A metal‐free, intermolecular syn‐addition of hexamethyldisilane across simple alkenes is reported. The catalytic cycle is initiated and propagated by the transfer of a methyl group from the disilane to a silylium‐ion‐like intermediate, corresponding to the (re)generation of the silylium‐ion catalyst. The key feature of the reaction sequence is the cleavage of the Si−Si bond in a 1,3‐silyl shift from silicon to carbon. A central intermediate of the catalysis was structurally characterized by X‐ray diffraction, and the computed reaction mechanism is fully consistent with the experimental findings.TU Berlin, Open-Access-Mittel - 201

Grimme, Stefan

Irran, Elisabeth

Klare, Hendrik F. T.

Oestreich, Martin

Qu, Zheng-Wang

Roy, Avijit

Wu, Qian

Angewandte Chemie International Edition

English

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German Edition: DOI: 10.1002/ange.201911282Homogeneous Catalysis Very Important PaperInternational Edition: DOI: 10.1002/anie.201911282Catalytic Difunctionalization of Unactivated Alkenes with UnreactiveHexamethyldisilane through Regeneration of Silylium IonsQian Wu, Avijit Roy, Elisabeth Irran, Zheng-Wang Qu,* Stefan Grimme, Hendrik F. T. Klare,and Martin Oestreich*Abstract: A metal-free, intermolecular syn-addition of hexa-methyldisilane across simple alkenes is reported. The catalyticcycle is initiated and propagated by the transfer of a methylgroup from the disilane to a silylium-ion-like intermediate,corresponding to the (re)generation of the silylium-ion catalyst.The key feature of the reaction sequence is the cleavage of theSi@Si bond in a 1,3-silyl shift from silicon to carbon. A centralintermediate of the catalysis was structurally characterized byX-ray diffraction, and the computed reaction mechanism isfully consistent with the experimental findings.There is a long-standing interest in the activation of the Si@Sibond in disilanes as evidenced by an ample body ofliterature.[1] The vast majority of Si@Si bond-breaking reac-tions is mediated by transition-metal complexes to ultimatelyenable the catalytic transfer of both silicon units to anunsaturated molecule. Intermolecular additions across C/Ctriple bonds are normally high-yielding but, unless conjugatedor strained, C=C double bonds demand intramoleculardelivery of the silyl groups. Also, oxidative addition of theSi@Si bond to a transition metal typically requires heteroatomsubstitution at the silicon atoms or incorporation of the Si@Sifragment into a small ring. There are just a few examples ofthe activation of unreactive hexamethyldisilane,[2–4] and thesewere successfully applied to the difunctionalization ofalkynes[2] while alkenes do not react.[3]We recently developed a broadly applicable protolysisapproach that reliably transforms otherwise inert fullyalkylated silanes[5] or hydrosilanes[6] into counteranion-stabi-lized silylium ions by making use of[C6H6·H]+[CHB11H5Br6]@[7] as a super-strong Brønsted acid(Scheme 1, top). By this, we were able to generate Me2-(Me3Si)Si+[CHB11H5Br6]@ , a previously elusive silyl-substi-tuted silylium ion (Scheme 1, bottom).[5] Interestingly, thesame outcome was obtained with Me3Si+[CHB11H5Br6]@instead of the protonated benzene (Scheme 1, bottom);Me3Si+ acts like a fat proton.We then reacted Me2(Me3Si)Si+[CHB11H5Br6]@ with allyl-benzene (1a ; Scheme 2, top), expecting the formation ofcarbenium ion 2a+ stabilized by the b-silicon effect.[8] How-ever, we isolated silylium ion 3a+ intramolecularly stabilizedby the arene ring. The molecular structure of3a+[CHB11H5Br6]@ was confirmed by X-ray diffraction,[9]and the 29Si chemical shifts of d= 7.5 and 100.7 ppm areconsistent with a tetraalkyl-substituted silicon atom and anarene-stabilized silylium ion, respectively. There is no bondbetween the two silicon atoms in 3a+ anymore. Knowing thatsilylium ions can chemoselectively cleave a Si@C(sp3) bond inhexamethyldisilane (see Scheme 1, bottom), we treated 3a+Scheme 1. Dealkylative and dehydrogenative protolysis approaches tocounteranion-stabilized silylium ions.[*] Dr. Q. Wu, A. Roy, Dr. E. Irran, Dr. H. F. T. Klare,Prof. Dr. M. OestreichInstitut ffr Chemie, Technische Universit-t BerlinStraße des 17. Juni 115, 10623 Berlin (Germany)E-mail: martin.oestreich@tu-berlin.deHomepage: http://www.organometallics.tu-berlin.deDr. Z.-W. Qu, Prof. Dr. S. GrimmeMulliken Center for Theoretical ChemistryInstitut ffr Physikalische und Theoretische ChemieRheinische Friedrich-Wilhelms-Universit-t BonnBeringstraße 4, 53115 Bonn (Germany)E-mail: qu@thch.uni-bonn.deHomepage: http://www.thch.uni-bonn.de/tcSupporting information and the ORCID identification number(s) forthe author(s) of this article can be found under:https://doi.org/10.1002/anie.201911282.T 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co.KGaA. This is an open access article under the terms of the CreativeCommons Attribution Non-Commercial License, which permits use,distribution and reproduction in any medium, provided the originalwork is properly cited, and is not used for commercial purposes.AngewandteChemieCommunications17307Angew. Chem. Int. Ed. 2019, 58, 17307 –17311 T 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheimwith hexamethyldisilane and alkene 1a (1 equiv each) andfound substantial formation of 1,2-bissilylated 4a (Scheme 2,bottom). Based on this stoichiometric reaction sequence, wedisclose here a catalytic alkene difunctionalization withunactivated hexamethyldisilane that does not requirea metal catalyst.The above stoichiometric experiments had establishedthat chemoselective cleavage of one of the Si@C(sp3) bonds inhexamethyldisilane could be used to initiate and thenmaintain a catalytic cycle. Hence, the catalytic version ofthe alkene difunctionalization would rely on the self-regen-eration of Me2(Me3Si)Si+[CHB11H5Br6]@ from the disilanereactant.[10] With 1.0 mol% of Me3Si+[CHB11H5Br6]@ as theinitiator,[11] we quickly identified suitable reaction setups(Scheme 3). Both chlorobenzene and benzene could beemployed as the solvent but yields were generally higherwith chlorobenzene. However, the low boiling point of severalalkenes and adducts made benzene the superior choice,particularly during purification. The reactions of volatileunfunctionalized substrates were performed at 50 instead ofat 80 8C. Under these reaction conditions, the parent com-pound 1a and various functionalized allylbenzenes 1b–j aswell as naphthyl-substituted 1k and 1 l were converted intothe 1,2-bissilylated adducts 4a–l in good yields; the yields ofisolated products were always slightly lower than thosedetermined by 1H NMR spectroscopy with an internalstandard because of the low polarity of the adducts. Thefunctional-group tolerance was as expected for silylium-ionchemistry: OMe and CF3 groups were not compatible. Aninteresting case was para-chloro-substituted 1g ; no reactionwas observed in chlorobenzene but 4g formed quantitativelyin benzene as the solvent. Styrene derivatives were prone topolymerization but 1m and 1n afforded decent yields of 4mand 4n, respectively, under more dilute conditions and usingan excess of hexamethyldisilane. These results show that theallylic aryl group in alkene 1 and thus its coordination to thecationic silicon center attached to the g-position in inter-mediate 3 are not necessary for the reaction to proceed. Thiswas corroborated by the reaction of other a-olefins such as1o–q ; these furnished bissilylated alkanes 4o–q in acceptableyields.Extension of the procedure to internal alkenes waspossible but limited (Scheme 4). Cyclic 1,2-disubstitutedalkenes 5a–c reacted in good yields and with superbdiastereoselectivity but acyclic substrates 5d–f did not. Tri-and tetrasubstituted alkenes decomposed. The cis relativeconfiguration of cis-6a–c is rationalized by our mechanisticScheme 2. Key finding with molecular structure (top; thermal ellip-soids set at 50% probability; hydrogen atoms are omitted for clarity)and application to alkene difunctionalization (bottom). Selected bondlengths [b] and angles [8]: Si1–C1 2.106(5), Si1–C2 1.848(5), Si1–C31.845(5), Si1–C4 1.859(4); C2-Si1-C3 114.9(2), C2-Si1-C4 115.0(2), C3-Si1-C4 113.3(2). B blue, Br red, C gray, Si yellow.Scheme 3. Catalytic difunctionalization of a-olefins with hexamethyldi-silane involving silylium ion self-regeneration. Yields determined by1H NMR spectroscopy with CH2Br2 as an internal standard; isolatedyields after flash chromatography on silica gel in parentheses.AngewandteChemieCommunications17308 www.angewandte.org T 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2019, 58, 17307 –17311model (see Scheme 5 and the Supporting Information for anillustration).Disilanes other than hexamethyldisilane were also testedin the model reaction with allylbenzene (Figure 1). However,heteroleptic disilanes such as 1,1,2,2-tetramethyl-1,2-diphe-nylsilane suffered from substituent redistribution.[12] A tert-butyl group at the silicon atom(s) prevents this but, relying ondearylation, 1,2-di-tert-butyl-1,1,2,2-tetraphenyldisilane didnot participate; a stoichiometric experiment showed thatMe3Si+[CHB11H5Br6]@ cannot facilitate the dearylation step.Notably, unsymmetrically substituted 1-isopropyl-1,1,2,2,2-pentamethyldisilane furnished both regioisomers of thebissilylated alkane with a 62:38 ratio in 47% combinedyield. The chemoselectivity of the demethylation obtainedfrom a stoichiometric experiment was low (40:60 for iPrMe-(Me3Si)Si+ and Me2(iPrMe2Si)Si+; see the Supporting Infor-mation for details).Density functional theory (DFT) calculations were per-formed at the PW6B95-D3/def2-QZVP+COSMO-RS(chlorobenzene)//TPSS-D3/def2-TZVP+COSMO(chlo-robenzene) level of theory[13] to provide insight into themechanism of the alkene difunctionalization in chloroben-zene solution using hexamethyldisilane as the reagent andMe3Si+[CHB11H5Br6]@ as the initiator. The computed Gibbsfree energy profiles (in kcalmol@1 at 298 K and 1 molL@1reference state) for the initiation and the catalytic bissilyla-tion of allylbenzene (1a) and but-1-ene (model compound foralkyl-substituted a-olefins) are shown in the SupportingInformation. The initiation step as well as the catalytic cyclefor 1a as the substrate are depicted in Scheme 5.As shown in Scheme 5, top, the methyl transfer fromhexamethyldisilane to Me3Si+[CHB11H5Br6]@ is exergonic by@0.9 kcalmol@1 over an overall barrier of 19.7 kcalmol@1 toform Me2(Me3Si)Si+[CHB11H5Br6]@ along with Me4Si. Themethyl-group exchange proceeds through intermediate INT+over two transition states (see the Supporting Information).Counteranion-stabilized Me2(Me3Si)Si+ then enters the cata-lytic cycle (Scheme 5, bottom). An SN2-like attack of alkene1a at the electrophilic silicon center of Me2-(Me3Si)Si+[CHB11H5Br6]@ is endergonic by 1.2 kcalmol@1over a low barrier of 13.8 kcalmol@1 to release the anion[CHB11H5Br6]@ and the unstable carbocation 2a+. A rapid 1,3-silyl shift occurs via transition state 7a+*, rearranging 2a+ into8a+ with a bridging methyl group. This transformation isexergonic by @7.7 kcalmol@1 over a barrier of 9.3 kcalmol@1.Another 16.7 kcalmol@1 of stabilization is gained fromreorganization of 8a+ into intramolecular arene–silylium ionadduct 3a+; the barrier with transition state 9a+* is again low(8.7 kcalmol@1). The coordination of [CHB11H5Br6]@ to 3a+by a Si@Br bond is endergonic by 10.3 kcalmol@1 and thusScheme 4. Catalytic difunctionalization of internal alkenes with hexa-methyldisilane involving silylium ion self-regeneration. Yields deter-mined by 1H NMR spectroscopy with CH2Br2 as an internal standard;yields of isolated products after flash chromatography on silica gelgiven in parentheses.Scheme 5. Initiation and catalytic cycle of the silylium-ion-promotedbissilylation of allylbenzene (see the Supporting Information forcalculated structures of relevant intermediates and transition states).For each reaction step, the Gibbs free reaction energies and barriers(labeled with an asterisk) in kcalmol@1 were computed at the PW6B95-D3 level of theory. The counteranion [CHB11H5Br6]@ is omitted forclarity when not acting as a stabilizing donor. rds= rate-determiningstep.Figure 1. Other disilanes investigated.AngewandteChemieCommunications17309Angew. Chem. Int. Ed. 2019, 58, 17307 –17311 T 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.orgunfavorable in solution. This reaction cascade is in fullagreement with the experimental observation that 2a+ wasnot detected and 3a+[CHB11H5Br6]@ is a bench-stable sepa-rated ion pair (see Scheme 2, top). This ion pair could beviewed as a frustrated Lewis pair that subsequently engages ina propagating methyl transfer from hexamethyldisilanesimilar to the initiation (see Scheme 5, top). This step isendergonic by 7.7 kcalmol@1 over a rate-limiting barrier of25.7 kcalmol@1 and regenerates the catalyst Me2-(Me3Si)Si+[CHB11H5Br6]@ with release of the 1,2-bissilylatedalkane 4a. The catalytic cycle is exergonic by@15.5 kcalmol@1with an overall barrier of 25.7 kcalmol@1, consistent with thegentle heating required in the experiment.[14]The unreactive Si@Si bond in hexamethyldisilane isusually activated by transition-metal complexes[2] or duringa transition-metal-catalyzed process.[4] Its intermolecularaddition across unactivated alkenes is unprecedented.[3] Wehave disclosed here a silylium-ion-promoted reaction thatenables the catalytic bissilylation of electronically unbiased a-olefins and selected disubstituted internal alkenes. Supportedby quantum-chemical calculations, the reactions mechanismis fully understood and in line with experimental observations.The transfer of a methyl group from a methyl-substitutedsilicon atom to a silylium-ion-like intermediate maintainscatalytic turnover in this transformation, and the cleavage ofthe Si@Si bond occurs in a 1,3-silyl shift from silicon to carbon.AcknowledgementsQ.W. gratefully acknowledges the Alexander von HumboldtFoundation for a postdoctoral fellowship (2017–2019) andA.R. thanks the Berlin Graduate School of Natural Sciencesand Engineering for a predoctoral fellowship (2018–2021). Z.-W.Q. and S.G. thank the Deutsche Forschungsgemeinschaft(Leibniz Prize to S.G.) for funding support. M.O. is indebtedto the Einstein Foundation (Berlin) for an endowed profes-sorship.Conflict of interestThe authors declare no conflict of interest.Keywords: alkenes · density functional calculations ·homogeneous catalysis · silylium ions · Si@Si activationHow to cite: Angew. Chem. Int. Ed. 2019, 58, 17307–17311Angew. Chem. 2019, 131, 17468–17472[1] For reviews on Si@Si bond activation, see: a) M. Suginome, Y.Ito, Top. Organomet. Chem. 1999, 3, 131 – 159; b) H. K. Sharma,K. H. Pannell, Chem. Rev. 1995, 95, 1351 – 1374.[2] a) H. Sakurai, Y. Kamiyama, Y. Nakadaira, J. Am. Chem. Soc.1975, 97, 931 – 932; b) H. Yamashita, M. Catellani, M. Tanaka,Chem. Lett. 1991, 20, 241 – 244; c) Y. Ito, M. Suginome, M.Murakami, J. Org. Chem. 1991, 56, 1948 – 1951; d) C. Gryparis,M. Kidonakis, M. Stratakis, Org. Lett. 2013, 15, 6038 – 6041;e) M. B. Ansell, D. E. Roberts, F. G. N. Cloke, O. Navarro, J.Spencer, Angew. Chem. Int. Ed. 2015, 54, 5578 – 5582; Angew.Chem. 2015, 127, 5670 – 5674.[3] T. Hayashi, T. Kobayashi, A. M. Kawamoto, H. Yamashita, M.Tanaka, Organometallics 1990, 9, 280 – 281.[4] For an account on C@H silylation using hexamethyldisilane, see:B. Zhou, A. Lu, Y. Zhang, Synlett 2019, 30, 685 – 693.[5] Q. Wu, Z.-W. Qu, L. Omann, E. Irran, H. F. T. Klare, M.Oestreich, Angew. Chem. Int. Ed. 2018, 57, 9176 – 9179; Angew.Chem. 2018, 130, 9317 – 9320.[6] Q. Wu, E. Irran, R. Mgller, M. Kaupp, H. F. T. Klare, M.Oestreich, Science 2019, 365, 168 – 172.[7] a) C. A. Reed, Acc. Chem. Res. 2013, 46, 2567 – 2575; b) C. A.Reed, Acc. Chem. Res. 2010, 43, 121 – 128.[8] a) For the reaction of an arene-stabilized silylium ion with analkene, see: J. B. Lambert, Y. Zhao, H. Wu, J. Org. Chem. 1999,64, 2729 – 2736; b) for an authoritative discussion of the b-siliconeffect, see: J. B. Lambert, Y. Zhao, R. W. Emblidge, L. A.Salvador, X. Liu, J.-H. So, E. C. Chelius, Acc. Chem. Res. 1999,32, 183 – 190; c) for a summary of the chemistry of silyl-substituted carbocations, see: H.-U. Siehl, T. Mgller in TheChemistry of Organic Silicon Compounds, Part 2 (Eds.: Z.Rappoport, Y. Apeloig), Wiley, Chichester, 1989, pp. 595 – 701.[9] The angle describing the position of the silicon atomwith respectto the plane of the arene ring is 106.58 (Si1-C1-Cpara). This valuecompares well with the angle of 1048 found for the toluene-stabilized silylium ion Et3Si(toluene)+[B(C6F5)4]@ ; see: J. B.Lambert, S. Zhang, C. L. Stern, J. C. Huffman, Science 1993,260, 1917 – 1918.[10] For selected silylium-ion-promoted reactions involving self-regeneration, see: a) Ref. [8a]; b) K. Mgther, M. Oestreich,Chem. Commun. 2011, 47, 334 – 336; c) K. Mgther, J. Mohr, M.Oestreich, Organometallics 2013, 32, 6643 – 6646 (C=X hydro-silylation); d) O. Allemann, S. Duttwyler, P. Romanato, K. K.Baldridge, J. S. Siegel, Science 2011, 332, 574 – 577; e) B. Shao,A. L. Bagdasarian, S. Popov, H. M. Nelson, Science 2017, 355,1403 – 1407 (C@C bond formation).[11] [C6H6·H]+[CHB11H5Br6]@ , Me2(Me3Si)Si+[CHB11H5Br6]@ , andthe intermediates 3+[CHB11H5Br6]@ can also be used as initia-tors.[12] L. Omann, B. Pudasaini, E. Irran, H. F. T. Klare, M.-H. Baik, M.Oestreich, Chem. Sci. 2018, 9, 5600 – 5607.[13] a) TURBOMOLE V7.3 2018, a development of the Universityof Karlsruhe and Forschungszentrum Karlsruhe GmbH, 1989 –2007, TURBOMOLE GmbH, since 2007; available from http://www.turbomole.com; b) A. Klamt, G. Schggrmann, J. Chem.Soc. Perkin Trans. 2 1993, 799 – 805; c) K. Eichkorn, F. Weigend,O. Treutler, R. Ahlrichs, Theor. Chem. Acc. 1997, 97, 119 – 124;d) F. Weigend, M. H-ser, H. Patzelt, R. Ahlrichs, Chem. Phys.Lett. 1998, 294, 143 – 152; e) F. Eckert, A. Klamt, AIChE J. 2002,48, 369 – 385; f) J. Tao, J. P. Perdew, V. N. Staroverov, G. E.Scuseria, Phys. Rev. Lett. 2003, 91, 146401; g) F. Weigend, F.Furche, R. Ahlrichs, J. Chem. Phys. 2003, 119, 12753 – 12762;h) P. Deglmann, K. May, F. Furche, R. Ahlrichs, Chem. Phys.Lett. 2004, 384, 103 – 107; i) F. Weigend, R. Ahlrichs, Phys. Chem.Chem. Phys. 2005, 7, 3297 – 3305; j) Y. Zhao, D. G. Truhlar, J.Phys. Chem. A 2005, 109, 5656 – 5667; k) F. Weigend, R.Ahlrichs, Phys. Chem. Chem. Phys. 2006, 8, 1057 – 1065; l) S.Grimme, J. Antony, S. Ehrlich, H. Krieg, J. Chem. Phys. 2010,132, 154104 – 154119; m) S. Grimme, S. Ehrlich, L. Goerigk, J.Comput. Chem. 2011, 32, 1456 – 1465; n) S. Grimme, Chem. Eur.J. 2012, 18, 9955 – 9964; o) F. Eckert, A. Klamt in COSMOtherm,Version C3.0, Release 16.01, COSMOlogic GmbH & Co.,Leverkusen, Germany, 2015.[14] The reaction mechanism is very similar for an a-olefin lackingthe intramolecularly stabilizing aryl group (see the SupportingInformation for the energy profile of the bissilylation of but-1-ene). The key difference is that the anion [CHB11H5Br6]@AngewandteChemieCommunications17310 www.angewandte.org T 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2019, 58, 17307 –17311assumes the role of the Lewis base, thereby intermolecularlystabilizing the rearranged silylium ion as an adduct with thecounteranion. It is less stable (@12.0 kcalmol@1 relative to but-1-ene) than the intramolecular arene adduct (@23.2 kcalmol@1relative to allylbenzene), resulting in a lower overall barrier of19.2 kcalmol@1 (vs. 25.7 kcalmol@1), and the catalytic cycle isexergonic by @14.2 kcalmol@1.Manuscript received: September 4, 2019Accepted manuscript online: September 30, 2019Version of record online: October 23, 2019AngewandteChemieCommunications17311Angew. Chem. Int. Ed. 2019, 58, 17307 –17311 T 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org

Catalytic difunctionalization of unactivated alkenes with unreactive hexamethyldisilane through regeneration of silylium ions

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Catalytic difunctionalization of unactivated alkenes with unreactive hexamethyldisilane through regeneration of silylium ions

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