Oxidative Addition of Alkenyl and Alkynyl Iodides to a AuI Complex

Article abstract of DOI:10.1002/anie.202000473

The first isolated examples of intermolecular oxidative addition of alkenyl and alkynyl iodides to Au^I are reported. Using a 5,5′-difluoro-2,2′-bipyridyl ligated complex, oxidative addition of geometrically defined alkenyl iodides occurs readily, reversibly and stereospecifically to give alkenyl-Au^III complexes. Conversely, reversible alkynyl iodide oxidative addition generates bimetallic complexes containing both Au^III and Au^I centers. Stoichiometric studies show that both new initiation modes can form the basis for the development of C?C bond forming cross-couplings.

Full text of DOI:10.1002/anie.202000473

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Accepted Article
Title: Oxidative addition of alkenyl and alkynyl iodides to a Au(I)
complex
Authors: Christopher Alan Russell, John Bower, jamie cadge, and
Hazel Sparkes
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202000473
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Oxidative addition of alkenyl and alkynyl iodides to a Au(I)
complex
Jamie A. Cadge, Hazel A. Sparkes, John F. Bower* and Christopher A. Russell*
Abstract: The first isolated examples of intermolecular oxidative
addition of alkenyl and alkynyl iodides to Au(I) are reported. Using a
5,5ʹ-difluoro-2,2ʹ-bipyridyl ligated complex, oxidative addition of
geometrically defined alkenyl iodides occurs readily, reversibly and
stereospecifically to give alkenyl-Au(III) complexes. Conversely,
reversible alkynyl iodide oxidative addition generates bimetallic
complexes containing both Au(III) and Au(I) centers. Stoichiometric
studies show that both new initiation modes can form the basis for the
development of CC bond forming cross-couplings.
determined, and, to the best of our knowledge, there are no
isolated examples of intermolecular Au-mediated oxidative
addition of either alkenyl or alkynyl-halides, although,
encouragingly, such steps have been invoked in recent reports of
gold catalyzed processes utilizing either alkynyl iodides or alkynyl
bromides.[11] The feasibility and nuances of such processes are
therefore inherently uncertain, especially given the unusual
mechanistic traits of ArX oxidative additions, where initial
electron donation from the arene to the Au(I) center was found to
be a key feature (Scheme 1B). In this report, we show that Au(I)-
complexes can indeed be induced to undergo intermolecular
oxidative addition of alkenyl C(sp2)I and alkynyl C(sp)I bonds,
and that, significantly, the former are stereospecific (Scheme 1C).
We also show that these processes can be integrated into cross-
couplings, setting the stage for the long-term development of
catalytic variants.
Significant recent progress has been made in demonstrating
elementary cross-coupling steps at Au-centers.[1] Bourissou,
Amgoune and co-workers and our group have shown that Au(I)-
systems modified with appropriate ligands are effective for
hitherto unknown ArX (X = I, Br) oxidative additions.[2-4] In
addition to these thermal pathways, significant progress has also
been made in oxidative addition at gold using photochemically
activated routes.[5,6] In part, such oxidative addition steps are
challenging[7] because of the high energy barrier associated with
the Au(I)/Au(III) redox couple (Ered: AuIII/I = 1.41 V vs PdII/0 = 0.92
V).[8] Catalysts that can overcome this are significant because
they dispense with the highly reactive external[9] or internal[10]
oxidants typically required in Au-mediated cross-couplings.
Indeed, using ArX oxidative addition as a platform, redox neutral
Au-based aryl CH arylations and Negishi-like cross-couplings
have now been achieved.[3,4] In these studies, reversible ArX
oxidative addition was faster for more electron rich arenes,
offering a counterpoint to the electronic trends typically seen in
Pd-based processes.[2-4] Accordingly, new Au-mediated CX
oxidative additions are not merely of interest from a reactivity
viewpoint but also have significant potential for addressing
broader selectivity challenges in cross-coupling chemistry.
Fundamental to the endeavors outlined above is the
detailed insight into the oxidative addition step provided by
stoichiometric studies (Scheme 1A). The underpinning feature of
Au-systems that are effective for ArX oxidative addition is the
use of bidentate ligands, namely a carboranyl diphosphine
(Bourissou & Amgoune),[2] a hemilabile P,N-system (Bourissou &
Amgoune),[3a,c] and 2,2ʹ-bipyridine (our group, Scheme 1B).[4]
These enforce sufficient changes to the properties of the Au-
center to allow ArX oxidative addition. However, the generality
of this tactic for other classes of CX bond has not been
Scheme 1.
J. A. Cadge, Dr. H. A. Sparkes, Prof. J. F. Bower, Dr. C. A. Russell
School of Chemistry, University of Bristol
Cantock’s Close, United Kingdom, BS8 1TS
Our studies to explore the feasibility of alkenyl C(sp2)I
oxidative addition began by examining 5,5ʹ-difluoro-2,2ʹ-bipyridyl
gold(I) ethylene complex 1 (Scheme 2A).[4] As evidenced by
spectroscopic analysis of the ethylene unit, the narrow bite angle
(75) bipyridyl ligand leads to significant back donation to the co-
E-mail: chris.russell@bris.ac.uk, john.bower@bris.ac.uk
Supporting information for this article is given via a link at the end of
the document.
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ligand, a feature that is desirable for promoting oxidative addition.
Reaction of 1 with iodostyrene (E)-2a under static vacuum[12] gave
a bright yellow solution whose analysis by 19F NMR spectroscopy
revealed two new multiplets (1:1 ratio), indicating
desymmetrization of the Au-complex, and mass spectrometry
styrenyl alkenes do not readily participate, e.g., oxidative addition
with (E)-2f (R = n-Bu) to form (E)-3f was sluggish, even at 50 C,
whereas (E)-2g and (E)-2h did not provide (E)-3g and (E)-3h.
Oxidative addition of -iodostyrene 2i was efficient and reversible,
giving Au(III) complex 3i in 76% yield (Scheme 2B). The fluorine
atoms of 1 were included as a reporter nucleus (19F NMR); similar
results were obtained using 2,2ʹ-bipyridine as ligand (see SI).
Relative initial rates were determined for the formation of
(E)-3a, d and e (see SI). Compared to parent (E)-2a (krel = 1.0),
the p-OMe analogue (E)-2d gave a higher rate of oxidative
addition (krel = 1.8), whereas electron poor p-CF3 analogue (E)-2e
reacted more slowly (krel = 0.02). This trend mirrors observations
made previously with aryl iodides,[4] and indicates that initial
electron donation from the vinyl unit to the Au center is a key factor.
One explanation is that an electron donating R-group is better
able to stabilize developing positive charge at the α-carbon of the
alkene during oxidative addition (cf. Scheme 1B). An alternate
explanation is that electron rich alkenes facilitate displacement of
ethylene from 1 to give a new -complex in advance of oxidative
addition. At atmospheric pressure, 19F NMR profiling of the
formation of (E)-3d revealed the rapid disappearance of 1 (F =
127.2 ppm) to give an intermediate (F = 121.8 ppm) en route
to the product; this intermediate is most likely the Au--complex
of (E)-2d.[14] When 1 was exposed to tert-butyl vinyl iodide (E)-2g
under analogous conditions, a similar intermediate formed, albeit
to a lesser extent, indicating that displacement of ethylene still
occurred, even though conversion to (E)-3g was not observed.
Overall, these observations indicate that oxidative addition, rather
than displacement of ethylene from 1, is the more demanding step
and that the alkenyl iodide must have the correct properties to
facilitate this.
Scheme 2. Oxidative addition of alkenyl iodides. [a] Conversion by 19F NMR
after heating at 50 C for 2 h.
Scheme 3. Transmetallation and reductive elimination from (E)-3a. [a]
Determined by GC analysis.
Figure 1. Thermal ellipsoid plot (50% probability level) of (E)-3b; counterion and
H atoms omitted for clarity. Selected bond lengths (Å) and angles (): N1-Au1
2.081(3), N2-Au1 2.119(4), N1-Au1-N2 78.65(14), Au1-I 2.5370(3) Au1-C11
2.007(4), Au1-C11-C12 120.6(3).
Based on the above, the stereospecificity of oxidative
addition with respect to alkenyl iodide geometry was uncertain: if
a substantial buildup of positive charge occurs at the -carbon
then scrambling of stereochemistry might ensue due to the
diminished double bond character. Accordingly, oxidative addition
studies were undertaken using (Z)-2a-c. In the event, Au(III)
complexes (Z)-3a-c were formed in 72-87% yield and with transfer
of alkenyl iodide stereochemistry (Z:E >9:1). Accordingly, the
process is diastereospecific, such that the geometry of the alkenyl
iodide determines the geometry of the alkenyl ligand of the Au(III)
complex. This feature is analogous to the synthetically valuable
diastereospecificity observed for oxidative addition of alkenyl
halides to Pd(0) complexes.[15,16] To validate subsequent CC
bond formations, (E)-3a was exposed p-FC6H4ZnCl 4 and this
(ESI+) gave m/z 618.9754 (calcd. 618.9757, [MNTf2]+). These
observations, as well as other data (see SI), are consistent with
oxidative addition to give Au(III) complex (E)-3a.[13] When a
solution of (E)-3a was exposed to an atmosphere of ethylene
reversal to starting complex 1 was observed, demonstrating the
reversibility of oxidative addition.[12] Similar behavior was
observed for p-substituted variants (E)-2b-e. The structure of (E)-
3b was confirmed by single crystal X-ray crystallogaphy, which
revealed a distorted square-planar geometry (Figure 1). Non-
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generated 5 in 41% yield (Scheme 3). Alkenyl iodide (E)-2a,
resulting from CI reductive elimination, was observed in 28%
yield, along with small quantities of homocoupling product 6.
Accordingly, transmetallation and CC reductive elimination are
feasible, such that all steps of a conventional Negishi cross-
coupling can be achieved at a single Au-center.
the +1 oxidation state, have been shown to be pivotal in dual
activation gold catalysis.[18] The Au(III) center of 8a has a distorted
square-planar geometry; the Au(I) center is symmetrically bonded
to the F2-bipy unit and asymmetrically bonded to the alkyne {8%
slippage towards C(11)}. The CC unit bonded to the Au(I) center
is lengthened {1.261(5) Å} and the Ph substituent of this unit is
bent back to 150.5(4). These data deviate more strongly from
those of a free alkyne in comparable cationic complexes
containing 2-coordinate gold for which the limited pool of X-ray
data show bond distances that average 1.21 Å and C≡C–C bend
20]
back angles that average 168.[19,
The more pronounced
deformation of the acetylide unit in 8a is consistent with previous
observations that 3-coordinate cationic Au(I)-complexes express
enhanced -back-donation compared to their 2-coordinate
counterparts.[4,21] Related bimetallic species, containing both -
and -bound Au-centers, have been generated previously by
distinct methods.[22] A complex similar to 8b can also be formed
using 2,2ʹ-bipyridine as the ligand (see SI).
Scheme 4. Oxidative addition of alkynyl iodides.
Figure 2. Thermal ellipsoid plot (50% probability level) of 8a with H atoms, the
counterion and solvent of crystallization omitted for clarity. Selected bond
lengths (Å) and bond angles (): N1-Au1 2.169(3), N2-Au1 2.172(3), N1-Au1-
N2 75.62(11), C11-Au1 2.029(4), C12-Au1 2.089(4), C11-C12 1.261(5), C11-
C12-C13 150.5(4), Au2-C11-C12 149.0(3), C11-Au2 1.976(4), I-Au2 2.5752(4),
Cl-Au2 2.241(6), N3-Au2 2.089(3), N4-Au2 2.080(3), N3-Au2-N4 79.75(12). The
structure contains disordered Cl/I atoms (I/Cl 0.78/0.22 occupancies
respectively), as observed with similar complexes.[2,17]
Scheme 5. Electronic effects on the oxidative addition of alkynyl iodides. [a]
Conversion by 19F NMR analysis (isolated as a mixture of 8e and 7e).
When a CH2Cl2 solution of 8a was pressurized with ethylene
(1 bar) complete conversion to 1 was observed, demonstrating
once again the reversibility of CI oxidative addition. Monitoring
of the forward process (1 to 8a) by 19F NMR spectroscopy
revealed initial conversion to an intermediate, symmetrical
complex, which was tentatively assigned as the Au--complex of
phenyliodoacetylene 7a. tert-Butyl and cyclohexyl systems, 7b
and 7c, behaved similarly, providing 8b and 8c in 60% and 72%
yield (Scheme 4). Interestingly, distinct outcomes were observed
for p-CF3 and p-OMe substituted systems 7d and 7e. For the
former, conversion to monometallic complex 8d occurred and a
bimetallic complex (cf. 8a) was not observed (Scheme 5A). Here,
the electron poor alkynyl unit of 8d is presumably a weaker ligand
for Au(I) such that competing -complexation is not
thermodynamically favored. Compared to parent system 7a, the
initial rate of oxidative addition with 7d was also much slower (see
SI). By contrast, p-OMe substituted system 7e underwent
oxidative addition faster, and, perhaps surprisingly, also delivered
Next, the feasibility of oxidative addition at sp-centers was
explored by exposing 1 to various alkynyl iodides (Scheme 4).
Under static vacuum, reaction of phenyliodoacetylene 7a with 1
generated a bright orange solution, and complete conversion was
verified by 19F NMR spectroscopy. Three new signals were
observed (1:1:2 ratio), consistent with the presence of two Au-
centers, one of which has been desymmetrized. A similar
outcome was observed at 20 C, whereas higher reaction
temperatures led to decomposition. Mass spectrometric analysis
(ESI+) gave m/z 616.9599, which corresponds to the initially
expected oxidative addition product (calcd. 616.9601, [MNTf2]+).
Ultimately, the structure of the new species was confirmed by
single crystal X-ray crystallography, which revealed a mixed
valence digold complex 8a, where CI oxidative addition has
occurred at one Au-center and the other is -bound to the alkynyl
unit (Figure 1). This result compliments the impressive body of
results where digold species, typically with both metal centers in
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a monometallic complex (8e) (Scheme 5B). Signals consistent
with alkynyl iodide 7e were also observed. We suggest that the
relatively electron rich alkyne of 7e can displace Au(I) from any
initially formed bimetallic species. The observed trends in the
rates of oxidative addition indicate that charge stabilization in the
transition state is once again a key factor (cf. Scheme 1B).
Reaction of 8a with alkynyl-zinc reagent 9 delivered
coupling product 10 in 47% yield; homocoupled products 11 and
12 also formed in 3% and 7% yield, respectively.[23] Other classes
of organometallic nucleophile resulted predominantly in
homocoupling products (see SI). The origin of this side process
was investigated using an analogue of 8a derived from 13C-
labelled phenyliodoacetylene (see SI). 13C NMR profiling of a
representative cross-coupling revealed the immediate formation
of alkynyl iodide 7a, providing direct evidence for CI reductive
elimination under the reaction conditions. The formation of
homocoupled products then ensued, and these increased over a
week, suggesting that their formation is promoted by
decomposition products of the Au-complex. Hashmi and co-
workers have developed catalytic CC bond formations where
alkynyl-Au(III) species were generated by CI cleavage; these
processes did not proceed via formal CI oxidative addition and
required highly reactive alkynyl-iodine(III) reagents.[20c] The
present study suggests that more available and atom economical
alkynyl-iodine(I) precursors can form the basis for future Au-
catalyzed alkynylations.[24]
Acknowledgements
We thank the Bristol Chemical Synthesis Centre for Doctoral
Training, funded by the EPSRC (EP/L015366/1) and the
University of Bristol (studentship to JAC) and the Royal Society
(URF to JFB) for funding. We thank Tom Leman and Dr Paul
Gates (Bristol) for their assistance with mass spectrometry.
Keywords: gold • oxidative addition • alkenyl iodide • alkynyl
iodide • bipyridyl
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7921.
[23] Exposure of 8a to p-FC6H4ZnCl delivered the expected Negishi-type
product in 14% yield (see SI).
[24] During the preparation of this manuscript
a process invoking
photosensitized oxidative addition of alkynyl iodides to Au(I) was
reported: Z. Xia, V. Corcé, F. Zhao, C. Przybylski, A. Espagne, L. Jullien,
T. Le Saux, Y. Gimbert, H. Dossmann, V. Mouriès-Mansuy, C. Ollivier, L.
Fensterbank, Nature Chem. 2019, 11, 797.
[25] Analogous processes with allyl iodide resulted in competing N-allylation
of the bipyridyl ligand. Alkenyl/alkynyl bromides are not suitable
electrophiles.
[18] See for example: a) A. M. Asiria and A. S. K. Hashmi Chem. Soc. Rev.
2016, 45, 4439; b) A. S. K. Hashmi Acc. Chem. Res. 2014, 47, 864.
[19] a) H. Schmidbaur and A. Schier, Organometallics 2010, 29, 2; b) M. S.
Nechaev, V. M. Rayon and G. Frenking, J. Phys. Chem. A, 2004, 108,
[26] Efforts to develop catalytic variants are ongoing. We suggest the key
impediment is the fragility of the [(bipy)Au]+ complex that is generated
upon CC reductive elimination.
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10.1002/anie.202000473
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Jamie A. Cadge, Hazel A. Sparkes,
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Oxidative addition of alkenyl and
alkynyl iodides to a Au(I) complex
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