Full Paper
Received: 15 June 2011
Revised: 2 August 2011
Accepted: 12 August 2011
Published online in Wiley Online Library: 7 September 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1842
Catalytic activity of cycloaurated complexes
in the addition of 2-methylfuran to methyl
vinyl ketone
∗
Kelly J. Kilpin, Bevan P. Jarman, William Henderson and Brian K. Nicholson
Cycloaurated gold(III) complexes derived from cycloaurated iminophosphorane, triphenylphosphine sulfide or selenide, or
pyridylsulfonamide ligands have been assessed for their catalytic activity in the addition of 2-methylfuran to methyl vinyl
ketone. AdditionofAgBF4 wasgenerallyneededforhighactivity, althoughthosewithcoordinatedatomsofhigh transinfluence
+
showed some activity in the absence of added Ag . Copyright ꢀc 2011 John Wiley & Sons, Ltd.
Keywords: gold complexes; cyclometallated ligands; iminophosphoranes; homogeneous catalysis
Introduction
investigated a series of less acidic, air- and moisture-stable C,N-
ꢁ
and N,N -cyclometallated gold(III) complexes as catalysts in the
From slow beginnings, homogeneous catalysis using gold(III)
complexes is an area of rapidly expanding research, with new
reports on the topic appearing on a weekly basis. Only recently,
an entire issue of the Journal of Organometallic Chemistry was
reaction,[
18,19]
and observed that the catalytic activity of neutral
C,N-cycloaurated complexes was greatly enhanced by added
silver salts. It was proposed that the catalytically active species
is a cationic solvated complex, generated by solvent- or silver-
assisted displacement of a chloride ligand. In contrast, the cationic
[
1]
dedicated to homogeneous gold catalysis. Reviews by Hashmi
cover homogeneous gold catalysis of organic reactions up until
ꢁ
N,N -cycloaurated iminophosphoranes showed little difference
late 2006,[ and the use of gold for heterocycle synthesis and
2]
[3]
in catalytic activity when silver cations were present; this was
attributed to partial dissociation of the ligand, effectively giving a
vacant coordination site on the gold atom.[
[
4,5]
earlier reviews on the subject are also available.
The history
of catalysis by gold (pre-1977) has also recently been reviewed
by Bond.[ These reviews indicate that the majority of the work
on homogeneous gold catalysis has been conducted using AuCl3,
which is very hygroscopic and acidic. Therefore the development
of air- and moisture-stable gold(III) complexes that can act as
catalysts in acid-sensitive reactions is desirable, and some recent
19]
6]
Experimental
2-Methylfuran and MVK (Aldrich) were used as received; o-xylene
[
7]
studies have been targeted in this area.
and acetonitrile were distilled before use. References for the
synthesis of the cycloaurated gold complexes are given in the
We have reported the synthesis and characterization of a
series of cycloaurated gold(III) complexes derived from pyridyl
◦
Tables 1–4. Catalytic reactions were carried out at 25 C under a
[
8]
sulfonamides,
phosphine sulfides and triphenylphosphine
nitrogen atmosphere. Duplicate runs showed reproducibility to
selenide,[ and phosphorimines,
9]
[10–12]
and wished to assess these
±
5%.
complexes as acid-free gold(III) catalyst sources. As a suitable reac-
tion we chose the gold(III)-catalysed addition of 2-methylfuran
Catalysis Conditions
(1) to methyl vinyl ketone (MVK) (2) to give 4-(5-methyl-2-
furanyl)butan-2-one (3) (Scheme 1). The reaction occurs under
a number of different conditions, including carrying out the re-
action in the presence of an ethereal solution of BF3 at low
2-Methylfuran (1) (2 mmol, 0.18 ml), MVK (2) (2 mmol, 0.17 ml),
the gold catalyst (0.02 mmol, 1% catalyst loading) and AgBF (if
4
needed) were stirred in acetonitrile (5 ml) for 18 h. The acetonitrile
temperatures[ or by using high pressures;
13]
[14]
Cu(O3SCF3)2 has
and any unreacted reactants were removed under vacuum. The
product was extracted into diethyl ether–hexane (3 : 1), made
up to 20 ml in a volumetric flask containing o-xylene (1 mmol,
0.12 ml), and passed through a silica column. A 0.050 ml aliquot of
wt hi et hs oh leu xt ai on ne sw. as transferred to a GC vial and made up to ∼1.5 ml
[
15]
also been shown to catalyse the reaction.
A mechanism for the catalytic cycle has been proposed on
the basis of NMR experiments, using AuCl3 as the catalyst;[16]
the first step in the reaction was proposed to be the auration
of the furan at the 2-position. AuCl3 catalysed the reaction very
efficiently with 80–90% yields of 3 after 40 min with 1 mol%
catalyst. The Au(I) complexes Et3PAuCl and (THT)AuCl (where
THT is tetrahydrothiophene) also catalyse the reaction, albeit less
∗
Correspondence to: Brian K. Nicholson, Department of Chemistry, University of
effectively and only in the presence of AgBF4 (80–90%, 1 mol%
Waikato, Private Bag 3105, Hamilton, New Zealand.
E-mail: b.nicholson@waikato.ac.nz
catalyst, 1 day).[
17]
It is unclear how the AgBF4 enhanced the
catalytic activity in this case; however, oxidation of Au(I) to Au(III)
by the silver cannot be excluded. Aguilar et al. subsequently
Department of Chemistry, University of Waikato, Private Bag 3105, Hamilton,
New Zealand
Appl. Organometal. Chem. 2011, 25, 810–814
Copyright ꢀc 2011 John Wiley & Sons, Ltd.