Carbene Complexes Made Easily: Decomposition of Reissert

Carbene Complexes Made Easily: Decomposition of Reissert Compounds and Further Synthetic Approaches. Agnes Bittermann, Denys Baskakov and ... DOI: 10...
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Organometallics 2009, 28, 5107–5111 DOI: 10.1021/om900354v

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Carbene Complexes Made Easily: Decomposition of Reissert Compounds and Further Synthetic Approaches Agnes Bittermann, Denys Baskakov,† and Wolfgang A. Herrmann* Department of Chemistry, Chair of Inorganic Chemistry, Technische Universit€ at M€ unchen, Lichtenbergstrasse 4, 85747 Garching, Germany. † Current address: Lanxess Deutschland GmbH, 51369 Leverkusen, Germany Received May 5, 2009

New methods for the preparation of NHC-metal complexes have been developed. It was shown that a nitrile group in Reissert analogous compounds can act as a good leaving group in thermal elimination, resulting in carbene formation. Moreover potassium carbonate was introduced as a highly efficient and mild reagent for the generation of N-heterocyclic carbene complexes in good to excellent yields, even in water, as the most environmentally friendly medium.

Introduction Since the discovery of the first N-heterocyclic carbene (NHC) complexes in 1968 by O¨fele1 and by Wanzlick2 and the isolation of the first stable free carbene in 1991 by Arduengo III et al.,3 NHCs are playing an increasingly important role in major areas of organometallic, organic, and polymer chemistry.4 While numerous powerful catalytic systems incorporating NHC ligands have been described since our first reports,5 methods of synthesizing them have advanced more slowly. Common routes are deprotonation of a precursor imidazolium or imidazolinium salt, either by a strong base or by a

basic ligand,6 oxidative addition of 2-chloro-1,3-disubstituted imidazolinium salts to appropriate metal complexes, transmetalation via silver complexes, and thermolysis of carbene adducts in the presence of an appropriate metal precursor.7 Generation of a free carbene intermediate in the first route necessitates dry, air-free conditions and provides limited tolerance of functionalities, while oxidative addition is known only for a number of specific cases.8 In an important advance, Wang and Lin showed that Ag2O can be used to form a Ag-NHC complex from an imidazolium salt that readily transfers the NHC to palladium.9 The great advantage of this method is the broad tolerance for sensitive N-substituents, which can be destroyed by conventional deprotonation of an imidazolium salt with strong bases. Transmetalation to various metal species gives a wide variety of NHCs coordinated to rhodium, copper, ruthenium, and iridium. However Ag-induced oxidative degradation of the imidazolium precursor severely limits the use of this method.10 Another successful method was established by Enders, who synthesized a triazol-2-ylidene by thermolysis of its methanol adduct in good yield.11 A significant drawback of this methodology is the extreme sensitivity of the

*Corresponding author. Tel.: þ49 89 289 13080; fax: þ49 89 289 13473. E-mail address: [email protected]. (1) O¨fele, K. J. Organomet. Chem. 1968, 12, P42. (2) Wanzlick, H. W.; Sch€ onherr, H. J. Angew. Chem., Int. Ed. Engl. 1968, 7, 141. (3) (a) Arduengo, A. J.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113, 361. (b) Arduengo, A. J.; Dias, H. V. R.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1992, 114, 5530. (4) (a) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (b) Jafarpour, L.; Nolan, S. P. Adv. Organomet. Chem. 2001, 46, 181. (c) Bourrissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39. (d) Herrmann, W. A.; K€ocher, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 2162. (e) Nair, V.; Bindu, S.; Sreekumar, V. Angew. Chem. 2005, 117, 1941. (f) Scholten, M. D.; Hedrick, J. L.; Waymouth, R. M. Macromolecules 2008, 41, 7399. (g) McGuinness, D. S.; Suttil, J. A.; Gardiner, M. G.; Davies, N. W. Organometallics 2008, 27, 4238. (h) Raynaud, J.; Ciolino, A.; Baceiredo, A.; Destarac, M.; Bonnette, F.; Kato, T.; Gnanou, Y.; Taton, D. Angew. Chem., Int. Ed. 2008, 47, 5390. (5) (a) Hill, J. E.; Nile, T. A. J. Organomet. Chem. 1977, 137, 293. (b) Lappert, M. F.; Maskell, R. K. J. Organomet. Chem. 1984, 264, 217. (c) Gardiner, M. G.; Herrmann, W. A.; Reisinger, C. P.; Schwarz, J.; Spiegler, M. J. Organomet. Chem. 1999, 572, 239. (d) Hillier, A. C.; Lee, H. M.; Stevens, E. D.; Nolan, S. P. Organometallics 2001, 20, 4246. (e) Lee, S.; Beare, N. A.; Hartwig, J. F. J. Am. Chem. Soc. 2001, 123, 8410. (f) Grasa, G. A.; Viciu, M. S.; Huang, J. K.; Zhang, C. M.; Trudell, M. L.; Nolan, S. P. Organometallics 2002, 21, 2866. (g) Jackstell, R.; Frish, A.; Beller, M.; R€ottger, D.; Malaun, M.; Bildstein, B. J. Mol. Catal. A: Chem. 2002, 185, 105. (h) Herrmann, W. A.; Baskakov, D.; Herdtweck, E.; Hoffmann, S. D.; Bunlaksananusorn, T.; Rampf, F.; Rodefeld, L. Organometallics 2006, 25, 2449. (i) Sholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1991, 1, 953. (6) Herrmann, W. A.; K€ ocher, C.; Goossen, L. J.; Artus, G. R. J. Chem. Eur. J. 1996, 2, 1627.

(7) (a) Wanzlick, H. W.; Esser, F.; Kleiner, H. J. Chem. Ber. 1963, 96, 1208. (b) Yamaguchi, Y.; Kashiwabara, T.; Ogata, K.; Miura, Y.; Nakamura, Y.; Kobayashic, K.; Ito, T. Chem. Commun. 2004, 2160. (c) Trnka, T. M.; Morgan, J. P.; Sanford, M. S.; Wilhelm, T. E.; Scholl, M.; Choi, T. L.; Ding, S.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2003, 125, 2546. (d) Boydston, A. J.; Xia, Y.; Kornfield, J. A.; Gorodetskaya, I. A.; Grubbs, R. H. J. Am. Chem. Soc. 2008, 130, 12775. (e) Courchay, F. C.; Sworen, J. C.; Ghiviriga, I.; Abboud, K. A.; Wagener, K. B. Organometallics 2006, 25, 6074. (f) G€unay, M. E.; Ayg€un, M.; Kartal, A.; Cetinkaya, B.; Kendi, E. Cryst. Res. Technol. 2006, 41, 615. (g) Xu, G.; Gilbertson, S. R. Org. Lett. 2005, 7, 4605. (h) Tuerkmen, H.; Cetinkaya, B. J. Organomet. Chem. 2006, 691, 3749. (8) Kremzow, D.; Seidel, G.; Lehmann, C. W.; F€ urstner, A. Chem. Eur. J. 2005, 11, 1833. (9) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972. (10) Baskakov, D.; Herrmann, W. A.; Herdtweck, E.; Hoffmann, S. Organometallics 2007, 26, 626. (11) Enders, D.; Breuer, K.; Raabe, G.; Runsink, J.; Teles, J. H.; Melder, J.-P.; Ebel, K.; Brode, S. Angew. Chem., Int. Ed. Engl. 1995, 34, 1021.

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methanol adduct. Thermal elimination of chloroform or pentafluorobenzene from their diamino adducts resulting in free NHCs was used by Nyce et al. The great advantage of this method is the high stability and complete lack of air and moisture sensitivity of these carbene precursors.12 Finally Crabtree et al. used imidazolium-2-carboxylates as a starting material in the synthesis of NHC complexes through elimination of carbon dioxide. Despite very mild conditions, a serious disadvantage of this route consists in the necessity to use strong bases for the generation of the corresponding CO2 adduct.13 Development of further methods is therefore eagerly sought. In our studies on homogeneous catalysis,14 we became interested in the development of new, simple, general synthetic approaches for the preparation of NHC complexes. We now report here that N,N-disubstituted imidazolium2-cyanides can transfer NHCs to metal complexes with release of HCN. Furthermore during the course of our studies we found that potassium carbonate, a cheap, toxicologically harmless, air- and moisture-stable substance, can be successfully used for the generation of a wide variety of NHC complexes in quantitative yield under very mild conditions.

Results and Discussion Synthesis of NHC-Rh(I) Complexes through Thermal Elimination of Hydrogen Cyanide. R-Acylaminonitriles (generally known in the literature as Reissert compounds) have proven to be useful intermediates in the synthesis of various heterocyclic compounds, such as derivatives of isoquinoline, quinoline, quinazoline, and benzimidazole, including alkaloids and other biologically active compounds.15 Their close structural relationship to carbene adducts commonly used as precursors in complex synthesis through thermal decomposition urged us to investigate their properties. In 1965 Brown briefly described dimerization of R,R-bis(N-pyrrolidino)acetonitrile under elevated temperature accompanied by the evolution of HCN gas with the formation of tetrakis(N-pyrrolidino)ethylene.16 This compound can be considered as NHC-dimer of the corresponding R,R-bis(N-pyrrolidino)carbene. To the best of our knowledge no further evidence for the application of Reissert analogous compounds in the NHC synthesis is published up to date. 1,3-Dibenzyl-2-cyano-2,3-dihydroimidazole (2) was prepared in excellent yield by the reaction of 1,3-dibenzylimidazolium bromide (1) with potassium cyanide absorbed on aluminum oxide in anhydrous methylene chloride (Scheme 1).17 To test the possibility of carbene generation, (12) (a) Nyce, G. W.; Csihony, S.; Waymouth, R. M.; Hedrick, J. L. Chem. Eur. J. 2004, 10, 4073. (b) Bittermann, A.; H€arter, P.; Herdtweck, E.; Hoffmann, S. D.; Herrmann, W. A. J. Organomet. Chem. 2008, 693, 2079. (13) (a) Voutchkova, A. M.; Appelhans, L. N.; Chianese, A. R.; Crabtree, R. H. J. Am. Chem. Soc. 2005, 127, 17624. (b) Voutchkova, A. M.; Feliz, M.; Clot, E.; Eisenstein, O.; Crabtree, R. H. J. Am. Chem. Soc. 2007, 129, 12834. (c) Delaude, L. Eur. J. Inorg. Chem. 2009, 13, 1681. (14) (a) Frey, G. D.; Reisinger, C. P.; Herdtweck, E.; Herrmann, W. A. J. Organomet. Chem. 2005, 690, 3193. (b) Sch€utz, J.; Herdtweck, E.; Herrmann, W. A. Organometallics 2004, 23, 6084. (c) Herrmann, W. A.; Öfele, K.; von Preysing, D.; Herdtweck, E. J. Organomet. Chem. 2003, 684, 235. (d) Denk, K.; Fridgen, J.; Herrmann, W. A. Adv. Synth. Catal. 2002, 344, 666. (e) Herrmann, W. A.; H€arter, P.; Gst€ottmayr, C. W. K.; Bielert, F.; Seeboth, N.; Sirsch, P. J. Organomet. Chem. 2002, 649, 141. (15) Jois, J. H. R.; Gibson, H. W. J. Org. Chem. 1991, 56, 865. (16) Brown, M. U.S. Patent 3214428, 1965. (17) (a) Regen, S. L.; Quici, S.; Liaw, S. J. J. Org. Chem. 1979, 44, 2029. (b) Salerno, A.; Ceriani, V.; Perillo, I. A. J. Heterocycl. Chem. 1997, 34, 709.

Bittermann et al. Scheme 1. Synthesis of Cyanide Adduct 2

this compound was subjected to thermal gravimetric analysis (Figure 1). The cyanide adduct began to show weight loss at 107 °C. In the TGA experiment, a 9.4% weight loss was observed at 107-218 °C. This correlates with the theoretical percent weight loss of HCN from 2 (9.8%). Carbene formation could also be confirmed by mass spectroscopy. In the range of 107-218 °C a flow of ions with a molecular mass of 27 amu was observed, which could be attributed to hydrogen cyanide (Mw =27 amu). However all attempts to obtain NHC-Rh(I) complexes through the reaction with [Rh(COD)Cl]2 at elevated temperatures in toluene were unsuccessful, resulting in formation of the corresponding imidazolium salt. Hydrogen cyanide is a weak acid (pKa =9.2-9.3), but it can still easily protonate an intermediate-free N-heterocyclic carbene (pKa =20-24) with generation of formamidinium salts. To neutralize HCN formed during the carbene formation, a large excess of potassium carbonate was added to the reaction mixture (Scheme 2). Indeed in this transformation rhodium-NHC complex 2Rh could be obtained in excellent yield. Potassium carbonate can play a double role in the carbene generation in this case. It can simply neutralize HCN formed in the course of the reaction through acid-base interaction, or alternatively it can deprotonate 2-cyanoimidazole, forming a carbanionic species stabilized through resonance with the pendant nitrile group. This species can quickly decompose with formation of NHC and release of cyanide ion. However addition of other, stronger bases such as KOtBu gave no positive effect on the reaction. Another possibility to remove HCN through addition of complexating agent such as FeCl3 to the reaction mixture was equally unsuccessful. Synthesis of NHC-Metal Complexes with Potassium Carbonate. Due to the astounding and profound effect potassium carbonate had on the NHC-complex formation described above, we decided to investigate the reaction of simple formamidinium salts with potassium carbonate as a model reaction. Alder has measured the pKa value of diisopropylimidazolin-2-ylidene in DMSO to be 24.18 For di-tert-butylimidazolin-2-ylidene Streitwieser found a pKa of 20 on the THF scale.19 Furthermore Amyes et al. have measured a pKa of 24 for the carbon acid of imidazolium cations in aqueous solution.20 Therefore, it is not surprising that the principal method used for NHC synthesis is deprotonation of the corresponding imidazolium or formamidinium salts using strong bases in the strict absence of water. For the isolation of the first NHC, Arduengo’s group used NaH/KH in THF in the presence of KOtBu and DMSO. Our group showed that milder conditions, such as sodium amide in liquid ammonia and THF at -40 °C, were also efficient.6 (18) (a) Alder, R. W.; Blake, M. E.; Oliva, J. M. J. Phys. Chem. A 1999, 103, 11200. (b) Alder, R. W. In Carbene Chemistry; 2002; p 153. (19) Kim, Y.-J.; Streitwieser, A. J. Am. Chem. Soc. 2002, 124, 5757. (20) Amyes, T. L.; Diver, S. T.; Richard, J. P.; Rivas, F. M.; Toth, K. J. Am. Chem. Soc. 2004, 126, 4366. (21) Herrmann, W. A.; K€ ocher, C.; Goossen, L. J.; Artus, G. R. J. Chem. Eur. J. 1996, 2, 1627.

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Figure 1. Thermogravimetric analysis of 2. Scheme 2. Synthesis of Rhodium Complex 2Rh by Thermal Decomposition of 2

The pKa of potassium carbonate (10.25) lies well below these values, and therefore this base was considered to be unsuitable for the generation of classical N-heterocyclic carbenes. During the course of our studies two publications concerning deprotonation of NHC precursors with potassium carbonate appeared in the literature. Thus You et al. reported formation of dimers upon addition of K2CO3 to triazolium salts.22 Also Kotschy et al. reported synthesis of quinoidal tetrazines through the action of inorganic carbonate on the mixture of dipyrazolyltetrazine and imidazolium salt under strictly inert conditions, indicating carbene generation as a possible mechanism of the formation of this fascinating compound class.23 Several reports describing the use of carbonates in the generation of NHC-metal complexes appeared in literature.24 However all described protocols suffered from narrow scope of used ligands, low yield, and significant decomposition of imidazolium salts to formamide derivatives of the corresponding amines. In our first trial, a simple 1,3-dimethylimidazolium salt (3) reacted with [Rh(COD)Cl]2 under slightly elevated temperature in toluene in the presence of a large excess of potassium carbonate to give the corresponding Rh-NHC complex (3Rh) in quantitative yield (Scheme 3). Due to this result, we tried to establish the scope of this transformation by reacting a series of NHC precursors (1, 3, 4) with [Rh(COD)Cl]2. The reaction rate showed a clear dependence on the nature of the N-substituents. Increasing steric demand (22) Ma, Y.; Wei, S.; Lan, J.; Wang, J.; Xie, R.; You, J. J. Org. Chem. 2008, 73, 8256. (23) Bostai, B.; Novk, Z.; Bnyei, A. C.; Kotschy, A. Org. Lett. 2007, 9, 3437. (24) (a) Liao, C. Y.; Chan, K. T.; Tu, C. Y.; Chang, Y. W.; Hu, C. H.; Lee, H. M. Chem. Eur. J. 2009, 15, 405. (b) Winkelmann, O.; N€ather, C; L€ uning, U. J. Organomet. Chem. 2008, 693, 923.

Scheme 3. Synthesis of Rhodium Imidazol-2-ylidene Complexes via K2CO3 Route

Scheme 4. Synthesis of Rhodium Imidazolin-2-ylidene Complex 7Rh via K2CO3 Route

of the substituent led to a marked decrease in the time needed to achieve 100% yield (1, 3, 4; Scheme 3). Experiments also revealed that the nature of the carbene backbone has a substantial influence on the reaction rate. Reactants based on the imidazole framework were all very efficient, and the products 5Rh and 6Rh were isolated in quantitative yield after only 2 h at a temperature of 70 °C (Scheme 3). The reaction rate of imidazolinium salt 7 on the other hand was drastically decreased. Even after 18 h only a 40% yield of 7Rh could be isolated (Scheme 4). Use of different counterions such as noncoordinating tetrafluoroborate brought no improvement. Backbone substituents however played an important role, which was demonstrated on Cl- and H-substituted N,N-dibenzylimidazolium salts 8 and 1 (Scheme 3). In these cases the time needed to achieve quantitative yield decreased from 5 to 2 h for 8Rh and 2Rh, respectively (1, 8; Scheme 3). In search of the scope and limitations of this simple synthetic approach we decided to perform it in the cheapest

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Scheme 5. Synthesis of Rhodium Imidazol-2-ylidene Complex via K2CO3 Route in Water

and environmentally friendliest solvent known: water. To our astonishment, the model reaction with 1,3-dibenzylimidazolium bromide takes place in this heterogeneous mixture under slightly elevated temperature and air exposure (Scheme 5). A quantitative yield could be achieved after 20 h.

Summary Two new distinct methods for the preparation of NHCmetal complexes were developed in the course of this study. It was shown that the nitrile group in Reissert analogous compounds can act as a good leaving group in thermal elimination, resulting in carbene formation. Also potassium carbonate was introduced as a highly efficient and mild reagent for the generation of N-heterocyclic carbene complexes in good to excellent yields, even in aqueous medium. Further studies of the scope and limitations, as well as of the mechanistic details of these important transformations, are in progress in our laboratory and will be reported in due time.

Experimental Section General Methods. Imidazolium and imidazolinium salts 1 and 3-8 used in this study were prepared according to literature procedures.6,12b,25,26 All other materials were obtained commercially and were used as received. 1,3-Dibenzyl-2-cyano-2,3dihydroimidazole (2) and its rhodium complex 2Rh were prepared through thermal elimination of HCN performed under an atmosphere of argon, using solvents dried on an alumina-based solvent purification system. NMR spectra were recorded on a JEOL JMX-GX 400 spectrometer operating at 400 MHz (1H NMR) and 100 MHz (13C NMR) at room temperature. Chemical shifts are given in ppm. The spectra are calibrated to the residual protons of the solvents. NMR multiplicities are abbreviated as follows: s=singlet, d=doublet, t=triplet, q= quartet, p=quintet, sp=septet, m=multiplet, br=broad signal. IR spectra were acquired using a Jasco FT/IR-460 Plus spectrometer. Thermogravimetric mass spectra (TGA) analysis measurements were conducted with a Netzsch TG209 system; typically about 10 mg was heated at 5 K min-1. Analytical data for 2Rh, 8Rh,12b 3Rh, 5Rh, 6Rh,26 and 7b27 correlate with the literature. KCN Supported on Alumina. One mole of potassium cyanide was dissolved in 1.3 mL of distilled water. 1.3 g of neutral aluminum oxide 90 was added, and solvent was removed in vacuo. For safety reasons sodium hypochlorite was added to a trap, to destroy nascent HCN. 1,3-Dibenzyl-2-cyano-2,3-dihydro-imidazole (2). 2,3-Dibenzylimidazolium bromide (1) (1 mmol) was dissolved in 15 mL (25) (a) Magill, A. M.; Cavell, K. J.; Yates, B. F. J. Am. Chem. Soc. 2004, 126, 8717. (b) Hindi, K. M.; Siciliano, T. J.; Durmus, S.; Panzner, M. J.; Medvetz, D. A.; Reddy, D. V.; Hogue, L. A.; Hovis, C. E.; Hilliard, J. K.; Mallet, R. J.; Tessier, C. A.; Cannon, C. L.; Youngs, W. J. J. Med. Chem. 2008, 51, 1577. (c) Starikova, O. V.; Dolgushin, G. V.; Larina, L. I.; Ushakov, P. E.; Komarova, T. N.; Lopyrev, V. A. Russ. J. Org. Chem. 2003, 39, 1467. (26) (a) Khramov, D. M.; Lynch, V. M.; Bielawski, C. W. Organometallics 2007, 26, 6042. (b) Herrmann, W. A.; Elison, M.; Fischer, J.; Kocher, C.; Artus, G. R. J. Chem. Eur. J. 1996, 2, 772. (27) Alder, R. W.; Blake, M. E.; Bufali, S.; Butts, C. P.; Orpen, A. G.; Sch€ utz, J.; Williams, S. J. J. Chem. Soc., Perkin Trans. 1 2001, 1586.

Bittermann et al. of dry dichloromethane. KCN supported on alumina (3 g, 0.017 mol KCN) was added during 10 min in two portions. After stirring the mixture for 17 h at room temperature, it was filtered and washed with 10 mL of dichloromethane. Solvent was removed in vacuo, and the resulting thick yellow oil was washed three times with 5 mL of THF and dried under reduced pressure. Yield: 256 mg (0.929 mmol, 93%). 1H NMR (CDCl3): δ 7.32 (s, 2H), 7.25 (m, 5H), 7.07 (m, 6H) (HCCH, CHbenzyl, HCCN), 5.29 (s, 4H, CH2). 13C NMR (CDCl3): δ 132.63, 128.80, 128.76, 128.39 (CHbenzyl), 121.87 (HCCH, CN), 77.40 (CCN), 52.51 (CH2). IR (CH2Cl2): νmax/cm-1 =2108 (CN). Rhodium Complex 2Rh through Thermal Elimination of HCN. Chloro(η4-1,5-COD)(1,3-dibenzyl-1H-imidazol-2-ylidene)rhodium(I) (2Rh). 1,3-Dibenzyl-2-cyano-2,3-dihydroimidazole (2) (256 mg, 0.930 mmol, 1 equiv) and 0.5 equiv of [Rh(COD)Cl]2 were dissolved in 10 mL of toluene. Potassium carbonate (10 equiv) was added in one portion, and the suspension was stirred at 70 °C for 19 h. Afterwards solvent was removed and the product was purified by column chromatography with dichloromethane as solvent. Yield: 91% (210 mg, 0.424 mmol). 1H NMR (CDCl3): δ 7.38 (m, 10H, CHbenzyl), 6.65 (s, 2H, CHimidazoline), 5.88 (d, 2H, 2J(H-C-H) = 14.8 Hz, CH2benzyl), 5.76 (d, 2H, 2 J(H-C-H)=14.8 Hz, CH2benzyl), 5.07 (m, 2H, codvinyl), 3.32 (m, 2H, codvinyl), 2.31 (m, 4H, codallyl), 1.88 (m, 4H, codallyl). 13C NMR (CDCl3): δ 183.60 (d, 1J(C-Rh) = 51.5 Hz, NCN), 136.54, 128.96, 128.26, 128.18 (Cbenzyl), 120.98 (Cimidazoline), 98.90, 98.83, 68.49, 68.35 (codvinyl), 54.69 (CH2benzyl), 32.94, 28.85 (codallyl). Rhodium Complexes through K2CO3 Route in Toluene. 0.222 mmol of the corresponding imidazolium salt was dissolved in 7 mL of toluene. Then 0.111 mmol of [Rh(COD)Cl]2 and 2.220 mmol of K2CO3 were added in one portion. The mixture was stirred at 70 °C, and the course of the reaction was followed by TLC. Reaction time is indicated in Scheme 3. Solvent was removed, and the product was purified using column chromatography, with dichloromethane as a solvent. The yellow solid product was dried under reduced pressure. In all cases quantitative yield was obtained. Chloro(η4-1,5-COD)(1,3-dimethyl-1H-imidazol-2-ylidene)rhodium(I) (3Rh). 1H NMR (CDCl3): δ 6.81 (s, 2H, CH), 5.16 (m, 2H, codvinyl), 3.94 (s, 6H, CH3), 3.43 (m, 2H, codvinyl), 2.29 (m, 4H, codallyl), 1.94 (m, 2H, codallyl), 1.77 (m, 2H, codallyl). 13C NMR (CDCl3): δ 181.95 (d, 1J(C-Rh)=48.9 Hz, NCN), 122.07 (Cimidazoline), 96.20, 96.14, 71.07, 70.93, 37.66 (codvinyl), 37.63 (CH3), 32.22, 29.41 (codallyl). Chloro(η4-1,5-COD)(4,5-dichloro-1,3-dimethyl-1H-imidazol2-ylidene)rhodium(I) (5Rh). 1H NMR (CDCl3): δ 5.39 (m, 2H, codvinyl), 4.17 (s, 6H, CH3), 3.46 (m, 2H, codvinyl), 2.36 (m, 4H, codallyl), 2.05 (m, 2H, codallyl), 1.89 (m, 2H, codallyl). 13C NMR (CDCl3): δ 183.40 (d, 1J(C-Rh)=50.3 Hz, NCN), 116.54 (CCl), 97.46, 97.39, 72.07, 71.93 (codvinyl), 36.63 (CH3), 32.33, 29.60 (codallyl). Chloro(η4-1,5-COD)(4,5-dicyano-1,3-dimethyl-1H-imidazol2-ylidene)rhodium(I) (6Rh). 1H NMR (CDCl3): δ 5.22 (m, 2H, codvinyl), 3.94 (s, 6H, CH3), 3.42 (m, 2H, codvinyl), 2.30 (m, 4H, codallyl), 1.96 (m, 2H, codallyl), 1.80 (m, 2H, codallyl). 13C NMR (CDCl3): δ 183.43 (d, 1J(C-Rh) = 50.5 Hz, NCN), 116.56 (CCN), 100.00 (CN), 97.49, 97.42, 72.07, 71.94 (codvinyl), 36.65 (CH3), 32.35, 29.62 (codallyl). Chloro(η4-1,5-COD)(4,5-dichloro-1,3-dibenzyl-1H-imidazol2-ylidene)rhodium(I) (8Rh). 1H NMR (CDCl3): δ 5.39 (s, 2H, codvinyl), 4.17 (s, 6H, CH3), 3.46 (s, 2H, codvinyl), 2.05 (m, 2H, codallyl), 1.89 (m, 2H, codallyl), 1.68 (m, 4H, codallyl). 13C NMR (CDCl3): δ 183.40 (d, 1J(C-Rh) = 50.3 Hz, NCN), 116.54 (CCl), 97.45, 97.39, 72.07, 71.93 (codvinyl), 36.62 (CH3), 32.33, 29.60 (codallyl). Chloro(η4-1,5-COD)(1,3-diisopropyl-1H-imidazol-2-ylidene)rhodium(I) (4Rh). 1H NMR (CDCl3): δ 6.85 (s, 2H, CHimidazole), 5.70 (sp, 2H, 3J(H-C-C-H)=7.2 Hz, CHisopropyl), 4.95 (s, 2H, codvinyl), 3.28 (s, 2H, codvinyl), 2.35 (m, 4H, codallyl), 1.89 (m, 4H,

Article codallyl), 1.45 (12H, 2J(H-C-H) = 9.2 Hz, 3J(H-C-C-H) = 7.2 Hz, 4 CH3). 13C NMR (CDCl3): δ 179.59 (d, 1J(C-Rh) = 50.8 Hz, NCN), 116.78 (CHimidazole), 97.86, 97.79, 67.51, 67.37 (codvinyl), 52.49 (CHisopropyl), 32.94, 28.84 (codallyl), 24.19, 23.26 (CH3). Rhodium Complexes through K2CO3 Route in Water. 0.222 mmol of 1,3-dibenzylimidazolium bromide (1) was dissolved in 7 mL of water. Then 0.111 mmol of [Rh(COD)Cl]2 and 2.220 mmol of K2CO3 were added in one portion. The heterogeneous mixture was stirred overnight at 70 °C. The aqueous phase was extracted three times with 10 mL of dichloromethane. The combined organic phases were dried over Na2SO4 and filtered. Solvent was removed. The yellow solid product was dried under reduced pressure. Quantitative yield was obtained. 1,3-Diisopropyl-4,5-dihydro-3H-imidazol-1-ium Chloride (7a). Concentrated HCl (10 mL) was slowly added under cooling to 1.0 mL of N,N-diisopropylethylenediamine (5.532 mmol). Solvent was removed by rotary evaporator. Then 10 mL of triethyl orthoformate and 2 drops of formic acid were added. The mixture was stirred overnight at 120 °C. Solvent was removed, and the white greasy solid was dried under reduced pressure. Yield: 1.00 g (5.255 mmol, 95%). 1H NMR (DMSO-d6): δ 8.91 (s, 1H, NCH), 3.91 (sp, 2H, 3J(H-C-C-H) = 6.8 Hz, CHisopropyl), 3.87 (s, 4H, CH2), 1.23 (dd, 12H, 2J(H-C-H) = 6.4 Hz, 3J(H-C-C-H)=2.0 Hz, 4 CH3). 13C NMR (DMSOd6): δ 115.14 (NCH), 49.30 (CHisopropyl), 44.79 (CH2), 20.23 (CH3). 1,3-Diisopropyl-4,5-dihydro-3H-imidazol-1-ium Tetrafluoroborate (7b). 1,3-Diisopropyl-4,5-dihydro-3H-imidazol-1-ium chloride (7a) (1.00 g, 5.243 mmol) was dissolved in 20 mL of water and poured into 20 mL of aqueous ammonium tetrafluoroborate solution (1 g of NH4BF4, 9.538 mmol). This mixture

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was extracted two times with 20 mL of dichloromethane. The organic solution was dried over Na2SO4 and filtered, solvent was removed, and the light yellow solid was dried under reduced pressure. Yield: 501 mg (4.131 mmol, 79%). 1H NMR ((CD3)2CO): δ 8.19 (s, 1H, NCH), 3.99 (s, 4H, CH2), 3.95 (sp, 2H, 3J(H-C-C-H)=3.4 Hz, CHisopropyl), 1.27 (dd, 6H, 2J(HC-H)=6.8 Hz, 3J(H-C-C-H)=2.0 (1.319, 1.302, 1.237, 1.220) Hz, 4 CH3). 13C NMR ((CD3)2CO): δ 154.77 (NCH), 50.22 (CHisopropyl), 45.18 (CH2), 19.84 (CH3). 19F (CDCl3): δ 151.10. Chloro(η4-1,5-COD)(1,3-diisopropyl-4,5-dihydro-1H-imidazolin-2-ylidene)rhodium(I) (7Rh). Imidazolidinium salt (0.222 mmol) was dissolved in 10 mL of toluene. Then 0.111 mmol of [Rh(COD)Cl]2 and 2.220 mmol of K2CO3 were added. The mixture was stirred overnight at 70 °C, and the course of the reaction was followed by TLC. Solvent was removed, and the product was purified using column chromatography with dichloromethane as solvent. The yellow solid product was dried under reduced pressure. Yield: 36 mg (0.090 mmol, 40%). 1H NMR (CDCl3): δ 5.55 (sp, 2H, 3J(H-C-C-H) = 6.8 Hz, CHisopropyl), 4.92 (m, 2H, codvinyl), 3.44 (m, 2H, codvinyl), 3.31 (m, 4H, CH2), 2.32 (m, 4H, codallyl), 1.88 (m, 4H, codallyl), 1.27 (dd, 6H, 2J(H-C-H)=19.6 Hz, 3J(H-CC-H)=6.8 Hz, 2 CH3). 13C NMR (CDCl3): δ 209.81 (d, 1J(CRh) = 46.4 Hz, NCN), 98.37, 98.30, 67.75, 67.60 (codvinyl), 50.91 (CHisopropyl), 41.70 (CH2), 32.96, 28.80 (codallyl), 21.07, 20.42 (CH3).

Acknowledgment. We gratefully acknowledge financial support from the BMBF (Bundesministerium f€ ur Bildung und Forschung).