3 For reviews, see: (a) E. Skucas, M.-Y. Ngai, V. Komanduri and M.
J. Krische, Acc. Chem. Res., 2007, 40, 1394; (b) M. Johannsen and
K. A. Jørgensen, Chem. Rev., 1998, 98, 1689; (c) R. B. Cheikh, R.
Chaabouni, A. Laurent, P. Mison and A. Nafti, Synthesis, 1983,
685; (d) A. Stutz, Angew. Chem., Int. Ed. Engl., 1987, 26, 320.
4 Importance of fluorine in medicinal chemistry: (a) I. Ojima, J. R.
McCarthy and J. T. Welch, Biomedical Frontiers of Fluorine
Chemistry, ACS Symp. Ser., No. 639, American Chemical Society,
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Eshwarakrishman, Wiley, New York, NY, 1991.
Scheme 3 Reduction of ketoimines 4 with DIBAL-H.
reduction. On the other hand, electrostatic repulsion between
the lone pair of sulfur atom and the electron-rich CF3 group
makes CF3 far away from sulfur atoms in both systems.
Hence, the six-membered transition state in which the sulfinyl
oxygen participates in the delivery of hydride in the DIBAL-H
system gives (Rs, R)-2 as the major product, while an open
transition state in the L-Selectride (poorly coordinating metal
hydrides) system affords the major product (Rs, S)-3. This
model may also explain why the addition of excess HMPA
tends to give higher selectivity. The probable role of HMPA in
the reaction system is to coordinate with lithium(I) ion and
disrupts the chelation of Li(I) with sulfinyl oxygen.
5 G. K. S. Prakash, M. Mandal and G. A. Olah, Org. Lett., 2001, 3,
2847.
6 (a) N. T. N. Tam, G. Magueur, M. Ourevitch, B. Crousse, J.-P.
Begue and D. Bonnet-Delpon, J. Org. Chem., 2005, 70, 699; (b) S.
D. Kuduk, C. N. D. Marco, S. M. Pitzenberger and N. Tsou,
Tetrahedron Lett., 2006, 47, 2377; (c) S. Fries, J. Pytkowiz and T.
Brigaud, Tetrahedron Lett., 2005, 46, 4761.
7 S. Fustero, J. Garcıa Soler, A. Bartolome and M. Sanchez Rosello,
Org. Lett., 2003, 5, 2707.
8 For reviews, see: (a) C. H. Senanayake, D. Krishnamurthy, Z.-H.
Lu, Z. Han and I. Gallou, Aldrichimica Acta, 2005, 38, 93; (b) J. A.
Ellman, T. D. Owens and T. P. Tang, Acc. Chem. Res., 2002, 35,
984; (c) F. A. Davis, J. Org. Chem., 2006, 71, 8993; (d) P. Zhou,
B.-C. Chen and F. A. Davis, Tetrahedron, 2004, 60, 8003; (e) J. A.
Ellman, Pure Appl. Chem., 2003, 75, 39.
9 For CF3-substituted N-tert-butylsulfinyl imines, see: (a) V. L
Truong, M. S. Menard and I. Dion, Org. Lett., 2007, 9, 683; (b)
H. Wang, X.-M. Zhao, Y.-H. Li and L. Lu, Org. Lett., 2006, 8,
1379; (c) See ref. 6b; (d) For CF3-substituted N-p-tolylsulfinyl
imines, see: A. Asensio, P. Bravo, M. Crucianelli, A. Farina, S.
Fustero, J. G. Soler, S. V. Meille, W. Panzeri, F. Viani, A.
Volonterio and M. Zanda, Eur. J. Org. Chem., 2001, 1449; (e) P.
Bravo, M. Crucianelli, B. Vergani and M. Zanda, Tetrahedron
Lett., 1998, 38, 7771; (f) M. Crucianelli, F. De Angelis, F. Lazarro,
L. Malpezzi, A. Volonterio and M. Zanda, J. Fluorine Chem.,
2004, 125, 573; (g) J. L. G. Ruano, T. Haro, R. Singh and M. B.
Cid, J. Org. Chem., 2008, 73, 1150.
10 (a) X.-J. Wang, Y. Zhao and J.-T. Liu, Org. Lett., 2007, 9, 1343; (b)
Z.-J. Liu, Y.-Q. Mei and J.-T. Liu, Tetrahedron, 2007, 63, 855; (c)
Y. Zhao, X.-J. Wang and J.-T. Liu, Synlett, 2008, 7, 1017.
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G. Liu, D. A. Cogan, T. D. Owens, T. P. Tang and J. A. Ellman, J.
Org. Chem., 1999, 64, 1278; (c) G. Liu, D. A. Cogan and J. A.
Ellman, J. Am. Chem. Soc., 1997, 119, 9913.
12 To our knowledge only a single example has been reported for the
reduction of a,b-unsaturated N-tert-butylsulfinyl ketoimines. See: G.
Borg, D. A. Cogan and J. A. Ellman, Tetrahedron Lett., 1999, 40, 6709.
13 For the stereoselectivity reversal of N-tert-butylsulfinyl imines in
hydride reduction, see: (a) J. T. Colyer, N. G. Anderson, J. S.
Tedrow, T. S. Soukup and M. M. Faul, J. Org. Chem., 2006, 71,
6859; (b) J. Tanuwidjaja, H. M. Peltier and J. A. Ellman, J. Org.
Chem., 2007, 72, 626; (c) B. Denolf, E. Leemans and N. De Kimpe,
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14 The absolute configuration of product 3a was assigned by cleaving
the sulfinamide with HCl and examining the optical rotation of the
known salt. See ESIw for details.
To demonstrate the importance of CF3 group in these
selective reduction reactions, non-CF3-substituted a,b-unsatu-
rated N-tert-butylsulfinyl ketoimines with a similar steric
hindrance to that of CF3 group 4a and 4b were synthesized
and subjected to both reduction reactions (Scheme 3). Using
the conditions of entry 3 in Table 1 for the DIBAL-H system,
the reaction proceeded well for both ketoimines and gave the
corresponding allylic amines in excellent overall yields, but
only moderate diastereoselectivity was obtained (de 60–68%).
With the optimized L-Selectride system, however, the reaction
became complicated in the case of 4a and no reaction was
observed for 4b.
In summary, we have developed a highly efficient method for
the asymmetric synthesis of either stereoisomer of trifluoro-
methylated allylic amines with good yield and high diastereo-
selectivity by the selective reduction of chiral trifluoromethyl
a,b-unsaturated N-tert-butanesulfinyl ketoimines
1 with
DIBAL-H and L-Selectride, respectively. Further studies on
the application of 1 in the preparation of a variety of chiral
amine analogues are in progress.
Financial support from the National Natural Science
Foundation of China (No. 20572124) is gratefully acknowledged.
Notes and references
z Crystal data: for 1c, C15H18F3NOS, M = 317.36, orthorhombic,
space group P212121, a = 6.0128(9), b = 8.4458(13), c = 32.013(5) A,
V = 1625.7(4) A3, T = 293 K, Z = 4, 8907 reflections measured, 3183
unique (Rint = 0.1349), R1 = 0.0545 [I 4 2s(I)], wR2 = 0.1175; for 3d,
C14H17ClF3NOS, M = 339.80, orthorhombic, space group P212121, a
= 5.8166(6), b = 14.9426(16), c = 18.2981(19) A, V = 1590.4(3) A3,
15 For HMPA used as Lewis base to improve the diastereoselectivity
in additions to imines, see: (a) X.-W. Sun, M.-H. Xu and G.-Q.
Lin, Org. Lett., 2006, 8, 4979; (b) Y.-W. Zhong, K. Izumi, M.-H.
Xu and G.-Q. Lin, Org. Lett., 2004, 6, 4747.
T = 293 K, Z = 4, 8401 reflections measured, 2948 unique (Rint
0.1417), R1 = 0.0510 [I 4 2s(I)], wR2 = 0.1108.
=
16 The competition of 1,2-addition vs. 1,4-addition has been observed
in the literature for the reduction of a,b-unsaturated imines using
DIBAL-H, NaBH4, etc. See, for example: (a) E. Aguilar, J. Joglar,
I. Merino, B. Olano, F. Palacios and S. Fustero, Tetrahedron, 2000,
56, 8179; (b) J. Barluenga, E. Aguilar, J. Joglar, B. Olano and S.
Fustero, J. Chem. Soc., Chem. Commun., 1989, 1132.
1 (a) For a review on the application of allylamines, see: S. Otsuka
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17 (a) K. Uneyama, Organofluorine Chemistry, Blackwell, 2006, pp.
81–89; (b) J.-A. Ma and D. Cahard, Chem. Rev., 2004, 104, 6119.
ꢁc
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