Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.A base-free, catalytic protocol for the dehydrogenative Si–N coupling of weakly nucleophilic N–H groups of heteroarenes or aryl-substituted amines with equimolar amounts of hydrosilanes is reported. Cooperative Si–H bond activation at a Ru–S bond generates a silicon electrophile that forms a Si–N bond prior to the N–H deprotonation by an intermediate Ru–H complex, only releasing H2.DFG, GRK 1143, Komplexe chemische Systeme: Design, Entwicklung und Anwendunge

Aiguabella, Nuria

Klare, Hendrik F. T.

Königs, C. David F.

Müller, Maria F.

Oestreich, Martin

Chemical Communications

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1506 Chem. Commun., 2013, 49, 1506--1508 This journal is c The Royal Society of Chemistry 2013Cite this: Chem. Commun.,2013,49, 1506Catalytic dehydrogenative Si–N coupling of pyrroles,indoles, carbazoles as well as anilines with hydrosilaneswithout added base†C. David F. Ko¨nigs, Maria F. Mu¨ller, Nuria Aiguabella, Hendrik F. T. Klare andMartin Oestreich*A base-free, catalytic protocol for the dehydrogenative Si–Ncoupling of weakly nucleophilic N–H groups of heteroarenes oraryl-substituted amines with equimolar amounts of hydrosilanesis reported. Cooperative Si–H bond activation at a Ru–S bondgenerates a silicon electrophile that forms a Si–N bond prior tothe N–H deprotonation by an intermediate Ru–H complex, onlyreleasing H2.The formation of Si–N bonds is relevant to various areas ofsynthetic chemistry.1 N–H bond silylation is one way to tem-porarily protect amino groups,2 in particular the N–H group inindoles and pyrroles.3 Their silylated derivatives are prevalentbuilding blocks for the construction of heteroarene-basedcomplex molecules. The Si–N bond is relatively labile though,and N-silylated anilines have been employed as silylatingagents themselves.4 The transition metal-catalysed synthesisof oligo-5 or polysilazanes6 is another significant application ofSi–N coupling.A silicon group is usually introduced at the nitrogen atom bya deprotonation/silylation sequence using a strong base and ahalosilane.2,7 The formation of stoichiometric amounts of saltis an issue in this approach. The direct coupling of N–H andSi–H bonds is an attractive alternative that, ideally, generatesdihydrogen as the sole by-product. Several dehydrogenativeSi–N couplings are known today8 but generality and controlof the chemoselectivity remain challenging. The latter wasrecently accomplished by Sadow et al.9a and Cui et al.9b in thedirect catalytic mono-coupling of RnSiH4n (with n = 1 and 2).Parallel to these current developments, progress has also beenmade toward the elusive dehydrogenative Si–N coupling ofheteroarenes.10 The methods reported by Tsuchimoto et al.10band Mizuno et al.10c are broadly applicable and robust butrequire the addition of an external base (stoichiometric andcatalytic, respectively) and a nitrile solvent as a H2 acceptor. Wedisclose herein a base- and H2 acceptor-free protocol for thecatalytic dehydrogenative Si–N coupling of both N–H group-containing heteroarenes and aryl-substituted amines thatfollows an unusual silylation/deprotonation sequence.Our laboratory, in collaboration with Ohki and Tatsumi,introduced the cooperative activation of Si–H bonds at thepolar Ru–S bond11 of the coordinatively unsaturated, tetheredruthenium complex 112 (left, Fig. 1). The heterolytic splitting ofthe Si–H bond (in analogy to H2 activation13) results in theformation of a Ru–H complex and a silicon electrophile, asulfur-stabilized silicon cation (right, Fig. 1). This catalyticentry into the chemistry of silicon cations has already allowedfor the development of a regioselective Friedel–Crafts-typeindole silylation11 and a chemoselective, dehydrogenative sily-lation of enolizable carbonyl compounds.14 The same reactionsetup applied to weakly nucleophilic N–H groups now results inSi–N bond formation together with liberation of H2.Our investigation began with the N-silylation of indole(2) using Me2PhSiH (3a) that we had used in our earlierC-3-silylation of N-protected indoles11 (Scheme 1). For anequimolar mixture of indole and silane, full conversion wasreached after 1 h at 60 1C using 1 mol% of 1. The expectedN-silylated indole 4a was, however, contaminated with sub-stantial amounts of the cognate indoline 5a in a ratio of 76 : 24.This ratio improved with prolonged reaction times in favour ofthe protected indole, e.g., 92 : 8 after 12 h. We have shownbefore that 1 is capable of (reversible) indoline-to-indoledehydrogenation.11Fig. 1 Tethered complex 1 with a polar Ru–S bond in Si–H bond activation[ArF = 3,5-bis(trifluoromethyl)phenyl and Si = triorganosilyl].Institut fu¨r Chemie, Technische Universita¨t Berlin, Strasse des 17. Juni 115,D-10623 Berlin, Germany. E-mail: martin.oestreich@tu-berlin.de;Fax: +49 (0)30 314 28829; Tel: +49 (0)30 314 29721† Electronic supplementary information (ESI) available: Mechanistic proposal,preparation and characterisation data as well as 1H and 13C NMR spectra of allcompounds. See DOI: 10.1039/c3cc38900fReceived 12th December 2012,Accepted 8th January 2013DOI: 10.1039/c3cc38900fwww.rsc.org/chemcommChemCommCOMMUNICATIONPublished on 17 January 2013. Downloaded by TU Berlin - Universitaetsbibl on 30/03/2016 12:42:41. View Article OnlineView Journal  | View IssueThis journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 1506--1508 1507We then turned our attention to the screening of diﬀerenttriorganosilanes 3 in the dehydrogenative Si–N coupling of3-methylindole15 as well as pyrrole (6 - 8 and 7 - 9,Table 1). For the indole, sterically less demanding silanes3a–3c aﬀorded excellent isolated yields whereas 3d and 3e didnot react even at higher temperature (columns 3–5). For pyr-role, similar observations were made, yet requiring highertemperature and excess pyrrole (columns 6–8). It is somewhatunfortunate that iPr3SiH (3e) is not activated by catalyst 1 forsteric reasons because the iPr3Si group is a commonly usedprotective group for these heteroarenes. By using an excess of3a (10 equiv.), we examined the possibility of double silylation,that is N-silylation and subsequent Friedel–Crafts C-3-silylationof indole and pyrrole (2 - 10a and 7 - 11a, Scheme 2).Upon prolonging the reaction time (18 h instead of 1 h), doublysilylated 10a (along with 5a) and 11a were formed in high yields.These findings are in accordance with previous results forN-protected indoles.11We next examined the scope of the indole motif, substitutedand unsubstituted at the C-3 atom (12–16, Table 2, entries 1–5).All of them underwent clean Si–N coupling in good to excellentyields. Aside from pyrrole and indole, carbazole also partici-pated in this dehydrogenative N-silylation (17, Table 2, entry 6).These results compare well with the reports by Tsuchimotoet al.10b and Mizuno et al.10c (vide supra). The fact that theseheteroarenes are suﬃciently nucleophilic encouraged us to alsotest aryl-substituted amines with enhanced nucleophilicity(24–30, Table 3, entries 1–7). Indoline, essentially an anilinederivative, was dramatically more reactive than indole, yieldingcomplete conversion in n-hexane after 5 min at ambienttemperature (entry 1); indoline-to-indole oxidation only occursat elevated temperatures (cf. Scheme 1).11 Various substitutedanilines displayed the same reactivity (Table 3, entries 2–7). It isworthy of note that the CF3 group in 29 remains intact underthese conditions.16The chemoselectivity of our catalytic Si–N coupling wasprobed in the selective monoamination of dihydrosilaneScheme 1 Dehydrogenative Si–N coupling of indole: reversible indoline-to-indole dehydrogenation catalysed by 1.Table 1 Screening of diﬀerent triorganosilanesEntry Si–H3-Methylindole (6) Pyrrole (7)Temp.(1C) ProductYieldb(%)Temp.(1C) ProductYieldb(%)1 Me2PhSiH (3a) 60 8a 97 90 9a 912 MePh2SiH (3b) 60 8b 90 90 9b 883c EtMe2SiH (3c) 60 8c 94 90 9c —d4 Et3SiH (3d) 90 8d —d 90 9d 905 iPr3SiH (3e) 90 8e —d 90 9e —da Conversion was monitored by GLC analysis and is based on theconsumption of 3. b Isolated yield after catalyst removal by filtrationthrough a short plug of deactivated silica gel. c Volatile at elevatedtemperatures, used in toluene (0.5 M). d No reaction.Scheme 2 Probing double silylation of indole and pyrrole.Table 2 Dehydrogenative Si–N coupling of indoles and carbazoleEntry Substrate Silylated product Yieldb (%)1 89c2 913 96d4 895 926 76a See Table 1. b See Table 1. c Contaminated with trace amounts of(Me2PhSi)2O.d Approximately 10% of the cognate indoline detected.Communication ChemCommPublished on 17 January 2013. Downloaded by TU Berlin - Universitaetsbibl on 30/03/2016 12:42:41. View Article Online1508 Chem. Commun., 2013, 49, 1506--1508 This journal is c The Royal Society of Chemistry 2013Ph2SiH2 (3f) and aniline (26- 32f, Scheme 3). With 1 equiv. of26 at room temperature, only one Si–H bond of 3f was cross-coupled. With subsequent addition of another equivalent of 26at elevated temperature, the remaining Si–H bonds also under-went the cross-coupling (26- 37f, Scheme 3).To summarize, we accomplished an eﬃcient protocol for thebase-free dehydrogenative Si–N coupling of monohydrosilanesand weakly to moderately nucleophilic N–H groups. Pyrroles,indoles and carbazoles fall into the former and anilines into thelatter category. Alkyl-substituted amines are not compatiblewith coordinatively unsaturated complex 1, thwarting Si–Hbond activation due to amine coordination (for a tentativemechanism, see the ESI†). Moreover, selective mono-couplingof a dihydrosilane and exactly 1.0 equiv. of aniline is feasible.This research was supported by the Deutsche Forschungs-gemeinschaft (International Research Training Group Mu¨nster–Nagoya, GRK 1143, with predoctoral fellowships to C.D.F.K.,2011–2012 andH.F.T.K., 2007–2010) and the Deutsche AkademischeAustauschdienst (Forschungsstipendium A/12/71607 to N.A.,2012). M.O. is indebted to the Einstein Foundation (Berlin)for an endowed professorship.Notes and references1 (a) J. A. Reichl and D. H. Berry, Adv. Organomet. Chem., 1998, 43,197–265; (b) D. A. Armitage, in The Silicon–Heteroatom Bond, Wiley,Chichester, 1991, pp. 365–484.2 (a) P. G. M. Wuts and T. W. Greene, Greene’s Protective Groups inOrganic Synthesis, Wiley, New York, 2007, 4th edn, p. 847;(b) P. J. Kocienski, Protecting Groups, Thieme, Stuttgart, 2004,pp. 595–599.3 (a) D. Dhanak and C. B. Reese, J. Chem. Soc., Perkin Trans. 1, 1986,2181–2186; (b) B. Jolicoeur, E. E. Chapman, A. Thompson andW. D. Lubell, Tetrahedron, 2006, 62, 11531–11563.4 A. Iida, A. Horii, T. Misaki and Y. Tanabe, Synthesis, 2005,2677–2682.5 (a) Y. Blum and R. M. Laine, Organometallics, 1986, 5, 2081–2086;(b) W.-D. Wang and R. Eisenberg, Organometallics, 1991, 10,2222–2227; (c) H. Q. Liu and J. F. Harrod, Organometallics, 1992,11, 822–827.6 (a) R. M. Laine, Platinum Met. Rev., 1988, 32, 64–71; (b) Y. D. Blum,K. B. Schwartz and R. M. Laine, J. Mater. Sci., 1989, 24, 1707–1718;(c) F. Lunzer, C. Marschner and S. Landgraf, J. Organomet. Chem.,1998, 568, 253–255.7 R. Fessenden and J. S. Fessenden, Chem. Rev., 1961, 61, 361–388.8 With amines but not anilines: (a) L. H. Sommer and J. D. Citron,J. Org. Chem., 1967, 32, 2470–2472; (b) J. X. Wang, A. K. Dash,J. C. Berthet, M. Ephritikhine and M. S. Eisen, J. Organomet. Chem.,2000, 610, 49–57; with amines including anilines: (c) H. Kono,I. Ojima, M. Matsumoto and Y. Nagai, Org. Prep. Proced. Int., 1973,5, 135–139; (d) E. Matarasso-Tchiroukhine, J. Chem. Soc., Chem.Commun., 1990, 681–682; (e) H. Q. Liu and J. F. Harrod, Can. J.Chem., 1992, 70, 107–110; ( f ) K. Takaki, T. Kamata, Y. Miura,T. Shishido and K. Takehira, J. Org. Chem., 1999, 64, 3891–3895;(g) K. Takaki, K. Komeyama and K. Takehira, Tetrahedron, 2003, 59,10381–10395; (h) A. Kawachi, M. Zaima, A. Tani and Y. Yamamoto,Chem. Lett., 2007, 36, 362–363; (i) F. Buch and S. Harder, Organo-metallics, 2007, 26, 5132–5135; ( j ) D. V. Gutsulyak, S. E.Vyboishchikov and G. I. Nikonov, J. Am. Chem. Soc., 2010, 132,5950–5951.9 (a) J. F. Dunne, S. R. Neal, J. Engelkemier, A. Ellern and A. D. Sadow,J. Am. Chem. Soc., 2011, 133, 16782–16785; (b) W. Xie, H. Hu andC. Cui, Angew. Chem., Int. Ed., 2012, 51, 11141–11144.10 (a) D. W. Robbins, T. A. Boebel and J. F. Hartwig, J. Am. Chem. Soc.,2010, 132, 4068–4069; (b) T. Tsuchimoto, Y. Iketani and M. Sekine,Chem.–Eur. J., 2012, 18, 9500–9504; (c) S. Itagaki, K. Kamata,K. Yamaguchi and N. Mizuno, Chem. Commun., 2012, 48, 9269–9271.11 H. F. T. Klare, M. Oestreich, J.-i. Ito, H. Nishiyama, Y. Ohki andK. Tatsumi, J. Am. Chem. Soc., 2011, 133, 3312–3315.12 Y. Ohki, Y. Takikawa, H. Sadohara, C. Kesenheimer, B. Engendahl,E. Kapatina and K. Tatsumi, Chem.–Asian J., 2008, 3, 1625–1635.13 Y. Ohki, M. Sakamoto and K. Tatsumi, J. Am. Chem. Soc., 2008, 130,11610–11611.14 C. D. F. Ko¨nigs, H. F. T. Klare, Y. Ohki, K. Tatsumi and M. Oestreich,Org. Lett., 2012, 14, 2842–2845.15 Used to prevent its reduction (cf. Scheme 1) and possible doublesilylation (cf. Scheme 2).16 Another version of 1 is also able to catalyse the hydrodefluorinationof 29: T. Stahl, H. F. T. Klare and M. Oestreich, J. Am. Chem. Soc.,DOI: 10.1021/ja311398j.Table 3 Dehydrogenative Si–N coupling of indoline and anilinesEntry Substrate Silylated product Yieldb (%)1 952 923 874 855 966 857 94a See Table 1. b See Table 1.Scheme 3 Probing chemoselective formation of silylamines with aniline (26)and Ph2SiH2 (3f).ChemComm CommunicationPublished on 17 January 2013. 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Catalytic dehydrogenative Si-N coupling of pyrroles, indoles, carbazoles as well as anilines with hydrosilanes without added base

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Catalytic dehydrogenative Si-N coupling of pyrroles, indoles, carbazoles as well as anilines with hydrosilanes without added base

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