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2
Tetrahedron
1. Introduction
The hydration of alkynes is a reaction of prime interest for
the formation of carbonyl derivatives considering the wide
availability of alkynyl substrates and the fundamental importance
of carbonyl compounds in organic synthesis [1-3]. Meanwhile,
the reaction proceeded by the addition of water to the metal–
alkyne complex with 100% atom economy is environmentally
friendly and consistent with the sustainable chemistry [4-6].
In the 19th century, the hydration of alkynes to ketones was
first accomplished through the mediation of Hg compounds as
the catalysts in acidic media [7]. In order to replace the highly
toxic mercury salts, other less harmful transition-metal-complex
catalysts including Rh[8], Pd[9], Ru[10], Cu[11], Pt[12], Co[13]
and Au[14-16] have also been applied in the transformation. All
metals lead to ketones following the Markovnikov rule except Ru
(Scheme 1).
Scheme 2 synthesis of the salen-Au(III) complex
2. Experimental
2.1. General
All reagents were purchased from commercial sources and
used without treatment. The products were purified by column
1
chromatography over silica gel. H NMR spectra were recorded
on
a
Bruker AMX500 (500 MHz) spectrometer and
tetramethylsilane (TMS) was used as a reference. Elemental
analysis was performed on a Vario EL III recorder. IR
spectroscopy was recorded on a Nicolet IS-10 spectrometer. Most
products were known compounds and were identified by
comparison of their physical and spectra data with those of
authentic samples.
Scheme 1 Catalytic hydration of alkynes
In the last two decades, the use of gold complexes as the
catalysts for organic reactions has increased substantially due to
their excellent catalytic activities in both heterogeneous and
homogeneous systems [17, 18]. In homogeneous gold catalysis,
two oxidation states of Au are used as pre-catalysts, Au(I) and
Au(III)[19]. Au(I) is isoelectronic to Hg(II) with lower toxic and
has the similar “alkynephilicity” to the latter[20]. Thus Au(I)
coordinated with the phosphines [21, 22] and nitrogen
heterocyclic carbene ligands [23, 24] has been widely applied in
the hydration of alkynes. For instance, Teles and Tanaka first
reported the [AuMe(PPh3)] as the catalyst for the hydration of
alkynes to ketones in acidic media [14, 21]. Recently, several
water-soluble Au(I) N-heterocyclic carbene complexes were
synthesized and also employed in the hydration of alkynes by
Silbestri [24]. However, only a limited number of examples of
well defined organogold(III) complexes acting as the catalysts for
the hydration of alkynes were reported[25,26]. Au(III) can easily
disproportionate to Au(I) and Au(0) and therefore a four-dentate
chelate ligand is helpful to stabilize the Au(III) center and show a
square planar environment, and this type of Au(III) complex
makes a stable catalyst.[25, 27-29]
Meanwhile, salen-type Schiff bases, which can be prepared
by condensation of aldehydes and amines [30], are able to
stabilize various metals in different oxidation states with four
coordinating sites and control the performance of metals in a
large variety of useful catalytic transformations [31]. Thus, the
coordination of Au(III) and salen ligand is eye-catching
especially for their more accessible synthesis conditions and the
great importance for catalysis, such as the cross-coupling
reaction[32, 33], Mannich reaction[28] and domino reaction[34].
In 2008, Iglesias reported the hydroamination reaction of alkyne
with amine catalyzed by the salen-Au(III) under acidic
condition[35]. However, there was no report about the
nucleophilic addition of water to alkynes catalyzed by salen-
Au(III). Herein, we report the easily available salen–Au(III)
2.2. Catalyst preparation
2.2.1. Preparation of salenH2
o-Phenylenediamine (108 mg, 1 mmol) in 5 mL MeOH was
added to a stirred mixture of salicylaldehyde (244 mg, 2 mmol)
in 10 mL MeOH. The resulting orange mixture was stirred
overnight at room temperature. The solid product was collected
by filtration, washed with cool alcohol and dried in vacuo (256
mg, yield: 81%).
2.2.2. Preparation of salen-Au(III) complex
The homogeneous salen-Au(III) complex was obtained as
follows. An ethanolic solution of HAuCl4·3H2O (1 mmol/15 mL)
was added to a solution of the ligand (316 mg, 1 mmol) in EtOH
(15 mL) at room temperature under inert atmosphere. The
resulting mixture was stirred under reflux for 4 h, then cooled to
room temperature and concentrated under vacuum. The residue
was washed several times with diethyl ether, filtered and dried to
afford the respective complex in good yield (493mg, yield: 88%).
IR: v (cm-1) = 1615, 1555.
2.3. Typical Procedure for the hydration of alkynes
Alkyne (0.5 mmol), catalyst (2.0 mol %), H2O (4.0 equiv, 0.04
mL) and CF3COOH (2.0 mol %) were dissolved in MeOH (0.4
mL) and the homogeneous solution was stirred in a sealed tube at
80°C for 5 h. After the completion of the reaction, the mixture
was cooled to room temperature, and then CH2Cl2 (10 mL) and
H2O (10 mL) were added to it. The organic layer were separated
and washed with brine, dried over Na2SO4, and concentrated
under reduced pressure. The residue was purified over silica gel
by column chromatography (25 % EtOAc in hexane).
3. Results and Discussion
complex
(salen=N,N’-bis(salicylidene)o-phenylenediamine)
(Scheme 2) as catalyst to promote the hydration of alkynes in the
presence of the CF3COOH as cocatalyst.
The salen-Au(III) complex was synthesized according to the
reaction sequence shown in Scheme 2 and applied as the catalyst
for the hydration of phenylacetylene. The reaction conditions
were optimized and the results were summarized in Table 1. Poor
Chen, Tingting
