Studies on gold-nitrone systems

Article abstract of DOI:10.1039/c8dt04254c

A series of novel Au(i)-nitrone complexes with specific catalytic properties were prepared. Furthermore, Au(i)-and Au(iii)-oxadiazole complexes were formed by a novel Au-generated nitrile-nitrone [3 + 2] cycloaddition, and the crystal structures of Au(i)-nitrones as well as the Au(i)-and Au(iii)-oxadiazole complexes were studied (X-ray). A useful one-pot Au(iii)-mediated cycloaddition method was developed for the formation of a number of dihydro-1,2,4-oxadiazoles, involving in situ formation of Au(iii)-oxadiazole complexes. The observed Au(i) and Au(iii) dual selective reactivity gives new understanding about the Au(i)-and Au(iii)-nitrone chemistry.

Full text of DOI:10.1039/c8dt04254c

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Studies on gold–nitrone systems†
Helgi Freyr Jónsson and Anne Fiksdahl
*
Cite this: Dalton Trans., 2019, 48, 142
A series of novel Au(I)–nitrone complexes with specific catalytic properties were prepared. Furthermore,
Au(I)– and Au(III)–oxadiazole complexes were formed by a novel Au-generated nitrile–nitrone [3 + 2]
cycloaddition, and the crystal structures of Au(I)–nitrones as well as the Au(I)– and Au(III)–oxadiazole com-
plexes were studied (X-ray). A useful one-pot Au(III)-mediated cycloaddition method was developed for
the formation of a number of dihydro-1,2,4-oxadiazoles, involving in situ formation of Au(III)–oxadiazole
complexes. The observed Au(I) and Au(III) dual selective reactivity gives new understanding about the
Au(I)– and Au(III)–nitrone chemistry.
Received 24th October 2018,
Accepted 19th November 2018
DOI: 10.1039/c8dt04254c
rsc.li/dalton
Introduction
Results and discussion
The chemistry of gold–nitrone systems is little explored.1a,b
Realizing the promising potential of this field of gold chemistry,
we have studied the complexation ability and synthetic utility of
gold–nitrone based systems.2,3 Terminal propargyl acetals and
nitrones afforded 1,2-oxazine derivatives in the presence of a gold
(I) catalyst (Scheme 1a)2 by an uncommon [3 + 3] cycloaddition
pathway. Moreover, we discovered that phosphane–gold–nitrone
complexes were catalytically active in a unique regio-/chemo-
selective [2 + 2 + 2] cyclotrimerization of 1,3-diarylpropargyl
acetals (1) to afford cyclohexylidene products (3, Scheme 1b).3 The
presence of a catalytic amount of Au–nitrone, prepared in situ,
was essential for successful selective cyclotrimerization, as a Au(I)
catalyst alone (I or II, Scheme 1d) was unable to afford the target
product, and complex product mixtures were obtained.
Au(I)–nitrone complexes
Several additional crystalline Au(I)–nitrone complexes were
prepared in moderate to high yields (38–95%) from Au(I)–
phosphane or Au(I)–NHC (pre)catalysts I–III and nitrones 2a–c
Scheme 2.
X-ray analysis of crystalline JohnPhos Au(I)–nitrone complex
I-2a, formed from Au(I)-JohnPhos I and nitrone 2a (equimolar
mixtures in DCM, Scheme 1c), confirmed for the first time the
O-binding of the nitrone ligand to the Au(I) centre with a
linear nitrone–O–Au(I)–P coordination mode. The crystalline
Au(I)-JohnPhos–nitrone complex I-2a showed similar catalytic
activity (60–74% yield of 3) as the catalysts prepared in situ
from Au(I)–phosphane I and nitrone 2a. Au–NHC catalysts III
or AuCl3 IV (with or without nitrones) gave full conversion into
unidentified product mixtures but failed to give trimerization.
We hereby report further investigations of the chemistry of
Au/nitrone based systems, which allow formation of new
Au complexes with specific properties.
Department of Chemistry, Norwegian University of Science and Technology,
NO-7491 Trondheim, Norway. E-mail: anne.fiksdahl@chem.ntnu.no
†Electronic supplementary information (ESI) available: Copies of 1H and 13C
NMR spectra. CCDC 1531556, 1548784, 1548787–1548791, 1548801 and
1579660. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c8dt04254c
Scheme 1 (a) Au(I) catalysed [3 + 3] cycloaddition2 of terminal propar-
gyl acetals and nitrones; (b) Au(I)–nitrone catalysed [2 + 2 + 2] cyclotri-
merization3 of 1,3-diarylpropargyl acetals; (c) formation of the Au(I)–
nitrone complex I-2a (X-ray); (d) nitrones 2a–c and Au complexes I–IV
included in the present study.
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When tested in the original cyclotrimerization reaction
(Scheme 1a), the catalytic activity of all the phosphane-based
crystalline complexes (I-2a, I-2a′, I-2a″, I-2b, I-2c, II-2a;
Scheme 2a and b) was similar to catalysts prepared in situ
(Scheme 1b).3 The crystalline Au(I)NHC-ligated nitrone
complex (III-2b) afforded a complex product mixture and no
trimerization, similar to the results by the in situ preparation
method.3 Nitrones did not coordinate to Au(III), as attempts to
coordinate the Au(III) complexes AuCl3 and PicAuCl2 (dichloro-
2-pyridinecarboxylato-gold) with nitrone 2a failed (NMR) and
only the parent compounds were recovered (Scheme 2d).
Au(I)–oxadiazoles
During the studies of the JohnPhos Au(I)–nitrone I-2a complex
(Schemes 2a and 3a), an additional crystalline Au(I) hetero-
cyclic complex, the Au(I)-dihydro-1,2,4-oxadiazole, I-4a, was
identified (Scheme 3b and 4a, X-ray). The product is apparently
formed by a transformation of JohnPhos-Au(I)–nitrile complex
I in a [3 + 2] cycloaddition of the Au-ligated acetonitrile ligand
with the present nitrone 2a. This is indicated by the oxadiazole
product coordination to Au(I) through the N(4)imine atom in
the Au(I) complex. The reaction of the acetonitrile (ACN) ligand
was slow in dichloromethane (9%, 24 h), but the product was
selectively formed (87%, 18 h) in refluxing acetonitrile. In the
absence of Au, the nitrone 2a in refluxing acetonitrile failed to
give the heterocyclic oxadiazole by cycloaddition in a possible
thermal reaction (48 h). Hence, the reaction is mediated by
gold.
1
In the H NMR spectrum of gold(I)–nitrone complexes I-2a,
a′,a″, the two tBu(JohnPhos) groups appear as one doublet
(1.36 ppm J = 16.0 Hz), due to 1H–31P coupling. In complex
Au(I)-I-4a, the two tBu groups are diastereotopic, shown as two
doublets, separated by approx. 0.25 ppm (1.07/1.31, J =
16.2 Hz), due to the oxadiazoline chiral centre.
The formation of both Au(I)-complex I-2a and Au(I) I-4a
indicates that the reaction between JohnPhosAu(I)–ACN I and
nitrone 2a has two possible competing outcomes (Scheme 3a
and b). Either nitrone 2a replaces the acetonitrile ligand to
give complex I-2a or, alternatively, nitrone 2a reacts with the
ligated acetonitrile to form Au(I)–oxadiazole complex I-4a.
According to NMR spectroscopy, the nitrile/nitrone ligand
exchange was both rapid and quantitative in CDCl3, while no
Au(I)-complex I-4a was seen after several hours. This indicates
a slow reaction and that a possible equilibrium between co-
ordinated and non-coordinated nitrone (2a/I-2a, Scheme 3) is
Scheme 2 (a)–(d) Preparation of crystalline Au(I)–nitrone complexes;
(e) crystal structures of Au–nitrone complexes I-2a, I-2a’, I-2a’’, I-2b,
I-2c, II-2a and III-2b.
Scheme 3 Competing dual reaction pathways of nitrone 2a with Au(I)–
ACN complex I.
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Since the Au(III) mediated reaction was faster than that with
Au(I), and the nitrile was not used as a solvent and would
enable the preparation of modified products 4 from different
nitriles, this procedure represented a convenient and versatile
method for nitrile–nitrone [2 + 3] cycloaddition.
One-pot preparation of dihydro-oxadiazoles
The initially formed Au(III)-4a/4d complexes were only stable in
EtOAc solutions for few hours, and attempts to isolate more
those minor samples for X-ray were not successful. Aiming at
the direct one-pot preparation of the target cycloaddition pro-
ducts 4 via Au(III)-4 complexes, a number of methods were
tested for cleavage of the Au(III)-4 intermediates. Treatment
with pyridine4e gave incomplete cleavage and the oxadiazole
products 4 partly decomposed. However, a method based on
treatment of the EtOAc reaction mixture with a combination of
Et3N and Al2O3 was successful. The complex was initially
cleaved by the addition of minor amounts of Et3N and sub-
sequent alumina adsorption decomposed the released AuCl3
and eliminated the problem of oxadiazole reattachment to
gold(III) during purification. The released stable dihydro-1,2,4-
oxadiazoles
4
were readily purified by silica flash
chromatography.
A number of oxadiazole products 4a,c–f (33–74%) were pre-
pared from nitrones 2a–a″ and Me/Ph-nitriles by this one-pot
procedure via the corresponding Au(III)4 complexes
(Scheme 4c). Electronic factors both from the nitrile and
nitrone affected the outcome of the reactions. Much higher
yields of the target products were formed from benzonitrile
(64–34–74%; 4d,e,f), than from acetonitrile (33–0–36%, 4a,b,c),
in accordance with other studies.5a,b This observation may be
explained by the fact that aromatic conjugation would increase
the rate of reaction by stabilization of the transition
state. Replacing the N-Me-nitrone 2a with the corresponding
N-Ph-nitrone failed to give any cycloaddition, indicating that
steric hindrance in the transition state dramatically lowers the
reaction rate.
Scheme 4 Formation of dihydro-1,2,4-oxadiazoles 4 by Au-mediated
[2 + 3] cycloaddition of nitrones and nitriles; (a) Au(I)-I-4a complex; (b)
Au(III)-4a/4d complexes; (c) one-pot process for the preparation and
release of products 4; (d) crystal structures of I-4a and Au(III)-4a
complexes.
strongly in favour of gold(I)–nitrone complex I-2a. A large
excess of nitrile, using acetonitrile as a solvent, indeed pushed
the equilibrium toward Au(I)–ligated nitrile and speeded up
the conversion into the desired cycloaddition product I-4a.
Au-Catalysed nitrile–nitrone [3 + 2] cycloaddition
In general, [3 + 2] cycloaddition reactions are concerted, peri-
cyclic processes controlled by the frontier molecular orbitals
on e.g. a nitrone 1,3-dipole and a dipolarophile.5 Nitrone–
Au(III)–oxadiazoles
As Au(III) did not coordinate to nitrones (Scheme 2d), the poss- nitrile cycloadditions are normal type II cycloadditions and
ible formation of a Au(III)-2a complex could be excluded. As seem to be controlled by HOMOnitrone–LUMOnitrile interactions,
desired, a rapid conversion of equimolar amounts of nitrone as the large HOMO coefficient of the nitrone–oxygen center is
2a and AuCl3 in CD3CN into Au(III)-d3-oxadiazole (Au(III)-d3-4a) expected to favor oxadiazole formation by HOMO1,3-dipole
–
was observed (NMR; >75%, 30 min, r.t.). The formation of LUMOdipolarophile interaction in [3
+
2] cycloadditions.5c
Au(III)-4a/-4d complexes from AuCl3, nitrone 2a and However, a crossover in the orbital control is observed. Thus,
Me-/Ph-nitriles was tested in a number of alternative reactivity of both possible interactions, HOMO1,3-dipole–LUMOdipolarophile
solvents. Most solvents were unsuccessful (CHCl3, DCM, Et2O, and HOMOdipolarophile–LUMO1,3-dipole, are reported to lead to
THF, acetone, MeOH) as either AuCl3 did not dissolve, the the actual target oxadiazole products by overlap of orbitals
formed Au(III)-complex or AuCl3 decomposed during reaction, or with comparable terminal coefficients, e.g. orbitals of nitrone-
a very slow reaction took place. However, the transformation of C and -O atoms with, respectively, nitrile-N and -C atoms for
AuCl3 and nitrone 2a with the respective Me/Ph-nitriles in EtOAc the preparation of oxadiazoles 4.5a
readily gave >85% conversion (NMR) into the Au(III)-4a/-4d com-
plexes in 2 hours at r.t. (Scheme 4b, Au(III)-4a X-ray).
It is also known that 1,3-dipolar cycloaddition reactions of
nitrones with both e-deficient and e-rich alkenes allow for
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Table 2 Bond lengths (Å) and angles in Au(I) and Au(III) oxadiazole 4a
complexes (Scheme 4d)
efficient syntheses of isoxazolidines by Cu-catalysis and have
shown that the frontier orbitals may be affected by coordi-
nation to a Lewis acid.5b,d,e Similarly, the scope of nitrileoxide–
nitrile cycloadditions to give the corresponding aromatic 1,2,4-
oxadiazole derivatives5f is limited to benzonitrile oxides and
e-deficient nitriles, unless in the presence of Lewis acids, e.g.
for relatively unreactive alkylnitriles.5b Modified carbamoyl-
nitrile oxides cause, however, inverse electron-demand 1,3-
dipolar cycloaddition LUMOnitrile oxide–HOMOnitrile with elec-
tron-rich aliphatic or aromatic nitriles to afford 3-functiona-
lized 1,2,4-oxadiazoles.5h
Complex
N–Au
Au–X
N–O
N/Cl–Au–X
Au(I)-I-4a
Au(III)-4a
2.066
2.0289
Au–P 2.250
1.487
1.485
N–Au–P 172.16°
Au–Cl1 2.2751
Au–Cl2 2.2843
Au–Cl3 2.2573
N–Au–Cl3 178.65°
Cl1–Au–Cl2 178.56°
allowing the nitrone to bind closer to the gold(I) centre
attached to the less bulky Ph3P ligand.
In our study, the electron-deficient p-CF3-substituted aro-
matic nitrone (2a″) indicates that decreased HOMO/LUMO
levels of the nitrone affords high reactivity, as shown by high
product yields (e.g. 74% of 4f from 2a″). The nitrone with a
non-substituted phenyl group (2a) showed similar reactivity
(e.g. 64% of 4d). The fact that ERG (p-OMe-phenyl) deactivated
for cycloaddition supports this theory, as nitrone 2a′ with an
electron rich phenyl group afforded no or low product yields
(0–34% of 4b and 4e from 2a′). Thus, the outcome of the pre-
sently reported nitrone/Au–nitrile [2 + 3] cycloadditions seems
to be controlled by the reverse LUMOnitrone–HOMOnitrile inter-
action, even if a reduced nucleophilicity of the nitrile–nitrogen
is expected, being coordinated to the gold centre.
The deviation from linearity increases going from Au(I)–
nitrone complexes I-2a (O–Au–P bond angles 174.8–176°,
Table 1) to the more rigid highly substituted oxadiazole cyclic
structure Au(I)-I-4a (N–Au–P bond angle 172.16°, Table 2). The
N–O bond lengths in the nitrone and oxadiazole gold com-
plexes indicate a N–O double bond character in all the nitrone
complexes (1.326–1.349 Å; Table 1, entries 1–7) in contrast to a
weaker single bond in the oxadiazole complexes Au(I)- and
Au(III)-4a (approx. 1.48 Å, Table 2).
As expected, the Au(III)-4a complex was shown to have a
nearly square planar structure. Interestingly, in the Au(III)-4a
complex (Table 2), the oxadiazole Au–Cltrans bond (Au–Cl3
2.2573 Å) is shorter than the Au–Clcis. bonds (Au–Cl1/–Cl2:
2.2751 and 2.2843 Å). This has been observed in other AuCl3
complexes with nitrogen ligands, e.g. with ammonia,4a
pyridine4b and an aromatic nitrile.4c This suggests that the
oxadiazole coordination strengthens the Au–Cltrans bond, in
contrast to that commonly observed in Pt(II) complexes.4d
Au(I) and Au(III) complexes; crystal structures
The crystal structures of the Au(I)–nitrone complexes (Table 1,
X-ray) showed the expected approximate linear nitrone–O–
Au(I)–P-coordination mode for Au(I)–phosphanes (I-2a, I-2a′,
I-2a″, I-2b, I-2c, II-2a; Scheme 2a and b) and, analogously,
O–Au–C(carbene) nearly linear complexation for the Au(I)–
NHC complex III-2b (Scheme 2c). The largest deviation from
linearity was observed for the most bulky non-aromatic
nitrones I-2a″, as shown by O–Au–P bond angles < 176°
(Table 1, entries 1–3). Both the nature of the Au(I)-ligand and
the coordinated nitrone affected the O–Au(I) bond lengths. The
pyridine-N-oxide O–Au bond length of complexes I-2b is 0.03 Å
longer than that in complex III-2b (Table 1, JohnPhos and
NHC ligands, respectively; entries 4 and 7), indicating that the
nitrone is stronger bonded to gold(I)–NHC than to gold(I)–
phosphane. Similarly, the shorter O–Au(I) bond length in
complex II-2a than in I-2a (Ph3P and JohnPhos ligands,
respectively; entries 6 and 1) may be caused by steric effects,
Dihydro-1,2,4-oxadiazoles
Transition metal Pd(II)6a–d and Pt(II)/(IV)7a–f nitrile complexes
are known to undergo similar [2 + 3] cycloaddition reactions
with nitrones to give dihydro-1,2,4-oxadiazole-Pd/-Pt com-
plexes. The Pd and Pt(II)/(IV)-oxadiazole methods require a sep-
arate step for cleavage of the isolated metal intermediate com-
plexes to release the heterocyclic products. The obtained yields
of released oxadiazoles by the Pt complex method are compar-
able to the obtained isolated yields in the one-step reaction via
the Au(III)-oxadiazole complex (Scheme 4c) with respect to
organic substrates. But two steps are required for product iso-
lation from the Pd/Pt complexes. Nevertheless, Pd and Pt(IV)
may coordinate two nitrile substrates to give two product
units. None of the methods has been developed into catalytic
processes.
Dihydro-1,2,4-oxadiazoles 4 represent a group of hetero-
cyclic compounds with interesting bioactive properties8a–c The
suitability of mixed ligand Pt(II) oxadiazoline complexes as
anticancer agents has also been screened; and in vitro cyto-
toxicity has been studied.8d–f There are a limited number of
non-metal-based preparation methods to afford the dihydro-
1,2,4-oxadiazole heterocycle, in general obtained in moderate
yields. Direct 1,3-dipolar cycloadditions of nitrones with
nitriles are reported to take place by vigorous reaction con-
ditions, such as thermal9a or at high pressure.5a,9b 2,3-Dihydro-
Table 1 Bond lengths (Å) and angles in gold(I)–nitrone complexes
(Scheme 2e)
Entry
Complex
O–Au
Au–L
N–O
O–Au–L
1
2
3
4
5
6
7
I-2a
2.096
2.087
2.090
2.074
2.085
2.068
2.045
2.2429
2.239
2.2363
2.2296
2.2358
2.222
1.340
1.343
1.340
1.349
1.326
1.344
1.348
174.87°
175.83°
175.85°
177.38°
178.20°
177.14°
177.51°
I-2a′
I-2a″
I-2b
I-2c
II-2a
III-2b
1.959
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1,2,4-oxadiazoles have also been prepared in two steps from formed on a “Synapt G2-S” Q-TOF instrument from Waters.
imines via oxaziridine and subsequent [3 + 2] cycloaddition Samples were ionized with an ASAP probe with no chromato-
with nitriles.8f,9c Some methods for the preparation of the graphy separation performed before mass analysis. IR spectra
corresponding aromatic 1,2,4-oxadiazole products are repor- were recorded with a Nicolet 20SXC FT-IR spectrometer using
ted9d–g in addition to cycloadditions.5b,f,g,h However, no access EZ OMNIC software to analyse the spectra and a Bruker Alpha
to dihydro-1,2,4-oxadiazoles 4 by reduction of the aromatic FT-IR spectrometer using OPUS V7 software to analyse the
oxadiazole compounds is known, as C–O or N–O ring cleavage spectra. Single crystal X-ray data were acquired using a Bruker
mostly takes place.
D8 Venture diffractometer with the APEX3 suit, integrated with
SAINT V9.32B, solved with XT and refined with XL using Olex2
as GUI. The cif files were edited with encipher 1.4 and mole-
cular graphics were produced with Mercury 3.8. ORTEP plots
are shown in the thesis and all metric data, including reflec-
Conclusions
In summary, crystalline Au(I)–nitrone complexes (I-2a′, I-2a″, tion data, are contained in the respective cif files.
I-2b, I-2c, II-2a, III-2b) were prepared from Au(I)–phosphane or
General procedure for the synthesis of Au(I)–nitrone complexes
Au(I)–NHC (pre)catalysts I–III and nitrones 2a–c (e.g. 63–95%
cryst. JohnPhos-Au(I)–nitrones). The Au(I)–phosphane–nitrones A solution of the appropriate nitrone (1.5 eq.) and gold
were catalytically active. All crystal structures (X-ray) showed a complex (1 eq.) in 0.2 mL (DCM) was shaken and left at room
nearly linear nitrone–O–Au(I)/–P(phosphane)/–C(NHC) coordi- temperature for 30 min. If applicable, anion exchange was per-
nation mode.
formed by adding 1.5 eq. AgSbF6 to the solution and the result-
Further studies on the gold–nitrone chemistry lead us to ing AgCl was filtered after 30 min. Crystalline Au(I)–nitrone
the discovery of a crystalline Au(I)-dihydro-1,2,4-oxadiazole complexes were obtained by adding Et2O (1 : 1 Et2O : pentane
complex I-4a (87%, 18 h, ACN, reflux), formed by a [3 + 2] cyclo- for I-2a′) until the solution became cloudy and left at r.t. until
addition of JohnPhos-Au(I)-ligated acetonitrile with nitrone 2a. full crystallisation (1–3 hours). Finally, the crystals were
Corresponding Au(III)-4a complexes were more readily formed washed with Et2O, re-dissolved in DCM and transferred to a
by conversion of AuCl3, Me/Ph-nitriles and nitrone 2a (>85%, round bottom flask. Removal of the solvent in vacuo yielded
2 h, EtOAc, r.t.). The 1,2,4-oxadiazole heterocycles coordinated the pure Au(I)–nitrone complexes.
to Au(I)/(III) through the N(4)imine atom (X-ray), indicating a
Complex I-2a. The title compound was prepared according
Au–nitrile origin. A convenient one-pot preparation method to the general procedure, using nitrone 2a (6 mg, 0.045 mmol)
for dihydro-1,2,4-oxadiazoles 4 (up to 74%) by Au(III) mediated and Au(I) complex I (23 mg, 0.030 mmol), yielding Au(I)–
1
[2 + 3] nitrone–nitrile cycloaddition and final product release nitrone complex I-2a (20 mg, 74%) as white crystals. H NMR
from the Au(III) complex was developed.
(400 MHz, CDCl3) δ (ppm) 8.15–8.16 (m, 2H, Ar), 8.01 (s, 1H,
The present study contributes to a new understanding of HCvN), 7.85–7.88 (m, 1H, Ar), 7.61–7.64 (m, 1H, Ar),
Au(I)–/Au(III)–nitrone chemistry, as shown by the dual selective 7.54–7.59 (m, 4H, Ar), 7.42–7.48 (b, 2H, Ar), 7.30–7.32 (m, 1H,
reactivity of such systems. The results demonstrate the versati- Ar), 7.20–7.21 (m, 2H, Ar), 3.87 (s, 3H, CH3N), 1.36 (d, 18H, J =
lity of gold–nitrone systems by the controlled ability to 16.0 Hz, (CH3)3C). 13C NMR (100 MHz, CDCl3) δ (ppm) 149.3
undergo complexation into (catalytic active) species and their (d, J = 12.6 Hz, CAr), 147.7 (CvN), 142.3 (d, J = 8.0 Hz, CAr),
selective synthetic utility.
134.0 (CHAr), 133.3 (d, J = 6.9 Hz, CHAr), 133.9 (d, J = 3.6 Hz,
CHAr), 132.6 (CHAr), 131.3 (d, J = 2.2 Hz, CHAr), 129.6 (CHAr),
128.8 (CHAr), 128.4 (d, J = 2.6 Hz, CHAr), 127.6 (CAr), 127.3 (d,
J = 9.4 Hz, CHAr), 123.8 (d, J = 50.5 Hz, PCAr), 54.2 (CH3N), 38.0
(d, J = 27.0 Hz, PC), 30.7 (d, J = 6.4 Hz, (CH3)3C). m.p:
160–162 °C (decomp.). IR (thin film, cm−1) 3065, 2950, 1480,
Experimental
General
All reactions were performed under a normal atmosphere. 1145, 924, 660. HRMS (ASAP) calcd for C20H27AuP
Commercial grade reagents were used without any additional [M• − nitrone]+ 495.1516, obsd 495.1715. X-Ray: CCDC 1531556.†
purification. Dry solvents were collected from a MB SPS-800
Complex I-2a′. The title compound was prepared according
solvent purification system. All reactions were monitored by to the general procedure, using nitrone 2a′ (7 mg, 0.040 mmol)
NMR and/or thin-layer chromatography (TLC) using silica gel and gold complex I (21 mg, 0.027 mmol), yielding Au(I)–
1
60 F254 (0.25 mm thickness). TLC plates were developed using nitrone complex I-2a′ (23 mg, 95%) as white crystals. H NMR
UV-light and/or p-anisaldehyde stain. Flash chromatography (600 MHz, CDCl3) δ (ppm) 8.15 (d, J = 8.8 Hz, 2H, Ar),
1
was performed with Merck silica gel 60 (0.040–0.063 mm). H 7.85–7.88 (m, 1H, Ar), 7.81 (s, 1H, CHvN), 7.53–7.58 (m, 2H,
and 13C NMR spectra were recorded with either a Bruker Ar), 7.41–7.44 (m, 2H, Ar), 7.29–7.31 (m, 2H, Ar), 7.20–7.21 (m,
Avance DPX 400 MHz or a Bruker Avance III 600 MHz spectro- 2H, Ar), 7.02 (d, J = 8.8 Hz, 2H, Ar), 3.91 (s, 3H, CH3O), 3.80 (s,
meter. Chemical shifts are reported in ppm (δ) downfield from 3H, CH3N), 1.37 (d, J = 16.1 Hz, 18H, (CH3)3C). 13C NMR
tetramethylsilane (TMS) as an internal standard. Melting (150 MHz, CDCl3) δ (ppm) 163.8 (CAr), 149.3 (d, J = 12.1 Hz,
points (mp) were determined using a Stuart SMP40 apparatus CAr), 142.3 (d, J = 6.7 Hz, CAr), 134.8 (b, CvN), 133.3 (d, J =
and are uncorrected. Accurate mass determination was per- 7.7 Hz, CHAr), 133.0 (d, J = 3.5 Hz, CHAr), 131.3 (d, J = 2.1 Hz,
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CHAr), 129.5 (CHAr), 128.4 (CHAr), 127.3 (d, J = 7.6 Hz, CHAr),
Complex II-2a. The title compound was prepared according
123.8 (d, J = 49.6 Hz, PCAr), 120.7 (b, CAr), 114.3 (CHAr), 55.6 to the general procedure, using nitrone 2a (9 mg, 0.066 mmol),
(CH3O), 53.5 (CH3N), 38.0 (d, J = 27.4 Hz, PC), 30.7 (d, J = 6.6 gold complex II (22 mg, 0.044 mmol) and AgSbF6 (15 mg,
Hz, (CH3)3C). IR (thin film, cm−1) 3061, 2959, 1600, 1510, 0.044 mmol) yielding Au(I)–nitrone complex II-2a (14 mg, 38%)
1265, 1177, 1142, 1022, 759. X-Ray: CCDC 1579660.†
as white crystals. 1H NMR (400 MHz, CDCl3) δ (ppm) 8.25–8.27
Complex I-2a″. The title compound was prepared according (m, 2H, Ar), 8.09 (s, 1H, NvCH), 7.54–7.60 (m, 4H, Ar),
to the general procedure, using nitrone 2a′′ (10 mg, 7.48–7.52 (m, 8H, Ar), 7.38–7.44 (m, 6H, Ar), 4.13 (s, 3H, CH3).
0.048 mmol) and gold complex I (19 mg, 0.024 mmol), yielding 13C NMR (100 MHz, CDCl3) δ (ppm) 147.8 (CvN), 134.0 (d, J =
1
Au(I)–nitrone complex I-2a′′ (19 mg, 86%) as white crystals. H 13.7 Hz, CHAr), 132.7 (d, J = 3.2 Hz, CHAr), 132.1 (CHAr), 129.6
NMR (600 MHz, CDCl3) δ (ppm) 8.29 (d, J = 8.3 Hz, 2H, Ar), (d, J = 12.5 Hz, CHAr), 128.9 (CHAr), 127.6 (CAr), 126.9 (d, J =
8.11 (s, 1H, CHvN), 7.88–7.85 (m, 1H, Ar), 7.79 (d, J = 8.3 Hz), 66.5 Hz, PCAr), 54.1 (CH3). m.p.: 152–156 °C (decomp.). IR
7.54–7.60 (m, 2H, Ar), 7.44–7.46 (m, 2H, Ar), 7.33–7.28 (m, 2H, (thin film, cm−1) 3059, 3021, 2926, 1697, 1438, 1143, 1102,
Ar), 7.21 (d, J = 7.5 Hz, 2H, Ar), 3.92 (s, 3H, CH3), 1.37 (d, J = 751. HRMS (ASAP) calcd for C18H15AuP [M• − nitrone]+
16.2 Hz, 18H, (CH3)3C). 13C NMR (150 MHz, CDCl3) δ (ppm) 459.0577 obsd 459.0576. X-Ray: CCDC 1548788.†
149.2 (d, J = 12.2 Hz, CAr), 146.0 (b, CHvN), 142.3 (d, J = 6.7
Complex III-2b. The title compound was prepared according
Hz), 134.5 (q, J = 35.2 Hz, CAr), 133.3 (d, J = 7.7 Hz, CHAr), to the general procedure, using nitrone 2b (5 mg,
132.9 (d, J = 4.4 Hz, CHAr), 132.6 (b, CHAr), 131.4 (d, J = 2.2 Hz, 0.056 mmol), gold complex III (23 mg, 0.037 mmol) and
CHAr), 130.7 (b, CAr), 129.6 (CHAr), 128.5 (CHAr), 128.4 (CHAr), AgSbF6 (13 mg, 0.037 mmol) yielding Au(I)–nitrone complex
127.4 (d, J = 7.7 Hz, CHAr), 125.6 (q, J = 4.1 Hz, CHAr), 123.7 (d, III-2b (34 mg, 47%) as white crystals. 1H NMR (400 MHz,
J = 50.4 Hz, PC), 123.3 (q, J = 273.2 Hz, CF3), 54.7 (CH3N), 38.1 CDCl3) δ (ppm) 7.90 (b, 2H, Ar), 7.62–7.68 (m, 4H, Ar), 7.59 (b,
(d, J = 27.5 Hz, PC), 38.7 (d, J = 5.9 Hz, (CH3)3C). m.p.: 2H, Ar), 7.39–7.40 (m, 4H, Ar), 7.37 (s, 2H, NHCvCHN), 2.41
160–165 °C (decomp.). IR (thin film, cm−1) 3061, 2959, 2903, (sept, 2H, J = 6.9 Hz, CH), 1.27 (d, 12H, J = 6.8 Hz, CH3),
1600, 1510, 1462, 1265, 1177, 1142. X-Ray: CCDC 1548791.†
1.25–1.27 (d, 12H, J = 7.0 Hz, CH3). 13C NMR (100 MHz, CDCl3)
Complex I-2b. The title compound was prepared according δ (ppm), the signals belonging to pyridine N-oxide cannot be
to the general procedure, using nitrone 2b (5 mg, 0.054 mmol) seen for some unknown reason. 162.4 (C–Au), 145.8 (CAr), 133.5
and gold complex I (28 mg, 0.036 mmol) yielding Au(I)–nitrone (CAr), 131.4 (CHAr), 124.6 (CHAr), 124.3 (NHCvCHN), 28.9 (CH),
complex I-2b (20 mg, 69%) as white crystals. 1H NMR 24.5 ((CH3)2), 24.0 ((CH3)2). m.p.: 174–176 °C (decomp.). IR (thin
(400 MHz, CDCl3) δ (ppm) 8.38–8.40 (m, 2H, Ar), 7.86–7.90 (m, film, cm−1) 3171, 2964, 2928, 2871, 1469, 1387, 1175, 1060, 761.
2H, Ar), 7.77–7.80 (m, 2H, Ar), 7.54–7.57 (m, 2H, Ar), 7.20–7.36 HRMS (ASAP) calcd for C27H37N2 [M• − (Au–nitrone)]+ 389.2957
(m, 6H, Ar), 1.44 (d, 18H, J = 16.4 Hz, CH3). 13C NMR obsd 389.2950. X-Ray: CCDC 1548789.†
(100 MHz, CDCl3) δ (ppm) 149.4 (d, J = 11.8 Hz, CAr), 142.5 (d,
J = 6.6 Hz, CAr), 139.1 (CHAr), 133.3 (d, J = 7.5 Hz, CHAr), 133.0
Synthesis of Au(I)–oxadiazole complex I-4a
(d, J = 3.8 Hz, CHAr), 131.4 (d, J = 2.8 Hz, CHAr), 129.7 (CHAr), A solution of nitrone 2a (9 mg, 0.064 mmol) and gold complex
128.6 (CHAr), 128.0 (CHAr), 127.8 (CHAr), 127.5 (CHAr), 127.4 (d, I (25 mg, 0.032 mmol) in 2 mL dry CH3CN was heated to 80 °C
J = 7.4 Hz, CHAr), 124.0 (d, J = 50.7 Hz, PCAr), 38.2 (d, J = 28.0 and stirred for 18 hours. The solvent was removed in vacuo and
Hz, PC), 31.0 (d, J = 6.0 Hz, CH3).
m.p.: 179–181 °C (decomp.). IR (thin film, cm−1) 3122, complex was crystallised by adding Et2O to the solution. The
2963, 2900, 1469, 1201, 1175, 765. X-Ray: CCDC 1548784.† resulting white crystals were washed with Et2O, which yielded
the product was re-dissolved in 0.2 mL DCM. The product
1
Complex I-2c. The title compound was prepared according the pure Au(I)–oxadiazole complex 20 (29 mg, 87%). H NMR
to the general procedure, using nitrone 2c (4 mg, 0.026 mmol) (600 MHz, CDCl3) δ (ppm) 7.77–7.82 (m, 1H, Ar), 7.60–7.64 (m,
and gold complex I (14 mg, 0.018 mmol), yielding Au(I)– 1H, Ar), 7.46–7.56 (m, 3H, Ar), 7.37–7.43 (m, 6H, Ar), 7.20–7.25
1
nitrone complex I-2c (10 mg, 63%) as yellow crystals. H NMR (m, 2H, Ar), 7.11–7.14 (m, 1H, Ar), 5.40 (s, 1H, CH), 3.06 (s,
(600 MHz, CDCl3) δ (ppm) 8.75–8.77 (m, 1H, Ar), 8.26–8.28 (m, 3H, CH3N), 2.13 (d, 3H, J = 0.5 Hz, CH3CvN), 1.31 (d, 9H, J =
1H, Ar), 7.87–7.91 (m, 2H, Ar), 7.80–7.83 (m, 1H, Ar), 7.65–7.69 16.2 Hz, (CH3)3C), 1.07 (d, 9H, J = 16.0 Hz, (CH3)3C). 13C NMR
(m, 2H, Ar), 7.54–7.57 (m, 2H, Ar), 7.14–7.28 (m, 6H, Ar), (150 MHz, CDCl3) δ (ppm) 169.7 (CvN), 149.0 (d, J = 12.3 Hz,
6.56–6.60 (m, 1H, Ar), 3.06 (s, 3H, CH3), 1.46 (d, 18H, J = 16.2 CAr), 143.1 (d, J = 6.5 Hz, CAr), 135.7 (CAr), 133.3 (CHAr), 133.2
Hz, (CH3)3C). 13C NMR (150 MHz, CDCl3) δ (ppm) 149.3 (d, J = (d, J = 4.7 Hz, CHAr), 131.1 (d, J = 2.2 Hz), 130.3 (CHAr), 129.9
12.1 Hz, CAr), 142.5 (d, J = 5.8 Hz, CAr), 139.8 (CHAr), 139.0 (CHAr), 129.8 (CHAr), 129.2 (CHAr), 128.9 (CHAr), 128.6 (CHAr),
(CAr), 136.4 (CHAr), 133.3 (d, J = 7.9 Hz, CHAr), 132.9 (d, J = 3.5 128.1 (CHAr), 127.7 (CHAr), 127.4 (d, J = 6.5 Hz, CHAr), 123.8 (d,
Hz, CHAr), 132.3 (CAr), 132.1 (CAr), 131.3 (d, J = 2.4 Hz, CHAr), J = 49.5 Hz, PCAr), 92.0 (CH), 45.9 (CH3–N), 37.9 (d, J = 26.4 Hz,
129.7 (CHAr), 129.5 (CHAr), 128.2 (CHAr), 127.7 (CHAr), 127.4 PC). 37.8 (d, J = 27.5 Hz, PC), 30.8 (d, J = 5.6 Hz, (CH3)3C), 30.4
(CHAr), 127.3 (CHAr), 127.2 (CHAr), 123.6 (d, J = 49.8 Hz, PCAr), (d, J = 6.6 Hz, (CH3)3C), 14.0 (CH3CvN). IR (thin film, cm−1
)
121.3 (CHAr), 38.2 (d, J = 27.4 Hz, PC), 30.9 (d, J = 6.3 Hz, 2961, 2927, 1647, 1460, 1260, 1173, 1020, 764. HRMS (ASAP)
(CH3)3C), 25.6 (CH3). m.p.: 178–183 °C (decomp.). IR (thin calcd for C30H39AuN2OP [M•]+ 671.2466, obsd 671.2474. Not
film, cm−1) 3059, 2961, 2902, 1584, 1462, 1201, 1160, 821.7. enough compound remained for m.p. determination. X-Ray:
X-Ray: CCDC 1548787.†
CCDC 1548790.†
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Dalton Trans., 2019, 48, 142–149 | 147

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Synthesis of Au(III)–oxadiazole complex Au(III)-4a
3-(4-Methoxyphenyl)-2-methyl-5-phenyl-2,3-dihydro-1,2,4-
oxadiazole 4e.7f The title compound was prepared according
to the general procedure from AuCl3 (22 mg, 0.073 mmol),
benzonitrile (37 mg, 0.36 mmol), and nitrone 2a′ (12 mg,
0.072 mmol) to give oxadiazole 4e (7 mg, 34%) as a colourless
A solution of nitrone 2a (6 mg, 0.043 mmol) and AuCl3 (13 mg,
0.043 mmol) in CDCl3 (0.6 mL) was added to acetonitrile
(8 mg, 0.19 mmol), shaken well and left for 90 min. The solvent
was evaporated and the product was re-dissolved in 0.2 mL aceto-
nitrile. The product complex Au(III)-4a was crystallised as yellow
crystals by addition of 1 : 1 Et2O : pentane to the solution. The
complex could only be isolated in amounts sufficient for X-ray
analysis, and the reaction yield could not be determined.
1H NMR (400 MHz, CDCl3) δ (ppm) Crude reaction mixture:
7.45–7.58 (m, 5H, Ar), 5.94 (s, b, 1H, CH), 3.05 (s, 3H, CH3N),
2.56 (d, J = 0.6 Hz, 3H, CH3). X-Ray: CCDC 1548801.†
1
oil. H NMR (400 MHz, CDCl3) δ (ppm) 7.97–7.99 (m, 2H, Ar),
7.51–7.53 (m, 1H, Ar), 7.43–7.45 (m, 2H, Ar), 7.35–7.37 (m, 2H,
Ar), 6.88–6.90 (m, 2H, Ar), 5.70 (s, 1H, CH), 3.79 (s, 3H, CH3O),
2.96 (s, 3H, CH3N). 1H NMR corresponds to previously
reported data.7f
2-Methyl-5-phenyl-3-(4-(trifluoromethyl)phenyl)-2,3-dihydro-
1,2,4-oxadiazole 4f. The title compound was prepared accord-
ing to the general procedure from AuCl3 (12 mg, 0.038 mmol),
benzonitrile (20 mg, 0.19 mmol), and nitrone 2a′′ (8 mg,
0.038 mmol) to give oxadiazole 4f (9 mg, 74%) as a colourless
oil. Rf = 0.32 (DCM). 1H NMR (400 MHz, CDCl3) δ (ppm)
General one-pot procedure for the synthesis of 1,2,4-dihydro-
oxadiazoles (4)
A solution of AuCl3 (1 eq.) and acetonitrile/benzonitrile (5 eq.) 7.96–7.98 (m, 2H, Ar), 7.59–7.63 (m, 4H, Ar), 7.52–7.55 (m, 1H,
in EtOAc (1 mL) was stirred for 10 min before adding the Ar), 7.43–7.46 (m, 2H, Ar), 5.80 (s, 1H, CH), 2.99 (s, 3H, CH3N).
appropriate nitrone (1 eq.) and stirred for 2 hours. The reac-
13C NMR (100 MHz, CDCl3) δ (ppm): 161.0 (CvN), 143.7
tion was quenched by addition of a few drops of Et3N, followed (CAr), 132.2 (CHAr), 130.2–130.9 (q, J = 32.9 Hz, CAr), 128.7
by adsorption of the reaction mixture on alumina. The mixture (CHAr), 128.6 (CHAr), 127.0 (CHAr), 125.5–125.6 (q, J = 4.3 Hz,
was left for 16 hours, followed by filtration with DCM CHAr), 125.3 (CAr), 121.1–126.8 (q, J = 272.2 Hz, CF3), 93.1
(50–100 mL). Concentration of the filtrate in vacuo followed by (CH), 47.1 (CH3N). IR (thin film, cm−1) 3064, 2964, 2992, 1777,
flash column chromatography (4 : 1 pentane : DCM) yielded 1325, 1165, 1124, 771. HRMS (ASAP) calcd for C11H12F3N2O
the pure oxadiazoles.
2,5-Dimethyl-3-phenyl-2,3-dihydro-1,2,4-oxadiazole,
[M + H]+ 245.0902 obsd 245.0898.
4a.7f
The title compound was prepared according to the general pro-
cedure from AuCl3 (24 mg, 0.079 mmol), acetonitrile (16 mg,
Conflicts of interest
0.38 mmol), and nitrone 2a (11 mg, 0.079 mmol) to give oxa-
1
diazole 4a (5 mg, 36%) as a colourless oil. H NMR (400 MHz, There are no conflicts to declare.
CDCl3) δ (ppm) 7.29–7.40 (m, 5H, Ar), 5.53 (s, 1H, CH), 2.88
1
(s, 3H, CH3N), 2.10 (s, 3H, CH3). H NMR corresponds to pre-
viously reported data.7f
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