Mechanistic study of gold(I)-catalyzed hydroamination of alkynes: Outer or inner sphere mechanism?

Article abstract of DOI:10.1002/anie.201402557

An experimental mechanistic study of the gold(I)-catalyzed hydroamination shows the formation of conformationally flexible auro-iminium salts Au-Im, which originate from the protonation of a vinyl gold species. Rotation around the C-CAu bond is the reason

Full text of DOI:10.1002/anie.201402557

Angewandte
Chemie
DOI: 10.1002/anie.201402557
Gold Catalysis
Mechanistic Study of Gold(I)-Catalyzed Hydroamination of Alkynes:
Outer or Inner Sphere Mechanism?**
Alexander Zhdanko* and Martin E. Maier*
Abstract: An experimental mechanistic study of the gold(I)-
catalyzed hydroamination shows the formation of conforma-
tionally flexible auro-iminium salts Au-Im, which originate
from the protonation of a vinyl gold species. Rotation around
À
the C CAu bond is the reason for the loss of stereospecificity
of protodeauration, which explains the stereochemical result of
the Stradiotto reaction. The ambiguity about inner or outer
sphere mechanism is thus resolved in favor of the outer sphere
mechanism.
G
old(I)-catalyzed hydroamination of alkynes is an impor-
[1]
ꢀ
tant synthetic tool for the functionalization of C C bonds.
Like some other processes in gold catalysis, it was already
described in 1987,[2] but did not receive much attention until
around 2000, when the interest in gold catalysis started to rise.
Nowadays, the scope of gold(I)-catalyzed hydroamination of
alkynes is not limited to the synthesis of imine or enamine
Scheme 1. Short representation of inner and outer sphere mechanism
proposed for the hydroamination of alkynes.
À
structures by the formal addition of various N H reagents
onto the C C bond.[3,4] Rather, a number of complex cascade
ꢀ
reactions were reported, using this addition as one step within
a complicated multistep process.[5] However, the hydroami-
nation reaction often requires harsh conditions, which limits
its scope. Last but not least, the development of the method is
also limited because of the lack of understanding of the
reaction mechanism. In several studies gold–amine complexes
[LAu(amine)]+ were identified as reaction intermediates. This
fact, together with the stereoselective formation of syn-E in
the intermolecular reaction described by Stradiotto and co-
workers, was considered as sufficient evidence for the inner
sphere mechanism (Scheme 1).[3a,6] However, ionic organo–
gold complexes formed upon intramolecular 5-endo-dig and
5-exo-dig addition of a tertiary amine moiety described by
Bertrand and co-workers[7] as well as a single vinyl–gold
complex described by Hashmi and co-workers,[8] provided
evidence for the outer sphere mechanism. It should be
stressed, that according to the current understanding of
protodeauration, the outer sphere mechanism should lead to
the product of anti-addition (anti-E), which is in contradiction
to the formation of syn-E described by Stradiotto and co-
workers.[3a] As it stands, there is still an unresolved ambiguity:
should the reaction generally be considered as proceeding
through an inner or outer sphere mechanism (Scheme 1)?
As part of our studies on mechanisms of gold catalysis,[9]
we report herein an experimental NMR investigation of the
mechanism of the gold(I)-catalyzed hydroamination of
alkynes. We describe the key organo–gold intermediates
originating from catalysts [Ph3PAuNCMe]+ SbF6 (1) and
À
[L2AuNCMe]+ SbF6À (2) (L2 = o-(di-tert-butylphosphino)bi-
phenyl) as well as the chemistry of gold complexes relevant to
the process, leading to a consensus: the gold(I)-catalyzed
hydroamination reaction is best described by the outer sphere
mechanism, whereas the inner sphere mechanism cannot be
accepted anymore as an explanation.
The addition of catalyst 2 (4 mol%) to alkyne-amine S1 in
CDCl3 led to a sluggish catalytic reaction. Monitoring by
1H NMR spectroscopy showed the immediate formation of
two organo–gold species Au1 and Au2, but after a few
minutes only Au2 remained (Scheme 2). After 8 h 60–70%
conversion of S1 was achieved, giving a mixture of enamines
E1/E2 as the main product.[10] In a stoichiometric study,
1.5 equiv of S1 reacted with 1 equiv of 2 in CDCl3 instantly
giving complex Au2 which was isolated in its individual state
(80% yield, 100% yield in situ) and fully characterized by
NMR spectroscopy and X-ray analysis. However, reaction of
a slight excess of 2 with S1 led to the immediate formation of
Au1 (> 90% in situ). Reaction of the sterically less hindered
catalyst 1 led to a mixture of diaurated species D1 and Au3.
Complexes Au1–Au3 are depicted as auro-iminium salts
and not as gold enamine p-complexes. This structure assign-
ment and understanding of bonding follows from the NMR
[*] Dipl.-Chem. A. Zhdanko, Prof. Dr. M. E. Maier
Institut fꢀr Organische Chemie, Universitꢁt Tꢀbingen
Auf der Morgenstelle 18, 72076 Tꢀbingen (Germany)
E-mail: Vinceeero@gmail.com
martin.e.maier@uni-tuebingen.de
[**] Financial support by the State of Baden-Wꢀrttemberg is gratefully
acknowledged. We thank Dr. K. Eichele and the Institut fꢀr Anor-
ganische Chemie for allowing us to use their NMR spectrometer.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201402557.
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Figure 3. Slippage mechanism and structure comparison of represen-
tative gold–enol ether,[11a] gold–enamine, and gold–ene-1,1-diamine
complexes.[11b]
Correspondingly, Au2 exhibits a high degree of h1-coordina-
Scheme 2. Reactions of amine S1 with catalysts 1 and 2.
tion mode to gold and a predominant loss of the double-bond
À
character of the C1 C2 bond; this enables easy rotation
spectra in solution and X-ray analysis. The 1H NMR spectrum
of pure Au2 in CD2Cl2 at room temperature contains simple
signals (including the doublet at 1.91 ppm, JP = 8.9 Hz of the
CH2Au group), but at low temperature the spectrum shows
a complicated pattern (Figure 1). Structural parameters
determined from the solid structure of Au2 are between the
typical values for known gold–enol ether complexes and
gold–ene-1,1-diamine complexes (Figures 2 and 3).[11] This
unambiguously characterizes the alkyl enamine E1 as a ligand
with a high degree of slippage when coordinated to gold.
around this bond and therefore Au2 should rather be
considered an alpha auro-iminium salt than a p-alkene
complex. This property is the key to understand the stereo-
chemical outcome of alkyne hydroamination reactions (see
below).
In order to conclude about the role of auro-iminium salts
in the hydroamination mechanism, the origin of these species
has to be established. First, the chemistry of Au1 and Au2 was
studied. The addition of free S1 to a solution of Au1 led to the
rapid rearrangement of Au1, thereby forming Au2 (with
concomitant slow catalytic reaction). The same occurs upon
addition of a moderately strong nucleophile (Me2S) to Au1.
However, the addition of ClÀ as a strong nucleophile either to
Au1 or Au2 led to complete liberation of [L2AuCl] and the
formation of free enamines E1/E2 as an equilibrium mixture
(Scheme 3). It becomes clear that the rearrangement of Au1
to Au2 is simply a ligand exchange with free E1, giving the
thermodynamically more stable Au2. This process can be
considered as catalytic in enamine (or another nucleophile).
Figure 1. 1H NMR spectra of Au2 at various temperatures in CD2Cl2.
Figure 2. Structure of the Au2 cation in the solid state. All hydrogens
are omitted except for the CH2Au fragment.
Scheme 3. Reaction of auro-iminium salts with nucleophiles.
2
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To confirm that Au2 arises as a product of ligand exchange
and not as a direct participant of the catalytic cycle, we
performed the hydroamination of S1 in the presence of an
acid (1,8-bis(dimethylamino)naphthalene triflate salt,
PrSpH+·OTfÀ). Under these conditions the formed enamines
are immediately protonated, giving iminium salt I1 as a single
final product (Scheme 4).[12] Because of the negligible con-
reduce the basicity of the relevant species. When S3 was
treated with 1,8-bis(dimethylamino)naphthalene (PrSp) and
catalyst 2, the diaurated species D2 was generated as the
single organo–gold product. Upon treatment with K2CO3,
Me2S, or picoline, D2 readily gives rise to B2 (Scheme 5). In
addition, diaurated species were found in the aforementioned
reaction of S1 with catalyst 1 (D1, Scheme 2). Even though B1
or B2 cannot be directly observed under normal catalytic
conditions, the formation of the diaurated species confirms
that vinyl–gold indeed has to be an intermediate of the
hydroamination process, because diaurated species are
known to be the product of trapping vinyl–gold intermediates
with an LAu+ unit.[13,9] Correspondingly, the auro-iminium
salt is considered to be the product of protonation of the
vinyl–gold species.
After the work of Bertrand and co-workers,[7] these
observations further confirm the outer sphere mechanism at
least for intramolecular gold-catalyzed hydroamination reac-
tions since the formation of vinyl–gold complex B as an
essential intermediate is simply not possible by an inner
sphere mechanism. However, there is no reason to assume the
mechanism to be different for the intermolecular version.[14]
So far, the stereochemical outcome of the intermolecular
hydroamination reaction as described by Stradiotto and co-
workers[3a] was the only experimental evidence which was
seemingly inconsistent with the outer sphere mechanism.
However, from now on with the knowledge of the nature of
auro-iminium salts, it is possible to correctly explain these
experimental results also with regard to this mechanism. Thus,
even though the anti-addition of an amine onto a gold–alkyne
complex would lead to vinyl–gold in a stereospecific manner,
this stereochemical information is completely lost upon
formation of the conformationally flexible auro-iminium
salt Au-Im, which makes the overall protodeauration process
no longer stereospecific, unlike it is generally considered for
other vinyl–gold species (Scheme 6a). It is clear, that via Au-
Im, gold will also catalyze the isomerization of one geo-
metrical isomer of an enamine into another until the
thermodynamic equilibrium is reached, regardless of the
exact stereochemical outcome of the ligand exchange itself
(Scheme 6b).[15] Likewise, the isomerization of enamines will
additionally occur through the classical protonation/deproto-
nation pathway promoted by traces of Brønsted acid in the
reaction mixture. In literature, the E-enamine is known to be
thermodynamically more stable than the Z-enamine, and this
simple fact explains the selective formation of E-enamine in
the reaction described by Stradiotto and co-workers.[16] The
Scheme 4. Observation of resting states.
centration of free enamine at all times, the rearrangement of
Au1 to Au2 should be suppressed. Indeed, gold was exclu-
sively present as Au1 (directly evidenced from NMR spectra).
This confirms that Au1 is the primary organo–gold inter-
mediate, originating from pure 5-endo-dig cyclization, and
that Au2 is a thermodynamically more stable isomer origi-
nating from ligand exchange with free E1. In addition, the
application of phenyl-substituted alkyne-amine S2 leads to
single enamine E3. Accordingly, the single auro-iminium salt
Au4 was observed as a resting state (Scheme 4).
In the next step the mechanism of the formation of the
auro-iminium salt was analyzed. Since the 5-endo-dig cycliza-
tion cannot be triggered by a proton, we hypothesized that the
auro-iminium salt Au1 forms as a result of the protonation of
vinyl–gold complex B1, which itself is undetectable because
of its very high affinity to protonation (Scheme 5). Indeed,
attempts to deprotonate Au2 to generate exo-vinyl–gold
failed, suggesting that this species is highly basic (and
definitely more basic than simple enamines). To confirm the
vinyl–gold complex as an essential organo–gold intermediate,
we studied substrate S3 having a sulfonamide functionality to
Scheme 5. Origin of auro-iminium. Confirmation of the intermediacy of
vinyl–gold.
Scheme 6. a) Loss of stereoselectivity due to protodeauration and
b) isomerization of enamines through auro-iminium salt Au-Im.
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overall stereochemical outcome of the reaction is thus
determined solely by the thermodynamics of the final
enamines. With this explanation, the consensus is reached
that gold-catalyzed hydroamination reactions should be
described by the outer sphere mechanism, same as other
hydrofunctionalization reactions of alkynes.[17]
To confirm this conclusion experimentally, we generated
auro-iminium salt Au6 from the corresponding E-E5. The
1H NMR spectrum of Au6 exhibits a similar temperature
dependence as was described above for Au2, confirming the
elevated temperature. However, the situation improves sig-
nificantly when amide substrates with a reduced binding
affinity to gold are used. For example, the reaction of S3 takes
only a few minutes (or less), whereas the reaction of S1
requires many hours (Schemes 5 and 2). The same accounts
for the reactivity series established by Tanaka and co-workers
(anilines with electron-deficient substituents react faster).[3b]
In summary, the gold-catalyzed hydroamination reaction
is described by the outer sphere mechanism which is also the
case for other hydrofunctionalization reactions of alkynes.
The process includes the formation of gold–enamine com-
plexes as intermediates. These complexes exhibit a loss of the
double bond character, which enables an easy rotation around
À
rotation around the C1 C2 bond (see the Supporting
Information). Addition of Me2S to a solution of Au6
regenerates E-E5 with > 95% stereoselectivity, Scheme 7.
À
the C CAu bond, and therefore they are viewed as auro-
iminium salts. Because of the rotation, the liberation of an
enamine from the auro-iminium salt upon ligand exchange is
not stereospecific. This is responsible for the fact that the
protodeauration of enamine-derived vinyl–gold complexes B
is not stereospecific, thereby constituting the fundamental
difference from other protodeaurations known to occur with
retention of the configuration at the double bond.[19] Via
intermediate auro-iminium salts, gold will also catalyze the
isomerization of one geometrical isomer of an enamine into
the other until the thermodynamical equilibrium is reached.
Furthermore, the positional rearrangement of the enamine
double bond can occur as a Brønsted-acid-catalyzed process.
It can be concluded, that the gold-catalyzed hydroamination
reaction yields enamines as a thermodynamically controlled
mixture of isomers and can only be selective if the dominant
isomer is considerably more stable than the other. The overall
mechanism of the gold-catalyzed hydroamination reaction
appears to be similar to hydroamination reactions catalyzed
by other transition metals (e.g. palladium(II)[20] and iridium-
(III)[21]).
Scheme 7. Preparation of an enriched sample of Au6 and its conver-
sion back to E-E5.
À
The fast rotation of the C1 C2 bond in Au6 implies that the
formation of this compound is independent of the initial
geometric configuration of the double bond at the enamine.
Therefore, the same compound would be formed from the
corresponding Z-isomer of E5. This argument together with
the stereoselective regeneration of E-E5 from Au6 confirms
the mechanism of enamine isomerization which is proposed in
Scheme 6 and that our explanation of the stereochemical
outcome of the intermolecular hydroamination reaction is
consistent with the outer sphere mechanism.
Received: February 18, 2014
Revised: April 7, 2014
To gain a deeper understanding of the course of the
hydroamination process, we determined the binding affinity
of the L2Au+ unit to representative neutral compounds using
the ligand exchange with auxiliary nucleophiles with known
binding affinities (Scheme 8).[18] From the ligand strength
series it is clear that simple alkyl amines and alkyl enamines
would be @ 105 stronger ligands than the alkyne substrate.
Not surprisingly, [LAu(amine)]+ complexes were previously
as well as in our research observed as resting states. It appears
that the ligand exchange equilibrium strongly disfavors the
formation of the activated gold–alkyne complex A. This is the
principal reason why hydroamination reactions often require
Published online: && &&, &&&&
Keywords: gold catalysis · hydroamination ·
.
reaction mechanisms
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Scheme 8. Binding affinity scale for L2Au+ (pic=4-picoline).
4
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Communications
Gold Catalysis
A. Zhdanko,*
M. E. Maier*
&&&& — &&&&
Mechanistic Study of Gold(I)-Catalyzed
Hydroamination of Alkynes: Outer or
Inner Sphere Mechanism?
Which way is right? Experimental mech-
anistic study of hydroamination reveals
the formation of conformationally flexible
auro-iminium salts Au-Im, which origi-
nate from protonation of the vinyl gold
bond is the reason for the loss of
stereospecificity of protodeauration,
which explains earlier stereochemical
results. This shows that the reaction
proceeds through the outer sphere
mechanism.
À
species B. Rotation around the C CAu
6
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