Dual H-bond activation of NHC-Au(i)-Cl complexes with amide functionalized side-arms assisted by H-bond donor substrates or acid additives

Article abstract of DOI:10.1039/d0cc05999d

Novel approach with amide-tethered H-bond donor NHC ligands enabled Au(i)-catalysis via H-bonding. The plain NHC-Au(i)-Cl complex catalysed conversions of terminal N-propynamides to oxazolines, and enyne cycloisomerization with an acid additive, in DCM at

Full text of DOI:10.1039/d0cc05999d

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S. J. Connon, M. O. Senge et al.
Conformational control of nonplanar free base porphyrins:
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DOI: 10.1039/D0CC05999D
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Dual H-bond activation of NHC-Au(I)-Cl complexes with amide
functionalized side-arms assisted by H-bond donor substrates or
acid additives
Received 00th January 20xx,
Accepted 00th January 20xx
Otto Seppänen,a# Santeri Aikonen,a# Mikko Muuronen,a† Carla Alamillo-Ferrer,b Jordi Burés,b and
Juho Helajaa
DOI: 10.1039/x0xx00000x
www.rsc.org/
Novel approach with amide-tethered H-bond donor NHC ligands utilised a pyridine side-arm to replace one chloride ion from the
enabled Au(I)-catalysis via H-bonding. The plain NHC-Au(I)-Cl Au(III) centre with a hemilabile N-Au -coordination (Scheme
complex catalysed conversions of terminal N-propynamides to 1a). Additionally, we noted that higher activity was achieved
oxazolines, and enyne cycloisomerization with an acid additive, in with a more nucleophilic pyridine moiety and in the presence of
DCM at RT. DFT calculations enlightened the function of the side- H-bond donor solvent or additive,10 which helped to stabilise
arm in the activation.
the cleaved chloride. Previously, Sen and Gabbai integrated the
H-bond donor moiety into a phosphine ligand and observed N-
H…Cl interactions in the dimer together with aurophilic
interaction (Scheme 1b).12 The dimer equipped with NHCOCF3
tethered ligand proved to be catalytically active for cyclization
of propargylamides, while under same conditions bare
PPh3AuCl with PhNHCOCF3 additive was inactive.12
Phosphine and N-heterocyclic carbene (NHC) ligands (L) have
brought about adjustable stability, activity, and selectivity for
homogeneous
gold-catalysed
carbon-carbon
-bond
activations.1,2 To access the active cationic gold catalyst from
ligated gold-chloride salt (LAuCl), the precatalyst is usually
activated with AgX salts by exchanging the Cl- counterion to a
Previous self-activation concepts
non- or weakly coordinating one, e.g., BF4 , OTf-, NTf2 , PF6 , SbF6
-
-
-
-
Improved activity
a) By nucleophilic NHC-tether
Ref. 10
.3 Many of the activated LAuX catalysts are also commercially
available, but these salts are often hygroscopic. In situ
activation, on the other hand, brings an extra step of removing
the precipitate or alternatively carrying out the reaction in the
presence of AgCl. The precipitating AgCl residues are non-
innocent in gold-catalysis4 and multiple roles have been
assigned for the counter ions, e.g., influence on chemo- and
regioselectivities operating as H-bond acceptors,5 or assisting in
intramolecular rearrangements.6 Silver-free activations of LAuCl
by stronger N-nucleophility
i)
O
Cl
N
Mes
N
Mes
N
N
Cl
Au
Cl
Nu =
Cl
Au
N
N
Nu
Nu
Cl
Cl
Donors:
CF3
CCl3
by H-bond donor additive
ii)
Activated catalyst
O
Cl
Cl
H
Donor
b) H-bond donor
activates Au(I)-Cl bond
Ref. 12
L
L
Au
Cl
Au
Cl
N
O
O
N
have been reported with sodium salts, e.g., Na[BArF ],7 and with
H
H
4
CF3
CF3
strong acid, HBF4.8
c) This work
R
H
R
Multifunctional NHC-ligands have shown to be capable to
deliver additional designed functions for homogeneous TM
catalysis.9 Recently, ambiphilic ligand Au(I/III) activation
strategies have been developed to generate active catalyst
without the need of ion exchange. We10 and others11 have
H-bond donor
N
N
tethered side-arm
N
N
H
N
N
Mes
Mes
Cl
H
Au
Cl
Au
H-bond donor
in substrate
or acid additive
Au(I)-Cl bond
N
H
R
N
O
R
O
Alkynes as
nucleophiles
Activated catalyst
Scheme 1 Ambiphilic ligand Au-catalyst activation modes
a. Department of Chemistry, University of Helsinki, A. I. Virtasen aukio 1, P.O. Box
55, University of Helsinki, 00014 Finland
Inspired by these findings, we hypothesised that amide-based
H-bond donor tethers in NHC ligands would help to activate the
Au(I)-Cl bond directly with alkynes via N-H…Cl interactions, see
E-mail: juho.helaja@helsinki.
b. The University of Manchester, School of Chemistry, Oxford Road, M13 9PL
Manchester, U.K. Address here.
# These authors have equal contributions
† M.M. current address: BASF SE, Carl-Bosch-Str. 38, 67056 Ludwigshafen, Germany. Scheme 1c. The approach would circumvent the need to create
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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the vacant site with a hemilabile ligand, thus offering a
straightforward approach to activated gold(I)-catalysts.
Importantly, the complexes 1a–2b showed comparable or
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superior activity in comparison to theD5O8I:%10y.1i0e3l9d/Do0fCC4a059w9i9tDh
To probe this hypothesis, we synthesized Au(I) NHC commercially available IPrAuNTf2 (entry 12). The performance
complexes 1a–2b with benzoyl and tosyl amide tethers bridged of catalysts 1a–2b was also better than what has been reported
with ethyl and propyl linkers. We selected the for other NHC or P ligands in homogeneous gold-catalytic
cycloisomerization of N-(prop-2-yn-1-yl)benzamide 3a to conversion with weakly or non-coordinative counter ions (Table
oxazoline 4a as a test reaction to investigate the catalytic S1).
efficiency due to its popularity in gold catalysts studies.13
2a
1.0 mol%
R2 R2
R2
R2
O
O
b
R1
N
CH2Cl2
R1
N
H
synthesized amide
tethered complexes for
Cl solvation by H-bond
pyridine tether complex
3
4
RT
reference complexes
ref 10
O
O
O
O
O
H
N
N
N
Mes
Mes
Mes
N
N
N
N
N
N
N
Mes
R
Mes
N
n
N
Au
(Cl)n
5a
5b
n=1
n=3
Au
(Cl)x
: n = 2, x = 1, R = Bz
: n = 3, x = 1, R = Bz
: n = 2, x = 1, R = Ts
: n = 3, x = 1, R = Ts
: n = 2, x = 3, R = Ts
: n = 2, x = 3, R = Bz
Au
Cl
Cl
Cl
7
4a
4b
4c
4d
: 94%, 5 h
: 98%, 3 h
O
: 98%, 2 h
O
: 93%, 3 h
1a
N
1b
2a
2b
8a
8b
N
butyl
Cl
N
N
O
N
O
N
Mes
Au
(Cl)n
6a
n=1
n=3
N
N
N
O
Au
Cl
Cl
6b
F
O
F3C
4f
Scheme 2 Studied complexes for catalysis. (Mes = mesityl)
Table 1 Catalysts screening
4e
4h
4i
: 83%, 8 h
: 89%, 24 h
: 21%, 4 h
: 97%, 7 h
O
O
N
N
[Au]
O
O
H
N
1 mol%
b
N
N
CD2Cl2
RT
3 h
H
a
O
4j
: 93%, 24 h
4g
: 96%, 20 h
3a
4a
a
Scheme 3 Substrate scope in oxazoline synthesis with isolated yields. Catalyst
Yielda [%] of 4a
Entry
Catalyst
loading 3.0 mol%. b Water content in all reactions 150 ppm.
1
2
1a
2a
1b
2b
5a
5b
6a
6b
7
73
95
58
67
Next, we studied the substrate scope for the catalytic
alkynyl amide oxazoline conversion (Scheme 3) with 1 mol%
3
4
t
5
0 (55)c
0 (8)c
0 (68)c
0 (30)c
trace
20
loading of 2a in DCM at RT. Several propynamides; Bu (3b),
6
electroneutral and rich aryls (3a,c-e) provided excellent yields
of oxazolines 4a-e, though an extended reaction time of 24 h
was necessary for 4-methoxy-phenyl oxazoline 4e. Curiously
electron deficient 4-CF3-phenyl amide (3f) provided an unclean
reaction and a poor yield of product 4f, while the 4-F-phenyl
oxazoline 4i was isolated in high 83% yield. Interestingly, bulky
functional groups such as triphenylmethyl as R1 substituent (4g)
and spirocyclohexyl as R2 substituent (4h) were well tolerated
and excellent yields of 96% and 97%, respectively, were
received after longer reaction times. Unlike complexes without
functional group tethers,14 the catalyst 2a proved to be
chemoselective towards terminal alkynes as terminally
functionalised alkynes 3k and 3l, see SI, were unreactive.
Similarly, in the case of substrate 3j that is equipped with both
types of alkynes, the terminal alkynylamide cyclised selectively
producing 4j with 93 % yield. Additionally, the catalyst was not
active for 6-exo-dig cyclisation in oxazoline 4m synthesis (SI). In
the case of terminally substituted alkynes 3j, 3k, and 3l, the
computational study indicated that steric hindrance between
the ligand and the alkynes’ terminal substituent limited the
reactivity (see Fig. S5).
7
8
9
10
11
12
8a
8b
5
58
IPrAuNTf2
Determined by 1H NMR using trimethoxybenzene as internal standard.
a
b Water content 150 ppm. c with 1 mol% of AgOTs
In catalysts screening (Table 1), the ethyl amide tethered
NHC-Au(I) complex 1a yielded a clean 73% conversion of 3a to
4a, while the corresponding tosyl functionalized amide 2a gave
an excellent 95% yield after 3 h monitoring period. Elongating
the ethyl arms to propyls lowered the yields to 58% and 67% for
Bz (1b) and Ts (2b) functionalized NHC-Au(I) catalysts,
respectively.
The NHC gold complexes without H-bond donors (5a–6b)
proved to be inactive, while the same complexes gave modest
(8%) to decent (68%) yields of 4a with AgOTs (entries 5–8).
Surprisingly, our previously developed self-activated Au(III)
complex 710 showed only negligible activity for the reaction, as
well as the gold(III)-catalysts 8a and 8b (entries 9–11). Although
AgOTs or TsOH additives promoted the reaction with 8a and 8b,
the catalysts were gradually reduced to the respective Au(I)-
complexes (see 8b + TsOH the reaction NMR monitoring in SI).
The solvent screening (Table S2) exposed that chlorinated
solvents CD2Cl2 and CDCl3 favour the catalysis, while the
performance was sluggish in acetone-d6, CD3CN and CD3OD.
The effect of the concentration of water in DCM for the
activation of the catalyst (2a) became a relevant issue, since
aqueous media was previously found necessary for Brønsted
acid self-activated ligands (Scheme 1a).11 We studied the effect
of small amounts of water in the solution for the catalytic
activity of 2a in the conversion of 3a to 4a in 1H NMR, see Fig. 1.
2 | J. Name., 2012, 00, 1-3
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The results clearly show that even in the absence of water (0 catalytic complex by hydrogen bonds or solvated bVyieswmAratilcllewOantliener
DOI: 10.1039/D0CC05999D
ppm), the catalyst 2a is activated. The gradual increase of the cluster. In the former case, the rate-determining step is the
water content (Fig. 1 and SI) up to 200 ppm increased the rate protodeauration with 21.3 kcal/mol activation free energy
and the yield of the reaction, but a higher water content of 250 barrier. In the latter case, the rate-determining step is
and 450 ppm decreased the rate or even inhibited the reaction, significantly faster with an activation free energy barrier of 17.2
respectively. A plausible explanation for the behaviour is that kcal/mol for the C-O bond formation (SI). This agrees with the
the small amount of water could assist in the anion solvation observed water effect in Fig. 1. The barriers for 5a are
(see SI) or in the proton transfer,15-17 meanwhile higher amount systematically higher, see SI.
of water lowers the proton’s acidity.15 An alternative
interpretation is that H-bonding interactions between the
amide tether and chloride anion are weakened by the water
content above 200 ppm thus inhibiting the activation step.
Fig. 1 Kinetic monitoring of 2a catalysed conversion of 3a to 4a conversion in
various water contents. [3a] = 0.226 M, and [2a/IPrAuNTf2] = 0.0023 M, in CD2Cl2
at 25 °C.
Fig. 2 Free energy profiles for Au-Cl bond activation with catalysts 1a (blue line), 2a
(yellow line), and 5a (black line). The free energies are calculated on PW6B95-D3/def2-
TZVPD//TPSS-D3/def2-TZVP in COSMO-RS for DCM level of theory. Full computational
details are in SI.
To understand the side-arm’s mechanistic role in the Au-Cl
bond activation, we compared the computational free energy
Beyond the oxazoline synthesis, we investigated how 2a
profiles for catalysts 1a, 2a, and 5a (Fig. 2). Noteworthy, both performed in other classic catalytic L-Au(I) transformations.
benzoyl and tosyl amide side-arms lowered the activation free Echavarren and co-workers have originally reported L-Au(I)
energy barrier in the alkyne addition to the gold, TS1, by 2.2 and catalysed cycloisomerization of enynes (9 → 10 + 11, Table 2)
3.6 kcal/mol, respectively, compared to 5a (Fig. 2). The chloride and observed no reactivity with bare [PPh3AuCl] complex, but
-
-
was hydrogen bonded to the side-arm’s NH with both 1a and 2a exchange of coordinative chloride counterion to SbF6 or BF4
while the NH of the substrate coordinated side-arm’s sulfonyl allowed smooth catalytic cycloisomerization at RT.18 In our case,
oxygen with 2a (Fig. 2) and the chloride with 1a. Similar the enyne 9 was unreactive with 2 mol% loading of 2a alone
bidentate coordination to chloride in 2a-TS1’ (SI) was close in (entry 1 in Table 2). However, a 5 mol% addition of mono- and
energy to 1a-TS1: 14.0 kcal/mol, whereas the barrier was 18.2 dichloroacetic acid additive gave selectively isomer 10 with 50%
kcal/mol in the absence of substrate’s hydrogen bond and 99% yields, respectively. When TsOH was used as an
coordination in 2a-TS1’’ (see SI).
additive a complete conversion to mixture of isomers 10 and 11
All catalysts then converged to a tricoordinate complex B, took place in 10 min.19 The acid additive was unable to activate
from which the chloride spontaneously cleaves (TS2s in Fig. 2). the non-functionalized complex 5a in similar efficiency and only
The chloride anion hydrogen bonded with NH of the substrate, traces of product was observed after 1 h (entry 10).
and with the side-arm’s NH if the catalyst was 1a or 2a (Cs in Fig.
Because chloride’s gold-affinity is higher20 compared to all
2). The bidentate hydrogen bond donation from the NHs of of the tested acid-additives (Table 2), we reason that the acidity
substrate and side-arm to chloride stabilised B, TS2, and C, over of the additive is important. Inspection of Table 2 reveals that
the monodentate hydrogen bonding with 5a and the as the acidity of the additive approaches HCl’s pKa(DCE) = 45.2,21
stabilisation was stronger with a better H-bond donor, tosyl chloride anion exchange takes place forming an active catalyst
amide (Fig. 2). Importantly, bidentate H-bonding with 1a and 2a for enyne 9 cycloisomerization. Based on the activation
favoured the bicoordinate gold-complex over tricoordinate by mechanism of 2a in Fig. 2 and in SI, we reason that the activation
1.3 and 2.0 kcal/mol, respectively, whereas the ∆G was of Au-Cl bond with enyne 9 is too high in energy since the enyne
thermoneutral between B and C for 5a.
substrate has no H-bond donors. Therefore, an acid-additive is
The rate limiting step of the reaction can either be the C-O needed to help in the activation via H-bonding, subsequent
bond formation or the protodeauration step for the catalyst 2a. protonation and release of HCl, and generation of a loosely
-
-
This will depend on whether the chloride anion is bound to the coordinating counterion (RCO2 or RSO3 ).
This journal is © The Royal Society of Chemistry 20xx
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The developed acid-assisted activation mechanism was then number 180234. C.A.-F. and J.B. expresses gratituVdieew fAortricleEOPnSlRinCe
utilised in enyne cycloisomerization and nucleophilic alcohol funding (EP/S005315/1). M.Sc. Karina MoDslOoIv:a10a.1c0k3n9o/Dw0leCdCg0e5d99f9oDr
(ROH) addition cascade reaction, following previous L-Au(I) measuring elemental analysis.
catalysis reports (Scheme 4, Table S3).18,22 Full conversion was
achieved for each case, and MeOH delivered the best yield of
98% for 12a while allyl and benzyl alcohols gave lower yields.
Conflicts of interest
Similarly, good yields were obtained for OMe-substituted vinyl
enyne substrates (9d-9f), but benzyl alcohol nucleophile
substituted the methoxy group in the product 12e.
There are no conflicts to declare.
Notes and references
Table 2 Screening of additives for enyne cycloisomerization
1
2
D. J. Gorin, B. D. Sherry and F. D. Toste, Chem. Rev., 2008, 108, 3351.
W. Wang, G. B. Hammond and B. Xu, J. Am. Chem. Soc., 2012, 134,
5697.
M. Jia and M. Bandini, ACS Catal., 2015, 5, 1638.
A. Zhdanko and M. E. Maie, ACS Catal., 2015, 5, 5994.
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Ranieri, I. Escofeta and A. M. Echavarren, Org. Biomol. Chem., 2015,
13, 7103; c) D. Zuccaccia, A. Del Zotto and W. Baratta, Coord. Chem.
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Eur. J. 2015, 21, 1328.
C. Nevado and A. M. Echavarren, Chem. Eur. J. 2005, 11, 3155.
E. Peris, Chem. Rev. 2018, 118, 9988.
O
O
O
O
O
O
O
2a
O
2 mol%
O
O
O
O
5 mol% acid
+
3
4
5
CH2Cl2, RT
R
9
10
11
R
R'
R'
R, R' = Me
R
R'
a
Entry
1
2
3
4
5
6
7
8
Additive
-
pKa
-
∆G(Au-X)b
-
t
Yieldc 10:11 (%)
trace:0
trace:0
trace:0
50:0
1 h
1 h
1 h
1 h
1 h
40 min
15 min
15 min
10 min
1h
TFE
AcOH
73.2
59.3
54.5
50.7
48.4
46.2
41.4
41.3
41.3
-
6
7
-0.5
4.2
7.2
9.3
9.4
ClCH2COOH
Cl2CHCOOH
åCl3CCOOH
TFA
99:0
99:0
99:0
45:54
66:33
trace:0
8
9
10 a) M. Muuronen, J. E. Perea-Buceta, M. Nieger, M. Patzschke and J.
Helaja, Organometallics, 2012, 31, 4320; b) M. Muuronen, PhD
thesis, University of Helsinki, 2015.
MsOH
12.0
14.1
14.1
9
p-TsOH
p-TsOH + 5a
10
11 a) E. Mendivil-Tomás, P. Y. Toullec, J. Díez, S. Conejero, V. Michelet
and V. Cadierno, Org. Lett., 2012, 14, 2520; b) E. Tomás-Mendivil, P.
Y. Toullec, J. Borge, S. Conejero, V. Michelet and V. Cadierno, ACS
Catal., 2013, 3, 3086.
a
b
Computed pKa values in DCM, see SI for details. Au-X bond strength with the
conjugate base relative to Au-Cl in kcal/mol, see SI for details. c Determined by 1H
NMR using trimethoxybenzene as an internal standard.
O
O
O
O
12 S. Sen and F. P. Gabbai, Chem. Commun., 2017, 53, 13356.
13 Key references: a) A. S. K. Hashmi, J. P. Weyrauch, W. Frey and J. W.
Bats, Org. Lett., 2004, 6, 4391; b) G. Verniest and A. Padwa, Org.
Lett., 2008, 10, 4379; c) D. Aguilar, M. Contel, R. Navarro, T. Soler
and P. E. Urriolabeitia, J. Organomet. Chem., 2009, 694, 486; d) J. P.
Weyrauch, A. S. K. Hashmi, A. Schuster, T. Hengst, S. Schetter, A.
Littmann, M. Rudolph, M. Hamzic, J. Visus, F. Rominger, W. Frey and
J. W. Bats, Chem. Eur. J., 2010, 16, 956; e) O. A. Egorova, H. Seo, Y.
Kim, D. Moon, Y. M. Rhee and K. H. Ahn, Angew. Chem. Int. Ed. 2011,
50, 11446.
O
2a
2 mol%
O
O
5 mol% TsOH
O
10 11
+ (
/
)
3:1
DCM:ROH
10min
R
R
R'
9a-b
R' OR
ROH = MeOH, BnOH
or allylOH
12a-g
O
O
O
O
O
O
O
O
O
R, R' = Me
O
O
O
OAllyl
OBn
OMe
98%
14 A. S. K. Hashmi, A. M. Schuster, M. Schmuck and F. Rominger, Eur.
J. Org. Chem. 2011, 4595.
15 R. BabaAhmadi, P. Ghanbari, N. A. Rajabi, A. S. K. Hashmi, B. F. Yates
and A. Ariafard, Organometallics, 2015, 34, 3186.
12a
12b
12c
74% + (20%)
30% + (55%)
R = H
R' = OMe
O
O
O
O
O
O
O
O
O
O
O
O
16 M. Chiarucci and M. Bandini Beilstein J. Org. Chem., 2013, 9, 2586.
17 C. M. Krauter, A. S. K. Hashmi and M. Pernpointner, ChemCatChem,
2010, 2, 1226.
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Cárdenas and A. M. Echavarren, Angew. Chem. Int. Ed., 2004, 43,
2402.
O
O
BnO
OMe
OAllyl
OBn
51%
12d
12e
12f
68%
85%
Scheme 4 Substrate scope in enyne cycloisomerization, with isolated yields.
Yield in parenthesis is the combined yield of products 10 and 11.
19 Previously, an acid additive (HSbF6) has been reported to cause
similar mixture of isomers in the NHC-Au(I) catalysis: S. Ferrer, A. M.
Echavarren, Organometallics, 2018, 37, 781.
20 Z. Lu, J. Han, O. E. Okoromoba, N. Shimizu, H. Amii, C. F. Tormena, G.
B. Hammond and B. Xu, Org. Lett., 2017, 19, 5848.
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Koppel, I. Krossing and I. Leito, Chem. Sci. 2017, 8, 6964.
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Belmont, V. César and V. Michelet, Angew. Chem. Int. Ed., 2019, 58,
7977.
In conclusion, we have developed H-bond donor tethered
NHC-ligands for in situ activated L-Au(I)Cl catalysis. Ethyl tosyl
amide functionalised Au(I) complex 2a catalysed the oxazole
synthesis selectively from terminal alkynes, and successful
enyne cycloisomerization was accomplished with an acid
additive. Computational analysis supports the mechanistic
scenario that the H-bond donor ligand assists in the Au-Cl bond
activation.
Financial support from Academy of Finland [project no. 129062
(J.H.)] is acknowledged. The Finnish National Centre for Scientific
Computing (CSC) is recognized for computational resources. O.S.
acknowledges the Emil Aaltonen foundation for funding, grant
22 Y. Tang, I. Benaissa, M. Huynh, L. Vendier, N. Lugan, S. Bastin, P.
Belmont, V. César, V. Michelet Angew. Chem. Int. Ed. 2019, 58, 7977.
4 | J. Name., 2012, 00, 1-3
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