Metal-Free Chiral Phosphoric Acid or Chiral Metal Phosphate as Active Catalyst in the Activation of N-Acyl Aldimines

Abstract

Whether metal-free chiral phosphoric acid or chiral metal phosphate functions as an active catalyst was confirmed in three reactions. In the aza-Friedel-Crafts and aza-ene-type reactions, a metal-free chiral phosphoric acid, namely, a chiral Brønsted acid, was verified to be the active catalyst. In contrast, the substitution ­reaction of α-diazoacetate with aldimine was accelerated by a salt-containing chiral phosphoric acid and hence chiral metal phosphate presumably functioned as an active catalyst.

Key words

asymmetric catalysis - enantioselectivity - Friedel-Crafts reaction - organocatalysis - substitution

References and Notes

• For reviews, see:
• 1a Doyle AG. Jacobsen EN. Chem. Rev.  2007,  107:  5713 
• 1b Akiyama T. Chem. Rev.  2007,  107:  5744 
• 1c Yu X. Wang W. Chem. Asian J.  2008,  3:  516 
• 1d Yamamoto H. Payette N. In Hydrogen Bonding in Organic Synthesis   Pihko PM. Wiley-VCH; Weinheim: 2009.  p.73-140  
• 1e Kampen D. Reisinger CM. List B. Top. Curr. Chem.  2010,  291:  395 
• For seminal studies, see:
• 2a Akiyama T. Itoh J. Yokota K. Fuchibe K. Angew. Chem. Int. Ed.  2004,  43:  1566 
• 2b Uraguchi D. Terada M. J. Am. Chem. Soc.  2004,  126:  5356 
• 3a Nakashima D. Yamamoto H. J. Am. Chem. Soc.  2006,  128:  9626 
• For a review, see:
• 3b Cheon CH. Yamamoto H. Chem. Commun.  2011,  47:  3043 
• For reviews, see:
• 4a Connon SJ. Angew. Chem. Int. Ed.  2006,  45:  3909 
• 4b Akiyama T. Itoh J. Fuchibe K. Adv. Synth. Catal.  2006,  348:  999 
• 4c Adair G. Mukherjee S. List B. Aldrichimica Acta  2008,  41:  31 
• 4d Terada M. Chem. Commun.  2008,  4097 
• 4e Terada M. Bull. Chem. Soc. Jpn.  2010,  83:  101 
• 4f Terada M. Synthesis  2010,  1929 
• 4g Zamfir A. Schenker S. Freund M. Tsogoeva SB. Org. Biomol. Chem.  2010,  8:  5262 
• 5 Xu S. Wang Z. Zhang X. Zhang X. Ding K. Angew. Chem. Int. Ed.  2008,  47:  2840 
• 6a Rueping M. Theissmann T. Kuenkel A. Koenigs RM. Angew. Chem. Int. Ed.  2008,  47:  6798 
• 6b Rueping M. Nachtsheim BJ. Koenigs RM. Ieawsuwan W. Chem. Eur. J.  2010,  16:  13116 
• 7 Terada M. Sorimachi K. J. Am. Chem. Soc.  2007,  129:  292 
• Ishihara and co-workers reported that the outcome of the Mannich reaction of N-Boc aldimine 6 with acetylacetone catalyzed by chiral calcium phosphate [G = 4-(β-naphthyl) phenyl] was comparable to our reported result (see ref. 2b) obtained using silica gel purified chiral phosphoric acid (G = same as above), see:
• 9a Hatano M. Moriyama K. Maki T. Ishihara K. Angew. Chem. Int. Ed.  2010,  49:  3823 
• Also see:
• 9b Hatano M. Ishihara K. Synthesis  2010,  3785 
• List and co-workers compared the catalytic activities of metal-free and silica gel purified, i.e., salt-containing phosphoric acids 1a in an asymmetric transfer hydro-genation of imine. They reported that both acids yielded the corresponding products with the same enantioselectivity. However, the metal-free acid displayed significantly higher catalytic activity than the salt-containing acid, see:
• 10a Klussmann M. Ratjen L. Hoffmann S. Wakchaure V. Goddard R. List B. Synlett  2010,  2189 
• Also see:
• 10b Lu G. Birman VB. Org. Lett.  2011,  13:  356 
• Selected examples of binaphthol-derived monophosphoric acids as the chiral ligand for enantioselective catalysis. For palladium catalysts, see:
• 11a Alper H. Hamel N. J. Am. Chem. Soc.  1990,  112:  2803 
• For rhodium catalysts, see:
• 11b McCarthy N. McKervey MA. Ye T. McCann M. Murphy E. Doyle MP. Tetrahedron Lett.  1992,  33:  5983 
• 11c Pirrung MC. Zhang J. Tetrahedron Lett.  1992,  33:  5987 
• For gold catalysts, see:
• 11d Hamilton GL. Kang EJ. Mba M. Toste FD. Science  2007,  317:  496 
• 11e LaLonde RL. Wang ZJ. Mba M. Lackner AD. Toste FD. Angew. Chem. Int. Ed.  2010,  49:  598 
• For copper catalysts, see:
• 11f Zhao B. Du H. Shi Y. J. Org. Chem.  2009,  74:  8392 
• For silver catalysts, see:
• 11g Zhang Q.-W. Fan C.-A. Zhang H.-J. Tu Y.-Q. Zhao Y.-M. Gu P. Chen Z.-M. Angew. Chem. Int. Ed.  2009,  48:  8572 
• For iron catalysts, see:
• 11h Yang L. Zhu Q. Guo S. Qian B. Xia C. Huang H. Chem. Eur. J.  2010,  16:  1638 
• For rare-earth metal catalysts, see:
• 11i Inanaga J. Sugimoto Y. Hanamoto T. New J. Chem.  1995,  19:  707 
• 11j Furuno H. Hanamoto T. Sugimoto Y. Inanaga J. Org. Lett.  2000,  2:  49 
• 11k Sugihara H. Daikai K. Jin XL. Furuno H. Inanaga J. Tetrahedron Lett.  2002,  43:  2735 
• 11l Jin XL. Sugihara H. Daikai K. Tateishi H. Jin YZ. Furuno H. Inanaga J. Tetrahedron  2002,  58:  8321 
• 11m Furuno H. Kambara T. Tanaka Y. Hanamoto T. Kagawa T. Inanaga J. Tetrahedron Lett.  2003,  44:  6129 
• 11n Furuno H. Hayano T. Kambara T. Sugimoto Y. Hanamoto T. Tanaka Y. Jin YZ. Kagawa T. Inanaga J. Tetrahedron  2003,  59:  10509 
• 11o Suzuki S. Furuno H. Yokoyama Y. Inanaga J. Tetrahedron: Asymmetry  2006,  17:  504 
• For aluminum catalysts, see:
• 11p Yue T. Wang M.-X. Wang D.-X. Masson G. Zhu J. J. Org. Chem.  2009,  74:  8396 
• For a combination of chiral phosphoric acid and magnesium salt as a binary catalytic system, see:
• 11q Lv J. Li X. Zhong L. Luo S. Cheng J.-P. Org. Lett.  2010,  12:  1096 
• After the report by Ishihara and co-workers, the enantioselective catalysis by chiral calcium phosphate was developed by three research groups, see:
• 12a Drouet F. Lalli C. Liu H. Masson G. Zhu J. Org. Lett.  2011,  13:  94 
• 12b Zhang Z. Zheng W. Antilla JC. Angew. Chem. Int. Ed.  2011,  50:  1135 
• 12c Rueping M. Bootwicha T. Sugiono E. Synlett  2011,  323 
• Also see chiral sodium phosphate:
• 12d Hennecke U. Müller CH. Fröhlich R. Org. Lett.  2011,  13:  860 
• In the reaction of trimethylsilyl cyanide, the formation of hypervalent silicate might be responsible for the acceleration of the reactions, see:
• 13a Hatano M. Ikeno T. Matsumura T. Torii S. Ishihara K. Adv. Synth. Catal.  2008,  350:  1776 
• 13b Shen K. Liu X. Cai Y. Lin L. Feng X. Chem. Eur. J.  2009,  15:  6008 
• 15 Uraguchi D. Sorimachi K. Terada M. J. Am. Chem. Soc.  2004,  126:  11804 
• 16 Uraguchi D. Sorimachi K. Terada M. J. Am. Chem. Soc.  2005,  127:  9360 
• 17 Terada M. Machioka K. Sorimachi K. Angew. Chem. Int. Ed.  2006,  45:  2254 
• For the enantioselective substitution reaction of α-diazo-acetates with aldimines catalyzed by chiral Brønsted acids, see:
• 20a Hashimoto T. Maruoka K. J. Am. Chem. Soc.  2007,  129:  10054 
• 20b Hashimoto T. Maruoka K. Synthesis  2008,  3703 
• For the enantioselective aziridine formation (aza-Darzens) reaction of α-diazoacetates with aldimines catalyzed by chiral Brønsted acids, see:
• 21a Hashimoto T. Uchiyama N. Maruoka K. J. Am. Chem. Soc.  2008,  130:  14380 
• 21b Akiyama T. Suzuki T. Mori K. Org. Lett.  2009,  11:  2445 
• 21c Zeng X. Zeng X. Xu Z. Lu M. Zhong G. Org. Lett.  2009,  11:  3036 

8

Sorimachi, K.; Terada, M. unpublished results.

14

List and co-workers reported that silica gel purified chiral phosphoric acid 1a contained substantial amounts of alkali and alkaline-earth metals along with several metal impurities as ascertained by ICP-OES elemental analysis. See ref. 10a.

18

For the Preparation of Acid-Washed 1b (Method A)
Silica gel purified 1b was dissolved in Et₂O. The resultant solution was washed with HCl aq solution (2 M) in a separatory funnel. The resultant ether layer was dried over Na₂SO₄ and evaporated to remove organic solvents. The resultant residue was dried under reduced pressure for more than 12 h to eliminate organic solvents completely.
For the Preparation of Acid-Washed 1c (Method A)
Silica gel purified 1c was dissolved in MeOH. Then HCl aq solution (2 M) was added to the resultant MeOH solution to give a white suspension. The resultant suspension was extracted with CH₂Cl₂ (twice or more), and the combined organic layer was dried over Na₂SO₄. This was followed by the procedure as shown in the preparation of acid-washed 1b (method A).

19

Extra pure silica gel (Silica gel 60 extra pure for column chromatography: Catalogue No. 1.07754) was purchased from Merck KgaA. Short-path column chromatography was performed using CH₂Cl₂-MeOH (10:1) mixtures as eluent.

22

A silica gel purified chiral phosphoric acid contains considerable amounts of alkali and alkaline-earth metals with other metal impurities. Therefore it should be considered that a problem of reproducibility in yield and selectivity would happen, because the composition of metals is dependent on the conditions of silica gel column chromatography conducted.