Assessing the influence of phosphine substituents on the catalytic properties of self-stabilised digold(i) complexes with supporting ferrocene phosphinonitrile ligands

Article abstract of DOI:10.1039/c9nj02555c

Gold(i) phosphine complexes are often used in catalysis, but the role of their auxiliary ligands still remains poorly understood. Thus, building on our previous research, we prepared a series of Au(i) complexes [Au2(μ-R2PfcCN)2][SbF6]2 (fc = ferrocene-1,1

Full text of DOI:10.1039/c9nj02555c

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Assessing the influence of phosphine substituents
on the catalytic properties of self-stabilised
digold(I) complexes with supporting ferrocene
phosphinonitrile ligands†
Cite this:DOI: 10.1039/c9nj02555c
ˇ
Ondrej Barta, Ivana Cısarova, Jirı Schulz and Petr Stepnicka
*
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´ ˇ ˇ´
´
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Gold(I) phosphine complexes are often used in catalysis, but the role of their auxiliary ligands still
remains poorly understood. Thus, building on our previous research, we prepared a series of Au(I)
complexes [Au2(m-R2PfcCN)2][SbF6]2 (fc = ferrocene-1,10-diyl) to assess the effect of phosphine groups
PR2 on the catalytic properties of these highly catalytically active, dimeric compounds. Catalytic testing
in Au-mediated cyclisation of N-propargyl amides to 2-substituted 5-methyleneoxazolines showed that
weaker donating phosphines gave rise to more active, albeit partly destabilised, catalysts. Nevertheless,
thanks to their self-stabilisation by reversible nitrile coordination, [Au2(m-R2PfcCN)2]+ cations readily
converted into catalytically active species (by dissociation) and, in addition, remained catalytically active
Received 17th May 2019,
Accepted 21st June 2019
DOI: 10.1039/c9nj02555c
1
even at very low metal loadings. The experimental results were supported by the trends in JPSe coupling
rsc.li/njc
constants for R2P(Se)fcCN as a measure of ligand basicity, and by DFT calculations.
(fc = ferrocene-1,10-diyl) using Au-mediated cyclisation of
Introduction
N-propargylamides to 5-methyleneoxazolines9 as a test reaction
Research interest in homogeneous gold-catalysed reactions and phosphine selenides to probe the ligand properties.10
has increased tremendously in recent decades, leading to
useful synthetic methods that efficiently exploit the ability of
Results and discussion
gold(I) species to activate unsaturated bonds via coordination.1
However, the influence of the auxiliary ligands (L) on the perfor- To achieve the goals of this study, additional phosphinonitrile
mance of the presumed active species LAu+ has been mostly ligands R2PfcCRN (1) bearing electronically distinct phos-
overlooked.2
phino substituents [R = iso-propyl (a), cyclohexyl (Cy, b), and
Recently, we have prepared dimeric complexes [Au2(m(P,N)-L)2]X2, 2-furyl (Fur; d)] were prepared to complement the archetypal
where L is 10-(diphenylphosphino)-1-cyanoferrocene (1c)3 or representative 1c.3,6 These compounds were obtained by successive
2-(diphenylphosphino)benzonitrile and X is a weakly coordinating lithiation/functionalisation11 of 1,10-dibromoferrocene (2), as shown
anion. These compounds are bench-stable and silver-free4,5 instant in Scheme 1.12 Ligands 1 were subsequently converted into
precursors of highly active Au(I) catalysts,6,7 which resemble the chloridogold(I) complexes 5 and then into the target digold(I)
commonly used pre-catalysts [(R3P)Au(MeCN)]X,2d,4 although their complexes [Au2(m(P,N)-R2PfcCN)2][SbF6]2 (6). To gain further
phosphinonitrile ligands provide both a strongly coordinating insight into the donor properties of ligands 1, selenides 7 were
donor moiety and a labile donor group. With previous work,8 we prepared from free phosphines via reactions with KSeCN13
have shown that the nitrile group plays a key role in determining (Scheme 1).
the catalytic activity of [Au2(m-L)2]X2 complexes. In this study,
In addition, syntheses performed on a relatively large scale,
we focus on the influence of phosphine substituents on the especially during the first steps, enabled us to isolate and
catalytic properties of complexes [Au2(m(P,N)-R2PfcCN)2][SbF6]2 ultimately characterise an intensely purple minor side-product
detected12 during the transformation of 2 into 3 as the cyanamide
Department of Inorganic Chemistry, Faculty of Science, Charles University,
Hlavova 2030, 128 40 Prague, Czech Republic. E-mail: petr.stepnicka@natur.cuni.cz
† Electronic supplementary information (ESI) available: Experimental details
including structure determination, additional structural diagrams and kinetic
plots, details on DFT calculations, copies of the NMR spectra and coordinates of
DFT computed structures. CCDC 1909789–1909795. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c9nj02555c
(Brfc)2CQNCRN (4). This compound arises by addition of the
intermediate BrfcLi to the cyanide group in the already formed
3, followed by reaction of the resulting imine salt with TsCN
(see the ESI†).
All compounds have been characterised by spectroscopic
methods and by elemental analysis. In their NMR spectra,
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Scheme 2 Au-catalysed cyclisation of propargyl amides 8 to oxazolines 9
[R = Ph (a), 4-MeC6H4 (b), 4-MeOC6H4 (c), 4-ClC6H4 (d), 4-CF3C6H4 (e),
and Cy (f)].
Au–P bonds in compounds with weaker donating phosphines
due to increased p-back donation (Au–P: 2.2361(6), 2.2319(7),
2.2287(6),6 and 2.2191(6) Å for 1a–d, respectively). Although
X-ray diffraction analysis confirmed the dimeric nature of 6 in
all cases, the structure of 6a could not be satisfactorily refined
due to disorder. The Au–P distances determined for 6b and 6d
are very close to those determined for 5b and 5d, while the CN
bond lengths of the coordinated nitrile unit show virtually no
difference from those in the respective free phosphines 1.
Complexes 6 were examined as (pre)catalysts in the Au-catalysed
cyclisation of propargyl amides 8 to 5-methylene-4,5-dihydrooxazoles
(oxazolines) 9 (Scheme 2).9 Gratifyingly, the cyclisation of the
prototypical substrate 8a proceeded selectively with all catalysts 6
to give 9a as the sole product, thus enabling us to monitor the
Scheme 1 Synthesis of complexes 6 and phosphine selenides 7 [R = iso-
propyl (a), cyclohexyl (b), phenyl (c), and 2-furyl (d); Ts = 4-toluenesulfonyl,
tht = tetrahydrothiophene]. Note: only compounds 1c, 5c and 6c have
been previously reported.
phosphines 1 showed characteristic signals due to the ferrocene
unit and to the phosphine substituents; the 31P NMR signals were
observed at dP 0.1 (1a), ꢀ8.0 (1b), ꢀ17.7 (1c)3 and ꢀ66.1 (1d) ppm.
The 13C NMR signals of the CN groups occurred at dC E 120, while
the IR bands attributable to the nCRN modes were observed in the
narrow range of 2224–2228 cmꢀ1. Coordination of 1, yielding
chloridogold(I) complexes 5, resulted in a shift of the 31P NMR
signals to a lower field, while the signature of the CN substituents
changed only marginally. Upon converting 5 into dimers 6, the
31P resonances shifted to slightly higher fields, while the nCRN
bands moved to higher energies.14
The molecular structures of 1d, 5d and 6d are depicted in
Fig. 1; those of the chloridogold(I) complexes 5a, 5b and 5d and
of dimer 6b are presented in the ESI,† along with additional
data and structural diagrams. The gold(I) ions in complexes 5
are linearly coordinated (Cl–Au–P = 175–1771), showing shorter
1
reaction conveniently by H NMR spectroscopy (Fig. S7, ESI†).
The resulting oxazolines were isolated, but they slowly decom-
posed when exposed to air.15
All pre-catalysts 6 gave full conversions within 3 h when used at
1 mol% Au loading (25 1C, [8a]0 = 0.25 M, CD2Cl2). Nonetheless, the
catalysts showed different kinetic profiles. In particular, cyclisation
reactions with catalysts bearing aromatic phosphine substituents
proceeded faster than those with catalysts bearing dialkylphosphine
moieties (Fig. 2). Although relatively minor, the difference in
performance between the catalysts can be easily demonstrated
by comparing the corresponding yields of 9a after 1 h, which were
approximately 90% for 6c and 6d, and 70% for 6a and 6b. When
using the common pre-catalyst [Au(PPh3)(MeCN)][SbF6], the cyclisa-
tion reaction also reached full conversion within 3 h, though at an
intermediate rate between 6c/6d and 6a/6b. The reaction rates (k)
determined by fitting the ln([8]/[8]0) vs. time plots (see ESI†)
corroborate this semi-quantitative assessment (Table 1).
Fig. 1 View of the molecular structures of 1d, 5d and of the complex
cation in the structure of 6d. Additional plots and further structural data are
available in the ESI.†
Fig. 2 Kinetic profiles of the cyclisation of 8a to 9a catalysed by different
Au(I) catalysts ([8a]0 = 0.25 M in CD2Cl2, 25 1C, [Au] = 1 mol%).
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Table 1 Reaction rates of the model cyclisation of substrate 8a and additional parameters illustrating the properties of ligands 1 and their Au(I)
complexes
Ligand
Parameter
1a
1b
1c
1d
PPh3
ka [10ꢀ3 minꢀ1
]
20.8(2)
2224
2263
n.a.
n.a.
27.4
715
20.2(2)
2225
2256
2.242 (av.)
2.050 (av.)
26.8
32.5(7)
2225e
32.9(10)
2228
2274
2.2139(7)
2.032(2)
32.9
25.7(7)
n.a.
nCN for 1b [cmꢀ1
nCN for 6b [cmꢀ1
Au–P for 6c [Å]
Au–N for 6c [Å]
]
]
2273 f
n.a.
2.225(2) f
2.035(4) f
29.4
2.228(1)g
2.038(5)g
n.a.
Edis for 6d [kcal molꢀ1
]
1JSeP for 7 (CDCl3) [Hz]
708
741
777
735h
a
Rate constants of the cyclisation of 8a. Conditions: [8]0 = 0.25 M in CD2Cl2, 1 mol% Au, and 25 1C. Determined from fitting the kinetic profile in
b
c
the range of 10–60 min. IR spectra recorded in Nujol mulls. Au–P and Au–N distances from X-ray diffraction analysis at 120 or 150 K.
d
e
f
g
h
Dissociation energies of isolated dimeric cations calculated by DFT. Ref. 3. Ref. 6. Data on [(Ph3P)Au(MeCN)][SbF6] from ref. 16. Ref. 17.
Fig. 4 Kinetic profile of the cyclisation of 8a to 9a mediated by 6d in
which additional 8a was added after 1 h of reaction (indicated by a red
arrow).
Fig. 3 Kinetic (ln([8a]/[8a]0) vs. time) plots of representative catalysts 6a
and 6d illustrating the departure from first-order behaviour.
Further measurements indicated that the cyclisation is a first-
order reaction with respect to the amide substrate (Fig. 3 and Fig. S8, proceeded reasonably well also under much lower catalyst
ESI†). Our findings thus contrast with previous measurements using loadings (cf. 65% conversion after 3 h with 0.25 mol% Au).
[Au(PR3)(OTf)] catalysts, which showed a linear decrease in substrate Remarkably, upon further decreasing the catalyst amount, the
concentration over time (0th order reaction).2b Such divergence, reaction slowed down but did not stop. Even with as little as
which was confirmed by independent experiments using the 0.03 mol% Au, the NMR yield of 9a was 4% after 90 min, and
[Au(PPh3)(OTf)] catalyst, may be attributed to the anion effect the reaction followed pseudo first-order kinetics. In other
([SbF6]ꢀ vs. OTfꢀ)18 and, mainly, to the stabilising role of the nitrile words, we have not observed any threshold value below which
moiety in 6, whose coordination can prevent decomposition and the reaction did not proceed.20 Instead, the reaction rate
the formation of gold(I) bis(phosphine) cations or similar species.19 tended towards zero at very low catalyst loadings (Fig. 5). This
The ln([8]/[8]0) vs. time plots for catalysts 6 were linear until observation again reflects the unique self-stabilising ability of
approximately 90% conversion, subsequently departing from a the Au(1)+ species resulting from reversible dimerisation by
linear relationship, which was more pronounced for the faster nitrile coordination. Unsurprisingly, strongly binding ligands
reacting catalysts (see Fig. 3 and Fig. S8, ESI†). This observation affected this process. For instance, addition of a stoichiometric
can be explained by catalyst decomposition, which is faster for the amount of chloride ions, which coordinate Au(I) stronger than
more reactive catalysts. Indeed, when an additional equivalent of alkynes,19 stopped the reaction, even at a relatively high catalyst
substrate 8a was added to the reaction mixture at nearly full concentration (1 mol% Au, [8a]0 = 0.25 M).
conversion (90%), using 1 mol% Au in the form of 6d, the reaction
continued but at a slower rate (approximately half of the original auxiliary ligands give rise to more active gold catalysts.21 The
reaction rate, see Fig. 4). reduced donor ability (basicity) of 1c and 1d is clearly expressed
On the whole, our results confirm that weaker donating
1
Additional reaction tests performed with the most efficient by the relatively large JPSe coupling constants (see Table 1),
catalyst 6d showed that the cyclisation is a first-order reaction which also suggest that the cyano substituent further reduces
in the catalyst (Fig. S9 and S10, ESI†). Using 2 mol% of Au, the the donor properties of the entire ferrocene ligand (compare
reaction with catalyst 6d reached full conversion within 1 h but the 1JPSe value of 1d in Table 1 with the value for FcP(Se)Ph2 of
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Table 2 Reactions rates (k) for the cyclisation of different substrates 8a
Substrate
k [10ꢀ3 minꢀ1
]
Substrate
k [10ꢀ3 minꢀ1
]
8a (Ph)
8b (4-MeC6H4)
8c (4-MeOC6H4)
30.9(7)
29.2(11)
25.1(15)
8d (4-ClC6H4)
8e (4-CF3C6H4)
8f (cyclohexyl)
45.5(3)
41.3(5)
12.4(3)
a
Conditions: [8]0 = 0.125 M, catalyst 6d (1 mol% Au), and reaction in
CD2Cl2 at 25 1C. The rate constants were determined by fitting the
kinetic profile over the range 10–60 min.
Indeed, such behaviour matches the proposed reaction
mechanism,9a which suggests that the coordination of the
substrate’s triple bond is the initial step. Although the acyl
groups in 8 are located far from the alkyne moiety (and are
separated by the non-conjugated methylene spacer), they can
nevertheless affect other reaction steps (for instance, the
nucleophilic attack of the oxygen atom on the coordinated
triple bond or the final protodeauration by changing the
partial charge at the oxygen atom and the acidity of the
NH hydrogen). Conversely, the reaction with amide 8f was
considerably slower (E80% conversion in 3 h), presumably
due to the lack of stabilisation of the reaction intermediates by
p-conjugation.
Fig. 5 Variation of the relative reaction rate (krel = k/k1mol%) as a function
of the concentration of gold in the reaction system ([8a]0 = 0.25 M, catalyst
6d).
733 Hz,22 Fc = ferrocenyl). As stronger p-accepting ligands, 1c and
1d form relatively shorter (stronger) Au–P bonds in dimers 6 while
the nCRN bands of 6c and 6d are shifted to higher energies due to
stronger s-donation from a weakly antibonding molecular orbital
corresponding to the lone pair at nitrogen.23 This blue shift in
nCRN frequencies is in line with the strengthening of the CN - Au
interactions suggested by DFT calculations, which show that
the energy cost associated with the dissociation of dimers 6
([Au2(m-1)2]2+ - 2[Au(1-kP)]+) is larger in complexes with
weaker donating phosphine ligands (1b o 1a o 1c o 1d).
Another set of experiments aimed at elucidating the influence of
the acyl groups in the substrate on the course of the cyclisation has
been performed with amides bearing different substituents in
position 4 of their benzene rings, compounds 8a–e, and with their
aliphatic analogue 8f (Scheme 2). All aromatic amides were effi-
ciently converted into the corresponding oxazolines when using 6d
as the pre-catalyst (1 mol% Au) under standard conditions (Fig. 6).
Even though compounds with electron-withdrawing substituents
reacted faster, the reaction rates (Table 2) did not correlate with
Hammett’s constants or with analogous substituent parameters.
Conclusions
The presence of a nitrile group as a secondary donor moiety in
the molecules of supporting phosphine ligands 1 markedly
influences the catalytic behaviour of cations [Au2(m-1)2]2+. These
cations, which become active catalysts by simple dissociation,
can form again and thus stabilise the active species. Such self-
stabilisation markedly differentiates [Au2(m-1)2]2+ cations from
their conventional counterparts [(R3P)Au(R0CN)]+, primarily by
their high catalytic activity, which is preserved even at very low
catalyst concentrations, and by the minimised influence of the
counter ion (anion coordination is suppressed by the compet-
ing nitrile donor). The catalytic activity of [Au2(m-1)2]2+ can be
further increased by introducing less basic phosphine moieties
into the structure of the supporting phosphinonitrile ligands.
However, such a change decreases the overall chemical stability
of the [Au2(m-1)2]2+ species. Therefore, a judicious choice of the
phosphine substituents is essential for achieving optimal cat-
alytic results with these catalysts.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This research was supported by the Czech Science Foundation
(project no. 17-02495S), by the Grant Agency of Charles Uni-
versity (project no. 130317), and by the Charles University
Research Centre program (project no. UNCE/SCI/014).
Fig. 6 Kinetic profiles of the cyclisation reactions of different substrates:
black – 8a, red – 8b, yellow – 8c, 8d – green, 8e – blue, and 8f – cyan.
Conditions: [8]0 = 0.125 M in CD2Cl2, catalyst 6d (1 mol% Au), and 25 1C.
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M. Rudolph, M. Hamzic, J. Visus, F. Rominger, W. Frey and
J. W. Bats, Chem. – Eur. J., 2010, 16, 956–963.
Notes and references
1 Selected reviews: (a) G. Dyker, Angew. Chem., Int. Ed., 2000,
39, 4237–4239; (b) A. S. K. Hashmi, Chem. Rev., 2007, 107,
3180–3211; (c) A. Fu¨rstner and P. W. Davies, Angew. Chem.,
Int. Ed., 2007, 46, 3410–3449; (d) Z. Li, C. Brouwer and C. He,
Chem. Rev., 2008, 108, 3239–3265; (e) N. D. Shapiro and
F. D. Toste, Synlett, 2010, 675–691; ( f ) R. Dorel and
A. M. Echavarren, Chem. Rev., 2015, 115, 9028–9072;
10 (a) C. A. Tolman, Chem. Rev., 1977, 77, 313–348; (b) D. W.
Allen and B. F. Taylor, J. Chem. Soc., Dalton Trans., 1982,
51–54; (c) A. Muller, S. Otto and A. Roodt, Dalton Trans.,
2008, 650–657; (d) U. Beckmann, D. Su¨slu¨yan and P. C.
Kunz, Phosphorus, Sulfur Silicon Relat. Elem., 2011, 186,
2061–2070.
11 (a) I. R. Butler and R. L. Davies, Synthesis, 1996, 1350–1354;
¨
(g) D. Pflasterer and A. S. K. Hashmi, Chem. Soc. Rev.,
ˇ
ˇ
ˇ
(b) P. Stepnicka in Ferrocenes: Ligands, Materials and Bio-
2016, 45, 1331–1367; (h) W. Zi and F. D. Toste, Chem. Soc.
Rev., 2016, 45, 4567–4589; (i) A. S. K. Hashmi and
F. D. Toste, Modern Gold Catalyzed Synthesis, Wiley-VCH,
Weinheim, 2012; ( j) F. D. Toste and V. Michelet, Gold
Catalysis: An Homogeneous Approach, Imperial College Press,
London, 2014.
2 (a) D. J. Gorin, B. D. Sherry and F. D. Toste, Chem. Rev., 2008,
108, 3351–3378; (b) W. Wang, G. B. Hammond and B. Xu,
J. Am. Chem. Soc., 2012, 134, 5697–5705; (c) D. Malhotra,
G. B. Hammond and B. Xu, Top. Curr. Chem., 2015, 357, 1–23;
(d) B. Ranieri, I. Escofet and A. M. Echavarren, Org. Biomol.
Chem., 2015, 13, 7103–7118; (e) C. A. Gaggioli, G. Ciancaleoni,
D. Zuccaccia, G. Bistoni, L. Belpassi, F. Tarantelli and
P. Belanzoni, Organometallics, 2016, 35, 2275–2285;
( f ) A. H. Christian, Z. L. Niemeyer, M. S. Sigman and
F. D. Toste, ACS Catal., 2017, 7, 3973–3978.
ˇ
ˇ
ˇ
molecules, ed. P. Stepnicka, Wiley, Chichester, 2008, ch. 5,
pp. 177–204.
ˇ
ˇ
ˇ
´ ˇ
´
´
ˇ
12 K. Skoch, I. Cısarova, F. Uhlık and P. Stepnicka, Dalton
Trans., 2018, 47, 16082–16101.
13 P. Nicpon and D. W. Meek, Inorg. Chem., 1966, 5, 1297–1298.
14 Coordination of a nitrile moiety is typically associated with
a shift of the nCRN band towards higher energies as the
result of electron density removal from a weakly antibond-
ing molecular orbital (lone pair at N). p-Back donation,
which increases the population of antibonding orbitals, is
rather weak. For references, see: K. Nakamoto, Infrared and
Raman Spectra of Inorganic and Coordination Compounds,
Part B: Applications in Coordination, Organometallic, and
Bioinorganic Chemistry, Wiley, New York, 5th edn, 1997,
sect. III-15, pp. 105–115.
15 A. S. K. Hashmi, M. C. B. Jaimes, A. M. Schuster and
F. Rominger, J. Org. Chem., 2012, 77, 6394–6408.
ˇ
ˇ
´ ˇ
´
ˇ
ˇ
3 K. Skoch, I. Cısarova and P. Stepnicka, Inorg. Chem., 2014,
53, 568–577.
´
´
16 E. Herrero-Gomez, C. Nieto-Oberhuber, S. Lopez, J. Benet-
Buchholz and A. M. Echavarren, Angew. Chem., Int. Ed.,
2006, 45, 5455–5459.
4 H. Schmidbaur and A. Schier, Z. Naturforsch., 2011, 66b,
329–350.
5 Additional references: (a) D. Wang, R. Cai, S. Sharma,
J. Jirak, S. K. Thummanapelli, N. G. Akhmedov, H. Zhang,
X. Liu, J. L. Petersen and X. Shi, J. Am. Chem. Soc., 2012, 134,
9012–9019; (b) A. Homs, I. Escofet and A. M. Echavarren,
Org. Lett., 2013, 15, 5782–5785; (c) Z. Lu, J. Han,
G. B. Hammond and B. Xu, Org. Lett., 2015, 17, 4534–4537.
17 R. P. Pinnell, C. A. Megerle, S. L. Manatt and P. A. Kroon,
J. Am. Chem. Soc., 1973, 95, 977–978.
18 (a) D. Zuccaccia, L. Belpassi, A. Macchioni and F. Taranteli,
Eur. J. Inorg. Chem., 2013, 4121–4135; (b) M. Jia and
M. Bandini, ACS Catal., 2015, 5, 1638–1652; (c) J. Schiessl,
¨
J. Schulmeister, A. Doppiu, E. Worner, M. Rudolph,
ˇ
ˇ
´ ˇ
´
ˇ
ˇ
6 K. Skoch, I. Cısarova and P. Stepnicka, Chem. – Eur. J., 2015,
R. Karch and A. S. K. Hashmi, Adv. Synth. Catal., 2018,
360, 2493–2502.
21, 15998–16004.
7 B. Michelet, D. Lebœuf, C. Bour, K. Skoch, F. Hork´y, P. Stepnicka
ˇ
ˇ
ˇ
ˇ
¨
19 A. Zhdanko, M. Strobele and M. E. Maier, Chem. – Eur. J.,
and V. Gandon, ChemPlusChem, 2017, 82, 442–448.
2012, 18, 14732–14744.
20 M. Kumar, G. B. Hammond and B. Xu, Org. Lett., 2014, 16,
3452–3455.
ˇ
ˇ
ˇ
´ ˇ
´
ˇ
8 K. Skoch, I. Cısarova, J. Schulz, U. Siemeling and P. Stepnicka,
Dalton Trans., 2017, 46, 10339–10354.
9 (a) A. S. K. Hashmi, J. P. Weyrauch, W. Frey and J. W. Bats, Org.
Lett., 2004, 6, 4391–4394; (b) A. S. K. Hashmi, M. Rudolph,
S. Schymura, J. Visus and W. Frey, Eur. J. Org. Chem., 2006,
4905–4909; (c) A. S. K. Hashmi, A. M. Schuster and F. Rominger,
Angew. Chem., Int. Ed., 2009, 48, 8247–8249; (d) J. P. Weyrauch,
21 Teles et al. concluded similarly in their seminal paper from
1998: J. H. Teles, S. Brode and M. Chabanas, Angew. Chem.,
Int. Ed., 1998, 37, 1415–1418.
22 B. Milde, M. Lohan, C. Schreiner, T. Ruffer and H. Lang,
¨
Eur. J. Inorg. Chem., 2011, 5437–5449.
A. S. K. Hashmi, A. Schuster, T. Hengst, S. Schetter, A. Littmann, 23 M. L. Kuznetsov, Russ. Chem. Rev., 2002, 71, 265–282.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019
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