602
Z. Zhou, W. Xue / Journal of Organometallic Chemistry 694 (2009) 599–603
By using in situ infrared technology, we try to continuously
The mixtures were refluxed for 4 h. The flask was then cooled to r.t.
and the Grignard reagent 1a was used directly in the following
experiment. Yield of mono-Grignard reagent determined by
quenching followed by HPLC analysis was 97%.
monitor the formation of the 4-methylphenylmagnesium chloride
and the corresponding biaryls. Fig. 1 shows an IR spectrum ‘‘water-
fall” plot obtained during the course of the reaction. The in situ IR
data for RMgX and the homo-couplings were profiled for the entire
reaction (Fig. 2). At the beginning of the reaction the Grignard re-
agents were formed quickly during the initiation period. After ini-
tiation the RMgX levels continue to rise until no aryl chloride was
left. Absorbance for biaryls increased immediately after the
catalyst was added as observed by the parallel profiles. The
homo-couplings stopped increasing when the Grignard Reagents
were consumed at 220 min. On-line monitoring by IR provided
confidence that the homo-coupling was successfully generated
by the increasing in absorbance at 850 cmꢁ1, as clearly shown in
Fig. 1.
A mechanism for the manganese catalyzed homo-coupling of
Grignard reagents is presented (Scheme 1), and is similar to the
iron catalyzed reaction [4]. Reaction of the aryl Grignard with man-
ganese chloride results in transmetallation affording a diarylman-
ganese intermediate D. Reductive elimination of the two aryl
groups gives the homo-coupling product and reduced manganese.
Oxidative addition of 1,2-dichloroethane to reduced manganese A
forms an alkyl manganese intermediate B which undergoes b-hal-
ogen elimination [42] giving ethylene and regenerating the catalyt-
ically active species C.
4.1.3. 4,40-Dimethyl-biphenyl
The Grignard reagent solution (1a containing 9.7 mmol mono-
Grignard chlorides) was added dropwise to a 50 mL-flask, charged
with MnCl2 (126 mg, 1 mmol), THF (10 mL), and 1,2-dichloroeth-
ane (1.19 g, 12 mmol), whilst maintaining the temperature at
25 °C under an atmosphere of N2. After stirring for 2 h, the reaction
mixtures were quenched with CH3OH (2 mL). The suspension was
filtered and the cake washed with THF. The combined organic layer
was concentrated under reduced pressure to give 900 mg of crude
4,40-dimethylbiphenyl. This crude material was purified by chro-
matography on silica gel (hexane: ethyl acetate, 50:1) to give
4,40-dimethyl-biphenyl (844 mg, 96% yield) as a white solid. M.p.:
123–124 °C (Lit.[2a]: 125 °C).
1H NMR (CDCl3/TMS): d 2.35 (s, 6H), 7.12–7.14 (d, J = 7.9 Hz,
4H), 7.36–7.38 (d, J = 7.6 Hz, 4H).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
Manganese complexes A and B should be considered as the
important intermediates because the yield of biaryls was de-
creased seriously (96% to 23%, 96% to 12%) without the catalyst
MnCl2 or the oxidant 1,2-dichloroethane respectively.
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demanding chlorides; generally good to excellent yields. We be-
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septum, heating mantle, magnetic stirring, and nitrogen atmo-
sphere. The mixture was stirred and heated to reflux, where reduc-
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4.1.2. 4-Methylphenylmagnesium chloride
A solution of 4-chloro-toluene (1.27 g, 10 mmol) in THF (5 mL)
was added dropwise to a 50 mL-flask, charged with activated mag-
nesium (292 mg, 12 mmol), THF (15 mL) and nitrogen atmosphere.