Elementary steps in gold catalysis: The significance of gem-diauration

Article abstract of DOI:10.1002/anie.201003349

Disturbing neighbors: Alkenylgold species with a heteroatom substituent are thought to be key intermediates in gold-catalyzed trans additions of protic nucleophiles to alkynes. One reason for the scarcity of such compounds lies in the non-innocence of the neighboring heteroatom, which may enforce the uptake of a second gold fragment with formation of surprisingly robust species with a gem-digold unit adjacent to a largely cationic center (see scheme).

Full text of DOI:10.1002/anie.201003349

Communications
DOI: 10.1002/anie.201003349
Homogeneous Catalysis
Elementary Steps in Gold Catalysis: The Significance of gem-
Diauration**
Gꢀnter Seidel, Christian W. Lehmann, and Alois Fꢀrstner*
Despite an impressive range of structural manifestations, all
the transformations of alkynes induced by carbophilic noble
metal catalysts are thought to proceed by a generic mecha-
nism (Scheme 1).[1,2] Coordination of the substrate to a PtII or
AuI center followed by attack of the nucleophile on the
resulting p-complex A generates an alkenylmetal species B by
net trans addition of the reactants.[3] Loss of H+ followed by
rapid protodeauration releases product D and regenerates the
catalyst. The efficacy of these latter steps thwarted attempts
to isolate complexes of type C, which are largely unknown
despite their central role within the standard catalytic cycle.
Only recently have selected examples of alkenylgold species
bearing a vicinal heteroatom substituent X been structurally
characterized (see Scheme 1).[4–7]
By considering the isolobal relationship between
a proton and a LAu+ fragment,[8] however, one may
speculate that the catalyst itself could also (reversi-
bly) react with C to give complexes of type E; this
pathway might seriously compete with protodeaura-
tion given the affinity of the carbophilic late tran-
sition metal to p bonds.[1] A recent report by Gagnꢀ
and co-workers lent credence to this notion. These
authors showed that the ordinary vinylgold com-
pound 2 is capable of intercepting a second gold
fragment to form the gem-diaurated complex 3
during the catalytic hydroarylation of allene 1
(Scheme 2).[9] Despite a three-center two-electron
bonding mode, the “noncanonical” complex 3 was
found to be relatively stable, but it reverted to
intermediate 2 on treatment with PPh3, Brꢀ, or
alumina. The constitution of 3 was inferred from mass
spectrometry and NMR data, even though the signal
of its gem-dimetalated carbon atom could not be
detected in the 13C NMR spectrum.[9] Related gem-
digold species were calculated to be competent
intermediates in gold-catalyzed cycloisomerization
reactions of 1,5-allenynes.[10,11] As part of our inves-
tigations on the elementary steps of noble metal
catalysis,[2,12,13] we now present the first two fully
characterized species of this type, the remarkable
stability of which may have implications for gold
Scheme 1. Generic catalytic cycle for the gold-catalyzed trans addition of a protic
catalysis in general. Moreover, it is shown that the
bias of an alkyne substrate to undergo geminal
diauration is strongly correlated with the nature of
ꢀ
nucleophile X H to an alkyne substrate. Structurally characterized complexes of
[4,5]
=
type C comprising a X-C C-Au substructure are shown; R=iPr; X=heteroele-
ment.
the incoming nucleophile and the chosen gold source.
Rather than trying to obtain alkenylgold species
of type C by the route depicted in Scheme 1, we
[*] Ing. G. Seidel, Prof. C. W. Lehmann, Prof. A. Fꢀrstner
Max-Planck-Institut fꢀr Kohlenforschung
45470 Mꢀlheim/Ruhr (Germany)
aimed to prepare such intermediates by exchanging gold for
boron (Scheme 3). Although such reactions are usually
performed in 2-propanol,[14] nonprotic media were found to
be equally suitable. Specifically, 1-ethoxypropyne (5)[15] was
hydroborated with catecholborane and the resulting alkenyl-
boronate 6[16] treated with the Gagosz complex
[(Ph3P)AuNTf2] (1 equiv)[17] in THF in the presence of
Cs2CO3. Much to our surprise, the 11B NMR spectrum of an
aliquot of the reaction mixture indicated that only half of the
starting boronate ester was consumed. The high-resolution
Fax: (+49)208-306-2994
E-mail: fuerstner@kofo.mpg.de
[**] Generous financial support from the MPG and the Fonds der
Chemischen Industrie is gratefully acknowledged. We thank the
analytical departments of our Institute for excellent support of our
programs.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003349.
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Chemie
tallizations, a sample of 8 suitable for X-ray diffraction
analysis was obtained.
As evident from Figure 1, the two gold atoms bind almost
ꢀ
ꢀ
equidistantly to the same C atom; the C1 Au1 and C1 Au2
bonds at 2.130(14) ꢂ and 2.181(16) ꢂ, respectively, are only
Scheme 2. A gem-diaurated compound with a three-center two-electron
bond as a competent intermediate in the hydroarylation of allenes, see
Ref. [9].
Figure 1. Structure of 8 in the solid state; disordered THF in the
crystal is omitted for clarity.[19]
[20]
ꢀ
slightly longer than an average C Au single bond.
The
conceivable alternative mode, in which only one LAu+
fragment is s bonded, while the second one engages in side-
on coordination, does not make any substantial contribution
to the ground-state structure of 8. As a consequence, the
=
former C C bond in 6 has lost much of its double-bond
ꢀ
character, as can be gleaned from the C1 C2 bond length of
Scheme 3. Preparation of an air-stable gem-diaurated complex and
Newman projection along its C2···C1 axis, which illustrates the
oxocarbenium center at C2 as well as the eclipsing orientation of the
EtO and Me substituents (the torsion angle O1-C2-C1-C5 is only 1.78):
a) catecholborane, neat, 708C, 55%; b) [(Ph3P)AuNTf2], Cs2CO3, THF,
RT, 85%.
ꢀ
1.41(2) ꢂ. Rather, the C2 O1 bond is very short (1.32(2) ꢂ)
ꢀ
compared with the reference bond length for -CH2 OR:
1.426 ꢂ,[21] thus confirming the build-up of considerable
charge density at this site. In the extreme, one can interpret
the structure of 8 as an oxocarbenium cation flanked by a
dimetalated center (see the Newman projection in Scheme 3).
These data indicate that complex 8—in contrast to Gagnꢀꢁs
compound 3—does not rely on a three-center two-electron
bond, but rather comprises two almost regular carbon–gold
s bonds, one of which is formed at the expense of the p system
of the precursor complex. Overall, the formation of 8 is
reminiscent of the reactivity pattern of organosilicon com-
pounds, in which metalation is directed toward the a position
and positive charge is stabilized at the b carbon atom by
electrospray mass spectrum of the resulting product showed a
signal at m/z = 1003.1801, consistent with the presence of the
diaurated cation 8. At no time could the expected mono-
metalated species 7 be detected, irrespective of the Au/B ratio
chosen. This finding implies that the interception of the
second gold fragment is faster than the formation of the
monoaurated complex. With this information in hand, the
exchange reaction was repeated with two equivalents of
[(Ph3P)AuNTf2], which afforded complex 8 in 85% yield after
purification by routine flash chromatography on silica gel.[18]
Thus, 8 is even more stable than Gagnꢀꢁs complex 3, which
was reported to rapidly decompose on silica or Florisil.
The diauration manifests in significant spectral changes.
Thus, the signal for the metalated atom C1 shifts from dC =
96.5 ppm in boronate 6 to dC = 116.6 ppm (t, 2JPC = 60.6 Hz) in
the gold complex 8; likewise, the signal for the alkoxylated
atom C2 experiences a pronounced downfield shift from dC =
159.9 ppm in 6 to dC = 174.8 ppm in 8. This significant
deshielding may reflect a build-up of an appreciable level of
positive charge at C2. Further information was sought to
corroborate this interpretation. After many attempted crys-
hyperconjugation with the C Si bond.[22] To what degree the
Au···Au interaction in 8 (Au1 Au2 2.7591(8) ꢂ)
butes to the unusual stability of this complex remains to be
elucidated.
Rendering the double bond of a vinylgold species of type
C less electron rich and/or reducing the ability of the
heteroelement to stabilize adjacent positive charge should
have an impact on its propensity to undergo such a diauration.
To probe this notion, the readily available compounds 9, 13,
and 15 were subjected to the gold-for-boron swap (Schemes 4
and 5). Most interestingly, the commercial pinacol boronate 9
with a cyclopropyl substituent rather than a heteroatom
closely mimicked the reaction behavior of its alkoxylated
cousin 6. On treatment with [(Ph3P)AuNTf2] in THF at low
ꢀ
[23]
ꢀ
contri-
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Communications
significantly contracted by virtue of the oxocarbenium
ꢀ
character of this site, whereas the corresponding C2 C3
bond in complex 10 (1.462(8) ꢂ) is hardly affected and falls
into the range of a regular single bond between a cyclopropyl
ring and an olefin.[26] Hence, we conclude that 10 is more
adequately described by a three-center two-electron bonding
motif of the kind previously proposed by the research groups
of Gagnꢀ and Toste[9,10] rather than as a cation flanked by a
gem-dimetalated center, as is the case in 8. The distinctly
different character of 8 and 10 shows that the charge density
resulting from the second auration step can either be “kept
within” the resulting {Au2C} entity or largely accumulated at
the adjacent position if a sufficiently stabilizing heteroele-
ment is present. This surprising modularity in the bonding
character suggests that gem-diauration might occur in many
structural environments of different chemical nature.
When treated with [(Ph3P)AuBr] in 2-PrOH, however,
boronate 9 transformed into the regular vinylgold species 12
(E/Z = 71:29, NMR spectroscopy), with no signs of dimeta-
lation discernible by spectroscopic means.[27] This result shows
that the counterion of the chosen gold source critically
determines the outcome of the reaction. [(Ph3P)AuBr] is not
electrophilic enough to be trapped by the primary intermedi-
ate, whereas the second auration is exceedingly fast if the
identical gold fragment [Au(PPh3)]+ is escorted by the less-
coordinating triflimide. Moreover, this experiment shows that
2-propanol (pKa = 17.1) is not capable of protonating the gold
center off complex 12 even at 508C.[28]
Scheme 4. a) [(Ph3P)AuNTf2], Cs2CO3, THF, ꢀ788C!RT, 51%;
b) CD2Cl2, 90% (NMR), see text; c) [(Ph3P)AuBr], 2-PrOH, Cs2CO3,
508C, 76%; d) [(Ph3P)AuNTf2], CD2Cl2, quantitative (NMR).
temperature, 9 converted into the corresponding diaurated
species 10, irrespective of the B/Au ratio. However, 10 was
found to be more fragile than its analogue 8. When kept in
CD2Cl2 solution, it readily decomposed to diene 11, with
[19]
concomitant formation of [Au(PPh3)2]NTf2 and colloidal
gold as the inorganic by-products.
The gem-diaurated character of 10 was evident from the
high-resolution mass spectrum (ESI+) and the characteristic
fingerprints in the NMR data. C2 is even more deshielded
(dC = 192.2 ppm) in this compound than in 8.[24] Surprisingly
though, the integrity of the cyclopropyl ring indicates that
insufficient charge density resides at this position to trigger
ring opening/rearrangement, as expected for an emergent
“cyclopropylmethyl cation”.[25] Therefore, it was of particular
interest to compare the structure of 10 in the solid state
(Figure 2) with that of 8 described above. Whereas the overall
Next, the characteristic b-alkoxyvinylboronate motif of 6
was embedded in the cyclic frame of compound 13,[29] in which
the double bond is conjugated to the ester carbonyl group and
is hence less electron rich (Scheme 5). Compound 13 under-
Scheme 5. a) [(Ph3P)AuBr], 2-PrOH, Cs2CO3, 508C, 86%;
b) [(Ph3P)AuCl], 2-PrOH, Cs2CO3, 508C, 60%.
went a clean gold-for-boron exchange with [(Ph3P)AuBr],
whereas the use of [(Ph3P)AuNTf2] led to decomposition. As
a result of the less nucleophilic character of the double bond,
only the corresponding monoaurated complex 14 was
obtained in a respectable 86% yield, and its constitution
was unambiguously confirmed by crystal-structure analysis
(Figure 3). Likewise, formal replacement of the oxygen atom
by sulfur, as in compound 15,[16b] afforded the corresponding
alkenylgold complex 16. This outcome is thought to reflect
the reduced nucleophilicity of a vinyl sulfide compared to a
Figure 2. Structure of complex 10 in the solid state.[19]
constitution of both complexes is similar and dominated by
the conspicuous gem-digold unit, the pattern of the bond
lengths in the backbone is strikingly different: thus, the C1
C2 bond in 8 (1.41(2) ꢂ) is notably elongated, whereas the
ꢀ
ꢀ
C1 C2 bond in 10 (1.366(8) ꢂ) is short and fairly close to that
ꢀ
of a regular olefin. In contrast, the C2 O1 bond in 8 is
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Figure 3. Structure of complex 14 in the solid state.[19] Selected bond
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regular enol ether, as well as the lower capacity of sulfur to
stabilize an adjacent carbocation.
Overall, this series of experiments has important ramifi-
cations for gold catalysis in general. The gem-dimetalation of
alkyne substrates by highly electrophilic gold complexes is
much more favorable than previously recognized;[30] it can
materialize either in a three-center two-electron arrangement
ꢀ
or in the form of two regular C Au bonds flanked by a
stabilized cationic center. For turnover of a given gold
fragment to occur in a catalytic setting, however, protodeau-
ration must outperform this facile side reaction, as the
resulting gem-digold complexes can be surprisingly resistant
to protolytic cleavage. Apparently, protons do not attack the
ꢀ
C Au bonds themselves with ease, at least in cationic species
such as 8, but rather seem to engage the p system of an
intermediate of type C as the primary site of interaction.[31]
Furthermore, our results show that the chosen gold catalyst
must be carefully matched with the donor capacity of the
ꢀ
incoming nucleophile X H to avoid such gem-dimetalation
pathways. The formation of stable gem-diaurated intermedi-
ates may partly sequester the catalyst and hence have an
impact on the necessary loading for a given transformation.
Since high turnover numbers are of eminent practical
relevance when working with precious noble metal species,
this competing process should be taken into consideration in
future catalyst optimization studies.
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Received: June 2, 2010
Published online: August 18, 2010
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Keywords: boron · gold · homogeneous catalysis ·
reactive intermediates · structure elucidation
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[18] By-products formed in small amounts are [Au(PPh3)2]NTf2 (dP =
45.3 ppm) and [(Ph3P)AuCl] (dP = 33.8 ppm); the chloride in the
latter product likely originates from activation of CH2Cl2 used in
the work up. The structure of [Au(PPh3)2]NTf2 in the solid state
is contained in the Supporting Information.
[19] The anisotropic displacement parameters are drawn at the 50%
probability level; hydrogen atoms are omitted for clarity.
CCDC-777626 (8), 779085 (10), 777624 (14), and 777625 ([Au-
(PPh3)2]NTf2) contain the supplementary crystallographic data
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stabilizing to about 5–10 kcalmolꢀ1
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[26] The average length for a single bond connecting a cyclopropyl
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deposited up to May 2010 in the Cambridge Crystallographic
Data File).
[27] Analogous attempts to react 6 with [Au(PPh3)]X (X = Cl, Br) in
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ꢀ
[20] A good comparison is the C Au bond length (2.081(4) and
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{Au2C} fragment is 2.12(6) ꢂ (344
structures deposited up to May 2010).
ꢀ
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8470 www.angewandte.org
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Angew. Chem. Int. Ed. 2010, 49, 8466 –8470