in general. The presented work discloses a very simple and efficient synthesis of tetrameric boron–imidazole macrocycles involving the reaction of imidazole or 2-methylimidazole and triethylborane, as shown in Scheme 1. The reaction occurs in two steps. In the first step, an equimolar amount of triethylborane in tetrahydrofuran is added to imidazole or 2-methylimidazole, followed by refluxing for about 1 hour. Next, the tetrahydrofuran is evaporated, mesitylene is added, and the mixture is refluxed at mesitylene boiling temperature for several
hours. After cooling the reaction mixture, pure tetrameric boron– imidazole macrocycles were obtained in about 60% yield.18 Recrystallisation of the obtained macrocycles from mesitylene afforded crystals suitable for performing X-ray crystal analysis. Crystal analysis of the tetramer formed from imidazole and triethylborane indicated that the compound exists in a 1,2-alternate conformation. In the case of the macrocycle formed from 2-methylimidazole, the product existed in a 1,3-alternate conformation. The reason for this is the much more favourable van der Waals interactions between the electron clouds of the methyl substituents in the 1,3-alternate conformation compared to the 1,2-conformation. This was indicated by simple calculations of heats of formation of this macrocycle in the corresponding conformations, employing the semi-empirical method AM1 using the GAMESS program.19 The 1,3-alternate conformation ( 33.43 kcal/mol) was the most stable, followed by the partial-cone conformation ( 27.79 kcal/ mol), and cone conformations ( 25.01 kcal/mol), and lastly the energetically least stable 1,2-alternate conformation ( 22.90 kcal/ mol). Table 1 summarises the results of the calculations for the boron–imidazole macrocycles.
Et N
NH
+ BEt3(THF)
mesitylene reflux
N
N
Et
⇑ Corresponding author. E-mail address: [email protected] (W. Iwanek). 0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2012.06.038
R
1: R = H 2: R = Me
B
4
R
Scheme 1. Synthesis of boron-imidazole macrocycles.
4527
W. Iwanek et al. / Tetrahedron Letters 53 (2012) 4526–4528 Table 1 Heats of formation, DH (kcal/mol) of the conformers of macrocycles 1 and 2 calculated by the semi-empirical method AM1 using the GAMESS program Cone conformation
Partial cone conformation
1,2-Alternate conformation
1,3-Alternate conformation
Boron–imidazole tetramer 1 18.57
17.34
18.76
16.62
Boron-2-methylimidazole tetramer 2 25.01
27.79
22.90
33.43
Figure 1. Side views of molecules 1 and 2 with hydrogen atoms excluded for clarity. Thermal displacement ellipsoids are drawn at the 50% probability level. The bond lengths in 1 are as follows: N(3)–C(2) 1.333(1) Å, C(2)–N(1) 1.333(1) Å, N(1)–C(5) 1.378(2) Å, C(5)–C(4) 1.353(2) Å, C(4)–N(3) 1.378(2) Å, and for 2: N(4)–C(8) 1.342(2) Å, C(8)–N(7) 1.343(2) Å, N(7)–C(6) 1.377(2) Å, C(6)–C(5) 1.349(2) Å, C(5)–N(4) 1.381(2) Å.
The preferred formation of the most thermodynamically stable conformers was in good agreement with the presented semiempirical calculations. Macrocycles 1 and 2 (Fig. 1) crystallise in the monoclinic P21/c and C2/c space groups, respectively, with halves of the molecules in the asymmetric parts of the unit cells.20 In the case of 1, the whole molecule is obtained by transformation of the independent part of it (half of the moiety) through the centre of symmetry, whereas the whole molecule 2 is obtained by applying a two-fold rotation axis. Both molecules contain a macrocyclic ring formed from four imidazole units connected through boron atoms. In compound 1, the N–B bond lengths are in the range 1.592(2)–1.601(2) Å, whereas, in 2 the
respective bond lengths are 1.598(2)–1.601(2) Å. The B–C bonds in 1 are in the range 1.611–1.615 Å, and between 1.613 and 1.619 Å in 2. The bond lengths in the imidazole rings are typical of those for imidazole derivatives. The introduction of the methyl group in 2 results in elongation of the C–N bonds (by ca. 0.01 Å) and similarly shortening of the C–C bonds. The conformations of the macrocyclic rings are different in the two products. In 2, the neighbouring imidazole rings have opposite orientations with the methyl groups above and below the best plane of the macrocyclic ring. In consequence, the methyl groups on the face-to-face opposite imidazole rings are oriented in the same manner bringing the methyl groups close to each other, with
Figure 2. Views of: (a) crystal packing along the z-axis for 2; (b) crystal packing along the y-axis for 1; (c) space-filling model of the molecular layer in 2 perpendicular to the y-direction—viewed along the x-axis, (d) space-filling model of the molecular layer in 1.
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W. Iwanek et al. / Tetrahedron Letters 53 (2012) 4526–4528
the methyl C C distance equal to 3.836 Å, whereas, the intercentroid distance for these rings equals 5.989 Å. In macrocycle 1, the two pairs of neighbouring imidazole rings have the same conformation, whereas the opposite imidazole rings have opposite conformations (see Fig. 1). The intercentroid distances are 6.093 and 6.311 Å for the rings containing C2 and C8, respectively, whereas the separation of the carbon atoms located between the two nitrogens from the centre of the C–C bond in the opposite imidazole rings is equal to 5.986 and 6.178 Å for C2 (the centre of the C4–C5 bond) and C8 (the centre of the C10–C11 bond), respectively. In the 3D crystal structure of 2, layers of molecules are formed perpendicular to the z-axis. These layers are ideally stacked thus forming columns of molecules along the z-axis direction. These columns create intramolecular channels. Each column of molecules is surrounded by eight neighbouring columns. The alkyl groups of molecules 2 create intermolecular channels as illustrated in Figure 2. Surprisingly, there are no close contacts shorter than the sum of the van der Waals radii within the layer which indicates that weaker interactions, particularly between the methyl and ethyl hydrogen atoms and imidazole aromatic rings are strong enough to hold the crystal together. The interactions between the layers are also very weak. They manifest themselves by short H H contacts existing between the boron bonded ethyl hydrogen atom and another hydrogen atom from one of the imidazole rings from the next molecular layer (H19 H13B = 2.32 Å). The molecules of 1 form stacks in the y-direction. However, in the case of 2, the molecules are arranged in such a way that neighbouring rows of molecules create an angle (75.4o) between the best leastsquare molecular planes as illustrated in the space-filling model ( Fig. 2d). There are several short intermolecular H H contacts, and one C H contact, shorter than the sum of the van der Waals radii. These are: H17B H17B = 2.16 Å, H13B H15B = 2.34 Å, H1 4A H19A = 2.31 Å and C13 H10 = 2.87 Å. The crystal data and structure refinement parameters for macrocycles 1 and 2 are provided.21 In conclusion, a simple, efficient and selective method for the synthesis of boron–imidazole macrocycles from imidazole (or 2methylimidazole) and triethylborane has been developed. The Xray analysis shows that the boron–imidazole macrocycle forms a 1,2-alternate conformer, whereas the boron–2-methylimidazole macrocycle forms a 1,3-alternate conformer. Supplementary data Supplementary data (crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications, CCDC 864908 (1) and 864909 (2). Copies of these data may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44 1223-336033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk/conts/retrieving/.html)) associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.06.038.
References and notes 1. Group 13 Chemistry I: Boron macrocycles; Roesky, H. W., Atwood, D. A., Eds.; Springer: Berlin, 2002. 2. Macromolecules Containing Metal and Metal-Like Elements; Abd-El Aziz, A. S., Carraher, C. E., Pittman, C. U., Zeldin, M., Eds.; John Wiley & Sons: New York, 2009; Vol. 9,. 3. Dahlhof, W. V.; Koester, R. Liebigs Ann. Chem. 1975, 1625. 4. Gerwarth, U. W. Z. Naturforschung 1977, B32, 1408. 5. Hoepfl, H.; Farfan, N. J. Organomet. Chem. 1997, 547, 11. 6. Hoepfl, H.; Sanchez, M.; Barba, V.; Farfan, N.; Rojas, S.; Santillan, R. Inorg. Chem. 1998, 37, 1679. 7. Barba, V.; Gallegos, E.; Santillan, R.; Farfan, N. J. Organomet. Chem. 2001, 622, 259. 8. Barba, V.; Höpfl, H.; Farfan, N.; Santillan, R.; Beltran, H. I.; Zamudio, L. S. Chem. Commun. 2004, 2834. 9. Ito, H.; Kono, Y.; Machida, A.; Mitsumoto, Y.; Omori, K.; Nakamura, N.; Kondo, Y.; Ishihara, K. Inorg. Chim. Acta 2003, 28, 344. 10. Barba, V.; Betanzos, I. J. Organomet. Chem. 2007, 692, 4903. 11. Niedenzu, K.; Woodrum, K. R. Inorg. Chem. 1989, 28, 4022. 12. Brock, C. P.; Comanion, A. L.; Kock, L. D.; Niedenzu, K. Inorg. Chem. 1991, 30, 784. 13. Iwanek, W.; Fröhlich, R.; Schwab, P.; Schurig, V. Chem. Commun. 2002, 2516. 14. Iwanek, W.; Urbaniak, M.; Gawdzik, B.; Schurig, V. Tetrahedron: Asymmetry 2003, 14, 2787. 15. Wzorek, A.; Iwanek, W.; Mattay, J. Tetrahedron: Asymmetry 2007, 18, 815. 16. Weiss, A.; Pritzkow, H.; Siebert, W. Angew. Chem., Int. Ed. 2000, 39, 547. 17. Weiss, A.; Barba, V.; Pritzkow, H.; Siebert, W. J. Organomet. Chem. 2003, 680, 294. 18. Synthesis of 1: To imidazole (500 mg, 7.34 mmol) was added an equimolar amount of a 1 M solution of triethylborane in THF (7.4 mmol, 7.4 ml) and the mixture was refluxed for about 1 h. THF was evaporated, mesitylene (10 ml) was added, and the mixture refluxed at the boiling point of mesitylene for 5 h. After cooling the reaction mixture, tetrameric boron–imidazole macrocycle 1 was obtained in 58% yield. Recrystallisation from mesitylene give pure compound 1 for spectroscopic measurements. 1H NMR (700 MHz, CDCl3) [ppm] (d): 0.61–0.63 (m, 24H, CH3–CH2), 0.68–0.69 (m, 16H, CH2–CH3), 6.83 (s, 4H, –CH–), 6.92 (d, J = 1.38 Hz, 8H, CH@CH); 13C NMR (175 MHz, CDCl3) [ppm] (d): 4.74, 8.17, 117.85, 141.65; HR-MS (ESI): m/z Calcd 544.4687. Found 567.2473 [M+Na]+. 19. Program GAMESS, version October 1, 2010 R3 for Microsoft Windows. 20. Experimental details are provided in the Supplementary data. 21. Crystal data for 1: Empirical formula C28H52B4N8, formula weight 544.02, T = 100(2) K, MoKa = 0.71073 Å, monoclinic, P21/c, a = 11.2168(4) Å, b = 6.9564(2) Å, c = 20.9190(7) Å, b = 96.271(4)o, vol = 1622.51(9) Å3, Z = 2, calculated density, 1.114 mg/m3, absorption coefficient 0.066 mm 1, F(0 0 0) = 592, crystal size 0.25 0.18 0.18 mm, H range 2.82–26.43o, limiting indices 14<=h<=14, 7<=k<=7, 26<=l<=26, reflections collected/ unique 13411 / 3336 [R(int) = 0.0246], completeness (to H = 26.43o) = 99.5%, semi-empirical absorption correction, max. and min. trans. 0.9882 and 0.9836, full-matrix least-squares on F2, data/restraints/parameters 3336/0/185, GoF (on F2) = 1.033, Final R indices [I>2d(I)] R1 = 0.0344, wR2 = 0.0870, R indices (all data) R1 = 0.0469, wR2 = 0.0894, largest diff. peak and hole 0.263 and 0.198 e A 3 Crystal data for 2: Empirical formula C32H60B4N8, formula weight 600.12, T = 100(2) K, MoKa = 0.71073 Å, monoclinic, C2/c, a = 15.0006(7) Å, b = 19.8708(10) Å, c = 13.0332(7) Å, b = 112.561(2)°, vol = 3587.6(3) Å3, Z = 4, calculated density, 1.111 mg/m3, absorption coefficient 0.066 mm 1, F(0 0 0) = 1312, crystal size 0.20 0.10 0.10 mm, H range 1.79–27.49o, limiting indices 19<=h<=19, 25<=k<=25, 16<=l<=16, reflections collected/ unique 33347/4122 [R(int) = 0.0366], completeness (to H = 27.49o) = 100.0%, semi-empirical absorption correction, max. and min. trans. 0.9935 and 0.9870, full-matrix least-squares on F2, data/restraints/parameters 4122/0/206, GoF(on F2) = 1.045, final R indices [I>2d(I)] R1 = 0.0430, wR2 = 0.1083, R indices (all data) R1 = 0.0552, wR2 = 0.1157, largest diff. peak and hole 0.414 and 0.212 e A 3.
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
A simple and efficient synthesis of boron–imidazole macrocycles and their crystal structures Waldemar Iwanek a,⇑, Agata Iwanek a, Krzysztof Woz´niak b, Maura Malin´ska b a b
Department of Environmental Protection and Modelling, The Jan Kochanowski University in Kielce, S´wie˛tokrzyska 15, 25-406 Kielce, Poland Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warszawa, Poland
a r t i c l e
i n f o
Article history: Received 6 February 2012 Revised 22 May 2012 Accepted 8 June 2012 Available online 23 June 2012
a b s t r a c t A simple and selective method for the synthesis of boron–imidazole macrocycles is described and their X-ray structures are presented. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Imidazoles Boron Macrocycles Crystallographic structures
The properties of boron as a Lewis acid are often used in the synthesis of macrocyclic compounds.1,2 Usually, such a synthesis is performed via one of three routes: (1) by formation of a boron–alkoxy or ester covalent bond between a boron atom and an oxygen atom;3–10 (2) by formation of a co-ordinate bond between a boron atom and a nitrogen atom;11,12 or (3) by modification of a macrocyclic platform, most often via formation of a boron-nitrogen co-ordinate bond. In the first two cases, the starting materials are appropriately selected boron compounds and monomeric units. In the latter case, modification of the already existing macrocyclic compound is achieved by formation of its derivative with boron compounds.13–15 Boron–imidazole macrocycles are known in the literature. However, they have been prepared in a complicated way, that is, from trimethylsilylimidazole and dichloroborane.16,17 This method was not tetramer-specific, rather it led to the formation of boron–imidazole oligomers in general. The presented work discloses a very simple and efficient synthesis of tetrameric boron–imidazole macrocycles involving the reaction of imidazole or 2-methylimidazole and triethylborane, as shown in Scheme 1. The reaction occurs in two steps. In the first step, an equimolar amount of triethylborane in tetrahydrofuran is added to imidazole or 2-methylimidazole, followed by refluxing for about 1 hour. Next, the tetrahydrofuran is evaporated, mesitylene is added, and the mixture is refluxed at mesitylene boiling temperature for several
hours. After cooling the reaction mixture, pure tetrameric boron– imidazole macrocycles were obtained in about 60% yield.18 Recrystallisation of the obtained macrocycles from mesitylene afforded crystals suitable for performing X-ray crystal analysis. Crystal analysis of the tetramer formed from imidazole and triethylborane indicated that the compound exists in a 1,2-alternate conformation. In the case of the macrocycle formed from 2-methylimidazole, the product existed in a 1,3-alternate conformation. The reason for this is the much more favourable van der Waals interactions between the electron clouds of the methyl substituents in the 1,3-alternate conformation compared to the 1,2-conformation. This was indicated by simple calculations of heats of formation of this macrocycle in the corresponding conformations, employing the semi-empirical method AM1 using the GAMESS program.19 The 1,3-alternate conformation ( 33.43 kcal/mol) was the most stable, followed by the partial-cone conformation ( 27.79 kcal/ mol), and cone conformations ( 25.01 kcal/mol), and lastly the energetically least stable 1,2-alternate conformation ( 22.90 kcal/ mol). Table 1 summarises the results of the calculations for the boron–imidazole macrocycles.
Et N
NH
+ BEt3(THF)
mesitylene reflux
N
N
Et
⇑ Corresponding author. E-mail address: [email protected] (W. Iwanek). 0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2012.06.038
R
1: R = H 2: R = Me
B
4
R
Scheme 1. Synthesis of boron-imidazole macrocycles.
4527
W. Iwanek et al. / Tetrahedron Letters 53 (2012) 4526–4528 Table 1 Heats of formation, DH (kcal/mol) of the conformers of macrocycles 1 and 2 calculated by the semi-empirical method AM1 using the GAMESS program Cone conformation
Partial cone conformation
1,2-Alternate conformation
1,3-Alternate conformation
Boron–imidazole tetramer 1 18.57
17.34
18.76
16.62
Boron-2-methylimidazole tetramer 2 25.01
27.79
22.90
33.43
Figure 1. Side views of molecules 1 and 2 with hydrogen atoms excluded for clarity. Thermal displacement ellipsoids are drawn at the 50% probability level. The bond lengths in 1 are as follows: N(3)–C(2) 1.333(1) Å, C(2)–N(1) 1.333(1) Å, N(1)–C(5) 1.378(2) Å, C(5)–C(4) 1.353(2) Å, C(4)–N(3) 1.378(2) Å, and for 2: N(4)–C(8) 1.342(2) Å, C(8)–N(7) 1.343(2) Å, N(7)–C(6) 1.377(2) Å, C(6)–C(5) 1.349(2) Å, C(5)–N(4) 1.381(2) Å.
The preferred formation of the most thermodynamically stable conformers was in good agreement with the presented semiempirical calculations. Macrocycles 1 and 2 (Fig. 1) crystallise in the monoclinic P21/c and C2/c space groups, respectively, with halves of the molecules in the asymmetric parts of the unit cells.20 In the case of 1, the whole molecule is obtained by transformation of the independent part of it (half of the moiety) through the centre of symmetry, whereas the whole molecule 2 is obtained by applying a two-fold rotation axis. Both molecules contain a macrocyclic ring formed from four imidazole units connected through boron atoms. In compound 1, the N–B bond lengths are in the range 1.592(2)–1.601(2) Å, whereas, in 2 the
respective bond lengths are 1.598(2)–1.601(2) Å. The B–C bonds in 1 are in the range 1.611–1.615 Å, and between 1.613 and 1.619 Å in 2. The bond lengths in the imidazole rings are typical of those for imidazole derivatives. The introduction of the methyl group in 2 results in elongation of the C–N bonds (by ca. 0.01 Å) and similarly shortening of the C–C bonds. The conformations of the macrocyclic rings are different in the two products. In 2, the neighbouring imidazole rings have opposite orientations with the methyl groups above and below the best plane of the macrocyclic ring. In consequence, the methyl groups on the face-to-face opposite imidazole rings are oriented in the same manner bringing the methyl groups close to each other, with
Figure 2. Views of: (a) crystal packing along the z-axis for 2; (b) crystal packing along the y-axis for 1; (c) space-filling model of the molecular layer in 2 perpendicular to the y-direction—viewed along the x-axis, (d) space-filling model of the molecular layer in 1.
4528
W. Iwanek et al. / Tetrahedron Letters 53 (2012) 4526–4528
the methyl C C distance equal to 3.836 Å, whereas, the intercentroid distance for these rings equals 5.989 Å. In macrocycle 1, the two pairs of neighbouring imidazole rings have the same conformation, whereas the opposite imidazole rings have opposite conformations (see Fig. 1). The intercentroid distances are 6.093 and 6.311 Å for the rings containing C2 and C8, respectively, whereas the separation of the carbon atoms located between the two nitrogens from the centre of the C–C bond in the opposite imidazole rings is equal to 5.986 and 6.178 Å for C2 (the centre of the C4–C5 bond) and C8 (the centre of the C10–C11 bond), respectively. In the 3D crystal structure of 2, layers of molecules are formed perpendicular to the z-axis. These layers are ideally stacked thus forming columns of molecules along the z-axis direction. These columns create intramolecular channels. Each column of molecules is surrounded by eight neighbouring columns. The alkyl groups of molecules 2 create intermolecular channels as illustrated in Figure 2. Surprisingly, there are no close contacts shorter than the sum of the van der Waals radii within the layer which indicates that weaker interactions, particularly between the methyl and ethyl hydrogen atoms and imidazole aromatic rings are strong enough to hold the crystal together. The interactions between the layers are also very weak. They manifest themselves by short H H contacts existing between the boron bonded ethyl hydrogen atom and another hydrogen atom from one of the imidazole rings from the next molecular layer (H19 H13B = 2.32 Å). The molecules of 1 form stacks in the y-direction. However, in the case of 2, the molecules are arranged in such a way that neighbouring rows of molecules create an angle (75.4o) between the best leastsquare molecular planes as illustrated in the space-filling model ( Fig. 2d). There are several short intermolecular H H contacts, and one C H contact, shorter than the sum of the van der Waals radii. These are: H17B H17B = 2.16 Å, H13B H15B = 2.34 Å, H1 4A H19A = 2.31 Å and C13 H10 = 2.87 Å. The crystal data and structure refinement parameters for macrocycles 1 and 2 are provided.21 In conclusion, a simple, efficient and selective method for the synthesis of boron–imidazole macrocycles from imidazole (or 2methylimidazole) and triethylborane has been developed. The Xray analysis shows that the boron–imidazole macrocycle forms a 1,2-alternate conformer, whereas the boron–2-methylimidazole macrocycle forms a 1,3-alternate conformer. Supplementary data Supplementary data (crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications, CCDC 864908 (1) and 864909 (2). Copies of these data may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44 1223-336033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk/conts/retrieving/.html)) associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.06.038.
References and notes 1. Group 13 Chemistry I: Boron macrocycles; Roesky, H. W., Atwood, D. A., Eds.; Springer: Berlin, 2002. 2. Macromolecules Containing Metal and Metal-Like Elements; Abd-El Aziz, A. S., Carraher, C. E., Pittman, C. U., Zeldin, M., Eds.; John Wiley & Sons: New York, 2009; Vol. 9,. 3. Dahlhof, W. V.; Koester, R. Liebigs Ann. Chem. 1975, 1625. 4. Gerwarth, U. W. Z. Naturforschung 1977, B32, 1408. 5. Hoepfl, H.; Farfan, N. J. Organomet. Chem. 1997, 547, 11. 6. Hoepfl, H.; Sanchez, M.; Barba, V.; Farfan, N.; Rojas, S.; Santillan, R. Inorg. Chem. 1998, 37, 1679. 7. Barba, V.; Gallegos, E.; Santillan, R.; Farfan, N. J. Organomet. Chem. 2001, 622, 259. 8. Barba, V.; Höpfl, H.; Farfan, N.; Santillan, R.; Beltran, H. I.; Zamudio, L. S. Chem. Commun. 2004, 2834. 9. Ito, H.; Kono, Y.; Machida, A.; Mitsumoto, Y.; Omori, K.; Nakamura, N.; Kondo, Y.; Ishihara, K. Inorg. Chim. Acta 2003, 28, 344. 10. Barba, V.; Betanzos, I. J. Organomet. Chem. 2007, 692, 4903. 11. Niedenzu, K.; Woodrum, K. R. Inorg. Chem. 1989, 28, 4022. 12. Brock, C. P.; Comanion, A. L.; Kock, L. D.; Niedenzu, K. Inorg. Chem. 1991, 30, 784. 13. Iwanek, W.; Fröhlich, R.; Schwab, P.; Schurig, V. Chem. Commun. 2002, 2516. 14. Iwanek, W.; Urbaniak, M.; Gawdzik, B.; Schurig, V. Tetrahedron: Asymmetry 2003, 14, 2787. 15. Wzorek, A.; Iwanek, W.; Mattay, J. Tetrahedron: Asymmetry 2007, 18, 815. 16. Weiss, A.; Pritzkow, H.; Siebert, W. Angew. Chem., Int. Ed. 2000, 39, 547. 17. Weiss, A.; Barba, V.; Pritzkow, H.; Siebert, W. J. Organomet. Chem. 2003, 680, 294. 18. Synthesis of 1: To imidazole (500 mg, 7.34 mmol) was added an equimolar amount of a 1 M solution of triethylborane in THF (7.4 mmol, 7.4 ml) and the mixture was refluxed for about 1 h. THF was evaporated, mesitylene (10 ml) was added, and the mixture refluxed at the boiling point of mesitylene for 5 h. After cooling the reaction mixture, tetrameric boron–imidazole macrocycle 1 was obtained in 58% yield. Recrystallisation from mesitylene give pure compound 1 for spectroscopic measurements. 1H NMR (700 MHz, CDCl3) [ppm] (d): 0.61–0.63 (m, 24H, CH3–CH2), 0.68–0.69 (m, 16H, CH2–CH3), 6.83 (s, 4H, –CH–), 6.92 (d, J = 1.38 Hz, 8H, CH@CH); 13C NMR (175 MHz, CDCl3) [ppm] (d): 4.74, 8.17, 117.85, 141.65; HR-MS (ESI): m/z Calcd 544.4687. Found 567.2473 [M+Na]+. 19. Program GAMESS, version October 1, 2010 R3 for Microsoft Windows. 20. Experimental details are provided in the Supplementary data. 21. Crystal data for 1: Empirical formula C28H52B4N8, formula weight 544.02, T = 100(2) K, MoKa = 0.71073 Å, monoclinic, P21/c, a = 11.2168(4) Å, b = 6.9564(2) Å, c = 20.9190(7) Å, b = 96.271(4)o, vol = 1622.51(9) Å3, Z = 2, calculated density, 1.114 mg/m3, absorption coefficient 0.066 mm 1, F(0 0 0) = 592, crystal size 0.25 0.18 0.18 mm, H range 2.82–26.43o, limiting indices 14<=h<=14, 7<=k<=7, 26<=l<=26, reflections collected/ unique 13411 / 3336 [R(int) = 0.0246], completeness (to H = 26.43o) = 99.5%, semi-empirical absorption correction, max. and min. trans. 0.9882 and 0.9836, full-matrix least-squares on F2, data/restraints/parameters 3336/0/185, GoF (on F2) = 1.033, Final R indices [I>2d(I)] R1 = 0.0344, wR2 = 0.0870, R indices (all data) R1 = 0.0469, wR2 = 0.0894, largest diff. peak and hole 0.263 and 0.198 e A 3 Crystal data for 2: Empirical formula C32H60B4N8, formula weight 600.12, T = 100(2) K, MoKa = 0.71073 Å, monoclinic, C2/c, a = 15.0006(7) Å, b = 19.8708(10) Å, c = 13.0332(7) Å, b = 112.561(2)°, vol = 3587.6(3) Å3, Z = 4, calculated density, 1.111 mg/m3, absorption coefficient 0.066 mm 1, F(0 0 0) = 1312, crystal size 0.20 0.10 0.10 mm, H range 1.79–27.49o, limiting indices 19<=h<=19, 25<=k<=25, 16<=l<=16, reflections collected/ unique 33347/4122 [R(int) = 0.0366], completeness (to H = 27.49o) = 100.0%, semi-empirical absorption correction, max. and min. trans. 0.9935 and 0.9870, full-matrix least-squares on F2, data/restraints/parameters 4122/0/206, GoF(on F2) = 1.045, final R indices [I>2d(I)] R1 = 0.0430, wR2 = 0.1083, R indices (all data) R1 = 0.0552, wR2 = 0.1157, largest diff. peak and hole 0.414 and 0.212 e A 3.