N-Heterocyclic Silylene Coordinated Dialkyl Borenium Equivalent

In an attempt to prepare the heavier analogue of NHC-stabilized borenium cation, we have prepared a silylene-coordinated borenium equivalent (1a), whi...
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N‑Heterocyclic Silylene Coordinated Dialkyl Borenium Equivalent Hsiu-Chen Tsai,† Ya-Fan Lin,‡ Wei-Chun Liu,† Gene-Hsiang Lee,† Shie-Ming Peng,† and Ching-Wen Chiu*,† †

Department of Chemistry, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan Department of Fragrance and Cosmetic Science, Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, No. 100, Shih-Chuan First Road, Kaohsiung, 80708 Taiwan

‡

S Supporting Information *

ABSTRACT: In an attempt to prepare the heavier analogue of NHC-stabilized borenium cation, we have prepared a silylenecoordinated borenium equivalent (1a), which can be viewed as a contact ion pair of borenium and triflate. The triflate anion coordinates to the oxophilic silicon center, leaving the boron center existing in a trigonal-planar geometry in the solid state and in solution. Nucleophilic substitution reactions of 1a all result in the release of free silylene ligand, demonstrating the borenium-ion-like reactivity of 1a. Chemical reduction of 1a leads to a neutral radical possessing a partial Si−B double bond.

Due to the less effective π stabilization of the Si−N bond, NHSi is markedly more π acidic than NHC. The introduction of NHSi to a borenium cation is expected to increase the Lewis acidity of the resulting cationic molecule. In addition, the silylene-coordinated borenium ion ([A]+ in Scheme 2) can be alternatively described as an α-boryl silylium ion ([B]+ in Scheme 2), which features a π-acidic boryl group adjacent to the highly electrophilic silylium center.37−39 To address the relative Lewis acidity of the boron and silicon center in the NHSi-borenium/boryl-silylium cation, hydride ion affinity (HIA)16 and chloride ion affinity (CIA)40 of a model compound were calculated with density functional theory (DFT). The optimized geometry of the B-NHSi-9-borabicyclo[3.3.1]nonane cation ([NHSi-9BBN]+) possesses a planar geometry with a Si−B bond distance of 204.9 pm (Figure 1). Natural population analysis (NPA) of [NHSi-9BBN]+ showed that the silicon center (+1.543) is actually bearing more positive charge than the boron atom (+0.58), which is consistent with the electropositive nature of silicon. Interestingly, the relative Lewis acidities of the boron and silicon centers of [NHSi-9BBN]+ were found to be dependent on the nucleophile. The HIAs of −43.4 and −37.8 kcal/mol for boronand silicon-centered reactions suggests that [NHSi-9BBN]+ functions as a boron-based electrophile for hydride addition. However, the CIA calculations (23.8 (B) vs 19.2 kcal/mol (Si)) support the description of [NHSi-9BBN]+ as a silicon-based electrophile in reaction with chloride (Table S3 in the Supporting Information). In other words, both boron and

B

orenium cation,1,2 the isoelectronic analogue of carbenium ion, has received considerable attention in the past few years due to its promising application in the activation of H− H,3−6 electrophilic borylation,7−12 and catalysis.13,14 The introduction of positive charge significantly enhances the electron deficiency and Lewis acidity of the tricoordinated boron center.15,16 In addition to the well-documented amineand pyridine-coordinated borenium ions, phosphine,17−19 Nheterocyclic olefin (NHO),20−22 and carbene have also been incorporated into boron cations.23−30 Interestingly, while the carbene-stabilized borenium ions have been proven to be effective in hydrogen activation, the introduction of the heavier group 14 congeners of carbene to boron cations has received much less attention. Highly reactive diarylsilylene was found to undergo B−E (E = B, H, Cl) bond insertions with various boranes.31−33 Metzler and Braunschweig have also reported similar reactions for stable N-heterocyclic silylenes (NHSi) (Scheme 1).34−36 The coordination of stable NHSi to B(C6F5)3 yielded a Lewis acid−base adduct that gradually transformed into a silyl borane via migration of the C6F5 group from boron to silicon. The B−Cl bond insertion was observed when NHSi was allowed to react with haloboranes. With the less sterically demanding PhBCl2, further expansion of the NHSi ring to a sixmembered silaborinine ring was obtained. In attempt to expand the ligand library of borenium cation, we decided to embark on the synthetic challenge of the silylene-stabilized borenium cation. Although the cationic species could not be generated due to the oxophilicity of silicon, reactivity investigations of 1a reveals the borenium-ion-like reaction pattern of the neutral molecule. In addition, chemical reduction of 1a also led to the observation of a neutral radical featuring an odd-electron π bond between boron and silicon. © XXXX American Chemical Society

Received: September 8, 2017

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DOI: 10.1021/acs.organomet.7b00687 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics

sharp signal at −29 ppm, which is indicative of the coordination of triflate anion at the silicon center in solution. The molecular structure of 1a was further verified with a single-crystal X-ray diffraction analysis. As shown in Figure 2, the boron center

Scheme 1. Reaction between N-Heterocyclic Silylene and Boranes

Figure 2. Molecular structure of 1a. Hydrogen atoms are omitted for clarity. Selected bond distances (in pm) and angles (in deg): Si(1)− O(1) 178.20(12), Si(1)−B(1) 200.92(19), Si(1)−N(2) 171.24(13), Si(1)−N(1) 171.39(14), C(11)−B(1) 155.7(2), B(1)−C(15) 156.2(2); N(2)−Si(1)−N(1) 93.73(7) O(1)−Si(1)−B(1) 100.07(7), N(2)−Si(1)−B(1) 121.71(7), N(1)−Si(1)−B(1) 121.11(7).

features a trigonal-planar geometry with a B−Si bond distance of 200.91(19) pm, which is comparable to the value reported by Braunschweig.35 The coordination of triflate to the silicon center with a Si−O distance of 178.20(12) pm confirms the tetrahedral geometry at the silicon center. In an attempt to diminish the cation−anion interaction, we have attempted the replacement of triflate with triflimide anion. Unlike the relatively stable, isolable 1a, 1b could only be generated from the reaction of NHSi and 9-BBN-NTf2 in hexane inside a J. Young NMR tube. A broad 11B NMR resonance centered at 94 ppm reminiscent of that for 1a is assigned to the anticipated triflimide derivative (1b). However, no 29Si NMR signal was observed for 1b. In addition, exposure of 1b to C6D6 led to rapid decomposition of the compound into a complicated mixture. Thus, no further reactivity investigation of 1b was performed. The coordination of NHSi to the boron center was found to be weak and reversible in 1a (Scheme 3). Upon addition of 1 equiv of MesLi to a C6D6 solution of 1a, the release of free NHSi was immediately identified in the 1H NMR spectrum of the reaction mixture (Figure S7 in the Supporting Informa-

Scheme 2. Canonical Structures of Silylene-Stabilized Borenium Cation

Figure 1. DFT optimized structure of [NHSi-9BBN]+.

Scheme 3. Substitution Reactions of 1a +

silicon atoms of [NHSi-9BBN] are susceptible to nucleophilic attack, and the regioselectivity of the addition reaction is governed by the relative bond strengths of the B−E and Si−E bonds. Fascinated by the theoretical predictions, we then carried out the synthesis of the silylene-coordinated borenium cation. Compound 1a could be readily obtained from the reaction of NHSi and 9-BBN-OTf in C6D6. Upon addition of NHSi, the 11 B NMR signal of 9-BBN-OTf shifted from 66 to 91 ppm. The detected 11B resonance is shifted 7 ppm downfield in comparison to that of the NHC-stabilized analogue (83.8 ppm), suggesting the exceptional electron-deficient nature of the boron center in 1a. The 29Si spectrum of 1a contains a B

DOI: 10.1021/acs.organomet.7b00687 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics tion). The reaction with MesLi cleanly transformed 1a to Bmesityl 9-BBN (2), which features a broad 11B NMR signal at 87 ppm. The introduction of neutral Lewis bases also results in the liberation of silylene (Figure S10 in the Supporting Information). Addition of 4-(dimethylamino)pyridine (DMAP) to 1a led to the formation of a boronium cation coordinated to two DMAP groups ([3][OTf]). A similar ligand replacement reaction was observed when 1a was treated with bulky NHO to afford the corresponding NHO-coordinated borenium cation ([4][OTf])(Figure S14 in the Supporting Information). As indicated in the work of Rivard, the coordinating ability of neutral ligands to borane follows the order NHC > DMAP > NHO.41 The substitution of NHSi with NHO shows that the coordinating ability of NHSi is the worst in the series. The NHSi-boron interaction is surprisingly weak, such that even an ester compound can replace the coordinated silylene (Figure S18 in the Supporting Information). In an attempt to test the catalytic ability of 1a in the ring-opening polymerization of ε-caprolactone, 1a was mixed with 25 equiv of εcaprolactone in C6D6. Once again, the release of free silylene was observed, and no polymerization of lactone was achieved. We reasoned that the boron−silicon interaction in 1a might be strengthened through population of the B−Si π-bonding orbital. Although molecules featuring a BSi double bond are rare,32,33,42 structural characterization of silaborene and its adduct has been accomplished a few times.43,44 As shown in Scheme 4, reduction of 1a with Cp*2Co in hexane at −78 °C

NHSi ring (N, 22%; C, 4%; H, 4%). The higher spin density on boron supports the description of 1a as a contact ion pair of silylene-coordinated borenium ion ([A]+). Unfortunately, such a partial B−Si π bond is still not strong enough to sustain a persistent radical. 5• underwent rapid decomposition at room temperature to free silylene and 9-BBN dimer, which might be the result of hydrogen atom abstraction of the 9-BBN radical from the environment. In summary, the reaction of free NHSi with 9-BBN-OTf results in a neutral Lewis adduct that reacts like a borenium cation. However, the weak silylene−boron interaction was found to be problematic in keeping the silylene in the coordination sphere of boron. Interestingly, one-electron reduction of 1a leads to the detection of a neutral radical that features a one-electron π bond between silicon and boron. To avoid the dissociation of silylene, we are currently examining molecules featuring more strongly σ donating silylene ligands or more electron deficient boron centers.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00687. Experimental details on the synthesis and characterization of 1a,b, NMR spectra of 1a,b and reactions of 1a, and theoretical calculations (PDF) XYZ coordinates of all optimized structures (XYZ)

Scheme 4. Chemical Reduction of 1a

Accession Codes

CCDC 1573407 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

resulted in the detection of an EPR signal centered at g = 2.0034 (Figure 3a). The signal can be simulated with hyperfine

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AUTHOR INFORMATION

Corresponding Author

*E-mail for C-W.C.: [email protected]. ORCID

Ching-Wen Chiu: 0000-0001-7201-0943 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of Taiwan (Grant MOST 104-2113-M-002-018MY3 and 105-2119-M-002-031-MY2) and National Taiwan University (NTU-CESRP-104R7619).

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Figure 3. (a) Experimental and simulated EPR spectra of 5 . (b) Calculated spin density distribution (isovalue 0.002) on the optimized structure of 5•.

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coupling constants of 15.8, 15.8, and 17.5/5.2 MHz to 1H, 14N, and 11B/10B nuclei, respectively. The coupling of the unpaired electron to one boron atom, two nitrogen atoms, and two hydrogen atoms is consistent with the description of 5• as a neutral radical having a one-electron π bond between boron and silicon. The optimized geometry of 5• features a B−Si bond of 192.8 pm, which is shorter than that of its cationic counterpart (204.9 pm in [NHSi-9BBN]+). This result is consistent with the partial π-bonding character of the B−Si bond in 5•. The calculated spin density of 5• is mainly localized in the B−Si π-bonding orbital (B, 52%; Si, 18%) with noticeable delocalization to the

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DOI: 10.1021/acs.organomet.7b00687 Organometallics XXXX, XXX, XXX−XXX