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DOI: 10.1055/s-0029-1217365
Novel Chiral C 2-Symmetric Bisimidazole-N-Oxides as Promising Organocatalysts for Enantioselective Allylation of Aromatic Aldehydes
Publikationsverlauf
Publikationsdatum:
12. Juni 2009 (online)
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Abstract
A series of new, chiral Lewis bases containing imidazole-N-oxide moiety were tested for purposes of asymmetric catalysis. Bisimidazole-N-oxides derived from (1R,2R)- and (1S,2S)-trans-1,2-diaminocyclohexane were used as catalysts in the allylation reaction of aromatic aldehydes with allyltrichlorosilane, which yielded homoallyl alcohols in good yields and with enantioselectivity up to 80% ee. Screening of catalysts revealed that the type of substituents and their location in imidazole ring has a crucial influence on enantioselectivity of the addition process.
Key words
allylation - asymmetric catalysis - chiral imidazole-N-oxides - nucleophilic addition - organocatalysis
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References and Notes
Typical Procedure
for the Preparation of Bisimidazole N
-Oxides - Synthesis of (1
R
,2
R
)-1d
To
a stirred soln of (1R,2R)-trans-1,2-diamonocyclohexane (114.0 mg,
1.0 mmol) in MeOH (3 mL), a portion of paraformaldehyde (63.0 mg,
2.1 mmol) was added and the soln was stirred overnight at ambient
temperature. After evaporation of the solvent in vacuum, the resulting,
viscous oil was dissolved in glacial acid (7 mL) containing 473
mg (2.1 mmol) α-benzil monoxime 3d and
the soln obtained thereby was stirred overnight at ambient temperature.
Next day, a gentle stream of gaseous HCl was bubbled through the soln
for ca. 1.5 h, and the separated colorless bisimidazole N-oxide hydrochloride was filtered off
and dried in vacuum exsiccator. The crude hydrochloride was suspended
in MeOH (ca 25 mL) and 1 g of the solid NaHCO3 was added; stirring
was continued for ca.1.5 h until evolution of CO2 was
complete. Precipitated solid of inorganic salts was filtered off,
and the filtrate was evaporated to dryness. The colorless solid
material was triturated with a little portion (ca. 5 mL) of a CHCl3-MeOH
(2:1) mixture. Suspended, solid material was separated, and the
filtrate was evaporated to dryness. Crude product was washed with
little amount of dry acetone to yield analytically pure sample of
(1R,2R)-trans-1,1′-(cyclohexane-1,2-diyl)bis(4,5-diphenylimida-zole)-3,3′-dioxide [(R,R)-1d]; yield 342 mg (62%);
colorless crystals; mp(dec) 208-210 ˚C. IR (KBr): ν = 3424-2867
(vs, br), 1635 (m), 1570 (m), 1506 (m), 1484 (m), 1446 (m), 1405 (m),
1339 (s), 1222 (m), 1193 (m), 767 (s), 711 (s), 658 (m), 636 (m)
cm-¹. ¹H NMR (CD3OD): δ = 8.08 [s,
2 H, HC(2), HC(2′) imidazole], 7.65-7.50
(m, 4 H, 4 arom. H), 7.38-7.23 (m, 12 H, 12 arom. H), 7.18-7.07
(m, 4 H, 4 arom. H), 4.37-4.26 (m, 2 H, 2CH, cHex), 2.38-1.40 (m, 8 H, 4CH2, cHex). ¹³C
NMR (CD3OD): δ = 131.8, 131.4, 131.2,
130.8, 129.9, 129.7 [6 d, 20 arom. CH, C(2), C(2′)
imidazole], 131.0, 129.6, 127.3 [3 s, 4 arom.
C, C(4), C(4′), C(5), C(5′) imidazole],
61.4 (d, 2 CH, cHex), 34.0, 25.3 (2 t,
4 CH2, cHex). ESI-MS: m/z = 575 (100) [M + Na]+.
ESI-HRMS:
m/z calcd for C36H32N4O2Na [M + Na]+:
575.2423; found: 575.2422. [α]D
²0 +6.0
(c 1.02; MeOH). For X-ray structure determination
of (1R,2R)-
and (1S,2S)-1d, see: Mloston G., Mucha P., Tarka R.,
Urbaniak K., Linden A., Heimgartner H.; Polish
J. Chem.; 2009, 83,
1105.
General Allylation
Procedure
To a stirred soln of the catalyst 1d (27.7 mg, 0.05 mmol) in dry CH2Cl2 (1
mL) the corresponding aldehyde 4 (0.5 mmol) and
dry diisopropylethylamine (260 µL, 1.5 mmol) were added.
After 5 min. magnetic stirring at 0 ˚C, allyltrichloro-silane
(90-95 µL, 0.6 mmol) was added to the reaction mixture.
Stirring was continued at 0 ˚C for another 20
h, and after this time the mixture was firstly diluted with Et2O, quenched
with aq NaHCO3 (1 mL) and next shaken with H2O.
Organic layer was separated and the aqueous soln was extracted again
with Et2O (2 × 10 mL); combined
ethereal soln were dried over MgSO4, filtered, and concentrated.
The residue was purified by silica gel column chromatography. Yields
refer to the isolated amount of alcohol 5.
The ee was determined using HPLC technique with chiral column (Chiralcel
OD-H or Chiralpak AS-H); mixture of 2-PrOH-hexane was applied
as an eluent.