Sly silyl caught in the act: Protonation of a mesitylene ring by the strongly acidic arenium carborane [CH(3)C(6)H(6)]- [CHB(11)Me(5)Br(6)] initiates a cascade reaction that results in a stable beta-silyl allyl cation (see picture, H yellow, C blue, silyl allyl group red). Remarkably, the driving force in the reaction suffices to disrupt a stable aromatic ring in favor of a cationic reactive intermediate

Baldridge, Kim K

Duttwyler, Simon

Linden, Anthony

Reed, Christopher A

Siegel, Jay S

Zhang, Yun

eScholarship - University of California

UC RiversideUC Riverside Previously Published WorksTitleSynthesis and Crystal Structure of a Silyl‐Stabilized Allyl Cation Formed by Disruption of an Arene by a Protonation–Hydrosilylation SequencePermalinkhttps://escholarship.org/uc/item/07x548vkJournalAngewandte Chemie International Edition, 48(21)ISSN1433-7851AuthorsDuttwyler, SimonZhang, YunLinden, Anthonyet al.Publication Date2009-05-11DOI10.1002/anie.200900098 Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaReactive IntermediatesDOI: 10.1002/anie.200900098Synthesis and Crystal Structure of a Silyl-Stabilized Allyl CationFormed by Disruption of an Arene by a Protonation–HydrosilylationSequence**Simon Duttwyler, Yun Zhang, Anthony Linden, Christopher A. Reed,* Kim K. Baldridge,* andJay S. Siegel*Dedicated to Josef Michl on the occasion of his 70th birthdayAllyl cations, which are well-known primarily from solutionNMR spectroscopy in traditional superacidic media,[1] haveremained elusive in crystalline form. Indeed, these rare guestsin the chemists lab have normally arrived unannouncedduring attempts to prepare other reactive species.[2] Recentinvestigations focused on the conversion of silane 1 into thesilylium ion 2+, which is stabilized by Si–p interactions(Scheme 1).[3] A side path of these studies led to the isolationof a crystalline bicyclic allylic cation 3+ that is formed withconcomitant disruption of an aromatic ring. Invited or not, thesolid-state structure of 3+ teaches us much about thisimportant class of organic intermediates and suggests anintricate stereochemistry, which reveals a likely mechanismfor this unexpected transformation.A generally successful method for producing silyl cationreactive intermediates involves the abstraction of hydridefrom a silane by the trityl cation. To that same end, treatmentof terphenylsilane 1 with the trityl salt [Ph3C][CHB11Me5Br6]afforded, in addition to the intended silylium carborane2[CHB11Me5Br6], a small amount of a second compound, asindicated by NMR spectroscopy.[3] Attempts to crystallizepure 2[CHB11Me5Br6] from ortho-dichlorobenzene/hexaneyielded instead yellow prisms of the minor product3[CHB11Me5Br6] (Scheme 1), the identity of which wasdetermined by X-ray crystallography (Figure 1).[4]The crystal structure of 3[CHB11Me5Br6] consists of well-separated cations and anions, and 3+ can be described as asilabicyclo[3.3.1]nonenyl cation with the formal positivecharge distributed over a five-atom unit Si-C18-C19-C20-C21. Selected bond lengths and angles are given in Table 1.The unusually short sp3–sp2 C18–C19 bond, and unusuallylong Si–C18 bond relative to normal lengths, set up a long-short-long-short distortion pattern from C18 to C21. Carbonatoms C16, C18–C21, C23, and C24 all lie in the mean planethrough them within 0.025(4) , and the sum of angles aroundC19 and C21 is 360.0(4)8 for each atom, indicating trigonalplanar coordination.Computations emulate well the experimental structureand provide insight into the orbitals over which the positivecharge is distributed. The structure suggests an interaction ofthe SiC s bond with the allyl p system. b-Silyl stabilization ofcarbenium ions has much precedence and has been exploitedsynthetically.[6]One hypothesis to account for the formation of 3+involves a protonation/hydrosilylation mechanismScheme 1. Formation of 2+ and 3+ by hydride removal from silane 1.Ar= 2,6-(2,4,6-trimethylphenyl)phenyl.Figure 1. ORTEP of 3+ with ellipsoids set at 30 % probability. Hydrogenatoms are omitted for clarity (except for those at C16 and C17).[*] Y. Zhang, Prof. C. A. ReedCenter for S and P Block Chemistry, Department of ChemistryUniversity of CaliforniaRiverside, CA 92521 (USA)E-mail: chris.reed@ucr.eduS. Duttwyler, Priv.-Doz. Dr. D. A. Linden, Prof. K. K. Baldridge,Prof. J. S. SiegelOrganic Chemistry Institute, University of ZurichWinterthurerstrasse 190, 8057 Zurich (Switzerland)E-mail: jss@oci.uzh.ch[**] We thank the Swiss and US National Science Foundation forresearch support.Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/anie.200900098.AngewandteChemie3787Angew. Chem. Int. Ed. 2009, 48, 3787 –3790  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim(Scheme 2). If a mesityl ring of 1, the most basic component inthe mixture, reacts with a proton source at the ipso position, aWheland intermediate 4+ would form. The acid source couldbe a protonated silanol formed from 2+ and trace water. H3O+is known to protonate mesitylene in benzene solution.[7] Asubsequent intramolecular hydride transfer from silicon to anortho carbon atom of 4+ affords silylium ion 5+. This formalsilylium ion–diene system collapses to give cation 3+. Theproposed reaction sequence offers an explanation for theobserved connectivity and relative stereochemistry in theproduct; external protonation with concomitant intramolec-ular hydride transfer leads to the observed configuration withan exo H at C16 and endo H at C17.A test of this mechanistic hypothesis comes from inten-tional treatment of 1 with strong Brønsted acids. In contrast tothe mechanistic hypothesis, addition of 1 equiv of triflic acid(TfOH) in benzene—one of the strongest oxyacids[8]—did notgive 3[OTf], but afforded the silyl triflate ArSiMe2OTf inquantitative yield. Apparently, the negatively polarized SiHhydrogen atom acts as a Brønsted base towards triflic acid,and trapping of a silylium ion species by the coordinatingtriflate anion occurs at a much faster rate than ringprotonation/hydrosilylation.An acid that would function as proposed in Scheme 2would be an arenium carborane [arene-H][CHB11X5Y6] (X =H, Me, halogen, Y= halogen), a class of redox-inert proticacids that are even stronger than triflic acid and with lesscoordinating anions, and which are capable of protonating amesityl ring to a high degree.[9] Would such an areniumspecies such as the toluenium ion (CH3C6H6+) convert 1 into3+? Indeed, treatment of 1 with 1 equiv of [CH3C6H6]-[CHB11Me5Br6] afforded 3[CHB11Me5Br6] as the single prod-uct [Eq. (1)]. It was isolated as a yellow powder in high yieldand characterized by a second X-ray crystallographic analysis,NMR spectroscopy (1H, 13C, 29Si, 11B), and IR spectroscopy.GC-MS analysis of the reaction quenched with differentnucleophiles supports the formation of neutral compoundsthat are consistent with 3+ as the precursor.One- and two-dimensional NMR experiments in[D2]dichloromethane allowed a detailed characterization of3[CHB11Me5Br6] in solution (Table 2). Carbon atoms C19,C20, and C21 resonate at 225, 129, and 197 ppm, respectively.The deshielding is slightly less pronounced than in compara-ble allyl cations,[1c] a fact that might be a result of s–pconjugation between the SiC bond and the allyl p system.Carbon atom C18 resonates at 69 ppm, which is shifteddownfield in comparison to the other saturated carbon atomsand is not in the range of usual sp3 13C signals.[10] Thisobservation might again be attributed to a certain dienecharacter of the C18–C21 system. Yet, to depict the silylfragment as cationic ArMe2Si+ is not justified: the 29Si NMRshift of 3+ is found at d =4.2 ppm, a value typically found intetracoordinate, neutral silicon species.[11]Experiments with deuterated reactants or reagents werecarried out to gain further insight into the mechanism thatleads to 3+. According to Scheme 2, acidic [D9]toluenium ion(CD3C6D6+) would afford a product with deuterium at theC16 exo position but not at C17. Conversely, deuterated silane[D]-1 in combination with toluenium ion (CH3C6H6+) shouldgive rise to the cation labeled only at C17.Table 1: Selected bond lengths [] and angles [8] for the calculated andthe single-crystal X-ray structure of 3+.Parameter Expt Calcd[a] Parameter Expt Calcd[a]C18C19 1.414(5) 1.4136 C19-C23 1.486(5) 1.4871C19C20 1.426(5) 1.4213 C21-C24 1.489(6) 1.4863C20C21 1.359(6) 1.3620 C21-C16 1.506(6) 1.5124SiC18 1.966(4) 2.0278 C19-C20-C21 120.4(4) 121.13SiC1 1.892(4) 1.8797 C18-C19-C20 120.9(4) 120.92SiC25 1.869(4) 1.8647 C23-C19-C20 117.3(4) 118.06SiC26 1.868(4) 1.8609 C20-C21-C16 121.8(4) 121.06C16C17 1.526(5) 1.5336 C20-C21-C24 120.6(4) 121.58C17C18 1.545(5) 1.5326 C1-Si-C18 104.4(2) 102.18C17C22 1.526(5) 1.5297 Si-C18-C19 107.8(3) 103.30[a] Computed at the M06-2X/DZ(2df,pd) level of theory (see Ref. [5]).Scheme 2. Proposed protonation/hydrosilylation mechanism for theformation of 3+.Table 2: Experimental and calculated 13C and 1H NMR shifts of 3+.Carbon atom 13Cexpt[a] 13Ccalcd[c] 1Hexpt[a,b] 1HcalcdC16 54.5 55.6 (53.8) 3.78 3.36 (3.47)C17 40.6 42.1 (41.5) 3.11 2.76 (2.74)C18 69.3 71.4 (69.2) 4.03 3.76 (3.84)C19 225.2 210.2 (208.6) – – –C20 129.4 121.2 (120.4) 6.98 6.28 (6.45)C21 196.8 193.0 (189.7) – – –C22 21.3 20.8 (20.7) 1.39 1.31 (1.16)C23 34.6 31.6 (31.2) 2.78 2.41 (2.33)C24 29.0 27.8 (27.2) 2.65 2.49 (2.35)[a] Determined by HSQC and HMBC experiments and given in ppm withrespect to solvent peaks of CD2Cl2 (54.0 ppm for13C, 5.32 ppm for 1H).[b] H attached to the corresponding carbon atom. [c] M06-L/DZ+(2df,pd)//M06-2X/DZ(2df,pd); number in parentheses is in dichloro-methane. C22, C23, C24 hydrogen values averaged: C22: 1.24, 1.30, 1.39(1.06, 1.13, 1.28); C23: 2.01, 2.48, 2.74 (1.94, 2.38, 2.67); C24: 2.05, 2.93,2.49 (1.94, 2.71, 2.39); see also the Supporting Information.Communications3788 www.angewandte.org  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2009, 48, 3787 –3790The reaction of [D]-1 under protic conditions yielded onlythe product endo-deuterated at C17 (Scheme 3a). Thenegatively polarized deuterium atom in [D]-1 does not seemto be involved in Brønsted acid–base chemistry and actsexclusively as a reducing agent towards the Wheland inter-mediate 4+ when arenium acids are used.In the experiment with acidic [D9]toluenium, positionsC16, C18, and C20 were partially deuterated, and the metapositions of the mesityl ring were fully deuterated(Scheme 3b). The degree of deuteration was inferred fromintegration of 1H NMR spectra and from 2H and 13C NMRmeasurements.[12] Reversible protonation of the mesityl ringsof 1 leads to H/D scrambling and some “washing in” of H tothe toluenium species. This scrambling accounts for thepresence of some H in the ipso position of the cationic ring of3+. After the hydride transfer has taken place, H/D exchangecontinues on the unchanged mesityl ring,[13] which is mostlikely due to a slight excess of toluenium ions. The central ringis not deuterated under the reaction conditions. Orbitalanalysis supports this observation; the basic HOMO of 3 isexpressed predominantly on the unchanged mesityl ring.The question arises as to whether 3+ is best described as asilyl-stabilized allyl cation or a diene-coordinated silylium ion(Scheme 4). From a valence bond perspective the principalresonance structures of 3+ to consider are 3a+, 3 b+, and 3c+.As pointed out above, the long-short-long-short pattern alongSi-C18-C19-C20-C21 and the 13C NMR signals of C18–C21are in agreement with a certain silylium ion–diene character,3c+. However, the observed structure deviates little from thehypothetical ideal allylic cation depicted by only 3a+ and 3b+.Silylium ions stabilized by p systems exhibit Si–C distances ofmore than 2.1  and chemical shifts d(29Si)> 55 ppm;[3, 14]neither is the case for 3+.A natural bond orbital (NBO) analysis of 3+ allows asegmentation of the Si-C18-C19-C20-C21 fragment intoLewis forms with a p orbital cation acceptor on C19 interact-ing strongly with the Si-C18 s bond and the C20-C21 p bondas donors. The analysis indicates significant delocalization of4 electrons among three orbitals (s, p, and p), with occupan-cies of 1.67, 0.67, and 1.71 electrons, respectively. Theeffective polarization of the SiC bond results in a percentagecontribution of 23% from Si and 77% from C compared to anormal SiC bond with around 30% from Si and 70 % fromC. The overall NBO picture of the cation is one in whichresonance form 3b+ dominates but 3 a+ and 3 c+ are significantcontributors of roughly equal importance.The reactivity of 3+ further supports silyl-stabilized allylcationic character. Treatment of 3+ with nucleophiles ofdifferent hardness (CN , H2O, or F) affords eliminationproducts formed by proton abstraction at C23 and C24, asevidenced by 1H NMR spectroscopy and GC-MS; addition tothe silicon nucleus is not observed. Cations with a highersilylium ion character, such as 2+, give addition productsR3SiNu exclusively upon treatment with nucleophiles.[3, 15] Inconclusion, although 3c+ has to be considered as a resonancestructure to account for the observed bond lengths and the13C NMR spectrum, other data suggest its relevance is limited.The picture of a silyl-stabilized allyl cation is more apposite.Received: January 7, 2009Published online: April 17, 2009.Keywords: allyl cations · arenes · carboranes · hyperconjugation ·silyl cations[1] a) G. A. Olah, P. R. Clifford, Y. Halpern, R. G. Johanson, J. Am.Chem. Soc. 1971, 93, 4219 – 4222; b) G. A. Olah, H. Mayr, J. Am.Chem. Soc. 1976, 98, 7333 – 7340; c) Carbonium Ions (Eds.: G. A.Olah, P. von R. Schleyer), Wiley, New York, 1968, chap. 18;d) Stable Carbocation Chemistry (Eds.: G. K. Surya Prakash,P. von R. Schleyer), Wiley, New York 1997, chap. 14; e) G. A.Olah, G. K. Prakash, Carbocation Chemistry, Wiley, New York,2004, chap. 8.[2] To our knowledge, there have been only three reports on crystalstructures of allyl cations that are not stabilized by heteroatomsor additional p systems. a) A cyclobutenyl cation: G. Maier, R.Emrich, K. D. Malsch, K. A. Schneider, M. Nixdorf, H. Irngar-tinger, Chem. Ber. 1985, 118, 2798 – 2810; b) the cyclopentenylcation: J. B. Lambert, L. Lin, V. Rassolov, Angew. Chem. 2002,114, 1487 – 1489; Angew. Chem. Int. Ed. 2002, 41, 1429 – 1431; M.Otto, D. Scheschkewitz, T. Kato, M. M. Midland, J. B. Lambert,G. Bertrand, Angew. Chem. 2002, 114, 2379 – 2380; Angew.Chem. Int. Ed. 2002, 41, 2275 – 2276; T. Mller, Angew. Chem.2002, 114, 2380 – 2382; Angew. Chem. Int. Ed. 2002, 41, 2276 –2278; J. B. Lambert, Angew. Chem. 2002, 114, 2382; Angew.Chem. Int. Ed. 2002, 41, 2278; J. N. Jones, A. H. Cowley, C. L. B.Macdonald, Chem. Commun. 2002, 1520 – 1521; c) A bicycliccyclopentenyl cation: K. Ogawa, S. Minigeshi, K. Komatsu, T.Kitigawa, J. Org. Chem. 2008, 73, 5248 – 5254.Scheme 3. Deuteration experiments using a) [D]-1 with CH3C6H6+, andb) 1 with CD3C6D6+.Scheme 4. Principal resonance forms of 3+.AngewandteChemie3789Angew. Chem. Int. Ed. 2009, 48, 3787 –3790  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org[3] S. Duttwyler, Q.-Q. Do, A. Linden, K. K. Baldridge, J. S. Siegel,Angew. Chem. 2008, 120, 1743 – 1746; Angew. Chem. Int. Ed.2008, 47, 1719 – 1722.[4] CCDC 715200 contains the supplementary crystallographic datafor this paper. These data can be obtained free of charge fromThe Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.[5] Structural, orbital, and property calculations were carried outusing the Gaussian03[5a] and GAMESS[5b] software packages.Details of the computational methods are given in the Support-ing Information. a) Gaussian03, Truhlar/Zhao module, Minne-sota Supercomputer Center; b) M. W. Schmidt, K. K. Baldridge,J. A. Boatz, S. T. Elbert, M. S. Gordon, J. H. Jensen, S. Koseki, N.Matsunaga, K. A. Nguyen, S. Su, T. L. Windus, S. T. Elbert, J.Comput. Chem. 1993, 14, 1347.[6] a) J. B. Lambert, Y. Zhao, R. W. Emblide, L. A. Salvador, X. Liu,J.-H. So, E. C. Chelius, Acc. Chem. Res. 1999, 32, 183 – 190;b) J. B. Lambert, Tetrahedron 1990, 46, 2677 – 2689; c) J. B.Lambert, L. Lin and S. Keinan, Org. Biomol. Chem. 2003, 1,2559 – 2565; d) J. B. Lambert, C. Liu, T. Kouliev, J. Phys. Org.Chem. 2002, 15, 667 – 671; e) A. Klaer, W. Saak, D. Haase, T.Mller, J. Am. Chem. Soc. 2008, 130, 14956 – 14957; f) T. Mller,M. Juhasz, C. A. Reed, Angew. Chem. 2004, 116, 1569 – 1572;Angew. Chem. Int. Ed. 2004, 43, 1543 – 1546; g) M. Sugawara, J.Yoshida, J. Org. Chem. 2000, 65, 3135 – 3142; h) K. S. Hassall,J. M. White, Org. Lett. 2004, 6, 1737 – 1739; i) J. B. Lambert, Y.Zhao, J. Am. Chem. Soc. 1996, 118, 7867 – 7868; j) M. Schor-mann, S. Garratt, D. L. Hughes, J. C. Green, M. Bochmann, J.Am. Chem. Soc. 2002, 124, 11266 – 11267.[7] C. A. Reed, Chem. Commun. 2005, 1669 – 1677.[8] R. D. Howells, J. D. Mc Cown, Chem. Rev. 1977, 77, 69 – 92.[9] a) C. A. Reed, K.-C. Kim, E. S. Stoyanov, D. Stasko, F. S. Tham,L. J. Mueller, P. D. W. Boyd, J. Am. Chem. Soc. 2003, 125, 1796 –1804; b) C. A. Reed, N. L. P. Fackler, K.-C. Kim, D. Stasko, D. R.Evans, J. Am. Chem. Soc. 1999, 121, 6314 – 6315.[10] H. Friebolin, Basic One- and Two-Dimensional NMR Spectros-copy, Wiley-VCH, Berlin, 2005.[11] NMR, Basic Principles and Progress, Vol. 17 (Eds.: P. Diehl, E.Fluck, R. Kosfeld), Springer, Berlin, 1981, chap. 2.[12] A reliable integration with errors of less than 5% was achievedwith the interval between two pulses d1 = 15 s. The2H NMRspectra, being “negatives” of the 1H spectra, were used toconfirm the position of deuteration. The 13C served the samepurpose; the carbon nuclei adjacent to 2H showed 1JC,D couplingand an isotope shift.[10][13] The deuterated solvent C6D6 was used as the reaction mediumbecause CD3C6D6+ would have undergone a fast H/D exchangein C6H6.[14] a) J. B. Lambert, S. Zhang, S. M. Ciro, Organometallics 1993, 13,2430 – 2443; b) T. Mller, C. Bauch, M. Ostermeier, M. Bolte, N.Auner, J. Am. Chem. Soc. 2003, 125, 2158 – 2168.[15] C. A. Reed, Acc. Chem. Res. 1998, 31, 325 – 332.Communications3790 www.angewandte.org  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2009, 48, 3787 –3790

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Synthesis and Crystal Structure of a Silyl‐Stabilized Allyl Cation Formed by Disruption of an Arene by a Protonation–Hydrosilylation Sequence

Synthesis and crystal structure of a silyl-stabilized allyl cation formed by disruption of an arene by a protonation-hydrosilylation sequence.

Synthesis and crystal structure of a silyl-stabilized allyl cation formed by disruption of an arene by a protonation-hydrosilylation sequence.

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