Synthesis and Structures of Sterically Congested Diarylsilanes ...

Received: December 26, 2015 | Accepted: January 22, 2016 | Web Released: January 28, 2016

CL-151191

Synthesis and Structures of Sterically Congested Diarylsilanes Bearing Two Bulky Rind Groups Naoki Hayakawa,1,³ Tatsuto Morimoto,1,³ Akihiro Takagi,1 Tomoharu Tanikawa,1 Daisuke Hashizume,2 and Tsukasa Matsuo*1,3 1 Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502 2 Materials Characterization Support Unit, RIKEN Center for Emergent Matter Science, 2-1 Hirosawa, Wako, Saitama 351-0198 3 JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012 (E-mail: [email protected]) The sterically overcrowded diarylsilanes, (Rind)2SiH2 and (Rind)2SiBr2, with the two bulky Rind groups (Rind: 1,1,3,3,5,5,7,7-octa-R-substituted s-hydrindacen-4-yl) have been synthesized by the coupling reaction of the Rind-substituted monochlorosilanes, (Rind)SiH2Cl, with (Rind)Li followed by bromination of the Si­H bonds. The key intermediates, (Rind)SiH2Cl, have been obtained by the reactions of the trihydrosilanes, (Rind)SiH3, with trichloroisocyanuric acid. The structural features of (Rind)2SiH2 and (Rind)2SiBr2 have been fully characterized by NMR spectroscopy and X-ray diffraction studies. Keywords: Diarylsilane | Sterically overcrowded | Monochlorination

Kinetic stabilization using a sterically large substituent or ligand is an essential concept to explore a variety of coordinatively unsaturated compounds.1 In recent years, we have demonstrated that a series of rigid, fused-ring bulky 1,1,3,3,5,5,7,7-octa-R-substituted s-hydrindacen-4-yl groups, called the “Rind” groups (Chart 1), can act as unique protecting groups not only in the main group chemistry, but also in the transition-metal chemistry.2 Among them, in 2012, we reported the first isolation of germanone, (Eind)2Ge=O, with a planar three-coordinate germanium atom and a terminal oxygen atom, stabilized by the two bulky Eind groups (R1 = R2 = Et).3 The germanone was obtained by the treatment of a germylene, (Eind)2Ge:, with trimethylamine N-oxide as an oxygen source. Practically, the precursor, two-coordinate Ge(II) species, (Eind)2Ge:, can readily be prepared by the reaction of a commercially available dichlorogermylene dioxane complex, GeCl2¢dioxane, with 2 equiv of the bulky aryllithium, (Eind)Li. Following the successful achievement of the monomeric germanone via the stable germylene with the two bulky Eind groups,3 we have been interested in extending this chemistry to silicon species. However, it appears to be rather difficult to introduce two bulky substituents on the silicon atom,4 which is mainly due to the smaller covalent atomic radius of silicon [1.11(2) ¡] relative to that of the heavier elements; i.e., germanium [1.20(4) ¡], tin [1.39(4) ¡], and lead [1.46(5) ¡]5

Chart 1.

Chem. Lett. 2016, 45, 409–411 | doi:10.1246/cl.151191

and the unavailability of general dihalosilylene reagents.6 In this context of highly reactive halogen-substituted silylenes, we also reported in 2014 the reactions of the dibromodisilenes with N-heterocyclic carbenes (NHCs) to produce the mono-NHC adducts of the bromosilylenes and the bis-NHC adducts of the silyliumylidene cations.7,8 We now describe the rational design and synthesis of sterically congested diarylsilanes having two bulky Rind groups, i.e., the less bulky EMind (R1 = Et, R2 = Me) and the bulky Eind (R1 = R2 = Et) groups. We also report the structural characteristics of the resulting bulky organosilicon(IV) compounds. The synthetic route to the diaryldibromosilanes, (Rind)2SiBr2 (5), is outlined in Scheme 1. The Rind-substituted trihydrosilanes, (Rind)SiH3 (1),9 can be conveniently obtained by the reactions of (Rind)Li with (EtO)3SiH followed by reduction with lithium aluminum hydride (LAH) in good yields.9b,9c,10 Selective monohalogenation of one of the three Si­H bonds in 1 was not easy. For example, the treatment of 1 with N-bromosuccinimide (NBS) resulted in the formation of a mixture containing the corresponding monobromo, dibromo, and tribromo compounds. After several attempts using possible halogenation reagents, we have finally succeeded in the synthesis of monochlorosilanes, (Rind)SiH2Cl (2), by the action of trichloroisocyanuric acid (TCCA).11 After centrifugation to remove any insoluble materials, monochlorosilanes 2 were isolated as air- and moisture-sensitive colorless crystals in 84% (2a) and 91% (2b) yields. We have found that the use of hexane as a reaction solvent is crucial for the effective control of the monochlorination of 1 to prevent the over-chlorination; the reactions of 1 with TCCA in CH2Cl2 afforded a mixture of monochloro and dichloro compounds. Compounds 2 have been characterized by NMR spectroscopy (Table 1) and mass spec-

Scheme 1. Synthesis of diaryldibromosilanes 5.

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Table 1. NMR data (benzene-d6, 298 K) 1

Compounds 1a9 1b9 2a 2b 3a 3b 4a 4b 5a 5b

H(¤) (Si­H) /ppm 4.77 4.71 5.61 5.82 6.07 5.90 5.95 5.85 ® ®

1

J (29Si­1H) /Hz 199 198 220 233 196 186 217 216 ® ®

29

Si(¤) /ppm ¹73.4 ¹74.3 ¹29.3 ¹30.2 ¹57.1 ¹59.2 ¹39.5 ¹37.0 ¹18.5 ¹21.1a

a

THF-d8, 298 K.

trometry (DART-HRMS). The 1H signal for the Si­H groups appeared at 5.61 (2a) and 5.82 ppm (2b) with satellite signals due to the 29Si nuclei [1J(29Si­1H) = 220 (2a) and 233 Hz (2b)]. In the 29Si NMR spectra, one resonance was observed at ¹29.3 (2a) and ¹30.2 ppm (2b). We have next investigated the possibility to perform the second introduction of the bulky Rind group to the Rindbonded Si(IV) center. We previously reported that the bulky Eind-substituted trihydrosilane, (Eind)SiH3 (1b), smoothly reacted with 1-naphthyllithium and 2-naphthyllithium to afford (Eind)SiH2(1-Naph) and (Eind)SiH2(2-Naph), respectively.12 Trihydrosilanes 1, however, did not show any sign of reaction with (Rind)Li. Accordingly, we planned to introduce another approach for the synthesis of the desired diarylsilanes, (Rind)2SiH2 (3), using monochlorosilanes 2. The addition of a solution of the less bulky EMind-based 2a in THF to a solution of (EMind)Li in THF, which was prepared by the reaction of (EMind)Br with 2 equiv of t-BuLi in Et2O followed by solvent exchange with THF, produced the diarylsilane, (EMind)2SiH2 (3a), as the major product. After purification by silica gel column chromatography using hexane as the eluent, air- and moisture-stable colorless crystals of 3a were isolated in 54% yield by recrystallization from a mixed solvent of ethanol and hexane. When the solvent exchange of (EMind)Li was not fully completed in this reaction, a disiloxane, [(EMind)H2Si]2O (4a), was also formed as a side product, which may be due to the hydrolysis of unreacted 2a. Although similar solvent exchange conditions were applied for the coupling reaction between the bulkier Eind-based 2b and (Eind)Li, the formation of a complicated mixture containing (Eind)H, (Eind)2SiH2 (3b), and [(Eind)H2Si]2O (4b) was observed, presumably due to the steric hindrance between the bulkier Eind groups. Since it was difficult to isolate 3b as a pure form, the mixture of 3b and 4b was used for the subsequent bromination reaction without further purification. The formation of diarylsilanes 3 and disiloxanes 4 was deduced on the basis of the NMR spectroscopic data (Table 1), and their molecular structures were confirmed by a singlecrystal X-ray diffraction analysis, as shown in Figures 1 and 2, respectively. In the 1H NMR spectra of 3 at 298 K, only one set of resonances due to the peripheral ethyl groups and the proximate methyl or ethyl groups of the Rind groups was observed on the NMR time scale, indicating the free-rotation around the Si­C bonds. The Si­H signal was found at 6.07 (3a) 410 | Chem. Lett. 2016, 45, 409–411 | doi:10.1246/cl.151191

Figure 1. Molecular structures of 3a (left) and 3b (right). The thermal ellipsoids are shown at the 50% probability level. The hydrogen atoms, except for the Si­H groups, are omitted for clarity.

Figure 2. Molecular structure of 4b. The thermal ellipsoids are shown at the 50% probability level. The hydrogen atoms, except for the Si­H groups, are omitted for clarity. Selected bond lengths (¡) and bond angles (°): Si1­O1 = 1.708(7), Si1*­O1 = 1.561(8), Si1­O1­ Si1* = 161.8(2). Table 2. Structural parameters

Compounds 3a 3b 4b 5a 5b

Si­C/¡ 1.911(2), 1.897(3) 1.950(4), 1.938(5) 1.8948(14) 1.912(5), 1.929(5) 1.934(3)

C­Si­C/° 122.70(14) 123.60(17) ® 116.64(19) 116.60(13)

and 5.90 ppm (3b) along with satellite signals [1J(29Si­1H) = 196 (3a) and 186 Hz (3b)]. The 29Si NMR signal was observed at ¹57.1 (3a) and ¹59.2 ppm (3b).4 The major structural parameters for 3 are summarized in Table 2. The Si­C bond lengths of the bulkier Eind-based 3b [1.950(4) and 1.938(5) ¡] are longer than those of the less bulky EMind-based 3a [1.911(2) and 1.897(3) ¡], ascribed to the steric congestion around the silicon atom. The C­Si­C bond angle of 3b [123.60(17)°] is slightly larger than that of 3a [122.70(14)°]. The two hydrogen atoms on the silicon atom are located based on the difference Fourier maps and are isotropically refined, thus giving the H­Si­ H bond angle of 107.1(19)° for 3a and 97.7(18)° for 3b. The molecule of 4b has an inversion center at the midpoint of the two Si atoms (Figure 2), in which the central oxygen atom is disordered over the two positions. The two bulky Eind groups are parallel to each other in the crystal. The bromination of the Si­H bonds of 3 can be performed with no difficulty. Thus, diarylsilane 3a was treated with a sufficient amount of NBS in hexane to produce the corresponding

© 2016 The Chemical Society of Japan

from MEXT and Kinki University. We are grateful to the Materials Characterization Support Unit, RIKEN Center for Emergent Matter Science (CEMS) for the elemental analyses of the samples synthesized in this study. Supporting Information is available on http://dx.doi.org/ 10.1246/cl.151191.

Figure 3. Molecular structures of 5a (left) and 5b (right). The thermal ellipsoids are shown at the 50% probability level. The hydrogen atoms are omitted for clarity.

diaryldibromosilane, (EMind)2SiBr2 (5a), as colorless crystals in 84% yield. We have also examined the reaction of the mixture of 3b and 4b with NBS in hexane, from which (Eind)2SiBr2 (5b) was obtained as colorless crystals in 7% isolated yield for 2 steps. It is worth mentioning that the straightforward coupling reactions between the Rind-substituted tribromosilanes, (Rind)SiBr3,9b,9c and (Rind)Li met with failure; no evidence was obtained for the formation of 5. The NMR data of 5 indicate the hindered-rotation around the Si­C bonds. For example, in the 13C NMR spectra of 5 at 298 K, the 12 s-hydrindacene skeletal carbon signals separately appear. The molecular structures of 5a and 5b have been clearly determined by X-ray crystallography (Figure 3). The molecule of 5b possesses a C2 symmetry with the twofold axis passing through the central silicon atom. As found in the crystals of 3a and 3b, the Si­C bond lengths of the bulkier Eind-based 5b [1.934(3) ¡] are slightly longer than those of the less bulky EMind-based 5a [1.912(5) and 1.929(5) ¡] (Table 2). The C­Si­ C bond angle of 5 [116.64(19)° for 5a and 116.60(13)° for 5b] is much smaller than that of 3 [122.70(14)° for 3a and 123.60(17)° for 3b], because of the steric repulsion from the additional bromine atoms. The Br­Si­Br bond angle of 5a [99.49(5)°] is somewhat larger than that of 5b [97.29(5)°], thus depending on the steric bulkiness of the Rind substituents. In conclusion, we have shown the stepwise introduction of the two bulky Rind groups (Rind = EMind and Eind) on the Si(IV) center to produce the sterically overcrowded diarylsilanes. Trichloroisocyanuric acid (TCCA) has been found to be extraordinarily useful for the selective monochlorination of trihydrosilanes 1. The reactions of monochlorosilanes 2 with (Rind)Li provided the long-sought after diarylsilanes 3 incorporating the two bulky Rind groups. The two kinds of stericallycongested molecules of the diaryldibromosilanes, the EMindbased 5a and the Eind-based 5b, are now available. Further studies on the reduction of 5 with various reducing agents are currently underway and will be reported elsewhere. We thank the Ministry of Education, Culture, Sports, Science and Technology of Japan for the Grant-in-Aid for Scientific Research on Innovative Areas, “Stimuli-responsive Chemical Species for the Creation of Functional Molecules” [#2408] (No. 24109003) and Scientific Research (B) (Nos. 24350031 and 15H03788). This work was also partially supported by MEXT-Supported Program for the Strategic Research Foundation at Private Universities 2014­2018, subsidy

Chem. Lett. 2016, 45, 409–411 | doi:10.1246/cl.151191

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