Intramolecular Cyclopropanation of Alkali-Metal-Substituted

Jul 11, 2019 - constitute an important class of compounds that have been widely studied.1a,b ... The SiN2C2 ring in the structure of 3 (Figure 1) is p...
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Intramolecular Cyclopropanation of Alkali-Metal-Substituted Silylene with the Aryl Substituent of an N‑Heterocyclic Framework Lizhao Zhu, Jianying Zhang, and Chunming Cui* State Key Laboratory of Elemento-Organic Chemistry and College of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China

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S Supporting Information *

carbenes and silylenes could cleanly react with alkynes and alkenes to give strained three-membered-ring systems,1a,b,5 while cyclopropanation of carbenes with an aromatic CC bond and subsequent insertion and ring expansion (Buchner reaction) to form cycloheptatrienes is well-known under photochemical and catalytic conditions,6 the same reaction of silylenes with aromatic systems has only been reported for an excited dialkylsilylene under photochemical conditions and a transient diarylsilylene generated by thermolysis of the corresponding disilene.7 Closely related reactions involving the insertion of a silylene into an aromatic CC bond to form silepins were only known for the excited dialkylsilylene and an acyclic iminosilylene with a small highest occupied molecular orbital−lowest unoccupied molecular orbital gap, as reported by Kira et al. and Inoue et al.7,8 Therefore, the design and synthesis of new types of silylenes with novel electronic structures, in particular with a triplet ground state, are of considerable interest for the discovery of new chemistry of silylenes. Herein, we report the synthesis, structure, and reactivity of the first silanorcaradienyllithium, which was presumably formed by intramolecular cyclopropanation of a lithium-substituted borylsilylene with the aromatic CC bond of an N-heterocyclic framework. Upon electrophilic substitution of the lithium with Me and H groups, the silanorcaradiene ring underwent insertion and ring expansion to give silepins. Density functional theory (DFT) calculations disclosed the triplet ground state of the lithium-substituted borylsilyene with a relatively large singlet−triplet energy gap. N-heterocyclic carbenes and the heavy group 14 analogues constitute an important class of compounds that have been widely studied.1a,b,2b,9 We are particularly interested in the divalent silicon species with bulky groups because they might be very useful for the stabilization of low-coordinate complexes or multiple-bonding species. Thus, the ethylenediamine [HNB(NArCH)2CH2]2 (1; Ar = 2,6-iPr2C6H3) with a bulky 1,3,2-diazaborolyl substituent on the nitrogen atoms was synthesized. Deprotonation of 1 with n-BuLi followed by treatment with SiCl4 led to formation of the N-heterocyclic dichlorosilane 2 in good yield (Scheme S1). Reduction of 2 with 2 equiv of lithium in the presence of 5 mol % naphthalene (Scheme S1) in tetrahydrofuran (THF) at room temperature afforded the NHSi Si[NB(NArCH)2CH2]2 (3) as colorless crystals in 66% yield. Silylene 3 is probably the most sterically demanding NHSi reported so far.2b,9b,c The 29Si NMR

ABSTRACT: Reaction of the bulky N-heterocyclic silylene Si[NB(NArCH)2CH2]2 (3; Ar = 2,6-iPr2C6H3) with BCl3 resulted in ring expansion with formation of the six-membered C2N2SiB heterocycle 4. Reduction of 4 with lithium afforded the first silanorcaradienyllithium 5, which presumably formed by intramolecular cyclopropanation of a lithium-substituted borylsilylene intermediate with one of the aryl CC bonds. Treatment of 5 with MeOTf and Et3NHCl led to electrophilic substitution accompanied by the SiC2 ring expansion to give silacycloheptatrienes (silepins) 6 and 7, respectively. The addition of 1,3,4,5-tetramethylimidazol-2-ylidene (NHCMe) to silepin 7 yielded the NHC-stabilized hydrosilylene 8 with the silicon atom free from the silepin ring. The formation of 5 could be attributed to the unique electronic structure of the lithium-substituted silylene, which has been predicted to be in a triplet ground state with a relatively large singlet−triplet energy gap by theoretical calculations.

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ilylenes (R2Si), silicon analogues of carbenes, have attracted a great deal of attention because of their interesting electronic structures and chemistry in comparison with carbenes.1 Since seminal work on the synthesis of SiCp*2 (Cp* = C5Me5) and the N-heterocyclic silylene (NHSi) Si(NButCH)2,2 a number of stable silylenes have been isolated. In contrast to the rich chemistry of carbenes, parallel studies on silylenes has remained very limited. So far, there has been very limited success in the chemistry of triplet silylenes. Theoretical calculations predicted that silylenes with electropositive substituents could be ground-state triplets.3 Experimental and theoretical studies on a couple of bulky bis(silyl)silylenes have shown that they may possess triplet ground states, while the alkali-metal-substituted silylsilylenes M-SiSiBut3 (M = Li, K), generated by photoextrusion and predicted to be ground-state triplets, have only been studied by electron paramagnetic resonance experiments in the temperature range from 14 to 50 K.4 We are particularly interested in alkali-metal-substituted silylenes because of not only their unique electronic structure but also their potential for the synthesis of low-valent silicon species via electrophilic substitution. With regard to reaction chemistry, silylenes may exhibit some distinct features from carbenes in the activation of unsaturated molecules. In contrast to the fact that both © XXXX American Chemical Society

Received: July 11, 2019

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DOI: 10.1021/acs.inorgchem.9b02069 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry spectrum of silylene 3 exhibited one singlet at 137.8 ppm, which was downfield-shifted compared to that reported for the heterocyclic silylene Si(NButCH2)2 (119 ppm).9b The structure of 3 was confirmed by single-crystal X-ray analysis. The SiN2C2 ring in the structure of 3 (Figure 1) is puckered

with a N1−Si1−N2 angle of 91.82(7)°. The two Si−N bond lengths of 1.7313 and 1.7397(15) Å are slightly longer than those in the silylene Si(NButCH2)2 (1.72 Å), whereas the C1− C2 bond distance of 1.509(3) Å is slightly shorter than that in Si(NButCH2)2 (1.520 Å).9b With silylene 3 in hand, we are interested in the exploration of its reaction with boron halides with the expectation to generate low-coordinate boron species.10 The reactions of silylenes with organoboranes such as B(C6F5)3, RBX2 and substituted diboranes were reported previously.11,12 Stirring a mixture of 3 and BCl3 in n-hexane from low to room temperature for 24 h led to formation of the ring-expansion product 4 in good yield (Scheme 1). 4 is obviously formed via

The formation of 5 by the reduction of 4 involved the unprecedented ring contraction of the six-membered C2N2SiB to the five-membered C2N2B system while maintaining the B− Si bond. It is assumed that the reduction of 4 may generate a highly reactive SiB double-bond intermediate in the cycle, which might isomerize to the stable five-membered C2N2B ring, as outlined in 5. Structural analysis of 5 (Figure 1) disclosed that the anionic silicon atom is bonded to one of the aryl CC bonds to form a silanorcaradienyllithium structure. The Si1−C31 and Si1−C36 bond lengths of 1.982 and 1.968(3) Å are noticeably longer than those observed in silacyclopropanes (1.868−1.898 Å).7a,c,d It can be reasonably envisioned that the silanorcaradienyllithium 5 was formed by intramolecular cyclopropanation of the lithiosilylene A (Scheme S2) with one of the aromatic C C bonds. This prompted us to investigate the electronic structure of A theoretically. DFT calculations [B3LYP/631G(d,p)] disclosed that the B−Si−Li angle is bent (128.97°) in the optimized structure of singlet A, while it is almost linear (178.74°) in the triplet A (Figures S5 and S6). Furthermore, the distance of Si−Li in the triplet A (2.488 Å) was noticeably shorter than that in the singlet A (2.604 Å). Energy analysis indicated that the triplet A was a ground state with a singlet− triplet energy gap (ΔES−T) of 20.23 kcal/mol. Facile intramolecular cyclopanation of A might result from the high reactivity of the intermediate A. The large ΔES−T value calculated for A compared to the relatively small ΔES−T of 9.4 kcal/mol calculated for But3SiSiLi suggested the significant effects of the boryl substituent on the electronic structure of the silylenes.4c,14 The synthesis of 5 provided a unique platform for studies of the effects of the silicon substituent on the structures of the cyclopropanation adducts by substitution reaction with suitable electrophiles. Reactions of 5 with MeOTf and Et3NHCl in n-hexane (Scheme 2) at room temperature

Scheme 1. Reaction of Silylene 3 with BCl3 and the Subsequent Reduction Reaction

Scheme 2. Reactions of 5 with Electrophiles and Reaction of 7 with NHCMe

the insertion of a BCl unit into one of the Si−N bonds accompanied by the migration of two chlorides from the boron atom to the silicon atom and a ring-expansion sequence. This type of ring expansion has been previously observed in the reaction of the silylene Si[NXylCH]2 (Xyl = 2,6-Me2C6H3) with PhBX2 by Braunschweig et al.11b The multiple nuclear NMR analysis of 4 is consistent with that of the six-membered C2N2SiB structure. Unfortunately, compound 4 is not stable and slowly decomposed in 24 h at room temperature. Chemical reduction of 4 with 4 equiv of lithium in THF at −78 °C for 2 days afforded the N-heterocyclic borylsubstituted silanorcaradienyllithium 5 as a red powder in 91% yield. Its 29Si NMR spectrum displayed a broad signal at −161.5 ppm, close to that for silacyclopropenyllithium (−153.9 ppm).13

afforded silacycloheptatrienes 6 and 7 in good yield, respectively. In both cases, SiC2 ring expansion to form the seven-membered ring occurred. The 29Si NMR spectra of 6 and 7 displayed one signal each at −40.1 and −56.1 ppm (1JSi−H = 206.2 Hz), respectively. Yellow crystals of 6 were obtained from n-hexane at 0 °C. X-ray crystal structural analysis confirmed the silacycloheptatriene structure of 6 (Figure 2). In order to elucidate the factors that control the ringexpansion reaction, the relative energies of lithium- and hydrogen-substituted silanorcaradiene (C) and silepin (D) structures were calculated at the B3LYP/6-31G(d,p) level (Figure S7). The relative energies of the transition states for mutual transformation in both cases are predicted to be very small (E = Li(THF)2, 11.0 kcal/mol; E = H, 9.2 kcal/mol). However, the lithium-substituted silanorcaradiene C is predicted to be more stable than the corresponding silepin

Figure 1. ORTEP drawings of 3 (left) and 5 (right) with 20% ellipsoid probability. Hydrogen atoms have been omitted for clarity. Selected bond parameters are given in Figures S1 and S2.

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DOI: 10.1021/acs.inorgchem.9b02069 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

ligands for the synthesis of triplet silylenes are currently being undertaken by our group.

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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.inorgchem.9b02069. Experimental procedures for the synthesis of compounds 1−8, X-ray crystallographic and computational data, Schemes 1 and 2, ORTEP drawings, optimized structures, and relative energy profiles (PDF)

Figure 2. ORTEP drawings of 6 (left) and 8 (right) with 20% ellipsoid probability. The hydrogen atoms except that of 8 on the silicon atom have been omitted for clarity. Selected bond parameters are given in Figures S3 and S4.

Accession Codes

CCDC 1923327, 1923358, 1923366, and 1923369 contain 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 [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

D by 5.9 kcal/mol, whereas the hydrogen-substituted silepin D is more stable than the silanorcaradiene C by 4.9 kcal/mol. These results indicated that the nature of the substituents has a significant effect on the relative stability of the C (SiC2) and D (SiC6) rings and their mutual transformation is thermodynamically controlled by their relative stabilities. Cycloaddition of a carbene with an aromatic CC bond could be reversible in some cases depending on the substituents.15 Thus, the structures of compounds 5−7 in solution have been studied by variable-temperature NMR studies (20−80 °C). However, a similar reversible process was not observed in these cases. Inspired by the work of Müller et al. for the synthesis of donor-stabilized silylenes by the reaction of 7-silanorbornadienes with strong nucleophiles,16 we investigated the reactions of the silanorcaradienyllithium 5 and silepins 6 and 7 with an N-heterocyclic carbene. The addition of 1 equiv of 1,3,4,5-tetramethylimidazol-2-ylidene (NHCMe) to 5 led to a complicated mixture, from which we were unable to isolate any pure product, while 6 was inert to NHCMe. To our delight, the reaction of 7 with NHCMe at room temperature cleanly yielded the NHC-stabilized hydrosilylene 8 in high yield. There are only a few examples of Lewis base-stabilized hydrosilylenes that have been reported.16,17 The 29 Si NMR spectrum of 8 showed one signal at −122.1 ppm, which falls in the range (−137.8 to −80.5 ppm) for NHCstabilized hydrosilylenes. The small Si−H coupling constant of 1 JSi−H = 108.2 Hz for 8 is characteristic of a divalent silicon species. The structure of 8 has been confirmed by X-ray diffraction studies and is shown in Figure 2. The divalent silicon atom in 8 is three-coordinate with a pyramidal geometry (the sum of the bond angles around the silicon atom is 321.1°). The B1−Si1 bond length of 2.014(5) Å is comparable to those determined for 5 and 6 [2.011(3) and 2.027(2) Å]. In summary, we have synthesized a sterically demanding NHSi, which reacted with BCl3 to give a ring-expansion product. Reduction of the product yielded a lithiumsubstituted silanorcaradiene in good yield. Reaction of the silanorcaradiene with MeOTf and Et3NHCl led to ring expansion to give silepins. The NHC-stabilized boryl hydrosilylene 8 was obtained by extrusion of the silicon atom from the SiC6 seven-membered silepin ring in the presence of a heterocyclic carbene. These reaction sequences and theoretical calculations disclosed the triplet ground state of the lithiumsubstituted borylsilylene. The reactivity of the lithiumsubstituted silanorcaradiene and the design of bulky boryl

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.C.). ORCID

Chunming Cui: 0000-0002-1493-7596 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (Grant 21632006) for their generous support. DEDICATION This work is dedicated to the 100th anniversary of Nankai University. REFERENCES

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DOI: 10.1021/acs.inorgchem.9b02069 Inorg. Chem. XXXX, XXX, XXX−XXX