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Synlett 2016; 27(01): 126-130
DOI: 10.1055/s-0035-1560540
letter
© Georg Thieme Verlag Stuttgart · New York

Preparation of C 2-Symmetric Biaryl Bisiminium Salts and Their Use as Organocatalysts for Asymmetric Epoxidation

Philip C. Bulman Page*
a   School of Chemistry, University of East Anglia, Norwich Research Park, Norwich, Norfolk NR4 7TJ, UK   eMail: p.page@uea.ac.uk
,
Mohamed M. Farah
b   Department of Chemistry, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK
,
Benjamin R. Buckley
b   Department of Chemistry, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK
,
Yohan Chan
a   School of Chemistry, University of East Anglia, Norwich Research Park, Norwich, Norfolk NR4 7TJ, UK   eMail: p.page@uea.ac.uk
,
A. John Blacker
c   School of Chemistry, University of Leeds, Leeds LS2 9JT, UK
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Publikationsverlauf

Received: 27. September 2015

Accepted after revision: 01. November 2015

Publikationsdatum:
16. November 2015 (online)

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Abstract

Two C 2-symmetric bisiminium salt species containing biphenylazepinium units and derived from two chiral diamines were prepared and tested as organocatalysts for asymmetric epoxidation.

Key words

iminium - diamine - organocatalysis - epoxidation - biaryl

Supporting Information

    Supporting information for this article is available online at http://dx.doi.org/10.1055/s-0035-1560540.
  • Supporting Information
 

  • References and Notes

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  • 25 General Procedure for the Synthesis of 5H-Dibenzo[c,e]azepinium Salts from 2-[2-(Bromomethyl)phenyl]benzene Carbaldehyde and Primary Amines A solution of the amine (1 equiv) in EtOH (10 mL per gram of amine), was added dropwise to an ice-cooled solution of 2-[2-(bromomethyl)phenyl]benzene carbaldehyde (17, 1.1 equiv) in EtOH (10 mL per gram of 17). The reaction mixture was stirred overnight while attaining ambient temperature. Sodium tetra­phenylborate (1.1 equiv) dissolved in the minimum amount of MeCN was added in one portion to the reaction mixture and after stirring for 5 min, the organic solvents were removed under reduced pressure. EtOH was added to the residue, followed by few drops of H2O. The resulting solid was collected by filtration and washed with additional EtOH followed by Et2O. If no solid materializes after the addition of the H2O the suspension is allowed to settle and the EtOH–H2O phase is decanted. The gummy residue is macerated in hot EtOH or MeOH. The organic salt may then precipitate, and in rare cases it does so upon slow cooling of the hot alcoholic solution. If solubility problems do arise, small amounts of MeCN may be added during this process. For catalyst 19 Prepared according to the general procedure using (1S,2S)-diaminocyclohexane (10, 0.10 g, 0.88 mmol) and 2-[2-(bromomethyl)phenyl]benzene carbaldehyde (17, 0.51 g, 1.85 mmol, 2.1 equiv). The product 19 was isolated as yellow powder (0.51 g, 52%); mp 145–147 °C; [α]D 20 –170.2 (c 0.59, MeCN). IR (film): νmax = 3052, 2933, 1639, 1597, 1552, 1480, 1445, 1425, 1332, 1265, 1208, 761, 731, 705 cm–1. 1H NMR (400 MHz, DMSO-d 6, 100 °C): δ = 1.77 (2 H, d, J = 12.0 Hz), 2.01 (2 H, d, J = 8.0 Hz), 2.30 (2 H, d, J = 8.0 Hz), 2.49–2.53 (2 H, m), 4.71 (2 H, d, J = 12.0 Hz), 5.06 (2 H, d, J = 12.0 Hz), 5.71 (2 H, s), 6.83 (8 H, t, J = 7.2 Hz), 6.97 (16 H, t, J = 7.2 Hz), 7.17–7.26 (18 H, m), 7.52 (4 H, br s), 7.73–7.79 (4 H, m), 7.87–8.06 (8 H, m). 13C NMR (100 MHz, DMSO-d 6, 100 °C): δ = 24.0, 32.1, 55.0, 72.1, 121.9, 125.1, 125.6, 126.7, 127.0, 128.7, 129.0, 129.5, 130.4, 130.4, 133.8, 135.9, 136.2, 136.8, 141.5, 171.2. HRMS–FAB: m/z calcd for C34H32N2 (cation): 468.2566; found: 468.2560
  • 26 General Procedure for Epoxidation Reactions The alkene (1 equiv) and the catalyst (5 mol%) were dissolved in a mixture of MeCN and H2O (10:1, 3 mL per mmol of alkene), and the mixture was cooled to 0 °C. A mixture of Oxone (2 equiv) and NaHCO3 (5 equiv) was added as a solid in one portion to the mixture with vigorous stirring. The mixture was stirred at 0 °C until complete conversion of the alkene was observed by TLC. Et2O (30 mL per mmol of alkene) was added, and the reaction mixture was filtered through a pad of mixed anhydrous MgSO4 and NaHSO3. The solvents were removed under reduced pressure, and the residue was purified using silica gel column chromatography using hexanes as the eluent.