We thank the Ecole Normale Supérieure (Paris) for a
fellowship (L. C.).
Notes and references
‡ Eu(hfc)3
=
Europium tris[3-(heptafluoropropylhydroxymethylene)-
(+)-camphorate].
1 (a) R. M. Williams, Synthesis of Optically Active a-Amino Acids,
Pergamon Press, Oxford, 1989; (b) R. O. Duthaler, Tetrahedron, 1994,
50, 1539.
2 For a review on boron analogues of biomolecules, see: C. Morin,
Tetrahedron, 1994, 50, 12521.
Fig. 2 X-Ray structure of adduct 8c.
3 Selected references: (a) D. S. Matteson, K. M. Sadhu and G. E.
Lienhard, J. Am. Chem. Soc., 1981, 103, 5241; (b) C. A. Kettner and
A. B. Shenvi, J. Biol. Chem., 1984, 259, 15106; (c) T. A. Kelly, V. U.
Fuchs, C. W. Perry and R. J. Snow, Tetrahedron, 1993, 49, 1009; (d) P.
Mantri, D. E. Duffy and C. A. Kettner, J. Org. Chem., 1996, 61, 5690;
(e) C. Lebarbier, F. Carreaux and B. Carboni, Synthesis, 1996, 1371; (f)
C. S. Brosz, J. C. Calabrese, C. A. Kettner and C. A. Teleha,
Tetrahedron: Asymmetry, 1997, 8, 1435.
4 Selected references: (a) B. F. Spielvogel, L. Wojnowich, M. K. Das,
A. T. McPhail and K. D. Hargrave, J. Am. Chem. Soc., 1976, 98, 5702;
(b) P. Wisian-Neilson, M. K. Das and B. F. Spielvogel, Inorg. Chem.,
1978, 17, 2327; (c) B. F. Spielvogel, F. U. Ahmed, G. L. Silvey, P.
Wisian-Neilson and A. T. McPhail, Inorg. Chem., 1984, 23, 4322; (d) A.
Sood and B. F. Spielvogel, Main Group Met. Chem., 1989, 12, 143; (e)
C. H. Sutton, M. W. Baize and L. J. Todd, Inorg. Chem., 1994, 33, 4221;
(f) W. J. Mills, C. H. Sutton, M. W. Baize and L. J. Todd, Inorg. Chem.,
1991, 30, 1046.
Table 1 Separation of the enantiomers of benzylcyanoborane adducts 8a–c
by chiral phase HPLC
Adduct
8a
Column
Eluent/flow rate
Rt/min
Chiracel OD
Chiracel OD
Chiracel OD
Chiracel OD
Chiracel OC
95:5 hexane–EtOH
1.2 mL min21
CO2, 1 mL min (180 bars)
MeOH, 0.2 mL min21
95+5 hexane–EtOH
1.2 mL min21
CO2, 2 mL min21 (250 bars)
MeOH, 0.15 mL min21
90+10 hexane–iPrOH
1 mL min21
23.2, 29.1
9.4, 10.6
26.8, 29.0
8.7, 9.8
8a
8b
8b
8c
54.1, 56.3
5 (a) W. J. Mills, C. H. Sutton, E. Libby and L. J. Todd, Inorg. Chem.,
1990, 29, 302; (b) W. J. Mills, L. J. Todd and J. C. Huffman, J. Chem.
Soc., Chem. Commun., 1989, 900.
6 H. C. Brown, M. Srebnik and T. E. Cole, Organometallics, 1986, 5,
2300.
7 B. Singaram, T. E. Cole and H. C. Brown, Organometallics, 1984, 3,
774.
8 J. Emri and B. Györi, J. Chem. Soc., Chem. Commun., 1983, 1303.
the presence of Eu(hfc)3‡ (2.4 eq.). In the same conditions,
single sets of peaks were observed for separated enantiomers.
Direct evidence of an SN2 substitution at the boron atom was
observed in our laboratory in the reaction of a diastereomer-
ically pure monoisopinocampheylcyanoborane adduct of tri-
phenylphosphine with a more nucleophilic phosphine, dime-
thylphenylphosphine.10,11
Having pure amine–benzylcyanoboranes enantiomers in
hand, we also studied similar substitution. Thus, one enantiomer
of trimethylamine–benzylcyanoborane 8a was dissolved in
pyridine, at rt or at 70 °C, and the reaction was monitored by
chiral HPLC. After 2 h at rt, one enantiomer of pyridine–
benzylcyanoborane 8b was formed, and after a longer time the
second enantiomer appeared, probably because of the racemiza-
tion of the first one in the presence of the excess of pyridine. At
70 °C, the same phenomenon occurred, but faster. The fact that
one enantiomer of reactant was transformed into one enantio-
mer of product is indicative of an SN2 mechanism occurring at
the boron atom.
9 Crystal data for racemic adduct 8a: C11H17BN2, M
= 188.08,
colourless, orthorhombic, a = 11.645(5), b = 16.506(3), c = 9.074(4)
Å, b = 95.07(2)°, T = 294 K, space group P212121, Z = 4, m(Mo-Ka)
0.586 cm21, 1222 reflections measured, 761 reflections observed [I
> s(I)], final wR = 0.048. Selected bond distances (Å): B–N 1.619(7),
B–CN 1.60(8). Crystal data for racemic adduct 8c: C13H12BN2Cl, M =
242.52, colourless, monoclinic, a = 8.539(2), b = 11.828(3), c =
12.989(4) Å, T = 293 K, space group P21/n, Z = 4, m(Mo-Ka) 2.68
cm21, 2592 reflections measured, 1180 reflections observed [I > s(I)],
final wR = 0.061. Selected bond distances (Å): B–N 1.578(8), B–CN
1.60(1). CCDC 182/1804.
10 P. Vedrenne, V. Le Guen, L. Toupet, T. Le Gall and C. Mioskowski,
J. Am. Chem. Soc., 1999, 121, 1090.
11 For other recent studies on substitution at a boron atom, see: (a) S.
Toyota and M. Oki, Bull. Chem. Soc. Jpn., 1990, 63, 1168; (b) S.
Toyota, T. Futawaka, H. Ikeda and M. Oki, J. Chem. Soc., Chem.
Commun., 1995, 2499; (c) S. Toyota, T. Futawaka, M. Asakura, H.
Ikeda and M. Oki, Organometallics, 1998, 17, 4155; (d) T. Imamoto and
H. Morishita, J. Am. Chem. Soc., 2000, 122, 6329; (e) E. Vedejs, S. C.
Fields, S. Lin and M. R. Schrimpf, J. Org. Chem., 1995, 60, 3028; (f) E.
Vedejs, S. C. Fields, R. Hayashi, S. R. Hitchcock, D. R. Powell and
M. R. Schrimpf, J. Am. Chem. Soc., 1999, 121, 2460.
In conclusion, several benzylcyanoborane adducts of amines
were prepared in four steps from toluene and separated using
chiral phase HPLC. The occurrence of an SN2 pathway at a
tetravalent boron atom was evidenced by the formation of a
single adduct in the treatment of an enantiopure trimethylamine
adduct with pyridine.
2276
Chem. Commun., 2000, 2275–2276