C O M M U N I C A T I O N S
Since 6a should be formed from 3a, the role of the pyridine base
is to inhibit desilylation of 3a during the reaction. The catalytic
reaction does not proceed at temperatures lower than 120 °C,
although the stoichiometric reaction takes place at 80 °C in 98%
yield. The higher temperature is therefore required for the regenera-
tion of 4a and not for the C-C bond formation or elimination of
GaCl3 from 5a. The yield of 3a decreases to 49% at 135 °C, and
an adduct of 3a and 1a, 2-(1-(2-methylphenoxy)ethenyl)-6-methyl-
phenol triethylsilyl ether, was obtained in 11% yield.
Several other ethynylating reagents were also investigated. The
reaction of iodotrimethylsilylethyne under the catalytic conditions
gave less than 10% yield of 2-(trimethylsilylethynyl)-6-methyl-
phenol (7a), while the yield increased to 71% using stoichiometric
amounts of GaCl3, butyllithium, and 2,6-di(tert-butyl)-4-methyl-
pyridine. Bromotrimethylsilylethyne behaves similarly. Phenyl-
(trimethylsilylethynyl)iodonium p-toluenesulfonate does not react.
The use of trimethylsilylated chloroethyne in place of 2 decreases
the yield of 7a to 82% with the concomitant formation of 6a in
10% yield. A reagent system of SnCl4 (100 mol %),10 butyllithium
(100 mol %), and 2,6-di(tert-butyl)-4-methylpyridine (100 mol %)
also promotes the ethynylation giving 3a in 91% yield, but not the
catalytic reaction. No reaction takes place with GaBr3, GaI3, AlCl3,
or InCl3 under the catalytic conditions.
The GaCl3-catalyzed ethynylation can be applied to various
substituted phenols 1 giving the products 3 in high yields (Scheme
2), and the turnover numbers based on GaCl3 are between 8 and
10. The optimum reaction temperature differs, depending on the
substrate; ortho-substituted phenols 1a-c react effectively at 120
°C in chlorobenzene; phenol (1d), para-substituted and meta-
substituted phenols 1e-n except for p-methoxyphenol (1f) require
a higher temperature of 160 °C in o-dichlorobenzene. Reactions of
meta-substituted phenols with tert-butyl-, iodo-, and chloro- groups
take place at the less hindered site exclusively to predominantly. It
is interesting that m-cresol (1m) reacts selectively at the more
hindered site, giving 3′m.
The mechanism of this catalytic ethynylation most likely involves
carbogallation of 2 and 4 as initially designed (Scheme 1), although
protonated 5 is not detected in the reaction mixture. â-Elimination
of 5 appears to be more rapid than the carbometalation of 2 and 4.
The lithium salt may be involved in the formation of 4 from GaCl3
and 1, liberating hydrogen chloride.
of organogallium species to chloroethyne and catalytic phenol
activation with GaCl3 are successfully employed in the catalytic
conversion of the organic C-H group to the C-CtC-H group.
Acknowledgment. This work was supported by grants from
JSPS. A fellowship to K.K. from JSPS for young Japanese scientists
is also gratefully acknowledged.
Supporting Information Available: Spectroscopic details of new
compounds (PDF). This material is available free of charge via the
References
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(9) Under an argon atmosphere, to a solution of o-cresol (1a) (1.08 g,
10.0 mmol) in chlorobenzene (50 mL) were added 1.6 M butyllithium
in hexane (1.90 mL, 3.0 mmol) and 1 M GaCl3 in methylcyclohexane
(1.0 mL, 1.0 mmol) at 0 °C successively. The mixture was stirred for
10 min at room temperature, and then 2,6-di(tert-butyl)-4-methylpyridine
(205 mg, 1.0 mmol) and chlorotriethylsilylethyne (2) (1.74 g, 10.0 mmol)
were added. The mixture was heated at 120 °C for 3 h. Water (25 mL)
and THF (25 mL) were added, and the organic materials were extracted
with ethyl acetate, washed with water and brine, dried over MgSO4,
filtered, and concentrated in vacuo. 6-Methyl-2-(triethylsilylethynyl)phenol
(3a) (2.22 g, 90%) was obtained by flash chromatography (hexane:ethyl
acetate ) 50:1) over silica gel.
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In summary, phenols are ethynylated at the ortho position with
chlorotriethylsilylethyne in the presence of a novel catalyst system,
GaCl3 (10 mol %), 2,6-di(tert-butyl)-4-methylpyridine (10 mol %),
and butyllithium (30 mol %). The addition-elimination process
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