Gold(I) Complexes Nuclearity in Constrained Ferrocenyl Diphosphines: Dramatic Effect in Gold-Catalyzed Enyne Cycloisomerization

Article abstract of DOI:10.1002/asia.202000579

Di-tert-butylated-bis(phosphino)ferrocene ligands bearing phosphino substituents R (R=phenyl, cyclohexyl, iso-propyl, mesityl, or furyl) allow tuning the selective formation of Au(I) halide complexes. Thus, dinuclear linear two-coordinate, but also rare mononuclear trigonal three-coordinate and tetrahedral four-coordinate complexes were formed upon tuning of the conditions. Both Au(I) chloride and rarer Au(I) iodide complexes were synthesized, and their X-ray diffraction analysis are reported. The significance of the control of structure and nuclearity in Au(I) complexes is further illustrated herein by its strong effect on the efficiency and selectivity of gold-catalysed cycloisomerization. Cationic linear digold(I) bis(dicyclohexylphosphino) ferrocenes outperform other catalysts in the demanding regioselective cycloisomerization of enyne sulphonamides into cyclohexadienes. Conversely, tetrahedral and trigonal cationic monogold(I) complexes were found incompetent for enyne cycloaddition. We used the two-coordinate linear electron-rich Au(I) complex 2 b (R=Cy) to extend the scope of selective intramolecular cycloaddition of different 1,6-enyne sulfonylamines with high activity and excellent selectivity to the endo cyclohexadiene products.

Full text of DOI:10.1002/asia.202000579

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Gold(I) Complexes Nuclearity in Constrained Ferrocenyl
Diphosphines: Dramatic Effect in Gold-Catalyzed Enyne
Cycloisomerization
Authors: T.-Anh Nguyen, Julien Roger, Houssein Nasrallah, Vincent
Rampazzi, Sophie Fournier, Hélène Cattey, Paul Fleurat-
Lessard, Charles H Devillers, Nadine Pirio, Dominique Lucas,
Jean Cyrille Hierso, and E. Daiann Sosa-Carrizo
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10.1002/asia.202000579
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Gold(I) Complexes Nuclearity in Constrained Ferrocenyl
Diphosphines: Dramatic Effect in Gold-Catalyzed Enyne
Cycloisomerization
Tuan-Anh Nguyen,[a] Julien Roger,*[a] Houssein Nasrallah,[a] Vincent Rampazzi,[a] Sophie Fournier,[a]
Hélène Cattey,[a] E. Daiann Sosa Carrizo,[a] Paul Fleurat-Lessard,[a] Charles H. Devillers,[a] Nadine Pirio,[a]
Dominique Lucas,[a] and Jean-Cyrille Hierso*[a]
[a]
T.-A. Nguyen, Dr. J. Roger, Dr. H. Nasrallah, Dr. V. Rampazzi, S. Fournier, Dr. H.Cattey, Dr. E. D. Sosa Carrizo, Prof. Dr. P. Fleurat-Lessard, Dr. C. H.
Devillers, Prof. Dr. N. Pirio, Prof. Dr. D. Lucas, Prof. Dr. J.-C. Hierso
Université de Bourgogne, Institut de Chimie Moléculaire de l’Université de Bourgogne, UMR-CNRS 6302 – Université Bourgogne Franche-Comté (UBFC)
9, avenue Alain Savary 21078 Dijon (France). E-mail: julien.roger@u-bourgogne.fr; jean-cyrille.hierso@u-bourgogne.fr
Abstract: Di-tert-butylated-bis(phosphino)ferrocene ligands bearing
phosphino substituents R (R = phenyl, cyclohexyl, iso-propyl, mesityl,
or furyl) allow tuning the selective formation of Au(I) halide complexes.
Thus, dinuclear linear two-coordinate, but also rare mononuclear
trigonal three-coordinate and tetrahedral four-coordinate complexes
were formed upon tuning of the conditions. X-ray diffraction analysis
of the resulting complexes are reported and both Au(I) chlorides and
rarer iodides were synthesized. The significance of the control of
structure and nuclearity in Au(I) complexes is further illustrated herein
by its strong effect on the efficiency and selectivity of gold-catalysed
and gem-diaurated species.[6,11] Advances in controlling the
formation of gold complexes from suited structuring polyfunctional
ligands is highly attractive for all these applications and especially
digold complexes formation.
cycloisomerization.
Cationic
linear
digold(I)
bis(dicyclohexylphosphino) ferrocenes outperform other catalysts in
the demanding regioselective cycloisomerization of enyne
sulphonamides into cyclohexadienes. Conversely, tetrahedral and
trigonal cationic monogold(I) complexes were found incompetent for
enyne cycloaddition. We used the two-coordinate linear electron-rich
Au(I) complex 2b (R = Cy) to extend the scope of selective
intramolecular cycloaddition of different 1,6-enyne sulfonylamines
with high activity and excellent selectivity to the endo cyclohexadiene
products.
Chart 1. General coordination modes of gold(I) with diphosphines
(top); Typical models for ferrocenyl diphosphines (bottom).
Introduction
Structural control towards the formation of gold(I) phosphine
complexes has been early on investigated using a bulky ferrocene
(Fc) platform. Among other valuable properties, ferrocenyl
phosphine ligands are widely functionalizable, redox active, and
amenable to chirality. The pioneering studies from the groups of
Hill,[12] Laguna,[13] Hor,[14] and others[8b,15] have established that by
the coordination of bis(diphenylphosphino)ferrocene ligand (dppf)
to Au−X fragments (X = Cl, I) digold(I) halide complexes or
polymers are formed. Originally, these complexes showed no
closed-shell d10−d10 intramolecular aurophilic Au···Au interaction
in the solid state (Chart 1, a1, c1 coordination modes).[12-15]
Looking for a better ligand-controlled gold complexes formation,
we recently used the conformation control of di-tert-butylated-
poly(phosphino)ferrocene ligands to favour the formation of
Au···Au intramolecular aurophilic interactions between Au–Cl
fragments in dinuclear and polynuclear gold(I) complexes (Chart
1, a2).[16] This conformational control, which clearly contrasts the
dppf coordination to gold, was achieved by the introduction of
bulky tert-butyl groups on the ferrocene platform (Chart 1, ligands
1-5).
Gold complexes activate unsaturated carbon-carbon π-bonds as
electrophiles, and gold is regarded as a mild carbophilic Lewis
acid highly suitable in atom-economic cyclization processes.[1-3]
Accordingly, ligand effect in homogeneous gold catalysis became
a topical subject of great interest.[4-6] AuI species are in large
majority stabilized in a two-coordinate linear geometry, which
contrasts with d10 AgI and CuI compounds for which the
complexes with coordination number greater than two are more
frequent.[7] Thus, gold(I) coordination schemes with
(bis)phosphino ligands mainly concerns mononuclear and
dinuclear two-coordinate linear complexes (Chart 1, A and B).
Comparatively, a small number of three-coordinate planar (C),[8]
and four-coordinate tetrahedral (D) gold(I) halide complexes
exist.[9,10] In addition to gold-catalysed C–C bond formation,
relevant photophysical,[8a,10a] and biomedical applications[10d]
requires a pertinent ligand control[9d] in forming AuI well-defined
complexes. Attractive closed-shell interactions between d10−d10
pairs of atoms have received increasing attention and gold
dinuclear cooperative properties and reactivity have been
evidenced, which encompass catalysis from dual -activation
1
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We now report the selective synthesis and structural
characterization (XRD) of AuI halide complexes (Cl, I), in which
each of the known structures (i. e. linear two-, trigonal three- and
tetrahedral four-coordinate; Chart 1, a2, c2, d) can be formed
depending on the same class of bulky ferrocenyldiphosphine
ligands. Both the substituents on phosphino groups and the halide
involved in the synthesis play a decisive role. The significance of
the control of structure and nuclearity in AuI complexes is further
illustrated herein by its strong effect on the efficiency and
selectivity of the elusive gold-catalysed cycloisomerization of
enyne sulphonamide to its corresponding cyclohexadiene.
while the constraints from t-Bu groups mostly favoured P–
Au…Au–P pairs proximity and thus aurophilic interactions.[16] In
XRD, the Au…Au distance is indicative of the presence, but is not
commensurate with the strength of aurophilic interactions.
Aurophilic interactions are mostly weak (comparable to H bonding
c.a. 5-10 kcal mol−1) and the sum of other weak interactions such
as crystal packing forces may play a role in the situation for 3a
and 4a (and partially 2a) in which no or weaker aurophilic
interactions are found. Notably, a strong electron-donating effect
of i-Pr, Mes, and Cy groups at each AuI center is common to these
complexes, which might possibly help offsetting M…M attractive
interactions.
Gold chloride complexes are by far the most often reported, and
we were intrigued by the influence of halides in the formation of
AuIX. The dinuclear complexes 1b-4b were isolated from the
reaction of two equiv of AuI with diphosphines 1-4 (Scheme 2,
yields 48-99%).
Results and Discussion
The ferrocenyl diphosphines rac-1-5 {Chart 1, t-Bu-Fc[PR2]2, R =
phenyl (Ph) 1, cyclohexyl (Cy) 2, iso-propyl (i-Pr) 3, mesityl (Mes)
4, [5-methyl]-2-furyl (Fu) 5},[17] where used for synthesizing mono-
and dinuclear gold complexes. We investigated the synthesis and
structure of dinuclear gold(I) halide complexes formed from 1-5,
paying special attention to aurophilic interactions formation. The
conformation control of Fc platform avoids the formation of gold
polymeric chains (Chart 1, c1). Two equiv of [AuCl(SMe2)] reacted
with diphosphines 1-5 formed dinuclear gold complexes 1a to 5a
in high yield (>95%, Scheme 1).
Scheme 2. Two-coordinate gold(I) iodide complexes 1b-4b and illustrative
molecular structures for 2b and 3b (H omitted for clarity). Ortep of 4b is available
in SI). Analogue 5b could not be obtained in pure form.
From XRD analysis, none of the iodide complexes 2b-4b
showed intramolecular aurophilic interaction (1b could not be
crystallized). This absence in 2b was a notable difference from
chloride analogue 2a. Gold(I) complexes 2a and 2b were thus
subjected to preliminary electrochemical analyses to possibly
identify on their redox properties further unexpected halide effect.
The cyclic voltammogram (CV) of the chlorinated complex 2a
(Figure 1) exhibits a single reversible Fc-centred oxidation at E1/2
= 0.49 V vs Fc/Fc+ (O1/R1 peaks, ΔEp = 90 mV), which is similar
to the analogous digold complexes [dppf(AuCl)2] stabilized by
1,1′-bis(phosphino)ferrocene ligands (dppf) reported by the group
of Nataro.[18] Conversely, on the CV of the iodinated complex 2b
two oxidation waves are observed. For complex 2b the first
system centered on the Fc moiety is not fully reversible (O2/R2
peaks, Epa = 0.43 and Epc = 0.28 V vs Fc/Fc+, ΔEp = 150 mV),
Scheme 1. Two-coordinate AuCl complexes 1a-5a and illustrative molecular
structures for 2a (major isomer 66%) and 4a (H, and one CH2Cl2 for 2a omitted
for clarity). Ortep of 1a and 3a are available in SI. XRD data for 5a were
previously reported.[16]
As expected, the XRD structures of complexes 1a (R = Ph), 2a
(Cy) and 5a (Fu) evidenced intramolecular aurophilic interaction
Au1…Au2 = 3.1790(4), 3.212(4), and 3.2349(6) Å, respectively
(see SI). Compound 2a (R = Cy) also crystallizes in a 34% second
minor isomeric form which displayed a weaker Au…Au interaction
(gold–gold separation 3.457(6) Å). By comparison, the dinuclear
complex formed from the meso-1 stereoisomer of 1 showed a
shorter aurophilic interaction Au1…Au2 = 3.0781(6) Å.[16] In the
solid state, complexes 3a (i-Pr) and 4a (Mes) adopted a
conformation that precludes intramolecular aurophilic interaction,
while the second redox system is reversible (O3/R3 peaks, E1/2
=
0.63 V vs Fc/Fc+, ΔEp = 90 mV). We attributed the lack of
reversibility for the first redox system in 2b to a fast follow-up
chemical reaction consecutive to the oxidation of the Fc moiety
2
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10.1002/asia.202000579
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and where the evolution product is oxidized at higher potential
(peak O3).[19]
Figure 1. CV scans for the oxidation of 0.5 mM 2a and 2b in CH2Cl2 at 20 °C
with 0.1 M (n-Bu)4NPF6 at 100 mV/s.
Scheme 3. Three-coordinate trigonal planar mononuclear AuII iodide
complexes 1c-3c and Cl analogue 2d, and illustrative molecular structures for
1c (H and one CH2Cl2 omitted for clarity) and 2c (depicted here one of the two
molecules of the asymetric unit). Ortep of 3c is available in ESI.
This redox behavior, previously unseen in gold(I) halide
complexes constitutes –in addition to the absence of aurophilic
interactions detected in the solid state– another noticeable
difference between these digold(I) halides. The first oxidation
potential for iodide 2b is significantly less positive than for chloride
2a, which is in full agreement with the electron-withdrawing effect
of I vs Cl.
Iodide precursor salts are preferable for the formation and stability
of these monogold compounds. From ferrocenyl diphosphines 1-
5 only the bis(dicyclohexyl)phosphino-substituted 2 led to the
formation of the mononuclear compounds of c2 type with AuICl,
illustrating a case of nuclearity controlled by tuning ferrocenyl
diphosphine ligands. This was further supported since we
eventually achieved the selective and quantitative synthesis of the
trimetallic four-coordinate tetrahedral AuI cationic complexes 5e,f
(Figure 2, top) by using 0.5 equiv of gold halide precursors with
bis(difurylphosphino)ferrocene ligand 5. Under the same
conditions, 1,1'-bis(phosphino)ferrocenes 1-4 did not form the
equivalent trimetallic four-coordinate complexes. The gold(I)
cationisation by spontaneous halide abstraction –without the
action of external additive such as sodium reagent, or silver salt–
was confirmed by NMR, without notable halide effects. Complex
5e crystallized as a gold-centered regular tetrahedron[10a] (dAu–P
= 2.3806(10) to 2.3874(10) Å, P–Au–P = 105.59(3)–114.11(4)°)
in the monoclinic P21/c group (Figure 2, middle), with an
intermetallic distance of 4.228(4) and 4.2331(8) Å between the
centers in the linear FeII…AuI…FeII motif. The structure seems to
be stabilized by - interactions between the four pairs of furyl
rings with centroid distances ranges between 3.481(3)–3.791(3)
Å. Weak bonding interactions were confirmed by analysing the
Non Covalent Interactions (NCI) in 5e,f (see Figure 2, bottom).
We further addressed the challenging issue of selective
synthesizing of mononuclear gold complexes from chelating
ferrocenyl diphosphines 1-5. We were intrigued about the typical
model that might be reachable (Chart 1, B, or c1 or c2). Are well-
defined monomeric three-coordinate gold mononuclear 1,1′-
bis(phosphino)ferrocenes accessible,[8,16,20] and does a selective
formation of stable species could be favoured? Under our
synthetic conditions, by reacting one equivalent of gold salt (either
AuCl or AuI) the diphosphines 4 and 5 unfortunately gave
intractable mixtures of complexes and uncoordinated Fc. Though,
we were glad to isolate from 1-3 the gold iodide mononuclear
complexes 1c-3c in high yield (85 to 97%). While the analogous
gold chloride 2d could be also isolated pure in 61% yield (Scheme
3), the selective synthesis of gold chloride mononuclear
complexes from ferrocenyldiphosphine 1 and 3 failed. Single
crystals of 1c allowed a XRD analysis that ascertained a c2
coordination mode with a fairly closed angle P1−Au−P2 =
115.33(5) °. Complexes 2c (two molecules with the same
conformation are present in the crystal unit, see SI) and 3c were
characterized by XRD, with trigonal P1−Au1−P2 = 117.77(5) °
and P3−Au2−P4 = 117.70(5) °, and P1−Au1−P2 = 116.75(3) °,
respectively. In the solid state, the gold chloride 2d achieved a
much larger trigonal angle P1−Au−P2 = 122.00(5) °. The
formation in good yield of the four trigonal planar mononuclear 1c-
3c and 2d, demonstrates that conformationally constrained
ferrocenyl diphosphines are a reliable class of ligands to
selectively produce these rare chelating three-coordinate gold(I)
mononuclear species.
3
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investigated comparatively the activity in intramolecular
cycloaddition of the 1,6-enyne 6 of AuI complexes 1a-c to 5a-f, to
further assess the interest in the structural control they provide.
Table 1. Intramolecular cycloaddition of 1,6-enyne 6.
Entry
Catalyst
Conversion
Selectivity
(2 mol%)
in 7 + 8 (%)
in 7 (%)
–
1[1e]
[PPh3AuNCMe+][SbF6
AuIa
]
50d
6d
<5d
0
91
/
2
3
AuCl(SMe2)a
/
b,c
4
AgSbF6
/
5
(2) FcPCy2c
0
/
6
(5e) [Au-(FcPFu2)2+][Cl-]
(5f) [Au-(FcPFu2)2+][I-]
(1c) [AuI-FcPPh2]
0
0
7
0
0
8
0
/
9
(2c) [AuI-FcPCy2]
16
6
71
/
10
11
12
13
14
15
16
17
18
19
20
(2d) [AuCl-FcPCy2]
(3c) [AuI-FcPi-Pr2]
30
19
33
72
88
65
23
49
41
43
77
77
92
86
87
83
88
80
80
81
(1a) [Au2Cl2-FcPPh2]
(1b) [Au2I2-FcPPh2]
(2a) [Au2Cl2-FcPCy2]
(2b) [Au2I2-FcPCy2]
(3a) [Au2Cl2-FcPi-Pr2]
(3b) [Au2I2-FcPi-Pr2]
(4a) [Au2Cl2-FcPMes2]
(4b) [Au2I2-FcPMes2]
(5a) [Au2Cl2-FcPFu2]
Figure 2. Synthesis and molecular structure of trimetallic 5e,f (H are omitted for
clarity). Non Covalent Interactions visualization (bottom right). Colors: C gray, P
orange, Au yellow, O red and iron dark orange.
Conditions: enyne 6 (0.20 mmol), gold(I) complex ([Au] 2 or 4 mol%), 2 mL dry
CH2Cl2 (0.04 M in 6), 20 °C, 3 h, AgSbF6 (10 mol%). Average yield from triplicate
experiments. a 4 mol%. b 50 mol%. c No gold added. d Determined from 1H NMR.
The salts AuI and AuCl(SMe)2 used as catalyst precursors in 2
mol% amount, and cationized by using AgSbF6 (10 mol%),
achieved enyne 6 cyclization in only trace amounts (Table 1,
entries 2 and 3). In the absence of gold, the use of AgSbF6 and
the most efficient ligand (entries 4,5) alone, or in combination,
does not achieve any reaction.
NCI visualization very clearly show that the strongest interactions
are developed at the borders of the complexes between face-to-
face pairs of furyl rings. This is consistent with the fact that -
interactions are favoured with electron-poor aromatics since -
repulsion is reduced. This suggests that peripheral furyl groups
might be an efficient heteroaromatic for further assembling such
d10 cage tetrahedral complexes by -stacking.
The tetrahedral cationic 5e and 5f used with or without AgSbF6
gave no cyclization reaction (entries 6,7) and remained
unchanged. This remarkable stability is attributed to the very
compact structure of the cationic complexes as attested by XRD
analysis.
Gold salts and complexes have emerged as the most powerful
catalysts for electrophilic activation of alkynes. For the activation
of enynes, gold(I) complexes typically outshine activities shown
by Pt(II) and other electrophilic metal salts and complexes.[1a] Yet,
the stronger Lewis acidity of gold complexes can be detrimental
in terms of selectivity and because of their low tolerance to certain
functional groups. The Echavarren group reported skeletal
rearrangement reactions of 1,6-enynes,[1e] with a sluggish
conversion of N-propargyl-N-allyl toluene-4-sulfonylamine 6 that
gave a mixture of endo (7) and exo (8) products with a conversion
Recent works examining the reactivity of Au(I)/Au(III)
complexes and their oxidative addition fundamental step have
shown the great potential of tricoordinate trigonal Au(I) complexes
using chelating ligands.[4e,f,8c] For instance, Au(I) complexes
featuring diphosphino-carborane ligand achieved the oxidative
addition of Csp2–I and strained C–C. However, the preference of
Au(I) for two-coordinate linear geometry considerably limits ligand
modulation and chelating Au(I) with reduced bite angle ligands
often lead to Au···Au dinuclear structures.[4f] Therefore, the
selective formation of the trigonal 1c-3c, 2d,[20] is an excellent
opportunity to further investigate their catalytic potential.
of ca 52% and >90% selectivity in
7
by using
[PPh3AuNCMe+][SbF6 ].[21] We indeed confirmed this result
(Table 1, entry 1). In our preliminary screening experiments, the
use of dppf and Fc analogues holding i-Pr or furyl groups instead
of phenyl was found to be inefficient in these conditions. We
–
4
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Especially, because of their rarity, the stability and catalytic
potential of such complexes in cyclization is still poorly unknown.
The well-defined trigonal mononuclear gold iodide complex 1c
was found incompetent for enyne cycloaddition (entry 8). The gold
iodide complexes with electron-rich phosphines 2c and 3c, and
chloride 2d, gave very poor and unselective enyne conversion
(entries 9-11). Clearly, their mononuclear trigonal structure does
not affect favourably the course of enyne activation.
Scheme 4. Intramolecular cycloaddition of 1,6-enyne 14.
The linear digold complexes showed an effective –albeit
contrasted– activity (entries 12-20) with for the electron-rich
iodide 2b (entry 15), 88% yield cyclization of enyne 6 with 87%
selectivity in 7. A still unclear positive effect of iodide compared to
chloride was in general observed. A strong influence of the
phosphino R substituent was evidenced, either lowering the yield
of 7 (PCy2>Pi-Pr2>PMes2PFu2>PPh2) or the 5/6-membered ring
cyclization selectivity (PCy2PPh2>Pi-Pr2PMes2PFu2).
Conclusion
Tuning the electronic/steric features of phosphino groups in
constrained di-tert-butylated 1,1'-bis(phosphino)ferrocenes is a
reliable lever for controlling gold coordination modes in mono-
nuclear and dinuclear AuI halide complexes. We further identified
that rarely reported three-coordinate and four-coordinate
mononuclear 1,1'-bis(phosphino)ferrocene AuI complexes are not
catalytically competent in the cycloisomerization of a challenging
sulphonylamine enyne to cyclohexadiene. Conversely, digold
halide bis(dicyclohexylphosphino)ferrocene 2b overtakes the
other AuI systems tested. The extension to other demanding
cycloisomerization reactions furnished the endo products with
high conversion and selectivity.
We thus used the two-coordinate linear complex 2b to extend
the scope of selective intramolecular cycloaddition of different
1,6-enynes (Table 2). Similar high activity and selectivity (up to
91%) to the endo products were obtained in presence of enynes
9-13 as observed with 6. Phenyl and 4-tertio-butylbenzene
sulfonates are tolerated as protecting group for the amine (Table
2, entries 1-3). In presence of benzyl and 4-fluorobenzyl group,
furnished excellent conversion of 94% and 98% respectively with
a 88% and 85% selectivity for 11a and 12a (Table 2, entries 3-4).
The mesyl sulfonate product (13a) was obtained in high selectivity
(91%) in dichloroethane at 60 °C (Table 2, entry 5).
Experimental Section
General conditions are provided as supporting information (SI), together
with details for all gold(I) complexes. The synthesis and characterization
of 1a is illustrated for example below. Details on the existence of gold
complexes conformers and complexes fluxionality (VT-NMR) as well as
NCI computation, are also provided as SI. XRD details are available in SI
for compounds with CCDC numbers: 1983785 (1a-CH2Cl2), 1983786 (1a-
CH3OH), 1983787 (1c), 1983788 (2a), 1983789 (2b), 1983790 (2c),
1983791 (2d), 1983792 (3a), 1983793 (3b), 1983794 (3c), 1983795 (4a-
C6H12), 1983796 (4a), 1983797 (4b) 1983798 (5e).
Table 2. Scope for the intramolecular cycloaddition of different 1,6-enyne 9-13.
Entry
R
Conversion
Selectivity
General procedure for the synthesis of dinuclear gold(I) complexes
of ferrocenyl diphosphines. A 50 ml round flask equipped with a
magnetic stirring bar was charged with ferrocenyl diphosphines (1 equiv)
and gold precursor [AuCl(SMe2)] or AuI (2 equiv) degassed before addition
of dichloromethane. The solution was stirred at room temperature for 5 min,
and the mixture was filtrated on Celite® and rinsed with dichloromethane.
The solvent was removed in vacuum to give the corresponding gold
complex, which was further purified by solvation in dichloromethane
followed by a precipitation after slow addition of pentane. The synthesis of
ferrocenyl diphosphines was achieved according literature reports.[17]
in Xa + Xb (%)
in Xa (%)
1
2
(C6H5) (9)
84
92
94
98
67
88
86
88
85
91
4-tBu(C6H4) (10)
Benzyl (11)
3
4
4-F(Benzyl) (12)
Me (13)
5a
Conditions: enyne 9-13 (0.20 mmol), gold(I) complex 2b (4 mol%), 2 mL dry
CH2Cl2 (0.04 M in enyne), 20 °C, 3 h, AgSbF6 (10 mol%). Average yield from
triplicate experiments. a 1,2-dichloroethane, 60 °C.
[1,1′-Bis(diphenylphosphino)-3,3′-di(tert-butyl)ferrocene]
digold(I)
dichloride (1a). The complexation of rac-1 (33 mg, 0.05 mmol) and
[AuCl(SMe2)] (29 mg, 0.10 mmol) affords 1a in 99% (56 mg) yield after
precipitation from CH2Cl2 by addition of pentane. Two conformers
coexisting in solution. 1H NMR (500 MHz, CD2Cl2), δ (ppm) = 8.19 (bs, 2H,
Ph), 7.54–7.27 (m, 18H, Ph), 4.78 (bs,1H, HCp), 4.51 (s, 3H, HCp), 4.24
(bs, 1H, HCp), 3.69 (bs, 1H, HCp), 1.24 (bs, 9H, t-Bu), 0.84 (bs, 9H, t-Bu).
31P{1H} NMR (202.5 MHz, CD2Cl2): δ (ppm) = 27.9 (bs, 42%), 24.2 (58%)
(bs, 2P). 13C{1H} NMR (125 MHz, CD2Cl2): δ (ppm) = 135.7 (CPh), 134.4
(CPh), 133.7 (CPh), 132.7 (CPh), 131.5 (CPh), 129.9 (d, JCP = 14 Hz, CPh),
129.4 (CPh), 111.6 (CCp), 108.2 (CCp), 74.4 (CCp), 73.3 (CCp), 73.0
(CCp), 70.6 (CCp), 32.5 [C(t-Bu)], 31.0 [CH3(t-Bu)]. Elemental analysis:
Calcd (%) for C42H44Au2Cl2FeP2·1CH2Cl2: C 42.46, H 3.81. Found: C 42.53,
H 3.86. HRMS + p ESI (m/z) [M-Cl] Calcd for C42H44Au2ClFeP2:
1095.128737. Found: m/z = 1095.12793.
The more challenging cycloisomerization of N-2-methylene-
propyl-N-propargyl-toluene-4-sulfonylamine
14
was
also
achieved in the presence of complex 2b (Scheme 4). The 3-
azabicyclo[4.1.0]hept-4-ene 14a was obtained as the major
product (51%) and isolated in 21% yield. Additionally, as
secondary product the endo compound 14b was also favored in
comparison to exo compound 14c.
Electrochemistry
reagents
and
instrumentation.
Tetra-n-
butylammonium hexafluorophosphate (TBAPF6) was synthesized by
5
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10.1002/asia.202000579
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mixing stoichiometric amounts of tetra-n-butylammonium hydroxide (Alfa-
Aesar, 40% w/w aq. sol.) and hexafluorophosphoric acid (Alfa-Aesar, ca
60% w/w aq. sol.). The supporting electrolyte (tetraethylammonium
tetrafluoroborate) was degassed under vacuum before use and then
dissolved to a concentration of 0.1 M. Voltammetry analyses were carried
out in a standard three-electrode cell, with an Autolab PGSTAT 302N
potentiostat, connected to an interfaced computer that employed
Electrochemistry Nova software. The reference electrode was a KCl
saturated calomel electrode (SCE) separated from the analyzed solution
by a sintered glass disk filled with the background solution. The auxiliary
electrode was a platinum wire separated from the analyzed solution by a
sintered glass disk filled with the background solution. The working
electrode was a platinum electrode disk, operating in CH2Cl2 (0.1 M
TBAPF6), with potential for ferrocene(+/0) couple +0.38 V vs. SCE.
General procedure for the cycloisomerization of enynes 6-14. Enyne
6-14 (0.20 mmol, 50 mg) and silver(I) salt AgSbF6 (0.1 equiv) were mixed
in dry CH2Cl2 (2 mL) and were added to a solution of gold(I) complexes (2
mol%) in dry CH2Cl2 (1 mL). The mixture was stirred at room temperature
(20 °C) for 3 h. The mixture was then filtered through SiO2. The solvent
was evaporated under reduced pressure to give the crude product. The
conversion was determinated by 1H NMR according spectrum given for
endo and exo according the literature. Compound endo was isolated by
using flash chromatography (heptane/ethyl acetate, 90/10). Due to the low
yields obtained in general during the reactions exo was not isolated
(<15%).
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Acknowledgements
This work was supported by the ANR program (ANR-16-CE07-
0007-02, Icare_1, grant for H. N.), by the CNRS, the Universite
́
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(COMICS project) and the FEDER which are all sincerely
acknowledged.
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Keywords: Functionalized ferrocenes • digold complexes •
halides • trigonal gold(I) • enyne cyclization
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6
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Entry for the Table of Contents
Linear, trigonal and tetrahedral gold(I) complexes are selectively formed depending on the R phosphino groups on
ferrocenyldiphosphines: linear digold(I) achieved cycloisomerization of enyne-sulphonamides into cyclohexadienes.
7
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