S. Yasuda et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 2 (2008)
289
stirred for about 20 min at room temperature. Bromocyclohexane
(0.082 g, 0.50 mmol) was added to the reaction mixture at 25 ꢃC.
While the Grignard reagent was being added, the mixture turned
dark-brown. After stirring for 1 h at 25 ꢃC, a saturated ammonium
chloride solution (0.2 mL) was added to the reaction mixture. The
mixture was filtered through a pad of Florisil, and the filtrate was
concentrated. Silica-gel column purification (hexane) of the crude
product provided cyclohexylbenzene (61 mg, 75% yield).
Characterization Data. The following products are well-
known compounds or found in the literature: 1b,8 2b,4 2c,9 3,10
4,11 and 5.12
VCl3
4 ArMgBr
[Ar4VMgBr]
R–X
[R–X]– •
ArMgBr
MgBr
[Ar3V]
[Ar4V]
X–
•
R
1-tert-Butyl-4-cyclohexylbenzene (1c): IR (neat) 2925, 2852,
R–Ar
1521, 1507, 1461, 1448, 1362, 1270, 1109, 825, 570 cmꢄ1
;
[RAr4V]
Scheme 4. Plausible mechanism.
1H NMR (CDCl3) ꢀ 1.21–1.29 (m, 1H), 1.31 (s, 9H), 1.36–1.45
(m, 4H), 1.72–1.75 (m, 1H), 1.82–1.88 (m, 4H), 2.45–2.49 (m,
1H), 7.13 (d, J ¼ 6:5 Hz, 2H), 7.31 (d, J ¼ 6:5 Hz, 2H); 13C NMR
(CDCl3) ꢀ 26.49, 27.25 (2C), 31.71 (3C), 34.58, 34.76 (2C),
44.26, 125.37 (2C), 126.67 (2C), 145.27, 148.67; Found: C, 88.58;
H, 11.38%. Calcd for C16H24: C, 88.82; H, 11.18%.
complexes such as [Ar4VMgBr] should serve as an active
species.
We propose the following mechanism for the catalytic reac-
tion as follows (Scheme 4). The reaction of VCl3 with four
molar amounts of ArMgBr gives [Ar4VMgBr]. The vanadate
complex undergoes single electron transfer to the alkyl halide
to yield the anion radical of the alkyl halide, [Ar4V],5d and
MgBrþ. Immediate loss of bromide from the anion radical
affords the corresponding alkyl radical intermediate Rꢁ,
and [Ar4V] then captures the carbon-centered radical to form
[RAr4V]. The pentavalent organovanadium complex should
be so unstable that it should undergo rapid reductive elimina-
tion to afford the coupling product and [Ar3V], which is recon-
verted into [Ar4VMgBr] by reacting with the remaining
ArMgBr.
In conclusion, we showed the catalytic activity of vana-
dium(III) chloride in the cross-coupling reactions of alkyl
halides with aryl Grignard reagents. Vanadium catalysts are
rarely used for carbon–carbon bond-formation reactions.7
The results herein will open up new possibilities for using
vanadium catalysts.
1-Cyclohexyl-4-fluorobenzene (1d): IR (neat) 2927, 2853,
1605, 1511, 1448, 1159, 827, 807 cmꢄ1 1H NMR (CDCl3) ꢀ
;
1.19–1.28 (m, 1H), 1.32–1.43 (m, 4H), 1.71–1.76 (m, 1H), 1.80–
1.88 (m, 4H), 2.44–2.50 (m, 1H), 6.94–6.98 (m, 2H), 7.13–7.16
(m, 2H); 13C NMR (CDCl3) ꢀ 26.33, 27.09 (2C), 34.88 (2C),
44.07, 115.12 (d, J ¼ 20:6 Hz, 2C), 128.28 (d, J ¼ 7:8 Hz, 2C),
143.94 (d, J ¼ 3:4 Hz,), 161.35 (d, J ¼ 241:5 Hz); Found: C,
80.91; H, 8.49%. Calcd for C12H15F: C, 80.86; H, 8.48%.
1-Cyclohexyl-3-trifluoromethylbenzene (1e):
2928, 2855, 1491, 1451, 1437, 1333, 1277, 1232, 1198, 1163,
1125, 1074, 799, 702, 668 cmꢄ1 1H NMR (CDCl3) ꢀ 1.24–1.31
IR (neat)
;
(m, 1H), 1.35–1.47 (m, 4H), 1.74–1.78 (m, 1H), 1.84–1.90 (m,
4H), 2.53–2.58 (m, 1H), 7.37–7.40 (m, 2H), 7.41–7.44 (m, 1H),
7.44–7.46 (m, 1H); 13C NMR (CDCl3) ꢀ 25.99, 26.73 (2C),
34.27 (2C), 44.42, 122.62 (q, J ¼ 3:9 Hz), 123.50 (q, J ¼ 3:4
Hz), 124.37 (q, J ¼ 270:6 Hz,), 128.63, 130.26, 130.28, 130.50
(q, J ¼ 31:5 Hz); Found: C, 68.57; H, 6.67%. Calcd for C13H15F3:
C, 68.41; H, 6.62%.
This work was supported by Grants-in-Aid for Scientific
Research and COE Research from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
Experimental
General. 1H NMR (500 and 300 MHz) and 13C NMR (125.7
and 75.3 MHz) spectra were taken on a Varian UNITY INOVA
500 spectrometer and a Varian GEMINI 300 spectrometer.
1H NMR and 13C NMR spectra were obtained in CDCl3 with tetra-
methylsilane as an internal standard. Chemical shifts (ꢀ) are in
References
1
1
parts per million relative to tetramethylsilane at 0.00 ppm for H
Jpn. 2004, 62, 1192. c) M. R. Netherton, G. C. Fu, Adv. Synth.
and relative to CDCl3 at 77.2 ppm for 13C unless otherwise noted.
IR spectra were determined on a JASCO IR-810 spectrometer.
TLC analyses were performed on commercial glass plates with
a 0.25 mm thick layer of Merck Silica gel 60F254. Silica gel
(Wakogel 200 mesh) was used for column chromatography. The
elemental analyses were carried out at the Elemental Analysis
Center of Kyoto University.
¨
2
Very recent examples in 2005–2007: a) M. Nakamura, S.
2005, 4161. f) N. Hadei, E. A. B. Kantchev, C. J. O’Brien,
Unless otherwise noted, materials obtained from commercial
suppliers were used without further purification. Anhydrous
VCl3 THF was purchased from Aldrich and was diluted to pre-
pare a 0.050 M THF solution.
Typical Procedure for Vanadium-Catalyzed Arylation Re-
ꢂ
actions of Alkyl Halides.
Vanadium(III) chloride (0.050 M
¨
Betham, D. W. Bruce, A. A. Danopoulos, R. M. Frost, M. Hird,
THF solution, 1.0 mL, 0.050 mmol) and phenylmagnesium bro-
mide (1.0 M THF solution, 1.0 mL, 1.0 mmol) were sequentially
added to a 20-mL reaction flask under argon. The mixture was
´