Gold(i) "click" 1,2,3-triazolylidenes: Synthesis, self-assembly and catalysis

Article abstract of DOI:10.1039/c0cc02185g

Novel gold(i) "click" carbene(1,2,3-triazolylidene) complexes have been synthesised, characterised and exploited for the self-assembly of a metallomacrocycle and as precatalysts for gold(i)-catalysed reactions.

Full text of DOI:10.1039/c0cc02185g

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Gold(I) ‘‘click’’ 1,2,3-triazolylidenes: synthesis, self-assembly and
catalysiswz
Kelly J. Kilpin,a Ursula S. D. Paul,b Ai-Lan Lee*b and James D. Crowley*a
Received 29th June 2010, Accepted 4th August 2010
DOI: 10.1039/c0cc02185g
Novel gold(I) ‘‘click’’ carbene(1,2,3-triazolylidene) complexes
have been synthesised, characterised and exploited for the
self-assembly of a metallomacrocycle and as precatalysts for
gold(I)-catalysed reactions.
1-Benzyl-4-phenyl-1H-1,2,3-triazole, 1,6 formed readily
through one-pot ‘‘click’’ reaction, was quantitatively
a
methylated in dichloromethane using Meerwein’s salt,8
providing the triazolium saꢀlt 2 (X = BF4ꢀ) (Scheme 1).
Conversion of 2 (X = BF4 ) into 20 (X = PF6ꢀ) allowed
the molecular structure to be determined unambiguously by
X-ray crystallography (see ESIz) and showed that the triazole
was methylated regioselectively at the N3 nitrogen atom. 2 or
20 was readily metalated with Ag2O providing the silver(I)
carbene.5 The Ag(I) complex was immediately treated with
Au(SMe2)Cl and the resulting transmetallation provided the
neutral 1,2,3-triazolylidene gold(I) chloride complex, 3, in 82%
yield (Scheme 1). The chloride anion of 3 can be readily
replaced by other ligands (Scheme 1). The neutral phenyl-
acetylide complex 4 was obtained by reacting 3 with the
potassium salt of phenylacetylide in methanol. Likewise the
cationic complex, 5, was synthesised by treating 3 with
AgSbF6 and N,N0-dimethylaminopyridine (DMAP) in an
acetone solution at RT. The gold(I)–1,2,3-triazolylidene
complexes, 3, 4 and 5, have been fully characterised by
elemental analysis, HR-ESMS, IR, 1H and 13C NMR (see
ESIz). X-Ray structural analyses of 3, 4 and 5 unambiguously
confirmed the connectivity patterns (Scheme 1 and ESIz). As
In the past 20 years N-heterocyclic carbenes (NHCs) have
evolved from chemical curiosities to standard organometallic
ligands. The strong covalent metal–carbene bond within these
complexes makes them very stable and this has led to applications
in a number of areas. Indeed, late metal–carbene complexes,
including those of gold(I), have been exploited to generate
new catalysts, medicines and materials.1 Arduengo-type
imidazolylidene ligands still dominate the area, because this
class of carbenes are relatively easily synthesised and handled.2
However, despite the availability of excellent synthetic methods
for the generation of imidazolylidenes the majority of these
ligands are symmetrically aryl- or alkyl-substituted.2 As such,
new modular synthetic methods that allow ready incorporation
of functionality and the controlled modification of steric and
electronic properties of NHCs could enable the rapid generation
of new catalysts, medicines and materials. One potential
method is the Cu(I)-catalyzed 1,3-cycloaddition of organic
azides with terminal alkynes (the CuAAC reaction).3 The mild
reaction conditions and wide substrate scope of this ‘‘click’’4
reaction has led to its extensive use in the synthesis of functional
molecules.3 Albrecht and co-workers have recently shown that
1,2,3-triazolylidenes, synthesised using the CuAAC reaction,
form metal–carbene complexes with Pd(II), Ag(I), Ru(II), Rh(I)
and Ir(I).5 Inspired by Albrecht’s discovery and building on
our own interest in the generation of functionalised ligand
architectures using CuAAC ‘‘click’’ chemistry,6 we report the
synthesis of novel gold(I) ‘‘click’’ carbene(1,2,3-triazolylidene)
complexes.7 Furthermore, we demonstrate for the first time
that these gold(I)–1,2,3-triazolylidene complexes can be
exploited to self-assemble a metallomacrocycle and are active
catalytic precursors for a variety of gold(I)-catalysed organic
transformations.
a Department of Chemistry, University of Otago, PO Box 56,
Dunedin, New Zealand. E-mail: jcrowley@chemistry.otago.ac.nz;
Fax: +64 3 479 7906; Tel: +64 3 479 7731
b School of Engineering and Physical Sciences, Chemistry – William
H. Perkin Building, Heriot-Watt University, Edinburgh EH14 4AS,
UK. E-mail: A.Lee@hw.ac.uk; Tel: +44 (0)131-4518030
w This article is part of the ‘Emerging Investigators’ themed issue for
ChemComm.
Scheme 1 (i) Me3OBF4, CH2Cl2, RT, 3 d; (ii) (a) Ag2O, NMe4Cl,
CH3CN–CH2Cl2 (1 : 1), RT, 18 h, (b) Au(SMe2)Cl, CH2Cl2, RT, 2 h;
(iii) KOtBu, phenylacetylene, MeOH, RT, 18 h; (iv) DMAP, AgSbF6,
acetone, RT, 1 h. ORTEP diagrams of 3 (centre), 4 (left) and 5 (right)
with thermal ellipsoids shown at the 50% probability level. Counterions
are omitted for clarity. Selected bond lengths (A) and angles (1): for 3,
Au1–C8 1.982(4), Au1–Cl1 2.2940(10), Cl1–Au1–C8 177.2(1); for 4,
Au1–C8 2.030(6), Au1–C17 2.024(6), C17–Au1–C8 177.8(5);
for 5, Au1–C8 1.962(8), Au1–N4 2.055(6), N5–Au1–C8 173.6(2).
z Electronic supplementary information (ESI) available: Experimental
procedures, 1H and 13C NMR and ESMS spectra, 1H NMR stacked
plots, and crystallographic data. CCDC reference numbers
782313–782316. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0cc02185g
c
328 Chem. Commun., 2011, 47, 328–330
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Table 1 The reaction of 10 (R1 = H or Ph) with alcohols, phenol,
aniline and benzaldehyde catalysed by complex 3/AgSbF6
Entry R1 R2Y
Product Yielda (%)
1b
2b
3b
4c
5b
6d
7e
8f
H
H
H
H
H
H
MeOH
EtOH
i-PrOH
t-BuOH
PhOH
11a
11b
11c
11d
11e
11f
11g
88
>95
82
81
50
62
Scheme 2 (i) Me3OBF4, CH2Cl2, RT, 3 d; (ii) (a) Ag2O, NMe4Cl,
CH3CN–CH2Cl2 (1 : 1), RT, 18 h, (b) Au(SMe2)Cl, CH2Cl2, RT, 2 h;
(iii) 7, Ag2O, NMe4Cl, CH3CN–CH2Cl2 (1 : 1), RT, 2 h.
expected the gold(I) ions are coordinated in a linear fashion
(L–Au–C bond angles range from 173.61–177.21) to the carbon
of the 1,2,3-triazolylidene and the other ligand. The Au–C and
Au–L bond lengths are similar to those observed in other
gold(I) N-heterocyclic carbene complexes (Scheme 1 and
ESIz).9,10
PhNH2
Ph EtOH
H
50
40% + 22% 11i + 27% 11jg
Ph(CQO)H 11h
a
Determined by 1H-NMR analysis using trimethoxybenzene as the
internal standard. No diethyl fumarate or maleate was detected in any
b
c
experiment. 20 1C, 3.5 h. Reaction was allowed to stir for 18 h.
d
e
f
g
40 1C, 70 h. 20 1C, 6 d. 20 1C, 1 h. 11i = ethyl 3-oxo-2-
phenylpropanoate, 11j = (Z)-ethyl 3-hydroxy-2-phenylacrylate (see
The chemical shift of the carbene carbon of 3, 4 and 5
appears between dC 174–155 ppm in the 13C-NMR spectra,
values which are similar to those observed for other reported
1,2,3-triazolylidene metal–carbenes,5 but significantly lower in
value than those observed for previously reported gold(I)–
carbene complexes.9
ESIz).
reactivity and selectivity of the gold(I)-catalysts.13 We were
thus keen to investigate whether gold(I)–1,2,3-triazolylidene
complexes such as 3 are viable catalytic precursors for gold(I)-
catalysed organic transformations.
Recently the use of NHCs as building blocks in the assembly
of metallosupramolecular architectures has been explored.11
As such, we next attempted to use the di-1,2,3-triazolylidene
gold(I) chloride complex, 8, to self-assemble a metallo-
macrocycle. 8 was synthesised from the known ditriazole, 66
(Scheme 2) using the same methodologies that provided the
parent compound 3. Treatment of 8 with 1 eq. of the in situ
generated (from 7) disilver(I) triazolylidene complex provided
the digold(I) metallomacrocycle 9 (see ESIz for full character-
isation). 1H and diffusion-ordered NMR spectroscopy
(DOSY) along with positive ion electrospray mass spectro-
metry (ESMS) experiments provide conclusive evidence for the
formation of metallomacrocycle, 9. The ESMS spectrum of 9
shows a series of isotopically resolved peaks consistent with
the formulation [M–n(BF4)]n+ (n = 1–2). The 1H NMR
spectrum (CD3CN, 298 K) of 9 shows a complicated pattern
of signals, however, the protons Ha–c of the phenyl spacer
units of the ligands are upfield shifted, characteristic of a face
to face p–p stacking interaction. 1H DOSY12 spectra provided
further support for the selective formation of the metallo-
macrocycle in CD3CN solution. All proton signals of 9
We were delighted to find that the air and moisture stable
complex 3 in the presence of AgSbF6 successfully catalyses the
carbene-transfer reaction from ethyl diazoacetate (EDA) 10
(R1 = H) (Table 1).15 The insertion of the :CHCOOEt unit
into O–H bonds of primary, secondary and tertiary alcohols
occurs readily (entries 1–4) and the insertion into the O–H
bond of phenol also proceeds albeit in a moderate yield (50%,
entry 5). Insertion into the N–H bond of aniline is also
successfully catalysed by complex 3/AgSbF6 (entry 6). As
reports on gold(I)-catalysed carbene-transfer reactions with
alcohols and anilines tend to focus only on EDA 10 (R1 =
H),15,16 we were keen to investigate whether more substituted
diazo compounds such as 10 (R1 = Ph) are also viable
substrates. In the presence of 3/AgSbF6, methyl 2-diazo-2-
phenylacetate 10 (R1 = Ph), does indeed react with ethanol to
give 11g, but more sluggishly when compared to EDA (entry
7). Finally, formal insertion of the carbene moiety into the
aldehyde C–H and C–C of benzaldehyde is also catalysed by
3/AgSbF6 (entry 8).17
showed the same diffusion coefficient of 4.8 ꢁ 10ꢀ10 m2 sꢀ1
,
Additionally, complex 3/AgSbF6 successfully catalyses the
hydroalkoxylation of allene 12 to furnish 13 in 80% isolated
yield (Scheme 3).18 Cycloisomerisation of enyne 14 also
proceeds smoothly in the presence of 3/AgSbF6 to produce
isomers 15 and 16 in 42% yield (Scheme 4). In the presence of
MeOH as the co-solvent, methoxycyclisation occurs to give an
excellent yield of methoxylated product 17 (98%, Scheme 4).
whereas a diffusion coefficient of 6.1 ꢁ 10ꢀ10 m2 sꢀ1 was
obtained for 8 alone in CD3CN solution, which is consistent
with the formation of the larger metallomacrocycle in solution
(ESIz).
Another obvious potential application of the gold(I) ‘‘click’’
carbene complexes such as 3 is in catalysis. There has recently
been much interest in gold(I)-catalysed synthetic organic
chemistry as gold(I)-complexes can act as efficient carbophilic
Lewis acids to activate p-systems such as alkynes, allenes,
dienes and alkenes.13 A combination of phosphine or NHC
ligated AuCl complexes in the presence of a silver salt (such as
AgOTf and AgSbF6) has been widely employed as highly
active cationic Au(I) species.1,13,14 It has also been shown
that the nature of the ancillary ligand can greatly affect the
Scheme 3 Hydroalkoxylation of allene 12, catalysed by complex 3/
AgSbF6.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 328–330 329

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B. Donnadieu and G. Bertrand, Angew. Chem., Int. Ed., 2010,
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Scheme 4 Cycloisomerisation and methoxycyclisation of enyne 15,
catalysed by complex 3/AgSbF6.
Following our successful preliminary results with the
unmodified parent gold(I)–1,2,3-triazolylidene complex 3, we
envisage that the easy access to analogues of 3 via the CuAAC
‘‘click’’ reaction should now open the door to ready and rapid
tuning of the steric and electronic properties of gold(I)-pre-
catalysts via modification of the substituents. Since the nature
of the ancillary ligand can greatly affect the reactivity and
selectivity of the gold(I)-catalysts, analogues of complex 3 with
different electronic and steric properties could be beneficial to
the development and optimisation of gold(I)-catalysed reactions.
Work is currently underway in this area and will be reported in
due course.
In conclusion, we have reported the synthesis and character-
isation of novel gold(I) ‘‘click’’ carbene complexes. These
complexes have been used for the self-assembly of a metallo-
macrocycle and as precatalysts for gold(I)-catalysed reactions.
Exploitation of the CuAAC methodology used to generate the
carbene ligands should provide the ability to readily and
rapidly tune the steric, electronic and functional properties
of the resulting gold(I)–1,2,3-triazolylidene complexes potentially
enabling the generation of new catalysts, medicines and
materials.
We would like to thank Erasmus (USDP), EPSRC, The
Royal Society, Heriot-Watt University and the University of
Otago for providing financial support for this work. Bevan
Jarman (University of Waikato) and the EPSRC Mass
Spectrometry services at Swansea are thanked for collecting
MS data.
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Notes and references
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330 Chem. Commun., 2011, 47, 328–330
This journal is The Royal Society of Chemistry 2011