Icosahedral carboranes as scaffolds for congested regioselective polyaryl compounds : the distinct distance tuning of C-C and its antipodal B-B

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY NC 3.0 https://creativecommons.org/licenses/by-nc/3.0/ Icosahedral carboranes as scaffolds for congested regioselective polyaryl compounds : the distinct distance tuning of C-C and its antipodal B-B © The Royal Society of Chemistry 2019 Published version Kelemen, Zsolt; Pepiol, Ariadna; Lupu, Marius; Sillanpää, Reijo; Hänninen, Mikko M.; Teixidor, Francesc; Viñas, Clara Kelemen, Z., Pepiol, A., Lupu, M., Sillanpää, R., Hänninen, M. M., Teixidor, F., & Viñas, C. (2019). Icosahedral carboranes as scaffolds for congested regioselective polyaryl compounds : the distinct distance tuning of C-C and its antipodal B-B. Chemical Communications, 55(61), 8927- 8930. https://doi.org/10.1039/C9CC04526K 2019 Registered charity number: 207890 rsc.li/chemcomm Showcasing research from Professor Clara Viñas’ laboratory, Institut de Ciència de Materials de Barcelona (CSIC), Barcelona, Spain. Icosahedral carboranes as scaffolds for congested regioselective polyaryl compounds: the distinct distance tuning of C–C and its antipodal B–B Four-fold aryl substituted o-carborane derivatives with defined patterns of substitution at the antipodal region of the cluster carbon atoms have been achieved. It is proven that this region is congested but lacks steric hindrance. As featured in: ISSN 1359-7345 COMMUNICATION Lingfeng Gao, Gengxiu Zheng et al . A radical capture mechanism for immediate Csp 2 –H bond hydroxylation via a heterogeneous Cu–graphene catalyst ChemComm Chemical Communications rsc.li/chemcomm Volume 55 Number 61 7 August 2019 Pages 8887–9052 See Clara Viñas et al ., Chem . Commun ., 2019, 55 , 8927. This journal is ©The Royal Society of Chemistry 2019 Chem. Commun., 2019, 55, 8927--8930 | 8927 Cite this: Chem. Commun., 2019, 55,8927 Icosahedral carboranes as scaffolds for congested regioselective polyaryl compounds: the distinct distance tuning of C–C and its antipodal B–B† Zsolt Kelemen, a Ariadna Pepiol, a Marius Lupu, a Reijo Sillanpa ¨a ¨, b Mikko M. Ha ¨nninen, b Francesc Teixidor a and Clara Vin ˜as * a Four-fold aryl substituted o-carborane derivatives with defined patterns of substitution at the antipodal region of the cluster carbon atoms have been achieved. It is proven that this region is congested but lacks steric hindrance. Also, the two antipodal sites C c –C c and B9–B12 are affected very distinctly by electron donor substituents. The closo C 2 B 10 H 12 icosahedral carboranes, o-(1,2), m-(1,7), or p-(1,12), are the most widely studied boron clusters; they can be functionalized by different reactions, 1 in a regioselective manner, with many possible sites of substitution. o-Carborane, 1,2-closo-C 2 B 10 H 12 , acts as a strong electron-withdrawing molecule through the substitution on the carbon, C c , 2 and as an electrondonating moiety through the boron vertices with a gradation depending on the distance to the carbon atoms. 3 On the other hand, it is well known that the C c –C c bond length in o-carborane and metalla-o-carborane strongly depends on the electronic nature of the immediate C c -substituents; 4 p-donor substituents bonded to the C c , as well as atoms with lone pairs, donate electron density to the s*(C c –C c ) antibonding orbital (negative hyperconjugation) of o-carborane, which lies in the C c –C c bond, increasing significantly the C c –C c bond distance. 5 Inthecaseof aryl substituents, the orientation of the substituents has a significant impact on the electron donation. 6 The longest C c –C c distance ever reported is 2.156(4) Å for the 1,2-(CR 2 F c ) 2 -1,2-closo- C 2 B 10 H 10 , {R = pentamethylene; F c =(Z 5 -C 5 H 4 )Fe(Z 5 -C 5 H 5 )} o-carborane cluster derivative. 7 While the plasticity of the C c –C c bond has been investigated, the possibility of a similar behaviour at the B–B bonds located antipodal to the cluster carbon atoms remained unexplored. In addition, if p-aromaticity and tridimensional aromaticity of the icosahedral boranes are two sides of the same coin, 8 could it be possible to obtain hybrid polyaryl compounds by merging the 2D organic aromatic groups at the dense antipodal region of the cluster carbon atoms with 3D inorganic icosahedral clusters? To produce B–C substitutions, a useful and general method is by electrophilic iodination of o-carborane followed by Kumada cross-coupling reaction. 9 This implies that one should start from the appropriate iodocarborane derivatives. A less regioselective B(8)/B(9)-aryl-o-carborane Pd-catalysed monoarylation with aryl iodides via B–H activation was reported, 10 as well as a palladiumcatalysed regioselective diarylation on B(4,5) of o-carborane directly from B–H with aryl halides, with the help of the traceless directing carboxylic group. 11 With hindered substitutions on the neighbouring C/B atoms, 3-Ph-1,2-Ph 2 -1,2-closo-C 2 B 10 H 9 and 3,6- Ph 2 -1,2-Ph 2 -1,2-closo-C 2 B 10 H 912 derivatives were obtained in low yield by using the sequential nucleophilic-capping reactions of BPhCl 2 in the precursors [7,8-Ph 2 -7,8-nido-C 2 B 9 H 9 ] 2 and [6-Ph- 7,8-Ph 2 -7,8-nido-C 2 B 9 H 8 ] 2 . It should be noted that disubstituted 9,12-Ph 2 -1,2-closo-C 2 B 10 H 10 , 13 1,9- and 1,12-(p-MeC 6 H 4 ) 2 -1,2-closo- C 2 B 10 H 10 were also reported. 14 We interpret that the presence of the four phenyl groups in the crowded area adjacent to the C c vertices is accessible because of the different behaviour of all the atoms connected to the C 2 B 2 region where the strong electronwithdrawing (C c ) and electron-donating moieties (B) of the o-carboranyl group co-exist. As mentioned, it was our main target to elucidate if the four iodo groups in the compact and electronrich region of the o-carborane cluster (Chart 1) could be fully or only partially replaced by the organic aryl (sp 2 ) groups such as phenyl to produce a ‘‘four phenyl congested site’’. Furthermore, it could provide a good picture of the B–B tuning possibility to the antipodal C c –C c unit. This would be a potentially useful heavily congested electron-rich region due to the regioselective formation of B–C(sp 2 ), B–C(sp) and B–C(sp 3 ). To achieve so, the crosscoupling reaction on the B-iodinated o-carboranes with Grignard reagents in the presence of the Pd(II) and Cu(I)catalystswas studied. Two were the starting iodinated derivatives, 9,12-I 2 -1,2- closo-C 2 B 10 H 10 (1) and 8,10,9,12-I 4 -1,2-closo-C 2 B 10 H 8 (2). In 2,the four iodo groups occupy a highly dense region antipodal to the a Institut de Cie `ncia de Materials de Barcelona, ICMAB-CSIC Campus U.A.B., 08193 Bellaterra, Spain. E-mail: [email protected] b Department of Chemistry, University of Jyva ¨skyla ¨, FIN-40351, Jyva ¨skyla ¨, Finland †Electronic supplementary information (ESI) available. CCDC 1562911, 1871937, 1871938, 1913782 and 1913783. Synthesis and characterization of 1–11. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/ c9cc04526k Received 12th June 2019, Accepted 21st June 2019 DOI: 10.1039/c9cc04526k rsc.li/chemcomm ChemComm COMMUNICATION Open Access Article. Published on 25 June 2019. Downloaded on 9/4/2019 10:59:44 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online View Journal | View Issue 8928 |Chem. Commun., 2019, 55, 8927--8930 This journal is ©The Royal Society of Chemistry 2019 C c –C c atoms. In 1, only two adjacent iodo groups are present, again antipodal to the C c –C c unit. Four Grignard reagents were used, phenylethynyl magnesium, phenyl magnesium and allyl magnesium chlorides, and 4-benzaldehyde dimethyl acetal magnesium bromide. The first, the second and the fourth would produce a highly electron dense and congested region in o-carborane derivatives, whereas the third would produce a more relaxed space. In a typical experiment, 1and cis-[PdCl 2 (PPh 3 ) 2 ] and CuI as catalysts were dispersed in anhydrous THF and treated with the appropriate Grignard reagent at low temperatures (Scheme 1). After refluxing overnight, the corresponding crude compounds were purified (see ESI†). The cross-coupling reaction of 1with 4-benzaldehyde dimethyl acetal magnesium bromide proceeded to give 9,12-(C 6 H 4 CH(OCH 3 ) 2 ) 2 -1,2-closo-C 2 B 10 H 10 ,3,in71% yield (Scheme 1a), which was confirmed by multinuclear NMR, FTIR, mass spectrometry and elemental analysis. Deprotection of the carbonyl group was achieved with diethylether in aqueous HCl 3 M solution to produce 9,12-(C 6 H 4 CHO) 2 - 1,2-closo-C 2 B 10 H 10 ,4, in 96% yield (Scheme 1a). Suitable crystals for X-ray diffraction of 3and 4, which were grown in chloroform (Fig. 1) unambiguously and unequivocally confirmed that 3and 4were obtained. Furthermore, tetrasubstitution on 2to yield 8,9,10,12-Ph 4 -1,2-closo-C 2 B 10 H 8 (10) was expected by reaction of 2with phenyl magnesium chloride but 8,10-Ph 2 -9,12-I 2 -1,2- closo-C 2 B 10 H 8 (5) was obtained with only two substitutions (Scheme 1b). 9d To get more insight into the reason for this atypical or at least unexpected result, we investigated 11 B{ 1 H}- NMR chemical shifts (Fig. 2) and performed DFT calculations. The 11 B{ 1 H}-NMR chemical shifts of 2were unambiguously assigned 9d with the aid of a 2D 11 B{ 1 H}– 11 B{ 1 H}-NMR COSY experiment. 15 The B(9,12) resonances of these B–I vertices at 2appear at 7.1 ppm that are not so different from the B–H resonances of the unreactive B(9,12) vertices in the pristine 1,2- closo-C 2 B 10 H 12 (3.1 ppm); on the contrary, the B(8,10) vertex resonances of 2appear close to 16 ppm being very similar to the B–I reactive B(9,12) vertices in 1that resonate at 14.9 ppm (Fig. 2). It can be seen in the 11 B{ 1 H}-NMR of 1,2-closo-C 2 B 10 H 12 , 9,12-I 2 -closo-1,2-C 2 B 10 H 10 (1) and 8,10,9,12-I 4 -1,2-closo-C 2 B 10 H 8 (2) that the four iodo groups in the congested area of 2are not electronically identical. Electronically, the B(8,10) vertices in 2 are strongly affected upon I substitution shifting 12 ppm upfield from B(8,10)–H to B(8,10)–I. These B(8,10)–I vertices are highly reactive and lead readily to disubstitution to obtain B–Ph in 5. The calculated Mulliken charges (Table S1 in ESI†) are in good agreement with the chemical shifts; the charge of the reactive B(8,10)–I vertices in 2is +0.66, while the charge of the B(8,10)–I vertices is +0.31 in 5. Clearly, the B(9,12) vertices in 5are not only less reactive electronically, but less accessible for the reactant (steric hindrance) due to the phenyl substituents at the B(8,10) vertices. Since the four-fold phenylation of 2was unsuccessful, a new synthetic strategy aiming at 8,9,10,12-Ph 4 -1,2-closo-C 2 B 10 H 8 was designed. The positional isomer of 5, 8,10-I 2 -9,12-Ph 2 -1,2-closo-C 2 B 10 H 8 (7) was synthesised starting from 6.In7the two iodo substituents would be ready for substitution at less crowded positions than in 5; moreover, DFT calculations suggest a more reactive B–I vertex. This led us to perform the cross coupling reaction of 2 with PhMgCl to obtain 9,12-Ph 2 -1,2-closo-C 2 B 10 H 10 ,6. 13 This was Chart 1 Different polyaryl-o-carboranes. Scheme 1 Kumada cross coupling reactions on 1(a) and 2(b) using cis-[Pd(PPh 3 ) 2 Cl 2 ] and CuI as catalysts, in THF refluxing overnight. (i) 4-Benzaldehyde dimethyl acetal magnesium bromide, (ii) PhMgCl, 13 (iii) solvent free electrophilic iodination with I 2 (10 eq.) at 210 1C during 4 h. (iv) Extraction with Et 2 O in 3 M aqueous solution HCl. Fig. 1 Crystal structures of 3(left) and 4(right). Thermal ellipsoids have been drawn with 20% probability. The B9–B12 distances are 1.804(4) and 1.798(3) Å for 3and 4. Communication ChemComm Open Access Article. Published on 25 June 2019. Downloaded on 9/4/2019 10:59:44 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online This journal is ©The Royal Society of Chemistry 2019 Chem. Commun., 2019, 55, 8927--8930 | 8929 treated with I 2 in a sealed tube at 210 1C for 4 h (73% yield, Scheme 1c). The 11 B NMR spectra of 7verified the more reactive B–I vertices (they resonate at 20.1 ppm), in agreement with the DFT calculations. Furthermore, as it was mentioned earlier, the different allocation of the four substituents in 7with respect to 5could decrease the steric effect of the two bulky phenyl groups since they are sited at the neighbouring vertices B(9,12), and the two remaining iodo substituents are placed one far from the other and at the lateral position, so the B–I substitutions by the phenyl groups are sterically feasible. However, the crosscoupling reaction of 7with PhMgCl did not lead to the sought four-fold phenylation, only to unreacted 7(Scheme 1c). To check the possibility of further substitution on the 5con- gested electron-rich region of the o-carborane cluster, we decided to combine the pre-existing sp 2 (Ph) in 5, with the sp (PhCRC) and sp 3 (allyl) units, respectively. The reaction of 5with phenylethynyl magnesium chloride to produce 8was unsuccessful as no iodo was exchanged (Scheme 2). Instead, when using a less crowded allyl magnesium chloride, the coupling reaction of 5 with allyl magnesium chloride leads to the four-fold substitution by the formation of 8,10-Ph 2 -9,12-(CH 2 CHCH 2 ) 2 -1,2-closo- C 2 B 10 H 8 ,9, in 75% yield (Scheme 2). 11 Band 1 H NMR spectroscopy, MALDI-TOF-MS and EA data along with its X-ray structure from the crystals grown from a chloroform solution (Fig. 3 and ESI†) confirmed the allylation of B(9,12) vertices on 5. Since the sp and sp 2 four-fold substitution is possible, the steric hindrance in the proposed tetra-substituted 8,10,9,12-Ph 4 -1,2-closo-C 2 B 10 H 8 (10, Scheme 3) compound was investigated by using different hypothetical isodesmic reactions (see more details in ESI†). The DFT calculations indicated that there is not enough steric hindrance in 10 to prevent four-fold phenyl substitution, therefore 10 should be thermodynamically available and so its synthesis should be restricted only by kinetic reasons. To prove this hypothesis, we have modified the reaction conditions by using 1,4-dioxane instead of THF, which increases the reaction temperature up to 101 1C (Scheme 3). After 16 hours of refluxing, the 1 H-NMR spectra of the crude mixture displayed new broad peaks in the region 4.5–5.5 ppm, in which the C c –H resonates, indicating the formation of additional compounds. 3a By working up (see ESI†), the isolation of four-fold substituted o-carborane derivative 10 as well as the trisubstituted 11 was achieved in 18 and 59% yield, respectively. To further increase the yield of 10, mesitylene (b.p. 165 1C) and diglyme (b.p. 162 1C) were used as solvents however, the yield was lower (12% and 16%, respectively) probably due to other side reactions. Finally, the reaction in toluene, slightly increased the yield of 10 to 21 and 34% after 16 h and 5 days’ reaction time, respectively. The reaction in toluene was carried out under microwave irradiation (see in ESI†) as well, but a similar ratio of 10 and 11 (comparing the reaction in toluene after 16 h reflux) was observed after 2 h irradiation at 120 1C according to the 1 H NMR spectra of the crude product. It is to be emphasized that the by-product of 10 (compound 11)is suitable for recycling, thus the overall yield can be significantly improved. Crystals of 10 and 11 suitable for X-ray diffraction Fig. 2 Diagram 11 B{ 1 H}-NMR spectra with the peak assignments for the o-carborane, 1,2and 5derivatives. Scheme 2 Kumada cross coupling reactions on 2by using (i) PhCRCMgCl, (ii) CH 2 QCHCH 2 MgCl, in the presence of cis-[PdCl 2 (PPh 3 ) 2 ]andCuIas catalysts, in refluxing THF overnight. Fig. 3 Crystal structures of 9. Thermal ellipsoids have been drawn with 20% probability. Scheme 3 Kumada cross coupling reactions on 2in refluxing dioxane for 16 h. ChemComm Communication Open Access Article. Published on 25 June 2019. Downloaded on 9/4/2019 10:59:44 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online 8930 |Chem. Commun., 2019, 55, 8927--8930 This journal is ©The Royal Society of Chemistry 2019 (Fig. 4) were obtained using the vapour diffusion technique (pentane/acetone, see ESI†). The four-fold phenyl groups at the dense antipodal region of the cluster carbon atoms of the closo-o- carborane in 10 parallels the nanohybrid [Z 5 -C 60 Ph 5 ]  , 16a that made the extremely hydrophobic C 60 fullerene soluble in water. 16b A search at the Cambridge Structural Database 17 shows only seven hits for 9,12-(aryl) 2 -1,2-closo-C 2 B 10 B 10 , whose B9–B12 distances are in the range 1.781–1.824 Å (Table S1 in ESI†), which are comparable, although a bit longer than that in o-carborane (1.776 Å). 18 Although a certain plasticity at the B–B bonds antipodal to the C c atoms is observed, the effect is considerably smaller than the plasticity found for the adjacent C c atoms in the o-carborane cluster. This is not surprising as the antibonding orbital is mainly located between the adjacent C c atoms for these derivatives. 5 Compared to the electrondonation to the s*(C c –C c ), whose extent in terms of distance has been found experimentally at 2.156(4) Å in 1,2-(CR 2 F c ) 2 -1,2- closo-C 2 B 10 H 107 and computed at 2.64 Å for 1,2-(CH 2  ) 2 -1,2- C 2 B 10 H 10 , 5 the maximum elongation found in the antipodal region of 10 (1.822 A) or 11 (1.824) indicates a minor effect, near 0.048 Å. In agreement, the second-order perturbation theory analysis on the NBO basis reveals weak interactions (sum of them B12 kcal mol 1 ) between the aryl p-systems and the antibonding orbitals of the boron atoms, which was in good agreement with enlargement of the B–B distances in the cluster. This enforces the concept of the electron back donation to the s*(C c –C c )vs. the very much abused steric hindrance. The proven, although weak, influence of the aryl groups in the plasticity on the B9–B12 bonds and the possibility of substitution at the C c –H vertices foresee these o-carborane derivatives as appropriate synthons for surface functionalization; research in this direction is underway in our laboratories. This work has been supported by the Spanish Ministerio de Economı ´a y Competitividad (CTQ2016-75150-R), the Generalitat de Catalunya (2017SGR1720) and European Union’s Horizon 2020 Marie Skłodowska-Curie grant agreement MSCA-IF-2016- 751587. Conflicts of interest There are no conflicts to declare. Notes and references 1(a) R. N. Grimes, Carboranes, Elsevier Inc, New York, 3rd edn, 2016; (b) P. C. Andrews, R. J. Hardie and C. L. Raston, Coord. Chem. Rev, 1999, 189, 169–198; (c) J. F. Valliant, K. J. Guenther, A. S. King, P. Morel, P. Schaffer, O. O. Sogbein and K. A. Stephenson, Coord. Chem. Rev., 2002, 232, 173–230; (d) T. J. Wedg and M. F. Hawthorne, Coord. Chem. 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Thermal ellipsoids have been drawn with 20% probability. Communication ChemComm Open Access Article. Published on 25 June 2019. Downloaded on 9/4/2019 10:59:44 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online