Chemoselective carbophilic addition of α-diazoesters through ligand-controlled gold catalysis

Article abstract of DOI:10.1002/anie.201404946

The chemoselective addition of arenes and 1,3-diketones to α-aryldiazoesters was achieved through ligand-controlled gold catalysis. Unlike a dirhodium catalyst (which promotes C sp 3-H insertion and cyclopropanation) and a copper catalyst (which catalyzes O-H and N-H insertions), the gold catalyst with an electron-deficient phosphite as the ancillary ligand exclusively gave the carbophilic addition product, thus representing a new and efficient approach to form carbophilic carbocations , which selectively react with carbon nucleophiles.

Full text of DOI:10.1002/anie.201404946

Angewandte
Chemie
DOI: 10.1002/anie.201404946
Gold–Carbenoids
Chemoselective Carbophilic Addition of a-Diazoesters through
Ligand-Controlled Gold Catalysis**
Yumeng Xi, Yijin Su, Zhaoyuan Yu, Boliang Dong, Edward J. McClain, Yu Lan,* and
Xiaodong Shi*
Abstract: The chemoselective addition of arenes and 1,3-
diketones to a-aryldiazoesters was achieved through ligand-
controlled gold catalysis. Unlike a dirhodium catalyst (which
À
promotes Csp3
H insertion and cyclopropanation) and
À
À
a copper catalyst (which catalyzes O H and N H insertions),
the gold catalyst with an electron-deficient phosphite as the
ancillary ligand exclusively gave the carbophilic addition
product, thus representing a new and efficient approach to
form “carbophilic carbocations”, which selectively react with
carbon nucleophiles.
T
he “push–pull” dual reactivity[1] of gold complexes renders
them versatile compounds for the synthesis of complex
molecules, because it allows the use of gold–carbenoid-type
reactivity. Despite the early debut of gold–carbenoids, the
direct proof of the existence of such an intermediate is largely
missing and mostly relies on indirect evidences[2] and compu-
tational studies.[3] The nature of gold–carbenoids, i.e. gold–
carbenes or gold-stabilized carbocations, remains elusive
(Scheme 1A). A recent comprehensive NMR study per-
formed by Widenhoefer and co-workers suggested that only
cationic gold intermediates were involved in gold–carbenoid-
related transformations.[4] More strikingly, Fꢀrstner and co-
workers later reported the first structure of a reactive gold–
carbenoid, which showed a very weak electron donation from
gold to the carbene center.[5] Whereas these results provided
insight into the nature of gold–carbenoids, most of the
reported gold-catalyzed carbene transfer reactions proceed
according to the typical carbene reactivity, i.e. in cyclo-
propanation and cyclopropenation reactions (Scheme 1B).
Herein, we report a chemoselective electrophilic aromatic
Scheme 1. Reactivity of gold–carbenoids: carbene versus carbocation.
substitution on a-diazoesters through ligand-controlled gold
catalysis. With an electron-deficient phosphite P(OAr)3 as the
ligand, the gold–carbene intermediate acts as a “carbophilic
carbocation”, leading to a selective nucleophilic addition on
carbon, without addition at typical carbene receptors such as
phenol and alkene (Scheme 1C).[6]
a-Diazoesters[7] are valuable compounds for carbene
À
À
transfer reactions and have been applied in C H and X H
insertion reactions catalyzed, e.g., by dirhodium[8] and
copper[9] complexes. The first gold-catalyzed decomposition
of a diazoester was reported in 2005[10] and since then many
more followed.[11] However, compared to the p-acid activa-
tion mode, this type of gold catalysis is less explored and little
mechanistic insight has been provided so far.
We hypothesized that in comparison to Rh and Cu cations,
gold cations have a very different coordination ability, which
was observed in numerous gold catalyzed reactions. Thus, the
[*] Y. Xi,[+] Y. Su,[+] B. Dong, E. J. McClain, Prof. X. Shi
C. Eugene Bennett Department of Chemistry
West Virginia University
Morgantown, WV 26506 (USA)
E-mail: Xiaodong.Shi@mail.wvu.edu
Homepage: http://community.wvu.edu/~xs007/
=
formation of a distinct Au C bond should translate into
Z. Yu, Prof. Y. Lan
School of Chemistry and Chemical Engineering
Chongqing University
400030, Chongqing (P.R. China)
E-mail: LanYu@cqu.edu.cn
a different reactivity when the proper conditions are found.
This hypothesis proved correct when we observed the
exclusive electrophilic aromatic substitution (EAS) of 4-
methoxytoluene with methyl phenyldiazoacetate (Sche-
me 2A), which was achieved through slow addition of the
diazoester in the presence of PPh3AuNTf2. This result clearly
indicated an orthogonal reactivity of the gold catalyst
compared to a dirhodium catalyst, which exclusively pro-
[+] These authors contributed equally to this work.
[**] We are grateful to the NSF (CAREER-CHE-0844602 and CHE-
1228336) and the NSFC (No. 21228204) for financial support. Y.L.
acknowledges support from the National Science Foundation of
China (No. 21372266 and 51302327).
À
moted the benzylic C H insertion without the occurrence of
electrophilic aromatic substitution.[12]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201404946.
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Table 1: Optimization of the reaction conditions.[a]
Entry Catalyst (mol%)
Conv. [%][b] 3aa [%][b] I [%][b]
1
2
Ph3PAuNTf2 (5)
IPrAuNTf2 (5)
100
30
48
3
13
5
3
4
5
6
JohnPhosAuNTf2 (5)
Di-Ad-BrettPhosAuNTf2 (5) 100
tBu3PAuNTf2 (5)
Me4tBuXPhosAuNTf2 (5)
MorDalPhosAuNTf2 (5)
(ArO)3PAuNTf2 (5)
(ArO)3PAuNTf2 (5)
100
24
18
19
12
3
70
87
82
74
0
6
8
50
16
37
5
Scheme 2. The distinct chemoselectivities observed with gold–, rho-
86
100
41
À
À
dium–, and copper–carbenoids in C H and O H insertion reactions.
Conditions: Nu (5 equiv) and Ph3PAuNTf2 (5 mol%) in dry DCM
(0.1 mL) was added to a solution of methyl phenyldiazoacetate
(0.2 mmol) in dry DCM (0.9 mL) over 1 h with a syringe pump, Ar.
DCM is degassed with Ar. The NMR yield was calculated using 1,3,5-
tribromobenzene as an internal standard.
7
8
83
9[c]
100
100
100
89
100
40
5
6
10[c,d] (ArO)3PAuNTf2 (5)
11
(ArO)3PAu(TA-Me)OTf (5)
(ArO)3PAuNTf2 (5)
AgNTf2 (5)
Cu(OTf)2 (5)
Rh2(OAc)4 (5)
12
22
3
16
41
12[e]
13
13
7
0
14
15
A different chemoselectivity was also observed for the
reaction between phenol and methyl phenyldiazoacetate:
100
whereas a copper catalyst has been shown to efficiently
[a] Reaction conditions: 2a (5 equiv) and catalyst in CDCl3 (0.1 mL) was
added to a solution of 1a (0.1 mmol) in CDCl3 (0.9 mL) over 30 min
using a syringe pump. [b] Conversion and yield were determined by
1H NMR spectroscopy using 1,3,5-tribromobenzene as internal stan-
dard. [c] Reaction time: 2 h. [d] Open flask (air, wet solvent), syringe-
pump-free. [e] 2a is absent. Ar=2,4-di-tert-butylphenyl. TA-Me: N-
methyl-benzotriazole.
[13]
À
catalyze the O H insertion reaction, the gold(I) complex
surprisingly promoted the para-alkylation in modest yield
(Scheme 2B).[6] These rather unexpected results prompted
our interest in exploring the carbocation reactivity of gold–
carbenoids to achieve a selective nucleophilic addition on
carbon, even in the presence of carbene-sensitive functional
groups such as phenol, alkene, and alkyne.
We began our study using methyl phenyldiazoacetate (1a)
and 1,3-benzodioxole (2a) as the model substrates. Since
gold(I) complexes exhibit a simple linear coordination
pattern, the reactivity of the resulting gold–carbenoid
should strongly depend on different ancillary ligands. As
shown in Table 1, the cationic gold(I) complex with Ph3P as
the ligand, led to the complete conversion of 1a and furnished
the desired product 3aa in 48% yield (entry 1). The catalyst
with the electron-rich ligand IPr showed inferior reactivity
toward the conversion of the diazo compound (entry 2).
Electron-rich phosphine ligands, on the other hand, showed
reactivities similar to that observed with the Ph3P-substituted
catalyst, although lower yields were obtained (entry 3–6).
Interestingly, MorDalPhos,[14] a superior ligand for non-diazo
gold–carbenoid chemistry,[15] also showed low reactivity
(entry 7). A careful examination of the crude reaction
mixture indicated the formation of at least five different by-
products, with the major one being the a-oxoester I, which we
propose to be a product of the reaction between the metal
carbene and oxygen, a common side reaction observed in
dirhodium–carbene chemistry.[16]
albeit a slower conversion was observed (entry 8). An
extension of the reaction time to two hours led to full
conversion and 87% yield of 3aa (entry 9). Notably, this
transformation (entry 10) was performed under open-flask
conditions without solvent pretreatment and syringe pump,
which is rarely seen in diazo chemistry.[18] Remarkably, the
observed chemoselectivity is unique for gold–carbenoids,
because dirhodium, copper, and silver catalysts gave only
marginal amounts of product (entries 13–15).[19]
With these results in hand, we examined the general
applicability of the reaction with regard to the chemoselec-
tivity and the substrate scope. The results are summarized in
Table 2. In general, electronically activated arenes undergo
this transformation leading to the corresponding products in
good to excellent yields. As shown in Scheme 2, the gold
catalyst promoted the electrophilic aromatic substitution with
4-methoxytoluene and phenol, whereas the rhodium and
À
À
copper catalyst promote the C H and O H insertion,
respectively (3ab, 3ac).[20] Notably, aniline, which is a poten-
tial ligand and a strong N nucleophile, also furnished the
Friedel–Crafts-type product 3ad, even though a low yield was
À
Based on these preliminary results, we hypothesized that
the product selectivity could be altered by fine-tuning the
electronic nature of the ligand.[17] Specifically, an electron-
donating ligand might lead to the gold–carbene resonance
form (Scheme 1A), whereas a p-acceptor ligand would favor
the formation of the gold-stabilized carbocation resonance
form. Thus, the latter type of catalyst could benefit the desired
electrophilic aromatic substitution by eliminating the by-
products resulting from the typical carbene pathway. Gratify-
ingly, the phosphite–gold(I) complex gave very good yields
obtained (the major product resulted from the N H inser-
tion).[21] This result supports the carbophilic nature of gold–
carbenoids compared to copper–carbenoids (which exclu-
À
sively gave the N H insertion products). Electron-rich
heterocycles, such as benzofuran and pyrrole, are also suitable
substrates for this aromatic substitution (3ae, 3af), whereas
the cyclopropanation reaction is dominant when the dirho-
dium catalyst is used.[22] Similar results were obtained with
anisole in an iron-promoted Buchner reaction with the
identical diazoester.[23]
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Table 2: Scope of arenes and chemoselectivity.[a]
Table 3: Scope of a-diazoesters.[a]
[a] General reaction conditions: 1 (0.2 mmol), 2a (0.8 mmol), 5 mol%
(ArO)3PAuNTf2 in DCM (1.0 mL), 30 min-2 h. Yield of the isolated
product.
Table 4: 1,3-Diketones as effective carbon nucleophiles.[a]
[a] General conditions: 1a (0.2 mmol), 2 (0.8 mmol), (ArO)3PAuNTf2
(5 mol%) in DCM (1.0 mL), 30 min–2 h. Yield of the isolated compound.
[b] 5 equiv of nucleophile was used, slow addition in 1 h, Ar. [c] Two
regioisomers were formed (ratio 2.4:1). [d] 2 equiv of nucleophile was
used. [e] 5 mol% Ph3PAuNTf2 was used. [f] Reaction time: 12 h.
[g] 2 equiv of 1a and 1 equiv of nucleophile were used.
The investigated transformation also shows good func-
tional group compatibility as illustrated in the second part of
Table 2. Indole, ester, and amide groups were well tolerated
under the reaction conditions. More impressively, substrates
bearing an olefin or an alkyne exclusively underwent Friedel–
Crafts alkylation instead of cyclopropanation and cyclopro-
penation, which have been reported previously for similar
gold catalysts (3al, 3am).[24] In the case of 3am, the alkyne
also remained intact, demonstrating the potential tandem use
of p-activation and diazo decomposition reactivity. Moreover,
a cholesterol derivative also gave the alkylation product in
satisfactory yield (3an), which highlights the potential usage
of this transformation for the late-stage modification of
complex molecules.
The scope of a-diazoesters was evaluated as shown in
Table 3. This reaction tolerates both electron-rich and elec-
tron-poor a-aryldiazoesters, when 2a is used as the nucleo-
phile. Electron-poor diazoesters (1b and 1d) showed better
reactivity, whereas the electron-rich diazoester 1c gave
dimerization products under standard conditions. A more
electron-rich nucleophile, 1,3,5-trimethoxybenzene, gave
a higher yield of the product (3ck). Notably, the diazoox-
indole worked well under the optimal conditions (yielding
3 fa), which provides an efficient approach to incorporate
[a] General reaction conditions: a solution of 1a (0.2 mmol) in 0.8 mL
DCM was added to a solution of 4 (1.0 mmol) and (ArO)3PAuNTf2
(5 mol%) in 0.2 mL DCM with a syringe pump over 1 h. Yield of the
isolated product.
various carbon-substituted groups at the C-3 position of
indoles.[25]
After the successful application of this gold-catalyzed
Friedel–Crafts-type alkylation, we envisioned that the use of
other carbon nucleophiles in conjunction with the gold–
carbenoid might lead to the discovery of novel methodologies.
As showcased in Table 4, 1,3-dicarbonyl compounds success-
fully serve as the nucleophile[26] to furnish tricarbonyl com-
pounds 5, which are rarely found in the literature.[27] Again,
this transformation gave exclusively the C-nucleophilic addi-
tion product over O-nucleophilic addition product,[28] which is
consistent with the chemoselectivity observed with phenol.
To gain more insight into how the ancillary ligand controls
the gold–carbenoid reactivity,[29] we briefly investigated this
new system with the density functional theory (DFT) method
M11-L[30] using the standard basis set 6-311 + G(d) (LanL08
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In summary, we have developed a ligand-controlled gold-
catalyzed a-diazoester decomposition for the formation of
a carbophilic carbocation. The observed chemoselectivity
differs from the typical metal–carbenoid reactivity and largely
depends on the choice of the ancillary ligand. Preliminary
DFT calculations and our experimental observations imply
that the nature of the gold–carbenoid, though varied with
supporting ligands, better fits the description of a gold-
stabilized carbocation, as Fꢀrstner recently presented.[5a] The
application of gold–carbenoids as carbocationic electrophiles
À
for C C bond forming reactions and detailed mechanistic
investigations are currently underway in our laboratory.
Scheme 3. Electron static potential maps for compounds A and B with
the tris(2,4-di-tert-butylphenyl)phosphite and triphenylphosphine
ligand, respectively. The values in parentheses are M11-L-calculated
NPA charges. The bond order for each gold–carbene bond is given
below.
Received: May 4, 2014
Published online: July 23, 2014
Keywords: carbenoids · carbocations · carbophilicity ·
.
diazo compounds · gold
basis set[31] for Au).[32] The calculated bond order of the gold–
carbene bond and the nature population analysis (NPA)
charge distribution for compounds A and B are shown in
Scheme 3. When the electron-deficient ligand tris(2,4-di-tert-
butylphenyl)phosphite is used (compound A), the gold–
carbene bond order is 0.439, which is 0.051 lower than that of
compound B with the Ph3P ligand. The difference of these
bond orders indicates that the use of phosphite (ArO)3P as
the ligand leads to a complex with a lower fraction of the
gold–carbene resonance form compared to the complex with
Ph3P as the ligand. The charge distribution shows that the
positive charge of the Au atom in compound A is 0.306, which
is 0.050 lower than in compound B. Accordingly, the charge of
the C1 atom in compound A is only 0.017 lower than that in
compound B.
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[32] Calculation results obtained with other DFT methods are
summarized in the Supporting Information.
[33] It is not clear at this moment whether this 1,2-proton shift is
concerted or stepwise.
Angew. Chem. Int. Ed. 2014, 53, 9817 –9821
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