Synthesis of 2,3-Disubstituted Pyrroles Initiated by 1,4-Addition of

Article pubs.acs.org/Organometallics

Synthesis of 2,3-Disubstituted Pyrroles Initiated by 1,4-Addition of Chelated Enolates onto Alkynyl Carbene Complexes Rupsha Chaudhuri and Uli Kazmaier* Institute of Organic Chemistry, Saarland University, Campus, Building C4.2, D-66123 Saarbruecken, Germany S Supporting Information *

ABSTRACT: Chelated glycine ester enolates, which have been found to be good nucleophiles, e.g., for Michael additions, undergo 1,4-addtition toward alkynyl Fischer carbene complexes. Instead of the expected unsaturated carbene amino acid derivatives, 2,3-disubstituted pyrroles are formed, probably by a domino process initiated by the 1,4-addition. Subsequent 1,3-H-shift, 1,2-addition, and reductive elimination during workup finalized the reaction cascade.

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INTRODUCTION Since their first description by Fischer and Maasbol in 1964,1 Fischer carbene complexes have been developed into very powerful tools for organic synthesis.2 In general, group 6 metals are incorporated into Fischer carbenes, of which chromium is the most popular metal with the most applications, while the chemistry of the analogous molybdenum and tungsten complexes is rich and complex. While alkyl carbene complexes show reaction behavior comparable to esters, the corresponding alkenyl and alkynyl carbene complexes are excellent substrates for cycloadditions.2 This can be explained by a strong activating effect of the metal fragment, comparable to a Lewis acid directly attached to the carbonyl group of the corresponding isolable ester.3 Therefore, the term “super ester” might describe the reaction behavior best. Alkenyl and alkynyl carbene complexes are also excellent Michael acceptors, and a wide range of nucleophiles can be added in a 1,4-fashion.4 Michael-induced ring-closing reactions are especially interesting from a synthetic point of view, giving access to a broad spectrum of carbo-5 and heterocycles.6 While alkyl carbene complexes behave like esters undergoing 1,2-addition (Scheme 1),7 alkenyl and alkynyl carbene complexes can undergo both 1,2- and 1,4-addition, and in most cases the 1,4-addition is the favored pathway.8 Our group is involved in the development of new protocols toward novel amino acids with particular interest in reactions of chelated amino acid ester enolates.9 These enolates show a higher thermal stability compared to unchelated enolates, and

in addition chelate formation causes high selectivities in a wide range of reactions.10 Especially good results are obtained with trifluoroacetylated (TFA) tert-butyl glycinate, which is almost a perfect nucleophile for transition-metal-catalyzed allylic alkylations,11 Michael additions,12 and Michael addition induced ring closures (MIRC).13 Scheme 2. Michael Additions and MIRC of Chelated Enolates18a

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RESULTS AND DISCUSSION Therefore, we were interested to see, if these enolates are also suitable nucleophiles for 1,4-additions toward alkynyl carbene complexes (1−3) (Scheme 3). The expected amino acid derivative with an alkenyl carbene complex in the side chain (4) should be an ideal substrate for further modifications such as cycloadditions or Dötz reactions.2 We started our investigations with the phenyl-substituted chromium carbene 1a.14 Surprisingly, the expected metalated amino acid 4 was not obtained, but the pyrrole derivative 5a was obtained with a reasonably good yield. To figure out if this unexpected result was a special effect of the chromium complex, we also subjected the analogous complexes of molybdenum (2a) and tungsten (3a)

Scheme 1. Nucleophilic Attack on Alkyl and Alkynyl Carbene Complexes

Received: August 13, 2013 Published: September 25, 2013 © 2013 American Chemical Society

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liberated alcoholate can cleave off the TFA-protecting group, giving rise to the mesomeric dianions G and H. During workup, protonation of this dianion should generate metal hydride I, which can undergo reductive elimination/isomerization toward the observed pyrrole L. To confirm this postulated mechanism, we undertook several control experiments. Quenching the reaction of the carbene complex 3a with D2O resulted in dideuterated compound 5a (d2-5a) with a d-incorporation of 100% in the 2-position and 62% in the 3-position. This is a very strong indication that a dideuterated carbene complex d2-I is formed from dianion H. To suppress the cleavage of the rather reactive TFAprotecting group, we also investigated a set of protected glycine esters in the 1,4-addition. Decomposition was observed only with carbamates (caused by the strong base), while a comparable product could be obtained with the N-tosylated glycine ester with the protecting group still connected, although the yield was lower. According to the proposed mechanism, the 1,3-H-shift should play a significant role, making comparable reactions with other amino acid ester enolates impossible. To prove this option, we also subjected the analogous phenylalanine ester to the same reaction conditions. While glycine esters can be deprotonated with lithium hexamethyldisilazide (LHMDS), other amino acid esters in general cannot for steric reasons. Here stronger bases are required, such as LDA,15 and indeed, under these conditions no pyrrole formation was observed and 90% of the glycine ester could be recovered. The only new product formed resulted from a 1,4-addition of the LDA on the carbene complex. In conclusion, we could show that chelated glycine ester enolates are also good nucleophiles for 1,4-additions toward alkynyl Fischer carbene complexes. Interestingly, the metalated amino acid derivatives are not formed, but 2,3-disubstituted pyrroles are formed via a sequence of 1,4-addition, 1,3-H-shift, 1,2-addition, and reductive elimination. Therefore, this new pyrrole synthesis differs mechanistically from other Fischer carbene-based syntheses starting from either amino16 or imino17 carbene complexes or are initiated by the coupling of alkyl Fischer carbenes with enyne-imines.18 Further mechanistic studies as well as synthetic applications are currently under investigation.

Scheme 3. Addition of Chelated Enolates to Alkynyl Carbene Complexes

to the same procedure. In both cases only the pyrrole 5a could be identified/isolated. The Mo-complex 2a was produced in trace amounts, but in the case of the W-complex 3a an even better yield was obtained. Obviously the central metal does play a role, but the reaction behavior of all three carbene complexes is more or less the same. Therefore, in our subsequent experiments we used a different set of substituted tungsten carbene complexes 3 (Table 1, Scheme 4). Both, electron-rich and electron-poor aryl Table 1. Pyrrole Formation via Nucleophilic Attack of Chelated Glycinates onto Alkynyl Carbene Complexes 3

entry

carbene

R

R′

product

yield [%]

1 2 3 4 5 6 7 8 9 10

3a 3b 3c 3d 3e 3a 3b 3c 3d 3e

Ph p-MeOPh p-ClPh n-Hex c-Pr Ph p-MeOPh p-ClPh n-Hex c-Pr

t-Bu t-Bu t-Bu t-Bu t-Bu Bn Bn Bn Bn Bn

5a 5b 5c 5d 5e 6a 6b 6c 6d 6e

72 71 73 71 57 76 82 62 75 68

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substituents have been investigated as well as linear and cyclic alkyl groups. In general, the tert-butyl esters of glycine are superior to other esters with respect to stability and especially selectivity (regio- and stereoselectivity), but in this new reaction selectivity issues probably do not play a role. Therefore, the corresponding benzyl ester was also investigated. No significant difference in yield was observed for the different esters, and the substituent on the triple bond in the carbene complex seems to be highly variable. The yields obtained were comparable to our initial experiment with complex 3a in most experiments in the range of 70−75%. To explain the outcome of this reaction, one might assume that the reaction starts with the expected 1,4-addition of the chelated enolate toward the triple bond (Scheme 4). The vinyl anion A that is formed is probably stabilized by mesomerism with the electron-deficient metal fragment B. Nevertheless, it should be more basic than the α-position of the amino acid ester, resulting in a 1,3-proton shift to generate enolate C or its mesomeric form, the “carbene enolate” D. Elimination of methanolate generates a vinylidene complex E, which can be attacked by the deprotonated amide, generating F. The

EXPERIMENTAL SECTION

All air- or moisture-sensitive reactions were carried out in dried glassware (>100 °C) under an atmosphere of argon. Dried solvents were distilled before use: THF was distilled from LiAlH4; dichloromethane was dried with CaH2. Alkynyl Fischer carbene complexes 1a, 2a, and 3a−3e were prepared following previously described procedures.19 The products were purified by flash chromatography on silica gel (Macherey-Nagel 60, 0.063−0.2 mm). Mixtures of ethyl acetate and petroleum ether were generally used as eluents. Analytical TLC was performed on precoated silica gel plates (Macherey-Nagel, Polygram SIL G/UV254). Visualization was accomplished with UV light, KMnO4 solution, or ninhydrin solution. Melting points were determined with a Dr. Tottoli (Büchi) melting point apparatus and are uncorrected. 1H and 13C NMR spectra were recorded with a Bruker AC-400 [400 MHz (1H) and 100 MHz (13C)] spectrometer in CDCl3. Chemical shifts are reported in ppm relative to TMS, and CHCl3 was used as the internal standard. General Procedure for the Synthesis of 2,3-Disubstituted Pyrroles. To a solution of trifluoroacetylated tert-butyl glycinate (68 mg, 0.29 mmol) and ZnCl2 (49 mg, 0.36 mmol) in THF was added dropwise LHMDS (1 M, 0.75 mL, 0.75 mmol) at −78 °C, and the reaction mixture was stirred for 30 min at the same temperature. 5547

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Scheme 4. Proposed Mechanism of Pyrrole Formation

complex 3a (140 mg, 0.29 mmol). Yield: 50.8 mg (0.21 mmol, 72%); off-white crystals, mp = 115−117 °C. 1H NMR (400 MHz, CDCl3): δ 1.33 (s, 9H, CH3), 6.20 (t, 3J = 2.8 Hz, 1H, NCH), 6.80 (t, 3J = 2.8 Hz, 1H, NCHCH), 7.16 (t, 3J = 7.6 Hz, 1H, CHPh), 7.24 (t, 3J = 7.2 Hz, 2H, CHPh), 7.41 (d, 3J = 7.2 Hz, 2H, CHPh), 9.21 (sbr, 1H, NH) ppm. 13 C NMR (100 MHz, CDCl3): δ 28.2 (q, CCH3), 81.1 (s, CCH3), 112.3 (d, NCH), 119.6 (s, NCCO), 121.1 (d, NCHCH), 126.6 (d, CHPh), 127.4 (d, CHPh), 129.6 (d, CHPh), 131.1 (s, CPh), 135.6 (s, CPh), 160.9 (s, CO) ppm. HRMS (CI): m/z calcd for C15H17NO2 (M)+ 243.1259, found 243.1259. tert-Butyl 4,5-Dideutero-3-phenyl-1H-pyrrole-2-carboxylate (d2-5a). According to the general procedure, d2-5a was obtained from phenyl alkynyl carbene complex 3a as described for 5a; the reaction was hydrolyzed with D2O. Offwhite crystals; mp = 128−130 °C. 1H NMR (400 MHz, CDCl3): δ 1.43 (s, 9H, CH3), 6.90 (d, 3J = 3.2 Hz, 0.38H, NCHCH), 7.26 (t, 3J = 7.6 Hz, 1H, CHPh), 7.33 (t, 3J = 7.2 Hz, 2H, CHPh), 7.51 (d, 3J = 6.8 Hz, 2H, CHPh), 9.19 (sbr, 1H, NH) ppm. 13 C NMR (100 MHz, CDCl3): δ 28.2 (q, CCH3), 81.0 (s, CCH3), 112.2 (d, NCD), 119.5 (s, NCCO), 121.0 (d, NCDCD), 126.6 (d, CHPh), 127.4 (d, CHPh), 129.6 (d, CHPh), 131.1 (s, CPh), 135.6 (s, CPh), 160.8 (s, CO) ppm. HRMS (CI): m/z calcd for C15H15D2NO2 (M)+ 245.1385, found 245.1389. tert-Butyl 3-(4-Methoxyphenyl)-1H-pyrrole-2-carboxylate (5b). Yield: 71%; yellow solid; mp = 98−100 °C. 1H NMR (400 MHz, CDCl3): δ 1.44 (s, 9H, CH3), 3.81 (s, 3H, OCH3), 6.26 (s, 1H, NCH), 6.87 (s, 1H, NCHCH), 6.88 (d, 3J = 8.8 Hz, 2H, CHAr), 7.46 (d, 3J = 8.0 Hz, 2H, CHAr), 9.19 (sbr, 1H, NH) ppm. 13C NMR (100 MHz, CDCl3): δ 28.3 (q, CCH3), 55.2 (q, OCH3), 81.0 (s, CCH3), 112.2 (d, NCH), 113.0 (d, CHAr), 119.2 (s, NCCO), 121.0 (d, NCHCH), 127.9 (s, CAr), 130.7 (s, CAr), 131.0 (d, CHAr), 158.5 (s, CAr), 160.7 (s, CO) ppm. HRMS (CI): m/z calcd for C16H19NO3 (M)+ 273.1365, found 273.1367. tert-Butyl 3-(4-Chlorophenyl)-1H-pyrrole-2-carboxylate (5c). Yield: 73%; yellow solid; mp = 76−78 °C. 1H NMR (400 MHz, CDCl3): δ 1.44 (s, 9H, CH3), 6.26 (s, 1H, NCH), 6.89 (s, 1H, NCHCH), 7.30 (d, 3J = 8.0 Hz, 2H, CHAr), 7.44 (d, 3J = 8.0 Hz, 2H, CHAr), 9.20 (sbr, 1H, NH) ppm. 13C NMR (100 MHz, CDCl3): δ 28.3

Scheme 5. Results of Deuterium Labeling Experiments

Scheme 6. Reaction of N-Tosylated Glycinate

Scheme 7. Attempt to Add α-Substituted Ester Enolates

Afterward, this Zn-chelated enolate was added dropwise to a solution of the alkynyl carbene complex 3 (0.29 mmol) in THF at −78 °C. The reaction mixture was stirred for one and a half hours while the temperature was slowly raised to −50 °C, after which the mixture was stirred another 2 h at 0 °C to complete the reaction. The reaction was quenched with distilled water, and the organic phase was extracted with ethyl acetate, dried over anhydrous Na2SO4, and evaporated in vacuo. The crude product was purified by column chromatography (silica, petroleum ether/EtOAc), affording the 2,3-disubstituted pyrrole. tert-Butyl 3-Phenyl-1H-pyrrole-2-carboxylate (5a). According to the general procedure 5a was obtained from phenyl alkynyl carbene 5548

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ppm. 13C NMR (100 MHz, CDCl3): δ 7.9 (d, CHCH2), 9.2 (t, CHCH2), 65.6 (t, OCH2), 106.2 (d, NCH), 119.2 (s, NCCO), 122.3 (d, NCHCH), 128.0 (2d, CHBn), 128.4 (d, CHBn), 136.1(s, CH2CBn), 136.3 (s, CCH), 161.4 (s, CO) ppm. HRMS (CI): m/z calcd for C12H17NO2 (M)+ 241.1103, found 241.1102. tert-Butyl 3-Phenyl-1-tosyl-1H-pyrrole-2-carboxylate (7a). Yield: 23%; yellow oil. 1H NMR (400 MHz, CDCl3): δ 1.33 (s, 9H, C(CH3)3), 2.41 (s, 3H, CArCH3), 6.32 (d, 3J = 3.2 Hz, 1H, NCH), 7.27−7.33 (m, 7H, CHPh, CHAr), 7.36 (d, 3J = 3.2 Hz, 1H, NCHCH), 7.90 (d, 3J = 8.4 Hz, 2H, CHAr) ppm. 13C NMR (100 MHz, CDCl3): δ 21.6 (q, CArCH3), 27.6 (q, C(CH3)3), 82.4 (s, C(CH3)3), 112.6 (d, NCH), 123.6 (s, NCCO), 123.9 (d, NCHCH), 127.3, 127.9, 128.1, 128.5, 129.5 (d, CHPh, CHAr), 131.9 (s, CCPh), 134.0 (s, CCPh), 135.3 (s, SCAr), 145.0 (s, CArCH3), 160.6 (s, CO) ppm. HRMS (CI): m/z calcd for C22H23NO4S (M)+ 397.1348, found 397.1306. Carbene Complex 8a. Yield: 82%; yellow crystals, mp = 125−127 °C. 1H NMR (400 MHz, CDCl3): δ 1.08 (d, 3J = 1.6 Hz, 6H, CHCH3), 1.68 (d, 3J = 1.6 Hz, 6H, CHCH3), 3.53 (s, 3H, OCH3), 3.69 (m, 1H, CHCH3), 3.89 (m, 1H, CHCH3), 6.91 (s, 1H, CHCW), 7.07−7.09 (m, 2H, CHPh), 7.35−7.41 (m, 3H, CHPh) ppm. 13C NMR (100 MHz, CDCl3): δ 27.7 (q, CHCH3), 37.2 (d, CHCH3), 53.8 (q, OCH3), 83.5 (d, CHCW), 127.3 (d, CHPh), 128.5, 129.3 (d, CHPh), 134.9 (s, CPh), 168.9 (s, CHCN), 199.6 (s, CO) ppm, signal of the carbene carbon could not be found. HRMS (CI): m/z calcd for C21H23NO6W (M)+ 569.1035, found 569.1039.

(q, CCH3), 81.4 (s, CCH3), 112.2 (d, NCH), 119.6 (s, NCCO), 121.1 (d, NCHCH), 127.6 (d, CHAr), 129.9 (s, CAr), 130.9 (d, CHAr), 132.5 (s, CAr), 134.0 (s, CCl), 160.4 (s, CO) ppm. HRMS (CI): m/z calcd for C15H16ClNO2 (M)+ 277.0870, found 277.0834. tert-Butyl 3-Hexyl-1H-pyrrole-2-carboxylate (5d). Yield: 71%; yellow oil. 1H NMR (400 MHz, CDCl3): δ 0.83−0.89 (m, 5H, CH2, CH3), 1.26−1.33 (m, 6H, CH2), 1.55 (s, 9H, C(CH3)3), 2.72 (t, 3J = 7.6 Hz, 2H, PyrCH2), 6.07 (t, 3J = 2.8 Hz, 1H, NCH), 6.77 (t, 3J = 2.8 Hz, 1H, NCHCH), 9.06 (sbr, 1H, NH) ppm. 13C NMR (100 MHz, CDCl3): δ 14.0 (q, CH2CH3), 22.6 (t, CH2CH3), 27.1 (t, CH2), 28.4 (q, CCH3), 29.2 (t, CH2), 30.9 (t, CH2), 31.8 (t, CH2), 80.5 (s, CCH3), 111.2 (d, NCH), 120.0 (s, NCCO), 120.9 (d, NCHCH), 132.3 (s, CCH2), 161.3 (s, CO) ppm. HRMS (CI): m/z calcd for C15H25NO2 (M)+ 251.1885, found 251.1882. tert-Butyl 3-Cyclopropyl-1H-pyrrole-2-carboxylate (5e). Yield: 57%; white crystals, mp = 115−117 °C. 1H NMR (400 MHz, CDCl3): δ 0.56−0.60 (m, 2H, CHCH2), 0.89−0.94 (m, 2H, CHCH2), 1.57 (s, 9H, CH3), 2.42−2.49 (m, 1H, CHCH2), 5.74 (t, 3J = 2.8 Hz, 1H, NCH), 6.74 (t, 3J = 2.8 Hz, 1H, NCHCH), 9.11 (sbr, 1H, NH) ppm. 13C NMR (100 MHz, CDCl3): δ 7.9 (d, CHCH2), 9.1 (t, CHCH2), 28.4 (q, CH3), 80.6 (s, CCH3), 106.1 (d, NCH), 120.9 (s, NCCO), 121.3 (d, NCHCH), 134.5 (s, CCH), 161.4 (s, CO) ppm. HRMS (CI): m/z calcd for C12H17NO2 (M)+ 207.1259, found 207.1258. Benzyl 3-Phenyl-1H-pyrrole-2-carboxylate (6a). Yield: 76%; yellow crystals, mp = 70−71 °C. 1H NMR (400 MHz, CDCl3): δ 5.23 (s, 2H, OCH2), 6.34 (t, 3J = 2.4 Hz, 1H, NCH), 6.92 (t, 3J = 2.8 Hz, 1H, NCHCH), 7.21−7.33 (m, 8H, CHBn, CHPh), 7.51−7.54 (m, 2H, CHPh), 9.27 (sbr, 1H, NH) ppm. 13C NMR (100 MHz, CDCl3): δ 65.9 (t, OCH2), 112.6 (d, NCH), 117.8 (s, NCCO), 121.9 (d, NCHCH), 126.9, 127.7, 128.0, 128.1, 128.4, 129.5 (d, CHBn, CHPh), 132.5 (s, CCPh), 135.1 (s, CPh), 135.8 (s, CH2CPh), 160.8 (s, CO) ppm. HRMS (CI): m/z calcd for C18H15NO2 (M)+ 277.1103, found 277.1085. Benzyl 3-(4-Methoxyphenyl)-1H-pyrrole-2-carboxylate (6b). Yield: 82%; orange-yellow oil. 1H NMR (400 MHz, CDCl3): δ 3.73 (s, 3H, OCH3), 5.14 (s, 2H, OCH2), 6.22 (s, 1H, NCH), 6.77 (d, 3J = 8.4 Hz, 2H, CHAr), 6.81 (s, 1H, NCHCH), 7.16−7.28 (m, 5H, CHBn), 7.38 (d, 3J = 8.4 Hz, 2H, CHAr), 9.22 (sbr, 1H, NH) ppm. 13C NMR (100 MHz, CDCl3): δ 55.2, (q, OCH3), 65.9 (t, OCH2), 112.3 (d, NCH), 113.1 (d, CHAr), 117.5 (s, NCCO), 122.0 (d, NCHCH), 126.9 (d, CHBn), 127.5 (s, CCAr), 128.2 (d, CHBn), 128.3 (d, CHBn), 130.6 (d, CHAr), 132.4 (s, CCAr), 135.8 (s, CH2CBn), 158.7 (s, OCAr), 160.8 (s, CO) ppm. HRMS (CI): m/z calcd for C19H17NO3 (M)+ 307.1208, found 307.1213. Benzyl 3-(4-Chlorophenyl)-1H-pyrrole-2-carboxylate (6c). Yield: 62%; light yellow solid, mp = 99−101 °C. 1H NMR (400 MHz, CDCl3): δ 5.19 (s, 2H, OCH2), 6.28 (t, 3J = 2.8 Hz,1H, NCH), 6.91 (t, 3J = 2.8 Hz, 1H, NCHCH), 7.18−7.30 (m, 7H, CHBn, CHAr), 7.40 (d, 3J = 8.8 Hz, 2H, CHAr), 9.17 (sbr, 1H, NH) ppm. 13C NMR (100 MHz, CDCl3): δ 66.1 (t, CH2), 112.4 (d, NCH), 118.0 (s, NCCO), 122.0 (d, NCHCH), 127.8 (d, CHBn), 128.2, 128.3, 128.4 (d, CHBn, CHAr), 130.8 (d, CHAr), 131.2 (s, CAr), 132.8 (s, CCAr), 133.5 (s, CCl), 135.6 (s, CH2CBn), 160.5 (s, CO) ppm. HRMS (CI): m/z calcd for C18H14ClNO2 (M)+ 311.0713, found 311.0663. Benzyl 3-Hexyl-1H-pyrrole-2-carboxylate (6d). Yield: 75%; orange oil. 1H NMR (400 MHz, CDCl3): δ 0.87 (t, 3J = 6.0 Hz, 3H, CH3), 1.22−1.34 (m, 6H, CH2), 1.53−1.60 (m, 2H, CH2), 2.78 (t, 3J = 8.0 Hz, 2H, CCH2), 5.30 (s, 2H, OCH2), 6.12 (t, 3J = 2.8 Hz, 1H, NCH), 6.80 (t, 3J = 2.8 Hz, 1H, NCHCH), 7.32−7.42 (m, 5H, CHBn), 9.13 (sbr, 1H, NH) ppm. 13C NMR (100 MHz, CDCl3): δ 14.0 (q, CH3), 22.6 (t, CH2CH3), 27.0, 29.2, 30.8, 31.6 (t, CH2), 65.7 (t, OCH2), 111.4 (d, NCH), 118.3 (s, NCCO), 121.9 (d, NCHCH), 128.0, 128.1, 128.4 (d, CHBn), 133.9 (s, CCH2), 136.2 (s, CH2CBn), 161.3 (s, CO) ppm. HRMS (CI): m/z calcd for C18H23NO2 (M)+ 285.1729, found 285.1739. Benzyl 3-Cyclopropyl-1H-pyrrole-2-carboxylate (6e). Yield: 68%; yellow oil. 1H NMR (400 MHz, CDCl3): δ 0.58−0.62 (m, 2H, CHCH2), 0.91−0.95 (m, 2H, CHCH2), 2.49−2.56 (m, 1H, CHCH2), 5.32 (s, 2H, OCH2), 5.77 (t, 3J = 2.8 Hz, 1H, NCH), 6.76 (t, 3J = 2.8 Hz, 1H, NCHCH), 7.31−7.42 (m, 5H, CHBn), 9.01 (sbr, 1H, NH)

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ASSOCIATED CONTENT

S Supporting Information *

Copies of NMR spectra for all new compounds are available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft. R.C. gratefully acknowledges a fellowship of the Alexander von Humboldt Foundation.

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REFERENCES

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Organometallics

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dx.doi.org/10.1021/om4008136 | Organometallics 2013, 32, 5546−5550