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Cite this: Chem. Commun., 2014, 50, 1980 Received 9th October 2013, Accepted 17th December 2013
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Ethene/alkyne exchange reaction at an intramolecular frustrated Lewis pair† Christina Eller, Kathrin Bussmann, Gerald Kehr, Birgit Wibbeling,‡ Constantin G. Daniliuc‡ and Gerhard Erker*
DOI: 10.1039/c3cc47769j www.rsc.org/chemcomm
A vicinal ethylene bridged S/B frustrated Lewis pair (FLP) in situ generated by a hydroboration reaction of phenyl vinyl sulfide, reacts with p-tert-butylphenylacetylene by ethene/alkyne exchange and subsequent 1,2-addition of a second alkyne equivalent to give a zwitterionic sulfonium/borate product.
Frustrated Lewis pair (FLP) chemistry has seen interesting developments in recent years.1 Intramolecular vicinal FLPs were shown to undergo a variety of specific reactions with small molecules. Some effect heterolytic cleavage of dihydrogen and may serve as metal-free hydrogenation catalysts of specific substrates.2,3 Vicinal P/B FLPs were shown to add nitrogen monoxide,4 carbon dioxide5 or even sulfur dioxide;4c,6 they may add alkenes and alkynes,7 carbonyl compounds8 etc. Many vicinal FLPs are usually prepared either by hydroboration or by recent variants of the 1,1-carboboration reaction.9 Although these reactions are often simple to perform and were shown to be useful, it would be desirable to extend the spectrum of synthetic methods to such species. We thought that one attractive scheme could be developed by utilizing the ability of many –CH2–CH2– bridged FLPs to add other olefins or alkynes to form heterocyclic products. If it were possible to subsequently eliminate CH2QCH2 one might arrive at a new FLP that formally has been formed by ethylene/alkene or alkyne exchange. We have now found first experimental evidence that such an exchange reaction proceeding by an addition/elimination sequence might be feasible. We found this at an ethylene-bridged sulfur/boron frustrated Lewis pair. We generated an ethylene-bridged thioether/borane FLP by treating phenyl vinyl sulfide (1) with Piers’ borane [HB(C6F5)2] (2).10 This apparently gave the S/B FLP 3 (4 h, r.t., pentane); however, product 3 was found to be sparsely soluble in noncoordinating solvents. Addition of a small quantity of THF-d8 ¨t Mu ¨nster, Corrensstrasse 40, 48149 Organisch-Chemisches Institut der Universita ¨nster, Germany. E-mail: [email protected]; Fax: +49-251-8336503 Mu † Electronic supplementary information (ESI) available: Experimental and analytical details. CCDC 964462 (4), 964463 (7), and 976687 (6). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc47769j ‡ X-ray crystal structure analysis.
1980 | Chem. Commun., 2014, 50, 1980--1982
Scheme 1
rapidly cleared a suspension of 3 in benzene-d6, probably by forming the respective THF-d8 adduct 3 (THF-d8) (see Scheme 1). This product shows 1H/13C NMR resonances of the bridging –CH2–CH2– moiety (1H: d 2.79, 1.59; 13C: d 31.2, 23.8) and a 11B NMR signal at d 6.9. For further characterization (including X-ray diffraction) we treated the in situ generated FLP 3 with tert-butyl isocyanide. This gave the isonitrile borane adduct 4 (see Scheme 1). Single crystals were obtained from a dichloromethane solution. The X-ray crystal structure analysis confirmed that the hydroboration reaction of phenyl vinyl sulfide (1) had indeed formed the ethylene-bridged intramolecular vicinal S/B FLP as anticipated. We found the Ph–S-group attached at the central –CH2–CH2– backbone. The B(C6F5)2 substituent was found attached at the other end. The central [S]–CH2–CH2–[B] core had attained an antiperiplanar conformation. The borane Lewis acid had the tert-butyl isonitrile donor attached to it (see Fig. 1). In solution compound 4 shows a 11B NMR feature at d 19.5 and the 1H/13C NMR resonances of the –CH2–CH2– bridge at d 2.84/1.61 and d 33.0/20.3, respectively. Compound 4 shows an isonitrile IR band at ˜v 2290 cm 1. When we added p-tert-butylphenylacetylene to a suspension of the in situ generated FLP 3 in dichloromethane the mixture
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Fig. 1 Molecular structure of the isonitrile–FLP adduct 4 (thermal ellipsoids are shown with 30% probability). Selected bond lengths (Å) and angles (grad.); S1–C11 1.764(2), S1–C1 1.806(2), C1–C2 1.527(2), C2– B1 1.632(2), B1–C3 1.623(2), C3–N4 1.140(2), C11–S1–C1 106.2(1), S1– C1–C2 107.0(1), C1–C2–B1 114.2(1), C2–B1–C3 106.3(1), B1–C3–N4 176.9(2), C3–N4–C5 174.2(2), y S1–C1–C2–B1 176.9(1).
turned red. It was stirred for 3 h at r.t. to allow the reaction to go to completion. Workup then gave the six-membered heterocylic zwitterionic sulfonium/borate product 7 in 61% yield. Compound 7 was characterized by X-ray diffraction (single crystals were obtained from dichloromethane). The X-ray crystal structure analysis (see Fig. 2) revealed that the ethylene moiety had been lost during the reaction. It was replaced by the acetylene building block and then a second equivalent of the p-tert-butylphenylacetylene reagent had
Fig. 2 A view of the molecular structure of compound 7 (thermal ellipsoids are shown with 30% probability). Selected bond lengths (Å) and angles (grad.): S1–C1 1.793(3), S1–C3 1.783(3), C1–C2 1.326(4), C3–C4 1.340(4), C2–B1 1.607(5), C4–B1 1.601(4), S1–C11 1.786(3), C1–S1–C3 107.4(1), S1–C1–C2 120.8(2), C1–C2–B1 130.1(3), C2–B1–C4 110.9(2), B1–C4–C3 129.3(3), C4–C3–S1 121.4(2), C2–C1–C41–C42 146.8(3), C4–C3–C51–C52 48.4(5), C1–S1–C11–C12 118.0(3), B1–C2–C1–S1 –3.8(5), B1–C4–C3–S1 –1.9(5).
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been added. The resulting six membered heterocyclic core structure is close to planar. Both alkyne moieties were added regioselectively with the small CH end bonded to boron and the bulky C–Ar acetylene terminus ending up being bonded to sulfur. The boron coordination geometry is pseudo-tetrahedral whereas the sulfonium substituent features a non-planar trigonal-pyramidal orientation. The planes of the aryl rings at C1 and C3 are markedly rotated out of the plane of the central heterocycle. The plane of the phenyl substituent at sulfur is oriented almost perpendicular to the central heterocyclic plane (see Fig. 2). Compound 7 shows a 1H NMR singlet of the symmetry equivalent pair of endocyclic QCH[B] units at d 7.87 [corresponding 13C NMR signal at d 157.2 (broad)] and a 11B NMR resonance at d 17.0. The non-planar geometry at the sulfonium unit renders the C6F5 groups at boron inequivalent. Consequently we observed two corresponding sets of 19F NMR (o, p, m) resonances (for details see the ESI†). We obtained experimental evidence for the formation of intermediate 6 by carrying out the reaction of 3 with p-tertbutylphenylacetylene in a ca. 1 : 0.9 molar ratio under carefully controlled reaction conditions. The starting material 3 was generated in situ by treatment of PhS–CHQCH2 (1) with [HB(C6F5)2] (2) in d2dichloromethane at r.t. (10 min) followed by the addition of the alkyne reagent. NMR spectroscopy revealed that 6 was the major product of the reaction [d 7.73 (br, B–CHQ, 1H), d 149.1 (br, 13C), 11 B NMR: d 15.7 (n1/2 B 500 Hz); for further details see the ESI†]. Single crystals of compound 6 suitable for the X-ray crystal structure analysis were obtained from a dichloromethane solution over several days at 40 1C. In the crystal compound 6 shows a central CQC double bond (two independent molecules in the crystal, C1A–C2A 1.334(9) Å). The B(C6F5)2 substituent is attached at C2, and C1 bears both the aryl group introduced with the p-tertbutylphenylacetylene reagent and the Ph–S functional group originating from the starting material 3. There is a sulfur–boron interaction9b (S1A–B1A 2.106(9) Å) (Fig. 3).
Fig. 3 Molecular structure of compound 6 (thermal ellipsoids are shown with 30% probability).
Chem. Commun., 2014, 50, 1980--1982 | 1981
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Notes and references
Scheme 2
Subsequent addition of another molar equivalent of p-tertbutylphenylacetylene eventually resulted in the formation of product 7. We assume that our reaction sequence is initiated by the 1,2-addition of the S/B FLP 3 to one acetylenic building block.11 For related 1,2-addition reactions of vicinal ethylene-bridged phosphane/ borane FLPs an asynchronous concerted pathway was suggested by DFT calculation.12 A low barrier of the 1,2-S/B-addition in the systems studied here makes the ‘‘quasi-reverse’’ elimination of ethylene from the respective zwitterionic intermediate likely to also proceed facilely (see Scheme 2). This directly leads to the new unsaturated vicinal S/B FLP 6, which can then be trapped by 1,2-S/ B FLP addition to a second alkyne equivalent to eventually yield the observed product 7. We note that this reaction sequence in a way is remotely reminiscent of the ‘‘Alder–Rickert–Spaltung’’,13 which is a useful variant in organic Diels–Alder chemistry, although the two reactions are topologically different. Our new exchange reaction of C2-building blocks may eventually become similarly useful as a novel synthetic tool in frustrated Lewis pair chemistry. Financial support from the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
1982 | Chem. Commun., 2014, 50, 1980--1982
1 (a) D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2010, 49, 46–76; (b) D. W. Stephan and G. Erker, Top. Curr. Chem., 2013, 85–110. ¨hlich, S. Grimme and D. W. Stephan, 2 (a) P. Spies, G. Erker, G. Kehr, R. Fro Chem. Commun., 2007, 5072–5074; (b) G. Kehr, S. Schwendemann and G. Erker, Top. Curr. Chem., 2013, 45–83. 3 (a) S. Schwendemann, T. A. Tumay, K. V. Axenov, I. Peuser, G. Kehr, ¨hlich and G. Erker, Organometallics, 2010, 4, 1067–1069; R. Fro ¨mming, G. Kehr, R. Fro ¨hlich and G. Erker, (b) K. Axenov, C. M. Mo Chem.–Eur. J., 2010, 16, 14069–14073. 4 (a) A. J. P. Cardenas, B. J. Culotta, T. H. Warren, S. Grimme, A. Stute, ¨hlich, G. Kehr and G. Erker, Angew. Chem., Int. Ed., 2011, 50, R. Fro 7567–7571; (b) M. Sajid, A. Stute, A. J. P. Cardenas, B. J. Culotta, J. A. M. Hepperle, T. H. Warren, B. Schirmer, S. Grimme, A. Studer, ¨hlich, J. L. Petersen, G. Kehr and G. Erker, C. G. Daniliuc, R. Fro J. Am. Chem. Soc., 2012, 134, 10156–10168; (c) M. Sajid, G. Kehr, ¨ttgen, A. J. P. Cardenas, T. Wiegand, H. Eckert, C. Schwickert, R. Po ¨hlich, C. G. Daniliuc and G. Erker, J. Am. Chem. T. H. Warren, R. Fro Soc., 2013, 135, 8882–8895. ¨mming, E. Otten, G. Kehr, R. Fro ¨hlich, S. Grimme, 5 (a) C. M. Mo D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2009, 48, 6643–6646; (b) I. Peuser, R. C. Neu, X. Zhao, M. Ulrich, ¨hlich, S. Grimme, B. Schirmer, J. A. Tannert, G. Kehr, R. Fro G. Erker and D. W. Stephan, Chem.–Eur. J., 2011, 17, 9640–9650. 6 M. Sajid, A. Klose, B. Birkmann, L. Liang, B. Schirmer, T. Wiegand, ¨hlich, C. G. Daniliuc, S. Grimme, H. Eckert, A. J. Lough, R. Fro D. W. Stephan, G. Kehr and G. Erker, Chem. Sci., 2013, 4, 213–219. ¨mming, G. Kehr, B. Wibbeling, R. Fro ¨hlich, B. Schirmer, 7 C. M. Mo S. Grimme and G. Erker, Angew. Chem., Int. Ed., 2010, 49, 2414–2417. ¨mming, G. Kehr, R. Fro ¨hlich and G. Erker, Dalton Trans., 8 C. M. Mo 2010, 39, 7556–7564. ¨hlich, G. Kehr and G. Erker, J. Am. Chem. Soc., 9 (a) O. Ekkert, R. Fro ¨hlich, 2011, 133, 4610–4616; (b) C. Eller, C. G. Daniliuc, R. Fro G. Kehr and G. Erker, Organometallics, 2013, 32, 384–386. 10 (a) D. J. Parks, R. E. von H. Spence and W. E. Piers, Angew. Chem., Int. Ed. Engl., 1995, 34, 809–811; (b) W. E. Piers, D. J. Parks and G. P. A. Yap, Organometallics, 1998, 17, 5492–5503. 11 (a) C. A. Tanur and D. W. Stephan, Organometallics, 2011, 30, 3652–3657; see also: (b) M. A. Dureen and D. W. Stephan, J. Am. Chem. Soc., 2009, 131, 8396–8397(c) M. A. Dureen, C. C. Brown and D. W. Stephan, Organometallics, 2010, 29, 6594–6607. ¨mming, S. Fro ¨mel, G. Kehr, R. Fro ¨hlich, S. Grimme and 12 C. M. Mo G. Erker, J. Am. Chem. Soc., 2009, 131, 12280–12289. 13 K. Alder and H. F. Rickert, Justus Liebigs Ann. Chem., 1936, 524, 180–189.
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Cite this: Chem. Commun., 2014, 50, 1980 Received 9th October 2013, Accepted 17th December 2013
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Ethene/alkyne exchange reaction at an intramolecular frustrated Lewis pair† Christina Eller, Kathrin Bussmann, Gerald Kehr, Birgit Wibbeling,‡ Constantin G. Daniliuc‡ and Gerhard Erker*
DOI: 10.1039/c3cc47769j www.rsc.org/chemcomm
A vicinal ethylene bridged S/B frustrated Lewis pair (FLP) in situ generated by a hydroboration reaction of phenyl vinyl sulfide, reacts with p-tert-butylphenylacetylene by ethene/alkyne exchange and subsequent 1,2-addition of a second alkyne equivalent to give a zwitterionic sulfonium/borate product.
Frustrated Lewis pair (FLP) chemistry has seen interesting developments in recent years.1 Intramolecular vicinal FLPs were shown to undergo a variety of specific reactions with small molecules. Some effect heterolytic cleavage of dihydrogen and may serve as metal-free hydrogenation catalysts of specific substrates.2,3 Vicinal P/B FLPs were shown to add nitrogen monoxide,4 carbon dioxide5 or even sulfur dioxide;4c,6 they may add alkenes and alkynes,7 carbonyl compounds8 etc. Many vicinal FLPs are usually prepared either by hydroboration or by recent variants of the 1,1-carboboration reaction.9 Although these reactions are often simple to perform and were shown to be useful, it would be desirable to extend the spectrum of synthetic methods to such species. We thought that one attractive scheme could be developed by utilizing the ability of many –CH2–CH2– bridged FLPs to add other olefins or alkynes to form heterocyclic products. If it were possible to subsequently eliminate CH2QCH2 one might arrive at a new FLP that formally has been formed by ethylene/alkene or alkyne exchange. We have now found first experimental evidence that such an exchange reaction proceeding by an addition/elimination sequence might be feasible. We found this at an ethylene-bridged sulfur/boron frustrated Lewis pair. We generated an ethylene-bridged thioether/borane FLP by treating phenyl vinyl sulfide (1) with Piers’ borane [HB(C6F5)2] (2).10 This apparently gave the S/B FLP 3 (4 h, r.t., pentane); however, product 3 was found to be sparsely soluble in noncoordinating solvents. Addition of a small quantity of THF-d8 ¨t Mu ¨nster, Corrensstrasse 40, 48149 Organisch-Chemisches Institut der Universita ¨nster, Germany. E-mail: [email protected]; Fax: +49-251-8336503 Mu † Electronic supplementary information (ESI) available: Experimental and analytical details. CCDC 964462 (4), 964463 (7), and 976687 (6). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc47769j ‡ X-ray crystal structure analysis.
1980 | Chem. Commun., 2014, 50, 1980--1982
Scheme 1
rapidly cleared a suspension of 3 in benzene-d6, probably by forming the respective THF-d8 adduct 3 (THF-d8) (see Scheme 1). This product shows 1H/13C NMR resonances of the bridging –CH2–CH2– moiety (1H: d 2.79, 1.59; 13C: d 31.2, 23.8) and a 11B NMR signal at d 6.9. For further characterization (including X-ray diffraction) we treated the in situ generated FLP 3 with tert-butyl isocyanide. This gave the isonitrile borane adduct 4 (see Scheme 1). Single crystals were obtained from a dichloromethane solution. The X-ray crystal structure analysis confirmed that the hydroboration reaction of phenyl vinyl sulfide (1) had indeed formed the ethylene-bridged intramolecular vicinal S/B FLP as anticipated. We found the Ph–S-group attached at the central –CH2–CH2– backbone. The B(C6F5)2 substituent was found attached at the other end. The central [S]–CH2–CH2–[B] core had attained an antiperiplanar conformation. The borane Lewis acid had the tert-butyl isonitrile donor attached to it (see Fig. 1). In solution compound 4 shows a 11B NMR feature at d 19.5 and the 1H/13C NMR resonances of the –CH2–CH2– bridge at d 2.84/1.61 and d 33.0/20.3, respectively. Compound 4 shows an isonitrile IR band at ˜v 2290 cm 1. When we added p-tert-butylphenylacetylene to a suspension of the in situ generated FLP 3 in dichloromethane the mixture
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Fig. 1 Molecular structure of the isonitrile–FLP adduct 4 (thermal ellipsoids are shown with 30% probability). Selected bond lengths (Å) and angles (grad.); S1–C11 1.764(2), S1–C1 1.806(2), C1–C2 1.527(2), C2– B1 1.632(2), B1–C3 1.623(2), C3–N4 1.140(2), C11–S1–C1 106.2(1), S1– C1–C2 107.0(1), C1–C2–B1 114.2(1), C2–B1–C3 106.3(1), B1–C3–N4 176.9(2), C3–N4–C5 174.2(2), y S1–C1–C2–B1 176.9(1).
turned red. It was stirred for 3 h at r.t. to allow the reaction to go to completion. Workup then gave the six-membered heterocylic zwitterionic sulfonium/borate product 7 in 61% yield. Compound 7 was characterized by X-ray diffraction (single crystals were obtained from dichloromethane). The X-ray crystal structure analysis (see Fig. 2) revealed that the ethylene moiety had been lost during the reaction. It was replaced by the acetylene building block and then a second equivalent of the p-tert-butylphenylacetylene reagent had
Fig. 2 A view of the molecular structure of compound 7 (thermal ellipsoids are shown with 30% probability). Selected bond lengths (Å) and angles (grad.): S1–C1 1.793(3), S1–C3 1.783(3), C1–C2 1.326(4), C3–C4 1.340(4), C2–B1 1.607(5), C4–B1 1.601(4), S1–C11 1.786(3), C1–S1–C3 107.4(1), S1–C1–C2 120.8(2), C1–C2–B1 130.1(3), C2–B1–C4 110.9(2), B1–C4–C3 129.3(3), C4–C3–S1 121.4(2), C2–C1–C41–C42 146.8(3), C4–C3–C51–C52 48.4(5), C1–S1–C11–C12 118.0(3), B1–C2–C1–S1 –3.8(5), B1–C4–C3–S1 –1.9(5).
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been added. The resulting six membered heterocyclic core structure is close to planar. Both alkyne moieties were added regioselectively with the small CH end bonded to boron and the bulky C–Ar acetylene terminus ending up being bonded to sulfur. The boron coordination geometry is pseudo-tetrahedral whereas the sulfonium substituent features a non-planar trigonal-pyramidal orientation. The planes of the aryl rings at C1 and C3 are markedly rotated out of the plane of the central heterocycle. The plane of the phenyl substituent at sulfur is oriented almost perpendicular to the central heterocyclic plane (see Fig. 2). Compound 7 shows a 1H NMR singlet of the symmetry equivalent pair of endocyclic QCH[B] units at d 7.87 [corresponding 13C NMR signal at d 157.2 (broad)] and a 11B NMR resonance at d 17.0. The non-planar geometry at the sulfonium unit renders the C6F5 groups at boron inequivalent. Consequently we observed two corresponding sets of 19F NMR (o, p, m) resonances (for details see the ESI†). We obtained experimental evidence for the formation of intermediate 6 by carrying out the reaction of 3 with p-tertbutylphenylacetylene in a ca. 1 : 0.9 molar ratio under carefully controlled reaction conditions. The starting material 3 was generated in situ by treatment of PhS–CHQCH2 (1) with [HB(C6F5)2] (2) in d2dichloromethane at r.t. (10 min) followed by the addition of the alkyne reagent. NMR spectroscopy revealed that 6 was the major product of the reaction [d 7.73 (br, B–CHQ, 1H), d 149.1 (br, 13C), 11 B NMR: d 15.7 (n1/2 B 500 Hz); for further details see the ESI†]. Single crystals of compound 6 suitable for the X-ray crystal structure analysis were obtained from a dichloromethane solution over several days at 40 1C. In the crystal compound 6 shows a central CQC double bond (two independent molecules in the crystal, C1A–C2A 1.334(9) Å). The B(C6F5)2 substituent is attached at C2, and C1 bears both the aryl group introduced with the p-tertbutylphenylacetylene reagent and the Ph–S functional group originating from the starting material 3. There is a sulfur–boron interaction9b (S1A–B1A 2.106(9) Å) (Fig. 3).
Fig. 3 Molecular structure of compound 6 (thermal ellipsoids are shown with 30% probability).
Chem. Commun., 2014, 50, 1980--1982 | 1981
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Notes and references
Scheme 2
Subsequent addition of another molar equivalent of p-tertbutylphenylacetylene eventually resulted in the formation of product 7. We assume that our reaction sequence is initiated by the 1,2-addition of the S/B FLP 3 to one acetylenic building block.11 For related 1,2-addition reactions of vicinal ethylene-bridged phosphane/ borane FLPs an asynchronous concerted pathway was suggested by DFT calculation.12 A low barrier of the 1,2-S/B-addition in the systems studied here makes the ‘‘quasi-reverse’’ elimination of ethylene from the respective zwitterionic intermediate likely to also proceed facilely (see Scheme 2). This directly leads to the new unsaturated vicinal S/B FLP 6, which can then be trapped by 1,2-S/ B FLP addition to a second alkyne equivalent to eventually yield the observed product 7. We note that this reaction sequence in a way is remotely reminiscent of the ‘‘Alder–Rickert–Spaltung’’,13 which is a useful variant in organic Diels–Alder chemistry, although the two reactions are topologically different. Our new exchange reaction of C2-building blocks may eventually become similarly useful as a novel synthetic tool in frustrated Lewis pair chemistry. Financial support from the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
1982 | Chem. Commun., 2014, 50, 1980--1982
1 (a) D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2010, 49, 46–76; (b) D. W. Stephan and G. Erker, Top. Curr. Chem., 2013, 85–110. ¨hlich, S. Grimme and D. W. Stephan, 2 (a) P. Spies, G. Erker, G. Kehr, R. Fro Chem. Commun., 2007, 5072–5074; (b) G. Kehr, S. Schwendemann and G. Erker, Top. Curr. Chem., 2013, 45–83. 3 (a) S. Schwendemann, T. A. Tumay, K. V. Axenov, I. Peuser, G. Kehr, ¨hlich and G. Erker, Organometallics, 2010, 4, 1067–1069; R. Fro ¨mming, G. Kehr, R. Fro ¨hlich and G. Erker, (b) K. Axenov, C. M. Mo Chem.–Eur. J., 2010, 16, 14069–14073. 4 (a) A. J. P. Cardenas, B. J. Culotta, T. H. Warren, S. Grimme, A. Stute, ¨hlich, G. Kehr and G. Erker, Angew. Chem., Int. Ed., 2011, 50, R. Fro 7567–7571; (b) M. Sajid, A. Stute, A. J. P. Cardenas, B. J. Culotta, J. A. M. Hepperle, T. H. Warren, B. Schirmer, S. Grimme, A. Studer, ¨hlich, J. L. Petersen, G. Kehr and G. Erker, C. G. Daniliuc, R. Fro J. Am. Chem. Soc., 2012, 134, 10156–10168; (c) M. Sajid, G. Kehr, ¨ttgen, A. J. P. Cardenas, T. Wiegand, H. Eckert, C. Schwickert, R. Po ¨hlich, C. G. Daniliuc and G. Erker, J. Am. Chem. T. H. Warren, R. Fro Soc., 2013, 135, 8882–8895. ¨mming, E. Otten, G. Kehr, R. Fro ¨hlich, S. Grimme, 5 (a) C. M. Mo D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2009, 48, 6643–6646; (b) I. Peuser, R. C. Neu, X. Zhao, M. Ulrich, ¨hlich, S. Grimme, B. Schirmer, J. A. Tannert, G. Kehr, R. Fro G. Erker and D. W. Stephan, Chem.–Eur. J., 2011, 17, 9640–9650. 6 M. Sajid, A. Klose, B. Birkmann, L. Liang, B. Schirmer, T. Wiegand, ¨hlich, C. G. Daniliuc, S. Grimme, H. Eckert, A. J. Lough, R. Fro D. W. Stephan, G. Kehr and G. Erker, Chem. Sci., 2013, 4, 213–219. ¨mming, G. Kehr, B. Wibbeling, R. Fro ¨hlich, B. Schirmer, 7 C. M. Mo S. Grimme and G. Erker, Angew. Chem., Int. Ed., 2010, 49, 2414–2417. ¨mming, G. Kehr, R. Fro ¨hlich and G. Erker, Dalton Trans., 8 C. M. Mo 2010, 39, 7556–7564. ¨hlich, G. Kehr and G. Erker, J. Am. Chem. Soc., 9 (a) O. Ekkert, R. Fro ¨hlich, 2011, 133, 4610–4616; (b) C. Eller, C. G. Daniliuc, R. Fro G. Kehr and G. Erker, Organometallics, 2013, 32, 384–386. 10 (a) D. J. Parks, R. E. von H. Spence and W. E. Piers, Angew. Chem., Int. Ed. Engl., 1995, 34, 809–811; (b) W. E. Piers, D. J. Parks and G. P. A. Yap, Organometallics, 1998, 17, 5492–5503. 11 (a) C. A. Tanur and D. W. Stephan, Organometallics, 2011, 30, 3652–3657; see also: (b) M. A. Dureen and D. W. Stephan, J. Am. Chem. Soc., 2009, 131, 8396–8397(c) M. A. Dureen, C. C. Brown and D. W. Stephan, Organometallics, 2010, 29, 6594–6607. ¨mming, S. Fro ¨mel, G. Kehr, R. Fro ¨hlich, S. Grimme and 12 C. M. Mo G. Erker, J. Am. Chem. Soc., 2009, 131, 12280–12289. 13 K. Alder and H. F. Rickert, Justus Liebigs Ann. Chem., 1936, 524, 180–189.
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