Activation of a σ-SnSn bond at copper, followed by double addition to an alkyne

Article abstract of DOI:10.1021/ja405516j

Many synthetically useful copper-catalyzed transformations involve the activation of apolar or weakly polar σ-bonds (E-H and E-E′ bonds, with E = C, B, Si, Sn, etc.). Yet, little is known so far about the associated elementary steps, and it is highly desirable to gain better knowledge regarding the way σ-bonds can be activated by copper to help further development in this area. To this end, we became interested in investigating the coordination and activation of apolar or weakly polar σ-bonds at copper using chelating assistance. Here we report investigations of gold and copper complexes deriving from the diphosphine-stannane [Ph₂P(o-C₆H ₄)Me₂Sn-SnMe₂(o-C₆H ₄)PPh₂] 1. The σ-SnSn bond of 1 readily undergoes oxidative addition at both gold and copper, giving bis(stannyl) Au⁺ and Cu⁺ complexes 2 and 3. Coordination of 1 to CuBr leads to the neutral complex 4 which features more σ-SnSn complex character. The ability of complex 3 to undergo insertion reactions with alkynes was then examined. With methyl propiolate, a clean reaction occurred, and the bis-stannylated alkene copper complex 5 was isolated. The structures of ligand 1 and complexes 2-5 have been unambiguously determined by multinuclear NMR spectroscopy and crystallography. These results substantiate the ability of copper to promote the addition of apolar σ-bonds to CC multiple bonds via a 2e redox sequence and draw thereby an unprecedented parallel with the group 10 metals.

Full text of DOI:10.1021/ja405516j

Article
pubs.acs.org/JACS
Activation of a σ‑SnSn Bond at Copper, Followed by Double Addition
to an Alkyne
Nicolas Lassauque,† Pauline Gualco,† Sonia Mallet-Ladeira,‡ Karinne Miqueu,§
Abderrahmane Amgoune,*,† and Didier Bourissou*,†
†Laboratoire Het
Narbonne, F-31062 Toulouse, France
‡Structure Fed
erative Toulousaine en Chimie Molec
Toulouse, France
§Institut Pluridisciplinaire de Recherche sur l’Environnement et les Mater
Pau et des Pays de l’Adour, Helioparc, 2 Avenue du President Angot, 64053 Pau cedex 09, France
́ ́ ́ ́
erochimie Fondamentale Appliquee, Universite de Toulouse, UPS, CNRS, LHFA UMR 5069, 118 route de
́
́
́ ́
ulaire, FR2599, Universite de Toulouse, UPS, 118 Route de Narbonne, F-31062
́
́
iaux (UMR 5254), Equipe Chimie Physique, Universite de
́
́
S
* Supporting Information
ABSTRACT: Many synthetically useful copper-catalyzed
transformations involve the activation of apolar or weakly
polar σ-bonds (E−H and E−E′ bonds, with E = C, B, Si, Sn,
etc.). Yet, little is known so far about the associated elementary
steps, and it is highly desirable to gain better knowledge
regarding the way σ-bonds can be activated by copper to help
further development in this area. To this end, we became
interested in investigating the coordination and activation of
apolar or weakly polar σ-bonds at copper using chelating
assistance. Here we report investigations of gold and copper complexes deriving from the diphosphine-stannane [Ph2P(o-
C6H4)Me2Sn−SnMe2(o-C6H4)PPh2] 1. The σ-SnSn bond of 1 readily undergoes oxidative addition at both gold and copper,
giving bis(stannyl) Au+ and Cu+ complexes 2 and 3. Coordination of 1 to CuBr leads to the neutral complex 4 which features
more σ-SnSn complex character. The ability of complex 3 to undergo insertion reactions with alkynes was then examined. With
methyl propiolate, a clean reaction occurred, and the bis-stannylated alkene copper complex 5 was isolated. The structures of
ligand 1 and complexes 2−5 have been unambiguously determined by multinuclear NMR spectroscopy and crystallography.
These results substantiate the ability of copper to promote the addition of apolar σ-bonds to CC multiple bonds via a 2e redox
sequence and draw thereby an unprecedented parallel with the group 10 metals.
Thanks to meticulous investigations, the groups of Stahl, Ribas,
and others have been able to characterize well-defined aryl
copper(III) species relevant to Ullmann-type coupling
reactions, and unambiguous evidence for Cu(I)/Cu(III)
redox sequences has been provided for C−N, C−O, C−S,
C−Se, C−P, and C−F bond formations.4−6 In addition,
studying photoinduced C−N couplings, Fu and Peters have
recently demonstrated the ability of copper to promote aryl
halide activation by single electron transfer.7
Comparatively, much less is known about the activation of
apolar or weakly polar σ-bonds (E−H and E−E′ bonds, with E
= C, B, Si, Sn, etc.) at copper, although these processes are
involved in a broad range of synthetically useful catalytic
transformations.8 It is highly desirable to gain better knowledge
on the way σ-bonds can be activated by copper to help further
development in this area, not only for the optimization of
catalytic properties but also for the discovery of new
transformations. To this end, we investigated the coordination
INTRODUCTION
■
Copper occupies a forefront position in transition metal
chemistry, and it is certainly one of the most, if not the most,
widely used metal in organic synthesis.1 Over the years, copper
complexes have found widespread applications in stoichio-
metric and catalytic transformations, with always increasing
activity, selectivity, and functional group tolerance. But the
considerable synthetic development of copper chemistry
contrasts with the relatively poor understanding of the involved
mechanisms. In fact, what is known for the other transition
metals often does not apply to copper because of the specific
properties of this metal, and our knowledge of the fundamental
behavior of copper complexes remains quite superficial.
However, the past decade has witnessed significant advances
in the understanding of copper chemistry.
This is particularly true for the activation of aryl halides and,
more generally, for the processes of C−X bond activation/
formation relevant to Ullmann-type couplings.2 In the two
most often invoked mechanisms, aryl halides are activated
either by a radical pathway or by concerted oxidative addition,
and direct evidence has been reported recently for both routes.3
Received: June 6, 2013
© XXXX American Chemical Society
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Chart 1. Interaction of σ-SiSi and σ-SiH Bonds with Copper and Gold, As Previously Observed in Phosphine-Chelated Systems
and activation of apolar or weakly polar σ-bonds at copper
using chelating assistance.9 Using phosphine anchors, we
recently discovered that σ-SiSi and σ-SiH bonds can coordinate
in a side-on fashion to copper, and thereby, the first σ-
complexes10 of copper11 have been isolated and structurally
characterized (Chart 1).12 According to geometric and
electronic data, the corresponding σ bonds are slightly
weakened but not strongly activated upon coordination to
copper. A markedly different situation was found in the related
gold complexes. Oxidative addition of the σ-SiSi bond occurs
spontaneously, affording bis(silyl) gold(III) complexes13 and
subsequent insertion of oxygen provided evidence for an
unusual 2e redox sequence at gold.14 To probe the ability of
copper to engage into similar 2e processes, and to compare
further the behavior of the two coinage metals, we decided to
explore the reactivity of other σ-bonds. Herein we report a
detailed investigation of gold and copper complexes deriving
from a diphosphine−distannane.15−17 Oxidative addition of the
σ-SnSn bond occurs spontaneously at gold to give a
bis(stannyl) gold(III) complex. The σ-SnSn bond is also
activated at copper, and the degree of activation increases with
the electrophilicity of the metal (CuBr/Cu+). Subsequent
reaction with methyl propiolate induces double stannylation of
the CC triple bond. The structures of all of the complexes
have been unambiguously determined by multinuclear NMR
spectroscopy and crystallography. These results substantiate the
ability of copper to promote the addition of apolar σ-bonds to
CC multiple bonds via a 2e redox sequence and draw thereby
an unprecedented parallel with the group 10 metals.18
RESULTS AND DISCUSSION
■
Theoretical Study of σ-SiSi and σ-SnSn Bond
Activation at Gold and Copper. The coordination of
Figure 1. Computational results for the σ-SiSi and σ-SnSn bond
diphosphine−disilane and diphosphine−distannane ligands
activation at gold and copper. Optimized geometries and relative
[Ph2P(o-C6H4)Me2E−EMe2(o-C6H4)PPh2] (E = Si, Sn) to
stabilities (ΔG in kcal/mol at 25 °C) of the M(I) complexes (with
gold and copper was first studied theoretically to gain some
pendant or side-on coordinated σ-EE bonds) and M(III) complexes
insight into how the nature of the σ-EE bond influences the
(resulting from oxidative addition of the σ-EE bonds).
activation process.19 DFT calculations were carried out at the
B3PW91/SDD+pol(Au,Sn, Cu),6-31G**(P,C,H,Ga,Si,Cl)
level of theory on complexes A−D (naked cationic complexes,
gas phase; see Supporting Information). The main results are
depicted in Figure 1. With gold, oxidative addition of the σ-EE
bond is energetically favored for both silicon and tin. The
Au(III) complexes AIII and BIII are more stable than their Au(I)
isomers AI and BI featuring pendant σ-EE bonds (by 9.8 kcal/
mol for silicon and 4.6 kcal/mol for tin). A different situation is
predicted for copper. The ground-state structures correspond
to the Cu(I) complexes CI and DI featuring side-on
coordinated σ-SiSi and σ-SnSn bonds, respectively. The Cu(III)
bis(silyl) complex CIII is found 7.8 kcal/mol higher in energy
than its Cu(I) isomer CI. Of note, the corresponding Cu(III)
B
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Figure 2. (a) Molekel plots (cutoff: 0.04) of the HOMO and LUMO for complexes DI and DIII. (b) Calculated electron-density maps for complexes
DI and DIII with relevant bond paths and bond critical points (green dots). Charge accumulation (∇2 ρ(r) < 0) is printed in blue and charge
depletion (∇2 ρ(r) > 0) in red.
bis(stannyl) complex DIII is only slightly higher in energy than
its σ-SnSn Cu(I) isomer DI (by 2.7 kcal/mol), indicating that
oxidative addition of the σ-SnSn bond at copper is about
−
thermoneutral. Moreover, when the GaCl4 counteranion is
included and the solvent effects are taken into account (single
point PCM calculations with dichloromethane), the Cu(III)
bis(stannyl) complex EIII becomes the ground state structure,
about 6.8 kcal/mol more stable than the corresponding Cu(I)
isomer EI.
The electronic structure of the diphosphine−distannane
copper complexes was then examined through analyses of the
molecular orbitals (MO) and atom-in-molecules (AIM)
calculations (Figure 2; see Supporting Information). The
highest occupied molecular orbital HOMO of DI is mainly
associated with the σ-SnSn orbital which is slightly distorted
toward copper (Sn, Sn, Cu three-center interaction). The
copper center has a larger contribution in the HOMO of DIII,
which corresponds to a symmetric combination of σ-SnCu
orbitals. The electron density maps of DI and DIII are also
Figure 3. Energy profile (ΔG in kcal/mol) and structure of the
noticeably different. While a bond path and a bond critical
point (BCP) are clearly observed between the two Sn atoms of
DI (electron density at the BCP: ρ(r) = 0.053 e·bohr−3), such
features are not found for DIII. In both complexes, BCP are
localized between the two Sn atoms and the copper center, but
the associated electron density is significantly higher in DIII
than in DI (0.048 vs 0.027 e·bohr−3). All of these data suggest
that DI is best described as a σ-SnSn complex, while DIII is best
described as a Cu(III) bis-stannyl complex.
transition states for the σ-SnSn bond activation at gold and copper
(model complexes B* and D*, respectively).
transition states (TS) differ significantly from Au to Cu. A
symmetric structure with well-developed AuSn bonds (2.875
Å) is found for gold, while the TS located for Cu is
dissymmetric and displays two very different SnCu distances
(2.282 and 3.317 Å), suggesting asynchronous formation of the
two CuSn bonds.
We then sought to gain more insights into the σ-SnSn bond
activation process at Au/Cu, and to this end, we explored the
energy profile of the oxidative addition reaction with model
complexes B* and D* (featuring Me instead of Ph groups at P)
(Figure 3; see Supporting Information). For gold, a low energy
pathway was found between BI* and BIII* (the corresponding
transition state is located only 4.1 kcal/mol above BI* in
energy). The activation barrier for σ-SnSn oxidative addition is
higher (18 kcal/mol) with copper but remains accessible under
mild conditions. The geometric features of the associated
From these DFT calculations, diphosphine−distannanes are
anticipated to behave quite differently from diphosphine−
disilanes upon coordination, and oxidative addition of the σ-
SnSn bond is envisioned to occur not only with gold, but also
with copper. This prompted us to prepare the diphosphine−
distannane 1 and to explore its coordination experimentally.
Synthesis of the Diphosphine−Distannane 1 and
Coordination to Gold. Whereas the related diphosphine−
disilane was readily obtained from the corresponding 1,2-
dichlorodisilane,11 the preparation of the diphosphine−
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a
Scheme 1. Preparation and X-ray Structure of the Diphosphine−Distannane 1
a
Ellipsoids set at 50% probability; hydrogen atoms omitted for clarity. Selected bond length (Å): SnSn 2.7682(2).
a
Scheme 2. Coordination of the Diphosphine−Distannane 1 to Gold, Oxidative Addition of the σ-SnSn Bond
a
Molecular structure of the ensuing bis(stannyl) gold(III) complex 2 (ellipsoids set at 50% probability; hydrogen atoms and BARF counteranion
omitted for clarity). Selected bond lengths (Å) and angles (deg): P1−Au, 2.3974(10); P2−Au, 2.3919(10); Sn1−Au, 2.6293(3); Sn2−Au,
2.6335(3); P1−Au−P2, 106.74(3); Sn1−Au−Sn2, 82.027(10); P1−Au−Sn1, 85.97(2); P2−Au−Sn2, 85.30(3).
distannane 1 required a multistep sequence (Scheme 1). The
key reaction is a tin−tin coupling between [2-(chlorodimethyl-
stannyl)phenyl]diphenylphosphine20 and the corresponding
stannyl lithium species (see Supporting Information). Reduc-
tive coupling of [2-(chlorodimethylstannyl)phenyl]diphenyl-
phosphine, as reported by Kang et al. for the preparation of
[Ph2P-(o-carboranyl)SnMe2]2,21 did not prove efficient in our
case. After purification by flash chromatography, the desired
ligand 1 was obtained as a white powder in 65% yield (see
Supporting Information). Key spectroscopic and structural data
of 1, such as the 119Sn NMR resonance signal at δ −122 ppm,
the JP−Sn coupling constants (130 and 82 Hz) and the SnSn
bond distance (2.7682(2) Å), will be important parameters to
consider later on when analyzing σ-SnSn bond activation at
gold and copper.
Coordination of the Diphosphine−Distannane to
Copper (Cationic and Neutral Complexes). Reaction of 1
with (Me2S)CuBr and NaBARF afforded the cationic Cu
complex 3, which was isolated in 90% yield after workup
(Scheme 3). Upon coordination, the 119Sn NMR resonance
Scheme 3. Coordination of the Diphosphine−Distannane 1
to Copper
The coordination properties of the diphosphine−distannane
1 were then investigated, starting with gold. The reaction of 1
with (Me2S)AuCl in the presence of NaBARF (BARF =
tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) results in spon-
taneous oxidative addition of the σ-SnSn bond and gives the
bis(stannyl) gold(III) complex 2 (Scheme 2). The cleavage of
the σ-SnSn bond is clearly apparent in NMR, from the
downfield shift of the 119Sn resonance signal (δ 114.6 ppm) and
the associated SnP coupling pattern (JP−Sn(trans) = 950 Hz and
JP−Sn(cis) = 96 Hz). The molecular structure of 2 was
unambiguously confirmed by X-ray diffraction analysis. The
gold center is tetracoordinate, and the complex adopts square-
planar cis geometry. The σ-SnSn bond is cleaved [the SnSn
distance reaches 3.4536(4) Å in 2], and short AuSn bonds are
formed [2.6293(3) and 2.6335(3) Å, vs 2.75 Å for the sum of
covalent radii22].23
Oxidative addition of the σ-SnSn bond upon coordination of
1 to gold is consistent with the computational study [the
gold(III) complex BIII is favored by 4.6 kcal/mol over the
corresponding gold(I) isomer BI] and parallels the σ-SiSi bond
activation observed with the related diphosphine−disilane.14a
To assess further the propensity of the σ-SnSn bond to undergo
oxidative addition, we then explored the coordination of the
diphosphine−distannane 1 to copper.
signal shifts to lower field (from δ −122 ppm for 1, to δ −13
ppm for 3), and two very different JSn−P coupling constants,
including a very large one, are observed (495 and 89 Hz).
Similar features were observed for the gold complex 2, and
thus, it is likely that the σ-SnSn bond has also been activated at
copper. To ascertain more precisely the molecular structure of
3, single crystals were grown from a dichloromethane/pentane
solution at −20 °C and analyzed by X-ray diffraction.
Accordingly, complex 3 features a discrete ion pair structure
−
(Figure 4, left). The related complex with GaCl4 as a
counteranion was also isolated and characterized crystallo-
graphically (see Supporting Information). The two structures
are very similar and fit well with that predicted computationally.
The copper center is tetracoordinate and adopts square-planar
geometry,24 with head-to-head arrangement of the two
phosphino-stannyl chelates. The maximum deviation from the
least-squares plane defined by the Cu, P1, P2, Sn1, and Sn2
atoms is of only 0.07 Å, and the CuSn bonds are short
(2.5988(5) Å and 2.6083(5) Å, vs 2.71 Å for the sum of
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The structural features of complexes 3 and 4 clearly indicate
substantial σ-SnSn bond activation. Coordination of the
diphosphine−distannane 1 to Cu+ proceeds with oxidative
addition of the σ-SnSn bond to give the bis(stannylated) Cu
complex 3. The neutral CuBr complex 4 features more σ-SnSn
complex character. The activation of the σ-SnSn bond at copper
advances when the electrophilicity of the metal increases,
suggesting that donation dominates over backdonation in this
process.
Having substantiated the ability of copper to activate the σ-
SnSn bond of 1, we were then interested to study the reactivity
of the bis(stannyl) Cu(III) complex 3, in particular its ability to
undergo insertion reactions so as to achieve a complete
sequence akin to that commonly encountered with Pd and Pt.18
For that purpose, we examined the reactivity of complex 3
toward alkynes.
Reaction of the Bis(stannyl) Copper Complex 3 with
Methylpropiolate. Reactions of 3 with phenyl acetylene and
dimethyl acetylene dicarboxylation led to a mixture of
unidentified products, but with methyl propiolate (10 equiv),
a clean reaction occurred within 10 h at room temperature. The
bis-stannylated alkene copper complex 5 was isolated in 70%
after workup (Scheme 4). The dissymmetric nature of 5 is
Figure 4. Molecular view of complexes 3 (left) and 4 (right) in the
solid state (thermal ellipsoids set at 50% probability; hydrogen atoms,
counteranion, and solvate molecules omitted for clarity). Selected
bond lengths (Å) and angles (deg): P1−Cu, 2.2741(9); P2−Cu,
2.2655(9); Sn1−Cu, 2.5988(5); Sn2−Cu, 2.6083(5); Sn1−Sn2,
3.0910(4); P1−Cu−P2, 112.23(3); Sn1−Cu−Sn2, 72.829(13); P1−
Cu−Sn1, 89.25(2); P2−Cu−Sn2, 85.74(3) for complex 3 and P1−Cu,
2.3059(14); P2−Cu, 2.2835(14); Sn1−Cu, 2.6698(7); Sn2−Cu,
2.6632(7); Sn1−Sn2, 2.9942(5); Cu−Br, 2.4767(8); P1−Cu−P2,
112.83(5); Sn1−Cu−Sn2, 68.311(18); P1−Cu−Sn1, 84.11(4); P2−
Cu−Sn2, 87.19(4) for complex 4.
covalent radii22).25 The distance between the two tin atoms
(3.0910(4) Å) is well beyond the sum of covalent radii (2.78
Å)22 but smaller than in the gold complex 2 (3.4536(4) Å),
suggesting the possible existence of some residual Sn···Sn
contact in 3.26 However, as mentioned above, AIM calculations
performed on the optimized structure DIII revealed the absence
of bond critical point between the two Sn atoms. Moreover, an
AIM analysis carried out on the crystallographic determined
geometry gave the same result (see Supporting Information).
Accordingly, complex 3 is best described as a bis(stannyl)
Cu(III) species.
Scheme 4. Reaction of Complex 3 with Methyl Propiolate
We then prepared a neutral complex related to 3 since the
electron density at copper is likely to influence the extent of σ-
SnSn bond activation. Indeed, we recently observed in a related
study that the coordination of σ-SiH bonds to copper
strengthens when the electrophilicity of the metal increases.12b
Thus, the diphosphine−distannane 1 was reacted with
(Me2S)CuBr in the absence of halide scavenger. The
coordination proceeded readily, but the ensuing complex 4
proved to be extremely sensitive, precluding its isolation.27
Decomposition occurs rapidly above −20 °C, but complex 4
could be characterized by multinuclear NMR spectroscopy at
−40 °C. Most diagnostic is the 119Sn NMR resonance signal
found at δ −37 ppm with JP−Sn coupling constants of 549 and
122 Hz. Fortunately, crystals were obtained from a pentane/
dichloromethane at −60 °C so that the structure of 4 could be
unequivocally assessed by X-ray diffraction analysis (Figure 4,
right). The coordination sphere around copper can be
assimilated to a tetrahedron, with the σ-SnSn bond capping
the pyramidal diphosphine copper bromide moiety (sum of
P1CuP2, P1CuBr, and P2CuBr bond angles = 323.28(13)°).
The two Sn atoms lie close to the metal center. The Cu−Sn
distances (2.6632(7) and 2.6698(7) Å) are longer than those of
complex 3 (∼2.60 Å) but below the sum of covalent radii (2.71
Å).22 The σ-SnSn bond is significantly elongated (2.9942(5) Å
vs 2.7682(2) Å in the free ligand 1 and 3.0910(4) Å in the
cationic complex 3). Here also, AIM calculations were
performed on the solid-state geometry, but in this case, a
bond path and a bond critical point (BCP) were found between
the two Sn atoms [electron density at the BCP: ρ(r) = 0.046 e·
bohr−3 vs 0.058 e·bohr−3 for a pendant σ-SnSn bond; see
Supporting Information].
apparent in 31P NMR spectroscopy, two doublets being
observed at δ = 14.9 and 9 ppm with a JP−P coupling constant
of 56 Hz. Consistently, the 119Sn NMR spectrum displays two
resonance signals at δ = −47 ppm and −65 ppm, in the same
range than those reported for free bis(stannyl) alkenes.28 Each
119Sn NMR signal appears as a doublet of a doublet due to Sn−
P couplings with the two inequivalent phosphorus atoms.
Single crystals were grown from dichloromethane/pentane at
−20 °C, and the molecular structure of complex 5 was analyzed
by X-ray diffraction (Figure 5), confirming insertion of the
Figure 5. Molecular view of complex 5 in the solid state (thermal
ellipsoids set at 50% probability; hydrogen atoms, counteranion, and
solvate molecules omitted for clarity). Selected bond lengths (Å) and
angles (deg): P1−Cu, 2.2817(13); P2−Cu, 2.2758(15); C1−Cu,
2.118(8); C2−Cu, 2.149(8); C1−C2, 1.394(12); P1−Cu−P2,
121.78(5).
E
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alkyne between the two tin atoms. Complex 5 adopts a discrete
ion pair structure. The copper center is surrounded by the two
phosphorus atoms (PCuP = 121.78(5)°) and the bis-
stannylated alkene, organized in a trigonal-planar geometry.
The CC double bond is essentially coplanar with the PCuP
skeleton (the corresponding dihedral angle is of 10.39(45)°).
The CuC distances (2.118(8) and 2.149(8) Å) slightly exceeds
the sum of covalent radii (2.05 Å),22 indicating relatively weak
coordination of the central alkene moiety to copper.29
Consistently, the CC bond is short (1.394(12) Å) and
only marginally deviates from planarity (the sum of bond angles
at C1 and C2 = 301.9(2)° and 305.9(2)°). Notably, complex 5
is obtained as sole product, and according to the X-ray
diffraction study, the two tin atoms display a trans-orientation.
Thus, complex 3 reacts with methyl propiolate via trans-
addition across the triple bond of the alkyne. Such a selectivity
contrasts with the syn-addition usually observed upon bis-
(stannylation) of alkynes with transition metals. This may not
be a specific feature of copper30 but rather the consequence of
geometric constraints associated with the chelate structure of
the PSnSnP system.
borastannation35 of CC multiple bonds. Copper(I) boryl
complexes are invoked as key intermediates, and redox neutral
catalytic cycles are proposed. The results reported here indicate
that 2e redox cycles are also viable with copper, which opens
new possibilities. Future work from our group will seek to
explore further the chemistry of the coinage metals and to gain
better knowledge on their behavior in all elementary steps.
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental conditions and procedures, theoretical
details, and analytical data and crystallographic data (CIF) for
compounds 1−5. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
amgoune@chimie.ups-tlse.fr
dbouriss@chimie.ups-tlse.fr
Notes
The authors declare no competing financial interest.
The reaction of the neutral copper complex 4 with methyl
propiolate was also examined, for comparison. No reaction
occurs at low temperature, and decomposition is observed
upon warming to room temperature. No 31P/119Sn NMR signal
attributable to an insertion product could be detected. The
degree of activation of the σ-SnSn bond thus plays a critical role
on the bis-stannylation reaction. The diphosphine−distannane
1 undergoes σ-SnSn bond activation upon coordination to Cu+,
and methyl propiolate then inserts between the two tin atoms.
In this reaction sequence, copper promotes activation of the
distannane followed by double addition to the CC triple
bond. Such 2e redox transformations are well-known for the
group 10 metals, in particular Pd and Pt,18,30−32 but have no
precedent with coinage metals.
ACKNOWLEDGMENTS
■
Financial support from the Centre National de la Recherche
Scientifique, the Universite de Toulouse, and the Agence
Nationale de la Recherche (ANR-10-BLAN-070901) is grate-
fully acknowledged. The theoretical work was granted access to
HPC resources of Idris under Allocation 2013 (i2013080045)
made by Grand Equipement National de Calcul Intensif
(GENCI). CCDC numbers for compounds 1−5, CCDC
940496−940501, contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
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■
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248, 2337. (c) Evano, G.; Blanchard, N.; Toumi, M. Chem. Rev. 2008,
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Taillefer, M. Angew. Chem., Int. Ed. 2009, 48, 333. (e) Chemler, S. R.
Science 2013, 341, 624.
The ability of copper to activate and functionalize σ-SnSn
bonds has been evidenced using chelating assistance.
Coordination of the diphosphine−distannane 1 to Au+ and
Cu+ proceeds with oxidative addition of the σ-SnSn bond and
affords the bis(stannyl) complexes 2 and 3. The extent of σ-
SnSn bond activation was found to increase with the
electrophilicity of copper. The related neutral CuBr complex
4 features more σ-SnSn complex character. The bis(stannyl)
copper complex 3 undergoes a clean insertion reaction with
methyl propiolate to give the bis-stannylated alkene copper
complex 5. The evolution of the bonding situation along the
entire sequence (activation/functionalization of the σ-SnSn
bond) has been thoroughly analyzed thanks to systematic X-ray
diffraction studies.
These results evidence the ability of copper to undergo two-
electron redox transformations33 and draw thereby unprece-
dented parallel with the group 10 metals, in particular Pd and
Pt that are habitually used to promote the addition of element−
element bonds to alkynes and alkenes. They also provide
valuable information for the development of difunctionalization
reactions with copper. Baker, Marder, and Wescott recognized
early on the ability of the coinage metals to promote double
addition to CC multiple bonds (diboration of vinylarenes with
gold in their pioneering contribution),34 but this remains a
relatively underexplored field of research. However, recent
contributions reported copper catalyzed diboration19,35 and
(3) As for most transformations, the operating scenario certainly
depends on the substrate and reaction conditions.
(4) (a) Huffman, L. M.; Stahl, S. S. J. Am. Chem. Soc. 2008, 130,
9196. (b) Casitas, A.; King, A. E.; Parella, T.; Costas, M.; Stahl, S. S.;
Ribas, X. Chem. Sci. 2010, 1, 326. (c) Casitas, A.; Poater, A.; Sola, M.;
Stahl, S. S.; Costas, M.; Ribas, X. Dalton Trans. 2010, 39, 10458.
(d) Huffman, L. M.; Casitas, A.; Font, M.; Canta, M.; Costas, M.;
Ribas, X.; Stahl, S. S. Chem.Eur. J. 2011, 17, 10643. (e) Casitas, A.;
Ioannidis, N.; Mitrikas, G.; Costas, M.; Ribas, X. Dalton Trans. 2011,
40, 8796. (f) Huffman, L. M.; Stahl, S. S. Dalton Trans. 2011, 40, 8959.
(g) Wang, Z. L.; Zhao, L.; Wang, M. X. Org. Lett. 2011, 24, 6560.
́
(h) Casitas, A.; Canta, M.; Sola, M.; Costas, M.; Ribas, X. J. Am. Chem.
Soc. 2011, 133, 19386. (i) Fier, P. S.; Hartwig, J. F. J. Am. Chem. Soc.
2012, 134, 10795. (j) Font, M.; Parella, T.; Costas, M.; Ribas, X.
Organometallics 2012, 31, 7976. (k) Casitas, A.; Ribas, X. Chem. Sci.
2013, 4, 2301.
(5) The copper-catalyzed α-arylation of β-diketones, known as the
Hurtley coupling, has also been proposed to involve oxidative addition
of aryl iodides. See: (a) Huang, Z.; Hartwig, J. F. Angew. Chem., Int. Ed.
2012, 51, 1028. (b) He, C.; Zhang, G.; Ke, J.; Zhang, H.; Miller, J. T.;
Kropf, A. J.; Lei, A. J. Am. Chem. Soc. 2013, 135, 488.
F
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(6) For recent computational studies on oxidation addition of aryl
halides at copper, see: (a) Zhang, S. L.; Liu, L.; Fu, Y.; Guo, Q. X.
Organometallics 2007, 26, 4546. (b) Tye, J. W.; Weng, Z.; Johns, A. M.;
Incarvito, C. D.; Hartwig, J. F. J. Am. Chem. Soc. 2008, 130, 9971.
(c) Giri, R.; Hartwig, J. F. J. Am. Chem. Soc. 2010, 132, 15860.
and E = Si, Sn). Classically, the two electrons of the (R3P)M bonds are
transferred to the P atom (χ = 2.19), whatever the electronegativity of
the metal (Cu: 1.90 and Au: 2.54). The two electrons of the M(ER3)
bonds are also transferred to the E atoms, despite the electropositive
character of Si (1.90) and Sn (1.96). Accordingly, (R3P)2M+ and
+
̀
(d) Lefevre, G.; Franc, G.; Tlili, A.; Adamo, C.; Taillefer, M.; Ciofini,
(R3P)2M(ER3)2 complexes are described as M(I) and M(III)
I.; Jutand, A. Organometallics 2012, 31, 7694. (e) Wang, M.; Fan, T.;
Lin, Z. Organometallics 2012, 31, 560. (f) Lv, H.; Cai, Y. B.; Zhang, J.
L. Angew. Chem., Int. Ed. 2013, 52, 3203.
complexes, respectively. The authors are grateful to the reviewer
who raised this formalism/terminology issue and invite the readers to
consult the following papers for detailed discussions about the
oxidation number and valence concepts: (a) Pauling, L. J. Chem. Soc.
1948, 1461. (b) Steinborn, D. J. Chem. Educ. 2002, 81, 1148.
(c) Parkin, G. J. Chem. Educ. 2006, 83, 791.
(14) (a) Gualco, P.; Ladeira, S.; Miqueu, K.; Amgoune, A.; Bourissou,
D. Angew. Chem., Int. Ed. 2011, 50, 8320. (b) Gualco, P.; Ladeira, S.;
Miqueu, K.; Amgoune, A.; Bourissou, D. Organometallics 2012, 31,
6001.
(7) (a) Creutz, S. E.; Lotito, K. J.; Fu, G. C.; Peters, J. C. Science
2012, 338, 647. (b) Bissember, A. C.; Lundgren, R. J.; Creutz, S. E.;
Peters, J. C.; Fu, G. C. Angew. Chem., Int. Ed. 2013, 52, 5129.
(c) Majek, M.; von Wangelin, A. J. Angew. Chem., Int. Ed. 2013, 52,
5919.
(8) For recent reviews on Cu catalyzed CH bond functionalization,
́
see: (a) Díaz-Requejo, M. M.; Perez, P. J. Chem. Rev. 2008, 108, 3379.
(b) Gephart, R. T.; Warren, T. H. Organometallics 2012, 31, 7728. (c)
For Si-B activation at copper, see: Oestreich, M.; Hartmann, E.;
Mewald, M. Chem. Rev. 2012, 113, 402. (d) For Cu catalyzed
hydroboration reactions, see: (e) Noh, D.; Chea, H.; Ju, J.; Yun, J.
Angew. Chem., Int. Ed. 2009, 48, 6062. (f) Feng, X.; Jeon, H.; Yun, J.
Angew. Chem., Int. Ed. 2013, 52, 3989.
(9) For previous contributions from our group on unusual metal−
ligand interactions and bond activation processes supported by
phosphine buttresses, see: (a) Bontemps, S.; Gornitzka, H.;
Bouhadir, G.; Miqueu, K.; Bourissou, D. Angew. Chem., Int. Ed.
2006, 45, 1611. (b) Sircoglou, M.; Bontemps, S.; Mercy, M.; Saffon,
N.; Takahashi, M.; Bouhadir, G.; Maron, L.; Bourissou, D. Angew.
Chem., Int. Ed. 2007, 46, 8583. (c) Bontemps, S.; Sircoglou, M.;
Bouhadir, G.; Puschmann, H.; Howard, J. A. K.; Dyer, P. W.; Miqueu,
K.; Bourissou, D. Chem.Eur. J. 2008, 14, 731. (c) Sircoglou, M.;
Bontemps, S.; Mercy, M.; Miqueu, K.; Ladeira, S.; Saffon, N.; Maron,
L.; Bouhadir, G.; Bourissou, D. Inorg. Chem. 2010, 49, 3983.
(d) Sircoglou, M.; Mercy, M.; Saffon, N.; Coppel, Y.; Bouhadir, G.;
Maron, L.; Bourissou, D. Angew. Chem., Int. Ed. 2009, 48, 3454.
(e) Gualco, P.; Lin, T. P.; Sircoglou, M.; Mercy, M.; Ladeira, S.;
Bouhadir, G.; Perez, L. M.; Amgoune, A.; Maron, L.; Gabbai, F. P.;
Bourissou, D. Angew. Chem., Int. Ed. 2009, 48, 9892. (f) Gualco, P.;
Mercy, M.; Ladeira, S.; Coppel, Y.; Maron, L.; Amgoune, A.;
Bourissou, D. Chem.Eur. J. 2010, 16, 10808. (g) Derrah, E. J.;
Ladeira, S.; Bouhadir, G.; Miqueu, K.; Bourissou, D. Chem. Commun.
2011, 47, 8611. (h) Derrah, E. J.; Martin, C.; Mallet-Ladeira, S.;
Miqueu, K.; Bouhadir, G.; Bourissou, D. Organometallics 2013, 32,
1121.
(10) For general reviews on σ-bond coordination to transition metals,
see: (a) Kubas, G. J., Metal dihydrogen and σ-Bond complexes; Kluwer
Academic/Plenum: New York, 2001. (b) Crabtree, R. H. Angew.
Chem., Int. Ed. 1993, 32, 789. (c) Coordination Chemistry of
Saturated Molecules special feature: Bercaw, J. E.; Labinger, J. A. Proc.
Natl. Acad. Sci. U.S.A. 2007, 104, 6899 and subsequent articles.
(11) The coordination of a σ-CH bond has been identified by pulsed
EPR in a copper(II) complex of a triazamacrocyclic ligand. See: Ribas,
X.; Calle, C.; Poater, A.; Casitas, A.; Gomez, L.; Xifra, R.; Parella, T.;
(15) For recent studies on complexes deriving from polypodal
ligands featuring SnSn bonds, see: (a) Nickl, C.; Eichele, K.; Joosten,
D.; Langer, T.; Schappacher, F. M.; Pottgen, R.; Englert, U.;
̈
Wesemann, L. Angew. Chem., Int. Ed. 2011, 50, 5766. (b) Henning,
J.; Wesemann, L. Angew. Chem., Int. Ed. 2012, 51, 12869.
(16) Phosphine chelation has been used to promote oxidative
addition of Sn−C bonds at Fe, Ni, and Pt. See: (a) Schubert, U.;
Grubert, S. Organometallics 1996, 15, 4707. (b) Gilges, H.; Kickelbick,
G.; Schubert, U. J. Organomet. Chem. 1997, 548, 57. (c) Gilges, H.;
Schubert, U. Eur. J. Inorg. Chem. 1998, 1998, 897.
(17) For oxidative addition of disulfide at copper, see: (a) Rokob, T.
A.; Rulisek, L.; Sogl, J.; Revesz, A.; Zins, E. L.; Schroder, D. Inorg.
̈
Chem. 2011, 50, 9968. (b) Konu, J.; Chivers, T.; Tuononen, H. M.
Chem.Eur. J. 2011, 17, 11844 and references therein.
(18) For reviews on transition metal-assisted activation and addition
of element-element bonds to unsaturated organic compounds, see:
(a) Sharma, H. K.; Pannell, K. H. Chem. Rev. 1995, 95, 1351.
(b) Beletskaya, I.; Molberg, K. Chem. Rev. 1999, 99, 3435.
(c) Beletskaya, I.; Molberg, K. Chem. Rev. 2006, 106, 2320.
(d) Suginome, M.; Ito, Y. Chem. Rev. 2000, 100, 3221.
(19) The reaction of the [Cu(NHC)+] fragment with σ-BH and σ-BB
bonds has been studied computationally and the corresponding
oxidative addition products were found quite unstable compared to the
related σ-borane complexes. See: Lillo, V.; Fructos, M. R.; Ramirez, J.;
Braga, A. C.; Maseras, F.; Mar Diaz-Requejo, M.; Perez, P.; Fernandez,
E. Chem.Eur. J. 2007, 13, 2614.
(20) For the related [2-(chlorodiphenylstannyl)phenyl]diphenyl-
phosphine, see: Lin, T.-P.; Gualco, P.; Ladeira, S.; Amgoune, A.;
Bourissou, D.; Gabbaï, F. P. C. R. Chim. 2010, 13, 1168.
(21) Lee, T.; Lee, S. W.; Jang, H. G.; Kang, S. O.; Ko, J.
Organometallics 2001, 20, 741.
(22) Cordero, B.; Gomez, V.; Platero-Prats, A. E.; Reves, M.;
Echeverria, J.; Cremades, E.; Barragan, F.; Alvarez, S. Dalton Trans.
2008, 2832.
(23) According to a Cambridge Database Search, only a few
compounds featuring short AuSn contacts have been structurally
authenticated so far. Most of these compounds are Au(I) stannyl
complexes and the respective AuSn distances range from 2.56 to 2.94
Å.
(24) Among crystallographically characterized copper(III) com-
plexes, coordination number four and square-planar arrangement
dominate. See Melnik, M.; Kabesova, M. J. Coord. Chem. 2000, 50,
323.
(25) Only one compound featuring a short CuSn bond was found in
the Cambridge Crystallographic database. It is a stannyl NHC
copper(I) complex, and the respective CuSn distance is shorter
(2.469(5) Å). Bhattacharyya, K. X.; Akana, J. A.; Laitar, D. S.; Berlin, J.
M.; Sadighi, J. P. Organometallics 2008, 27, 2682.
(26) A similar situation was previously encountered in the
bis(stannyl) Pd(II) complex deriving from [Ph2P-(o-carboranyl)
SnMe2]2 (Sn···Sn contact = 3.2612(5) Å).21
Benet-Buchholz, J.; Schweiger, A.; Mitrikas, G.; Sola,
Stack, T. D. J. Am. Chem. Soc. 2010, 132, 12299.
̀
M.; Llobet, A.;
(12) (a) Gualco, P.; Amgoune, A.; Miqueu, K.; Ladeira, S.; Bourissou,
D. J. Am. Chem. Soc. 2011, 133, 4257. (b) Joost, M.; Mallet-Ladeira, S.;
Miqueu, K.; Amgoune, A.; Bourissou, D. Organometallics 2013, 32,
898.
(13) The oxidation state is a useful formalism to describe the
electronic structure and reactivity of molecules, but assignment of
oxidation numbers of metals in coordination complexes is sometimes
challenging and somewhat arbitrary. The common rule consists in
transferring each pair of electrons engaged into metal/ligand bonds to
the more electronegative atom. As recognized early on, this leads
sometimes to inappropriate assignments, and alternative descriptions
have to be preferred. In this context, and for sake of consistency and
homogeneity, a common terminology has been used throughout this
+
manuscript to describe (R3P)2M+ and (R3P)2M(ER3)2 (M = Au, Cu
G
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(27) Similarly, complexes deriving from the coordination of
diphosphine-silanes to (Me2S)AuCl tend to rapidly decompose in
the absence of chloride scavengers.12
(28) Mancuso, J.; Lautens, M. Org. Lett. 2003, 5, 1653.
(29) These are general features of copper alkene complexes. See
(a) Dias, H. V. R.; Lovely, C. J. Chem. Rev. 2008, 108, 3223.
(b) Adiraju, V. A. K.; Flores, J. A.; Yousufuddin, M.; Dias, H. V. R.
Organometallics 2012, 31, 7926 and references therein.
(30) A mixture of trans and cis addition products have been observed
upon oxidative addition of a diphosphine-silane-stannane to Pd(0),
followed by insertion of dimethyl acetylene dicarboxylate: Murakami,
M.; Yoshida, T.; Kawanami, S.; Ito, Y. J. Am. Chem. Soc. 1995, 117,
6408.
(31) For Pd-catalyzed bis(stannylation) of CC triple bonds, see ref
28 and Yoshida, H.; Tanino, K.; Ohshita, J.; Kunai, A. Angew. Chem.,
Int. Ed. 2004, 43, 5052.
(32) For mechanistic studies on the activation/functionalization of σ-
SiSi, σ-SnSn, and σ-SiSn bonds with Pd and Pt, see: (a) Pan, Y.;
Mague, J. T.; Fink, M. J. Organometallics 1992, 11, 3495. (b) Murakami,
M.; Yoshida, T.; Ito, Y. Organometallics 1994, 13, 2900. (c) Obora, Y.;
Tsuji, Y.; Nishiyama, K.; Ebihara, M.; Kawamura, T. J. Am. Chem. Soc.
1996, 118, 10922. (d) Ozawa, F.; Kamite, J. Organometallics 1998, 17,
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H.; Fink, M. J. J. Am. Chem. Soc. 2006, 128, 9054.
(33) The 2e redox sequence evidenced here at copper is also relevant
to the catalytic cycle proposed to account for the gold-catalyzed
oxidative cycloaddition of dihydrodisilanes to alkynes, as recently
developed by Stratakis. See: (a) Lykakis, I. N.; Psyllaki, A.; Stratakis,
M. J. Am. Chem. Soc. 2011, 133, 10426. (b) Kotzabasaki, V.; Lykakis, I.
N.; Gryparis, C.; Psyllaki, A.; Vasilikogiannaki, E.; Stratakis, M.
Organometallics 2013, 32, 665.
(34) Baker, R. T.; Nguyen, P.; Marder, T. B.; Westcott, S. A. Angew.
Chem., Int. Ed. 1995, 34, 1336.
(35) (a) Takemoto, Y.; Yoshida, H.; Takaki, K. Chem.Eur. J. 2012,
18, 14841. (b) Yoshida, H.; Kawashima, S.; Takemoto, Y.; Okada, K.;
Ohshita, J.; Takaki, K. Angew. Chem., Int. Ed. 2012, 51, 235.
H
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