The 2013 Impact Factor is 2.820 according to the Thomson Reuters Journal Citation Reports.


 
 

Imidazolinium and amidinium salts as Lewis acid organocatalysts

Oksana Sereda1, Nicole Clemens1, Tatjana Heckel2 and René Wilhelm1,2
1Institute of Organic Chemistry, Clausthal University of Technology, 38678 Clausthal-Zellerfeld, Germany
2Department of Chemistry, University of Paderborn, 33098 Paderborn, Germany
Email of corresponding author Corresponding author email     
This article is part of the Thematic Series "Organocatalysis".
Guest Editor: B. List
Beilstein J. Org. Chem. 2012, 8, 1798–1803.
doi:10.3762/bjoc.8.205

Part of Thematic Series

Organocatalysis
 
 
 
 

Abstract

The application of imidazolinium and amidinium salts as soft Lewis acid organocatalysts is described. These salts were suitable catalysts for the activation of unsaturated thioesters in a Diels–Alder reaction and in the ring opening of thiiranes and epoxides. The products were isolated in good yields. The mild catalysts did not cause desulfurization of the products containing a thiol or thiocarbonyl group.

Keywords: Diels–Alder; ionic liquids; organocatalysis; soft Lewis acids; thiiranes; thioesters

Top

Introduction

Salts with melting points below 100 °C are known as ionic liquids and are often used as novel solvents for reactions and electrochemical processes [1]. Several of these solvents can contribute to the research field of “green chemistry” [2]. The most common used ionic liquids are based on imidazolium cations. Next to their application in catalytic reactions [3-6], they are also capable of catalyzing reactions themselves, either in substoichiometric amounts or as reaction medium due to hydrogen-bond activation of the protons of the imidazolium cation [7-10] and other variables, such as π-orbital and charge–charge interactions [8-10].

Recently, we applied saturated imidazolium salts with an aryl substituent at the C(2) position as catalysts for the aza-Diels–Alder reaction [11,12]. These catalysts do not activate the substrate through hydrogen bonding, but instead, like other carbon-cation based catalysts, through their positive center [8-10,13-15], and belong also to the field of organocatalysis [10,16,17].

Taking the soft Lewis acidic character of imidazolinium salts into consideration, we were interested to apply these salts in a Diels–Alder reaction with ethyl crotonthioate as dienophile. Due to the low electronegativity of sulfur it would be difficult to catalyze this reaction with hydrogen-bond activation catalysts [18]. The utilization of the soft Lewis basic sulfur groups in this reaction is rare. Often mercury Lewis acids are applied [19-22] to activate the sulfur group and there is also the risk of desulfurization [23].

The only known asymmetric Diels–Alder reaction with ethyl crotonthioate (1) and cyclopentadiene (2) in the presence of optically pure 2,2'-dimercurio-1,1'-binaphthyl compound 3 gave the product 4 in 44% yield with 58% ee (Scheme 1) [20].

[1860-5397-8-205-i1]
Scheme 1: Diels–Alder reaction with ethyl crotonthioate (1) and cyclopentadiene (2).
Top

Results and Discussion

Since product 4 has a low boiling point and proved difficult to purify on a small scale, the model reaction shown in Scheme 2 was evaluated. The α,β-unsaturated thioester 5 was prepared from ethyl cinnamate and 2,4-bis(4-methoxyphenyl)-1,3,2,4-dithiadiphosphetane-2,4-disulfide (Lawesson’s reagent) [22]. The cycloaddition of ethyl thionocinnamate (5) with 1.5 equiv of cyclopentadiene (2) was performed in different solvents, at varied temperatures, by using a broad range of catalysts, which are summarized in Table 1.

[1860-5397-8-205-i2]
Scheme 2: Diels–Alder reaction catalysed with imidazolinium salts.
Table 1: Diels–Alder reaction with 10 mol % catalyst according to Scheme 2. Reactions with CH2Cl2 were carried out in a pressure vessel.
entrysolventcatalysttemp [°C]time [d]yield %a (endo:exo)
1CH2Cl2Hg(OAc)2−10–4540
2CH2Cl2BF3.Et2O−10–45490 (99:1)
3CH2Cl2TMSOTf−10–45455 (99:1)
4CH2Cl2725–4548 (13:1)
5toluene725–11040
6dioxane725–11046 (3:1)
7CH2Cl2825–4545 (9:1)
8CH2Cl290–45412 (14:1)
9CH2Cl210−10–45469 (99:1)
10CH2Cl2110–45366 (99:1)

First a few metal-based Lewis acids were applied in the reaction. BF3•Et2O is known to activate also sulfur carbonyl groups [24] and gave the product in 90% yield, while TMSOTf gave the product in 55% yield. In both cases the endo product was the major product. The reaction was followed by TLC and the temperature was gradually increased from –10 to 45 °C over 4 days in ca. 10 °C steps.

Next, imidazolinium salt 7 [25] was used in the reaction. The formation of the product was observed at 45 °C; however, the product was isolated only in a low yield of 8%. In toluene no reaction was observed even under reflux. In dioxane under reflux a low yield of 6% was obtained with a low endo/exo ratio of (3:1). Salt 8 [25] and 9 [25] gave similar poor results as salt 7 [25]. Finally, the bis-imidazolinium salt 10 [12] gave the product in 69% yield with an endo/exo ratio comparable to BF3•Et2O. The reaction started slowly at 10 °C. In addition, the similar salt 11 [12] gave the product in a comparable yield of 66%. Although in all cases the product was racemic, the results show that bis-imidazolinium salts can be applied as Lewis acid organocatalysts to activate a thiocarbonyl group.

Next, the behavior of these salts in the ring opening of a thiirane was explored. No activity was observed with salt 9 in the ring opening with aniline. Therefore, the more active bis-imidazolinium salt 10 was applied with thiirane 12 and aniline in CH2Cl2. A yield of 18% was obtained. However, in the absence of a solvent the yield increased to 98% after 16 h (Scheme 3). In the absence of a catalyst a yield of 12% was obtained under neat conditions. Product 13 was obtained in all reactions as racemate in the error range of the HPLC measurements.

[1860-5397-8-205-i3]
Scheme 3: Ring opening of thiirane 12.

Next, several salts were explored in the ring opening of cyclohexene epoxide with aniline (Scheme 4, Table 2). A reaction in CH2Cl2 in the absence of the catalyst gave a yield of 10% after 24 h (Table 2, entry 1).

[1860-5397-8-205-i4]
Scheme 4: Ring opening of epoxide 14.

The camphor based salt 16 gave a yield of 24%, while bis-imidazolinium salt 17 resulted in a yield of 60% (Table 2, entries 2 and 3). The bis-imidazolinium salts 10 and 11 gave lower yields compared to 17. However, when the reaction was carried out in the absence of a solvent, the yield increased with salts 10 and 11 to 98 and 96%, respectively (Table 2, entries 6 and 7).

Table 2: Ring opening of epoxide 15 with aniline and 10 mol % salt.
entrysolventcatalysttimeyield %a
1CH2Cl224 h10
2CH2Cl21624 h24
3CH2Cl21748 h60
4CH2Cl21048 h34
5CH2Cl21148 h48
6neat1024 h98
7neat1124 h96
8neat24 h0
9CH2Cl21848 h12
10CH2Cl21924 h78
11CH2Cl22024 h38
12CH2Cl2216 h99
13toluene2 h0
14toluene212 h89

Compared to sulfur, oxygen is a good hydrogen-bond acceptor. Hence, also imidazolinium salts with a C(2)H unit were applied in the reaction. Here, an activation of the epoxide can be also possible through hydrogen bonding next to the direct interaction with the positively charged NCN center. While salt 18 displayed very low catalytic activity, salt 19 gave the product in 78% yield (Table 2, entry 10). This phenomenon may be explained by the better delocalization of the positive charge in the planar sp2-centered imidazoline scaffold in salt 18. Meanwhile the special geometry of 7-membered 1,3-diazepinium cations such as 19 does not allow the planar conformation to be kept, thus the positive charge is less delocalized over the NCN atoms (Scheme 4) [26-29]. Salt 16, incorporating a six-membered ring as the smallest ring, displayed a catalytic activity between salt 18 and 19. Salt 20, with a larger steric environment around the amidinium unit next to the nitrogen atoms, gave a yield of only 38% (Table 2, entry 11). On the other hand salt 21 gave the product in 99% yield after 6 h (Table 2, entry 12). By changing the solvent from dichloromethane to toluene the reaction time was even further reduced with salt 21, and the product was isolated in 89% yield (Table 2, entry 14). In all cases product 15 was racemic in the error range of the HPLC measurements.

Salt 17 was prepared according to Scheme 5. The synthetic route involved a tetraamine formation via amidation under neat conditions and reduction with LAH, furnishing the product in 98% yield over two steps. Next, the aminal formation under neat conditions was performed. The aminal was further oxidized to a bromide salt by N-bromoacetamide in dimethoxyethane. The bromide counter anion was further efficiently replaced by a tetrakis(3,5-bis(trifluoromethyl)phenyl)borate anion providing the salt 17 by anion metathesis.

[1860-5397-8-205-i5]
Scheme 5: Synthesis of bis-imidazolium salt 17.

The 7-membered 1,3-diazepinium salts were prepared according to Scheme 6. Salts 19 and 20 were prepared in a standard way by treatment with triethyl orthoformate in the presence of ammonium tetrafluoroborate followed by anion metathesis in 33 and 29% yield, respectively.

[1860-5397-8-205-i6]
Scheme 6: Synthesis of amidinium salt 21.

Due to the low yield, salt 21 was prepared through a different route. First amine 27 [30] was transformed with formaldehyde to the aminal 28. The latter could be oxidized to the corresponding bromide salt, which was transformed directly by anion metathesis to the salt 21.

Top

Conclusion

It was possible to prepare new metal-free Lewis acids containing bis-imidazolinium cations and investigate the salts as organocatalysts. Although with enantiopure chiral salts no enantiomeric excess was observed, it was shown for the first time that these salts can interact with thiocarbonyl groups and thiiranes in order to activate these substrates. In addition, it was found that 7-membered 1,3-diazepinium cations are good catalysts for the ring opening of epoxides and that these salts are more reactive than imidazolinium salts.

Top

Supporting Information

Supporting Information File 1: Experimental part.
Format: PDF   Size: 205.7 KB   Download

Top

Acknowledgements

Financial support by the DFG is gratefully acknowledged. In addition, Dr. Dräger from the University of Hannover is acknowledged for HRMS measurements.

Top

References

  1. Wasserscheid, P.; Welton, T., Eds. Ionic Liquids in Synthesis, 2nd ed.; Wiley-VCH: Weinheim, Germany, 2008; Vol. 1–2.
    Return to citation in text: [1]
  2. Rogers, R. D.; Seddon, K. R., Eds. Ionic Liquids as Green Solvents: Progress and Prospects; American Chemical Society: Washington, DC, 2003.
    Return to citation in text: [1]
  3. Zhao, D.; Wu, M.; Kou, Y.; Min, E. Catal. Today 2002, 74, 157. doi:10.1016/S0920-5861(01)00541-7
    Return to citation in text: [1]
  4. Baudequin, C.; Baudoux, J.; Levillain, J.; Cahard, D.; Gaumont, A.-C.; Plaquevent, J.-C. Tetrahedron: Asymmetry 2003, 14, 3081. doi:10.1016/S0957-4166(03)00596-2
    Return to citation in text: [1]
  5. Jain, N.; Kumar, A.; Chauhan, S.; Chauhan, S. M. S. Tetrahedron 2005, 61, 1015. doi:10.1016/j.tet.2004.10.070
    Return to citation in text: [1]
  6. Pârvulescu, V. I.; Hardacre, C. Chem. Rev. 2007, 107, 2615. doi:10.1021/cr050948h
    Return to citation in text: [1]
  7. Aggarwal, A.; Lancaster, N. L.; Sethi, A. R.; Welton, T. Green Chem. 2002, 4, 517. doi:10.1039/b206472c
    Return to citation in text: [1]
  8. Vidiš, A.; Ohlin, C. A.; Laurenczy, G.; Küsters, E.; Sedelmeier, G.; Dyson, P. J. Adv. Synth. Catal. 2005, 347, 266. doi:10.1002/adsc.200404301
    Return to citation in text: [1] [2] [3]
  9. Tsuzuki, S.; Tokuda, H.; Mikami, M. Phys. Chem. Chem. Phys. 2007, 9, 4780. doi:10.1039/b707419k
    Return to citation in text: [1] [2] [3]
  10. Sereda, O.; Tabasum, S.; Wilhelm, R. Top. Curr. Chem. 2010, 291, 349. doi:10.1007/128_2008_17
    See for a review.
    Return to citation in text: [1] [2] [3] [4]
  11. Jurčík, V.; Wilhelm, R. Org. Biomol. Chem. 2005, 3, 239. doi:10.1039/b415023f
    Return to citation in text: [1]
  12. Jurčík, V.; Wilhelm, R. Tetrahedron: Asymmetry 2006, 17, 801. doi:10.1016/j.tetasy.2006.02.021
    Return to citation in text: [1] [2] [3]
  13. Mukaiyama, T.; Yanagisawa, M.; Iida, D.; Hachiya, I. Chem. Lett. 2000, 606. doi:10.1246/cl.2000.606
    Return to citation in text: [1]
  14. Chen, C.-T.; Chao, S.-D.; Yen, K.-C.; Chen, C. H.; Chou, I.-C.; Hon, S.-W. J. Am. Chem. Soc. 1997, 119, 11341. doi:10.1021/ja970900o
    Return to citation in text: [1]
  15. Chen, C.-T.; Chao, S.-D.; Yen, K.-C. Synlett 1998, 924. doi:10.1055/s-1998-1815
    Return to citation in text: [1]
  16. Dalko, P. I.; Moisan, L. Angew. Chem., Int. Ed. 2001, 40, 3726. doi:10.1002/1521-3773(20011015)40:20<3726::AID-ANIE3726>3.0.CO;2-D
    See for a review.
    Return to citation in text: [1]
  17. Berkessel, A.; Gröger, H. Asymmetric Organocatalysis; Wiley-VCH: Weinheim, Germany, 2005.
    See for a review.
    Return to citation in text: [1]
  18. Wittkopp, A.; Schreiner, P. Chem.–Eur. J. 2003, 9, 407. doi:10.1002/chem.200390042
    Return to citation in text: [1]
  19. Lopez, P.; Oh, T. Tetrahedron Lett. 2000, 41, 2313. doi:10.1016/S0040-4039(00)00157-X
    Return to citation in text: [1]
  20. Oh, T.; Lopez, P.; Reilly, M. Eur. J. Org. Chem. 2000, 2901. doi:10.1002/1099-0690(200008)2000:16<2901::AID-EJOC2901>3.0.CO;2-0
    Return to citation in text: [1] [2]
  21. Lee, H.; Diaz, M.; Hawthorne, M. F. Tetrahedron Lett. 1999, 40, 7651. doi:10.1016/S0040-4039(99)01582-8
    Return to citation in text: [1]
  22. Alper, H.; Brandes, D. A. Organometallics 1991, 10, 2457. doi:10.1021/om00053a058
    Return to citation in text: [1] [2]
  23. Wuest, J. D.; Zacharie, B. J. Am. Chem. Soc. 1985, 107, 6121. doi:10.1021/ja00307a058
    Return to citation in text: [1]
  24. Braddock, D. C.; Brown, J. M.; Guiry, P. J. J. Chem. Soc., Chem. Commun. 1993, 1244.
    Return to citation in text: [1]
  25. Clemens, N.; Sereda, O.; Wilhelm, R. Org. Biomol. Chem. 2006, 4, 2285. doi:10.1039/b601104g
    Return to citation in text: [1] [2] [3] [4]
  26. Alder, R. W.; Blake, M. E.; Oliva, J. M. J. Phys. Chem. A 1999, 103, 11200. doi:10.1021/jp9934228
    Return to citation in text: [1]
  27. Magill, A. M.; Cavell, K. J.; Yates, B. F. J. Am. Chem. Soc. 2004, 126, 8717. doi:10.1021/ja038973x
    Return to citation in text: [1]
  28. Scarborough, C. C.; Popp, B. V.; Guzei, I. A.; Stahl, S. S. J. Organomet. Chem. 2005, 690, 6143. doi:10.1016/j.jorganchem.2005.08.022
    Return to citation in text: [1]
  29. Iglesias, M.; Beetstra, D. J.; Knight, J. C.; Ooi, L.-L.; Stasch, A.; Coles, S.; Male, L.; Hursthouse, M. B.; Cavell, K. J.; Dervisi, A.; Fallis, I. A. Organometallics 2008, 27, 3279. doi:10.1021/om800179t
    Return to citation in text: [1]
  30. Angelovski, G.; Keränen, M. D.; Eilbracht, P. Tetrahedron: Asymmetry 2005, 16, 1919. doi:10.1016/j.tetasy.2005.03.035
    Return to citation in text: [1]
Top
Scheme 1: Diels–Alder reaction with ethyl crotonthioate (1) and cyclopentadiene (2). Move  Close
Scheme 2: Diels–Alder reaction catalysed with imidazolinium salts. Move  Close
Scheme 3: Ring opening of thiirane 12. Move  Close
Scheme 4: Ring opening of epoxide 14. Move  Close
Scheme 5: Synthesis of bis-imidazolium salt 17. Move  Close
Scheme 6: Synthesis of amidinium salt 21. Move  Close
Table 1: Diels–Alder reaction with 10 mol % catalyst according to Scheme 2. Reactions with CH2Cl2 were carried out in a pressure vessel. Move  Close
entrysolventcatalysttemp [°C]time [d]yield %a (endo:exo)
1CH2Cl2Hg(OAc)2−10–4540
2CH2Cl2BF3.Et2O−10–45490 (99:1)
3CH2Cl2TMSOTf−10–45455 (99:1)
4CH2Cl2725–4548 (13:1)
5toluene725–11040
6dioxane725–11046 (3:1)
7CH2Cl2825–4545 (9:1)
8CH2Cl290–45412 (14:1)
9CH2Cl210−10–45469 (99:1)
10CH2Cl2110–45366 (99:1)
Table 2: Ring opening of epoxide 15 with aniline and 10 mol % salt. Move  Close
entrysolventcatalysttimeyield %a
1CH2Cl224 h10
2CH2Cl21624 h24
3CH2Cl21748 h60
4CH2Cl21048 h34
5CH2Cl21148 h48
6neat1024 h98
7neat1124 h96
8neat24 h0
9CH2Cl21848 h12
10CH2Cl21924 h78
11CH2Cl22024 h38
12CH2Cl2216 h99
13toluene2 h0
14toluene212 h89
10.Sereda, O.; Tabasum, S.; Wilhelm, R. Top. Curr. Chem. 2010, 291, 349. doi:10.1007/128_2008_17
See for a review.
16.Dalko, P. I.; Moisan, L. Angew. Chem., Int. Ed. 2001, 40, 3726. doi:10.1002/1521-3773(20011015)40:20<3726::AID-ANIE3726>3.0.CO;2-D
See for a review.
17.Berkessel, A.; Gröger, H. Asymmetric Organocatalysis; Wiley-VCH: Weinheim, Germany, 2005.
See for a review.
Go to references 10,16,17
18.Wittkopp, A.; Schreiner, P. Chem.–Eur. J. 2003, 9, 407. doi:10.1002/chem.200390042
Go to reference 18
11.Jurčík, V.; Wilhelm, R. Org. Biomol. Chem. 2005, 3, 239. doi:10.1039/b415023f
12.Jurčík, V.; Wilhelm, R. Tetrahedron: Asymmetry 2006, 17, 801. doi:10.1016/j.tetasy.2006.02.021
Go to references 11,12
8.Vidiš, A.; Ohlin, C. A.; Laurenczy, G.; Küsters, E.; Sedelmeier, G.; Dyson, P. J. Adv. Synth. Catal. 2005, 347, 266. doi:10.1002/adsc.200404301
9.Tsuzuki, S.; Tokuda, H.; Mikami, M. Phys. Chem. Chem. Phys. 2007, 9, 4780. doi:10.1039/b707419k
10.Sereda, O.; Tabasum, S.; Wilhelm, R. Top. Curr. Chem. 2010, 291, 349. doi:10.1007/128_2008_17
See for a review.
13.Mukaiyama, T.; Yanagisawa, M.; Iida, D.; Hachiya, I. Chem. Lett. 2000, 606. doi:10.1246/cl.2000.606
14.Chen, C.-T.; Chao, S.-D.; Yen, K.-C.; Chen, C. H.; Chou, I.-C.; Hon, S.-W. J. Am. Chem. Soc. 1997, 119, 11341. doi:10.1021/ja970900o
15.Chen, C.-T.; Chao, S.-D.; Yen, K.-C. Synlett 1998, 924. doi:10.1055/s-1998-1815
Go to references 8-10,13-15
7.Aggarwal, A.; Lancaster, N. L.; Sethi, A. R.; Welton, T. Green Chem. 2002, 4, 517. doi:10.1039/b206472c
8.Vidiš, A.; Ohlin, C. A.; Laurenczy, G.; Küsters, E.; Sedelmeier, G.; Dyson, P. J. Adv. Synth. Catal. 2005, 347, 266. doi:10.1002/adsc.200404301
9.Tsuzuki, S.; Tokuda, H.; Mikami, M. Phys. Chem. Chem. Phys. 2007, 9, 4780. doi:10.1039/b707419k
10.Sereda, O.; Tabasum, S.; Wilhelm, R. Top. Curr. Chem. 2010, 291, 349. doi:10.1007/128_2008_17
See for a review.
Go to references 7-10
8.Vidiš, A.; Ohlin, C. A.; Laurenczy, G.; Küsters, E.; Sedelmeier, G.; Dyson, P. J. Adv. Synth. Catal. 2005, 347, 266. doi:10.1002/adsc.200404301
9.Tsuzuki, S.; Tokuda, H.; Mikami, M. Phys. Chem. Chem. Phys. 2007, 9, 4780. doi:10.1039/b707419k
10.Sereda, O.; Tabasum, S.; Wilhelm, R. Top. Curr. Chem. 2010, 291, 349. doi:10.1007/128_2008_17
See for a review.
Go to references 8-10
2.Rogers, R. D.; Seddon, K. R., Eds. Ionic Liquids as Green Solvents: Progress and Prospects; American Chemical Society: Washington, DC, 2003.
Go to reference 2
3.Zhao, D.; Wu, M.; Kou, Y.; Min, E. Catal. Today 2002, 74, 157. doi:10.1016/S0920-5861(01)00541-7
4.Baudequin, C.; Baudoux, J.; Levillain, J.; Cahard, D.; Gaumont, A.-C.; Plaquevent, J.-C. Tetrahedron: Asymmetry 2003, 14, 3081. doi:10.1016/S0957-4166(03)00596-2
5.Jain, N.; Kumar, A.; Chauhan, S.; Chauhan, S. M. S. Tetrahedron 2005, 61, 1015. doi:10.1016/j.tet.2004.10.070
6.Pârvulescu, V. I.; Hardacre, C. Chem. Rev. 2007, 107, 2615. doi:10.1021/cr050948h
Go to references 3-6
1.Wasserscheid, P.; Welton, T., Eds. Ionic Liquids in Synthesis, 2nd ed.; Wiley-VCH: Weinheim, Germany, 2008; Vol. 1–2.
Go to reference 1
12.Jurčík, V.; Wilhelm, R. Tetrahedron: Asymmetry 2006, 17, 801. doi:10.1016/j.tetasy.2006.02.021
Go to reference 12
25.Clemens, N.; Sereda, O.; Wilhelm, R. Org. Biomol. Chem. 2006, 4, 2285. doi:10.1039/b601104g
Go to reference 25
25.Clemens, N.; Sereda, O.; Wilhelm, R. Org. Biomol. Chem. 2006, 4, 2285. doi:10.1039/b601104g
Go to reference 25
22.Alper, H.; Brandes, D. A. Organometallics 1991, 10, 2457. doi:10.1021/om00053a058
Go to reference 22
24.Braddock, D. C.; Brown, J. M.; Guiry, P. J. J. Chem. Soc., Chem. Commun. 1993, 1244.
Go to reference 24
25.Clemens, N.; Sereda, O.; Wilhelm, R. Org. Biomol. Chem. 2006, 4, 2285. doi:10.1039/b601104g
Go to reference 25
25.Clemens, N.; Sereda, O.; Wilhelm, R. Org. Biomol. Chem. 2006, 4, 2285. doi:10.1039/b601104g
Go to reference 25
30.Angelovski, G.; Keränen, M. D.; Eilbracht, P. Tetrahedron: Asymmetry 2005, 16, 1919. doi:10.1016/j.tetasy.2005.03.035
Go to reference 30
19.Lopez, P.; Oh, T. Tetrahedron Lett. 2000, 41, 2313. doi:10.1016/S0040-4039(00)00157-X
20.Oh, T.; Lopez, P.; Reilly, M. Eur. J. Org. Chem. 2000, 2901. doi:10.1002/1099-0690(200008)2000:16<2901::AID-EJOC2901>3.0.CO;2-0
21.Lee, H.; Diaz, M.; Hawthorne, M. F. Tetrahedron Lett. 1999, 40, 7651. doi:10.1016/S0040-4039(99)01582-8
22.Alper, H.; Brandes, D. A. Organometallics 1991, 10, 2457. doi:10.1021/om00053a058
Go to references 19-22
12.Jurčík, V.; Wilhelm, R. Tetrahedron: Asymmetry 2006, 17, 801. doi:10.1016/j.tetasy.2006.02.021
Go to reference 12
23.Wuest, J. D.; Zacharie, B. J. Am. Chem. Soc. 1985, 107, 6121. doi:10.1021/ja00307a058
Go to reference 23
26.Alder, R. W.; Blake, M. E.; Oliva, J. M. J. Phys. Chem. A 1999, 103, 11200. doi:10.1021/jp9934228
27.Magill, A. M.; Cavell, K. J.; Yates, B. F. J. Am. Chem. Soc. 2004, 126, 8717. doi:10.1021/ja038973x
28.Scarborough, C. C.; Popp, B. V.; Guzei, I. A.; Stahl, S. S. J. Organomet. Chem. 2005, 690, 6143. doi:10.1016/j.jorganchem.2005.08.022
29.Iglesias, M.; Beetstra, D. J.; Knight, J. C.; Ooi, L.-L.; Stasch, A.; Coles, S.; Male, L.; Hursthouse, M. B.; Cavell, K. J.; Dervisi, A.; Fallis, I. A. Organometallics 2008, 27, 3279. doi:10.1021/om800179t
Go to references 26-29
20.Oh, T.; Lopez, P.; Reilly, M. Eur. J. Org. Chem. 2000, 2901. doi:10.1002/1099-0690(200008)2000:16<2901::AID-EJOC2901>3.0.CO;2-0
Go to reference 20
© 2012 Sereda et al; licensee Beilstein-Institut.
This is an Open Access article under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The license is subject to the Beilstein Journal of Organic Chemistry terms and conditions: (http://www.beilstein-journals.org/bjoc)