oxidative kinetic resolution of secondary alcohols. The mild
reaction conditions of the catalytic system provide access to a
wide range of benzoins (a-hydroxy ketones) in good selectivity
(s = 6.7–10.6) and excellent enantiomeric excess (90–98% ee).
This method is very versatile in that the sole by-product
accompanying the oxidation process is water, making our
system more eco-friendly and green as well. Efforts are
currently underway to provide a detailed mechanistic insight
into the catalytic cycle to expand the scope and synthetic
utility of the enantioselective oxidation.
5 (a) Comprehensive Organic Synthesis, ed. B. M. Trost and I. Fleming,
Pergamon Press, Oxford, 1991; (b) F. A. Luzzio, Org. React., 1998,
53, 1; (c) T. T. Tidwell, Org. React., 1990, 39, 297; (d) M. Hudlicky, in
Oxidations in Organic Chemistry, ACS Monograph Series, American
Chemical Society, Washington, DC, 1990.
6 (a) P. Muller, in Advances in Catalytic Processes, JAI Press Inc.,
¨
Greenwich, CT, 1997, vol. 2, pp. 113–151; (b) J. B. Arterburn,
Tetrahedron, 2001, 57, 9765; (c) D. E. J. E. Robinson and
S. D. Bull, Tetrahedron: Asymmetry, 2003, 14, 1407.
7 For general discussions, see: (a) H. B. Kagan and J. C. Fiaud, in
Topics in Stereochemistry, ed. E. L. Eliel, Wiley & Sons, New York,
1988, vol. 18, pp. 249–330; (b) A. C. Spivey, A. Maddaford and
A. J. Redgrave, Org. Prep. Proced. Int., 2000, 32, 333; (c) E. Vedeja
and M. June, Angew. Chem., Int. Ed., 2005, 44, 3974.
8 S. D. Rychnovsky, T. L. McLernon and H. Rajapakse, J. Org.
Chem., 1996, 61, 1194.
Notes and references
y Typical experimental procedure for OKR: A mixture of L5 (35.8 mg,
0.05 mmol) and iron(II)acetate (8.7 mg, 0.05 mmol) in 8 mL of hexanes
was taken in a reaction tube and stirred at room temperature for
10 min, then TEMPO (3.9 mg, 0.025 mmol) was added to the reaction
mixture. After stirring for 5 min, benzoin (106 mg, 0.5 mmol) was
added and then the reaction mixture was stirred under an O2
atmosphere (using an O2 balloon) at 60 1C for 25 h. The reaction
mixture was concentrated and the resulting residue was purified by
silica gel column chromatography (eluents: hexanes–ethyl acetate) to
give the benzil (77 mg, yield 73%) and recovered benzoin (22 mg,
yield 21%). Benzoin: Rf 0.23 (hexanes–ethyl acetate, 90 : 10 v/v);
9 (a) E. M. Ferreira and B. M. Stoltz, J. Am. Chem. Soc., 2001, 123,
7725; (b) J. T. Bagdanoff, E. M. Ferreira and B. M. Stoltz, Org.
Lett., 2003, 5, 835; (c) J. T. Bagdanoff and B. M. Stoltz, Angew.
Chem., Int. Ed., 2004, 43, 353.
10 (a) D. R. Jensen, J. S. Puglsey and M. S. Sigman, J. Am. Chem.
Soc., 2001, 123, 7475; (b) S. K. Mandel, D. R. Jensen, J. S. Puglsey
and M. S. Sigman, J. Org. Chem., 2003, 68, 4600; (c) S. K. Mandel
and M. S. Sigman, J. Org. Chem., 2003, 68, 7535.
11 T. A. Radosevich, C. Musich and D. F. Toste, J. Am. Chem. Soc.,
2005, 127, 1090.
12 S. Arita, T. Koike, Y. Kayaki and T. Ikariya, Angew. Chem., Int.
Ed., 2008, 47, 1.
13 M. L. Kantam, T. Ramani, L. Chakrapani and B. M. Choudary,
J. Mol. Catal. A: Chem., 2007, 274, 11.
14 K. Masutani, T. Uchida, R. Irie and T. Katsuki, Tetrahedron Lett.,
1995, 36, 9519.
15 (a) G. Sekar and H. Nishiyama, J. Am. Chem. Soc., 2001, 123,
3603; (b) G. Sekar and H. Nishiyama, Chem. Commun., 2001,
1314.
16 S. K. Alamsetti, S. Mannam, P. Muthupandi and G. Sekar,
Chem.–Eur. J., 2009, 15, 1086.
[a]D = ꢃ76.0 (c = 1 in acetone); 1H NMR (400 MHz, CDCl3):
25
d 7.88–7.93 (m, 2H), 7.48–7.53 (m, 1H), 7.23–7.41 (m, 7H), 5.95
(d, J = 6 Hz, 1H), 4.55 (d, J = 6 Hz, 1H); 13C NMR (100 MHz,
CDCl3): d 199.1, 139.2, 134.0, 133.7, 129.3, 129.2, 128.8, 128.7, 127.9,
76.4; IR (neat) 3418, 1679, 1261, 1068 cmꢃ1; HRMS (m/z): [MNa]+
calcd for C14H12O2Na1, 235.0735; found, 235.0727. The enantiomeric
excess (% ee) was determined to be 98% by HPLC using a Daicel
ChiralPAK AS-H column (15% i-PrOH–hexanes, 1 mL minꢃ1
,
220 nm): tR (major, 13.250 min), tR (minor, 8.500 min). Benzil:
Rf 0.43 (hexanes–ethyl acetate, 90 : 10 v/v); 1H NMR (400 MHz,
CDCl3): d 7.96–8.01 (m, 4H), 7.63–7.69 (m, 2H), 7.49–7.55 (m, 4H);
13C NMR (100 MHz, CDCl3): d 194.7, 135.0, 133.3, 130.1, 129.2; IR
17 Benzoins are used as urease inihibitors and building blocks for the
synthesis of different heterocyclic compounds: (a) T. Tanaka,
M. Kawase and S. Tani, Bioorg. Med. Chem., 2004, 12, 501;
(b) W. M. Abdou, Y. O. El-Khoshnieh and A. A. Kamel, Heteroat.
Chem., 1999, 10, 481; (c) W. W. Pei, S. H. Li, X. P. Nie, Y. W. Li,
J. Pei, B. Z. Chen, J. Wu and X. L. Ye, Synthesis, 1998, 1298;
(d) M. S. Singh and A. K. Singh, Synthesis, 2004, 6, 837.
18 For asymmetric benzoin condensations and enzymatic dynamic
(neat): 3064, 1656 cmꢃ1
;
HRMS (m/z): [MNa]+ calcd for
C14H10O2Na1, 233.0578; found, 233.0585.
1 (a) Transition Metal Reagents and Catalysts: Innovations in
Organic Synthesis, ed. J. Tsuji, John Wiley and Sons, New York,
2002; (b) Transition Metals for Organic Synthesis, ed. M. Beller and
C. Bolm, Wiley-VCH, Weinheim, 2004; (c) Lewis Acids in Organic
Synthesis, ed. H. Yamamoto, Wiley-VCH, Weinheim, 2008.
2 (a) Comprehensive Asymmetric Catalysis, ed. E. N. Jacobsen,
A. Pfaltz and H. Yamamoto, Springer-Verlag, Berlin, 1999,
vol. 1–3; (b) J. K. Whitesell, Chem. Rev., 1992, 92, 953.
kinetic resolutions of racemic benzoins: (a) P. Dunkelmann,
¨
D. Kolter-Jung, A. Nitsche, A. S. Demir, P. Siegert, B. Lingen,
M. Baumann, M. Pohl and M. Muller, J. Am. Chem. Soc., 2002,
¨
124, 12084; (b) D. Enders, O. Niemeier and T. Balensiefer, Angew.
Chem., Int. Ed., 2006, 45, 1463; (c) D. Enders and U. Kallfass,
Angew. Chem., Int. Ed., 2002, 41, 1743; (d) X. Linghu and
J. S. Johnson, Angew. Chem., Int. Ed., 2003, 42, 2534;
3 For review, see: (a) C. Bolm, J. Legros, J. L. Paih and L. Zani,
Chem. Rev., 2004, 104, 6217, and references therein; (b) Iron
Catalysis, ed. B. Plietker, Wiley-VCH, Weinheim, 2008.
4 Selected papers on iron catalyzed asymmetric reactions:
(a) E. J. Corey, N. Imai and H. Y. Zhang, J. Am. Chem. Soc.,
1991, 113, 728; (b) M. Nakamura, A. Hirai and E. Nakamura,
J. Am. Chem. Soc., 2000, 122, 978; (c) S. Matsukawa, H. Sugama
and T. Imamoto, Tetrahedron Lett., 2000, 41, 6461;
(d) Y. Yamashita, M. Ueno, Y. Kuriyama and S. Kobayashi,
Adv. Synth. Catal., 2002, 344, 929; (e) G. Du, B. Andrioletti,
E. Rose and L. K. Woo, Organometallics, 2002, 21, 4490;
(f) T. Itoh, K. Kawai, S. Hayase and H. Ohara, Tetrahedron Lett.,
2003, 44, 4081; (g) M. Redlich and M. M. Hossain, Tetrahedron
Lett., 2004, 45, 8987; (h) J. Jankowska, J. Paradowska, B. Rakiel
and J. Mlynarski, J. Org. Chem., 2007, 72, 2228; (i) C. Sui-Seng,
F. Freutel, A. J. Lough and R. H. Morris, Angew. Chem., Int. Ed.,
2008, 47, 940; (j) K. Suzuki, P. D. Oldenburg and L. Que, Jr,
Angew. Chem., Int. Ed., 2008, 47, 1887; (k) N. S. Shaikh,
S. Enthaler, K. Junge and M. Beller, Angew. Chem., Int. Ed.,
2008, 47, 2497.
(e) P. Hoyos, M. Ferna
A. R. Alca
´
´
ndez, J. V. Sinisterra and
ntara, J. Org. Chem., 2006, 71, 7632.
19 Conversion was measured by 1H NMR analysis of the crude
reaction mixture. See ESI for full detailsz.
20 The selectivity factor (s) was determined using the equation
s = krel(kfast/kslow) = ln [(1 ꢃ C)(1 ꢃ ee)]/ln [(1 ꢃ C)(1 + ee],
where C = conversion. See ESI for full detailsz.
21 (a) D. A. Evans, J. M. Janey, N. Magomedov and J. S. Tedrow,
Angew. Chem., Int. Ed., 2001, 40, 1884; (b) N. Takenaka, Y. Huang
and V. H. Rawal, Tetrahedron, 2002, 58, 8299; (c) K. Bernardo,
S. Leppard, A. Robert, G. Commenges, F. Dahan and B. Meunier,
Inorg. Chem., 1996, 35, 387; (d) I. P. Holmes and H. B. Kagan,
Tetrahedron Lett., 2000, 41, 7457; (e) K. C. Gupta and A. K. Sutar,
Coord. Chem. Rev., 2008, 252, 420.
22 The absolute configurations of benzoins were determined from the
signs of specific rotations in comparison with the literature values.
See ESI for further detailsz.
ꢁc
This journal is The Royal Society of Chemistry 2009
3290 | Chem. Commun., 2009, 3288–3290