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A Heavy Analogue of the Smallest Bridgehead Alkene Stabilized by a Base

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A Heavy Analogue of the Smallest Bridgehead Alkene Stabilized by a Base ARTICLE Received 18 Jun 2014 | Accepted 22 Sep 2014 | Published 4 Nov 2014 DOI: 10.1038/ncomms6353 A heavy analogue of the smallest bridgehead alkene stabilized by a base Takeaki Iwamoto1, Naohiko Akasaka1 & Shintaro Ishida1 Bicyclo[1.1.0]but-1(2)-ene (BBE), one of the smallest bridgehead alkenes and C4H4 isomers, exists theoretically as a reactive intermediate, but has not been observed experimentally. Here we successfully synthesize the silicon analogue of BBE, tetrasilabicyclo[1.1.0]but-1(2)- ene (Si4BBE), in a base-stabilized form. The results of X-ray diffraction analysis and theoretical study indicate that Si4BBE predominantly exists as a zwitterionic structure involving a tetrasilahomocyclopropenylium cation and a silyl anion rather than a bicyclic structure with a localized highly strained double bond. The reaction of base-stabilized Si4BBE with triphenylborane affords the [2 þ 2] cycloadduct of Si4BBE and the dimer of an isomer of Si4BBE, tetrasilabicyclo[1.1.0]butan-2-ylidene (Si4BBY). The facile isomerization between Si4BBE and Si4BBY is supported by theoretical calculations and trapping reactions. Structure and properties of a heavy analogue of the smallest bridgehead alkene are disclosed. 1 Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aramakiazaaoba, Aoba-ku, Sendai 980-8578, Japan. Correspondence and requests for materials should be addressed to T.I. (email: [email protected]). NATURE COMMUNICATIONS | 5:5353 | DOI: 10.1038/ncomms6353 | www.nature.com/naturecommunications 1 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6353 ridgehead alkenes have a strained doubly bonded carbon at base coordination. Their structures and properties have the bridgehead positions in the bicyclic skeleton. Their contributed to further understanding of the bonding and Bsyntheses, highly strained structures and high reactivities structures of organic and heavy main group element p-electron 19 have widely attracted the attention of both theoretical and compounds. Among the Si4R4 species, tetrasilatetrahedrane and experimental chemists since the pioneering work of Bredt1–5. tetrasilacylobutadiene13 have been isolated. However, the tetrasila Bicyclo[1.1.0]but-1(2)-ene (BBE, E ¼ C, Fig. 1a) is a bridgehead version of BBE (Si4BBE, E ¼ Si, Fig. 1a) has been only alkene with the smallest number of bridge atoms and several theoretically investigated as one of the isomers among the Si4R4 possible canonical structures containing a strained double bond2, molecules20,21, whereas it’s detailed molecular and electronic even though BBE has been considered to be an exceptional structures and reactivity remain elusive. bridgehead alkene (‘zero-bridge alkene’) because of the lack of a In this study we report the synthesis of Si4BBE 1, which was twisted double-bond geometry observed in typical bridgehead stabilized by coordinating to 4-(N,N-dimethylamino)pyridine alkenes. Although BBE and its derivatives have been proposed as (DMAP) (1ÁDMAP). The results of X-ray diffraction (XRD) 6 reactive intermediates and theoretically investigated as the analysis and theoretical study indicate that Si4BBE predominantly isomers of C4H4 molecules present in important fundamental exists as a zwitterionic structure containing a tetrasilahomocy- organic molecules such as tetrahedrane and cyclobutadiene7–11, clopropenylium cation22 and a silyl anion, rather than a bicyclic the synthesis and experimental observation of BBE and its structure with a localized double bond. The elimination of DMAP derivatives have not been reported yet. from 1ÁDMAP with BPh3 afforded the [2 þ 2] cycloadduct of Recently, the silicon analogues of alkenes and structurally 1 (4) and the dimer of tetrasilabicyclo[1.1.0]butan-2-ylidene 0 related unsaturated organic compounds that have never been (Si4BBY) 1 (8), whose parent molecule was predicted as the 20,21 experimentally observed or synthesized previously such as global minimum on a Si4H4 surface . These results indicate a pentasilaspiropentadiene12, tetrasilacyclobutadiene with a planar reversible valence isomerization between 1 and 10, which was rhombic charge-separated structure13, trisilavinylcarbene14, supported by the thermal isomerization of 8 to 4 and reactions of 15 silavinylidene , Si(0)2 (ref. 16) and isomers of 8 with SiCl4 and DMAP. Thus, the synthesis, structure, bonding hexasilabenzene17,18 have been synthesized as isolable molecules and reactivity of a heavy analogue of the smallest bridgehead by taking advantage of kinetic stabilization and/or stabilization by alkenes are reported. R R R R E E E E + R E R E 2 ER 2 ER R2E ER R2EER+ etc. E E E E IIIIII IV Bicyclo[1.1.0]but-1(2)-ene (BBE, E = C) Tetrasilabicyclo[1.1.0]but-1(2)-ene (Si4BBE, E = Si) R t-Bu t-Bu t-Bu t-Bu R R R R R R Si R Si Si t-Bu SiCl R Si R R 4 40 °C, 10 h Si Si Si Si + R Si Si Si Si benzene Si Si Si Si R K R R R R R R t-Bu R R R R R t-Bu NNMe 322 45 (DMAP) RR H RRt-Bu 40 °C, 5h, RRt-Bu R t-Bu Si benzene Si R Si Si t-Bu Si Si Si + Si Si Si DMAP Me2N R R R R RR N RR 1 6 1•DMAP 7 RRt-Bu t-Bu RR BPh3 Si Si 1•DMAP 4 + SiSi Si Si –DMAP•BPh3 Si Si R R t-Bu t-Bu RR Δ 8 t-Bu R R t-Bu RR SiCl Si DMAP SiCl Si 3 4 Si Si 1 Si Si 1•DMAP 8 Si Si CI R R R R t-Bu t-Bu 1' 9 Figure 1 | Synthesis and reactions of a silicon analogue of the smallest bridgehead alkene. (a) BBE and its silicon analogue (Si4BBE). (b) Synthesis of DMAP-stabilized Si4BEE (1?DMAP) (R ¼ SiMe3) and the dimer of Si4BBE (4). (c) Reaction of 1?DMAP with triphenylborane (R ¼ SiMe3). (d) A possible 0 equilibrium between Si4BBE 1 and tetrasilabicyclo[1.1.0]butan-2-ylidene (Si4BBY) 1 (R ¼ SiMe3). (e) Reactions of disilene 8 supporting generation 0 of 1 and 1 (R ¼ SiMe3). 2 NATURE COMMUNICATIONS | 5:5353 | DOI: 10.1038/ncomms6353 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6353 ARTICLE H H Results (t-Bu)Si and R 2Si nuclei (R 2 ¼ 1,1,4,4-tetrakis(trimethylsilyl) 1,3 Thermolysis of hexaalkyltricyclo[2.1.0.0 ]pentasilane.Si4BBE butan-1,4-diyl) are d À 26.8, ( À 18.4 and À 5.5) and 24.6 p.p.m., 1 was initially suggested to be generated by the thermolysis of respectively. As 5 is a thermal isomerization product of hexaalkyltricyclo[2.1.0.01,3]pentasilane 2, which was prepared in dialkylsilylene 6 (ref. 25), the generation of 1 and 6 was 63% yield by the reaction of the corresponding potassium indicated by the thermolysis of 2. trialkyldisilenide 3 (ref. 23) with 0.5 equiv. of SiCl4 (Figs 1b and 2a; for the detailed molecular structure of 2, see Synthesis of 1 . DMAP. To obtain monomeric Si BBE 1,we Supplementary Information), similar to the synthesis of 4 investigated the thermolysis reaction in the presence of a Lewis Scheschkewitz’s hexaaryl derivative Tip Si 20 (Tip ¼ 2,4,6- 6 5 base, because coordinating to a Lewis base may stabilize Si BBE 1, triisopropylphenyl)24. Compound 2 was isolated as reddish 4 similar to other unsaturated silicon compounds14–16,26. The purple crystals at ambient temperature. However, heating 2 at reaction of 2 in the presence of 7.6 equiv. of DMAP, which is a 40 °C for 10 h in benzene afforded a mixture of typical Lewis base for unsaturated silicon compounds, at 40 °C for pentacyclo[5.1.0.01,6.02,5.03,5]octasilane 4 and silene 5 (Fig. 1b), 5 h afforded an orange solution. The 1H NMR spectrum of the whereas Scheschkewitz24 observed an ion peak assignable to a mixture indicated the existence of a new compound with 1 and cluster expansion product with formula Si R during the 7 6 DMAP units together, and 2-silyl-(N,N-dimethylamino)pyridine measurement of the electron ionization mass spectrometry of 7, which is a product of the reaction of silene 5 with DMAP. 20. Compound 4 was isolated as orange crystals in 61% yield and Removal of the volatiles from the reaction mixture and repeated identified by nuclear magnetic resonance (NMR) spectroscopy, recrystallization from hexane at À 35 °C and then from benzene mass spectrometry (MS), XRD analysis and elemental analysis at room temperature afforded analytically pure 1ÁDMAP as (Fig. 2b). The results of XRD analysis indicate that the silicon orange crystals in 89% yield. The structure of 1ÁDMAP was skeleton of 4 has a four-membered ring fused by two determined by NMR spectroscopy, MS spectrometry, XRD tetrasilabicyclo[1.1.0]butane units in a syn manner, which is a analysis and elemental analysis. formal head-to-tail [2 þ 2] cycloadduct of 1. Notably, the geometry around naked silicon atoms (silicon atoms without substituents), Si3 and Si6 atoms, in 4 are very far from the X-ray analysis of 1 . DMAP. The results of the XRD analysis of tetrahedral geometry similar to the naked silicon atoms in 2 and 1ÁDMAP indicate that the central Si4 ring has a folded structure 20: For Si3 atom, the angles Si1 À Si3 À Si2, Si2 À Si3 À Si4, with the dihedral angle Si2–Si1–Si3–Si4 of 127.03(2)° (Fig. 2c). Si4 –Si3 À Si5 and Si1 À Si3 À Si5 are 60.98(3)°, 57.22(3)°, The nitrogen atom of DMAP coordinates to Si2 atom with the 94.29(4)° and 163.97(4)°, respectively, and for Si6 atom, Si2 À N1 distance of 1.8764(14) Å, which is moderately longer the angles Si4 À Si6 À Si5, Si5 À Si6 À Si7, Si7 À Si6 À Si8 and than the typical Si À N distance (1.71–1.74 Å)27.
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