Synthesis, structure, and reactivity of a gold carbenoid complex that lacks heteroatom stabilization

Article abstract of DOI:10.1002/anie.201404882

Hydride abstraction from the neutral gold cycloheptatrienyl complex [(P)Au(η¹-C₇H₇)] (P=P(tBu) ₂(o-biphenyl)) with triphenylcarbenium tetrafluoroborate at -80 C led to the isolation of the cationic gold cycloheptatrienylidene complex [(P)Au(η¹-C₇H₆)]⁺ BF ₄^- in 52 yield, which was characterized in solution and by single-crystal X-ray diffraction. This cycloheptatrienylidene complex represents the first example of a gold carbenoid complex that lacks conjugated heteroatom stabilization of the electron-deficient C1 carbon atom. The cycloheptatrienylidene ligand of this complex is reactive; it can be reduced by mild hydride donors, and converted to tropone in the presence of pyridine N-oxide. Hydride abstraction from the neutral gold cycloheptatrienyl complex 1 (P=P(tBu)₂ (o-biphenyl)) with Ph₃C⁺ BF ₄^- formed the cationic gold cycloheptatrienylidene complex 2, which was characterized by single-crystal X-ray diffraction. The cycloheptatrienylidene ligand of 2 is reactive; it can be reduced by mild hydride donors, and converted into tropone in the presence of pyridine N-oxide.

Full text of DOI:10.1002/anie.201404882

Angewandte
Chemie
DOI: 10.1002/anie.201404882
Carbenoids
Synthesis, Structure, and Reactivity of a Gold Carbenoid Complex That
Lacks Heteroatom Stabilization**
Robert J. Harris and Ross A. Widenhoefer*
Abstract: Hydride abstraction from the neutral gold cyclo-
heptatrienyl complex [(P)Au(h1-C7H7)] (P = P(tBu)2(o-
biphenyl)) with triphenylcarbenium tetrafluoroborate at
À808C led to the isolation of the cationic gold cyclohepta-
trienylidene complex [(P)Au(h1-C7H6)]+ BF4 in 52% yield,
À
which was characterized in solution and by single-crystal X-ray
diffraction. This cycloheptatrienylidene complex represents the
first example of a gold carbenoid complex that lacks con-
jugated heteroatom stabilization of the electron-deficient C1
carbon atom. The cycloheptatrienylidene ligand of this com-
plex is reactive; it can be reduced by mild hydride donors, and
converted to tropone in the presence of pyridine N-oxide.
C
ationic gold carbenoid complexes have been widely
proposed as reactive intermediates in a range of gold(I)-
catalyzed transformations, including alkene cyclopropanation
and enyne cycloaddition.[1] Nevertheless, there remains con-
siderable debate regarding the nature of these complexes,[2–4]
which is in large part due to the lack of direct experimental
information regarding the structure and behavior of cationic
gold carbenoid complexes.[5] Rather, information regarding
the nature of cationic gold carbenoid complexes was for many
years restricted to indirect experimental evidence and com-
putational studies.[1–5] In 2008, Hammond reported the
spectroscopic detection of a gold carbenoid/oxonium com-
plex,[6] and since then, a number of cationic gold carbenoid
complexes have been isolated,[7–10] including Fꢀrstnerꢁs recent
Figure 1. Representative cationic, heteroatom-stabilized gold carbenoid
complexes (L=N-heterocyclic carbene or tertiary phosphine).[6–9]
of 1 either through hydride abstraction from a gold diphenyl-
methane precursor or through the reaction of a mixture of
[(P)AuCl] and AgSbF6 with diphenyldiazomethane failed to
form detectable quantities of 1, with the latter reaction giving
only the hydrazine complex 2 (Scheme 1).[11,12] Fꢀrstner
likewise noted the failure to generate complex 1 through
transmetalation from Group 6 diphenylcarbenoid com-
plexes.[8]
disclosure of
a
reactive dianisylcarbenoid complex
(Figure 1).[9] However, in all cases, these complexes rely on
the conjugation of heteroatoms to the electron-deficient C1
carbon atom for stabilization. Herein, we report the synthesis,
structure, and reactivity of a gold cycloheptatrienylidene
complex, which represents the first gold carbenoid complex
that lacks stabilizing, conjugated heteroatom groups.
Toward the synthesis of a gold carbenoid complex that
lacks conjugated heteroatom groups, we initially targeted the
gold diphenylcarbenoid complex [(P)AuCPh2]+ SbF6À (1; P =
P(tBu)2(o-biphenyl)). Unfortunately, the attempted synthesis
Scheme 1. Attempted synthesis of gold diphenylcarbenoid complex 1.
[*] R. J. Harris, Prof. R. A. Widenhoefer
French Family Science Center, Duke University
Durham, NC 27708–0346 (USA)
In an effort to impart greater stability in a heteroatom-
free gold carbenoid complex, we sought to exploit the
potential aromatic stabilization of the cycloheptatrienylidene
ligand.[13] An initial experiment was encouraging, and the
treatment of a solution of triphenylcarbenium tetra(perfluor-
ophenyl)borate in CD2Cl2 with the neutral gold cyclohepta-
trienyl complex [(P)Au(h1-C7H7)] (3; 10:4:3 mixture of
isomers) at À808C led to the immediate formation (ꢀ
5 min) of an approximately 3:1 mixture of the desired gold
E-mail: rwidenho@chem.duke.edu
[**] We acknowledge the NSF (CHE-1213957) for support of this
research. R.J.H. was supported in part through a Kathleen Zielek
fellowship (Duke University). We thank Dr. Rachel E. M. Brooner for
performing initial experiments directed toward the synthesis of
complex 1.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201404882.
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cycloheptatrienylidene complex [(P)Au(h1-C7H6)]+ B(C6F5)4
À
(4·B(C6F5)4) and an isomeric mixture of the bis(gold)
bicycloheptatrienyl complexes [{(P)Au}2(h1,h2-C14H13)]+ B-
(C6F5)4À (A·B(C6F5)4), which are presumably formed through
the attack of unreacted 3 on 4·B(C6F5)4.[14,15] On the basis of
this latter hypothesis, we reasoned that the formation of A
could be mitigated through the slow addition of 3 to a solution
of the triphenylcarbenium ion. Indeed, dropwise addition of
a solution of 3 in CH2Cl2 over 30 minutes to a solution of
triphenylcarbenium tetrafluoroborate in CH2Cl2 at À808C led
À
to the isolation of pure [(P)Au(h1-C7H6)]+ BF4 (4) in 52%
Figure 2. ORTEP diagram of 4·0.25CH3CN. One of two crystallograph-
ically independent molecules is depicted with ellipsoids shown at the
50% probability level with counterion and hydrogen atoms omitted for
clarity. Selected bond distances (ꢀ) and bond angles (deg) for
4·0.25CH3CN: Au1-C1=2.035(6), Au1-P1=2.3008(13), C1-
yield as a yellow microcrystalline solid (Scheme 2).
C7=1.406(9), C2-C3=1.385(9), C3-C4=1.396(11), C4-C5=1.327(11),
C5-C6=1.418(11), C6-C7=1.392(9), P1-Au1-C1=177.0(2), C7-C1-
C2=123.3(6), C7-C1-Au1=118.9(5), C2-C1-Au1=117.8(5).
atoms C20 and C24 of the tert-butyl group. The mean C-C-C
bond angle of the cycloheptatrienylidene ring is 128.5 Æ 1.98,
which is not significantly different from that of a regular
heptagon (128.68). The carbene carbon atom deviates most
significantly from the mean with a C7-C1-C2 angle of
123.3(6)8, which is closer to the idealized value for an sp2-
hybridized carbon atom than to that of a regular heptagon,
which may suggest a contribution of the gold carbene form
Scheme 2. Synthesis of gold cycloheptatrienylidene complex 4.
Complex 4 was thermally stable and characterized by
NMR spectroscopy and single-crystal X-ray diffraction. The
1H NMR spectrum of 4 displayed a 1:1:1 ratio of vinylic
resonances at d = 9.02, 8.79, and 8.64 ppm, which were shifted
downfield relative to the alkene protons of 3 (d ꢀ 7.9 ppm),
consistent with a contribution of the aromatic canonical form
4a (Scheme 3). In the 13C NMR spectrum of 4, the C1
4b.[14] Unfortunately, any systematic deviations in the C C
À
bond distances within the cycloheptatrienylidene ligand that
would be indicative of carbene contributor 4b are obscured
by the relatively large standard deviations associated with
these bond lengths.
The cycloheptatrienylidene ligand of 4 underwent reduc-
tion in the presence of mild hydride donors. For example, the
treatment of 4 with triethylsilane (2.4 equiv) in CD2Cl2 at
room temperature for 4.5 hours led to the formation of free
cycloheptatriene in 50% yield and the bis(gold) hydride
complex [{(P)Au}2(m-H)]+ BF4 (5) as the exclusive phos-
À
phine-containing species. This outcome is consistent with the
initial reduction of the cycloheptatrienylidene ligand of 4
followed by protodemetalation of 3 to form free cyclohepta-
triene and the cationic gold fragment [(P)Au]+, which reacts
with HSiEt3 to form 5 (Scheme 3).[19] Similarly, the treatment
of 4 with the Hantzsch ester gave a 1:1.2 mixture of free
cycloheptatriene and free 7,7’-bicycloheptatriene in 71%
combined yield as exclusive organic products, and [(P)Au-
Scheme 3. Reaction of 4 with hydride donor reagents.
(py’)]+ BF4 (py’ = dimethyl pyridine-3,5-dicarboxylate) as
À
carbenoid resonance appeared as a phosphorus-coupled
doublet at d = 225.5 ppm (JCP = 101.8 Hz), which is similar
to the C1 resonances of cationic cyclopentadienyl Group 8
dicarbonyl (d = 224–242 ppm)[14] and neutral PdII h1-cyclo-
heptatrienylidene complexes (d = 224–229 ppm),[16] and
slightly downfield of the C1 resonance of the cationic PtII
complex [trans-(PPh3)2PtBr(h1-C7H6)] (d = 210 ppm).[17]
Vapor diffusion of diethyl ether into a concentrated
solution of 4 in acetonitrile at 258C provided crystals of
4·0.25CH3CN suitable for X-ray analysis (Figure 2).[18] Com-
plex 4 adopts a near-linear conformation about gold with a P-
Au-C1 angle of 177.28 and with the cycloheptatrienylidene
ligand positioned perpendicular to the plane that reflects the
the sole phosphine-containing species (Scheme 3). This out-
come is consistent with the initial reduction of 4 followed by
either protodeauration of 3 to form free cycloheptatriene, or
the attack of 3 on 4 to form A, followed by protodeauration to
form free bis(cycloheptatriene).[15] Interestingly, no reduction
of 4 was observed in the presence of substituted cyclo-
heptatrienes such as p-anisylcycloheptatriene.
We also investigated the reactivity of 4 toward nucleo-
philes. Complex 4 reacts rapidly with triethylamine or ethyl
vinyl ether below room temperature to form intractable
mixtures of products. In contrast, treatment of 4 (21 mm) with
pyridine N-oxide (1.2 equiv) in CD2Cl2 at 258C for 20 minutes
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Scheme 4. Reaction of 4 with pyridine N-oxide.
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led to a selective oxygen atom transfer to form tropone in
77% yield and [(P)Au(py)]+ BF4À as the exclusive phosphine-
containing species (Scheme 4). This outcome is consistent
with a nucleophilic attack at the C1 atom of 4 followed by
elimination from intermediate I.[20]
In summary, we have synthesized the gold cyclohepta-
trienylidene complex 4 through hydride abstraction from the
neutral gold cycloheptatrienyl complex 3. Complex 4 repre-
sents the first example of a gold carbenoid complex lacking
heteroatom-stabilizing groups in conjugation with the C1
carbon atom. Downfield shifts of the cycloheptatrienylidene
protons of 4 relative to the cycloheptatrienyl protons of 3 in
the 1H NMR spectra point to the contribution of the
tropylium canonical form 4a, whereas the compressed C7-
C1-C2 bond angle (123.38) of the cycloheptatrienylidene
À
ligand of 4 relative to the mean C-C C bond angle (128.5 Æ
1.98) may suggest a contribution of gold carbene form 4b. The
cycloheptatrienylidene ligand of 4 is reactive; it can be
reduced by mild hydride donors such as triethylsilane and
Hantzsch ester, and converted to free tropone in the presence
of pyridine N-oxide.
Received: May 1, 2014
Published online: July 2, 2014
[15] In a separate experiment, reaction of a 1:1 mixture of 3 and 4 in
CD2Cl2 at 258C for 80 minutes formed isomers A as the
exclusive products, which were indistinguishable from the by-
products formed in the synthesis of 4. Complex A was
characterized without isolation by spectroscopy, mass spectrom-
etry, and through degradation studies (see the Supporting
Information).
[16] K. Mantas-Öktem, K. L. Öfele, A. Pothig, B. Bechlars, W. A.
Herrmann, F. E. Kꢀhn, Organometallics 2012, 31, 8249 – 8256.
[17] Z. Lu, W. M. Jones, W. R. Winchester, Organometallics 1993, 12,
1344 – 1350.
À
Keywords: carbene ligands · carbenoids · C C coupling · gold ·
reactive intermediates
.
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