Todd:Lewis Acid Catalysed Pictet-Spengler Reaction Model

Home        Contact        Internal        People        Papers        Research        Teaching        Links        News        Construction       

The Lewis Acid-Catalysed Pictet-Spengler formation of Substituted 1,2,3,4-Tetrahydroisoquinolines

This is an undergraduate project running from March-June 2012. Current plan of action here. Final document due: 17:00 +10 GMT (AEST), 15 June 2012.

Abstract

• Mini-intro
• Mini-summary
• Mini-conclusion

Introduction

• What the Pictet-Spengler reaction is.
  • Carbon-carbon bond forming reaction. Cyclisation.
  • Condensation of a aryl-amine with an aldehyde or ketone.
  • Original reaction.
  • Variations of the PS reaction.
    • Acyl.
    • Tryptamine, thiophene, oxa.
    • Ketone - Spirocycle formation.
• PS in Nature
  • Saframycin A.
  • Pictet-Spenglerases: STR and NCS.

Figure X:

• Other useful molecules
• Limitations(?) for catalytic PS - non-electron rich. None currently identified in Nature and synthetically (for homogenous but mention zeolite and clay).
• Mini-lit-review
  • The current scope of the catalytic (& asymmetric) Pictet-Spengler reaction. Lots of tryptamine but not much with THIQ.
  • Few LA catalysed. No asymmetric LA catalysed.
• Typical LA catalysts - metal triflate catalysts. Lanthanoid Lewis acid-catalysts. Studies of the Ln - Lewis acidity - Youn, Kobayashi, Stambuli, THIQ scaffold.
  • Precedent for asymmetric: Friedel-Crafts using BOX ligands[Tang], Yb(OTf)3 catalysed PS from m-tyramines[Kobayashi], also Ca(HFIP)2 [Stambuli] and AuCl3/AgOTf [Youn]
  • But asymmetric first needs racemic model to... model.
• The focus of the project: model PS system on not widely explored THIQ.
  • Most literature focus for the PS formation of the THIQ scaffold starts from the aryl-amine and aldehyde. In this project, the model substrate materials were the corresponding imines. The reason: reaction less complicated. Minimising the amount of water in the reaction.
• What was found:
  • THIQ from simple imines using Bronsted acids (mediated). Not so good. Catalytically - negative.
  • Sentence on the choice of M(OTf)s. Lewis acids (catalytically). Negative.
  • N-acyl.
• Bullet: Yb(OTf)3 effective catalyst in the acyl-Pictet Spengler. THIQ scaffold.

Results and Discussion

The first phase of the project required synthesis of the imine model substrates (1a-d). were easily prepared in excellent yield using very mild conditions (Scheme X, Table X).

Scheme X: Synthesis of the Pictet-Spengler model substrate imines.

Table X: Synthesis of the Pictet-Spengler model substrate imines.

• Bronsted acid mediated cycliations - because lit. trend in BA catalysed reactions (even though it's tryptamine).
  • All literature describes harsh conditions.
  • Attempted neat MSA - based on PZQ results. Did not proceed. Possibly a results of non-acylated?
• Poor conversion. Only able to monitor conversion by NMR<--describe how. Lit HNMR in CDCl3. Used DMSO to better see imine proton in dimethoxy substrates. THIQ difficult to handle. Behaved unexpectedly on silica.
  • Zero conversion under catalytic and stoichiometric (double check stoich.) acid loads.
• Qualitative assay for the cyclisation of imine 1d only (by TLC).
• Move to stronger(?) Lewis acids. But why?. Choice of LA. Triflates + Yb(OTf)3. High LA. f13.

Scheme X: Expected outcome for the...

Table X

• Screen of catalysts (4 x MOTf) - no conversion in all cases.

The model substrates 1b and 1d were most likely to react. The screen was adapted from the conditions described by Kobayashi.[ref]

Scheme X

Scheme X

• Focussed on the Yb(OTf)₃ - adapted conditions from literature. Attempts to optimise reaction conditions. Zero conversion.
  • Substrate specific? Compare hydroxy + and Stambuli's replicated result with altered reaction conditions.

• Move to acyl-Pictet-Spengler:
  • Why change to acyl?
    • Inertness of the imines.
    • Acyl EWG increase
• Literature for dimethoxy THIQ - Youn. AuCl3/AgOTf. Gold triflate? Lit results (pretty much) reproduced. Requirement of lutidine - reaction still proceeds, however yield is reduced. Also suspect formation of salts that complicated work up. Lutidine has high affinity for HCl [check ref].

Given the difficulties in effecting the PS formation of the THIQ scaffold from the dimethoxy. The move to acyl-PS. The acyl-PS cyclisaiton to give 2d is catalysed by AuCl3/AgOTf. The identical model substrate and expected product made this ideal to template the Yb reaction on. Minimal adaptation of the procedure gave similar results to the literature (Entry 1, table X). The by-products isolated from the reaction were the aldehyde (4) and the monoacetylated phenethylamine (5). An additional byproduct was recovered, the H-NMR of which suggested it was the diacetylated phenethylamine.

Scheme X

1H-NMR of the isolated reaction by products revealed sufficient peak separation. Different to previous monitoring of the conversion to non-acylated THIQ. DMSO was worse. Started with CDCl3, Ideally, conversion was by use of tetrachloroethane as an internal standard and peak integrals of the aldehydic proton, the acyl or CH2 environment of 5 and either the stereogenic proton or the CX aromatic proton of 3d.
Moved to Yb(OTf)3 and implementatin of NMR assay. The first attempts to gauge the yield by NMR analysis of the crude product in CDCl₃. Mention the 81% yield by NMR vs. 50% isolated product. Two problems with assay: Insoluble white solid in CDCl3. Therefore mass balance of NMR was wrong. And intereference of 2,6-lutidine peaks. The lutidine was easy to deal with - minimise presence by performing a mildly acidic acid work up. This made the insoluble bit worse, since there is Cl and some H+ in reaction mixture and I knew there was monoacetylated (from TLC).... adding acid made formation of suspected salt worse. So added basic workup after acidic work up. Result --> Lutidine muted or removed and all of crude product goes into CDCl3.

Error creating thumbnail: File with dimensions greater than 12.5 MP

Figure X

Once the NMR assay was worked out efforts to minimise hydrolysis were more efficient. Allowed gauge of yield. Reducing temperature of reaction Increased scale slightly - hydrolysis went down. Where's the water coming from? Still unsure when the hydrolysis occured. Requires NMR studies.
Confirmed byproducts of Yb after column. Got 60% isolated yield from 7 mol% load. Doesn't matter when hydrolysis happens?
Probing the effectiveness of teh catalyst - 1 mol% load. NMR yield - X. Isolated yield, 77%.

• Byproducts isolated from reaction. Once the product was isolated, and the other spots on the TLC were identified, an NMR assay was developed. Allowed increased efficiency in evaluationg outcomes of the optimisation process for the subsequent Yb(OTf)₃ catalysed acyl-PS reactions.
• Adapted conditions to Yb(OTf)3 catalysed - Why use? easier to handle than AuCl3/AgOTf. V. strong LA.
• Describe the choice of Yb in more detail... Move back to Yb based on positive result of Au.
  • Byproducts only aldehyde + acetamide. Hydrolysis during reaction or workup? Unknown.
  • Development of NMR assay to aid screening process <-- integrate above somewhere.
    • Downsides to the NMR assay:
      • Requires DRY crude product - can fix this by creating standard curves of TCE:Product and TCE:Aldehyde.
  • Development of workup procedure.

Table X

Scheme X

Conclusion

• Summary of what I just talked about.
• Comment on robustness(?) of model system.
• Acyl Pictet-Spengler + LA catalyst = happy, effective combination.
• What next?
  • Optimising reaction conditions - order of addition? Reaction time? Typically, the solvents used to effect Pictet-Spengler cyclisations are toluene, dichloromethane and dichloroethane.

88Suggested formation of iminium by Δ colour intensity. Possible UV-Vis monitoring of rxn? Kinetic NMR. See how far the catalyst can be pushed = increase temp, decrease load.

• • Evaluation of other MOTfs in acyl PS reaction (Esp. CuOTf2).
  • CuOTf2-BOX complexes for asymmetric.
  • enantioselective - pyBOX. Yb(OTf)3
  • PZQ?

Experimental

NMR specs. What CDCl₃ and DMSO-d₆ was used. All melting points were recorded using on a Standford Research Systems OptiMelt (ø = X mm, 90?? mm (pyrex?) capillaries, ramp rate 1 °C min^-1). Glassware used in anhydrous reactions were (or was?) dried >2 hours at 130 °C. All molecular sieves were microwave activated and cooled under nitrogen before immediate use.

N-benzylidene-2-phenylethanamine (1a)

To a stirring solution of benzaldehyde (4 mL, 40 mmol, 1 equiv.) in diethyl ether (10 mL) was slowly added 2-phenylethanamine (5 mL, 40 mmol, 1 equiv.). The clear yellow solution was stirred at room temperature for 5 hours, dried (MgSO₄) and concentrated in vacuo to yield a yellow oil that solidified on standing to give a yellow crystalline solid (8.3 g, 99%). M.p. 32.8-35.3 °C. ¹H-NMR (300 MHz; DMSO-d₆): δ 8.26 (s, 1H), 7.71 (dd, J = 6.6, 2.9 Hz, 2H), 7.45-7.16 (m, 10H), 3.81 (t, J = 7.3 Hz, 2H), 2.93 (t, J = 7.3 Hz, 2H). ¹³C-NMR (75 MHz; CDCl₃): δ 161.5, 139.9, 136.2, 130.6, 129.0, 128.6, 128.3, 128.1, 126.1, 63.2, 37.5. Relevant lab book entries: KAB18-1, KAB18-2.

N-benzylidene-2-(3,4-dimethoxyphenyl)ethanamine (1b)

To a stirring solution of benzaldehyde (3.1 mL, 30 mmol, 1 equiv.) in diethyl ether (10 mL) was slowly added 2-(3,4-dimethoxyphenyl)ethanamine (5.0 mL, 30 mmol, 1 equiv.). The mixture was stirred at ambient temperature for 5 hours, diluted with diethyl ether (20 mL), dried over magnesium sulfate and concentrated under reduced pressure to yield a yellow oil that crystallised on standing (7.3 g, 90%). M.p. X-X °C. ¹H-NMR (300 MHz; CDCl₃): δ 8.15 (s, 1H), 7.73 (dd, J = 6.7, 3.0 Hz, 2H), 7.43 (dt, J = 5.3, 2.6 Hz, 3H), 6.83-6.77 (m, 3H), 3.89-3.84 (m, 8H), 2.99 (t, J = 7.2 Hz, 2H). ¹³C-NMR (75 MHz; CDCl₃): δ 161.5, 148.7, 147.4, 136.2, 132.6, 130.6, 128.6, 128.0, 120.9, 112.6, 111.2, 77.6, 77.1, 76.7, 63.3, 55.89, 55.71, 37.0. Relevant lab book entries: KAB19-1, KAB19-2.

N-(4-nitrobenzylidene)-2-phenylethanamine (1c)

To a stirring suspension of 4-nitrobenzaldehyde (6.0 g, 40 mmol, 1 equiv.) in diethyl ether (40 mL) was slowly added 2-phenylethanamine (5.0 mL, 40 mmol, 1 equiv.). The mixture was stirred at room temperature for 1 hour before a yellowish solid precipitated. The mixture was concentrated under reduced pressure and the residue recrystallised from diethyl ether to afford the pure product as pale yellow needles. M.p. 70.6 - 71.1 °C. ¹H-NMR (300 MHz; CDCl₃): δ 8.25 (d, J = 8.7 Hz, 2H), 8.20 (s, 1H), 7.85 (d, J = 8.7 Hz, 2H), 7.31-7.26 (m, 2H), 7.20-7.18 (m, 1H), 3.93 (t, J = 7.2 Hz, 2H), 3.04 (t, J = 7.3 Hz, 2H). ¹H-NMR (300 MHz; DMSO-d₆): δ 8.41 (s, 1H), 8.28 (d, J = 8.7 Hz, 2H), 7.96 (d, J = 8.7 Hz, 2H), 7.31-7.16 (m, 5H), 3.89 (t, J = 7.2 Hz, 2H), 2.96 (t, J = 7.3 Hz, 2H).¹³C-NMR (75 MHz; DMSO-d₆): δ 160.1, 149.0, 142.1, 140.1, 129.30, 129.26, 128.7, 126.5, 124.4, 62.4, 37.1. Relevant lab book entry: KAB22-1.

2-(3,4-dimethoxyphenyl)-N-(4-nitrobenzylidene)ethanamine (1d)

4-Nitrobenzaldehyde (3.7 g, 23.5 mmol, 1 equiv.) was suspended in diethyl ether (50 mL). 2-(3,4-dimethoxyphenyl)ethanamine (4.0 mL, 24.5 mmol, 1 equiv.) was added dropwise, with stirring. The mixture was left to stir at ambient temperature for 6 hours resulting in the precipitation of a fine light yellow solid. The solvent was removed in vacuo to give the crude product as a fine, yellow powder (7.95 g, 103%). Recrystallisation of the crude product from ethanol (~200 mL) afforded the pure product as yellow needles (7.1 g, 23 mmol, 92%). M.p. 123.3-124.2 °C. ¹H-NMR (200 MHz; CDCl₃): δ 8.29-8.23 (m, 2H), 8.19 (s, 1H), 7.86 (d, J = 8.8 Hz, 2H), 6.78-6.72 (m, 3H), 3.91 (td, J = 7.1, 1.1 Hz, 2H), 3.85 (s, 3H), 3.81 (s, 3H), 2.99 (t, J = 7.1 Hz, 2H). ¹H-NMR (300 MHz; DMSO-d₆): δ 8.40 (s, 1H), 8.29 (d, J = 8.7 Hz, 2H), 7.98 (d, J = 8.7 Hz, 2H), 6.84 (dd, J = 4.8, 3.3 Hz, 2H), 6.75 (dd, J = 8.2, 1.4 Hz, 1H), 3.86 (t, J = 7.1 Hz, 2H), 3.69 (d, J = 2.1 Hz, 6H), 2.90 (t, J = 7.2 Hz, 2H). ¹³C-NMR (75 MHz; DMSO-d₆): δ 160.0, 148.9, 147.6, 142.1, 132.5, 129.2, 124.4, 121.1, 113.3, 112.2, 62.7, 55.94, 55.80, 36.6. Relevant lab book entries: KAB23-1, KAB23-2.

Attempts at the synthesis of (2a)

Relevant lab book entries: KAB20-2, KAB20-3, KAB20-4.

Bronsted acid synthesis of (2b)

Methanesulfonic Acid

Relevant lab book entry: KAB21-1.

Trifluoroacetic Acid

Relevant lab book entries: KAB21-2, KAB21-3, KAB21-4.

Bronsted acid synthesis of (2d)

Relevant lab book entries: KAB24-1, KAB24-2, KAB24-3, KAB24-4.

Procedure for the Lewis Acid-Catalyst Screen

Substrate stock solutions (0.20 M in dichloromethane) were prepared N-benzylidene-2-(3,4-dimethoxyphenyl)ethanamine (0.20 M) and 2-(3,4-dimethoxyphenyl)-N-(4-nitrobenzylidene)ethanamine. Relevant lab book entry: KAB21-5, KAB21-6, KAB21-7, KAB21-8, KAB24-5, KAB24-6, KAB24-7 & KAB24-8.

Yb(OTf)₂ catalysed synthesis of 2c

Procedures were adapted from the literature.[ref and ref]

Procedure 1

To a mixture of Yb(OTf)₃ (120.5 mg, 0.194 mmol, 0.2 equiv.) and microwave activated 3 Å powdered molecular sieves (~20 mg) was added dry dichloromethane (30 mL). Benzaldehyde (0.10 mL, 0.97 mmol, 1 equiv.) and 3,4-dimethoxyphenethylamine (0.16 mL, 0.97 mmol, 1 equiv.) were added. The reaction mixture was stirred under nitrogen for 24 hours. Saturated sodium bicarbonate solution (30 mL) was added to quench the reaction. The organic layer was separated and the alkaline aqueous fraction was extracted with ethyl acetate (3 × 50 mL). The organic fractions were combined, dried over magnesium sulfate and concentrated under reduced pressure yielding a yellow oil (392 mg, 150%). ¹H-NMR of the oil indicated the isolated product was a wet, 1:0.15 mixture of imine (1d) and 4-nitrobenzaldehyde (5). Relevant lab book entries: KAB25-1

Procedure 2

Relevant lab book entry: KAB25-2

HAuCl₄·3H₂O/AgOTf catalysed synthesis of 1-(6,7-dimethoxy-1-(4-nitrophenyl)-3,4-dihydroisoquinolin-2(1H)-yl)ethanone (3d)

A solution of Gold(III) chloride trihydrate (31 mg, 0.079 mmol, 0.01 equiv.) and silver(I) trifluoromethanesulfonate (30 mg, 0.12 mmol, 0.02 equiv.) in acetonitrile (15 mL) was vigorously stirred at ambient temperature (~17 °C) for 1 hour. To the now yellow reaction mixture was added a pale yellow solution of 2-(3,4-dimethoxyphenyl)-N-(4-nitrobenzylidene)ethanamine (1d) (1.9 g, 6.1 mmol, 1 equiv.), acetyl chloride (0.40 mL, 6.1 mmol, 1 equiv.) and 2,6-lutidine (0.70 mL, 6.1 mmol, 1 equiv.) in acetonitrile (250 mL). The reaction mixture was stirred for 14 hours at ambient temperature (~12 °C), concentrated in vacuo and purified by silica gel column chromatography (50-100% ethyl acetate/hexane, v/v). The expected product (3d) was isolated (0.86 g, 41%) in addition to the byproducts,4 and 5. Relevant lab book entries: KAB26-1 & KAB26-2, KAB26-3, KAB26-10.
1-(6,7-dimethoxy-1-(4-nitrophenyl)-3,4-dihydroisoquinolin-2(1H)-yl)ethanone (3d)
M.p. 176.8 - 178.7 °C. Two amide rotamers (91:9). Signals corresponding to the major rotamer: ¹H-NMR (200 MHz; CDCl₃): δ 8.12 (d, J = 8.7 Hz, 2H), 7.42 (d, J = 8.7 Hz, 2H), 6.90 (s, 1H), 6.69 (s, 1H), 6.48 (s, 1H), 3.89 (s, 3H), 3.76 (s, 3H), 3.74-3.70 (m, 1H), 3.42-3.27 (m, 1H), 2.95 (ddt, J = 15.9, 10.7, 5.2 Hz, 1H), 2.81-2.71 (m, 1H), 2.18 (s, 3H). Signals corresponding to the minor rotamer: ¹H-NMR (200 MHz; CDCl₃): δ 8.17 (s, 2H), 6.60 (s, 1H), 5.94 (s, 1H), 2.32 (s, 3H). Spectroscopic data matched those in the literature.[ref]
4-nitrobenzaldehyde (4)
M.p. 103.2 - 104.3 °C. ¹H-NMR (300 MHz; CDCl₃): δ 10.18 (s, 1H), 8.42 (d, J = 8.6 Hz, 2H), 8.10 (d, J = 8.5 Hz, 2H).¹³C-NMR (75 MHz; CDCl₃): δ 190.2, 140.0, 130.5, 124.3. Spectroscopic data matched those in the literature.[ref]
N-(3,4-dimethoxyphenethyl)acetamide (5)
M.p. °C. ¹H-NMR (300 MHz; CDCl₃): δ 6.82-6.79 (m, 1H), 6.74-6.71 (m, 2H), 5.66 (s, 1H), 3.86 (s, 3H), 3.86 (s, 3H), 3.48 (q, J = 6.6 Hz, 2H), 2.76 (t, J = 7.0 Hz, 2H), 1.94 (s, 3H). ¹³C-NMR (75 MHz; CDCl₃): δ 170.1, 149.0, 147.7, 131.4, 120.6, 114.7, 111.9, 111.4, 55.91, 55.86, 40.8, 35.2, 23.3.

Yb(OTf)₃ catalysed synthesis of 1-(6,7-dimethoxy-1-(4-nitrophenyl)-3,4-dihydroisoquinolin-2(1H)-yl)ethanone (3d)

The acetonitrile (HPLC grade, something 190?) was dried over microwave activated 3A molecular sieves (2.5-5.0 mm, 30 %(w/v)) for >24 hours. All glassware was ovendried (130 °C) for >2 hours prior to use. 2,6-lutidine was dried over 3A molecular sieves (diameter, 2.5-5.0 mm, 50 %(w/v)).

Procedure 1

2-(3,4-dimethoxyphenyl)-N-(4-nitrobenzylidene)ethanamine (X g, X mmol, 1 equiv.) was dissolved in anhydrous acetonitrile (X mL). Relevant lab book entries: KAB26-4

Procedure 2

- Added citric acid workup. Relevant lab book entry: KAB26-5

Procedure 3

Acetonitrile/liquid N2 bath. http://www.ourexperiment.org/racemic_pzq/5712 KAB26-6 http://www.ourexperiment.org/racemic_pzq/5714 KAB26-7 http://www.ourexperiment.org/racemic_pzq/5755 KAB26-9

Procedure 4

To a mixture of 3 Å molecular sieves (~30 g) in dry acetonitrile (160 mL), under nitrogen, was added 2-(3,4-dimethoxyphenyl)-N-(4-nitrobenzylidene)ethanamine (1.50 g, 4.77 mmol, 1 equiv.). Once dissolved, the mixture was cooled in a brine ice bath. Acetyl chloride (0.34 mL, 4.8 mmol, 1 equiv.) and 2,6-lutidine (0.55 mL, 4.8 mmol, 1 equiv.) were added, dropwise. Yb(OTf)₃ (0.034 g, 0.048 mmol, 0.01 equiv.) was added. Thereaction mixture was allowed to warm to ambient temperature (~12 °C) and stirred under argon for 23 hours. The mixture was filtered through a bed of Celite, eluting with ethyl acetate (~50 mL). The filtrate was washed with saturated sodium bicarbonate solution (40 mL). The aqueous layer was extracted with ethyl acetate (3 × 40 mL). The organic fractions were combined, dried over magnesium sulfate, filtered and concentrated in vacuo to yield a yellow oil that partially crystallised on standing (1.8 g, 106%). The crude product was dissolved in hot methanol, dry loaded onto a silica gel column (ø = 6.5 cm, 15 cm) and purified by chromatography (70-100% ethyl acetate/hexane) yielding the expected product as a yellow powder (1.3 g, 77%). Relevant lab book entries: KAB26-11.

Typical procedure for the ¹H-NMR Assays

Tetrachloroethane (

References (arranged by date)

• Ueber die Bildung von Isochinolin-derivaten durch Einwirkung von Methylal auf Phenyl-aethylamin, Phenyl-alanin und Tyronsin, A. Pictet and T. Spengler, Ber. Dtsch. Chem. Ges. 1911, 44, 2030-2036. Paper
• Highly efficient Lewis acid-catalysed Pictet-Spengler reactions discovered by parallel screening, N. Srinivasan, A. Ganesan, Chemical Communications 2003, 916. DOI: 10.1039/B212063A Paper
• One-step preparation of 1-substituted tetrahydroisoquinolines via the Pictet–Spengler reaction using zeolite catalysts, Adrienn Hegedus and Zoltan Hell, TetLett 2004 45, 8553–8555. DOI: 10.1016/j.tetlet.2004.09.097 Paper
  
• Catalytic Pictet-Spengler reactions using Yb(OTf)₃, Manabe, K., D. Nobutou, et al. Bioorganic & Medicinal Chemistry 2005 13(17): 5154-5158. DOI: 10.1016/j.bmc.2005.05.018 Paper
  
• Development of the Pictet-Spengler Reaction Catalyzed by AuCl3/AgOTf, S. W. Youn, The Journal of Organic Chemistry 2006 71(6), 2521-2523. DOI: 10.1021/jo0524775 Paper
  
• Principal properties (PPs) for lanthanide triflates as Lewis-acid catalystsC. G. Fortuna, G. Musumarra, M. Nardi, A. Procopio, G. Sindona, S. Scirè, Journal of Chemometrics 2006, 20, 418-424.
  
• Trisoxazoline/Cu(II)-catalyzed asymmetric intramolecular Friedel-Crafts alkylation reaction of indoles, Zhou, J.-L., M.-C. Ye, et al. Tetrahedron 2009 65(34), 6877-6881. DOI: 10.1016/j.tet.2009.06.071 Paper
• Calcium-Promoted Pictet-Spengler Reactions of Ketones and Aldehydes, M. J. V. Eynden, K. Kunchithapatham, J. P. Stambuli, Journal of Organic Chemistry 2010, 75, 8542. DOI: 10.1021/jo1019283 Paper
  
• Pictet-Spengler condensation reactions catalyzed by a recyclable H(+)-montmorillonite as a heterogeneous Bronsted acid. Wang, Y., Z. Song, et al. Science China-Chemistry 2010, 53(8), 562-568. DOI: 10.1007/s11426-010-0073-4 Paper
• J. Stöckigt, A. P. Antonchick, F. Wu, H. Waldmann, Angewandte Chemie International Edition 2011, 50, 8538-8564.